Woodhead Publishing Series in
Electronic and Optical Materials
Graphene Quantum
Dots
Biomedical and Environmental Sustainability
Applications
Edited by
Mohammad Oves
Khalid Umar
Iqbal M. I. Ismail
Mohamad Nasir Mohamad Ibrahim
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List of contributors
Rohana Adnan, School of Chemical Sciences, Universiti Sains Malaysia, Penang,
Malaysia
Rameez Ahmad Aftab, Department of Chemical Engineering, Zakir Hussain College
of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh,
India
Shaikh Ziauddin Ahammad, Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology IIT Delhi, New Delhi, India
Hilal Ahmad, Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New
Delhi, India
Mohd Ashraf Alam, Department of Pharmacology, IIMS & R, Integral University,
Lucknow, Uttar Pradesh, India
Syed Wazed Ali, Department of Textile and Fibre Engineering, Indian Institute of
Technology Delhi, New Delhi, India
Marai Almari, Department of Surgery, Faculty of Medicine, University of Tabuk,
Tabuk, Saudi Arabia
Mohammad Azam Ansari, Department of Epidemic Disease Research, Institute for
Research & Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia
Mohammad Omaish Ansari, Centre of Nanotechnology, King Abdulaziz University,
Jeddah, Saudi Arabia
Abdul Hakeem Anwer, School of Mechanical Engineering, Yeungnam University,
Gyeongsan, Gyeongaan, Republic of Korea
Mohd Arshad, Department of Physics, Aligarh Muslim University, Aligarh, Uttar
Pradesh, India
Abdullah M. Asiri, Chemistry Department, Faculty of Science, King Abdulaziz
University, Jeddah, Saudi Arabia; Centre of Excellence for Advanced Materials
Research, King Abdulaziz University, Jeddah, Saudi Arabia
Mohd Ayaz, Ministry of Higher Education, Applied Biotechnology Department, Sur
College of Applied Sciences, Sur, Oman
Ogechukwu Bose Chukwuma, School of Industrial Technology, Universiti Sains
Malaysia, Penang, Malaysia
Farha Fatima, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar
Pradesh, India
xi
xii List of contributors
Sufia ul Haque, Advance Functional Material Laboratory, Department of Applied
Chemistry, Zakir Husain College of Engineering and Technology, Faculty of
Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh,
India
Uzma Haseen, Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar
Pradesh, India
Bahaa A. Hemdan, Environmental Microbiology, Laboratory, Water Pollution
Research Department, National Research Centre, Dokki, Giza, Egypt
Fahad Mabood Husain, Department of Food Science and Nutrition, Faculty of Food
and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia
Iqbal M. I. Ismail, Centre of Excellence in Environmental Studies, King Abdulaziz
University, Jeddah, Saudi Arabia; Department of Chemistry, King Abdulaziz
University, Jeddah, Saudi Arabia
Mohd Jameel, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar
Pradesh, India
Kiran Jeet, Electron Microscopy and Nanoscience Laboratory, Punjab Agricultural
University, Ludhiana, Punjab, India
Mangala Joshi, Department of Textile and Fibre Engineering, Indian Institute of
Technology Delhi, New Delhi, India
Sakshi Kapoor, Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia,
New Delhi, India
Mohammad Zain Khan, Industrial Chemistry Research Laboratory, Department of
Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, Uttar Pradesh,
India
Mohamad Nasir Mohamad Ibrahim, School of Chemical Sciences, Universiti Sains
Malaysia, Penang, Malaysia
Mohammad Nazim, Department of Chemical Engineering, Kumoh National Institute
of Technology, Gumi-si, Gyeongbuk-do, Republic of Korea
Mohammad Oves, Centre of Excellence in Environmental Studies, King Abdulaziz
University, Jeddah, Saudi Arabia
Tabassum Parveen, Department of Botany, Aligarh Muslim University, Aligarh, Uttar
Pradesh, India
Aftab Aslam Parwaz Khan, Chemistry Department, Faculty of Science, King
Abdulaziz University, Jeddah, Saudi Arabia; Centre of Excellence for Advanced
Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia
Huda A. Qari, Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; Department of Biological Science, King Abdulaziz
University, Jeddah, Saudi Arabia
Mohammad Amir Qureshi, Department of Chemistry, Faculty of Natural Science,
Jamia Millia Islamia, New Delhi, India
List of contributors
xiii
Mohd Rafatullah, School of Industrial Technology, Universiti Sains Malaysia, Penang,
Malaysia
Rani Rahat, College of Dentistry, University of Illinois, Chicago, IL, United States
Mohd Ahmar Rauf, Use-Inspired Biomaterials & Integrated Nano Delivery (U-Bind)
Systems Laboratory, Department of Pharmaceutical Sciences, Eugene Applebaum
College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI,
United States
Asif Saud, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia;
Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India
Mohammad Shahadat, School of Chemical Sciences, Universiti Sains Malaysia,
Penang, Malaysia
Muhammad Taqi-uddeen bin Safian, School of Chemical Sciences, Universiti Sains
Malaysia, Penang, Malaysia
Shuchita Tomar, Department of Textile and Fibre Engineering, Indian Institute of
Technology Delhi, New Delhi, India
Khalid Umar, School of Chemical Sciences, Universiti Sains Malaysia, Penang,
Malaysia
Mohammad Faisal Umar, School of Industrial Technology, Universiti Sains Malaysia,
Penang, Malaysia
Sadiq Umar, College of Dentistry, University of Illinois, Chicago, IL, United States
Waris, Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of
Science, Aligarh Muslim University, Aligarh, Uttar Pradesh, India
Mohammad Zubair, Department of Medical Microbiology, Faculty of Medicine,
University of Tabuk, Tabuk, Saudi Arabia
Preface
The book provides an overview of the fundamentals and advances in applications of graphene quantum dots. Concepts covered include a brief introduction, an overview of the structure and chemistry, fundamental properties of
different characterization techniques, and methods of preparation of graphene
quantum dots. The book has also included graphene and quantum dots recent
and emerging applications in various fields: antimicrobial therapy and medical
device development, bioimaging, biomedical tools development and clean
energy for environmental sustainability. It has a helpful, informative, and
valuable resource for researchers, scientists, and postgraduate and undergraduate students from various sectors. Thus, there is an immediate urge to
educate the young masses and professionals about the newer materials capable
of biomedical and environmental sustainability applications as well as various
advanced imaging, disinfectant, and environmental remediation technologies.
Graphene quantum dots, light yet mechanically strong materials, has the potential of being among the pioneer materials for the aforementioned field due
to wide possibilities such as large surface area, ease of functionality, fabrication from natural and synthetic materials, and so forth. In spite of these
qualities, the graphene quantum dots have yet to gain attention and wide
popularity, and this is the reason for this book. The intended potential audience
of this book is materials scientists and engineers working on biomedical tool
development and environmental remediation, scientists, practising engineers
and students working in the field of biomedical, energy, and environmental
science.
The book content covers the basic properties of graphene quantum dots,
their special properties and fabrication techniques. The book has discussed
most of the major biomedical and environmental remediation applications of
graphene-based quantum dots. The advantages will be readers of graphene
quantum dots in biomedical tool development and clean energy, and environmental sustainability to aid in materials selection. Apart from this,
prospects about utilizing quantum dots of graphene in modern day-to-day
life and the fabrication of tangible products have also been deliberated.
xv
xvi Preface
The information presented about the graphene quantum dots in simple and
lucid form and much attention has been paid to covering all aspects of the title
through to the present era.
Dr. Mohammad Oves
Dr. Khalid Umar
Prof. Iqbal Mohammad Ibrahim Ismail
Prof. Mohamad Nasir Mohamad Ibrahim
Chapter 1
Graphene and its quantum
dots: fabrication and
properties
Sakshi Kapoor1, Uzma Haseen2 and Hilal Ahmad1
1
2
Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi, India;
Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India
1.1 Introduction
“Nano” is a Latin word meaning “dwarf.” Scientifically materials that have at
least one dimension between 1 and 100 nm are considered a nanomaterial. The
name nanotechnology was proposed by Norio Taniguchi in 1974 and the
theory was explained by physicist Richard P. Feynman in 1959 in his lecture
“There’s Plenty of Room at the Bottom” [1]. Nanotechnology has entered
fields such as electronics, medicines, agriculture, aerospace, and many more,
highlighting its importance in every sector [2]. The physicochemical properties are enhanced when reducing from bulk to nanodimensional particles [3].
This implies particles smaller than the Bohr exciton radius, where the
movement of excitons is confined, energy band splits up into discrete energy
levels due to quantum confinement and size effect [4]. Therefore, it is significant to understand the properties of nanostructured materials to integrate
them into nanoscale devices for higher efficiency [5].
The carbon nanomaterials possess exemplary properties and diversified
applications, where graphene is one of the most popular allotropes of this
family due to its remarkable performance [6] in every stratum vis-à-vis other
classic nanomaterials [7]. The name “graphene” was designated by Mouras
et al. in 1987 [8], while the material was extensively explored after A. K Geim
and K. S Novoselov in 2007 isolated graphene from highly oriented pyrolytic
graphite (HOPG) [9]. Two-dimensional (2D) graphene nanomaterial is formed
of a single layer of sp2 hybridized carbon atoms organized in a honeycomb
matrix and possesses excellent properties at room temperature (RT) [10e12]
as shown in Table 1.1.
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00006-6
Copyright © 2023 Elsevier Ltd. All rights reserved.
1
2 Graphene Quantum Dots
TABLE 1.1 List of graphene properties at room temperature.
S.
No.
Property
Approximate value
1.
Large surface area
2630 m2 g1
2.
Young’s modulus
2.0 TPa
3.
Superior tensile strength
130 GPa
4.
High electron mobility
15,000 cm2 V1 s1
5.
Enhanced electrical conductivity
Due to zero-bandgap
6.
Great thermal conductivity
3500 W mK1e5300 W mK1
7.
Chemically stable with high optical
transparency and flexibility
88%e97.7% and (20%
elongation)
Monolayer, transferable, device quality graphene sheets (GSs) are obtained
by mechanical exfoliation [9] and epitaxial chemical vapor deposition [10,13].
These methods suffer a major drawback of large-scale production plus low
throughput due to time and cost constraint [14]. Therefore, this led to the
development of structurally similar compounds; graphene oxide (GO) and
reduced graphene oxide (rGO) that has great scientific benefits [15,16]. In the
case of GO, the aromatic crystal lattice of graphene is obstructed by epoxides,
alcohols, ketones, and carboxylic groups, marked by a rise in interlayer
spacing from 0.335 nm for graphite to more than 0.625 nm for GO and hydrophilicity [17,18]. The structure of GO and rGO possess a conjugated system of sp2 carbon atoms, few hundreds or thousands of nanometres wide and
domains of conjugation are up to few centimeters in length as in the case of
carbon nanotubes (CNTs) [11,19]. The unprecedented features are attained in
case of either bilayer or few-layer graphene. The reason is that functional
groups and defects attached on the aromatic carbon ring during various synthesis methods dramatically alter the structure of the carbon plane, thereby
making it inappropriate to use for device assembly [20]. Therefore, numerous
attempts have been made to achieve mono-to-bilayer graphene structure at
mass scale and reasonable cost through ecofriendly synthesis technique [21],
but the final target is still a dream. But it is hoped that soon efficient growth
methods will be developed, as happened for CNTs [22].
GS has extraordinary physical, chemical, and electronic properties as listed
earlier. Because of the hydrophobic nature of graphene, the suspension in
almost all solvents gets agglomerated in short period of time, inhibiting its
application in different areas. Many attempts had been made in the past to
enhance its dispersion by attaching more oxygen functionalities, cutting it into
one-dimensional (1D) graphene nanoribbons (GNRs) or zero-dimensional (0D)
Graphene and its quantum dots: fabrication and properties Chapter | 1
3
graphene quantum dots (GQDs) [23]. Therefore, more emphasis is laid down in
converting micron sized graphene sheets into GQD and graphene nanoribbons
[24]. A quantum dot (QD) is a crystal possessing dimension <10 nm, derived
from either metal (gold, silver, etc.), semiconductors (CdSe, CdTe, ZnS, etc.),
or carbon materials (carbon dots, graphene, etc.) [25]. GQDs, a new nontoxic
material under the category of 0D nanomaterials, hold the qualities of quantum
dots as well as graphene, that is possess a band gap required for distinct
electrical, mechanical, thermal, and optoelectronic properties [26]. The theme
of this chapter lies in discussing basic static characteristics of graphene
quantum dots relatively to graphene, surface-passivated GQDs and semiconductor QDs. The GQDs are prepared not only through synthetic protocols,
but are also procured after treatment of natural resources; all the preparation
protocols are also elaborated in this report.
1.2 Fabrication of graphene and its quantum dots
Innumerable strategies have been followed in the past for the synthesis of GSs
and GQDs [11]. Top-down techniques involve disintegration of source material
(like graphite rod/foil/pencil, graphene oxide, carbon black, coal, fullerene C60,
etc.) into nanofragments through electrochemical, chemical, and physical
incision. Contrarily, synthesis from organic precursors [11,27] indicates the
bottom-up approach, while certain drawbacks are associated with both the
techniques. The need of costly, high-end equipment’s and high purity raw
materials; tedious separation methods of GSs and GQDs from impurities and
larger size particles; and defects induced while preparation are few limitations
that are still faced by the researchers [25]. We had synthesized GSs and GQDs
through various techniques and studied the basic mechanism behind them with
comparative properties [5]. During acid refluxing of GSs, epoxy groups are
attached on the basal plane and edges of the aromatic crystal lattice. Further, the
epoxy lot oxidize at RT to form strong carbonyl group, making GSs brittle. The
reduction process during hydrothermal treatment gradually breaks the delicate
GSs into GQDs due to high pressure at elevated temperature [4]. In case of
electrolysis, electric field acts as a scissor in causing intercalation of ions and
cleavage of parent electrode dipped in electrolyte, into smaller nanoparticles
[5]. The numerous applications of graphene and GQDs in various fields are
drawn in Fig. 1.1. The major techniques followed to synthesize graphene, its
derivatives, and GQDs are discussed below in Tables 1.2 and 1.3.
1.3 Relative properties of graphene quantum dots
The features of the produced GQDs in comparison to its parent material (such
as GSs, GO, rGO, etc.), surface-passivated GQDs and semiconductor QDs are
classified under subsequent sections below. The properties discussed broadly
includes topography, structural, chemical, optical, biological and energy
conversion under static conditions.
4 Graphene Quantum Dots
FIGURE 1.1 Schematic showing applications of (a) graphene and (b) GQD in various fields.
1.3.1 Morphological and structural elucidation
(a) The uniform size distribution with number of layers and high transparency
of GQDs and GSs were acquired from field emission scanning electron
microscope (FESEM) as shown in Fig. 1.2. The micron sized graphene
sheets were employed in making GQDs (ca. 6e10 nm) by solvothermal
method as explained in Table 1.3(II), the structural parameters of which
are further noted in below (b-c) points.
(b) The cutting of graphite, oxidation of graphene sheets and GQDs, no. of
graphitic layers, crystallographic properties are determined by X-ray
diffraction (XRD), where enhanced inter layer spacing interprets increase in oxygen bonds [5]. GQDs exhibits two large peaks nearly at 9.6
degrees (001) and 25.8 degrees (002) having interlayer spacings of 9.16
and 3.43 Å, respectively, vis-à-vis GS (3.37 Å) as illustrated in Fig. 1.3a.
Therefore, there is a combination of few-layer GQDs (002); specifying
reduction due to hydrothermal process, and monolayer GO-GQD (001)
indicating oxidation, because of protic solvent (H3PO4). In contrast,
electrochemical process caused reduction during GSs exfoliation as seen
in Fig. 1.3a.
(c) Resonant Raman spectroscopy (RRS) is considered as the safest machine
to assess the structural nature of the material as the laser heating has
minimal impact on chemical structure of the sample. The four deconvoluted peaks at 2651, 2677, 2701, and 2721 cm1 wavenumber (Fig. 1.3b),
marks the prepared GSs as bilayer agreeing with XRD results [5,41]. The
presence of two subbands (2D1 and 2D2) confirms bulk graphite electrode
(GE) used for the fabrication of GSs as depicted in Fig. 1.3b. The ID/IG
(sp3/sp2) intensity ratio for surface-passivated GQDs (GQD 2) is 0.78 and
1.32 for source GSs (PGS) as seen in Fig. 1.3c. The bonding of oxygen
groups at the time of acid refluxing, increased ID/IG for purified GS (PGS)
[5]. Further, during solvothermal synthesis of GQD two all the defects on
the edges of the structure of PGS were encapsulated, thereby enhancing
crystallinity [42].
Graphene and its quantum dots: fabrication and properties Chapter | 1
5
TABLE 1.2 List of major techniques followed to synthesize graphene and its
derivatives.
S.No.
Approach
Protocol followed
References
1.
Chemical vapor
deposition (CVD)/
Epitaxial growth
(a) Cu foil substrate in a quartz tube
furnace at 1030 C with CH4 and H2
mixture
[10]
(b)Ni substrate in a sealed reaction
tube at 625 C with N2/IPA flow for
>1 min.
[13]
2.
Microwave heating
irradiation
Graphite flakes:(2,2,6,6tetramethylpiperidine 1-oxyl)
TEMPO:H2O2::1:0.1:2 heated at
1200 W for 100 s
[12]
3.
Mechanical cleavage
Tapping process is continuous
attaching and detaching of
exfoliated graphite (HOPG) using
scotch tape.
[9]
4.
Solvothermal
synthesis
Pentachloropyridine and metallic
potassium processed at 160 C for
10 h in Teflon-lined autoclave.
[28]
5.
Liquid-phase
exfoliation
(a) Graphite powder dispersed in
NMP/PVA/surfactant NaC in a mixer
with variable rotor diameter and
durations,
[29]
(b)H2SO4 and HNO3 refluxed
graphene sheets dispersed in DMF,
followed by multi-frequency
ultrasonication and centrifugation,
[30]
(c)Graphite flakes dispersed in NMP
and o-DCB were sonicated at
40 2 C (600 W) for 6 h at RT,
[31,32]
(d)Graphite dispersed in NMP and Cl
was produced in the mixture by
adding HCl and KMnO4.
[33e35]
(a) Graphite rod in
H2SO4:HNO3::3:1 under 2e10 V at
RT
[6]
(b)Graphite sheet in H2SO4 under
0e8 V at RT,
[36]
(c)Graphite foil in NaOH under 3 V,
H2SO4, Na2SO4, and LiClO4 under
2e10 V at RT and
[14,37,38]
6.
Electrochemical
exfoliation
Continued
6 Graphene Quantum Dots
TABLE 1.2 List of major techniques followed to synthesize graphene and its
derivatives.dcont’d
S.No.
Approach
Protocol followed
References
H2SO4 þ KOH þ DW under 10 V at
80 C,
7.
Chemical synthesis
(d)Graphite powder in sodium
chloride, sodium hydroxide, PVP,
SDBS and DTAB,
[39]
(e)HOPG in Na2SO4, K2SO4,
(NH4)2SO4, H2SO4 under 10 V at RT,
[20,21]
(f)Graphite foil in TBA/HSO4 under
10 V, 0.1 Hz in an ice bath.
[40]
GO synthesis
(oxidant þ solvent þ additiveheating at 90 C):
(a) Brodie: KClO3 þ HNO3,
(b) Staudenmaier: KClO3 þ Fuming
HNO3,
(c) Hummers:
KMnO4 þ H2SO4 þ NaNO3,
(d) Tour: KMnO4 þ H2SO4 þ H3PO4
and many more methods.
[15,17,18]
rGO synthesis:
(a) Thermal annealing at 900
e1100 C in ultra-high vacuum
(UHV) and Ar/H2,
(b) Microwave and photoreduction,
(c) Chemical reduction with
hydrazine hydrate and NaBH4,
(d) Photocatalytic reduction,
(e) Solvothermal reduction at
supercritical temperature of solvent
and many more methods.
[16,19]
After the process, the obtained graphene or its derivatives were sonicated, centrifuged, vacuum
filtrated, heated or freeze dried.
1.3.2 Surface-enhanced Raman scattering (SERS)
The GQDs were synthesized by electrochemical oxidation of graphene and
GQD-NTs by electrodeposition of GQDs into 200 nm anodic aluminum oxide
(AAO) membrane as shown in Fig. 1.4. GQD microbowls (Fig. 1.5d, inset)
were fabricated by utilizing the microspheres as electrodeposition template
TABLE 1.3 List of synthesis techniques and features of GQD comparative to its source and derivatives.
Products
Method
Yield
(%)
C/O or O/C
atomic ratio
Functional
groups attached
Absorption
shoulder (nm)
Fluorescence
Ex/Em (nm)
References
11.43
e
eOH, NeH, C]
O, CH2
420
Green
[23]
I. GQD versus CQD
Microwave- CA and urea
heated at 750 W for 4
e5 min
CQD
Electrochemical- ablation of
graphite rods under 50 V at
RT
0.81
e
eOH, NeH, C]
O, CH2
420
Green
GQD
Solvothermal-GO dispersed
in DMF heated at 200 C for
8h
9.81
e
eOH, NeH, C]
O, CH2, CeH
420
Green
II. GQD versus graphene
Electrochemical- stripping of
GEs in H2SO4, NaCl, NaOH
under 10V at RT
50
4.03
eOH, C]O, C]
C, CeO, CeH
208, 286
e
PGS
Refluxing-HNO3 acid
refluxing of GS at 100 C for
24 h
e
4.02
eOH, C]O, C]
C, CeO
236, 275
Blue
300/397
GQD5
Electrochemical- ablation of
GE in NaOH under 10 V at
RT
e
11.64
eOH, C]O, C]
C, CeO
251, 290
Blue
300/375, 394,
and 414
GQD4
Hydrothermal-PGS and DI
water heated at 200 C for
8h
e
6.59
eOH, C]O, C]
C, CeO
214, 263
Blue
300/405
GQD2
Solvothermal- PGS, H3PO4
and ethylenediamine heated
at 200 C for 8 h
e
2.30
eOH, C]O, C]
C, CeO
220, 265
Blue
300/400
[5]
7
GS
Graphene and its quantum dots: fabrication and properties Chapter | 1
CQD
Continued
Products
Method
Yield
(%)
C/O or O/C
atomic ratio
Functional
groups attached
Absorption
shoulder (nm)
Fluorescence
Ex/Em (nm)
References
III. GQD versus GO
GO
Pyrolysis- thermal heating of
citric acid at 200 C for 2 h
2.2
1.16
eCOOH, CeO
eC, C]O, eOH
Broad peak
below 600
Blue
330e480/450
e542
GQD
Pyrolysis- thermal heating of
citric acid at 200 C for
30 min
9
0.92
eCOOH, CeH,
C]O, eOH
362
Blue
300e440/362
and 460
[48]
IV. GQD versus surface-passivated GQD
GQD
Hydrothermal- HNO3 acid
refluxed GO heated at
200 C for 24 h
13.1
e
No polymer
chains
320
Blue
300e400/340
e320
GQDPEG
Hydrothermal- HNO3 acid
refluxed GO and PEG
heated at 200 C for 24 h.
28
e
eC]C, eOH, C
eH
No clear
absorption
peak
Blue
300e400/377
e493
[50]
After the process, the obtained GQDs were isolated by either column chromatography, decantation of supernatant, dialysis, centrifugation, vacuum filtration, evaporation,
or lyophilization.
8 Graphene Quantum Dots
TABLE 1.3 List of synthesis techniques and features of GQD comparative to its source and derivatives.dcont’d
Graphene and its quantum dots: fabrication and properties Chapter | 1
9
FIGURE 1.2 FESEM images: (a) electrochemically exfoliated thin graphene sheets and
(b) unseparated GQDs scattered densely on the surface of graphene sheets.
FIGURE 1.3 (a) XRD spectra of GS versus GQD, (b) Lorentzian peak fitted 2D band of GS
versus GE and (c) Complete Raman Spectra of PGS versus GQD with D, G, 2D, D þ G and 2G as
resonant modes. Reproduced with authorization from Kapoor S, Jha A, Ahmad H, Islam SS. Avenue
to large-scale production of graphene quantum dots from high-purity graphene sheets using
laboratory-grade graphite electrodes. ACS Omega 2020;5(30):18831e41.
FIGURE 1.4 Fabrication of GQDs and GQD-NTs for SERS analysis. Imprinted with authorization from Cheng H, Zhao Y, Fan Y, Xie X, Qu L, Shi G. Graphene-quantum-dot assembled
nanotubes: a new platform for efficient Raman enhancement. ACS Nano 2012;6(3):2237e44.
10 Graphene Quantum Dots
FIGURE 1.5 632.8 nm stimulated Raman plot of (a) 4 106 mol L1 R6G and (c)
2 102 mol L1 2,4-DNT dripped on: bare Si substrate, GQDs film (GQDs/Si) and GQD-NTs
(GQD-NTs/Si) adhered on Si substrate, respectively, (d) 4 106 mol L1 R6G adsorbed on (i)
GQD-NTs/Si, and (ii) GQD microbowls/Si, respectively inset: FESEM image depicting synthesized GQD microbowls with a width of 500 nm and a height of ca. 250 nm distinct from that of
GQD-NTs (ca. 25). and (b) 623.8 nm stimulated Raman plot of different concentration R6G
8 1011, 8 109, 8 107, 4 106, and 4 105 mol L1 (bottom to up) adsorbed on
GQD-NTs/Si, respectively. (e) 532 nm excited Raman spectra of GSs/glass (magenta) and GSs/
AAO (green), i.e., graphene sheets spin coated on glass and AAO membrane (90 nm), respectively.
Imprinted with authorization from Cheng H, Zhao Y, Fan Y, Xie X, Qu L, Shi G. Graphenequantum-dot assembled nanotubes: a new platform for efficient Raman enhancement. ACS Nano
2012;6(3):2237e44.
Graphene and its quantum dots: fabrication and properties Chapter | 1
11
under similar conditions as that for GQD-NTs, with a greatly lower aspect
ratio of ca. 0.5 unlike GQD-NTs (ca. 25). Rhodamine 6G (R6G) and 2,4dinitrotoluene (2,4- DNT) were used as the model molecules, to investigate
GQDs, GQD-NTs, GQD microbowls for SERS applications [43].
The adsorption of molecules on different SERS substrates are discussed
below:
(a) GQD-NTs: Intense peaks with high signal-to-noise (S/N) ratio were
recorded for R6G adsorbed on GQD-NTs, while GQD film depicted only
D and G bands and no peaks were observed on Si wafer under similar
environment (Fig. 1.5a). The Raman signal was 40- to 74- fold amplified
on GQD-NTs as compared to only 2- to 17-fold increase on graphene film,
with Si substrate as normalization reference. The limit of detection
measured for R6G concentration was found to be 109 M at the level of
(S/N) ¼ 3 (Fig. 1.5b). Hence, these results inferred that arrangement of
GQDs into geometrically well-defined nanotubes (NTs) proved to be a
promising substrate for SERS. The Raman peaks for 2,4-DNT on GQD
film were also found seven times less intense via-a-vis GQD-NTs as
shown in Fig. 1.5c, with no relevant peak detected on Si surface [43].
(b) GQD Microbowls: The much effective trap of R6G molecules on GQDNTs than GQD microbowls (Fig. 1.5d), played a vital part in strengthening the charge transmission among target molecules and graphene dots,
which is accountable for attaining intensified Raman signals [43].
Although, all the three assemblies are composed of GQDs (GQD film, NTs,
microbowls) but recorded measurements concludes that Raman signal strongly
depend on morphology, surface and interface chemistry. This is due to the fact
[43]:
(a) The porous walls and hollow nanostructure of GQD-NTs provided large
surface area for adsorption of molecules, effective charge transfer, and its
rough surface captured incident light more constructively. In addition,
GQD-NTs electrodeposited into porous AAO membrane exhibited a uniform array structure with GQDs (ca. 5 nm) systematically confined in the
NTs; meeting the requirement of ordered and regular display of the
nanoparticles for SERS.
(b) Contrarily, drop-casted GQD film had an uneven surface comprising of
agglomerated GQDs with a comparatively big size of ca. 100e200 nm
caused due to wet-to-dry process. The adsorbed molecules struggle to
establish a homogeneous molecular layer, coating the surfaces of the
loosely packed GQD group in this scenario.
Further, we inspected recently SERS study by spin-coating synthesized
GSs (Table 1.3(II)) on the surface of lab-made porous alumina membrane [44].
The Raman modes of graphene sheets (D, G, and 2D) on AAO membrane
(90 nm) were greatly enhanced vis-à-vis GSs coated on glass substrate as seen
12 Graphene Quantum Dots
in Fig. 1.5e. The Raman intensity increased 10 times due to the presence of
nanohole matrix of AAO membrane below the freestanding film of graphene
sheets [45].
1.3.3 Chemical study of nitrogen (N)- doping
The type of functional elements or molecules attached to the structure of GSs
and GQDs depends on the fabrication method, raw materials and chemicals
used for its synthesis. This can be evaluated via Fourier transform infrared
spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and energydispersive X-ray (EDX) as described below and listed in Table 1.3. The heteroatoms, functional groups, and ligands on the basal plane and edges of the
polyaromatic structure of GQDs and GSs helps them to gain hydrophilicity in
solvents [5].
The degree of oxidation and N-doping after following complete synthesis
mechanism as depicted in Fig. 1.6 was analyzed through XPS. The magnified
view of the survey range for C 1s (Fig. 1.7a,b) depicted rise in the quantity of
FIGURE 1.6 Mechanism followed for the preparation of N-doped GO and GQDs.
FIGURE 1.7 (A) Complete XPS survey constituting carbon, oxygen, hydrogen, nitrogen and
magnified deconvoluted peaks of C 1s for (a) GO, (b) OeGO, (c) ReOeGO, (d) GQD1 and (e)
GQD2. Imprinted with authorization from Zhao M. Direct synthesis of GQD with different fluorescence properties by oxidation of GO using nitric acid. Appl Sci 2018;8(8):1303.
Graphene and its quantum dots: fabrication and properties Chapter | 1
13
CeO and decrease in CeC and C]C, after oxidation of GO by HNO3 (i.e.,
OeGO). These results confirmed that oxidation causes cleavage of the CeC
and C]C bonds and attachment of oxygen functional batch. Further, hydrothermal procedure caused loss of oxygen groups in the form of CO2, CO, and
H2O in ReOeGO and GQD1 (Fig. 1.7c,d) due to reduction process [46]. The
complete survey in Fig. 1.7a, revealed introduction of N atoms into GO after
oxidation by HNO3 in the form of CeNeC (w399.5 eV) and Ne(C)3
(w401.5 eV) as seen in Fig. 1.8aed. Further, oxidation of R-O-GO disclosed
increase in oxygen and additional nitrogen molecules (eNO2 w406.4 eV) in
GQD2 as seen in Figs. 1.7e and 1.8d due to refluxing in HNO3. It was observed
that nitrogen content was small in case of O-GO obtained after 8M HNO3 acid
reflux treatment [46] and negligible in 5 M HNO3 refluxed GSs [5], which
confirms the fact that degree of N-doping also depends upon the molarity of
the source dopant.
1.3.4 Optical analysis
1.3.4.1 pH-dependent properties
GQDs were obtained by heating refluxed graphene sheets (H2SO4 and HNO3)
in DI water at 200 C for 10 h [4]. The absorption peak around 230 and 320 nm
was observed for oxidized GSs, that is attributed to pep* jump of aromatic
sp2 zones as shown in Fig. 1.9a. The photoluminescence (PL) peak red shifts
on changing the excitation wavelength from 320 to 400 nm, with sudden
decrease in intensity as shown in Fig. 1.9b. The intense PL emission peak
(430 nm) was recorded from the absorption bands of 320 and 257 nm excitation as depicted in Fig. 1.9c. The PL quenching phenomenon in GQDs is
observed in pH variant solutions [47], with strong emission in alkaline medium. The PL is quenched in acidic medium and depicts reversible phenomenon on changing the pH between 13 and 1 (Fig. 1.9d). The strong PL is
originated from the unbound zigzag sites on GQDs with a carbene-like triplet
ground state as explained by Pan et al. (Fig. 1.10a,b). The zigzag sites are
protonated under acidic environment due to reversible compound formed
FIGURE 1.8 Magnified deconvoluted XPS spectra of N 1s for (a) OeGO, (b) ReOeGO, (c)
GQD1 and (d) GQD2. Imprinted with authorization from Zhao M. Direct synthesis of GQD with
different fluorescence properties by oxidation of GO using nitric acid. Appl Sci 2018;8(8):1303.
14 Graphene Quantum Dots
FIGURE 1.9 (a) Absorption and PL combined plot of GQDs and oxidized GSs (Inset: digital
image of dispersed GQDs in water beneath visible light), (b) PL plot of the GQDs excited at
variable wavelengths, (c) PLE and PL plot with sensing wavelength of 430 nm and excited at
257 nm, respectively (Inset: digital image of dispersed GQDs in water under UV light) and (d) pHdependent PL plot where pH is switched between 13 and 1. Imprinted with authorization from Pan
D, Zhang J, Li Z, Wu M. Hydrothermal route for cutting graphene sheets into blue-luminescent
graphene quantum dots. Adv Mater 2010;22(6):734e8.
between the zigzag sites and H⁺ and restored in alkaline conditions producing
enhanced PL.
The ground state of carbene possess two electronic configurations: singlet
and triplet. The s1 and p1 orbitals are singly occupied in the triplet condition,
while two electrons that aren’t bound together are coupled in s2 orbital with
p2 orbital empty in singlet state. The valence band spin orbital energy difference in the ground state of the carbene is dE<1.5 eV. Since, the zigzag
edges are the most regular location for triple carbenes, the two electronic
transitions of 320 nm (3.86 eV) and 257 nm (4.82 eV) recorded in the PLE
spectra of triple state carbene can be interpreted as transitions from the s and
p orbitals (highest occupied molecular orbitals, HOMOs) to the lowest unoccupied molecular orbital (LUMO), as illustrated in Fig. 1.10c. The dE is
hence evaluated to be 0.96 eV within the needed value (<1.5 eV) for triple
carbenes, implying that the two transitions were correctly assigned. The blue
Graphene and its quantum dots: fabrication and properties Chapter | 1
15
FIGURE 1.10 (a) Mechanism for the hydrothermal cutting of oxidized GSs into GQDs: a mixed
epoxy chain formed of epoxy and carbonyl pair groups (left) is turned into a full cut (right) under
the hydrothermal treatment as explained in Section 1.2. (b) Models of the GQDs in acidic (right)
and alkali (left) media. The two models can be converted reversibly depending on pH. The pairing
of s (l) and p (B) localized electrons at carbene-like zigzag sites and the presence of triple bonds
at the carbyne-like armchair sites are represented. (c) Typical electronic transitions of triple carbenes at zigzag sites seen in the optical spectra. Imprinted with authorization from Pan D, Zhang J,
Li Z, Wu M. Hydrothermal route for cutting graphene sheets into blue-luminescent graphene
quantum dots. Adv Mater 2010;22(6):734e8.
emission is the irradiation decay of activated electrons from the LUMO to the
HOMO [4] because the two transitions are directly associated with the
measured blue PL.
The excitation independent blue radiation of the GQDs usually infer that
both the dimension and surface condition of sp2 bundles are consistent, ordered, and passivated, as illustrated in Table 1.3(III). Conversely, broad absorption, maximum emission wavelength, and low yield can be attributed to
nonuniform sp2 clusters contained in the nonpassivated GQDs, graphene, and
its derivatives [48].
16 Graphene Quantum Dots
1.3.4.2 Computational PL theory
Computational study is carried out through density-functional theory (DFT) to
imitate the fluorophore assembly inside the carbon dots and its functionalized
form. The band gap decreases eventually as the dimension of polyaromatic
structure expands (Fig. 1.11a); in sync with practical observations. This means
that emission red shifts from blue to red on increasing size of GQDs, as
emission is stimulated from polyaromatic lattice within the dots. Additionally,
the band gap of functionalized polyaromatic crystals of GQDs were remarkably elevated vis-à-vis original structure, concluding that surface can also
modify their fluorescence. The pristine GQDs (P-GQDs) functionalized with
organic molecules like 1,2-glycol, 1,2-dithioglycol, 1,2-ethylenediamine, 1,3propanediamine, 1,4-butanediamine and ethylamine are designated as (GGQDs), (DTG-GQDs), (EDA-GQDs), (PDA-GQDs), (BDA-GQDs), and
(EA-GQDs), respectively. As seen, the fluorescence of functionalized GQDs is
much stronger than P-GQDs [49]. The optimized design of EDA-GQDs, PDAGQDs, BDA-GQDs, their protonated products, and their portions are shown in
Fig. 1.11b.
The protonation process from ammonium to polyaromatic assembly via
carboxyl batch prevents electron transfer of the excited state; as a result, strong
influence seen on the electronic band structure of EDA-GQDs. Contrarily,
zigzag line of carbon atoms is available instead of cyclic structure when 1,2
ethylenediamine was not protonated. This cyclic structure was absent in the
protonated PDA-GQDs and BDA-GQDs, therefore, stronger enhancement of
fluorescence was found in the case of 1,2- ethylenediamine than by 1,3-
FIGURE 1.11 (a) The graph depicts the impact of increasing the size of polyaromatic assembly
and functionalized diamines on the band gap of redeveloped fluorophore structures inside carbon
dots and (b) complex representing EDA-GQDs, protonated EDA-GQDs, protonated PDA-GQDs
and protonated BDA-GQD, respectively. Imprinted with authorization from Qian Z, Ma J, Shan
X, Shao L, Zhou J, Chen J, Feng H. Surface functionalization of GQD with small organic molecules from photoluminescence modulation to bioimaging applications: an experimental and
theoretical investigation. RSC Adv 2013;3(34):14571e9.
Graphene and its quantum dots: fabrication and properties Chapter | 1
17
propanediamine and 1,4-butanediamine. The theoretical results concluded that
both size and surface effects are responsible in affecting the fluorescence
emission properties of GQDs [49].
1.3.4.3 Up-conversion PL emission
The surface passivation enhances the band gap and fluorescence properties,
majorly in 0D nanostructures [44]. This was demonstrated through upconverted emission spectra of GQDs-PEG (polyethylene glycol) from 431 to
544 nm when excited by 700e980 nm light, with a 10-fold rise in intensity
than GQDs as shown in Fig. 1.12a and Table 1.3(IV). The difference between
upconverted emission (Em) and excitation energy (Ex) remained constant
(dE ¼ 1.1 eV) as displayed in Fig. 1.12b, signifying multiphoton active process and anti-Stokes PL [50]. Zhu et al. published similar upconverted emission shift of GQDs in the visible region when excited from 600 to 900 nm [47].
1.3.4.4 Temperature-dependent PL
The GQDs were synthesized by heating glucose, DI water and H2SO4 at 200 C
for 3 h in Teflon-lined autoclave. The emission peaks positioned at w3.25 eV
(peak A) and w2.54 eV (peak B) in the PL spectra of GQDs film are shown in
Fig. 1.13a. The rise in temperature causes a blueshift of w80 meV in peak A
and redshift of w100 meV in peak B as displayed in Fig. 1.13b. In reality, the
photoexcited electrons are either eased into sp2 domains or confined at CeC
bonds with vibration relaxation; resulting in excitation free blue and ultraviolet
emissions, respectively. Temperature-dependent PL experiments reveal substantial electron-electron dispersion and electron-phonon interactivity,
implying that GQDs behave likely to inorganic semiconductor QDs [51].
FIGURE 1.12 (a) Up-conversion PL spectra of GQDs-PEG (inset: magnified view of emission
spectrum stimulated by 980 nm laser) and (b) relation between excitation and emission energy
fitted in equation Em ¼ 1.00Ex þ dE, (R2 ¼ 0.9747) with dE ¼ 1.1 eV. Imprinted with authorization from Shen J, Zhu Y, Yang X, Zong J, Zhang J, Li C. One-pot hydrothermal synthesis of GQD
surface-passivated by polyethylene glycol and their photoelectric conversion under near-infrared
light. New J Chem 2012;36(1):97e101.
18 Graphene Quantum Dots
FIGURE 1.13 (a) Temperature-dependent PL spectra of GQDs with radiations nearly at 3.25 and
2.54 eV are marked as peak A (magenta) and peak B (green), respectively excited by 325 nm laser.
Temperature-dependent (b) emission peak, (c) FWHM, and (d) integrated PL intensity (inset:
histogram of temperature-dependent intensity weight of peaks A and B). Imprinted with authorization from Yang P, Zhou L, Zhang S, Wan N, Pan W, Shen W. Facile synthesis and photoluminescence mechanism of GQD. J Appl Phys 2014;116(24):244306.
The reason for the red shift observed in peak B can be attributed to
interband transition; resembling decrease in bandgap of inorganic semiconductors. The FWHM of peak A remains unchanged, while it increases for
peak B with increasing temperature as depicted in Fig. 1.13c. The integrated
intensity of both the peaks narrows down with rising temperature (below
240 K) as shown in Fig. 1.13d, suggesting carrier relaxation through thermally
activated process. The nonradiative emission is thermally deactivated at low
temperatures. Further, intensity ratio remains the same at varied temperature
(Fig. 1.13d inset), signifying that both radiative emissions are not competitive
Graphene and its quantum dots: fabrication and properties Chapter | 1
19
with each other [51]. The blue quantum emission is 22% at RT that is nearly
17%, computed with quinine sulfate as standard.
1.3.5 Photoelectrochemical (PEC) cell
The photoelectric response is analyzed from as-prepared and passivated
GQDs, under 365 nm ultraviolet (UV) and 808 nm near-infrared (NIR) irradiation. This was realized using GQDs and GQDs-PEG photoelectrodes
fabricated on ITO by spin-coating, respectively. The basic phenomenon involves generation and dissociation of an electron-hole pair upon light exposure
followed by electron transport to ITO electrode; measured by three-electrode
system. The photocurrent density generated by GQDs under 365 nm light was
less than half the value produced by passivated GQDs (GQDs-PEG) electrode;
consistent with PL quantum yield as shown in Fig. 1.14. Further, the photonto-electron conversion potential of the GQDs-PEG below NIR laser was less
and steady than under UV source. This proves GQDs-PEG to be a promising
dopant nanomaterial for solar cell, where the light of the photon-to-electron
change expand from UV to NIR [50].
1.3.6 Cytotoxicity assay
Laser scanning confocal microscope (LSCM) was used to view cell imaging
and interaction of GQDs with live human and stem cells. Methyl thiazolyl
FIGURE 1.14 The response of photoelectrochemical cells beneath 365 nm UV light and 808 nm
NIR laser. Imprinted with authorization from Shen J, Zhu Y, Yang X, Zong J, Zhang J, Li C. One-pot
hydrothermal synthesis of GQD surface-passivated by polyethylene glycol and their photoelectric
conversion under near-infrared light. New J Chem 2012;36(1):97e101.
20 Graphene Quantum Dots
diphenyl-tetrazolium bromide (MTT) was used to detect their cytotoxicity
with different cells in comparison to other QDs as listed in Table 1.4. The
nanomaterial holds antibacterial properties if the survival rate of cells remains
high under its larger dose and longer exposure time [19].
1.3.6.1 GQDs versus CdTe and CdS semiconductor QDs
Zhang. et al. compared the cellular toxicity of GQDs with cadmium telluride
quantum dots (CdTe QDs) (Table 1.4(1)), former prepared by solvothermal
synthesis in DMF at 200 C for 8 h [47,52]. According to MTT detection results (Fig. 1.15a) and microscopic images (Fig. 1.15bec), the toxicity of
GQDs was less than CdTe QDs, and the cytological compatibility of GQDs
was finer [52,53]. It was also noted that stem cells expired due to poisonous
Cd2þ, after incubation with CdS nanoparticles [51].
1.3.6.2 GQDs versus C60 QDs
Electrochemically stripping of parent GE under 80e200 mA cm2 in NaOH,
synthesized GQDs. The stem cells, neurospheres cells (NSCs), pancreas progenitor cells (PPCs) and cardiac progenitor cells (CPCs) were investigated
(Table 1.4(2)) through GQDs and C60 nanoparticles [51]. Along with the stem
cells, tumor cells human lung carcinoma (A549), and human breast cells
TABLE 1.4 Live cells viability with their different concentrations under
GQDs and other QDs depicted by MTT assay.
Survival
rate (%)
S.No.
QDs
Concentration of cells
References
1.
GQD versus
CdTe QD
Tongue cancer cells
(a) 300 mg mL1
(b) 400 mg mL1
>80
versus<60
48 versus
49
[52]
2.
GQD versus
C60 QD
Stem cells
100 mL
(a) NSCs and CPCs
(b) PPCs
>80 versus
all dead
65 versus
all dead
[51]
3.
Fluorescent
GQDs
Bacteria S. aureus
(a) 1 mg 10 mL1 b-GQD,
g-GQD and y-GQD
(b) 3 mg 10 mL1 b-GQD,
g-GQD and y-GQD
(c) 5 mg 10 mL1 b-GQD,
g-GQD and y-GQD
97, 96, 98
85
<70
[19]
4.
P-GQD versus
EDA-GQD
Human Hela cells
(a) <125 mg mL1
>80 versus
>80
[49]
Graphene and its quantum dots: fabrication and properties Chapter | 1
21
FIGURE 1.15 (a) The cytotoxicity of GQDs * (P < .05), LSCM cell morphologies observed at
300 mg mL1: (b) CdTe QDs group-most cell ecptomas disappeared eventually and cell bodies
became tiny and smoother and (c) GQDs group: cells appeared cobblestone in shape and cellular
tuber was well-defined. Imprinted with authorization from Zhang J, Ma YQ, Li N, Zhu JL, Zhang T,
Zhang W, Liu B. Preparation of GQD and their application in cell imaging. J Nanomater 2016:52.
(MCF-7) were also examined. The confocal fluorescence images revealed
complete marking of the cytoplasm of the tumor cells (A549 and MCF-7) and
stem cells by GQDs easily. Also, the PL spots were comparatively feeble in the
central part of the NSCs nucleus, concluding that GQDs avoid disturbing the
genetic coding of the alive stem cells. The GQDs translocated the stem cells
smoothly without disrupting its internal structure and traits; desired quality
required in the area of regenerative medicines [51]. Contrarily, C60 nanoparticles could only label A549 and MCF-7 and were not found in stem cells
besides owing to their smaller size (ca. 5 nm). This concludes that size is not
the main reason for GQDs penetration and C60 dullness into the stem cells.
1.3.6.3 Fluorescent GQDs
The two variants of bacterial cells were investigated S. aureus and E. coli using
three different kinds of GQDs as the bio imaging materials [19]. The b-GQDs
emitted blue fluorescence after electrolysis in sodium methoxide and ethanol.
After reducing with hydrazine monohydrate and glucose, yellow (y-GQD) and
green (g-GQD) color emitting GQDs were obtained, respectively. The LSCM
images suggested effortless penetration of all the three types of GQDs into
both the bacterial cells through the outer cell membrane; dispersed uniformly
and homogenously over the entire cell surface. In case of S. aureus and E. coli
bacterial cells, former displayed higher quality bio-images for all the three
variants of GQDs. This can be attributed to productive electrostatic interplay
among GQDs and S. aureus bacterial cell wall membrane than E. coli [19].
Further, y-GQDs gave enhanced fluorescence images among all types of
GQDs because of its uniform size and great quantum yield, but its higher
concentration led to the death of cells as shown in Table 1.4(3).
22 Graphene Quantum Dots
The order in GQDs fluorescence and cell viability is observed (Table 1.4(3))
in below pattern:
(a) Fluorescence: y-GQDs > b-GQDs > g-GQDs
(b) Bacterial cells survival rate: g-GQDs < b-GQDs < y-GQDs
1.3.6.4 GQDs versus surface-passivated GQDs
The interaction of pristine GQDs (P-GQDs) and functionalized GQDs marked
the cytoplasm of Human HeLa cells smoothly (Table 1.4(4)). P-GQDs were
developed by heating graphite powder in H2SO4 and HNO3 at 120 C for 24 h.
1,2-ethylenediamine functionalized GQDs (EDA-GQDs) were prepared by
mixing P-GQDs and SOCl2 at 80 C for 2 h and heating again after adding 1,2ethylenediamine at 100 C for 4 h [49]. The fluorescence cell imaging performance obtained from P-GQDs were exactly obtained from relatively
smaller quantity of EDA-GQDs because of its improved emission efficiency
(17.6%). The impact of diamine functionalized GQDs was comparatively less
harmful with superior bioimaging execution than P-GQDs, as inferred from
toxicity practicals. This concluded that organic molecule functionalization on
GQDs effects little on the cell toxicity [49].
1.4 Conclusion and future prospects
This chapter described physical phenomena observed in graphene and its QDs
in static form. The traits of graphene quantum dots were studied comparatively
with graphene and its derivatives, semiconductor (Cd X¼ S, Te) QDs, and
surface-passivated GQDs. The fundamental properties are vital to understand
and utilize the material extensively in every field of application. The defects
originated and functional groups attached while synthesis procedure are
evaluated through structural and chemical characterization tools. The
mannerism in which PL is dependent on temperature, pH and passivation was
thoroughly studied with experimental and theoretical aspects and profound
results of SERS were obtained from uniformly arranged GQD assemblies plus
AAO membrane. The remarkable qualities of robust and nontoxic GQDs as
discussed in this chapter are compelling to explore this wonder nanostructure
material extensively in every field. The future scope of GQDs can be foreseen
in all fields of electronics, medicines, agriculture, and biological applications.
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Chapter 2
Graphene quantum dots
characterization and surface
modification
Muhammad Taqi-uddeen bin Safian1, Khalid Umar1,
Tabassum Parveen2, Iqbal M. I. Ismail3, 4, Huda A. Qari3, 5 and
Mohamad Nasir Mohamad Ibrahim1
1
School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; 2Department of
Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 3Centre of Excellence in
Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; 4Department of
Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia; 5Department of Biological Science,
King Abdulaziz University, Jeddah, Saudi Arabia
2.1 Introduction
Graphene quantum dots (GQDs) are the tiny version of graphene represented
by zero-dimensional (0D) nanomaterial structures with its length only reaches
less than 100 nm [1e3]. Given that it’s graphene structurewise, it has comparable physical and chemical properties of graphene, which include the likes
of high surface area, chemically stable, and high electronic mobility [4]. The
quantum size of GQDs enables quantum confinement characteristics of the
Broglie wavelength nanosystem. This phenomenon can be attributed to the
Bohr radius of the bulk exciton. When the size of quantum dot structures
reducing its dimension, the exciton is free to move in any direction due to less
restriction [5]. This leads to the unique optical and electrical properties of
quantum dots structures. Depending on the size of the GQDs, it might illuminate unique photoluminescence (PL) spectra that fit that particular size [6].
In this case, it was possible to customize a GQD based on its PL spectra to get
a specific size of the GQDs. Apart from the typical graphene properties, GQDs
also possessed NIR light absorption, photothermal properties, fluorescence
emission, and many more. Moreover, due to the tiny structure of GQDs, the
surface-to-volume ratio indicates there are many sp2 networks that available
which allow functional group attached from p-p stacking interactions [7].
Therefore, GQDs can be dispersed easily in water due to the functional group
available around the edges. In the characterization of GQDs, a related
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00001-7
Copyright © 2023 Elsevier Ltd. All rights reserved.
27
28 Graphene Quantum Dots
approach to graphene characterization can be used due to similarity in the
structure. In addition to that, optical studies such as photoluminescence are
useful to determine the particle size.
Surface modification of GQDs is essential to open up the restriction in
applications. Even size modification can also be classified as a surface
modification. Surface modification is also known as changing the functionalization of the GDPs. These functionalized methods are done to tailor-fit
GDPs to various applications. The basic properties of GDPs can be changed
via doping with heteroatoms, composites with polymers or inorganic materials, and changing the shape and size of the nanomaterial structures [8]. In this
chapter, we would touch upon the modification toward GQDs, which potentially address some challenges of pure GQDs limitation.
2.2 GQDs characterization
The characterization of GQDs is similar to graphene. The properties of GQDs
can be measured by using optical, microscopy, and surface state characterization. The optical characterizations are used to monitor the fluorescence
properties and vibration patterns as provided by UV-Vis spectroscopy, Raman
spectroscopy, and photoluminescence. The microscopy characterizations are
used to study its surface morphology can be provided by transmission electron
microscopy (TEM) and atomic force microscopy (AFM). While the surface
state characterizations to study its functional group composition are provided
by Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron
spectroscopy (XPS).
2.2.1 Optical characterizations
Optical characterization techniques investigate the change in the intensity,
polarization, energy, or phase of the light wave when interacting with the
studied sample. The advantages of this characterization are that it required
little sample preparation while being nondestructive, fast, and can be performed at room temperature. Optical absorption of graphene has been used to
monitor the feedback of the graphene extraction since the size, defects, and
concentration plays an essential role in defining the optical properties [9]. In
addition to that, fluorescence properties added to the quantum structure of
GQDs can be studied by using the PL spectrum. Combining all the optical
characterizations data, we can get a meaningful analysis to paint a picture of
GQDs from such a brief and short analysis.
2.2.1.1 UV-Vis spectroscopy
The study of UV spectra is based on the BeereLambert law, which obeys a
linear relationship between the absorbance and the concentration of one solution [10]. This means that the intensity of the beam that passed through a
Graphene quantum dots characterization Chapter | 2
29
solution is influenced by the concentration of the solution. The molecule inside
that solution absorbing certain wavelength energy from the beam, which
transitions electrons into higher energy orbital. Thus, by observing a sample at
a different wavelength, one can get the absorbance of the sample. UV-Vis
comprises the spectrum of the ultraviolet, visible, and IR regions. For graphene, it produces multiple responses depending on the energy. At lower
energies, classical electrodynamics responses from the electric field interaction
of graphene structure responsible for the absorption. While at higher energies,
the electronic excitation due to photon absorption resulting in interference
[11]. Graphene reflects very little given its constant absorbance value of 2.3%.
The difference in incident light reflection between a monolayer and multilayer
graphene is 0.1%e2% for 10 layers [12]. Hence, the absorption data can be
used as a good measurement to determine the number of layers for graphene
derivatives. Graphene feature a prominent peak in the range of 250e300 nm
due to the excitation of the p-electrons [13]. The same peak can be observed
for GQDs as the pep* transition of graphitic sp2 is the domain structure for
graphene relative [14]. Another peak that can be observed mainly for GQDs is
at around 300e390 nm, attributed to the oxygenated functional group present
on the GQDs [15].
2.2.1.2 Raman spectroscopy
Raman spectroscopy is based on the interaction of the visible light with the
chemical bonds of the sample. It is a proven tool to provide details such as
chemical structure, crystallinity, and molecular interaction based on the band
structure. Raman has been widely used for graphene characterization as its
responsiveness toward carbon-based materials [16]. The mechanic of Raman
lies in the resonance effect that increases when the laser source excitation
equal to the electronic band structure translation to the Raman intensity [17].
The Raman intensity reveals the functional groups attached, modification of
the matrix, the edge types, presence and type of defects, and the number of
layers. The signals formulated for the Raman intensity came from the vibration
of the atoms in the structure during the scattering of two phonon modes. For
any graphitic materials, the specific position and intensity of two bands known
as D band and G band are used to determine the graphitic structure. The same
applies to GQDs. From Fig. 2.1, the G band located around 1581 cm1 is
generated from the vibration of the carbon atoms among each other in the
lattice plane [18]. It also represents an aromatic structure in a 2D plane. At the
same time, the D band located around 1350 cm1 represents the disorder of
graphene matrix or defects [18]. Another important frequency often used in
graphitic materials is D0 band around 1620 cm1 is a weaker frequency which
generates from the carbon on the same plane as the G band originated from,
but relative toward the longitude of the neighbor carbon atoms. The other band
that predominant in graphene is called 2D band which does not represent
30 Graphene Quantum Dots
FIGURE 2.1 Raman spectroscopy of
GQDs. Reproduced with permission
from Liu D, Chen X, Hu Y, Sun T, Song
Z, Zheng Y, et al. Raman enhancement
on ultra-clean graphene quantum dots
produced by quasi-equilibrium plasmaenhanced chemical vapor deposition.
Nat Commun 2018;9:1e10. https://doi.
org/10.1038/s41467-017-02627-5.
defects in the graphene lattice but due to the wavelength excitation dispersion
located at 2700 cm1. For GQDs, most functional groups attached are tallied
in the D band intensity [19]. Hence, the ID/IG ratios can be used to determine
the oxidation level proportion to the carbon atoms.
The 2D band also signifies the number of layers in the graphene. Increasing
layers will shift the 2D band while it also becoming broader and shorter.
Fig. 2.2 showed the change in the 2D band with the increasing of layers up to
graphite. At the same time, noticeable G band intensity was also observed due
to the interaction between carbon atoms to its neighboring carbon atoms
perpendicular to the plane. The horizontal displacement produces phonons
with an elevated energy level resulting in the intensity of the G band [18]. In
the case of doping, the assimilation of heteroatoms into the GQDs lattice will
prompt more defective sites. This can be monitor through the ID/IG ratio,
where more than one ratio equal to highly functional GQDs. Raman spectroscopy is an essential tool in graphene derivative characterization, including
GQDs. Furthermore, a deep understanding of the Raman spectra can generate
lots of information regarding the GQDs characteristics, including its
properties.
2.2.1.3 Photoluminescence
Photoluminescence (PL) relies on the unyielding scattering of the lightning
source. The process of the light source illuminating the sample and reemission
causes a delay which may vary depending on the structure of the sample. The
mechanism is quite familiar to that of Raman spectroscopy. However, the
distinctive difference is that while Raman causes excitation to the molecules,
PL is determined by the emission energy by the electronic structure of the
sample [22]. GQDs accommodate quantum confinement effect due to link up
p-domains, especially around the edge of the lattice [23]. This behavior is
Graphene quantum dots characterization Chapter | 2
31
FIGURE 2.2 2D bands intensity in response to the number of layers. Reproduced with
permission Wu JB., Lin ML, Cong X, Liu HN, Tan PH. Raman spectroscopy of graphene-based
materials and its applications in related devices. Chem Soc Rev 2018;47:1822e1873. https://
doi.org/10.1039/c6cs00915h. Published by The Royal Society of Chemistry.
quite the opposite of the graphene due to its naturally high electron mobility
with zero band-gap. The photoexcited carriers entering the graphene structure
would typically slow down and recombined, which reduced the emission
process [24,25]. However, this scenario can be overcome by introducing bandgap toward graphene structure with defects. Defects do play an essential role
in affecting the electronic transitions and optical properties of the graphene
structure. GQDs are considered defected graphene derivatives as it cuts graphene sheets into smaller pieces. Any domains other than the perfect sp2
networks are regarded as defects in graphene and its derivative structures [26].
Therefore, PL can be confirmation tools for GQDs or any other functionalized
graphene traceable by the PL measurement. It should be noted that GQDs are
characterized differently than other functionalized graphene due to their
dissociation with isolated sp2 islands [26]. In many reports, color variations
were used to determine the variance of GQDs [1,6,27]. The GQDs are built
with light atoms resulting in great carrier-carrier interactions and a defining
spin multiplicity due to its weak spin-orbit coupling [23]. Therefore, it
possessed a much larger energy band than other compounds with a comparable
build. The range of spectrum it possessed is ranging from blue to green. In
addition to that, the PL is also strongly affected by pH. In acidic conditions,
the PL intensity was found to diminished, which probably due to the
32 Graphene Quantum Dots
protonation of Hþ ions at the edge of the GQDs. Vice versa in the alkaline
condition as the edge sites remain in this condition [28]. The size of the flakes
also is a big part of the energy gap. As such, PL can be used to determine the
size of GQDs fragments. It was found that the energy gap of GQDs is
disproportion to the diameter of the GQDs, as shown in Fig. 2.3 [29].
FIGURE 2.3 (a) TEM images of the GQDs and their shape and size. (b) PL intensity-dependent
to the size of the GQDs. Reprinted (adapted) with permission from Kim S, Hwang SW, Kim MK,
Shin DY, Shin DH, Kim CO, et al. Anomalous behaviors of visible luminescence from graphene
quantum dots: Interplay between size and shape. ACS Nano 2012;6:8203e8208. https://doi.org/
10.1021/nn302878r. Copyright (2012) American Chemical Society.
Graphene quantum dots characterization Chapter | 2
33
2.2.2 Microscopy characterization
Microscopy characterization analyzes and draws the surface of the sample
using photons, electrons, ions, or physical probes. In graphene characterization, TEM and AFM are often used as the tools to study the surface, which also
can be used to study any graphene derivatives such as GQDs.
2.2.2.1 Transmission electron microscopy (TEM)
TEM has been used to study the surface level of a sample down to the atomic
scale by using imaging techniques. In short, the electron-optical beam is
focused onto the sample attaining a magnified picture projected to the screen
or film. The picture is captured by converting the electron’s intensity on the
detector. The high-resolution picture taken can be used to identify ultrastructure as minuscule as 0.1e0.2 nm. TEM has been used in graphene and its
derivative to study the structure size and morphology [3,30e32]. A thin and
light structure such as graphene or its derivatives would require a free-standing
and stronger contrast for a better image. Layers can be analyzed by firing the
primary electron beam toward the graphene structure a bit tilted; just exterior
to the intention aperture region causing a small angle of electron scattered
highlighting the thickness of graphene film with dark field images. A better
sample preparation that includes transferring the graphene onto the TEM grids
needed to be addressed by either using a simple mechanical transfer of direct
synthesis transfer via chemical vapor deposition (CVD) [33,34]. For GQDs,
the lattice spacing around 0.24 nm can be used as a successful confirmation of
synthesis depending on the layers. The lattice spacing of GQDs should be the
same as the graphene it’s derived from [35]. Characterization of GQDs should
be straightforward as graphene with some addition of high degree in
crystallinity.
2.2.2.2 Atomic force microscopy (AFM)
AFM utilized a sharp tip of a cantilever to analyze the surface of a sample. The
sensitivity of the cantilever will react toward any surface-level changes
causing a better view of the nanoscale topography of a solid surface. A rise in a
surface will bend the cantilever, which in turn changing the direction of the
reflection beam tracking the cantilever movement. At the same time, the
cantilever is pulled down when there is a decline in level by attractive forces
by the surface [36]. The lever moves from point A to B, and the back and forth
movement of the cantilever was recorded describing the morphology of the
solid surface. In graphene characterization, the sensitivity of the cantilever can
detect the differences in height between functionalized-riddled graphene oxide
and graphene [37]. This is based on the principle that functionalized graphene
is slightly thicker than flat pure graphene due to the oxygenated functional
groups present across the surface. Other than that, AFM can be used as
34 Graphene Quantum Dots
indentation by microbending to test the mechanical properties of a solid
structure [38]. By using an indenter tip, force is introduced by pushing hard the
tip onto the surface. It is the only way to measure the physical properties of a
nanoscale structure for graphene. Graphene also responds to electrical interaction on its surface. Applying various voltages onto the AFM allows the study
of the potential barrier and other electrical properties of graphene [39]. For
GQDs, AFM can be used to determine the average diameter of the GQDs
presented between point A to B.
2.2.3 Surface state characterization
Surface state characterization study the electronic states or elements decorated
the surface of graphene and its derivatives. Two surface state characterization
normally used are FTIR and XPS. Both utilized the changing of the electronic
band structure on the surface due to the mass material forming an electronic
state called surface state.
2.2.3.1 Fourier transform infrared spectrometer (FTIR)
The mechanism of FTIR lies in the ability of molecules in absorbing certain
infrared light regions due to the bonds present in the molecule. Therefore,
functional groups attached to the surface of graphene sheets vibrate at a
different wavelength. The FTIR is normally used as a confirmation for GO and
RGO synthesis by monitoring the functional groups present in each structure.
In Fig. 2.4, the typical oxygenated functional groups discovered on GO are the
same can be found on GQDs; such as OeH, eCeO, C]O, C]OH, etc.
[1,40]. However, the variety of GQDs sizes may result in a variant of oxidation
exposure [41]. Table 2.1 showed the functional groups and their representative
absorption frequency. The hydroxyl group from a phenolic OH or OH from a
carboxylic group presents an intense peak around 3700e3000 cm1. There is
also a tendency for vicinity hydrogen being bonded together with other OH
groups, which creates the intensity [42]. Another peak around 1300 to
1000 cm1 is responsible for the existence of CeO with toward 1300 cm1
attributed by the bending of CeOeC [43]. A multilayer GQDs will also see
the existence of eCH2 and eCH peaks around 2900 to 2800 cm1.
2.2.3.2 X-ray photoelectron spectroscopy (XPS)
XPS is a surface quantitative spectroscopic that can identify the elements
within the material surface based on the photoelectric effect. The average
depth of XPS analysis is approximately 5 nm. The instrument measured spatial
distribution information within a resolution of w7 mm by scanning the
microfocused X-ray beam across the sample surface. The X-rays excite the
sample surface causing photoelectrons to be emitted that will be analyzed by
the binding energy and the intensity of a photoelectron peak. The
Graphene quantum dots characterization Chapter | 2
35
FIGURE 2.4 FTIR spectra of various carboxyl functionalized GQDs and GO [41]. Published by
The Royal Society of Chemistry.
TABLE 2.1 Major functional groups in GQDs and their absorption
frequency.
Functional group
Absorption frequency
OeH stretching
3400 cm1
CeH stretching
2910 cm1
CeH system stretching
2875 cm1
C¼O stretching
1687, 1710 cm1
C¼C stretching
1542, 1568 cm1
CeOeH bending
1409 cm1
CeO stretching
1208 cm1
CeOH stretching
1113 cm1
functionalized graphene across its surface can be studied using this method by
measuring its elemental composition, empirical formula, and electronic state
of the elements present. In most cases, XPS was used to study the morphology
of the graphene composite, especially with metal [44]. GQDs posed a typical
carbon structure signal such as CeC, C]C, and CeO. Besides the oxygenated functionalized group provide the intensity of the peaks. Hence, two
36 Graphene Quantum Dots
intense peaks of C1s and O1s were typically used to characterized GQDs [45].
The measurement of C1s usually are build-up from corresponding to sp2 and
sp3 C of C]C, CeC, CeOH, CeOeC, etc., as shown in Fig. 2.5. Moreover,
the presence of sp3 suggested that GQDs are filled with defects. The C/O ratio
can be calculated from this result to analyze the richness of oxygen in GQDs.
2.3 Surface modifications
Pure GQDs have many limitations that constrain their ability to be used in
many applications. Surface modification has been used to alter the GQDs
morphology in a way that enhanced its chemical, electronic, and optical
properties. This can be done by either controlling the size and shape of the
GQDs or merging GQDs with other substances such as heteroatoms, or
polymers. In other terms, changing the functionalization of GQDs will
improve their properties.
2.3.1 Tunable through size
Previously, it was mentioned that size played an essential role in synthesizing
GQDs. Some applications can only use a specific size of the material. For
example, a larger size GQDs can be used to reduce immune-mediated liver
damage which comparatively smaller sized GQDs would not work [47]. This
is due to large nanoparticles accumulated more in the liver after intravenous
injection for the intended procedure to work. In the other hand, smaller size
GQDs promote more edge states that can serve as a position to store charge
carriers suitable for energy storage applications [7]. Moreover, the color
changes can be observed depending on the structure size of GQDs due to
functional groups surrounding the GQDs [6]. More functional groups such as
OH, C]O, and COOH attaching themselves at the edge of the GQDs can shift
the tunes emissions from green to red. This also affects their chemical moieties
which will add to the potential in various applications. The size difference is
also emitting light differently due to the band-gap reduction, as shown in
FIGURE 2.5 The XPS spectra of C 1s for GQDs [46].
Graphene quantum dots characterization Chapter | 2
37
Fig. 2.6. GQDs are capable of emitting light ranging from blue-green (2.9 eV)
to orange-red (2.05 eV) [48]. The band-gap differences by the size and
morphology of GQDs have been studied intensively using density-functional
theory (DFT). This characteristic is attributed to the band-gap difference of
the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO). A large GQD mainly consists of more functional
groups, defects, and zig-zag sites that generate a different energy level than
smaller GQDs. A smaller GQD will possess a large pep* energy gap, thus
emitted higher energy blueish light [49]. The HOMO-LUMO gaps are also
being affected by the presence of functional groups, typically on the surface of
the GQDs. Edge-functionalization tends to boost only a little on the optical
and electronic properties, whereas a much more massive boost can be examined in surface-functionalized GQDs [50]. This is due to the great shift in the
electron density distribution for surface-functionalized GQDs compared to
pure GQDs and edge-functionalized GQDs. Bigger GQDs also can host more
functional groups on its surface, hence more favorable for more significant
electron density.
2.3.2 Doping of GQDs with heteroatoms
Doping is an essential process in the graphene family, especially in the
semiconductor industry. Modifying the structure with heteroatoms means to
enhance the chemical, electronic, and optical properties. The insertion of
FIGURE 2.6 Illustration of the reduction in band-gap with increasing size of GQDs.
38 Graphene Quantum Dots
heteroatoms such as nitrogen, phosphorus, oxygen, sulfur, etc. into the carbon
network structure can shift the electron balance and the Fermi level [51]. This
typically will improve the material toward a specific reaction which can be
utilized as a catalyst [52]. Similarly, the assimilation of heteroatoms onto a
GQD surface will modify its properties. This doping can be categorized into
three groups based on the number of type heteroatoms in GQDs lattice, single
heteroatom, and double heteroatoms.
2.3.2.1 Single heteroatom
Single heteroatom doping has been used to produce materials in energy-related
applications. The doping is done by following either substitution of the C
atoms from the lattice with the heteroatoms or attachment on the surface
through adsorption. The substitution method is more stable due to the stronger
covalent bonds created compare to the attachment on the lattice. It should also
be noted that the substitution also includes filing the empty carbon holes of the
defective sites in the GQDs lattice. Depending on the atom size and concentration, it might damage the honeycomb structure. This concern can be address
by properly monitor the balance of the doping concentration. Nitrogen in
GQDs is quite popular and widely used due to its reliable performance in
many fields. N has high electronegativity and will create polarization in the
GQDs structure [53]. Hence, a high concentration of N will heavily influence
the band-gap. However, in the acidic condition, the band-gap of N-GQDs will
reduce due to the N protonate. This phenomenon created a gap between the
protonated N atoms and the unprotonated one, which will result in a high p electron density, thus reducing the band-gap [54]. This means that the bandgap of N-GQDs can be fine-tuned through the pH level. PL is an essential
characteristic possessed by GQDs. This property enables GQDs to be utilized
as photoelectrochemical (PEC) cells materials. Although the performance for
GQDs in producing hydrogen and oxygen via solar energy is less efficient
from the practical operation, N-GQDs proved to be effective due to the N
dopants acting as active sites to promote kinetics of PEC mechanics [55].
Further improvement of longer carrier life-time was also noticeable with the
increasing of N heteroatom concentration. This was primarily due to the
intrinsic sp2-bonded carbons was further strengthened by the existence of
quaternary N in the network. Fig. 2.7 showed the PL spectra of the pure GQDs
and various concentrations of N-GQDs. A longer life-time can be observed as
the N dopant significantly participate in redox reactions, therefore extended
the carrier relaxation of the PEC cells.
A boron atom can be easily introduced into the GQDs lattice as the size is
almost similar to carbon. Furthermore, B is situated near carbon in the periodic
table while having fewer valence electrons. This means that carbon is more
electronegative than the B atom. As such, a B-doped GQDs will result in a ptype extrinsic semiconductor as B atoms are more likely to become an electron
Graphene quantum dots characterization Chapter | 2
39
FIGURE 2.7 PL spectra of pristine GQDs and various concentrations of N-GQDs at (a) 430 nm
and (b) 525 nm. Reprinted with permission from Tsai KA, Hsieh PY, Lai TH, Tsao CW, Pan H, Lin
YG, et al. Nitrogen-doped graphene quantum dots for remarkable solar hydrogen production. ACS
Appl Energy Mater 2020;3:5322e5332. https://doi.org/10.1021/acsaem.0c00335. Copyright 2020
American Chemical Society.
acceptor [51]. These electrons shift will create voids disrupting other selectrons which will further create more electron holes. More holes equal to
more shortage of electrons which in turn leads to better electrocatalytic performance [56]. However, there is a certain amount of B concentration allowed
before oversaturation will break the honeycomb structure. Optoelectronic is
one of the applications suitable for GQDs due to its PL properties. Doping
with heteroatoms enhanced said properties by the strong electron-withdrawing
effect. This phenomenon different from B heteroatoms as B-doped GQDs
would offer more active sites. As such, B-GQDs can be used as sensor
providing its intramolecular chain structure. This has been supported by a
study that showed the PL observed increased upon the interaction of glucose
with B-GQDs [57]. The boronic acid on the B-GQDs surface acted as a
glucose sensor where it aggregates with glucose interaction, thus increasing
the PL intensity. Apart from that, sulfur also has been demonstrated to be
useful once doped into GQDs lattice [58,59]. Although, the size of the S atom
is bigger compare to the C atom makes it difficult for the assimilation process.
Furthermore, the electronegativity of the S atom is almost identical to that of
the C atom; 258 and 2.55 respectively, which offer less significant electron
transfer compare to the other heteroatoms [59]. However, it was reported that a
significant fluorescent characteristic was observed from S-GQDs [58]. The SGQDs solution is on the softer side of yellow color but shown strong blue
fluorescence around 365 nm UV. S-GQDs was shown to favor selectivity toward Ag ions owing to the synergistic effect of the S and O functional groups
on S-GQDs [59].
40 Graphene Quantum Dots
2.3.2.2 Double heteroatoms
Double heteroatoms are also known as codoped, where two heteroatoms were
introduced into the GQDs network structure. The ability to utilized multiple
enhance abilities provided by two different heteroatoms has been the staple in
graphene doping scenes. Similar to the single-doped GQDs, codoped affected
the UV-VIS spectra of the structure. For example, a codoped N, P-GQDs
efficiently absorb shorter wavelengths due to the pep* shifts of the sp2
networks in the structures, which appear as brown under visible light. At the
same time, the PL spectrum exhibited a noticeable peak and a shoulder peak
around 460 and 360 nm, respectively [60]. The same scenario occurred with S,
N-GQDs UV-VIS spectra, where absorption peak due to the pep* transition
was observed at 237 and 369 nm [40]. In both cases, the codoped GQDs
showed an excellent ability as sensor materials. This enhancement has to do
with the PL quenching effect, which can increase efficiency in detection and
selectivity; both are good attributes for a sensor. The PL quenching effect
came from the transition of the localized bonds due to the dopants insertion.
Hence, codoped has the ability to increase the PL quenching factors by
creating more localized energy levels [61]. This phenomenon has been proven
based on the selectivity of Hg2þ ions sensing. The experiment showed that the
sensitivity of codoped S, N-GQDs is four times higher than only one
heteroatom-doped of N-GQDs [62]. The S, N-GQDs and N-GQDs exhibit blue
and bright green color under 360 nm UV irradiation before it became darker
after exposure with increasing of Hg2þ ions due to the PL quenching. However, the S, N-GQDs showed better sensitivity as it took only 30 ppm of the
Hg2þ ions for the quenching process to start while 75 ppm for N-GQDs, as
shown in Fig. 2.8. The different efficiency attributed to the S, N-GQDs are
having S and N atom more electron-rich than the C network. The doping
process increased the electron density of the structure resulting in strong PL
quenching with additional levels possibly were generated during the transition
phase [61].
Meanwhile, Yang et al. [63] utilized B, N-GQDs as a sensor material for
Hg2þ detection. The multi heteroatoms doping factor improves the detection
limit for Hg2þ ions as low as 0.16 mM. Furthermore, it was found that the
selectivity of the sensor also improves as the PL quenching efficiency does not
react toward other metal ions. This indicates that the material suitable in most
environments due to its selectivity but also highlighted the one-dimensional
(1D) of its usage. Apart from its fluorescence capability, GQDs also has
been developed heavily in the oxygen reduction reaction (ORR). This is due to
its surface structure, enabling quantum confinement and edge effects, resulting
in better electrocatalyst reactions [7]. However, it was found that is electrocatalytic activity is limited due to the nature of GQDs, which is too small as
the base material in the electrode as it posed significant percolation threshold
values with low electrical conductivity [64]. Fan et al. [65] compared codoped
Graphene quantum dots characterization Chapter | 2
41
FIGURE 2.8 The quenching effect shown by (a) N-GQDs and (b) S, N-GQDs with the increasing
of Hg2þ ions. Reprinted with permission from Gu S, Hsieh C T., Tsai YY, Ashraf Gandomi Y, Yeom
S, Kihm KD, et al. Sulfur and nitrogen co-doped graphene quantum dots as a fluorescent
quenching probe for highly sensitive detection toward mercury ions. ACS Appl Nano Mater
2019;2:790e798. https://doi.org/10.1021/acsanm.8b02010. Copyright 2020 American Chemical
Society.
GQDs against multiple single-doped GQDs and commercially used ORR
material. The electrocatalytic performance of N, S-GQDs worked significantly
better than N-GQDs, S-GQDs and undoped GQDs with 3.82 compared to 3.19,
3.41 and 3.19, respectively. It was equally performed Pt/C, which is the
commercial ORR electrode. Codoping creates more active sites available for
catalytic reaction than a single-doped material forcing more synergistic
coupling effects between heteroatoms. Moreover, a few recent articles also
demonstrate the application of graphene derivative for various applications
toward sustainability [66e71].
2.4 Conclusions
GQDs are still in the early stages of their development. However, GQDs
possess unique abilities due to their optical, physical, and chemical properties.
Given the development of this material, it is essential to know how to characterize and analyze its structure and morphology. Raman spectroscopy is the
best way to know the layers and defect of GQDs. At the same time, TEM and
AFM reveal the surface morphology, which is essential in GQDs utilization.
The many functional groups attached on the GQDs can be identified by using
FTIR and together with XPS, giving a better understanding of the oxygenated
functional groups. The PL phenomena possessed by GQDs is one of the main
attributes that open up this material toward all types of applications. The PL
spectra also help in interpreting the size of the GQDs sheets as it is one of the
42 Graphene Quantum Dots
important features of the material. No materials are perfect. However, GQDs
have the ability to be manipulated in such a way to fit certain applications.
Surface manipulation can be implemented to alter GQDs properties. The
ability to be tuned by controlling the size of GQDs showed how flexible GQDs
are as prospect materials. Even then, there still some concerns that needed to
be answered in GQDs’ development. Understanding the limit and finding a
way to overcome it is a positive step in moving forward.
Acknowledgments
This article was financially supported by Universiti Sains Malaysia, (Malaysia) under short
term grant; 304/PKIMIA/6315580.
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temperatures on the structural and conversion of thin films of reduced graphene oxide. Phys
B Condens Matter 2019;572:296e301.
Chapter 3
Graphene quantum dots
application in bacterial and
viral pathogen disinfection
Shuchita Tomar3, Mohammad Shahadat1, Rohana Adnan1, Syed
Wazed Ali3, Shaikh Ziauddin Ahammad2 and Mangala Joshi3
1
School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia;
Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology IIT
Delhi, New Delhi, India; 3Department of Textile and Fibre Engineering, Indian Institute of
Technology Delhi, New Delhi, India
2
3.1 Introduction
Water is one of the most vital substances for all living beings on earth and the
utmost valuable resource for human civilization. With the rapid increase in
global population and industrialization, people are experiencing various
climate change and environment-related issues [1]. Among these environmental issues, water pollution or contaminated water has become the most
serious issue all over the world. Several anthropogenic activities like mining,
pharmaceuticals, textile industries, chemical industries, etc., are indirectly
producing wastewater in the environment [2]. Such wastewater includes
various types of organic and inorganic compounds, and microbial species.
Contaminated water generally contains different types of pathogens such as
viruses, microalgae, helminths, bacteria, protozoa and fungi that may lead to
severe waterborne diseases like diarrhea, typhoid, cholera, etc. According to a
recent survey of the World Health Organization (WHO), around 2 billion
people (less than 25% of the worldwide population) are taking polluted water
[3,4]. Approximately, 155 million people are still dependent on unprocessed
surface water. However, the water supply at worldwide level, particularly in
developing countries is rigorously stressed due to bad weather conditions and
contaminated water sources [5,6]. Also, the United Nations World Water
Development Report (2020), reveals that the water consumption demand is
getting increased by 1.0% every year [7]. The water consumption demand in
industrial field as well as in domestic field are rising more quickly than the
agricultural field (UN-Water, 2020) and this will undoubtedly continue for the
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00009-1
Copyright © 2023 Elsevier Ltd. All rights reserved.
47
48 Graphene Quantum Dots
next two to 3 decades [8]. Therefore, the requirement for clean and reliable
treated drinking water increases constantly over the years. Unfortunately, those
people who are living in rustic and remote areas generally have less incomes
and are not enough capable to access better-quality drinking water sources,
thus, higher chances of getting infections are there through pathogenic microorganisms. Hence, there is a persuasive requirement to produce an effective
and low-priced technique for bacterial control and water decontamination [4].
Conventional techniques used for water treatment mostly involves
adsorption (ion-exchange), microbial fuel cell (MFC), photocatalytic degradation, chemical oxidation (for example, chloramines, chlorine/dioxide, and
ozone), that can efficiently control the infection of waterborne microorganisms
[2,9e13]. Moreover, some of the corrosive substances raise a concern during
operational challenges in the generation, storage and transport processes.
Many consuming water efficacies are now progressing toward the use of UV
disinfection to overwhelm the limitations. Though, uses of UV radiation for
disinfection purpose can be more expensive and energy-intensive rather than
oxidation (chemical), and also this method could be less effective to UVresistant pathogens. In addition to this, no remaining disinfectant could be
maintained afterward UV disinfection [14]. Thus, the world needs a green,
superficial, steady, and sustainable water disinfection technique having less
cost and high efficacy (chi zang et al., 2018). This chapter mainly focuses on
graphene quantum dots (GQDs) and their applications. GQDs carbon-based
materials recently grabbed more attention of researchers because of having
special advantages for example robust chemical inertness, low noxiousness,
high fluorescent activity, and excellent photostability.
3.2 What are quantum dots?
The quantum dot (QD) is one of the beneficial discoveries by researchers in
nanotechnology, which has emerged as a semiconductor inorganic crystal that
contain several numbers of electrons well-defined and discrete quantum state.
QDs are basically the arrangement of atoms as in the bulk materials, but more
of the atoms are present on their surfaces due to the 3-dimensional (3D)
truncation. Moreover, QDs are small-sized particles that offer a wide range of
variable element ratios, which may result in fluorescent properties. QDs are
generally semiconductor nanoparticles that contain various unique properties
such as size-dependent emission wavelength, broad excitation range, and also
capable to produce glowing light when they are stimulated by UV light to
create interesting phenomena. Moreover, the structure of QDs can be
controlled simply while obeying the principle of quantum confinement.
The emission and absorption spectra conforming to the energy bandgap of
the QDs are basically controlled by quantum confinement principles, which is
the energy essential to excite the electrons from the electronic band to higher
energy levels. The excited electrons instinctively form an electron-hole pair in
GQD application in bacterial and viral pathogen disinfection Chapter | 3
49
which this excitation can emit energy in the form of a fluorescent photon. QDs
can also be considered as artificial atoms that are able to produce distinct
energy levels, and their bandgap can be moderated exactly via changing their
sizes. Bandgap can be related to the nanocrystalline size because it depends on
the number of atoms that make up the band [15]. Thus, QDs show optical
properties which are dependent on size, where smaller nanocrystals can have
larger band gaps. Generally, the energy bandgap increases with a decrease in
QD particle size and corresponding wavelength [16]. QDs mostly contain
distinctive luminescent characteristics and electric properties for example,
narrow emission, wide and continuous absorption spectra, and high light
stability. Their ability to captivate white light and further reemission of
particular colors in a few nanoseconds later, generally depends on band gap of
the materials. Moreover, QDs are potentially used in several devices such as
telecommunication lasers, light-emitting diodes, and also in biomedical applications (which can be used as tools for monitoring cancerous cells, tumor
imaging, and therapy). Though, confinement of the QDs can be understood by
fabricating the semiconductors in extremely small sizes (within hundreds to
thousands of atoms per particle). Due to this confinement effect, QDs have the
potential to show controllable discrete energy levels as well as have a tendency
to emit different colors of light at various wavelengths. QDs also exhibit stable
and tunable wavelengths [17]. Thus, the potential properties of QDs grab the
attention of the researchers toward its applications in wastewater treatment,
adsorption treatment of environmental pollutants, detection of heavy metals,
etc. Here, we focus more on the application of GQD.
3.3 Graphene quantum dots (GQDs): structure, synthesis,
and Characteristics
With rise in nanotechnology, GQDs have become a novel associate with the
nanocarbon category. Usually, GQDs could be observed as the smaller particles of graphene. Graphene is basically a, 2D single-layer nanostructure with
sp2-hybridized conjugated carbons, become an exciting material to inspect
because of its unusual thermal, electrical and mechanical properties. In
addition to this, its surface area and chemical stability are very high/and has
low fabrication cost. Also, graphene is suitable and good alternative component to silicon for nanoelectronic applications as it shows high carrier agility.
However, to implement graphene sheets always challenging because of some
unresolved restrictions: firstly, the surface reactivity becomes reason for easy
aggregation; secondly, it is quite difficult to get them disperse in eminent
solvents; thirdly, during etching method, graphene rearranges into 1D nanoribbons. Moreover, it is a zero-bandgap component but still the optical properties of graphene made it inappropriate for optoelectronics application.
Though, by changing the sheet size it is possible to tune the bandgap of
graphene from 0eV to that of benzene [18].
50 Graphene Quantum Dots
Recently, many researchers have focused on converting 2D to 0D GQDs
and further observing the edge effects along with quantum confinement impact
on the characteristic of this innovative substance. GQDs can be generally
defined as 0D and sp2 bonded carbon atoms that are organized in a flat and
honeycomb assembly like graphene having adjacent dimensions less than
10 nm. Similar to graphene, GQDs can also be a single or few-layered
structures. Generally, GQDs shows characteristic which is consequent to
both graphene and GQDs. However, sometimes there is misperception among
(CDs) and GQDs, that occurs because they both belongs to carbon-based
nanomaterials having lateral dimensions <10 nm. Although, there is a
distinct difference among them, CDs are sphere-shaped and mostly amorphous
in character. While, GQDs having sheet-like assembly and crystalline in
character. In addition to this, CDs and GQDs are oxygen enriched and also
having functional groups that builds them appealing components for numerous
modifications. GQDs contain several distinctive electronic, chemical, and
physical characteristics which includes solubility, chemical and thermal solidity, large surface area, less cytotoxicity, active functional sites which provide simple variation, and high photoluminescence characteristics. GQDs that
have a nonzero band gap are normally capable to modify through regulating
the size of the dots. Their electrical characteristics, photoluminescence, and
catalytic behavior can be improved by modifying GQDs with heteroatoms.
3.3.1 Synthesis of GQDs
There are basically two methods available that are used to synthesize GQDs:
Firstly is a top-down approach, where GQDs can be instigated from exfoliated
graphene layers by using chemicophysical, and electrochemical process;
Secondly is bottom-up approach method, here the production of GQD medieties could be managed through a step-by-step reaction method by using
molecular precursors.
3.3.1.1 Top to down approach
The top-down approach includes various processes such as hydrothermal
process, solvothermal process, ultrasound technique process, lithography
process, exfoliation by nanotomy technique, chemical exfoliation process,
microwave-assisted exfoliation, slitting of graphene sheets through an electrochemical. Here, we discussed some of the processes in detail.
Hydrothermal process
The process was initially tried by Pan et al., which includes the scissoring of
graphene sheets up to 5e13 nm range size GQDs [18]. Firstly, reduced graphene oxide (rGO) sheets were pretreated thermally in the presence of
concentrated sulfuric acid in order to get small-scale sheets having size
2e50 nm range, furthermore it was solubilized in water by adding oxygen rich
GQD application in bacterial and viral pathogen disinfection Chapter | 3
51
functional groups such as epoxy, carboxylic, and hydroxyl groups. After that
the modified graphene sheets hydrothermally treated around at 200 C for 10 h.
The substance is then dialyzed to attain 5e13 nm diameters which is approx.
one to three layers GQDs. This was again modified to develop smaller GQDs,
thus 1.1 nm (lateral height) GO layers were chosen to produce proper crystallized GSs. Afterward of the pretreatment of GO layers (sheets), it undergoes
hydrothermal process of cutting at higher pH (more than 12). Thus, resulting in
the production of GQDs of 1.5e5 nm range in size. In this method, carboxylic,
carbonyl and epoxy groups enclose to the carbon lattice resulting in acid oxidization. After the oxidation of mixed linear chains bounding the sp2 groups
changes it into carbonyl pairs as shown in Fig. 3.1a. These selected sites
ruptured at the subsequent hydrothermal deoxidation of the border oxygen
FIGURE 3.1 (a) A proposed mechanism for the transformation of oxygen-rich GS into GQD.
52 Graphene Quantum Dots
atoms over the mark of the epoxy group, where fine parts finally fragment to
GQDs. The structure (lateral) of the graphene layers kept unchanged; hence, the
concluding GQDs resulting a proper ordered monolayer crystalline morphology
analogous to the initial substantial.
Solvothermal method
This method has different synthetic ways through which we able to produce
GQDs by the means of solvents namely benzene, dimethyl sulfoxide (DMSO)
and dimethylformamide (DMF). Herein, solvent depicts the physicochemical
nature and makes a straight influence on the ultimate size and morphology of
the substance. GQDs can be prepared through reducing agent like DMF to
break down GO into fluorescent green emitting GQDs. Firstly, GO was put for
sonication in DMF solution, then the solution was heated in an autoclave at
200 C for approximate 5 h. The transparent suspension (brown) was filtered,
and solvent got evaporated after heating it. Thus, the resultant GQDs having
approx. 5.3 nm diameter and around 1.2 nm thickness. Moreover, the photoluminescence characteristics can be enhanced by carrying out the reaction in
presence of methanol and methylene chloride mixture around 8h where, water
was used as a mobile phase [19].
Lithography process
This process includes huge graphene flakes which can be simply gained
through microchemical flat breaks of the graphite crystals by means of conventional “scotch-tape” exfoliation process. Over the thick 300 nm SiO2/
silicon-wafer, the graphene flakes can be placed. After that GQDs of
30e50 nm range could be obtained through graphene using electron beam
lithography. However, least size could be restricted through lithography
apparatus resolution. Moreover, graphene flakes isolation could be a finicky
process: Firstly, the graphene flakes thin-layer can be recognized through
optical microscopy. Secondly, Raman spectroscopy is used to observe singlelayer graphene flakes. Besides this, standard of the component profoundly
reliant over various aspects like surface cleanliness, SiO2 layer thickness and
technique applied to riven the chemical bonds within the graphite [19].
Exfoliation using “nanotomy” technique
In this process, a two-step shattering method is preferred in which graphite is
initially anatomize by using “nanotomy” diamond shape edge-induced method
to nano blocks of graphene. After that, chemical exfoliation using superacid
occurs that can breakdown the chemical bonds of extremely well-organized
graphite (pyrolytic), though obtaining nanostructures of graphene having
precise profiles such as nano-squares, nano-rectangles, nano-triangles, and its
size ranges between 10 and 50 nm. Thus, resultant GQDs have zig-zag and
even edges morphology. The process could be encouraged at large-level
GQD application in bacterial and viral pathogen disinfection Chapter | 3
53
development by means of fast-speed, self-regulating nanotoning process; still,
the GQDs are greater as compared to those prepared chemically and
electrochemically.
Electrochemical method used to scissor graphene sheets
The technique was found to be efficient to create fluorescent C-dots. Thus, this
technique becomes an effective method to produce GQDs. In this method,
oxygen plasma was used to treat the graphene film and thus improve water
solubility and it also shows electrochemical cleavage. Then, GSs formed
through rGO, and oxidized by KMnO4 and further reduced using aqueous
N2H4. Thereafter, a 3 V potential was applied that initiate the electrolyte ions to
enter the layers of graphene and thus, stimulate carbon-carbon bond oxidation
of in the structure of graphene. Furthermore, positive and negative (0.3, 0.3)
voltage was given externally over the graphene sheet (film), i.e., working
electrode, and for cyclic voltammetry (CV) phosphate-buffer solution (PBS)
was dipped in 0.1m. After that resultant GQDs were accumulated by using
filtration and dialysis both through a membrane bag. Hence, the process produces monodispersed GQDs having 3e5 nm diameter and 1e2 nm diameter of
about three graphene layers. Thus, the faults occur through the chemical and
physical treatment of graphene sheets become energetic positions for electrochemical oxidation causing shattering of the sheet into GQDs [20].
3.3.1.2 Bottom-up approaches
Precursors pyrolysis
The pyrolysis process comprises of organic molecules decomposition at high
temperature that are utilized in the form of raw material in the absent of oxygen. The method is responsible for irretrievable changes in chemical
composition as well as in physical phase. The precursor pyrolysis like citric
acid has the ability to yield GQDs having a diameter of 15 nm along with
0.5e2.0 nm thicknesses by enhancing the carbonization situations and thus
pyrolyzed products were dispersed in alkaline media. The whole carbonization
process could be accomplished through heating precursors persistently, hence
making GO nanostructures having 100 nm width and nm thickness [21]. A
proposed outline for the pyrolysis of GQDs preparation is shown in Fig. 3.2.
Step-by-step synthetic route
In this method smaller graphene type particles are involved, such as polycyclic
aromatic hydrocarbon molecule (PAH). Although, to prepare large GQDs
showing good solubility is quite tough. Thus, in many conditions, they
aggregate because of insufficient lateral aliphatic side chains which can
interrelate to aromatic molecule edge. In order to reduce the surface interaction among the layers of graphene, the graphene moieties edge could be
decorated through 2,4,6-trialkyl phenyl group by means of covalent bonding.
54 Graphene Quantum Dots
FIGURE 3.2 Pyrolysis method to produce GQDs and GO using citric acid (CA). Dong Y., Shao J,
Chen C, Li H, Wang R, Chi Y, Lin X, Chen G. Blue luminescent GQD and graphene oxide prepared
by tuning the carbonization degree of citric acid, Carbon 2012;50:4738e4743 adopted by
permission (Elsevier).
Thus, resulting 3D cages adjoining graphene moieties. Hence, gap within
respective layer increases, thus enhancing the GQDs solubility containing
subsequent conjugated carbon atom such as 168 (C168, 2), 132 (C132, 1), and
170 (C170, 3). Currently, GQDs functionalization of surface through electron
donating or electron withdrawing groups were implemented to control its
bandgap and redox potential. Through, by employing this approach a variety
of functionalized GQDs can be synthesized, which becomes a distinctive
benefit for wet-chemistry routes while preparing well-organized nanostructures of graphene. However, producing, cleansing, and illustrating large
GQDs by such approaches remains a challenge [22].
Decomposition of fullerene
In this method, fullerene C60 can be utilized in the form of carbon source in
order to develop uniform GQDs on a Ru (0001) substrate. Later, it behaves like
catalyst that can collapse the C60 cage, through which the segments are
rearranged into clusters. The result of scanning tunneling microscopy (STM)
GQD application in bacterial and viral pathogen disinfection Chapter | 3
55
reveals that molecules show thermal hopping and separation on surface, started
via adatom-vacancy process as C60 samples annealed up to 500e550 K. Thus,
C60 particles finally decomposed completely at 650 K to carbon clusters. This
has been observed that clusters resulting from C60, having diffusion coefficient
in 1015e1016 cm2 s1 range, comprised of uniform GQDs atomically, else it
is tough to achieve after derived from other precursors like C2H4. It was also
found that when C60 annealed for 2 min at 725 K, GQDs of various shape start
forming namely triangular, parallelogram, and trapezoid morphology. Moreover, when temperature (annealing) increased up to 825 K for other 2 min lead
to create GQDs having hexagonally morphology of 5e10 nm. Furthermore,
Chen et al., stated synthesis of GQDs through oxidization using aqueous KOH.
The preparation of GOLQDs at large-scale was also stated by the means of
oxidizing C60 molecule by applying improved “Hummer’s method,” thus
achieving a benefit of 25 wt% [16]. These were vastly soluble in water and
provide hexagonal morphology; they also contain other carbon rings. After
drying the substrate, they show normal height of 1.2 nm and diameter
0.6e2.2 nm, respectively [23].
3.4 GQDs for water treatment
GQDs showed a varied range of utilization regarding nanostructures fabrication for eliminating various impurities from water. Numerous information has
appeared with the use of doped GQDs and its resulting nanocomposites for
photocatalytic deprivation of various contaminants, the photo electrocatalytic
degradation of contaminants, their adsorption and membrane filtration [24].
Thus, the GQDs and their nanocomposites fabrication shows substantial
enhancement to eliminate water contaminations. Several impurities like
various kind of dyes, heavy metals can be removed from polluted water
through several nanocomposites derived from GQDs. Thus, the photocatalytic
water treatment has capability to eradicate organic as well as inorganic contaminants and, also the microbial pollutants from water, respectively. Still,
there are some major issues related to this process like charge carrier’s fast
recombination, poor visible light use, small surface area, and the accumulation
of nano photocatalysts in water. However, it was observed that when several
semiconductors coupled by carbon nanomaterials comprising graphene, GO,
rGO, CQDs and GQDs shows the great tactic for enhancing the complete
action of numerous semiconductors. GQDs have emerged as the most efficient
nanomaterials of carbon in photocatalysis due to its photostability, visible light
absorption, virtuous adsorption properties, and charge parting ability [25]. Anh
et al., 2019 found it a reliable method for the quick recognition and screening
of 4-Nitrophenol (4 NP), a publicized pollutant. They established an easy and
worthwhile one-step pyrolysis process for fabricating S-GQDs by means of
citric and 3-mercaptosuccinic acid as form of bases of carbon and sulfur,
correspondingly. Therefore, developed S-GQDs were further utilized as
56 Graphene Quantum Dots
fluorescence probe to effectually spot 4-NP in an aqueous medium having a
complex system. Hence, developed S-GQDs show distinctive fluorescence
peak around 450 nm under excitement of 330 nm UV light over the pH range
5e9, that is extremely profound and careful to 4-NP recognition. The evenly
S-GQDs distribution of diameter 1e5 nm has the potential to produce fluorescence around 450 nm and behave as sensing element and doping by S
molecules improves fluorescence quantum production.
On the other hand, nitrophenol has the ability to act like fluorescence
quencher to vanish fluorescence concentration through pep interaction from
GQDs backbone. Therefore, preparing S-GQD an effective sensing component
for extremely sensitive recognition of 4NP. The S-GQDs shows outstanding
performance toward 4-NP recognition and an active limit of three to four order
scale having a limit of detection (LOD) around 0.7 nM in DI water and 3.5 nM
in wastewater. Furthermore, S-GQD related sensing stage could also be used
to detect 4-NP in various waters and the approximate significant amount
(98 4)e (108 2) % are achieved by using 0.05e100 mM of 4-NP. Moreover, S-GQDs paper related sensor demonstrate the pertinence to a rapid
screen of 0.1e500 mM 4-NP in wastewater deprived of apparent matrix
intrusion [24].
Another study done by Qian et al., used ultrasonic exfoliation synthesis
method for modifying nonmetal (P, N, S, B). Nanostructures of GQDs-gC3N4 and their estimated visible light-aided disintegration of RhB. However,
excitingly, no apparent improvement in action of g-C3N4 when GQDs, SGQDs, and N-GQDs coupled purely, by the action of B-GQDs which is
lesser as compared to bulk g-C3N4. Therefore, it was attributed to poor
visible light absorption and feebler charge parting. Also, in another study by
Qian et al., 2018, they developed modified GQDs with CNNSs (GQDs/
CNNSs) through a facetious and effective GQDs-assisted exfoliation method
within a standard ultrasonic water bath in which they dispersed uniformly.
The exfoliated process was further observed via changing the dopant in
GQDs, time for ultrasonification, and also the GQDs amount. The achi
eved colloidal GQDs/CNNSs exhibits distinctive 2D morphology having
adjacent 100 nm size and 1.5e1.8 nm thickness. Thus, show semiconductive
nature of GQDs via the doping of heteroatom and attain a modified pen-type
p-doped GQDs CNNSs colloidal system. Therefore, pen GQDs/CNNSs
substantial shows improved parting efficacy of photoexcited carters and
photocatalytic action as compared to g-C3N4 (CN) and different types of
CNNSs component through acid or base (alkali) exfoliation. Moreover,
improved photocatalytic deprivation of methyl orange (MO) [26] and MB
and RhB [27] has been stated. Therefore, the enhancement in the action
might be credited to fabrication of GQDs that absolutely affect the optical
properties and charge parting efficiency in a positive way [28,29]. Table 3.1
shows different studies on GQDs based nanocomposites and their application for wastewater treatment.
GQD application in bacterial and viral pathogen disinfection Chapter | 3
57
TABLE 3.1 Preparation of GODs nanocomposites and their applications.
Method
Applications/Types
of study
Ti -TiO2/GQDs
Carbothermal
reduction using
citric acid
Enhanced
photocatalytic
performance
[26]
S-GQDs
One-step
pyrolysis method
Detection of 4-NP
organic pollutant
[28]
g-C3N4 (PCNO)-oxGQDs nanocomposite
Facile selfassembly method
Photocatalytic (visible
light) deprivation and
pollutant disinfection
[30]
Synthesis of GQDs
using green and natural
citric acid as precursor
Bottom-up
approach
Photo catalytic
degradation of
cationic dye
[31]
Graphene quantum
dots (GQDs)
Synthesis of
GQDs using
pyrocatechol at
100oC
Photo catalytic
degradation of MB and
MO
[32]
s-g-C3N4@GQDs
nanohybrid
One-step
hydrothermal
method
Removal of pollutants
[33]
N-GQDs/CNeU
Facile method
Enhanced
photocatalytic
performance
[34]
MB-GQDs
Laser ablation
method
Photodeactivation of
different strains of
bacteria
[35]
PVK-GO modified
membrane
Synthesis of GO
using Hummers’
method.
Enhanced antibacterial
property
[36]
TiO2/Sb2S3/GQDs
Synthesis of
GQDs by facile
method using
corn powder
Enhanced antibacterial
activity
[37]
Materials
3þ
References
3.5 GQD nanostructures for reduction of heavy metals
and water disinfection
The most significant steps in water treatment involve disinfection. Deprived of
this decontamination process, the infective microorganisms like bacteria,
fungi, algae, and viruses would be able to search their path in consuming water
and can become the reason for severe waterborne diseases. Therefore, it needs
58 Graphene Quantum Dots
to be confirmed that water is protected from bacterial contamination. Also, the
hefty utilization and antibiotics discharge within the atmosphere may lead to
the growth of superbugs, that can possess a vast risk to health and strength.
Thus, water requires to be adequately sterile in order to overcome the escalate
of these types of superbugs. Other disinfection method of water that is photocatalytic method provides a favorable substitute for disinfection of chlorine
that frequently cause the development of potentially poisonous decontamination by-products. Though, the corresponding method that is photocatalytic
decontamination has the capability to abolish microorganisms and also the
decontaminated by-product developed at the time of chlorination [38].
Recently, solvothermal synthesis method has been employed to prepare
ternary nanostructure of TiO2/Sb2S3/GQDs and examined its antimicrobial
performance for Escherichia coli (E. coli) and Staphylococcus aureus
(S. aureus) under visible light irradiation [37]. Hence, they observed that
when GQDs coupled by TiO2 (GQDs/TiO2) and Sb2S3 (GQDs/Sb2S3) efficiently decreased the minimum inhibition concentration (MIC) for E. coli and
S. aureus respectively. This is because of the enhanced utilization of visible
light and charge parting efficacy. Moreover, the ternary nanostructures of
TiO2/Sb2S3/GQDs exhibited lowest MIC values for bacterial strains in contrast
to pure TiO2, GQDs-TiO2, and GQDs-Sb2S3. MIC values of E. coli and S.
aureus was 0.03 and 0.1, correspondingly. Thus, it can be related to cosensitization consequence of GQDs and Sb2S3 over TiO2 along with enhanced
charge parting and creation of reactive species that cause bacterial growth
inhibition. As shown in Figure 3.3 the reduction in growth of bacteria by
increasing the irradiation time, till approximately no growth was analyzed
after 24 h. Also, Kholikov et al. estimated GQDs antimicrobial performance
that were mixed with MB in visible light radiance. When GQDs coupled with
MB the creation of singlet oxygen was increased, and high production was
found at 1:1 ratio of GQDs:MB. It was found that in dark situation GQDs
existence did not show any effect on cell capability thus, shows the better
potential of MB-GQDs under photodynamic treatment. Furthermore, the
illumination through light (660 nm), Micrococcus luteus (M. luteus) and
E. coli both were almost decontaminated entirely next 5 min [37].
Another major problem for wastewater treatment is the removal of heavy
metal to ensure clean drinking water. Thrash metals like Zn, Cr (VI), Pb (II)
and Hg (II), etc., were found in atmosphere in numerous concentrations; many
of them reported due to industrial activities. Various thrash metals were
categorized as type of carcinogenic, teratogenic, embryotoxic, mutagenic
which are harmful to immune system. Sometimes, they are so harmful that
they can also cause cardiac disorder, liver damage, and other health issues.
Catalytic method for removing thrash metals shows better result because of
nonselective procedure type and has ability to convert poisonous metal ions to
nonpoisonous metal ions. For instance, Cr (VI) photocatalytic reduction was
examined on nanowires of Bi2S3/GQDs/TiO2 in existence of MO within
GQD application in bacterial and viral pathogen disinfection Chapter | 3
59
FIGURE 3.3 Inhibition growth of bacteria in the presence of (a) TiO2/Sb2S-GQDs in 24 h, (b)
GQDs/MB for 2 min, and (c) GQDs-MB for 5 min at 660 nm. Teymourinia H, Salavati-Niasari M,
Amiri O, Yazdian F. Application of green synthesized TiO2/Sb2S3/GQDs nanocomposite as high
efficient antibacterial agent against E. coli and Staphylococcus aureus, Mater Sci Eng C
2019;99:296e303 adopted by permission (Elsevier).
visible light radiance. Remarkably, maximum Cr (VI) depletion was found in
the existence of MO (97%) rather than lacking MO (92%). Furthermore,
elimination of MO was around 52% in the presence of Cr (VI) rather than 43%
lacking Cr (VI). Therefore, it can be attributed to an effective tricking of
photogenerated light (electrons) through Cr (VI), that depleted to Cr (III),
remaining the holes allowed to undertake oxidation. Thus, causing MO deficiency. Hence, considerably enhanced MO elimination in existence of Cr (VI),
that behave like an electron forager. Hence, GQDs occurrence proved
enhanced visible light retort and as well as exhibit better charge parting efficacy in nanocomposites. Moreover, it was reported that electrocatalytic (EC),
photocatalytic (PC) and photo electrocatalytic (PEC) ability of ternary nanostructures of Fe2O3-GQDseNFeTiO2 was explored for depletion of Cr (VI)
and deprivation of EDTA in an aqueous solution under visible light radiation.
Therefore, maximum PEC action was found of ternary nanostructures in place
of binary nanostructures. Furthermore, PEC method also displayed greater
action rather than EC and PC methods. When relating to deceptive rate constant, action of PEC was found to be 7.67 times and 4.6 times greater as
compared to PC and EC, correspondingly. The interaction between EDTA and
Cr (VI) as well as Cr (VI)-phenol and Cr (VI)-MB, that showed the
60 Graphene Quantum Dots
appropriability of Fe2O3-GQDs/NFeTiO2 to PEC elimination of organic
metals from effluent. GQDs mainly act as an electron accelerators among
NFeTiO2 and Fe2O3 [39]. The interaction among the organics and hefty
metals throughout the elimination offers a better vision toward industrial
wastewater treatment comprising this type of impurities.
3.6 Mechanism
Some important graphene and carbon dots (C-dots) supported on rGO sheets
have been developed by the addition of TiO2 due to the fast photo-excitation of
electron toward C-dot through rGO [40]. Thus, the C-dots act as an electron
donor in the reduction of oxygen which facilitate the generation of anionic
superoxide for decontamination as shown in Fig. 3.4. Another study also
involved the degradation of dye was using TiO2-graphene which was act as a
light harvesting adsorbent [41]. Moreover, TiO2-rGO photocatalyst associated
with MnOx quantum dots has been effectively employed as an energy-storage
material owing to the significant electron storage capacity of quantum dots
(MnOx). Interestingly, the photoexcited electrons obtained from TiO2 showed
reversible nature toward MnOx which release reactive species in the dark.
Another reduced graphene oxide based Ag-AgX-rGO nanocomposite material
has been used for the disinfection of antimicrobial agents in water suing
visible light [42]. The nanocomposite material (AgeAgeBr-rGO (0.5%)
showed significant inactivation for E coli (2 107 cfu mL1) in 8 min as
compared to AgeAgeBr (in 35 min). The mechanism of microbial
FIGURE 3.4 A proposed photocatalytic bacterial inactivation mechanism. Xia D, An T, Li G,
Wang W, Zhao H, Wong PK. Synergistic photocatalytic inactivation mechanisms of bacteria by
graphene sheets grafted plasmonic AgAgX (X¼ Cl, Br, I) composite photocatalyst under visible
light irradiation, Water Res 2016;99:149e161 adopted by permission (Elsevier).
GQD application in bacterial and viral pathogen disinfection Chapter | 3
61
inactivation using AgeAgBr-rGO (0.5%) was confirmed by the releasing of
Agþ, radical scavengers, and radical degradation. In the photodegradation
process, both hþ and e_ are engaged in the process of photocatalyst which
played a key role to enhance the activation in the inhibition after addition
of oxalate (hþ scavenger) and Cr6þ (e_ scavenger) in the suspension of
AgeAgeBr-rGO (0.5%). The bactericidal potential of e_ was found higher
than hþ which might be owing to the plasmon-induced electron transfer as
shown in Fig. 3.4. It was observed that under visible light irradiation, the e_
and holes hþ are generated from AgBr, and the nanoparticles of Ag are also
excited to produce electron-hole (e and hþ) pairs owing to the localization of
surface plasmon resonance (SPR) as shown in Eqs. (3.1) and (3.2).
Ag þ hv (ʎ > 400 nm) /Ag*
(3.1)
AgBr þ hv (ʎ > 400 nm) /e þ hþ
(3.2)
-
The excited e s have potential to transfer from conduction band of AgBr
toward the nanoparticle of Ag owing to the high conductivity of Ag nanoparticle. Additionally, the electrons obtained in the conduction band (AgBr)
also transfer to the rGO surface and facilitate the fast charge transfer along the
p-p graphitic carbon network. Moreover, the e generated from the nanoparticles of Ag could also flow toward rGO [43]. Consequently, the nanoparticle of silver and rGO act as a medium to transport e-s from AgBr/RGO
and/or AgBr/Ag/RGO, which reduced the recombination chance with holes.
Therefore, high density (high concentration) of e-s having high reduction
potential (CBAgBr1/4 1.04 V vs. Normal Hydrogen Electrode (NHE)) amass
onto the surface of RGO, which can readily stimulate the generation of 1O2
with the help of energy transfer to O2, or it also reduce chemisorbed (surface)
O2/H2O to produce .O2 (E0 (O2/.O-2) ¼ 0.33 V vs. NHE) [44]. Consequently,
. O2 go through simplistic disproportionation reaction to generate .OH, H2O2
and 1O2 as shown in Eqs. (3.3) to (3.6) [45,46]. Furthermore, this kind of
transportation of photogenerated electrons stabilize the photogenerated
hþ onto the surface of AgBr and Ag nanoparticles [47]. Thus, the generation of
reactive species namely e, hþ, .O-2, .OH, 1O2, and H2O2, play an important
role to inactive E. coli K-12; and scavengers test exposed that the plasmoninduced H2O2 is the main reactive species in this photocatalytic inactivation
process.
O2 þ e_
1
O2/.O-2
(3.3)
O-2 þ H2O2 þ 2OH
(3.4)
OH þ OH
(3.5)
OH þ O2 þ OH
(3.6)
hþ, e þ O2, OH, O2, H2O2 þ E. coli
Bacterial cells (Organic debris)
(3.7)
2.O2þ 2H2O
H2O2 þ e
.
H2O2 þ .O-2
1
.
.
.
-
62 Graphene Quantum Dots
3.7 Conclusions
Water pollution caused by the presence of heavy metal ions, microbial species,
and other organic and inorganic pollutants have become a serious threat to the
clean and safe drinking water supply. Nanostructures derived from GQDs
appeared like one of the efficient potential solutions toward water pollution
extenuation. GQDs and their nanocomposites have been magnificently
developed and assessed for catalytic elimination of various organic pollutants
like dyes and other evolving contaminants, pollutants adsorption, filtration,
and decontamination. Fabricating GQDs within several nanocomposites lead
to nanocomposite properties modification and enhanced the elimination efficacies of various contaminants. Here, we discussed different methods of
synthesizing GQDs such as hydrothermal method, solvothermal method, and
so forth that could be utilized for treating wastewater. The amount of the
incorporation of GQDs should be optimized carefully to confirm the optimistic
effect in pollutant elimination efficacies of various nanocomposites. Although
there are many developments on nanostructures of GQDs, but still many works
left that is to be done to confirm the pattern and use of these materials at
extensive level. However, nanostructures derived from GQD has capability as
pollutant remedy tool, because of its nontoxicity, decomposable, and ample
functional groups. Still, it is essential to make enhanced synthesis situations
having ability to produce GQDs that are even in relation to size and surface
functionalities as well as to make better synthesis methods which confirm
suitable spreading of GQDs in nanocomposite matrix. Further, this can come
up with consistency regarding the stated performances of several nanostructures derived from GQD for water effluence reduction.
Acknowledgments
The authors would like to express their appreciations to Science and Engineering Research
Board (DST) fast tract young scientist scheme (SB/FT/CS-122/2014) for providing Postdoctoral Fellowship to Mohammad Shahadat.
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Chapter 4
Microbial sensing and
antimicrobial properties of
graphene quantum dots
Mohammad Oves1, Mohammad Azam Ansari2, Mohd Ahmar Rauf3,
Bahaa A. Hemdan4 and Iqbal M. I. Ismail5
1
Centre of Excellence in Environmental Studies, King Abdulaziz Univesity, Jeddah, Saudi Arabia;
Department of Epidemic Disease Research, Institute for Research & Medical Consultations
(IRMC), Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia; 3Use-Inspired
Biomaterials & Integrated Nano Delivery (U-Bind) Systems Laboratory, Department of
Pharmaceutical Sciences, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne
State University, Detroit, MI, United States; 4Environmental Microbiology, Laboratory, Water
Pollution Research Department, National Research Centre, Dokki, Giza, Egypt; 5Department of
Chemistry, King Abdulaziz Univesity, Jeddah, Saudi Arabia
2
4.1 Introduction
Most communicable diseases are caused by viruses, bacteria, fungi, or parasites, and these are the prominent leading source of mortality around the world
[1]. If infections become multidrug resistant (MDR), many of these diseases
become more difficult to treat, resulting in increased mortality rates hospital
bills [2]. The number of Gram-negative bacteria caused MDR bacterial infections; they are progressing at an alarming rate, all available antibacterial
treatments become useless due to developing resistance against antibiotics. The
Centers for Disease Control and Prevention (CDC) has characterized multidrug
resistance pathogen as methicillin-resistant Staphylococcus aureus (MRSA),
Enterococcus faecium, or Vancomycin-resistant Enterococci (VRE), and other
antibiotics-resistant of Streptococcus pneumonia [3]. Pathogen-caused MDR
has been accompanied by a steady drop in discovering and developing novel
medicines, providing significant worldwide difficulties. According to some
estimates, antibiotic resistance might be responsible for 300 million deaths and
an additional cost of trillion 100 dollars by 2050, necessitating immediate and
comprehensive action to solve the MDR problem [4].
The skin is the biggest organ in the human body and defends and touches
harmful bacteria. Bacteria can enter a wound through a cut or abrasion,
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00003-0
Copyright © 2023 Elsevier Ltd. All rights reserved.
67
68 Graphene Quantum Dots
producing local infection and even systemic sepsis. Antibiotics can treat superficial wound infections on a local level [5]. Because of abuse and quick
activation of resistance genes, toxic bacteria have developed resistance to
conventional antibiotics. A novel drug class with broad antibacterial action and
adequate biocompatibility is required.
In clinics and other healthcare institutions, various antiseptics and disinfectants are widely employed to inactivate harmful bacteria and prevent infections. On the other hand, most disinfectants are poisonous and irritating,
causing health concerns and dermatitis [6]. Some are losing effectiveness as
germs evolve and develop resistance to them. As a result, there is a sense of
urgency. There is a growing demand for alternate antimicrobial methods with
improved characteristics and reduced harmfulness for infection treatment [7].
The antimicrobial photodynamic inactivation (PDI) has revealed considerable
promise in the decontamination of several microbe types, with essential
compensations such as little intrusiveness, low side effect incidence, and
adaptability for quick and recurrent presentation [8]. Because it caused
nonspecific oxidative damage of proteins, lipids, enzymes, and nucleic acids
within the cells and in the cellular membrane, the PDI therapy is less likely to
elicit resistance by the targeted microorganisms [9]. When photosensitizers are
stimulated by mild light of the proper wavelength, then reactive oxygen species (ROS) are formed in PDI [10]. The ROS can comprise free radical ions of
,O2 (superoxide), ,OH (hydroxyl radical), resultant lipid ions, and singlet
oxygen (1O2), and the generation is related with type I and/or type II photodynamic special effects [11].
The dye moleculesdporphyrins, phthalocyanines, bacteriochlorins, phenothiazines, and derivativesdhave all been used as photosensitizers [12]. The
alternative, emerging, and effective antimicrobial medicine are developed
from nanoscale materials because it is a carrier for selective transport and
diffuse photosensitizers in targeted cells [13]. While semiconductors and metal
nanoparticles have been extensively researched for the conveyance of medicine at a particular site, carbon nanomaterials have lately received much
attention in PDI-related applications due to their wide-ranging optical spectrum exposure and other favorable aspects material features [14]. Beyond the
well-known fullerenes, nanotubes, and graphenes of carbon allotropes, the
current identification of C-nanoparticles as a distinct zero-dimensional Callotrope, as opposed to fullerenes as “zero-dimensional C molecules.” The
well-defined chemical structures and stoichiometry have opened new avenues
for contesting antimicrobial agents [15]. CDs are tiny carbon nanoforms with
varying surface passivation. They have emerged as a potential raised area for
natural light-activated antimicrobial drugs by utilizing and increasing their
inherent optical features and photoinduced redox characteristics [16e18]. In
this sense, GQDs are exceptionally visible photosensitizers for operative PDI,
with extra benefits owing to nontoxicity, photostability, plasticity in surface
functioning for preferred contagious linkage and interactions, and so on [19].
Microbial sensing and antimicrobial properties Chapter | 4
69
4.2 GQDs for bacterial sensing
GQDs are versatile revolutionary bionanomaterials with many functional
groups and the ability to generate multidonor ligands with macromolecules,
which brings a lot of opportunities for biomedical purposes [20]. Another
fascinating and prospective implementation of GQDs mentioned here seems to
be photothermal/photodynamic therapy (PTT/PDT). PDT highly depends on
molecular oxygen, which can kill malignant cells immediately. Ge et al.
described a GQD-based PDT molecule exhibiting widespread uptake, significant emission of deep-red, and a quantum value of almost 1.3 [21]. Radiation
exposure produced cell shrinkage, the development of numerous benign
growths, and the mortality of HeLa cells in the experiments. With increasing
GQDs dosage, cell viability diminished, although GQDs had no impact on the
rate fluctuations of cells (HeLa) in the dark. In addition, in vivo experiments
on cytotoxicity, photothermal activity, imaging, and PDT have been undertaken. In vivo and in vitro studies disclosed that simultaneously, GQDs might
be deployed as a PDT and image analysis agent.
Moreover, heteroatom doped GQDs have a greater chance of succeeding
PDT effectiveness than free GQDs. Kuo and his colleagues investigated Ndoped GQDs with a 3-min photoexcitation period [22]. The findings
revealed that higher N content in N-GQDs extra effectively implemented PDT
actions than lesser N content N-GQDs treated similarly. The amalgamation of
chemotherapy with PTT has developed an immediate research priority to
achieve a better therapeutic impact with more secondary detrimental effects.
Synergistic chemophotothermal chemotherapy has been successfully performed using silica nanoparticles capped with GQDs (GQDMSNs) [23]. DOXloaded GQDMSNs (DOXGQDMSNs) had a perfect temperature and pH,
sensitivity, regulated drug target release, and exceptional near IR-absorption,
indicating that GQDs could be useful in cancer treatment. DOX extraction was
quicker in the solution at pH 7.4 than in the media at a pH of 5.0. The DOX
discharge rate was significantly larger at 50 C than at 37 C, demonstrating
that DOXGQDMSNs had an excellent pH and temperature responsiveness.
With near-infrared (NIR) irradiation, the surface temperature of the
GQDMSNs increased and was substantially more significant than that of the
suspension, showing that the GQDMSNs had a remarkable photothermal
outcome. As a result of the successful chemo-photothermal synergetic therapy,
DOXGQDMSNs may result in increased cancer cell death. Following the
chemophotothermal treatment study, the PDT/PTT synergistic cancer treatment has become a challenging topic in cancer cure research. Cao et al.
recently published a report using aptamer-conjugated GQD/porphyrin hybrid
drug delivery applications [24]. These GQD-PEG-P had several unique,
essential materials, including high photothermal transformation performance,
high 1O2 formation capacity, decent luminescence characteristics, and a large
specific surface area, all of which make them suitable for intracellular miRNA
revealing and photodynamic treatment.
70 Graphene Quantum Dots
4.2.1 Antimicrobial property of carbon dots
Carbon dots (CDs) have emerged as a prominent, interesting revolutionary
framework for microbicidal remedies triggered by visible/natural light. CDs
have recently gained considerable attention for their antimicrobial properties
due to their preferred visual qualities, minimal toxic effects to cell cultures,
and bifunctional contact capabilities with bacteria. The formulations, architectures, and features of CDs are considered in this section, and comparative
investigations on their antibacterial, antifungal, and antiviral activity and
related mechanistic considerations [13].
4.2.2 Potential of CDs for combating bacteria
CDs with favorable surface qualities might react with microbes with a negative
surface charge, causing superficial damage, intracellular permeability, and,
eventually, bacterial cell loss. To make complex þ ve charged CDs, bioactive
polyamines were used as precursors or functionalization agents right away
[25]. Polyamines, such as cadaverine, spermine, putrescine, and spermidine,
are tiny fragments with two or more amine groups created at the most
excellent density of millimoles per liter in existing cells. They can be used in
nanoparticle surface functionalization because of their strong positive charge
and high-grade biocompatibility [26]. When SC-dots were combined with
microorganisms, internal ROS increased, contributing to their antibacterial
characteristics. The CDs made with identical components, including spermine,
appeared to have no antibacterial properties. Straight pyrolysis of spermidine
powder followed by dry annealing produced an alternative category of
supercationic CDs (CQDSpds) [27]. The non-MDR bacterium E. coli, S.
aureus, Salmonella enterica, P. aeruginosa, and the multidrug-resistant MRSA
exhibit outstanding antibacterial activity [28]. The inhibitory activity (MICs)
of CQDSpds against these microbes is around 2e4 g mL1, which is about
2500-fold lower than free spermidine. Due to their solid biological properties
in both in vitro and in vivo systems, such CDs were effectively applied for
keratitis treatment, indicating a novel nanoantibiotic agent for the topical
application of eye-related infectious disorders [29]. The CDs were discovered
to break bacterial membranes and connect to bacteria’s genome, resulting in
cell death. The CDs’ antibacterial properties were reportedly aided by their
positive ions. On the other hand, spermidine was used to enhance the
characteristics of CDs and give them highly positive charges [30]. The
spermidine-capped CQDs have produced and exhibit remarkable antibacterial
effectiveness against E. coli, B. subtilis, S. aureus, MRSA, and P. aeruginosa,
with a zeta potential of þ60.6 mV [31].
Photoexcited CDs might produce ROS designed to wipe out or inhibit
microbes. According to research, the underlying processes implicated in CDs’
antibacterial effects are commonly linked to ROS formation. The action and
Microbial sensing and antimicrobial properties Chapter | 4
71
mechanism of CDs to the bacterial cell surface, photoexcitation of ROS,
interruption, and entry of the microbial cell wall/membrane, induction of
oxidative stress with DNA/RNA losses, resulting in changes or restrictions of
essential transcriptional activation, and initiation of oxidative damages to
intracellular biomolecules like proteins and other, as shown in Fig. 4.1 [32].
The EDA-CDs could significantly reduce E. coli cells in liquids and on the
agar surface, demonstrating the CDs’ visible/natural light-activated antibacterial capabilities. When E. coli cells are exposed to light for 30 min, the
number of viable cells drops by four logs, whereas dark therapy only reduces
possible cell concentrations by one record [33].
In approximately the same study, treatment with EDA-CDs with photoexcitation significantly suppressed the proliferation of Escherichia coli in
solution and decreased population density on agar medium. Photosensitizer
reactions, analogous to those experimentally demonstrated in the destruction
of tumor cancer cells by CDs in photoirradiation, are attributable to these
antibiotic capabilities [34]. Subsequently, numerous analyzes of CDs using
different combinations revealed results similar to those described above. Lee
and colleagues used a single-step electrochemical technique. Vitamin C was
used as an originator to CDs and discovered that they had broad antibacterial
action against B. subtilis, S. aureus, Bacillus sp., and E. coli [35]. For example,
B. subtilis and Bacillus sp. cells can be inhibited by CDs at a dose of
FIGURE 4.1 illustrates the mechanism of action for CDs with photoactivated antimicrobial
properties. (a) CD adsorption to bacterial surfaces and ROS production induced by natural light,
(b) ROS generated resulting in bacterial cell rupture.
72 Graphene Quantum Dots
50 g mL1. The MIC rate significance in the point sample was lesser for E.coli
than for S. aureus, indicating that the model had a more negligible antibacterial effect on Gram-positive than Gram-negative bacteria. In a similar hydrothermal carbonization process, penicillin was exploited as a precursor to
CDs, but at a temperature well below only 120 C [33]. CDs were also synthesized using a separate nonpenicillin-containing precursor array, followed by
dot surface-binding of penicillin, for comparison. The antibacterial activity of
two CDs, which should include penicillin but indifferent skeletal configurations, was tested against S. aureus, MDR E. coli, and MRSA. Despite this,
metronidazole CDs have not been able to prevent the development of
S. aureus. The antimicrobial action of CDs is influenced by a variety of factors,
including their optical properties, photosynthetic properties, and surface
functions, as expected, and these requirements provide additional possibilities
to deceive and enhance their bactericidal capabilities in light [36].
4.2.3 Combination with other antimicrobial reagents
The antimicrobial activities of nanostructures, especially carbon hybrid dots
with metal oxides in the dot formation, were characterized. Nanoscale carbon
was effectively connected with a multitude of nanometer semiconductors, such
ZnO, TiO2, Cu2O, Na2W4O13/WO3, and others, for hybrid CDs. Under UV/
vis photoirradiation, a few mixed dots exhibited markedly higher activity
versus bacterial infections, which was ascribed to much more effective charge
transference and a suppressive influence on electron-hole pair combination for
enhanced ROS generation [13]. Spotless TiO2 nanoparticles, in particular, are
recognized for their photovoltaic performance. They’ve been used in antimicrobial and overall disinfection applications because of their chemical inertness, sizable specific surface extent, less toxicity, and capacity to make an
electric charge when exposed to UV light [37]. Because of the expansive
bandgap energy and the comparatively fast recombination of electron-hole
pairs, colloidal TiO2 also requires UV stimulation. The former is a significant constraint for the more ideal natural light activation application conditions, while the latter links to less operative ROS generation and antimicrobial
effectiveness. Because carbon in CDs effectively produces visible photons,
other carbon/TiO2 hybrid dots extend the light stimulation into the visible
spectrum to initiate nanoscale C and TiO2 photocatalytic properties in the dot
architecture. The nanoscale carbon and TiO2 domains have a structural
configuration in the core of each carbon/TiO2 hybrid dot which played an
essential part in the practical optical and photoinduced redox characteristics
[38]. On the entire carbon/TiO2 hybrid dots, properties were comparable to
TiO2 systems with dye-sensitized, here C-domains serving as photonharvesting by the dye function in the visible spectral region, and while TiO2
has no absorptions, and the collected photon energies sensitizing the TiO2 in
the hybrid dots [38]. Yan et al. has developed CDs furnished with TiO2 via
Microbial sensing and antimicrobial properties Chapter | 4
73
hydrothermal process and explained the antibacterial potentials toward
S. aureus and E. coli. The pure TiO2 antimicrobial results were rationalized
compared to the combination’s CeTiO2 because the mixture has high visible
light uptake, enhanced dispersibility, enhanced ROS generation [39]. Moreover, Zhang et al. proved that the designed CDs with Na2W4O13/WO3 layers
to be utilized as an ecofriendly photo-disinfection substance and revealed that
the material might decrease E. coli cells by seven logs in 100 min under solar
light illumination, connected with one log and two log reductions by WO3/
Na2W4O13 and WO3, respectively (Fig. 4.2). The photocatalyst’s developments for the creation of ROS species can be observed in reactive species
scavenging studies and electron spin resonance spectroscopy, which have a
significant role in the enhanced photocatalytic disinfection efficiency [40].
4.2.4 Potential of CDs for combating the virus
The application of CDs to neutralize viruses and diminish contagion rates has
garnered limited investigation. The interferons (IFNs) are the most distinguished antiviral cellular defense components inside the human body, exhibiting a potent antiviral response to viral illness [41]. EDA-CDs had more
powerful suppressive activities on VLPs’ adsorption to HBGA and their corresponding antibodies than EPA-CDs, displaying a similar charge transfer
impact. Huang et al. recently discovered CDs could be made from the
monomer of benzoxazine, which may block the infectious disease caused by
FIGURE 4.2 Schematic depiction of tungsten oxide-coated CDs for photocatalytic disinfection
purposes.
74 Graphene Quantum Dots
flaviviruses (Dengue, Zika, and Japanese encephalitis virus) and withoutenveloped viruses (parvovirus, porcine, and adenovirus-associated virus)
in vitro, most likely by straight coupling to the virion’s exterior and ultimately
obstructing the virus-cell communication [42].
By changing the cell surface membrane and binding protein, invasion and
viral entrance can be blocked. According to the plaque reduction assay, CDs
from curcumin had numerous concentration-dependent suppressive activities
on the swine endemic diarrhea virus. The curcumin-derived C-dots can inhibit
infectious disease via viruses at a preliminary phase. According to the Raman
spectroscopy and fluorescence investigation, the electrostatic interaction
of þve charged C-dots causes viral accumulation and deactivation [43].
The Garg et al. observed suppressive strategy of coronaviruses using heterogeneous CDs in a recent publication. Using a succession of unique
bioactive, the authors recommended developing triazole-based CDs to fight
SARS CoV-2 illness. CDa are suitable for a wide range of biomedical applications in line with the significant number of hydrophilic functional groups on
their edges. Besides, the surface functioning of these mysterious nanomaterials
is effective for fine-tuning the level of virus contact [44]. Coronavirus infection can be effectively inhibited by curcumin þ ve C-dots (CCM-CDs). CCMCDs were made by hydrothermally reacting citric acid and curcumin in a
Teflon-coated autoclave, then centrifuging and dialysis to purify the product. It
was also determined to prevent virus entrance synthesis of the ve strand of
RNA. ROS accumulation and stimulation of exciting interferon genes and
proinflammatory cytokines suppressed viral replication. This was found to be a
multisite enteric coronavirus inhibitor (Fig. 4.2). CDs can successfully stop the
replicating RNA viruses, for example, respiratory syndrome virus and porcine
reproductive virus. In a Teflon-coated autoclave chamber, the PEG-diamine
and ascorbic acid require hydrothermal reaction to produce CDs. The antiviral activity was assessed on monkey kidney cells infected in vitro and WUH3
virus strain of the pig cardiovascular and respiratory illness. Higher interferon
combination and increased expression of interferon-stimulating genes stop
viral replication [45]. The polyamine-modified CQDs could inhibit the white
spot syndrome virus infection by adhering to the viral particle in a dosages
manner [46]. The C-dots for the antiviral property has summarized in
Table 4.1.
4.2.5 GQDs application in wound pathogen disinfection
Recently, drug delivery via GQDs has sparked renewed attention due to their
enormous precise surface area and constant contact with various molecules via
electrostatic interactions and hydrophobic stacking. The acidic or hydrophobic
environment may weaken the drug placed on GQDs. As expected, unmodified
GQDs could efficiently deliver the anticancer medication DOX to the nucleus
via GQDs/DOX conjugates. The augmentation of DOX nuclear accumulation
Microbial sensing and antimicrobial properties Chapter | 4
75
TABLE 4.1 Summarizes of antiviral mechanisms of CDs and their
derivatives.
CD
Antiviral
mechanism
Antiviral against
References
CCM-CDs
(1.5 nm)
Penetration,
multiplication,
and budding
Porcine epidemic diarrhea virus
(coronavirus model)
[43]
DCs (4.7 nm)
Multiplication
Porcine respiratory and
reproductive syndrome virus
[45]
Functionalized
CQD
Penetration and
multiplication
Human coronavirus
[47]
Boronic acid/
aminefunctionalized
CD
Penetration
Herpes simplex virus type 1
[48]
Benzoxamine
CD (4.4 nm)
Virus adhesion
Porcine parvovirus, adenovirusassociated virus, Zika virus, and
Dengue virus
[46]
CCM-CDs
(4.2e5.2 nm)
Penetration,
and
multiplication
Enterovirus
[49]
CDs
Blocking of
binding
Human norovirus virus-likeparticles
[41]
Glycyrrhizic
acid CD
(11.4 nm)
Invasion and
replication
Porcine reproductive and
respiratory syndrome virus,
coronavirus, and herpes viridae
[50]
Bluefluorescent CQD (1.9 nm)
Cyan
fluorescent
C-QD (2.7 nm)
mRNA
expression
level of IFN-a,
IFN-b,
Pseudorabies virus
[51]
Polyaminemodified CQD
Penetration
White spot syndrome virus
[46]
by GQDs results in increased cytotoxicity against cancer cells. In the presence
of GQDs, the luminous properties of DOX were quenched [52].
Additionally, GQDs passivated with PEG exhibited a remarkable capacity
(2.5 mg mg1) to deliver the anticancer medication DOX. The PEG-passivated
surface increased the solubility of the GQDs and drug load via hydrogen
bonding. Nahain et al. described a method for efficiently and precisely
76 Graphene Quantum Dots
delivering GQDs using hyaluronic acid (HA) as a targeting agent [53]. DOX
has a quenching effect, and quantitative investigation revealed that 75% of
DOX was loaded onto the surface of GQDs. DOX release increased to 42%
during 6 h of mildly acidic accommodation (pH 5.0), and the GQDs-HA
matrix released all DOX within 48 h. At the same time, DOX release was
significantly slower (only 20%) at pH 7.4 after 48 h. This pH-dependent
behavior proved favorable due to the tumor’s somewhat acidic environment.
Additionally, in vitro cellular imaging and in vivo distribution can verify
the target specificity in tumor tissue and cancer cells. Later in the study, Wang
and colleagues produced folic acid (FA)-conjugated GQDs for loading DOX
[54]. Even after DOX loading, the GQDs-FA could reliably identify HeLa
cancer cells from normal cells and deliver DOX to targeted cells. DOX had
loading effectiveness of 689 wt percent on the GQD-FA surface. Additionally,
because GQDs have an intrinsic steady fluorescence, the delivery complex
should be observed in real-time via two channels comparable to DOX and
GQDs. Within 30 min of internalization of the DOX-GQD-FA nanomaterials
by HeLa cells, the fluorescence of DOX was lost because its adsorption on the
GQDs superficial, but the GQDs green fluorescence was perceived in the cell
cytoplasm and then incubated for 8 h, free DOX fluorescence was visible in
both the cytoplasm and nucleus.
4.3 The live cells real-time molecular tracking by GQD
The GQDs have enormous potential as emerging bioimaging nanomaterials for
real-time molecular tracking because of their excellent conjugation properties
with protein and robust photostability in light. Zheng et al. used insulin-GQD
conjugates as fluorotags to monitor the real-time dynamics of insulin receptors
in alive adipocytes tissue [55]. They also investigate the distribution, internalization, and recycling of insulin receptors in adipocytes tissue. The graphene materials have been used to reveal regulated insulin receptor trafficking
in adipocytes and demonstrate that apelin facilitates insulin receptor dynamics
in adipocytes. At the same time, TNF inhibits it indicates the first time and
opens the gate for more investigation of insulin cellular interaction. Ananthanarayanan et al. revealed that GQD transferrin-conjugated (Tr-GQD) might
be used to detect transferrin receptors in human cervical carcinoma (HeLa)
cells in real-time [56]. Since HeLa cells can overexpress transferrin receptors,
they were used to investigate the reutilizing of iron-bound transferrin molecules and internalization. Confocal microscopy revealed that selective binding
of Tr-GQDs to transferrin receptors induced their endocytosis.
Moreover, GQDs might be applied for specific drug carriers and bioimaging nanomaterials to monitor the real-time release kinetics of drug
doxorubicin (DOX). The stocking and release characteristics of DOX from
GQDs have been demonstrated using GQDs coupled hyaluronic acid (GQDHA). Due to the extraordinary attraction between HA and the CD44
Microbial sensing and antimicrobial properties Chapter | 4
77
receptor, HA acts as a targeting molecule and boosts the bright fluorescence.
As a result, GQD-HA penetrated the cells more than GQD. According to the
findings, pharmaceuticals agents were released from the QGD-HA, indicating
that GQDs could be used as drug transporters and fluorescent probes in the
future. Kim et al. demonstrated the utility of GQDs for in vivo tracking of
human adipose-derived stem cells [57] and revealed that GQDs were noncytotoxic, had no substantial effect on their survival or functionality, were
primarily dispersed in the cytoplasm via endocytosis, and retained their
fluorescent signal for 24 h. Most of the characteristics mentioned earlier meet
the primary criterion for stem cell tracking molecules. Hollow-structured
nanospheres created on GQDs with Pd nanoparticles were utilized to
monitor trace levels of H2O2 in real-time, a viable stage for cancer diagnostics
due to the increased H2O2 generated by active tumor cells. Due to the high
electrocatalytic activity of Pd and the innate peroxidase mimicking the action
of GQDs, nanospheres can be used as H2O2 tracking agents.
Imaging in vivo, numerous photoluminescent nanomaterials have been
described for labeling cells in vitro. However, in vivo imaging and biomaterial
application are well established because their excitation-dependent photoluminescence makes multi-color fluorescent GQDs possible. Autofluorescence of GQD significantly affects imaging superiority because it is
critical to validate the ideal excitation and emission wavelengths before imaging. The Nurunnabi et al. initially showed that carboxylated GQDs collected
mainly in the liver, spleen, lung, and tumor simultaneously and did not produce acute toxicity [58] as the fluorescence intensity increased and then
decreased with increasing time, which was primarily due to the GQDs’ time
dependence and excretion. Analyses of the organs from GQDs injected mice
revealed that the GQDs were disseminated throughout the body and gathered
in various probable locations. Biomaterials that integrate imaging and photodynamic treatment (PDT) have increased interest. For instance, a study
showed that due to their extensive absorption, high singlet oxygen (1 O2)
generation yield, strong deep-red emission, GQDs could be used as PDT
biomaterials for real-time imaging and highly effective cancer treatment.
Where GQD was injected in the mice, a spot of high red fluorescence intensity
was reported, and the power remained constant for one week, establishing the
parameters for PDT. This study examined the photothermal outcome and 1O2
quantum yields of GQDs in tumor cell death. The abovementioned findings
showed that GQDs should be used in imaging and cancer therapy. The
nanocomposite of GQDs and annexin V antibody (AbA5-GQDs) has been used
to sense apoptotic cells required for homeostasis. Due to the transparency of
the zebrafish, it was chosen as a unique model for studying apoptotic cell death
start and progression. GQDs have emerged as a special material for building
biosensors due to their intrinsic flaw structure, ease of modification, tunable
PL property, and superior electrochemical property. They can be used as
fluorescence probes to detect various substances with excellent selectivity and
78 Graphene Quantum Dots
sensitivity, including metal ions, hydrogen peroxide (H2O2), glucose, cholesterol, protein, and nucleotides.
4.4 Conclusion
Nanoscience breakthroughs are now being studied for their potential to
improve and create detection methods and adapt therapy for a variety of
complex ailments. In particular, QDs have been designed as cutting-edge
raised areas aimed at high-throughput computable investigations of various
indicators in clinical tissue samples and biomarkers in cells, in vivo assessments of cells with illnesses, and possibly tailored and perceptible medicine
administration. Quantum dots indeed offer great potential in pharmacy, bioimaging, medical, and photoluminescent uses. They can be used as capable
fluorescent probes for imaging with little toxicity in cells other applications,
such as bioanalysis and others. Due to its good chemical inertness, outstanding
biocompatibility, and resistance to photobleaching that graphene and carbon
quantum dots have become more popular. Their optical characteristics may
also be modified for specialized and specific applications through size control,
chemical doping, and functionalization, among other techniques.
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2013;7(8):6858e67.
Chapter 5
Graphene quantum dots for
drug biodistribution and
pharmacokinetics
Mohammad Zubair1, Fahad Mabood Husain2, Farha Fatima3,
Mohammad Oves4, Mohammad Azam Ansari5 and Marai Almari6
1
Department of Medical Microbiology, Faculty of Medicine, University of Tabuk, Tabuk, Saudi
Arabia; 2Department of Food Science and Nutrition, Faculty of Food and Agricultural Sciences,
King Saud University, Riyadh, Saudi Arabia; 3Department of Zoology, Aligarh Muslim University,
Aligarh, Uttar Pradesh, India; 4Centre of Excellence in Environmental Studies, King Abdulaziz
University, Jeddah, Saudi Arabia; 5Department of Epidemic Disease Research, Institute for
Research & Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, Dammam,
Saudi Arabia; 6Department of Surgery, Faculty of Medicine, University of Tabuk, Tabuk, Saudi
Arabia
5.1 Introduction
Quantum dots (QDs) are semiconducting nanoparticles gaining ground in
numerous applications, counting the biomedical field, because of their
exclusive optical characteristics. Recently, graphene quantum dots (GQDs)
have earned some attention in biomedicine and nanomedicine because of their
higher biocompatibility and low cytotoxicity compared to other QDs. GQDs
share the optical characteristics of QD and have the established capability to
cross the bloodebrain barrier (BBB). For this reason, GQDs are now being
employed to develop our understanding of neuroscience diagnostics and
therapeutics. Their size and surface chemistry that ease the loading of
chemotherapeutic drugs makes them the perfect drug delivery systems through
the bloodstream, across the BBB, and up to the brain. GQDs-based neuroimaging techniques and theragnostic applications, such as photothermal and
photodynamic therapy alone or in combination with chemotherapy, have been
designed [1].
Ever since the first synthesis that occurred in the 1980s QDs have been
studied in-depth and applied in numerous devices such as optical devices and
solar cells. However, more recently, the interest in QDs has deepened further
and spread in various branches of medicine and biology primarily because of
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00010-8
Copyright © 2023 Elsevier Ltd. All rights reserved.
83
84 Graphene Quantum Dots
their photophysical properties. Due to this property, QDs are best suited to be
used in drug delivery, bioimaging, and much more important because they can
be used in theragnostic applications such as photothermal therapy [2].
Quantum dots are semiconducting nanomaterials with dimensions below
100 nm. The chemical and physical characteristics of QDs are dependent on
their size, this is because of the quantum confinement effect. When an electron
gets promoted to conduction band from valence band what happens is that an
empty electron state is left which is known as a “hole.” Electrons and holes are
attracted to each other by electrostatic Coulomb force, resulting in a bound
state, called excitons, which are neutral quasiparticles. QDs can also be treated
as particles-in-a-box and the reason is the dimensions of QDs are comparable
to the exciton diameter and this aspect can be expressed through the following
formula [3]:
In the aforementioned formula, h is reduced Planck constant, m is the mass
h2 p2 leads to a
of the particle and L is the length of the box. In QDs, this E ¼ 2mL
2
dependence of the bandgap energy on the size: the smaller the size, the bigger
the bandgap energy [3]. This generally becomes evident by quantifying the
variations in absorption and emission as a function of the decreasing size and
this is called “blue-shifts” and these shifts indicate an increase in bandgap
energy. Similar to particle-in-a-box, electrons can occupy only specific
discrete levels of energy and because of this they are also known as “artificial
atoms.” As compared to organic fluorophores, QDs have unique features and
one such feature is the broader absorption of spectra, this enables excitation by
a wide range of wavelengths and narrower emission spectra and this helps to
reduce the signal overlap. The methods involving synthesis primarily involve
surface passivation of an inner layer and this is done by deposing a capping
layer, which generally is an inorganic semiconductor material [4].
The core layer has a slender bandgap as compared to the shell. It has been
found that the synthesis of QDs generally occurs in a coordinating solvent and
the presence of core material from the 16th group and of an organometallic
shell precursor at great temperatures. QDs are also less vulnerable to photobleaching as compared to other molecules and the reason is that they have a
very stable light emission. This has been showcased in various biological labeling experimentations where the photostability of QDs was likened with
generally used fluorophores [5].
As the popularity of QDs is increasing day by day, concerns have also risen
because of their cytotoxicity. The primary concern is regarding the inherent
toxicity of the core elements of QDs and they include selenium and cadmium,
both of these elements can affect the cell cultures and as well as live animals.
In the last couple of years, QDs have displayed tremendous photophysical
properties as well as good biocompatibility. This suggests that their character
is more similar to the molecule as compared to other QDs, and this is the
reason that their popularity has increased drastically in life sciences [6].
Graphene quantum dots for drug Chapter | 5
85
Graphene is a bidimensional material, a single-atom-thick sheet of
honeycomb-arranged, sp2 -bonded carbon atoms. Graphene and graphene
oxide (GO) has been employed in several biomedical applications, as antibacterial agents, scaffolds for bone regeneration and diagnostic tools. Despite
the wide range of applications of graphene and GO in the biomedical field and
their biocompatibility, the use of GQDs comes with further advantages. GQDs,
as QDs, have unique optical properties which strictly depend on their size,
shape and surface chemistry, making them highly suitable for bioimaging
compared to other organic dyes. Furthermore, these nanoparticles’ small dimensions easily allow them to cross biological barriers and target specific
anatomical regions inaccessible for graphene and GO, making them good
candidates for drug delivery [7].
5.2 Graphene quantum dots
Graphene quantum dots (GQDs) are small flakes of graphene in which
quantum confinement of excitons becomes prevailing, and this causes a casual
spacing Coulomb blockade peaks as compared to a periodic distribution. The
interesting fact is in GQDs quantum confinement is not just given by the size,
the reason is that different boundaries lead to a diverse spectrum of energy and
as well as the polarization of spin. Furthermore, have absorption peaks that are
similar to those of other graphene-based materials. These two separate summits depend on two precise electronic transitions: the first is determined by
pp* alterations within the aromatic rings and, in general, by the sp2 -hybridized portions. The second, less concentrated, the peak is due to pp*
evolutions, and it is given by the existence of lone pairs contained by oxygens.
As evidenced, pristine graphene exhibitions only a pp* transition peak at
270 nm, and no np* [8].
The first absorption peak of GQDs is generally in the range of 200 and
270 nm. One occurs at the wavelengths of more than 280 nm which occur
within pp* and np* transitions, respectively. The primary characteristics
that determine the photoluminescence and absorption features of graphenebased materials comprise its sp2-hybridized fraction, the existence of efficient groups comprising, specifically, oxygen or nitrogen, and the comparative
synthesis process and the dimension of the molecule. Since the dimensions of
GQDs are similar to the exciton diameter, they can be considered as particlesin-a-box for which the bandgap energy is contrariwise proportional to the
squared size. It is also shown that higher bandgap energies match up to minor
emissions wavelengths. Correspondingly, it has been testified that the peak of
absorption of GQDs with lateral proportions ranging from 1 to 4 nm is situated
at 270 nm, while 7e11 nm GQDs displayed a maximum located at 330 nm [9].
This shift could simply be elucidated by taking into account the quantity of
sp2 constructions inside the molecule. The carbon materials that contain a
mixture of sp3 and sp2 bonding, the optoelectronic characteristics are
86 Graphene Quantum Dots
determined by the p conditions of the sp2 sites, which are engrained in the
unvaryingly advanced bandgap of s and s* orbitals and this is the reason that
repeat combination of electron-hole couples in sp2 clusters results in PL. To
tune the emission of PL, nature and as well the quantity of sp2 sites can be
manipulated, this reason is that bandgap depends not only on size and shapes
but also on the portion of sp2 domains [10].
Carbon materials that have disordered sp2 clusters generally act as unstructured semiconductors. In these semiconductors the density of states of
aromatic chains falls in contained states within the bandgap, plummeting the
energy gap among valence and transmission band. The quantity of functional
clusters comprising nitrogen gives as well in GQDs optical features: their lone
pairs upsurge the second absorbance peak connected to n p* conversions.
Other agents of bioimaging, such as QDs or other organic compounds lack
specific to surface functionalization that can be tuned. Because the chemistry
of carbon is the most characterized and studied this allows a more sophisticated surface engineering [11]. It has also been discovered that the method of
synthesis can upset the optical characteristics as well. The methods of synthesis that involve hydrothermal approaches can reduce the mean number of
oxygens, therefore, the quantity of aromatic rings also gets increased and the
result is that bandgap energy is reduced [12].
5.3 Synthesis of GQDs
GQDs are blocks of graphene with a two-dimensional (2D) cross-sectional
size and excellent chemical, physical and biological characteristics. An ideal
form of GQD consists of a single atomic shell of carbon atoms. Though, most
of the synthesized GQDs also contain functional groups such as oxygen and
hydrogen and generally have multiple layers of atoms with sizes less than
10 nm. Because of its small size, GQD has better prospects than graphene,
graphene oxide, or graphene in applications related to biomedicine. Though,
before scheming GQD for applied applications, its biocompatibility and
toxicity continue to remain foremost trepidations. Studies have revealed that
GQDs have decent low and biotoxicity biocompatibility [13].
Currently, the methods that are used for GQD synthesis can usually be
categorized into two processes which names top-down or bottom-up. The
bottom-up method requires reaction steps that are more complicated and it also
requires precise organic materials and because of these reasons the top-down
approach is preferred as compared to the bottom-up. The top-down approach
primarily involves cutting big blocks of carbon materials into lesser fragments.
Furthermore, there are other numerous methods for top-down processes as
well such as hydrothermal method, chemical oxidation method, chemical
vapor deposition, electrochemical oxidation method and pulsed laser ablation,
even a combination of these methods could be used as well [13]. The details of
these methods are as follows.
Graphene quantum dots for drug Chapter | 5
87
5.3.1 Chemical oxidation method
This method is also identified as an oxidation cutting method. In this method,
the carbon bonds of graphene or sometimes carbon nanotubes are generally
destroyed by oxidants. An experimental system was developed by Liu et al.
[14]. This method uses Vulcan XC-72 carbon black as a carbon source and
robust oxidant concerted nitric acid reflux to formulate high concentration
GQDs. The yield and the purity of this method is generally 75 wt% and
99.96 wt% correspondingly. As compared to other methods, this method of
synthesis is faster and as well as eco-friendlier. However, since this method
uses strong oxidants such as HNO3 and H2SO4 it is not considered that safe
and also, the chemical waste generated is also considered to be hazardous [15].
5.3.2 Hydrothermal method
This method is quicker and simpler to prepare GQDs. In this method, GQDs
can be attained using a diversity of macromolecular or small molecular materials. Very pure GQD can be obtained by evaporation or dissolution and
filtration without dialysis. The results showed that the diameter and thickness
of the GQDs were mainly distributed in the range of 20e40 nm and 1e1.5 nm,
respectively. The PL signal has shown good stability under various pH conditions, indicating that it has broad application prospects in various environments. This method offers many advantages, such as low cost, high quantum
efficiency, no need for dialysis and cleaning, simple experimental setup, etc.
The GQDs produced were environmentally friendly and demonstrated solubility in solid water to illustrate their promising applications in biomedical and
bioelectronics devices. The hydrothermal method can be used to dot many
elements or groups, and the raw materials come from a wide variety of
compounds. Furthermore, the hydrothermal process can be combined with the
chemical oxidation process to produce various GQDs. However, it suffers from
the high temperature and high-pressure safety problem, and it usually takes a
long time, typically at least 5 h [15].
5.3.3 Ultrasound assisted method
This is an innovative technology that uses ultrasound to extract many compounds from a variety of matrices. Ultrasound propagation causes bubbles to
burst, known as the cavitation phenomenon, large turbulence, high-velocity
collisions between particles and perturbations in the arrays induce small,
porous particles in the sample. As a result, the solute rapidly expands from the
solid phase to the solvent. This method offers a clean, ecological extraction
with several benefits. This technique is simple, effective and has a lower cost
as compared to other methods. Its main advantages are related to increased
extraction performance and speeded up mobility compared to conventional
88 Graphene Quantum Dots
extraction. However, the tool has a significant disadvantage with age, since the
performance gradually decreases with decreasing intensity, thus reducing the
possibility of repeat experiments [15].
5.4 Applications of GQDs
The exceptional properties of GQDs facilitated their promising application in
biosensors. Though, the electrochemical properties of the biosensor have
shown an excellent benefit for GQD in electrochemical biosensors. The high
stability and accuracy of the biosensor upon glucose assay and the low toxicity
of GQD for enzyme control are major advantages of its practical application.
The glucose sensing system was developed based on the electrostatic attraction
between anionic fluorescent GQDs and a cationic boronic acid dipyridinium
salt [16].
This showed that the electrostatic attraction between GQD and BBV
occurred in transferring electrons in the excitation state from GQD to BBV and
also reduced the fluorescence intensity of GQD. A glucose biosensor has also
been developed that uses GQD as a modified carbon-ceramic electrode with an
enzyme fixation substrate. Razmi et al. confirmed the excellent accuracy of the
glucose study and noted the practicality of the glucose biosensor in clinical
studies. It has also been used to determine glycemic reflection in human plasma
models. Trypsin is the main gastric enzyme produced by pancreatic acinar cells
and breaks the peptide bonds at the C-end. However, these approaches have
certain drawbacks such as complications of conjugate electrolyte synthesis,
fluorophore classification, photobleaching, and as well as requirements for
dissimilar kinds of substances and cytotoxicity for GQDs [16].
In modern times, GQD applications have increased in drug delivery, sensor
imaging, magnetic hyperthermia, phototherapy, antibacterial activity, catalyst,
environmental protection, and energy. To better apply GQDs to drug delivery,
some researchers have used functional density theory calculations, molecular
dynamics simulations, or other methods to study the properties of GQDs in
theory. There are many ways to administer medications, but focusing on
administering and ignoring medications cannot enhance the therapeutic effect
of medications. Therefore, more and more researchers paid attention to the
close relationship between drug delivery and drug release and tried to develop
a variety of drug delivery methods to enhance the effect of therapeutic drugs
by enhancing drug delivery and discharge efficiency [14].
GQDs are innovative and effective nanomaterials for biological treatment.
There have been reports of graphene or graphene-based nanomaterials for drug
delivery and drug delivery to improve conduction efficiency and enhance
therapeutic effects. Compared to graphene, they have better water solubility,
lower cytotoxicity, and higher specific surface area, making them more efficient
cores in the molecular loading of the drug. As a member of the graphene and
carbon family of nanomaterials, it shows several advantages over other
Graphene quantum dots for drug Chapter | 5
89
nanoparticles in drug delivery applications due to their low toxicity, high surface
ratio or size, and ability to operate a massive surface. Compared with other
traditional nano plants such as polyethylene glycol, it can offer more binding
sites for chemical coupling and improve cell absorption capacity. Compared to
quantum dots of similar size, it has superior properties such as quantum
confinement effects and simultaneous tracking due to its modifiable optical
luminescence, but relatively lower toxicity due to lack of heavy metal components. Therefore, they have great potential for biomedical application. Besides, due to its flat structure, it has a large surface area-to-volume proportion,
which allows for more efficiency in loading and administering drugs [17].
5.5 Drug delivery methods
One of the major issues in medical treatment to lower the side effects of
chemotherapy is localized drug delivery. There are numerous ways in which
targeted drug delivery could be used such as through temperature control, pH
control, or the use of ultrasonic and optical waves [18]. Numerous nanoparticles have been used for this purpose as shown in Fig. 5.1 below.
Sample formulations of treatment agents require membrane cells that do
not fit into the hydrophilic type. Some proteins and peptides can penetrate
membrane cells and carry bio-conjugated nanocarriers including total cells.
Cells embedded in peptides are the most popular biomolecules used for the
synthesis of nanoorganisms including agents. A large amount of protein,
nucleic acid, synthetic drugs, and liposomes are compatible with CPPs, so they
can be used for a wide range of loads. However, some studies have suggested
that certain factors include size, charge and the properties of the final dots in
FIGURE 5.1 Different nanoparticles used for localized drug. Adopted from Itani R, Al Faraj A.
siRNA conjugated nanoparticlesda next generation strategy to treat lung cancer. Int J Mol Sci
2019;20(23):6088.
90 Graphene Quantum Dots
the cell without the need for bio-fictionalization [20]. They showed that a good
total number of tumors fit into simple cells and that a large number of spots are
stored in the cytoplasm while small particles pass through the vital component.
Depending on the target, the bio-molecule attached should be selected by the
number of points. For the establishment of specific antigens, the dots should be
set with the appropriate drug. As well as detecting a unique immune system in
any type of cell requires total bio-contact points in molecular molecules,
peptides or aptamers [21].
Foliate is a useful quantitative compound of the best bioconjugation studies
for the synthesis of folate receptor, overexpressed in many types of tumor
cells. Similarly, total molecules can act as biomolecular sensors, such as
maltose sugar, by binding the sum of points to a protein that has a close affinity
for maltose. Recent studies have demonstrated the potential of dots concerning
the delivery of chemicals to specific cells while monitoring the administration
of drugs [22].
Chakravarthy et al. [23] proved to deliver a beneficial total score with
doxorubicin in alveolar macrophages, severe cells in diseased lungs. Savla
et al. use the mucin one quantum dot aptamer to target ovarian cancer cells and
administer the anticancer drug doxorubicin. Quantum dots encapsulated in
biodegradable polymers such as chitosan are candidates for regulating the
delivery of cancer drugs. The use of chitosan dots in chitosan prevents the drug
from being released before the body part of the body is implanted. The effect
of a large number of points on its television transmission is very useful for
many different images of different parts of the body.
5.5.1 Fluorescent graphene quantum dots application
Detection of heavy metal ions is significant within the environmental setting.
Various graphene quantum dots (GQD) based fluorescence sensors are made
for determining sensitivity of different metal ions. The mechanics to detect
Fe3þ considering GQD fluorescence sensor feature the special relationship
between phenolic hydroxyl groups and Fe3þ [24]. The efficient use of functionalized GQDs along with amine and heteroatom doping groups is observed
to detect Fe3þ. A wide linear range of sulfur and nitrogen codoped GQDs are
used ranging between 0 and 130 mM for sensing fluorescent material for the
presence of Fe3þ. It possess low detection limit of 0.07 mM. A sensitive
response toward Fe3þ is exhibited by amino acid-functionalized and nitrogen
GQDs [25]. The anomaly is caused not only by the high thermodynamic
attraction between Fe3þ and nitrogen atoms in GQDs, but also by Fe3þ
paramagnetism. This suggests a related detection mechanism. The block
copolymer-integrated GQDs can also be used for multifunctionally colorimetric temperature, metal ion sensing, and pH. The bcp-GQDs obtained
exhibit rapid fluorescence quenching when 100 M Fe3þ is added. A turn-on
orange-red fluorescence nanosensor based on rhodamine B derivative-
Graphene quantum dots for drug Chapter | 5
91
functionalized GQDs (RBD-GQDs) has been developed for the detection of
Fe3þ in cancer stem cells, unlike the previously described turnoff fluorescent
nanosensor [26]. With the addition of Fe3þ, a new solid peak of orange-red
emission oriented at 580 nm occurs, owing to the on-switch of the spirocyclic moiety in the RBD.
There is production of different GQDs, which are sensitive toward
detecting Cu2þ, besides Fe3þ. Cu2þ has been detected using a sensing technique based on amino-functionalized GQDs (af-GQDs). The facilitation of
nonradiative electron/hole recombination annihilation by an efficient electron
transfer mechanism, which is due to static and dynamic phenomena, causes
fluorescence quenching. GQDs obtained by a hydrothermal system as a result
of the forming of a complex between Cu2þ ions and the GQDs have been
formed as an efficient Cu2þ sensing approach. The static (rather than dynamic)
kind of fluorescence quenching mechanism tends to be the most common.
Furthermore, pitch graphite fibers could be chemically oxidized to produce
pristine GQDs. These GQDs may be used to determine Cu2þ in water samples
in a simple and environmentally safe manner [27]. A new “off-on” fluorescence probe using Eu-GQDs was recently published for the label-free determination of Cu2þ with high sensitivity and selectivity. The use of coffee
grounds as a precursor to prepare fluorescent GQDs functionalized by poly(ethylene imine) (PEI) has been suggested as an environmentally safe and
simple method. As a fluorescent probe, PEI-GQDs are extremely vulnerable to
Cu2þ ions.
The methods of doping and functionalization of GQDs are widely used to
increase selectivity. For instance, QDs modified with thymine-rich DNA
(DNA-GQDs) increase selectivity; however, their fluorescence is quenched in
the presence of Hg2þ. The binding of Hg2þ to the thymine bases of the DNA
inhibits transfer of electrons to DNA-GQDs. A related approach can be used to
make cysteine-doped GQDs (Cys-GQDs). A highly selective “on-off” fluorescence sensing technique for the detection of Hg2þ was designed on the
premise that the aggregation of GQDs can be validated using dynamic light
scattering in the presence of Hg2þ and that the aggregation induces the fluorescence quenching of GQDs [28]. It is possible to constrict a ratiometric
fluorescence sensor using CdTe as the internal standard, along with GQDs and
a CdTe-based core-satellite hybrid to detect Hg2þ as it is easy to observe
ratiometric fluorescence sensing with the naked eye. Fig. 5.2 shows the steady
decline in fluorescence intensity of GQDs as the concentration of Hg2þ rises.
The test solution undergoes a vibrant visual color transition from blue to red
with high sensitivity and selectivity (limited to 3.3 nM) [30].
Synthesis of 3,9-dithia-6-monoazaundecane (DMA)-functionalized GQDs
is carried on with the help of hydrothermal method via electrostatic interaction
on its surface. A microfluidic sample pretreatment system for Pb2þ preconcentration has recently been developed, which includes a large-volume
SPE chamber and a peristaltic pneumatic micropump for automated sample
92 Graphene Quantum Dots
FIGURE 5.2 (a) Structure of the ratiometric fluorescence probe and its working principle; (b)
CdTe@SiO2@GQD ratiometric probes; (c) GQDs probes. Adopted from Hua M, Wang C, Qian J,
Wang K, Yang Z, Liu Q, Wang K. Preparation of graphene quantum dots based core-satellite
hybrid spheres and their use as the ratiometric fluorescence probe for visual determination of
mercury (II) ions. Anal Chim Acta 2015;888:173e81 with License no 5096510894333 (for a) &
5098180378004 (for b & c).
loading and recovery. Fig. 5.3 shows the process in which DNA aptamer
functionalized GQDs serve as “on-off” sensing units to detect transfer of
electrons from GQDs to Pb2þ. In this process, high sensitivity is exhibited
toward Pb2þ with the sensor with a linearity ranging between 1 and 1000 nM,
and detection limit of 0.64 nM [30]. An innovative platform for on-site water
emission screening is provided by on-chip preconcentration of trace metal ions
from a large-volume sample accompanied by metal ion detection using the
fluorescence GQD sensor.
AgNPs developed on GQDs quench the FL of GQDs by charge-transfer
processes, resulting in a novel GQD-based probe for the detection of Agþ.
The protocol may also be used to detect Au on a limited basis (III). Based on
Pearson’s HSAB theory [32], amine-functionalized GQDs (Am-GQDs) have
been synthesized as a “on-off-on” probe for the selective detection of Agþ.
Soft acids react faster and form stronger bonds with soft bases, according to
Pearson’s HSAB principle, while hard acids react faster and form stronger
bonds with hard bases. According to the HSAB principle, S and Ag form
stronger bonds than the other metal ions, since they are soft base and soft acid,
resulting in recovery of Agþ ions selectively.
5.5.2 Long-term biodistribution
Fig. 5.4 shows the rate and degree of substance absorption, distribution, metabolism, and elimination (ADME) determine the behavior of any nanomaterial at
the tissue or cellular level. These processes are known as biokinetics in nanoscience, and they include uptake, biodistribution, and removal [34].
Nanomaterials have a dramatically different volume-to-surface ratio than
bulk materials, so their physicochemical properties can vary significantly.
These variations have an effect on not only biodistribution but also
Graphene quantum dots for drug Chapter | 5
93
FIGURE 5.3 (a) Schematic illustration of a five-layered sample pretreatment microdevice; (b)
Digital image of the cation exchange resin; (c) Experimental scheme for Pb2þ detection. Adopted
from Park M, Ha HD, Kim YT, Jung JH, Kim SH, Kim DH, Seo T. Combination of a sample
pretreatment microfluidic device with a photoluminescent graphene oxide quantum dot sensor for
trace lead detection. Anal Chem 2015;87:10969e75.
FIGURE 5.4 Biokinetics of nanomaterials. Adopted from Zhao FD, Yao, Guo R, Deng L, Dong A,
Zhang J. Composites of polymer hydrogels and nanoparticulate systems for biomedical and
pharmaceutical applications. Nanomaterials 2015;5(4):2054e130.
94 Graphene Quantum Dots
biodegradation rates in vivo. The oxidation products of soluble nanomaterials,
mainly ions, follow the same path as their organic solutes. This results in a
global dissemination, but it can also result in localized preservation of tissues
or cells. In summary, several factors influence nanomaterial biodistribution,
including the nanomaterial’s size, surface properties, and dissolution rate, as
well as tissue- or organ-dependent factors including barrier tightness or
permeability [35]. Generally, nanomaterials are found in spleen, bone marrow,
lymph nodes, liver, kidneys, and central nervous system after being exposed
orally, through inhalation, of intravenous injections.
The method of exposure is also important. There is variation in biodistribution and biokinetics of nanomaterials from the administered ones and
the respiratory tract after intravenous administration. Furthermore, it is
demonstrated that a surrogate biokinetic approach for oral or pulmonary routes
of exposure is not presented through biokinetics of intravenously injected
nanomaterials [36]. After 1 h, the liver, spleen, carcass, skeleton, and blood
had the greatest titanium accumulation, followed by the spleen, spleen,
carcass, skeleton, and blood, after which the blood content quickly declined,
while the distribution in the other organs and tissues remained steady until day
28. Following oral administration, the majority of the dose was excreted in the
feces, but 0.6% of the dose translocated through the GI tract and was eventually contained in the lung, brain, spleen, skeleton, liver, kidney, and uterus.
After 1 h, a 4% translocation rate of the original administered dose occurred,
with the majority of the dose remaining in the carcass, and this level decreased
to 0.3% after 28 days. The liver and kidney organ fractions remained unchanged. For all translocated/absorbed particles, the clearance from the lungs
through the larynx increased from 5% to 20%.
The development of physiologically based pharmacokinetic (PBPK)
models can reliably predict such relationships have been established. Most of
these models characterize the fate of nondegradable nanomaterials injected
intravenously [37]. The most significant considerations have been determined
to be the nanomaterials’ physicochemical properties (such as shape and size),
blood/tissue permeability coefficients, and macrophage phagocytosis. A novel
two-step method for assessing the biokinetics of inhaled nanomaterials was
recently proposed for the lung. The translocation kinetics of aerosolized gold
nanoparticles through the epithelial tissue barrier were determined in vitro, and
the distribution to secondary organs was predicted using a PBPK model in a
second phase [37].
5.5.3 Biodistribution and toxicology of carboxylated graphene
quantum dots
GQDs have gathered a lot of attention as a new kind of quantum dot because of
their chemical stability, electrical properties, and photoluminescence.
Furthermore, because of the high quantum confinement and edge effects,
Graphene quantum dots for drug Chapter | 5
95
transforming two-dimensional graphene sheets into 0-dimensional GQDs will
expand the electronic and optoelectronic applications. The majority of GQD
research has been based on biomedical studies, and experimental synthesis is a
relatively new endeavor. There have been various studies of the production of
graphene-based biosensors aimed at detecting biomolecules with high sensitivity using GQDs in biomedicine [38]. GQDs with sufficient surface functionalization can be used for drug and gene distribution due to their ultrahigh
sensitive surface region.
A substance that incorporates the modalities of medical imaging and
therapy is referred to as theranostic. It concurrently releases medicinal drugs
and medical imaging agents in a single dosage. Theranostic methods have the
ability to solve the unfavorable biodistribution and selectivity gaps that
currently occur between different imaging and therapeutic agents. Graphene is
reports as a carrier for a drug delivery system; for instance, the hybrid SiOe
2
coated quantum dots (HQDs)-conjugated graphene is used to monitor drug
delivery ad cancer therapy [39]. The most exciting feature of using GQDs as a
theranostic nanoparticle is that they have large surface area-to-volume ratios,
which means they can hold a lot of information. Since a variety of drug
molecules could be found on both surfaces, the large planar surface of GQDs
is called an additional window for drug distribution. Chemical conjugation
may also be used to conjugate the edge of drug molecules, as previously
mentioned. Furthermore, based on their surface functionalization, nondegradable nanoparticles prevent them from being easily cleared by the kidneys
without producing harmful moieties. As a result, it is one of the most
appealing materials for advancing precision medicine and imaging in the
biomedical field.
To make photoluminescent GQDs, exfoliation of carbon fiber first occurred
during ultrasonication in acidic media, as reported previously. Synthesis process of GQDs from carbon fiber is described in our previous report. However,
in brief, due to sonication in acid, the carbon fiber partially exfoliated and
formed multilayered and/or monolayered graphene. The development of a
zigzag-shaped graphene was aided by these conditions, which resulted in a
nearly transparent gray-black color in solution. Fig. 5.5 shows the result of
long-term exposure to strong acid and intense stirring that produces hexagonal
nanosized graphene shaped particles [40]. Some of the synthesis methods and
its applications were sum up in Tables 5.1 and 5.2.
5.6 Critical issues
Nanomaterials such as GQDs alone or in combination with many other
nanomaterials to form hybrid nanocomposites have remarkable properties due
to their unique properties and/or synergies required in the fabrication of biosensors. Over the past decade, these biosensor platforms have offered many
options for diagnosing a wide range of diseases, from autoimmune diseases to
96 Graphene Quantum Dots
FIGURE 5.5 (a) Synthesis of PL GQDs from carbon fiber; (b) PL intensities of the carboxylated
GQDs; (c) TEM images of the carboxylated GQDs; (d) HR-TEM image; (e) Size distribution of
the carboxylated GQDs; (f) confocal laser scanning microscopic (CLSM) images. Adopted from
Zeng Z, Chen S, Tan TTY, Xiao FX. Graphene quantum dots (GQDs) and its derivatives for
multifarious photocatalysis and photoelectrocatalysis. Catal Today 2018;315:171e83 with
Licence no 5098190151952.
TABLE 5.1 Biomedical applications of Bottom-Up synthesis methods GQDs
[41].
Synthesis methods
Applications
Size
obtained
GQD-RhB-silka
Diagnosis
3e20 nm
Mango leaf extract
mGQDs
NIR responsive fluorescence
bioimaging
2e8 nm
PEGylated GQD
Fluorescence imaging of tumors
2.75 nm
GQD-PEI
Gene transfection
3e4 nm
GQDs
Drug delivery and bioimaging
w12 nm
Graphene quantum dots for drug Chapter | 5
97
TABLE 5.2 Biomedical applications of Top-Down synthesis methods GQDs
[41].
Synthesis methods
Applications
Size obtained
GQDs
Diagnosis
5 nm
Durian extract GQDs
Bioimaging
2e6 nm
NP-GQD
Cysteine detection
10e30 nm
GQD-PEG-AG
Radiotherapy
3e4 nm
Lignin-GQDs
Bioimaging
2e6 nm
neurodegenerative diseases, cardiovascular disease, infectious diseases and
even cancer diagnostics. Regarding the degree of specificity of this pathophysiology, early detection of it through the use of biomarkers of diseases,
toxins, and pathogens in biological, environmental, and nutritional specimens
shows important and concrete ideas about their severity. The sensitivity,
selectivity, accuracy, and reliability of disease-related molecules always seem
difficult due to their extremely low concentrations. However, our review article
demonstrates that GQD-based sensors have now clearly reached the threshold
for detecting some target biomolecules. These biosensors for the early
detection of diseases have achieved tremendous popularity in many areas,
including clinical treatment, disease surveillance, the discovery of preventive
treatments, and the development of treatment-based drugs [42].
GQDs are in a position to operate multiple electrodes with ease and enable
fast, simple, stable, reproducible, and cost-effective sensor systems for clinical
and practical applications. Moreover, these GQD sensors are known for
excellent specificity, super selectivity and sensitivity in biological matrices
such as human blood, urine, sputum, saliva, milk, hard water, soil, etc. Which
can be attributed to both the material and the material content. GQD chemical
properties. On the other hand, the sharing of more expensive nanomaterials
with GQD, complex test methods, insufficient storage stability, and a host of
unpleasant factors at the nanoscale are obstacles that are obstacles to their
production. Moreover, most of these recently reported sensors have yet to be
verified for their clinical application. Therefore, strategies must be developed
to create large groups of sensors and to have mass production and approval in
the real world [19].
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Chapter 6
Graphene quantum dots:
application in biomedical
science
Rani Rahat1, Khalid Umar2, Sadiq Umar1, Mohd Jameel3, Mohd
Ashraf Alam4, Tabassum Parveen5 and Rohana Adnan2
1
College of Dentistry, University of Illinois, Chicago, IL, United States; 2School of Chemical
Sciences, Universiti Sains Malaysia, Penang, Malaysia; 3Department of Zoology, Aligarh Muslim
University, Aligarh, Uttar Pradesh, India; 4Department of Pharmacology, IIMS & R, Integral
University, Lucknow, Uttar Pradesh, India; 5Department of Botany, Aligarh Muslim University,
Aligarh, Uttar Pradesh, India
6.1 Introduction
Carbon is one of the most prevalent elements on the planet and found in a
variety of allotropes. The discovery of football-shaped fullerenes by Kroto in
1985, and needle-like carbon nanotubes (CNTs) by Iijima in 1991, sparked a
huge and escalate the interest of using this material in carbon material field.
Geim and Novoselov got the Nobel Prize in Physics in 2010 for their discovery
of graphene. They spun a graphite flake into graphene using a particular type
of tape with a single layer of carbon atoms. In comparison to zero-dimensional
(0D) fullerenes and one-dimensional (1D) carbon nanotubes, two-dimensional
(2D) graphene has unlocked new possibilities for studying several fundamental
quantum relativistic phenomena that were previously thought to be highly
unusual. Two-dimensional graphene has a high inherent carrier mobility, a
huge surface area, outstanding mechanical characteristics, and exceptional
flexibility. It has some drawbacks such as easy agglomeration and poor
dispersion. Graphene quantum dots (GQDs) belong to the graphene family,
recognized as a unique form of zero dimensional luminous nanomaterials
produces a quantum-size effect excitation in 3e20 nm particle’s range [1].
They also have remarkable optoelectronic characteristics, as well as good
biocompatibility and a low-cost production technique, and so have the potential to replace the well-known metal chalcogenides-based quantum dots [2].
Furthermore, the pep bonds below and above the atomic plane provide
graphene excellent thermal and electrical conductivity when compared to
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00002-9
Copyright © 2023 Elsevier Ltd. All rights reserved.
101
102 Graphene Quantum Dots
traditional semiconductor quantum dots, allowing GQDs to have their advantageous properties without the weight of inherent toxicity [3,4]. The
quantum confinement effect, as well as changes in the density and type of the
sp2 sites accessible in GQDs, cause their optical characteristics to be highly
dependent on their size, allowing GQDs’ energy band gaps to be adjusted by
varying their size [5].
Over the last few decades, quantum dots have a distinctive place in
nanomaterial areas, with continuously expanding research, and have achieved
significant progress [6]. GQDs’ remarkable chemical, physical, and biological
characteristics permit them to flourish in a broad spectrum application in the
field of nanomedicine. The exclusive electronic structure of GQDs deliberates
functional qualities onto these nanoparticles like a robust and tunable photoluminescence for its usage in fluorescence bioimaging and biosensing processes. In this chapter, importance and various application of GQD have been
discussed.
6.2 Applications of GQDs in biomedical sciences
6.2.1 Immunological assay based on GQDs
Immunosensors are quick and easy analytical techniques for determining
numerous clinical illnesses and different biochemical compositions that rely
on traditional method of interaction of antibody-antigen (Ab-Ag) and provide
a promising clinical diagnostic as their precise and delicate characteristics.
Generally, immunosensors work by detecting the antigen’s complexity with a
definite antibody pairing, one of which might be mounted on a solid surface
and Ab-Ag complex formation change the signal. Enzymatic processes involve
fixing enzyme-labeled antigens and can therefore be used to create very sensitive immunosensors [7]. Because of graphene’s advantageous structural and
compositional synergy, GQDs are suitable materials for constructing different
immunosensing systems. According to the kind of transduction, immunosensors can be categorized as amperometric immunosensors, electrochemical
immunosensors [8], piezoelectric immunosensors [9], thermometric immunosensors, or magnetic immunosensors.
6.2.1.1 Electrochemical immunosensors
Electrochemical immunosensors have sparked a lot of interest in the research
community as they have the benefits of label-free and interaction of antigenantibody at the detection surface of device, permitting change in potential
that reveals the presence of a particular protein or peptide that is to be
measured. Yang and colleagues developed a highly sensitive electrochemical
immunosensor stranded on nitrogen-doped graphene quantum dots (N-GQDs)
sustained PtPd nanoparticles (PtPd/N-GQDs) to the detect the carcinoembryonic antigen, showing a wide range ranging from 5 fge50 ng mL 1 [10].
Graphene quantum dots: application in biomedical science Chapter | 6
103
Moreover, GQD-based immunosensors for detecting biomarker in human heart
attacks have piqued the curiosity of researchers. When compared to existing
detection methods, the results of a GQDs fluorescence resonance emergency
transference (FRET) based biosensor for detecting the cardiac Troponin I
(cTnI; Fig. 6.1), exhibited greater specificity, and low limit of detection
(0.192 pg mL 1), with a shorter detection time of 10 min [11].
In other research, for the detection cTnI by means of a GQD-PAMAM
nanohybrid a kind of improved gold (Au) screen printed electrode (Fig. 6.2)
generated similar results [13]. GQD-based electrochemical immunosensors
have become the most popular research topic in recent years, and interest in
this field is not going to diminish.
6.2.1.2 Amperometric immunosensors
Amperometric immunosensors are considerably more widely researched than
other types of immunosensors, owing to their ease of manufacture, contraction,
toughness, and economy [14]. Huang et al. made an amperometric immunosensor for the detection of protein (Fig. 6.3.). The immunosensor has been
developed on chitosan, TiO2-graphene, and the composite of gold nanoparticles (AuNPs) film-modified glassy carbon electrode (GCE). Electrostatic
adsorption allows negative charged AuNPs to get adsorbed on the charged
chitosan/TiO2-graphene nanocomposite film, which may subsequently be
utilized to restrain a-fetoprotein antibodies for a-fetoprotein testing (AFP).
FIGURE 6.1 Showing the immobilization of the cardiac troponin I antibody (anti-cTnI) probe on
the Au/GQD/PAMAM nanohybrid electrode and electrochemical detection of cTnI. SPE: Screen
printed electrode (CE: counter electrode, WE: working electrode, RE: reference electrode) [12].
104 Graphene Quantum Dots
FIGURE 6.2 Showing the schematic mechanism of immunosensing based on specific interaction
of anti-cTnI/afGQDs with graphene [12].
This approach yields a broad detecting range (0.1e300 ng mL 1) for the
model to target AFP [15]. It has been also reported that the study on immunosensors of amperometric nature has similarity to immunosensors of electrochemical nature.
6.2.1.3 Other types of immunosensors
The concepts of optical immunosensors are based on linking immunoassay
with surface plasmon resonance (SPR) technology. An alteration in the
FIGURE 6.3 Schematic showing the preparation of immunosensor [12].
Graphene quantum dots: application in biomedical science Chapter | 6
105
medium’s refractive index may be detected when Ag (tumor marker) interacts
with particular antibodies mounted on the sensor surface. Instead, antibodies
might be fixed to the surface of an optical fiber, allowing some modification in
refractive index, luminescence, or fluorescence associated with a various antigen and their interaction to the antibodies to be detected [16]. The concept of
piezoelectric immunosensors is used by evaluating the variation in oscillation
frequency due to variation in mass and antigens interact with antibodies
mounted on quartz crystal [17e21]. Immunosensors of this category are not
extensively utilized in biomedical applications, owing to their excessive cost,
difficulties in bulk manufacture, and the typical issues related to electromagnetic and mechanical interference.
6.3 GQD-based platforms for drug delivery
It has been well noted that various nanocarriers have been developed to increase drug solubility and its precise targeting. Drug carriers and targeted
cellular imaging are generally combined in multifunctional GQDs, which can
be utilized in the cancer. To comprehend cellular uptake, drug delivery systems
might be observed by utilizing semiconductor quantum dots, and organic
fluorophores, by using intrinsic fluorescence of GQDs. We could be able to
readily track movement inside the cells in full detail deprived of using any
external dyes [22]. In a study, synthetic folic acid (FA)-conjugated GQDs were
used to load doxorubicin (DOX), an anticancer drug. The prepared nanoassembly can differentiate cancerous from healthy cells and proficiently
deliver the drugs to target tissues. HeLa cells immediately import the nanoassembly with the help of receptor-mediated endocytosis, however release and
accumulation of DOX take longer time. The results of in vitro toxicity show
that the nanoassembly DOXeGQDeFA may selectively target the HeLa cells
with minimum toxicity in nontarget cells (Fig. 6.4a and b) [23].
FIGURE 6.4 GQDs TEM image (a). (b) GQD and GQDeFA FTIR spectra [12].
106 Graphene Quantum Dots
A novel hybrid nanosystem is multimodal device for treating and obtaining
image of cancerous cell. The GQDs were employed as a carrier to load
lutetium texaphyrin and gadolinium texaphyrin for biological redox therapy as
well as increased photothermal and photodynamic therapy [24]. Khodadade
et al. has developed a drug delivery system by making 10 nm nitrogen-doped
GQDs (N-GQDs) having 10 graphitic layers also loaded with methotrexate
(MTX). The findings indicate that utilizing GQDs as a nanocarriers had better
antitumor activity as they can extend the cytotoxic effects of loaded drug
(Fig. 6.5a and b) [25]. GQD receptor-mediated endocytosis offered a more
precise and selective cancer diagnosis method.
6.4 Bioimaging applications of GQDs
Bioimaging is considered to be one of the vital field of science where GQDs
have various advantage [26,27]. Because bioimaging is a technique of seeing,
observing, and detecting targeted tissues, cells, as well as molecules in the
body devoid of using any intrusive techniques [28], which provide us broad
knowledge of the biochemical processes within the body of organism [29]. In
1896, Wilhelm Roentgen was able to record the first X-ray image [30], a new
door for future applications of bioimaging has opened to identify and observe
different type of diseases and its symptoms such as bone fractures [31], cancer
[32], Parkinson’s disease [33], tumor imaging [34], and so on. GQDs are
useful and desired material in bioimaging process because of their outstanding
adjustable PL properties, strong photostability, chemical inertness, and great
biocompatibility [35]. Moreover, GQDs have superior characteristics when
compared to other carbon-derived materials [36], such as notable resistance to
photobleaching, which allows for effective bioimaging applications [37].
Inorganic semiconductor QD-based fluorophores and organic dyes have
traditionally been used for cell visualization and bioimaging system. While
there low extinction coefficient and photobleaching and inherent toxicity, are
the primary barriers to their use in bioimaging [38].
FIGURE 6.5 N-GQDs TEM image (c). (d) FTIR spectra of N-GQDs and MTX-GQDs (N-GQDs)
[12].
Graphene quantum dots: application in biomedical science Chapter | 6
107
6.4.1 Fluorescence imaging
GQDs have been designed and introduced as a probe of fluorescent for
observing cellular activities and taking images for cell based and in vivo tumor
cells [39]. Researcher develop a redox-sensitive fluorescence probe which can
record the dynamic fluctuations in the intracellular redox status caused by
reductive or oxidative stress in real time. Chen and coworkers later determined
the GQDs functionalization by sugar monosaccharide in order to regulate
trafficking as well as general dispersal of carbohydrate receptors of cell surface [26]. The same group recently completed a real-time estimation of
intracellular hydrogen sulfide (H2S) level changes inside the living cells [40].
The suggested fluorescence turn-on approach allows the specific evaluation of
H2S as compared to earlier produced GQD-based fluorescent probes. For
in vitro tumor cell imaging, Gao et al. reported GQDs coated with polyethyleneimine (PEI) instead of monitoring cellular dynamics [41]. The produced GQDs emitted red, yellow, or blue light depending on the molecular
weight (MW) of the PEI, permitting in vitro multicolor imaging of U87 tumor
cells [6]. GQDs are water soluble, have high biocompatibility, and are
nontoxic. Ding et al. synthesized a doxorubicin (DOX)-loaded GQD-based
theranostic nanoagent [42]. The blue fluorescence generated by the GQDs
tracked the nanoagent’s internalization, but the DOX fluorescence was
considerably muted by GQDs owing to their near proximity. However, after
internalization, DOX emits a strong green fluorescence, indicating that DOX
was effectively released from the nanoagent, bring about significant chemotherapeutic death noteworthy inhibition of cancerous cell [43].
Campbell et al. demonstrated in vitro multicolor emissive N, S-doped, as
well as B, N-co-doped GQDs mutually for NIR-I and visible imaging [44].
These doped GQDs produced red, green, and blue, light at various excitation
wavelengths. These emissions of multicolor is ascribed to the GQDs of
quantum size as well as their electronic state or surface defect configurations.
Furthermore, the ratiometric identification of healthy (HEK-293 cell) and
malignant cells was possible because to the pH-dependent fluorescence
emission of these GQDs (HeLa and MCF-7 cell). Different research groups
have also shown heteroatom codoped GQDs for cellular imaging, like Fe, N
and P, N codoped GQDs [45e48].
6.5 Toxicity of GQD materials
The major challenges of nanomaterials used is their toxicity. As illustrated in
Fig. 6.6, graphene influences the biological system at the cellular, protein as
well as genetic levels. The toxicity of graphene is based on its uptake in
various particular organs as well as on its physical and chemical interference.
Accumulation of graphene in such organs affects cellular performance [49].
Their impeachment, dispensation, and excretion after entering in a cellular
environment retrieve the information about their cytotoxicity.
108 Graphene Quantum Dots
FIGURE 6.6 Schematic representation of the effective mechanisms by which ROS are linked
with the toxicity of graphene at a cellular level.
6.6 Conclusion
GQDs are recognized as a novel type of nanomaterials at 0D and are gaining
attention in the field of biomedical science due to their unique physiochemical
and biocompatibility properties. They are used for in vitro biosensing as well
as in vivo imaging applications such as magnetic resonance imaging, dualmodel imaging fluorescence imaging, and two photon imaging. Moreover,
GQDs were also helpful directly or indirectly to human life. However,
regarding safety as well as toxicity of these nanomaterials is one of the biggest
challenge arising in using these in the field of biotechnology. Therefore, the
future research must be oriented based on the outcomes arising in this field and
whether or not these will be beneficial to human beings.
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Chapter 7
Graphene quantum dot
application in water
purification
Mohammad Oves1, Mohammad Omaish Ansari2 and Iqbal M. I. Ismail3
1
Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia;
Centre of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia; 3Department of
Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia
2
7.1 Introduction
Providing safe drinking water and enough freshwater resources is becoming
increasingly problematic in many parts of the world’s population [1,26].
Despite several substantial challenges and constraints, using environmentally
friendly nanomaterials with unique features including high precision and
specificity, earth-abundance, reusability, and low-cost fabrication routes [2].
The first carbon nanomaterials used in wastewater treatment and purification
were activated carbon, multiwalled carbon nanotubes, and single-walled carbon nanotubes [3]. Now, more advanced graphene and graphene oxide-based
nanomaterials and graphene quantum dots-based nanomaterials have shown
significant promise for wastewater treatment [4].
The graphene quantum dots (GQDs) have aroused much interest in science
since they are zero-dimensional graphene derivatives. It has excellent optical
and electrical properties because of its edge effects and strong quantum
confinement. The GQDs have considerable benefits over typical semiconductor quantum dots (QDs) in terms of inexpensive, least toxic, high water
solubility, persistent fluorescence, adjustable band-gap, and good biocompatibility, making them a genuine competitor. GQDs have a lot of potential in
various fields, including medical diagnosis, sensors, catalysis, bioimaging,
energy storage, and optoelectronics [5]. Scientists have synthesized GQDs
using various carbon-based resources, including graphite flakes and other
nanotubes, graphene and carbon fiber, and coal and other materials. Different
synthesis techniques like hydrothermal, electrochemical, sonochemical, solvothermal, laser ablation, microwave cutting, etc., have been widely adopted
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00012-1
Copyright © 2023 Elsevier Ltd. All rights reserved.
113
114 Graphene Quantum Dots
in recent years [6]. Microwave treatment is the most advantageous of these
methods because it heats uniformly and quickly, resulting in a terse reaction
time. Therefore, an approach to synthesizing high yields of GQDs from lowcost precursors that are simple, fast, appropriate, and ecofriendly. Wang et al.
synthesized white light-emitting GQDs using a two-step microwave-assisted
hydrothermal approach from corrosive acids within a 14-h reaction time [7].
Toxic and corrosive chemicals (H2SO4, KMnO4) were used in large quantities
in the process, but the yield was good. In the main principle of the photocatalytic operation, electron-hole pair formation is driven by light with a
particular frequency and energy more significant than the band-gap of semiconductors [8]. Recombining or assorting the produced charge carriers can
lead to hydroxyl and superoxide radicals, respectively, in the valence and
conduction bands. The electron has a higher þ0.5e1.5 V reductive potential
than a hydrogen electrode. A typical hydrogen electrode has an oxidative
potential of about þ1.0 to þ3 V, while holes have a much higher value. In the
presence of reactive oxidation species, pollutants can be converted to harmless
byproducts [9].
Electrons reduce oxygen in the conduction band to generate a superoxide
anion, whereas holes oxidize water in the valence band to create hydroxyl
radicals [10]. Many contaminants and biomolecules can be oxidized by either
the superoxide anion or the hydroxyl radical, two oxidation species that are
incredibly reactive themselves. Disinfectants and deodorants can be used to
clean the air and water. Contaminants deteriorate over time as reactive
oxidation species continually attack them. During photocatalysis, most of the
organic water contaminants were oxidized by free radicals or reactive oxidation species (OH,, O2,, and H2O2). Cadmium is the primary component of
conventional QD, and this contributes to cytotoxicity by allowing cadmium
ions to seep out [11]. As a result, scientists create QD derived from cadmium,
such as carbon QD, GQDs, and silicon QD [12e14]. Carbon-coated nanostructures have the advantage of improving charge carrier mobility because the
carbonaceous material acts as an electron sink/acceptor [4]. Carbon framework
codoping through N & S also persuades surface imperfections that intensify
electron delocalization [15]. For example, a metal-free carbon-based photocatalytic system is famous for water treatments. Carbohydrates are primarily
carbon, ranking second in the periodic table only to oxygen. Carbon QD are
getting a lot of attention because they are used as photocatalytic nanomaterials, which use the most abundant carbon.
7.2 The worldwide water crisis
The World Water Development Report (WWDR) from the United Nations
(UN) provides insight into current safe water supply developments and plans
[16]. Water protection, described as a population’s ability to maintain longterm access to sufficient quantities of water of appropriate quality, is already
Graphene quantum dot application in water purification Chapter | 7
115
in jeopardy for many, and the problem is only going to get worse in the coming
decades [16]. In today’s world of 7.7 billion inhabitants, clean water shortage
is a big concern. By 2050, the global population will have risen by 22%e34%
to 9.4e10.2 billion people, burdening the water supply [17]. Uneven population increase in various regions, which is unrelated to local capital, would
exacerbate the burden. The majority of this population boom will occur in
developing countries, first in Africa and then in Asia, where clean water
shortage is already a significant problem.
As the world’s population and economy accelerate, safe, adequate freshwater demand increases. Nevertheless, the global water system has been
seriously strained, particularly in developing countries, due to extreme climate
situations (high temperatures and droughts) and contamination of pure water
supplies. Water demand has increased by 1% annually since 2000, conferring
to the UN World Water Development unit [18]. This could be due to economic
growth, population growth, and changing water use habits. This upward industrial and residential water use trend is predictable to duplicate for the next
2 decades, outpacing agricultural needs [18]. Up to 26 African countries, home
to half of the continent’s population, are expected to experience water stress or
shortage by 2025. Water shortages will affect nearly 3.6 billion people
annually, and that number is expected to rise to 5.7 billion by 2050 [16]. The
UN Water Agency (2018) Most water still goes to agricultural operations,
which is vital if we have enough food to feed the world. Because of this, a
steady source of freshwater is needed to ensure food safety [19]. Currently,
approx. 3.6 billion people, somewhat less than half of the world’s population
(47%), live in places where water shortage occurs at least once a year. According to the percentage, up to 52% of the world’s population, or w4.0
billion inhabitants, face water crisis problems [20]. By 2050, more than half of
the world’s population (57%) will live in areas where water shortage occurs at
least 1 month a year. Many geopolitical variables are challenging to forecast
regarding water production, water supplies, and water quality. The deterioration of water supply and water quality was only briefly explored, maybe even
more challenging to regulate [21].
7.2.1 Source of water pollution and impact on life
The global population is increasing significantly, and parallel the demand for
quality life leads to industrialization and other anthropogenic activities, and
this development generates high pollution and climate changes [22]. The
heavy industrialization and megacities municipality activities release a
tremendous amount of wastewater that directly and indirectly pollute natural
resources because wastewater from the pharmaceutical, pesticide, and fertilizer industries contains a high amount of potentially toxic compounds
responsible for freshwater eutrophication [23]. The mining industries also
release wastewater with a high load of heavy metals, textiles, and tanning
116 Graphene Quantum Dots
industries known for the colored effluent discharge, which is harmful to plant
and animal life because most of the dyes are used in the coloring of fabric and
leather are carcinogenic. Mining and metal purification processing released a
considerable amount of heavy metal-rich wastewater; most heavy metals are
harmful to life. Pharmaceutical industrialization is also booming because drug
abuse increased in middle-income countries due to pandemic coronavirus [24].
Due to the high consumption of drugs, most of them are released through the
excreta into the wastewater. Most antibiotics and medicines belong to a specific group of pollutants because they disrupt the natural function of an
aqueous ecosystem and sustain a long time in the water. Some lifestyle-based
contaminants are emerging in wastewater which quickly skips from the
existing water treatment plants and reaches our tap water, where they cause
severe health problems to human health [25]. Nowadays, most agricultural
lands use high pesticides and artificial fertilizers during irrigation or high rain.
These pollutants leach out and contaminate water bodies like lakes and rivers.
Instead, these pollutants accumulated in crop plant parts and directly reached
the food web. When these organic and inorganic pollutants are in different
cellular environment becomes transformed into various forms, they may be
more toxic or less toxic to the surrounding environment. So, urgent need to
develop a specific wastewater treatment protocol and management to ensure
decontamination of pure water and maintain wastewater discharge quality with
a limited amount of pollutants. Recently, a developed nanomaterials-based
water purification system has been more efficient in treating and purifying
wastewater. This chapter provides extended details about the graphene and
GQD application and photocatalytic activity in wastewater treatment.
7.3 Graphene quantum dot (GQD)
The GQD is the small block of nanocarbon and emerging as a primary
structure of graphene [26]. Carbon chain arranged honeycomb-like construction, and each carbon attached with sp2 bond with zero-dimensional (0D) and
10 nm in lateral dimensions [27]. GQD is a single and less than 10 layers
honeycomb structure is typically known as GQD [28]. The optical properties
of graphene QD are derived from both quantum dots and graphene. Still,
carbon dots (CD) are different from the GQD in lateral dimensions, always
less than 10 nm and CD amorphous in nature, while GQD has crystalline
properties [29]. For the surface, modification GQD contains oxygen as a
functional group, making it more feasible for diverse applications because it
possesses chemical, physical, thermal, and electronic properties [30].
However, the band-gap of graphene is 0, but the band-gap of GQDs is
nonzero and may be altered by altering the size of the dots [31,32]. The
electrical, catalytic, and photoluminescence characteristics of GQD in heteroatoms may be tweaked [33]. Bottom-up or top-down approaches can be
used to make graphene quantum dots. Larger graphene sheets can be
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117
fragmented using the top-down approach to create GQDs, while chemical
precursors may be used to fuse the smaller ones using the bottom-up method.
A distinctive top-down synthesis method includes electrochemical exfoliation,
liquid exfoliation, e-beam lithography, microwave cutting, ultrasonic shearing,
and hydrothermal/solvothermal cutting. Intermolecular interaction, precursor
pyrolysis, and opening fullerene cages are examples of bottom-up techniques.
Bioimaging, photocatalysis, sensors, solar cells, tissue engineering, anticorrosion materials, and pollutant sorption are just a few examples of the many
uses for these materials [34,35]. This chapter discusses the use of GQDderived nanomaterials in water management to assure the accessibility of
clean and sufficient water for agricultural production and other critical industries. The capacity of nanostructures to remove contaminants will be
demonstrated to be significantly impacted by GQDs.
7.3.1 GQDs application
The GQD nanostructures have been widely used to eliminate different contaminants from polluted water [36]. For the past few years, numerous researchers
have investigated the use of doped-green quantum dots and GQD-based nanocomposites in pollutant removal via various methods, including photocatalysis
and photoelectrocatalysis [37,38]. All of the experiments found that adding
GQDs to the nanocomposites significantly improved their capacity to remove
pollution. For removing colors, emerging pollutants, and heavy metals, several
GQD-derived nanocomposites have been employed. Inorganic, organic, and
microbiological impurities may be removed from water via photocatalytic water
treatment [39]. However, significant obstacles are charge carrier recombination,
a small surface area, poorly visible light consumption, and water-based nano
photocatalyst aggregation. Carbon nanomaterials such as graphene and graphene oxide may be combined with a wide range of semiconductors to increase
the overall activity of the semiconductors. This strategy has been used to
advance the semiconductor potential. The GQD seems to be the highly fascinating and valuable of all carbon nanomaterials because of their extraordinary
adsorption and charge separation abilities as well as their ability to absorb visible
light [40].
7.3.2 GQD for organic pollutants degradation
The researcher uses GQD-derived nanostructures in photocatalytic dye
degradation is one of the most intended uses widely [41]. Dye contamination is
a major environmental problem in areas where dye pollution colors freshwater
and poses a toxic threat to aquatic life. Studies have also shown that certain
dyes can cause cancer and mutations in the human body, kidney, and skin
irritation, and a wide range of allergies. The organic day such as rhodamine B
can be degraded photocatalytically by the applied hydrogel immobilized with
118 Graphene Quantum Dots
the GQD. In another study, Methylene blue dye was degraded by the N-doped
GQDs (NGQDs) in visible light.
Similarly, NGQDs combined with bismuth oxyhalides (BiOX, X ¼ Br, Cl)
can be used for the RhB degradation. The RhB can be eliminated from the
wastewater by adding GQD-TiO2 material synthesized by ultrasonic and hydrothermal synthesis methods [41]. Both materials were shown to be more
active against RhB than pure TiO2 in general. Additionally, the hydrothermalprepared binary nanostructures had somewhat more significant activity than
the materials generated by ultrasonic. Many of the flaws caused by ultrasound
might serve as recombination centers for the charge carriers, explaining this
observation. Using the binary nanostructures, this was accomplished in
135 min, which reduced RhB to almost zero in 30 min. The inclusion of GQDs
boosted RhB adsorption via the interactions and changed the band-gap of
TiO2, that enhanced visible light absorption and benefited by charge split-up
[42]. Nonmetal (P, N, S, B) doped GQDs/g-C3N4 nanostructures were
tailored using an ultrasonic exfoliation to the breakdown of RhB in visible
light-assisted reaction. Oddly enough, the activity of g-C3N4 was not enhanced
when combined with pure Gqds, Sqds, or Nqds, with B-Gqds having even less
activity than bulk g-C3N4. Insufficient visible light absorption and poorer
charge separation were blamed for this. However, combining g-C3N4 with PGQDs improved RhB degradation significantly, with a degradation rate 17
times faster than bulk g-C3N4 in just 40 min [44].
Under visible light, RhB showed improved photocatalytic activity over
PGQDs/g-C3N4 similarly. When compared to pristine g-C3N4, the composite
had more significant action due to better visible light consumption and the
development of close p-n heterojunction that facilitated effective charge splitup [44]. Other studies have shown enhanced photocatalytic degradation of MO
and MB and RhB GQDs, which improved optical characteristics and charge
separation efficiency, may be to blame for the increasing inactivity [46].
Recent studies have shown that nonmetal doped and pure GQDs exhibit
photocatalytic capabilities, pointing to possible uses for metal-free photocatalysts in dye degradation [47]. The basic fuchsin (BF) >80% was removed
in 120 min over S-GQDs, considerably >18% higher elimination reported
over only GQDs. That’s because S-GQDs and BF both use more visible light
and have similar energy levels [47]. Another study found that after 100 min of
sunlight exposure, clean GQDs could only remove 45% of the MB. The
photocatalyst remained stable after three cycles despite the poor removal efficiency and showed a slight activity decline [48]. Even if activity levels are
down, the sun’s fantastic stability and capacity to harness its energy may make
up for it. An in-depth examination of the photodegradation findings, in
conjunction with the response of optical analysis, most prevalent reactive
species identification, and the band structure of the catalyst, offers crucial
information for understanding the charge transfer process. As the photocatalyst complexity increases, so does the charge transfer route. Because of
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this, ternary semiconductor nanocomposites degrade more slowly than binary
nanostructures. While in the conduction band of BiVO4, for example, electrons
inhabited the region upon visible light stimulation, parting holes in the adjacent valence region. Subsequently, the electrons were transported to NGQDs,
someplace O2 trapped them to produce superoxide radicals, although the holes
interacted by H2O and generated OH radicals. Dye molecules degraded due to
the presence of both radical species. The RhB breakdown through the BiOBr/
NGQDs and BiOCl/NGQDs has also been proposed to be similar to the dye
sensitization method. By the dye sensitization approaches, in visible light,
RhB molecules could absorb and generate an excited state that might introduce
einto the conduction band of BiOX’s. Conspicuously, even though BiOCl is
not light-sensitive, this methodology was postulated as the sole cause of its
activity [49]. Instead, the conduction band of GQDs/ZnO, in this case, consisted of GQD and ZnO conduction bands with energies of 3 and 4.09 eV in
opposition to vacuum energy, respectively, and valence bands with powers of
6.62 and 7.39 eV in opposition to vacuum energy. The charge transfer route of
the photocatalyst must be thoroughly understood when constructing nanomaterials with extraordinary charge separation efficiency, suitable optical
characteristics, and corresponding band potentials. Improved photodegradation achievement may be the result of this development. Instead of dye
pollution, micropollutants (MPs), and emerging pollutants (EPs) are the main
classes of organic pollutants that have gotten considerable interest due to the
absence of regulatory oversight and the unknown fate of these new and
emerging pollutants in the environment (MPs). There are both recently
discovered and long-established pollutants in this category and the chemicals’
metabolites.
The personal care and detergents EPs, hormonal, antiviral, fire retardant,
and narcotics are also included in the list of possible EPs. Drug-resistant
bacteria have emerged due to antibiotics in the environment, posing a significant danger to health care. These microorganisms have been related to
cancer- and mutation-causing properties. GQD-derived nanostructures have
been utilized to study the photocatalytic breakdown of EPs in water. The solar
light promoted the research of 4-nitrophenol (4-NP), ciprofloxacin (CIP), and
diethyl phthalate (DEP) because the researcher found that all emerging pollutants degraded simultaneously. At the same time, hydrogen was generated
with the treatment of GQDs/MneN TiO2/g-C3N4 using a cocatalyzer of Pt
[50]. A fascinating side note to this study is the possibility of solving pollution
prevention and fuel creation simultaneously. g-C3N4 (S/GQDs/TCN-0.4) was
shown to be the most effective photocatalyst for pollution elimination and H2
evolution. To our surprise, the pollutant solution exhibited a greater rate of H2
generation than clean water, showing that the two phases are interconnected.
According to DFT calculations and GCeMS analyses, electrons were implicated in the breakdown of 4-NP, but not CIP or DEP. As a result, decreased H2
evolution in the 4-NP solution was observed [50]. According to the researchers
120 Graphene Quantum Dots
working with Ag/NGQDs/g-C3N4 ternary nanostructures, the light source
affected the antibiotic tetracycline (TC) breakdown. For the same circumstances, ternary nanomaterials eliminated five times the TC amount as g-C3N4
did. The TC removal productivities were 92.8% under full-spectrum illumination and 32.3% under NIR illumination.
NGQDs and Ag cannot aggregate due to g-C3N4 acting as a scaffold for
their anchoring, which prevents them from doing so. Another work combined
hydrothermal and physical mixing to produce an NGQD and nano cubic TiO2
sunlight-responsive photocatalyst. Bisphenol A (BPA) was degraded entirely
in 30 min using a photocatalyst comprising 0.5 weight percent NGQDs. When
exposed to visible light, BPA degradation was also detected in ternary nanocomposites composed of BiOCl/BiVO4/NGQDs. Charge separation in the
ternary nanostructure was enhanced above that in the binary and sole semiconductors due to the many heterojunctions generated therein. When it comes
to radical scavenging, both holes and hydroxyl radicals were found to be active
in the GQDs/MneNeTiO2- g-C3N4 investigations. This information, combined with the band structure and optical response of the photocatalyst, led to
the hypothesis that, when exposed to solar light, electrons in both g-C3N4 and
MneNeTiO2 were advanced to their conduction bands while holes remained
in their valence bands. The GQDs could also absorb and emit illumination
with different wavelengths, such as light with an 800-nm absorption wavelength created by MnNeTiO2 and light with a 500-nm emission wavelength
generated by g-C3N4. g-C3N4 will then move its electrons to the MneNeTiO2
conduction band, although the holes will go in reverse. The Pt nanoparticles
deposited on the surface that caught the electrons reduced 4-NP and generated
H2. Hydroxide ions formed hydrogen peroxide in the holes of g-C3N4 and
MneNeTiO2, whereas hydrogen peroxide itself was oxidized in the pits of
MneNeTiO2. Degradation byproducts might potentially develop in the perforations, targeting organic contaminants directly [50]. The internal Z-scheme
mechanism was proposed to explain the charge transfer pathway in BiOCl/
BiVO4/NGQDs. The conduction bands of NGQDs and BiVO4 were filled with
electrons when the visible light irradiated numerous heterojunctions between
the three semiconductors. However, electrons from BiOCl’s valence band
could transfer to BiVO4 and recombine with holes from BiVO4’s valence band,
while electrons from BiVO4 can transferal to the NGQDs’ valence band. When
using the Z-scheme approach to create these radicals, the electrons and holes
on NGQDs were preserved in their highly reductive conduction band and their
highly oxidative state on BiOCl. Due to the invasiveness of these radical
species, the emerging pollutant degraded. Materials that can remediate a wide
range of contaminants are required because wastewater is complex [51]. The
development of materials with multiple functions may also compensate for the
expense of synthesis/precursors.
Many materials may be used to remove dye molecules and other impurities
throughout the development process. RhB, TC, and BPA were all degraded
Graphene quantum dot application in water purification Chapter | 7
121
photocatalytically using NS-GQDs, and (BiO)2CO3 (NS-GQDs/(BiO)2CO3),
with elimination efficiencies that were greater than those of pure (BiO)2CO3.
The fact that the photocatalyst was activated by both light and shade was an
important finding. The biomimetic action of N,S-GQDs, which might break
down H2O2 into hydroxyl radicals in a way similar to peroxidase, was
attributed to dark activity [45,52]. To show that black TiO2/N,S-GQDs are
adaptable, many dyes were degraded using sunlight aided sunlight and EPs,
such as phenol, aniline, nitrobenzene, and dimethyl ophthalate these dyes
included RhB, MO, Flu, MB, and Bpb. Researchers also looked at using the
nanocomposite in sewage treatment. The photocatalyst exhibited excellent
durability, retaining 95% of its original effectiveness after 30 cycles of light
exposure in addition to a more significant removal percentage (k > 0.68 min1) for all contaminants. Photocatalysts show great promise for wastewater
treatment because they can remove nearly 85% of the total organic compounds
(TOC) in just 30 min [53]. Because dyes (colored substances) and EPs coexist
in natural water, the degradation experiments were conducted separately,
despite the impressive results of both pollutants’ degradation. As a result,
studying the degradation process in solutions containing multiple pollutants
will provide a more accurate picture of how each pollutant interacts with the
others. However, the future seems bright in utilizing GQD-based structures to
eliminate organic contaminants from water photocatalytic. Table 7.1 summarizes various studies on photocatalytic degradation of organic pollutants by
applying GQD-based nanocomposites.
7.3.3 Microbial and heavy metal load reduction by graphene
quantum dot
Disinfection is critical in water treatment since it ensures that the water is free
of microorganisms. Bacteria, fungi, algae, and viruses might enter the drinking
water supply system if it is not disinfected, resulting in waterborne diseases
and epidemics [54]. Drug-resistant bacteria have emerged due to the widespread use and disposal of antibiotics in the environment, creating a severe
threat to public health. To prevent the spread of such superbugs in water needs
to specific disinfectants [55,56]. As an alternative or supplement to chlorine
disinfection, photocatalytic water disinfection presents the possibility of
creating potentially harmful disinfection byproducts. Photocatalytic disinfection can be used in addition to chlorination to destroy bacteria and disinfection
byproducts [57]. According to recent research, the antibacterial properties of
the ternary nanostructure made of TiO2/Sb2S3/GQDs against Escherichia coli
and staphylococcus aureus using a solvothermal synthesizing technique in a
visible light environment [58,59]. When GQDs were combined with TiO2 and
Sb2S3, the inhibitory concentration for S. aureus and E. coli was significantly
reduced (GQDs/TiO2 and GQDs/Sb2S3) [59]. Charge separation efficiency and
greater use of visible light were cited as reasons for the rise. In addition, the
GQDs or GQDsnanocomposite
Synthesis rout
Precursor
material
Rate of dye
degradation
GQDs
Hydrothermal
Graphene
oxide
GQDs
Hydrothermal
GQDs
Dye degradation
References
In sunlight driven
photocatalysts degrade
upto 45% of MB
Photodegradation of MB dye
[74]
Graphite
powder
827.5 mg g1 highest
adsorption capacity
Adsorption of MB
[70]
Microwaveassisted
hydrothermal
route
Citric acid
Upto 150 mg
Adsorption of pesticide
compound oxamyl
[75]
S-doped GQDs
Hydrothermal
1,3,6Trinitropyrene
and Na2S
81% after 2 h
Photodegradation of fuschin dye
[47]
GQDs/AgVO3
Hydrothermal
1,3,6Trinitropyrene
90% after 120 min
Degradation of Ibuprofen
[76]
QDs@ZnO-NRs and
GQDs@ZnO-NFs
Pyrolysis
Citric acid
80% after 180 min
Photodegradation of BB dye A
[77,78]
NGQDs/BiOCl and
Hydrothermal
Citric acid
Nearly 100% in
75 min
Photodegradation of RhB
[79]
NGQDs/BiOBr
Hydrothermal
Citric acid
Nearly 100% in
60 min
Photodegradation of RhB
[80]
122 Graphene Quantum Dots
TABLE 7.1 GQDs-nanocomposites synthesis methods and use in dye and pollutant degradation.
Citric acid
365 nm for 92.8%;
420 nm for 90.1%;
and760 nm for 31.3%;
removal under
irradiation light
Tetracycline (TC)
[81]
NGQDs-BiVO4
Hydrothermal
Citric acid
90% after 200 min
irradiation
Degradation of MB
[82]
Bi2S3-GQDs/TiO2
Pyrolysis
Citric acid
Under irradiation
52%e92%
Removal of Cr (VI) and methyl
orange
[83]
S,N:GQDs-(BiO)2CO3
Hydrothermal
Citric acid
GQDs create ROS
production and
degrade pollutant in
light
Degradation of Rhb, tetracycline
and BPA
[84]
GQDs-mpg-C3N4
Hydrothermal
Pyrene
97% removed
Degradation of RhB and
tetracycline
[85]
GQDs þ SDS-(dodecyl
sulfates)-LDHs (layered
double hydroxides)
Hydrothermal
Citric acid
Adsorption capacity of
80%
Adsorption of 2,4,
6- trichlorophenol
[68]
GQDs/TiO2
Pyrolysis
Citric acid
100% of the RhB
within 30 min
Photodegradation of Rhb
[43]
Ti3þ-TiO2/GQDs NSs
Hydrothermal
Citric acid
Enhanced
photocatalytic
efficiency
RhB and MB dye
[86]
ZnO-GQD
Hydrothermal
1,3,6Trinitropyrene
95% of both MB and
CZ within 70 min
MB and a colourless pollutant;
carbendazim (CZ)
[87]
GQDs/MneNeTiO2/g- C3N4
Pyrolysis
Citric acid
The photodegradation
rate of p- nitrophenol
was the highest; the H2
evolution rates in
solutions system
f p- nitrophenol, diethyl
phthalate and ciprofloxacin and
fabrication of H2
[88]
123
Pyrolysis
Graphene quantum dot application in water purification Chapter | 7
Ag/N-GQDs/g-C3N4
124 Graphene Quantum Dots
ternary nanostructure of TiO2/Sb2S3/GQDs recorded the minor minimum
inhibitory concentration for both bacterial strains when equated to the standard
TiO2, GQDs/TiO2, and GQDs/Sb2S3. E. coli had a minimum inhibitory concentration of 0.03, whereas S. aureus had a minimum inhibitory concentration
of 0.1. A possible explanation is that GQD and Sb2S3 on TiO2 operate in
concert to promote split-up charge and the generation of free radicals or
reactive species that inhibit bacterial development. As the irradiation duration
increased, the amount of bacterial growth decreased until there was almost no
growth left after 24 h [59]. The antibacterial effectiveness of GQDs in combination with methylene blue (MB) was studied by Kholikov et al. during light
irradiation [60]. The greatest singlet oxygen production was when the
GQDs:MB ratio was 1:1 when combined GQDs and MB. The inclusion of
GQDs did not affect cell viability in low-light conditions, suggesting that MBGQDs might be used in photodynamic therapy. Both M. luteus and E. coli
were almost disinfected in about 5 min when exposed to 660 nm light, as well.
However, the disinfection time for M. luteus was substantially less than that
for E. coli. In contrast to the M. luteus as a Gram-positive, E. coli as Gramnegative strains have an additional complicated and intense cell wall, which
offers more defense. An antibacterial method developed from a combination of
GQDs could disrupt bacteria’s cell membrane, allowing up passageways for
MB to enter and create free radicals of oxygen that transformed protein synthesis and Gene [60]. The variation in bacterial cells deactivation rates between Gramþve and Grameve bacteria appear to be more complex than just
changes in the cell envelope structure. The photocatalyst’s surface charge
concerning the bacterial cell may be crucial in the interactions between bacteria and photocatalyst. In comparison to Gramþve bacteria, the repulsive
interface with Grameve bacteria on a negatively charged photocatalyst surface
might lead to reduced inactivation kinetics. Heavy metal pollution is a significant obstacle to safe drinking water and fertile agricultural land.
Heavy metals, including Cr(VI), Pb(II), and Hg(II), among several others,
have been found in variable amounts in the milieu; most of the metals are
frequently the result of industrial activity [61]. The heavy metals have been
identified as carcinogenic, mutagenic, embryotoxic, teratogenic, and reproductive system toxicity; some metals are linked to heart disease, liver
impairment, and various health complications [61e63]. Furthermore, heavy
metals can be delivered to crops by contaminated water irrigation and then
reach people by consuming such goods. Because of the process’s nonselective
nature and capacity to convert dangerous metal ions to harmless metal species,
catalytic heavy metal removal is a viable approach. Under visible light illumination, Cr(VI) photocatalytic reduction was investigated over Bi2S3/GQDs/
TiO2 nano assembly as nanowires with metal oxide. Intriguingly, there was a
more significant drop in Cr(VI) in the presence of MO (97%) than in the
absence of MO (92%) percent.
Graphene quantum dot application in water purification Chapter | 7
125
Meanwhile, when Cr(VI) was present, MO removal was about 52%,
compared to 43% when Cr(VI) was absent (VI). According to this idea, photoinduced electrons were efficiently captured by Cr(VI), which was then reduced
to Cr(III), freeing up the holes for oxidation processes that resulted in MO’s
MO degradation. An electron collector, such as Cr, made it much easier just to
get rid of MO (VI). GQDs to the nanocomposite improved the visible light
sensitivity and charge split-up potential [64]. The photocatalytic (PC), electrocatalytic (EC), and photoelectrocatalytic (PEC) ability of Fe2O3-GQDs/
NFeTiO2 nanocomposite for Cr(VI) reducing and EDTA disintegration in
aqueous solutions. PEC activity was substantially higher in the ternary
nanostructure than in the binary. Furthermore, the PEC process surpassed the
EC and PC procedures in terms of efficiency. PEC activity had an apparent rate
constant of 7.67 times greater than PC and EC activity. Cr(VI) and EDTA were
eliminated instantaneously during the PEC experiment, with Cr(IV) reduction
enhanced from 40% to 91% in 80 min. FeeO3-GQDs/NFeTiO2 are adequate
for the PEC decomposition of pollutants with heavy metals in wastewater. The
GQDs work primarily as electron accelerators between Fe2O3 and NFeTiO2
[65]. In industrial wastewater containing these pollutant species, the power of
collaboration offers an attractive possibility to treat organic pollutants and
heavy metals simultaneously time.
7.3.4 Membrane filter based on graphene quantum dot
The adsorption phenomenon of materials can remove a wide range of organicinorganic and microbiological contaminants [66]. Activated charcoal has long
been used in industry as an adsorbent during water treatment. Different adsorbents based on graphene-related materials demonstrating outstanding absorption capabilities have also been investigated. GQD-based adsorbents in
recent times appeared as possible adsorbents among these graphene materials
due to their high surface area, cost-effectiveness, biocompatibility, and
abundance of functional groups attachment sites [67]. Because GQDs are
greatly soluble in aqueous solutions, more research into their ability to adsorb
contaminants in the aqueous environment is crucial before using them. Yao
and coworkers exploited the coinciding intercalation of citrate and sodium
dodecyl sulfate (SDS) in the number of layers of the layer with double hydroxides to create GQDs in two-dimensional (2D) hydrophobic space. The
adsorbent (GQDs þ SDS)-LDH was used to extract 2,4,6-trichlorophenol
(2,4,6-TCP) [68]. To put this in perspective, the nanocomposite removed
80% of the 2,4,5-TCP, which was 65% greater than elimination using GQDsLDHs and 40% higher than removal using SDS-LDH. Pseudosecond-order
dynamics were employed to model adsorption. The results matched the
Langmuir model’s equilibrium adsorption capacity of 119.00 mg g1. The
intercalation technique addresses both GQD aggregation and water solubility
[67]. Furthermore, the best extraction results were obtained with 150 mg of
126 Graphene Quantum Dots
nano sorbent. The extraction of PAHs from real water samples spiked with
known analyte quantities using GQDs/eggshell nano sorbent was investigated
further to ensure the extraction procedure’s efficiency. The procedure’s
extraction efficiency ranged from 92.4% to 112.84%. This demonstrated that
the adsorbent might be employed in the solid-phase extraction of PAHs.
Because of their adsorption properties, GQDs and eggshells functioned well
together, resulting in high extraction yields [69]. In previous research, GQDs
have been used as adsorbents for MB, RhB, and oxamyl, their water solubility
makes it difficult to separate and recover the GQD in its pristine condition
[70]. Centrifugation was often employed to remove the sorbent from the
treated solution. Pure GQD, on the other hand, does not provide an appealing
alternative as an adsorbent when compared to GQD-based nanocomposites
because it is difficult to recover and recycle.
Recent research on modified membranes of GQD has revealed that these
nanocarbons play an essential role in the performance of the membrane.
Interfacial polymerization was used to make polyethyleneimine augmented
with GQDs in the presence of trimethyl chloride linked to polyacrylonitrile
ultrafiltration backings. PEI chains were covalently bonded to GQDs with an
average diameter of 2.19 nm and a thickness of one to three layers [71]. When
tested under ideal conditions, the membrane of mixed matrix possesses a
neutral surface, strong hydrophilicity, high water flow, and suitable antifouling
(0.05 wt% GQDs). In addition, the membrane rejected salt (MgCl2) at a rate of
96.80.4%, showing that it can desalinate. In addition, the membrane could be
used in wastewater treatment [72]. TMC and GQDs were polymerized in situ
in the pores of ultrafiltration (UF) membranes, then thermally processed to
form nanofiltration membranes in other research locations. The membrane
chemical stability, porosity, and water flow were all influenced by the addition
of GQDs [73]. Compared to the nanofilter membrane, Na2SO4 demonstrated
improved water flow and rejection properties toward OGII, MBII, AB, and
CR.
7.4 Conclusion
Threats to water and food security come from organic, inorganic, and
microbiological species polluting the water. Water pollution prevention may be
made more accessible with graphene quantum dot-derived nanostructures.
Organic pollutants like dyes and emerging contaminants may be removed
catalytically using these materials, and they can be adsorbed, filtered, and
disinfected with them. Nanocomposites modified by the addition of GQDs
have increased removal efficiency for various contaminants. GQD incorporation levels must be carefully monitored and optimized to positively impact
the pollutant-removal efficiency of multiple nanocomposites. Yet more effort
must be made to ensure the design and use of GQD-based nanostructures for
large-scale applications. The numerous functional groups, nontoxicity, and
Graphene quantum dot application in water purification Chapter | 7
127
biodegradability of GQD-derived nanostructures make them promising as
pollution remediation agents. To achieve uniform GQDs in terms of chemical
stability, size, and surface functions, it is necessary to establish optimal synthesis conditions and develop synthesis methods that enable the appropriate
scattering of the GQDs inside the nanocomposite matrix. As a result, there
may be more uniformity in the claimed water pollution abatement effectiveness of diverse GQD-derived nanostructures.
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Chapter 8
Graphene-based organicinorganic hybrid quantum dots
for organic pollutants
treatment
Asif Saud1, 3, Mohammad Oves2, Mohammad Shahadat1,
Mohd Arshad4, Rohana Adnan1 and Mohammad Amir Qureshi5
1
School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; 2Centre of
Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia;
3
Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India;
4
Department of Physics, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 5Department
of Chemistry, Faculty of Natural Science, Jamia Millia Islamia, New Delhi, India
8.1 Introduction
Water pollution caused by pollutants, including dyes, polyaromatic hydrocarbons, hormones, major organic waste, and pesticides. Most of the pollutants are
highly nonbiodegradable having toxic nature that can potentially be transformed into carcinogenic, teratogenic, and become hazardous, and also can
increase the risk of chronic diseases [1e4]. Thus, releasing pollutants containing water into natural water resources is severely affected the life of aquatic
animals. On the other hand, the consumption of contaminated water and aquatic
creatures by human beings has become another serious cause of diseases and
ecosystem disturbance. Therefore, attention has been paid to treating wastewater containing organic and inorganic waste before releasing it into water
systems. Nanotechnology’s advent and application in wastewater treatment has
been a benefit, and many metal nanoparticles (NPs) have been employed to
decolorize harmful effluent dyes [5,6]. Alternatively, semiconducting metal
oxides have a large enough bandgap to accelerate photochemical breakdown,
the majority of them are carcinogenic [7]. Nano-metal oxides are widely known
for their inability to dissolve in water or disperse properly in any solvent. As a
result, after multiple photocatalytic cycles, a considerable number of these
metal nanoparticles are called exhausted leftovers. Although these metal NPs
are efficient photocatalysts, their fatal effect on living beings should not be
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00005-4
Copyright © 2023 Elsevier Ltd. All rights reserved.
133
134 Graphene Quantum Dots
overlooked [8]. These metal nanoparticles are either created with a lot of heat,
or a more precise method that deposit chemical with the help of vapor deposition technique, or their starting materials are quite expensive [9]. GQDs as a
supplement emerges as low-cost, easily manufactured materials that reduce
organic pollutants effectively. Some organic pollutants namely methyl blue,
bisphenol-A methyl orange, parachlorophenol, tetracycline, 4-nitrophenol, triclocarban [10], and carbendazim (CZ) [11] have been degraded using GQDs.
GQDs are a new type of nanocarbon that has appeared in the field of
nanomaterials. GQDs are graphene fragments that are smaller in size. A GQD
is defined as a zero dimension, sp2 bonded carbon atom array, well-arranged
hexagonal structure with sideward dimensions less than 10 nm, similar to
graphene [12e14]. GQDs exhibit diverse, unique physical, chemical, and
electrical properties. Chemical Stability and heat resistance, a great specific
surface area, solubility, minimal cell damage, instinctive functional groups that
allow for facile modifications are just a few of the advantages. Moreover,
GQD’s bandgap can be altered by varying the dimension of the dots, among
other variables as it has a nonzero band gap [13,15e18]. GQDs have been
prepared by using different types of materials such as graphite flakes [19],
carbon nanotubes [20], graphene [21], carbon fiber [22], coal [23], and others
[24,25] have all been employed to synthesize GQDs to far [26].
A bottom-up and a top-down technique are commonly used to create graphene quantum dots [27]. GQDs are created by fragmenting bulk graphene
material and respectively. Lithographic patterning techniques, electrochemical
exfoliation, ultrasonic shearing, microwave-influenced cutting, Liquid exfoliation, hydrothermal cutting are all commonly used top-down synthesis techniques [16,28,29]. Intermolecular coupling, revere-micelle route, precursor
pyrolysis, sol-gel synthesis, cage opening of fullerenes, colloidal precipitation
are examples of bottom-up methods [28,29]. GQDs show various applications
due to their exceptional features including photocatalysis, sensors, solar cells,
adsorption of pollutants, lithium-ion batteries, and organic synthesis [30]. An
outline of the top-down and bottom-up approaches is shown in Fig. 8.1 [31].
The present chapter consists of the synthesis and application of GQD-derived
nanostructures for dye and organic waste treatment in wastewater.
8.2 Synthesis of quantum dots (GQDs)
8.2.1 Synthesis of (GQDs) using pyrocatechol
The GQD were prepared by using precure; pyrocatechol, or 1,2dihydroxybenzene, having a molecular formula C₆H₄(OH)₂ of the three benzenediol isomers (it is the ortho isomer). For the preparation of GQDs, the
suspension of pyrocatechol was heated in the temperature range 100e105 C
for 45 min. Heat treatment turned the ash-gray powder into a pale dark syrupy
liquid. The molten liquid was held for an hour at room temperature to the
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
135
FIGURE 8.1 An outline of the top-down and bottom-up approaches.
decreased temperature of up to 60 C to continue heating. Before moving on to
the next step, the syrupy liquid had to be brought to room temperature. To
modify and get multiple independent pH values of prepared GQD solution,
prepared a 0.5 M standard sodium hydroxide (NaOH) solution was prepared
and introduce dropwise over the molten brown color solution [32].
8.2.2 Graphene quantum dot using citric acid coated with iron
codoped TiO2
The synthesis of GQDs decorated with iron codoped TiO2 was carried out
using citric acid; 2 g citric acid (as a precursor) was pyrolyzed on a mantle
heating at 200 C for 10 min in a typical synthetic manner. Dong et al.
established [33], that the appearance of the solution was pale yellow and
product formation was confirmed by observing the color change which appears
to be orange, indicating desired GQDs synthesis. Under vigorous stirring, the
liquid state GQDs were neutralized by CH3CH2OH [34]. The prepared GQD
solution was modified using a modified sol-gel technique. For the preparation
of GQD-Fe-TiO2 composite photocatalysts, the solution of titanium tetraisopropoxide (37 mL) was mixed with 60 mL pure ethanol. Moreover, solution 2 was prepared by mixing a fixed quantity of GQD solution, Fe (iron)
precursor, acetic acid (15 mL), DI (deionized) water, and ethanol (20 mL).
Then, solution 2 was added to solution 1 with vigorous stirring until gel formation at room temperature (25 2 C). Before being dried in an oven, the gel
was aged for 24 h between 25 and 28 C. The obtained material was then
calcined at 300 C [35,36]. The dried material was kept in a desiccator for
future use.
136 Graphene Quantum Dots
8.2.3 Preparation of graphene quantum dots (GQDs) using spent tea
Pyrolysis accompanied by microwave thermochemical cutting produces lowordered array of carbon from spent tea [26]. Waste tea is rinsed with
distilled water (DW), dried for 12e13 h at 80.5 C, and then compressed into a
fine powder (more than 89 mm), by the controlled rate of heating 10 C min1,
the powder then pyrolyzed at 500 C in a passive environment. The pyrolysis
reaction was carried out for 3 h, yielding a rich carbon biochar precursor. After
the pyrolysis reaction of precursor, the prepared solution gets mixed with
0.1 M HCl after rinsing with DI water and the solution gets exposed to boiling
for the reduction of contaminants. The sample was rinsed again with deionized
water before being dried in a 60 C to yield the final product, following that,
around 20 mg of the C-rich sample as prepared was kept in a reactor containing 11 mL DI water, for oxidative cutting of carbon spheres around
1e2.5 mL of nitric acid was supplemented for lowering the pH. The reactor
was then microwave-assisted for 15e180 min under reflux at a power of
100e900 W, to discrete the bigger unreacted particles, the resultant browncolored solution was thinned with 100 mL DI water and filtered through
0.1 um PVDF filtering films after the preadjusted duration was completed. The
pale-yellow appearance confirmed the formation of GQDs in the filtrate. The
acidic character of the solution necessitated the purification of the synthesized
GQDs. To make the pH of GQDs solution (w7), NaOH solution was used and
the neutral solution get dialyzed for 24 h, then freeze-dried to give dry GQDs
in order to achieve the pure GQDs solution.
8.2.4 Synthesis of graphene quantum dots by using ground coffee
Using a hydrothermal method, GQDs were prepared by using spent coffee
grounds of coffee beans [37]. For the GQDs preparation, the powdered form of
beans were obtained by crushing them in the grinder and then converted to hot
coffee using a coffee machine. Used coffee grinds were cleaned and dried in an
oven at 80 C. After that 0.1 g coffee was ground, and 1 mL H6N2O was dissolved in 10 mL water in an ultrasonic bath for 30 min in a traditional process.
After that, the solution was transferred to a stainless autoclave lined with
Teflon. The sealed autoclave was heated to 150e200 C in an electric oven and
then let to cool for 6e10 h. The water-soluble GQD-containing product was
filtered through a micro dimension membrane to remove insoluble carbon
product and dialyzed for 2 days to remove unfused small molecules after
cooling to ambient temperature. GQDs were purified and dried at 80 C.
8.2.5 Synthesis of rice husk derived GQDs
The fixed content of rice husks (5.0 g) was washed and cleaned in DI water
before being ground to powders [38]. After that, the powders were processed
for 2 h in a tube furnace at 700 C in a N2 environment. Then, 1.95 g of rise
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
137
husk ash was obtained, which contains both carbon and silica. The rice husk
ash was then treated with a high concentrated NaOH solution for 2 hat 900 C
in a protected environment. Rice husk ash was transformed to rice husk carbon
and sodium silicate throughout this process. The residual dark black sample,
after being properly washed with DI water and vacuum filtered, it was dried at
80 C for 12 h. Moving on, a 50 mg rice husk carbon sample was combined
with 10 mL conc. H2SO4 and 3 mL DI water, for the next 5 h the dark colored
dispersion was ultrasonically cleaned. After that, nitric acid (69%e70%) was
gradually added (measured volume 20 mL), and the dispersed mixture was
filtered by applying vacuum through a filtrate and thoroughly cleaned after
another 10 h of ultrasonication. To make the pH (w8) 1 M aqueous NaOH
solution along with prepared black colored sample and 30 mL of DI water
were mixed. The prepared mixture was then ready to move into Teflon-lined
autoclave and the temperature should be maintained at (200 C) for about
10 h. The resultant dispersion was filtered using the microporous membrane
after cooling to room temperature, yielding the desired GQD in the filtrate.
8.2.6 Synthesis of lignin-based graphene quantum dots [39]
GQDs have also been prepared using lignin by using 2 g of o-aminobenzenesulfonic acid and to achieve a consistent solution the temperature was
raised to 80 C by mixing it with 38 g of DI water. Then, while stirring at 80 C
for 20 min, 2 g of alkali lignin was manually added to the mixture. The
dispersion was vacuum filtered through the filter membrane as soon as
possible. After air cooling to ambient temperature, the filtrate was vacuum
filtered to form a new filtrate with lignin nanoparticles. To neutralize the residual o-aminobenzenesulfonic acid, sodium hydroxide (0.5 g) was ultrasonically added to the obtained filtrate residue for next the 2 h. The mixture was
placed in a Teflon-lined autoclaved for 12 h at 200 C. The generated graphene
quantum dots were put into the process of dialyzation with the help of DI
water for continuous 72 h, before this the filtering process was performed with
the appropriate pore size filtrate and lowering the temperature to 25e27 C.
GQDs based on lignin were made in solid form by lowering the temperature to
the extent of freezing (16 C) and this step requires about 12 h, proceeding
with the step the obtained samples were then freeze-drying by keeping the
temperature at 50 C and pressure about 20 Pa.
8.2.7 Synthesis of N, S codoped commercial TiO2/GQDs [40]
In this method, carbon assembly and thiourea were dissolved in 8 mL dimethylformamide at varying molar ratios and agitated to make a clear solution
[41]. The solution was then added to and agitated to obtain a homogeneous
suspension of 100 mg P25 (20% rutile and 80% anatase). The suspension was
then placed in a stainless-steel autoclave with Teflon lining for additional
138 Graphene Quantum Dots
processing. At 180 C, the solvothermal treatment lasted for 8 h. After allowing
the products to cool to ambient temperature, they were separated using ethanol
and centrifugation at 9000 rpm for 15 min.
8.2.8 Development of CdS/GQDs using g-C3N4 nanosheet
To prepare CdS/GQDs nanocomposite, fluctuating quantity of g-C3N4 (GCN)
nanosheet and an established/measured quantity (133 mg) of Cd(Ac)2H2O
were mixed with known volume (50 mL) of Dimethyl Sulfoxide solvent under
the continuous dynamic condition [10]. The generated suspension was then put
into the Teflon-lined stainless-steel autoclave and kept on heating for 12 h at a
constant temperature of about 180 C. The obtained sample was then treated
several times with water and CH3CH2OH solutions to make it contaminantfree, after cleaning up the complete sample it gets dried with the help of
vacuum ovum, by making changes in the weight proportion of CdS/GQD,
photocatalyst samples with varying molar percentages can be created.
8.2.9 Synthesis of metal free N dopped carbon quantum dots
The nitrogen doped CQDs were obtained by putting a 25% ammonia hydroxide with a 10% glucose water solution. Aqueous glucose/NH4OH (5:1)
solutions were exposed to high a temperature environment (heating) for 60 s
by keeping the temperature constant at 100 C [42]. The reaction was
microwave-assisted with the two variable values of power (200 and 100 W).
The obtained solution was colourless and transparent in nature, but after the
MW heating was performed the color of the solution will appear as light brown
indicating the formation of NGQDs. The prepared solution was in an aqueous
form, to get the desired QD (N-GQD) in the dry form the solution gets cooled
under the controlled temperature to make it around 27 C, proceeding with the
dialyzation step (300 Da) which was performed for the next 120 h, at last, the
sample was obtained by filtering it through the manual filtration process
keeping the pore size in the range of (450e10 nm).
8.2.10 Synthesis of GQDs using graphene oxide (GO)
A straightforward method was employed used to prepare GQDs by using the
modified Hummers’ method. To obtain a brown-colored graphene oxide (GO)
solution, GO was dispersed in DI water and then sonicated [43]. The assorted
solution was then preserved at 60 C for 24 h after H5IO6 was introduced to the
GO dispersion. After centrifugation the precipitate out, it was rinsed with
deionized water until to get a clear solution. After that, sodium polystyrene
sulfonate was used to sonicate the graphene oxide nanosheet solution for 2 h.
Finally, the temperature was raised to 50 C for the agitating purpose with the
addition of L-ascorbic acid to the prepared solution. As soon as the appearance
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
139
of the solution will change to dark black after the reaction has been completed,
suggesting that the Graphene oxide nanosheets had been efficiently reduced
into GQDs of diverse sizes (5e15 nm) (Table 8.1).
8.3 Application for the removal of organic pollutants
GQDs can be used to degrade a wide range of organic pollutants and dyes.
such as Methyl Orange, Methylene blue, 4-NP, p-chlorophenol, bisphenol-A,
tetracycline, triclocarban [10], CZ [11], and reactive black 5 (RB5) dye, with
the help of different types of GDQs which can be monitored by the change in
color/absorbance of the solution. UV irradiation was used to test organic dyes
degradation such as MO and MB with GQDseZnO nanocomposites [55]. The
nanostructures are thought to attain impressive photocatalytic activity because
of the high surface-to-volume ratio [57]. GQDseZnO nanocomposites show
variable morphology for different GQD concentrations, and the nanostructure
production of nanocomposites was established by the dominant count frequency of unit size in the 80e100 nm range. The (unit) particle size distribution of these ZnO-GQDs nanocomposites with different concentrations of
GQDs are shown in Fig. 8.2 [55].
The absorbance of MO and MB at different time intervals of irradiation
was observed After UV irradiation, both organic dyes show a drop in absorbance, indicating that dye molecules are degrading. The lower absorbance
could indicate that GQDseZnO nanocomposites could be used as photocatalytic materials. The purpose of the study was to look into the degradation
efficiency (DE) of organic waste [55] Another observation for the degradation
was, after the process of decomposition had been going on for a while, in 2 h
absorbance spectra for MB and MO dyes showed enhancement in dye
degradation to 52% and 79.4%, respectively. The rate constant (k) for MO and
MB was found to be 0.00519 min1 as well as 0.01133 min1 and so forth
[32,58]. The degradation of MO by TiO2 NTAs can be estimated by observing
the absorption and degradation of MO under UV-vis radiation. The high
photocatalytic activity of composite is due to the broad absorption in the
visible wavelength region, significant photoinduced charge separation through
the transfer of photogenerated electrons from TiO2 NTAs to GQDs, and the
high adsorption capacity of GQDs toward MB molecules. At varied UV
exposure times, optical absorption profiles were obtained (365 nm, 6 W).
GQDs infilled TiO2 annealed samples disassembled 99.8% of MB in 180 min,
which could be owing to the tight bandgap, low charge carrier recombination
rate, stability, and quick electron transport (Fig. 8.3)
Another type of dye is Reactive Black 5 (RB5) dye. RB5 is a synthetic
reactive dye that is widely used in the dyeing business. It is water-soluble and
has reactive groups that can establish a covalent bond between the dye and
fiber. RB5 is discharged into the environment in a variety of ways, causing
severe ecological issues [59]. A considerable reduction of RB5 can be noticed
TABLE 8.1 Preparation of QDS using different chemical routes.
N-CQDs
GQD
Raw materials
Citric acid
Citric acid
GQD
GQD-DMA
Graphite powder
Reaction condition
Methods
References
Hydrothermal
[44]
160 C for 4 h
Hydrothermal
[45]
15e180 min
Oxidative cutting &
microwave heating
[3,26]
120 C for 24 h
Microwave irradiation.
[30,46]
200 C for 5 h
GQD/CdSe
Citric acid
180 C for 24 h
Hydrothermal
[29,47]
GQD
Natural graphene
powder
600 W for 1 h
Microwave irradiation
[48]
GQD
Graphene
12 h
Ultrasonic
[49]
GQD
Glucose
MW heating at 700 W for 11 min
Precursor pyrolysis
[50]
Amino functionalized
GQDs
GO, ammonia
150 C for 5 h
Acidic oxidation
[50]
NGQDs/TiO2
Citric acid, urea,
180 C for 8 h
Hydrothermal and
ultrasonic dispersion
[51]
GQD
Citric acid and alkali
hydroxide
Electrochemical
exfoliation
[52]
CQDs/Bi2O2CO3
Bi(NO3)3$5H2O,
HNO3, citric acid
Centrifugation with CH3CH2OH and distilled wate
subsequently dries at 80 C for 8 h
Dynamic-adsorption
precipitation
[53]
Multi layered
graphene quantum
dot
Graphene oxide
200 ⁰C for 4 h
Hydrothermal method
[54]
GQDseZnO
Citric acid,
Zn(NO3)2.6H2O
180 for 6 h
Hydrothermal
[55]
NGQDs
Citric acid
Hydrothermal
[56]
140 Graphene Quantum Dots
Type of QDs
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
141
FIGURE 8.2 Particle size distribution of GQDseZnO nanocomposites for different GQD concentrations of: (a) 0.0 M, (b) 0.1 M, (c) 0.2 M, (d) 0.5 M, and (e) 1.0 M.
142 Graphene Quantum Dots
FIGURE 8.3 Photodegradation of MB using GQDs. Adopted with permission from Kalkan E,
Nadaro
glu H, Celebi N, Tozsin G. Removal of textile dye Reactive Black 5 from aqueous solution
by adsorption on laccase-modified silica fume. Desalin Water Treat 2014;52:6122e34. https://doi.
org/10.1080/19443994.2013.811114.
using GQDs and Fe-codoped TiO2. The degradation of the RB5 dye was aided
by the holes and OH radicals. Additionally, both GQDs and Fe enhanced
TiO2’s photon energy harvesting capability. GQD-0.1FeeTiO2-300 was shown
to be more robust and energy-efficient than undoped photocatalysts after four
cycles. Photolysis of a 4-chlorophenol solution in the presence of ZnPc-(NH2)
GQDs-PS-membrane (adjourned in solution) can be detected [60] during
various irradiation times. 4-chlorophenol is characterized by the presence of
two absorption peaks [61]. Within 1 min, noticeable degradation was detected,
which is a desired attribute to the material because the window for catalytic
degradation in actual membrane applications (continuous process system) will
be partial. The kinetic restrictions of the experiment caused by membrane
confinement in the catalytic system should be minimized by irradiating for
short periods compared to reported intervals. On the other hand, when
compared to pure TiO2, synthetic composite dots (N-GQDs) achieve full
elimination of bisphenol-A (BPA) after 30 min of sunshine irradiation [56].
Degradation of glyphosate using GQDseZnO nanoparticles [55] was
observed, they found that when compared to pure ZnO, GQDs degrade
glyphosate very well. Photocatalytic reaction is responsible for the absorbance
of glyphosate sample in the presence of sunlight, and it can evaluated on that
basis of the photocatalytic reaction. Both pure ZnO and composite of
GQDseZnO showed a slight decrease in absorbance peak around 435 nm,
representing the photo degradation property of GQDs. To assess photocatalytic
activity, the DE (Degradation efficiency) was considered from the peak shown
during the absorption. After 179 min of photocatalytic processing, DEs for
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
143
ZnO and GQDseZnO nanocomposites were determined to be 12.07% and
14.05%, respectively [62].
Degradation of New Fuchsin (NF) [(H2N(CH3)C6H3)3C]Cl with GQD are
as follows [63]; NF dye solution at a fixed amount was formed and treated
ultrasonically for 10 min. The pH of the solution was changed to the desired
value by adding NaOH or HCl solution. By taking 10 mL of GQDs which is
supplemented to a quartz cell holding solution of NF dye, further the amount
of NF degradation was examined. UV-Vis measurements at a predetermined
time interval (ʎmax ¼ 553 nm) were used to track the change in NF concentration. The Co Ct/Co 100 was used to compute the percentage of
degradation (D %), where Co and Ct were the dye concentrations at 0 and t,
respectively.
In the presence of visible light, GQDs shows the photocatalytic activity and
this step results into the degradation of pollutant as shown in Fig. 8.4. Data was
collected for NF dye degradation in the presence of 10 mL photocatalyst.
These observations were taken at various concentrations of the dye solution
(2.0e40.0 mg L1). In the presence of GQD, NF (20.0 mg L1) decolorizes as
seen in Fig. 8.4a at various contact time intervals. Ibrahim et al., [64] shows
the different degradation data model of dye rhodamine B (RhB) (for 6 h) with
FIGURE 8.4 (a) Show the decrease in the dye’s corresponding absorption spectra in the presence
of photocatalyst, (b) Change in appearance before and after the introduction of GQDs [63].
144 Graphene Quantum Dots
three different types of graphene quantum dots GQD-1 (oxygen content
37.0%), GDQ-2 (nitrogen content 15.0%), GDQ-3 (oxygen 25.4% and nitrogen 10.8%) for the three forms of synthesized GQD in the dark and under CFL
irradiation, indicating considerable activity variations under equivalent
experimental conditions (Table 8.2).
GQD-2 (55% degradation in 6 h), GQD-1 (25% degradation in 6 h), and
GQD-3 (No degradation observed). The degradation of RhB can be investigated with the decrement in the concentration and the reason behind this was
either absorption of RhB dye molecules on the surface of photocatalyst (GQD3) or the photocatalysis of the dye molecule. A small amount of dye (8% avg.)
were absorbed on the surface of three type photocatalyst (GQD-1,2,3) under
the no photonic condition and this was also observed in other similarly charged
dyeeGQD systems due to the interaction (electrostatic) between the positively
charged dye surface and negatively charged GQDs surface. Indeed, there were
no significant variations in RhB degraded under dark and light irradiation
conditions for the GQD-3 [65,66]. The large negative charge of GQD-2
(56.0 mV, about double that of the other two GQDs) is responsible for its
increased photocatalytic activity, the photodegradation of RhB with GQD-2
during 6 h is shown in Fig. 8.5 [24,64].
Nitrogen doped graphene quantum dots NGDQ/TiO2 composites degrade
RhB [51] solutions and the degradation was compared with different NGQDs
indicated as green and yellow (gNGQD/TiO2 and yNGQD/TiO2) and also with
TiO2. After 2 h of irradiation (>420 nm), the degradation capability of the
composites was examined, and it was found that the max % of degradation was
observed with y-NGQDs/TiO2 i.e., 94% while on the other hand the minimum
degradation was shown by pure TiO2 is 10% only [51]. 17% and 37% of
degradation of RhB molecules was shown by GQDs/TiO2 and g-NGQDs/TiO2
photocatalysts respectively (Fig. 8.6).
8.4 Proposed mechanisms
Different mechanisms of the interaction of dye pollutants on the surface of the
GQDs have been proposed as described below:
8.4.1 Photocatalytic activity of ZnO-GQD
In the presence of sunlight, both ZnO & GQD by UV radiation and visible
incoming photons-are photoexcited respectively, resulting in the formation of
electronehole pairs [11]. Photoexcited electrons pass from the more negative
conduction band of GQD to the less negative conduction band of ZnO due to
the high interfacial contact between the two materials. Meanwhile, the
generated holes accelerated from ZnO to GQD, triggering induced
electronehole separation across the heterojunction effectively [72]. The highly
oxidizing species (*OH radicals) were produced by the breakdown of H2O2
Percentage of degradation
(%)
Time of
degradation
References
100
12 min
[53]
89
1h
[54]
10
99
4h
[67]
5
81
2h
[68]
e
73
30 min
[69]
100
80
80 min
[70]
95
1h
[71]
4
1h
[71]
14.05
3h
[55]
25
6h
[64]
Concentration (mg L1)
GQDs types
Pollutant
O, N, S-GQD
Rhodamine
B
O-GQD
Methyl blue
917
O, N-GQD
Methyl blue
O, S-GQD
Basic fuchsin
GQDs/Bi2WO6
Nitric oxide
GQD- maltose
Imipramine
GQDs/ZnPor
Methyl blue
GQDs
Methyl blue
GQDseZnO
Glyphosate
17.8
GQDs with 37%
oxygen
Rhodamine
B
10
4.8
17.8
e
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
TABLE 8.2 Graphene quantum dot photocatalytic efficiency with different organic pollutants.
145
146 Graphene Quantum Dots
FIGURE 8.5 Photodegradation of RhB (C0 ¼ 10 mg L1) with GQD-2 (C ¼ 100 mg L1)
during 6 h.
FIGURE 8.6 Photocatalytic degradation rate of Rhodamine B-Hyaluronate solution [51].
molecules in the presence of light energy and electrons generated in the
process [55,58]. Breaking of pollutants (MB/carbendazim) into CO2 and H2O
is caused by these hydroxide radicals. The hþ generates from Valance Band of
GQD degraded pollutants directly. As a result, the generation of *OH and hþ
as active species on the photocatalyst surface oxidizes organic contaminants,
resulting in improved photocatalytic performance. Furthermore, by harvesting
solar energy, higher solar spectrum utilization from UV to visible, quick
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
147
charge transfer across heterojunction, and high specific surface area of GQD
lead to strong photocatalytic activity [11]. The proposed mechanism of the
charge carrier transitions in ZnO-GQD toward photocatalytic pollutant
degradation under natural sunlight irradiation is shown in Fig. 8.7.
8.4.2 Degradation of MO and MB
Fig. 8.8 shows the photocatalytic activity of GQDs in the presence of MO and
MB. The following processes could have occurred during the photocatalytic
degradation of organic pigments: The e jumped into the conductive band
(CB) from the excited state of GQD, and as a result, radicals from water and
O2 were produced, and dyes were oxidized through the formed active radicals.
FIGURE 8.7 Photocatalytic activity of ZnO-GQD.
FIGURE 8.8 Mechanism of the photocatalysis undergoing on the surface of the GQDs [32].
148 Graphene Quantum Dots
8.4.3 Degradation of New Fuchsin dye [63]
After irradiation of visible light, generation of e/hþ pair takes place. The
electron that generates due to the irradiation of visible light can then rapidly
reduce (O2) near the edge of photocatalyst and the formation of superoxide
radicals (O2 ) and hydrogen peroxide radicals (lOOH) takes place, whereas
the hþ can either directly degrade the dye pollutants which were present on
the surface of the photocatalyst or circuitously degrade through formed hydroxyl radicals (lOH) during the process. GQDs, for starters, have a large
surface area that allows them to effectively adsorb NF molecules. The
movement of an electron from an excited state to conducting band of NF
absorbed dye was assisted by the presence of visible light during the process
of degradation. As a result, photoinduced active species (free radicals) would
have an easier time oxidizing or decomposing the organic pollutants absorbed
on the surface. The oxygen radicals are then dissociated as NFþl reacts
quickly with them (Fig. 8.9).
l
8.4.4 Photodegradation of dye rhodamine-B RhB catalyzed by
GQD
Irradiation of light will direct the electron to get jumped into the accommodative VB (Valance ban), this particular step will generate e and hþ pair and
in this scenario electron and hole accelerated toward the active site present on
GQD. GQDs show excellent donor and acceptor capability of electron and
hole respectively and this character of GQDs will definitely open up the way
for some of the charge carriers to move toward the RhB molecule and gives
way to the process of degradation. The carriers, on the other hand, might react
with H2O and dissolved O2 to form the radicals lOH and lO2, respectively
(Fig. 8.10).
FIGURE 8.9 The hypothesized method for NF degradation under visible light irradiation by
(a) photocatalytic and (b) photosensitization routes is depicted schematically. Adopted with
permission from Roushani M, Mavaei M, Rajabi HR. Graphene quantum dots as novel and green
nanomaterials for the visible-light-driven photocatalytic degradation of cationic dye. J Mol Catal
Chem 2015;409:102e9. https://doi.org/10.1016/j.molcata.2015.08.011.
Graphene-based organic-inorganic hybrid quantum dots Chapter | 8
149
FIGURE 8.10 Proposed photocatalytic degradation of RhB using GQD. Adapted with permission
from Ibarbia A, Grande HJ, Ruiz V. On the factors behind the photocatalytic activity of graphene
quantum dots for organic dye degradation. Part Part Syst Char 2020;37:1e9. https://doi.org/10.
1002/ppsc.202000061.
8.4.5 Pathway proposed for catalytic oxidative degradation of
amines on dimethylamino functionalized graphene dot (GQDDMA)
The mechanism behind the degradation is shown in Fig. 8.11 [46], based on
the observations and earlier reports [73e76]. GQD-DMA has sufficient
oxidation potential and can easily oxidize benzylamine in the given condition.
FIGURE 8.11 Pathway representation followed by GQD-DMA photocatalyst for coupling reaction. “s” and “t” represents singlet and triplet state respectively.
150 Graphene Quantum Dots
The electrons get separated from the hole and the mixing of these pairs (e/hþ)
pair occurs with the benzylamine and surrounding oxygen as the sample gets
exposed to the light. As soon as the formed radical cation reacts with oxygen
species, the formation of benzyliminium and H2O2 takes place. After
removing the ammonia, benzyliminium can be added to another benzylamine
to produce N-benzyl-1-phenylmethanediamine, which is the final linked
product.
8.5 Conclusions and prospects
GQDs due to the unique properties gained attention in the field of biosensing,
supercapacitors, sensors, light-emitting diodes, bioimaging, nanocarriers for
gene transport, optoelectronic devices, electrochemical application, medical
diagnosis, and energy storage devices but the excellent property of GQDs, i.e.,
photocatalytic activity have been somehow ignored, we have tried to enlighten
the emergence of GQDs as an effective organic waste degradant as they have
unique photocatalytic properties, in this article the different methods are
shown to synthesize GQDs and explain the mechanism behind stimulation of
e to the conduction band, resulting in electronehole pairs and proposed many
hypothesized route for showing photocatalytic activity of GQDs. Because of
their simple manufacturing method and distinctive photocatalytic properties,
GQDs are a promising contender in the field of waste treatment.
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Chapter 9
Graphene quantum dots for
heavy metal detection and
removal
Sufia ul Haque1, Mohammad Faisal Umar2, Ogechukwu
Bose Chukwuma2 and Mohd Rafatullah2
1
Advance Functional Material Laboratory, Department of Applied Chemistry, Zakir Husain
College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim
University, Aligarh, Uttar Pradesh, India; 2School of Industrial Technology, Universiti Sains
Malaysia, Penang, Malaysia
9.1 Introduction
9.1.1 Background
In 1974, Norio Taniguchi coined the term nanotechnology, which has drawn
the attention of researchers to a new area of knowledge and application [1,2].
The plethora of materials can be synthesized and designed with optimized
properties for various applications by playing with the dimensions of the
material at atomic scale [3,4]. The most suitable example is nanomaterials of
carbon. The discovery of nanomaterials of carbon like graphene [5], carbon
nanotubes (CNTs) [6], and fullerene [7] had a massive influence in science and
technology development, which resulted in Nobel Prize. That unquestionably
shows the interest of scientists in nanostructures of carbon and their applications in many other fields of application [8]. In 2004, another example of
nanomaterial of carbon i.e., luminescent carbon nanoparticle with diameter
(<10 nm) was reported which was later known as carbon quantum dots.
Further, many fundamental and theoretical studies regarding changes
occurred upon reducing graphene dimensions carried out by several researchers. Ponomarenko et al. [9] reported the graphene quantum dots (GQDs)
with bandgap effects and quantum confinement [10e13]. The luminescence
properties of the GQDs were reported by Pan and coworkers [14] and the first
nitrogen-doped GQDs was reported by Zhao and coworkers [15].
The vast interest in GQDs raises from its carbon composition, which is one
of the most common elements on earth. Besides, the GQDs is considered as
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00007-8
Copyright © 2023 Elsevier Ltd. All rights reserved.
157
158 Graphene Quantum Dots
the best substitute for semiconducting quantum dots for some applications [8].
Also, the carbon-based composition of GQDs eases their utilization in
biomedical and biological applications due to the low toxicity [16,17], ecofriendly [18,19], superior photostability [20e22], and biocompatibility
[23,24]. Furthermore, GQDs have good solubility and dispersibility in H2O
and many other solvents, easing their processing and applications [25,26].
GQDs is obtained from two-dimensional graphene and formed into zerodimensional material, which caused edge effects and quantum confinement.
In which the electron distribution modifies because of crystal’s dimension
reduction to nanometer scale. Also, unlike graphene, GQDs have nonzero
bandgap. So, the semimetal graphene can be converted to an insulator or
semiconductor GQDs. In contrast to different nanomaterials of carbon
including carbon nanotubes, graphene, and fullerene, GQDs exhibited physical
and chemical characteristics due to quantum and edge confinement effects.
The synthesizing route determine the nanomaterials physical and chemical
characteristics. Based on preparation methods established in recent years, the
bottom-up and the top-down methods are classified on the basis of the precursor material. Fig. 9.2 shows the top-down method in which graphene is
directly cut into quantum size through various processes. In contrast, the
stepwise chemical reaction involves a bottom-up method, where graphene-like
material like fullerene, benzene, and glucose are transformed to GQDs
[27e29].
The fundamental characteristic of GQDs is edge and quantum confinement
effects. The exceptional properties of GQDs like solubility, dispersibility,
surface grafting, stable PL, nontoxicity, biocompatibility, and inertness are due
to the aforementioned essential characteristics (Fig. 9.1).
9.1.2 Outlook for GQDs
The research on GQDs is still in its infancy, and several challenges need to be
resolved (Fig. 9.2). Further research on GQDs is required to enhance their
properties for proper implementation in various applications. Hence, the investigations of GQDs are going on to attend the issues stated in Fig. 9.3. For
industry usage, the mass production of GQDs is required at a relatively low
cost. Yet, the GQDs obtained from currently available methods is significantly
low (mostly less than 10%) [30]. Therefore, new procedures should be adopted
like facile fabrication technique using coals [31] and photo-Fenton reaction
method (45% yield) [30], to enhance the yield of GQDs. Also, mostly the
recorded quantum yield of GQDs ranges between 2% and 22% [30] less than
the traditional semiconductor quantum dots. So, the low yield of quantum dot
can be improved by making surface modifications like carbon-metal
nanocomposites.
Additionally, the optoelectronic properties of GQDs are relied on their
shapes and sizes [32,33]. Thus, modifying the in-built characteristics of GQDs
depends on how precisely its shapes and sizes can be controlled. GQDs
showed different PL mechanisms. It varied from blue to yellow so far [30].
Graphene quantum dots for heavy metal detection and removal Chapter | 9
159
FIGURE 9.1 The GQDs related inherent effects, properties and applications.
FIGURE 9.2 According to recent researches several challenges need to be catered.
This narrow spectrum of GQDs restricts their use in optoelectronic devices.
The extension of the visible wavelength and near-infrared spectrum coverage
of GQDs is an essential area in future study. Some research labs discovered
nitrogen doping in GQDs in the near-infrared PL spectrum.
In this chapter, the current synthesis methods of GQDs and their utilization
in heavy metal removal or detection will be discussed.
160 Graphene Quantum Dots
FIGURE 9.3 Schematic illustration of several GQD preparation techniques.
9.2 Common methods used for the synthesis of GQDs
Based on the literature the preparation methods of GQDs are of two main
categories, viz. bottom-up and top-down approaches. Fig. 9.3 exhibits the topdown approach in which electron beam lithography and exfoliation techniques
are utilized to cut carbon material into nanoscale GQDs. Such methods usually
create functional groups that contain oxygen, at the edge, which facilitates
functionalization and solubility. However, this method has drawbacks like the
immense volume of defects, uncontrollable size/shape, and low yield. This
approach showed fewer defects and control on morphology and size in contrast
to the top-down method. On the reverse side, the bottom-up method showed
aggregation issue and poor solubility, and as shown in Fig. 9.3. Further, details
about GQDs preparation methods are explained.
9.2.1 Bottom-up approach
The bottom-up method involved growth of graphene and graphene-like smaller
polycyclic aromatic hydrocarbons into GQDs. This approach can be divided
into four methods: metal-catalyzed method, hydrothermal method,
microwave-assisted hydrothermal method, and soft-template method,
depending on how the external energy is involved.
Graphene quantum dots for heavy metal detection and removal Chapter | 9
161
9.2.1.1 Hydrothermal method
It entails the crystallization of appropriate substance using high temperatures
and high vapor pressures. Various research labs have studied the preparation of
GQDs via hydrothermal methods. In 2012, Dong et al. [34] synthesized GQDs
from citric acid via the hydrothermal method and the findings were a photoluminescence quantum yield of 9.0%. Yang et al. [35] utilized citric acid and
ethylenediamine as a source of carbon to successfully prepare nitrogen-doped
GQDs having a photoluminescence quantum yield of 75.2%. In the preparation of GQDs, there are other graphene-like, smaller polycyclic aromatic hydrocarbons, asides citric acid that can serve as carbon source. Pyrene while
emitting a bright green fluorescence had a GQDs yield of 63% [19]. Guo’s
research group used 1,5-dinitronaphthalene as source to prepare GQDs with
tunable photoluminisence emission [36]. There was unique tunable photoluminisence emission that saw a color change from blue-green to yellow with
concurrent pH change from 5 to 10 respectively, leading to production of
amine-functionalized GQDs. This can be attributed to amine-groups found in
acid or alkaline solutions being protonated or deprotonated.
9.2.1.2 Hydrothermal method using microwave
The hydrothermal preparation method is unsuitable for industrially mass
producing GQDs due to its huge time demands. This led to its modification by
introducing the microwave making it more suitable for mass production of
GQDs as it became faster and more efficient. Now there is the possibility of a
shorter duration for the synthesis of GQDs to even seconds and minutes. This
method is a combination of both hydrothermal and microwave-assisted approaches. These improvements can be attributed to microwave heating promoting a quicker and more uniformed heating, giving rise to nonsurface
passivation and equal size distribution.
9.2.1.3 Soft-template method
This method is used to synthesize the facile, environmentally friendly, and
economic GQDs. This preparation involves a nanoscale reaction cavity
deprived of purifying and complicated separation processes. Hence, this
method is utilized for bulk production. In 2016, Yang and co-author synthesize
GQDs by this method. The 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB), is a
graphite type pattern with planar and highly symmetric molecule having six
intramolecular hydrogen bonding between amino and nitro functional moieties. The breaking of various chemical bonds was carried out by thermal
process which cause the generation of expanding gases like nitrogen monoxide, nitrogen dioxide, and water. As a result, the graphite type TATB multilayered structure was converted to single layer. Last, dispersed GQDs was
obtained via oxidative exfoliation. The major asset of the soft-template
approach is that the GQDs size can be controlled.
162 Graphene Quantum Dots
9.2.1.4 Metal-catalyzed method
This is the rarely used method for the synthesis of GQDs. By this method the
shape of GQDs can be modified by the annealing temperature. However,
particularly metal-catalyst and raw material were mandatory for synthesizing
GQDs, making this approach rare.
9.2.2 Top-down methods
This method usually synthesizes the GQDs via break down of the whole
material by chemical or physical methods. For the first time, GQDs was
prepared by this approach. Top-down approach commonly utilizes liquid
exfoliation through hydrothermal, ultrasonic, electrochemical, and electron
beam lithography method. Other rare routes include as magnetron sputtering
technique.
9.2.2.1 Liquid exfoliation method
Recently, many research groups have been using liquid exfoliation methods to
prepare GQDs [37e41]. Peng’s research group produced GQDs by this
approach from carbon fibers. They also revealed that synthesis temperature
affects the PL emission. Also, Tour et al. [31] succeeded to enhance the
production yield of the GQDs by the utilization of coal. Another research
group utilized coal for the synthesis of GQDs [42]. But, the nonuniformity of
the GQDs size’s distribution was observed via TEM images due to the preparation surroundings.
In contrast, graphite, when used to prepare GQDs, emerged as the preferred
source [43,44]. A study was carried out for the synthesis of GQDs by the
sonication of graphite in liquid [45]. Besides, Zuo et al. [46] used cotton that
was annealed prior to exfoliation in liquid to produce GQDs. Similarly, Zhao
et al. [47] also utilized the cotton as a carbon source to synthesize GQDs with
chlorine acting as dressing agent. So, the GQDs obtained were doused in
chlorine, which showed different properties from the pure GQDs. Besides
graphite and cotton, carbon nanotube can also be utilized as a source of carbon
for the preparation of GQDs. The three-step chemical oxidation of single
walled carbon nanotube was also used to prepare GQDs-1 and GQDs-2. The
aqueous solution of GQDs-1 and GQDs-2 were yellow under white light with
dissimilar UV-vis absorption.
Usually, the liquid exfoliation method takes place in liquid, and the surroundings affect the final products’ characteristics. Hence, the physicochemical properties of the GQDs can be checked/regulated by synthesis conditions.
Liu’s research group has attempted a different technique using magnetron
sputtering. Magnetron sputtering of graphite and ZnO composite generated a
GQDs and ZnO films, which were then treated with acid and dialyzed to
produce ZnO from GQDs.
Graphene quantum dots for heavy metal detection and removal Chapter | 9
163
9.2.2.2 Electron beam lithography method
For the first time GQDs was synthesized by this method. However, this method
is rarely utilized due to the usage of costly apparatus. Also, quantum dots size
was restricted by the processing scale of lithography. Table 9.1 below
described some properties of GQDs synthesized by top-down and bottom-up
approach. It can be seen that the production yield from top-down method is
lower than the bottom-up method. In bottom-up approach, the physicochemical properties of the GQDs can be easily altered. Additionally, the
sources for bottom-up approach are broader compared to top-down approach.
However, the preparation method is chosen according to the application of
GQDs.
9.3 Applications of GQDs
In various fields the GQDs finds many applications such as medical, optical
and energy as shown in (Fig. 9.4), which will influence the quality of life in a
huge way [48,49]. A number of research analysis have been carried out to look
into the exceptional properties of GQDs. The properties of GQDs have been
tailored by doping and controlling of shape/size. Over the coming decade,
GQDs could recognized as a fascinating material for the exploitation of many
applications like drug delivery [50,51] biological imaging [52,53], LEDs
[54,55], photodetector [56,57], battery [58] and heavy metals detection. The
detailed applications of GQDs in various fields are explained below:
9.3.1 Medical applications
GQDs have the ability to replace traditional materials used for medical applications due to their non-toxicity and biological compatibility. The recent
outstanding results obtained from GQDs have been encouraging continuous
exploitation in the various fields of medical applications like biological imaging, drug delivery, photodynamic therapy, photothermal therapy, and antimicrobial materials. The various characteristics like pep interactions and
functional moieties (carboxyl, carbonyl, hydroxyl and epoxy) make the GQDs
an ideal applicant for the drug delivery systems [59]. Furthermore, the
nanosystems for drug delivery with various functions have gained significance
for cancer therapy because of their therapeutic efficacy [59]. The effects of the
particles on cell viability were evaluated by incubating the GQDs and sizechangeable nanoaircrafts with a rat glioma cell.
9.3.2 Optical applications
The GQDs have unique optical properties which led to the development of
various optodevices like photodetector [60] photocatalysis [61] and lightemitting diode [62]. This application is based on the upconversion, tuning
164 Graphene Quantum Dots
TABLE 9.1 GQDs-based sensors for the sensing of Hg2þ.
Variety of
GQDs
Starting
materials
Optical
method
LOD
(nM)
References
GQDs
CA
Fluorescent
chemosensor
3360
[86]
GQDs
Graphene
FP
100
[87]
GQDs
CA
Dual
fluorescent
sensor
0.439
[81]
DNA-GQDs
Graphite
powder
FP
0.25
[88]
N-OGQDs
CA/L-DOPA
FP
8.6
[90]
GQDs-SR
Graphite
powder/SR
Fluorescent
chemosensor
230
[91]
GQDs-DNA-AuNP
CA/DNA/
AuNP
ECL sensor
0.00248
[89]
RhB-GQDs
B/
ethylenediamie
FP
0.16
[92]
N, S/GQDs
CA/Dpenicillamine
FP
0.69
[93]
N-GQDs
CA/ammonia
FP
4.7
[94]
PEHA-GQDs-DPA
CA/DPA/PEHA
FP
0.046
[95]
2þ
Mn(II)-NGQDs
Glycine/Mn /
sodium citrate
FP
0.34
[96]
N, S-GQDs
CA/thiourea
FP
0.14
[97]
MEA-GQDs
CA/MEA
Fluorescent
“off-on”
10
[98]
PEI-GQDs
Graphite
powder/PEI
Fluorescent
“off-on-off”
0.25
[99]
Fe3O4@SiO2@GQDs
CA/
Fe3O4@SiO2
Fluorescence
detection
30
[100]
GQDs-Pcs
Graphite
powder
Fluorescent
“turn ON”
0.12
[101]
Valine-GQDs
CA/valine
FP
0.4
[102]
GQDs-T-ZnPc
GO/T-ZnPc
Fluorescent
“turn ON”
0.05
[103]
cys-GQDs
CA/Cysteine
FP
20
[104]
Graphene quantum dots for heavy metal detection and removal Chapter | 9
165
TABLE 9.1 GQDs-based sensors for the sensing of Hg2þ.dcont’d
Variety of
GQDs
Starting
materials
Optical
method
LOD
(nM)
References
CdTe@SiO2@GQDs
CA
Ratiometric
fluorescent
probe
3.3
[105]
where AuNP, Au nanoparticle; CA, Citric acid; FP, Fluorescent probe; DPA, D-penicillamine; ECL,
electrochemiluminescence; GO, graphene oxide; L-DOPA, 3,4-dihydroxy-L-phenylalanine; LOD,
limit of detection; MEA, monoethanolamine; Mn(II)eN, (Mn2þ)-bonded nitrogen; N, nitrogen; Pcs,
phthalocyanines; PEHA, pentaethyleneheaximine; PEI, polyethyleneimine; RhB, rhodamine B; S,
sulfur; SR, spirolactam rhodamine; T-ZnPc, thymine-appended zinc phtalocyanine.
FIGURE 9.4 Schematic presentation for several applications of GQDs
of energy gap, and strong photoluminescence, which differ from conventional
quantum dots (ZnO and TiO2 quantum dots).
9.3.3 Energy-related applications
Rising populations and the dwindling of resources have led to an urgency in
finding sustainable and alternative energy sources. Carbon-based quantum
166 Graphene Quantum Dots
dots, regarded as one of the most abundant globally, are seen by many as an
excellent material for energy production. Despite being a clean and renewable
form of energy, solar energy has the limiting quality of daytime availability,
which diminishes its use in providing electrical energy to meet man’s needs.
Overcoming the challenge of storing solar energy is now an important and
challenging topic of research. GQDs show potential in that regard as they are
employed in solar cells and energy storage batteries. The properties of GQDs
like strong fluorescence, intensive absorption at UV range, down conversion,
and easier functionalization are being exploited to enhance the performance of
solar cells. Hybrid of GQDs and Si solar cells are very efficient in power
conversion (showing enhancement to 16.55%) making them on of the best
solar cells [63].
9.3.4 Heavy metal detection and removal
Over the last few decades, the term heavy metal with a specific density greater
than 5 g cm3 has been gained attention [64]. The toxic nature of heavy metals
cause considerable environmental pollution [65]. The consumption of toxic
heavy metal via drinking water, plant derived food, and beverages can cause
irreversible damage to the human. It can enter to the food chain with a subsequent biomagnification. Hence, the World Health Organization (WHO)
enacted some laws to shield fauna and flora from pollution due to toxic nature
of heavy metals. Cadmium (Cd), lead (Pb), and mercury (Hg) are three toxic
heavy metals that must be secluded from the soil, ground water, and
atmosphere.
Presently the heavy metal ions can be analyzed by several approaches like
inductively coupled plasma atomic emission spectrometry, atomic absorption
spectrometry, inductively coupled plasma mass spectrometry, enzymatic inhibition method, and high-performance liquid chromatography. Since these
methods need (a) expensive bulky equipment, (b) involve a cumbersome
detection process, (c) extensive sample preparation, and (d) are nonspecific.
The above-stated techniques still suffering from long-term stability, specificity,
ease of onsite sampling, and compatibility with aqueous environments.
Some other advanced methods like electrochemical and electronic analysis
are also utilized to detect the toxic heavy metals [66,67]. But the downsides of
these methods include its on-site sampling being complex, instability, low
selectivity and its compatibility being negligible when in an aqueous medium.
Conversely, optical method has grabbed attention for the utilization of the
sensing purpose of toxic metal ions and this method showed compatibility in
an aqueous medium along with being highly selective and sensitive. In these
circumstances, GQDs possessing optical information such as fluorescent
emissions, absorbance and the change in their intensity due to the interaction
with metal ions are useful for heavy metals detection. The change in properties
of GQDs when interacted with metal ions are observed by fluorescent,
Graphene quantum dots for heavy metal detection and removal Chapter | 9
167
photoluminescent, electrochemiluminescence, colorimetric, and surface plasmon resonance sensing.
Recently, extensive research has been carried out to design and synthesize
sensitive devices and materials to be used to detect and remove toxic heavy
metals [68e76]. The nanomaterials possessing unique and novel properties for
detecting heavy metals are being considered as feasible options [77]. Among
various nanomaterials, GQDs have shown outstanding properties that make
them potential candidate in the sensor platforms.
To detect toxic heavy metals in real-time, GQDs are considered a good
nanomaterial. The mechanism of heavy metal adsorption on the surface of
GQDs needs a clear understanding of these heavy metal’s behavioral changes
and response to the adsorption actions. The detection of heavy metals on
GQDs surface depends on the sensitivity of the GQDs toward toxic heavy
metals. For example, A research work reported the adsorption of toxic
heavy metals (Cd, Hg, Pb) on the GQDs and defective GQDs surface
coordinated with nitrogen by using density functional theory [78]. The thermochemistry evaluations revealed that the adsorption of Pb was more favorable compared to the Hg and Cd adsorption on the surface of GQDs
(GQDs@4N > GQDs@3N > GQDs@1N > GQDs@2N > GQDs). This result
was validated by the noncovalent interaction plots and quantum theory of atoms
in molecules analysis.
However, several research groups were investigated the interaction of
graphene oxide quantum dots (GOQDs) with heavy metals form a specific
point of view (fluorescence quenching effect) [79e83]. It was observed that
the intensity of intrinsic blue/green fluorescent emission of GOQDs gradually
decreased with the increase in heavy metal ions (Hg2þ, Fe3þ, Cu2þ, Pb2þ)
concentration. Chelation of metal ions by various functional moieties of
GOQDs might be the reason for this effect, as it causes reduction in fluorescence emission rate owing to the appearance of nonradiative processes.
Further, research based on the coating of GQDs on quartz sand was carried out
for removing Hg2þ and Pb2þ in an aqueous solution [84]. The maximum
adsorption capacity reported was 24.65 and 24.92 mg g1 for Hg2þ and Pb2þ,
respectively. That showed better adsorption capacity of the GQDs coated on
quartz sand compared to the quartz sand alone. The experimental outcomes
showed the adsorption of Hg2þand Pb2þ on the GQDs coated on quartz sand
pursued pseudo-second order kinetic model whereas the experimental data at
equilibrium followed Langmuir isotherm (R2 > 0.99). It was also observed
that the heavy metal removal performance of the GQDs coated on quartz sand
relied on the GQDs particle size. This adsorbent can be used to treat
contaminated water at industrial scale due to its low cost, effectiveness, and
stability.
Research work focused on the interaction between heavy metals (Cd, Pb,
and Hg) and GQDs was carried out. The binding energy and height of heavy
metal (neutral and charged) ions on GQDs was determined and the findings
168 Graphene Quantum Dots
showed adsorption energy of donors like physisorbed neutral Pb atoms was
greater than that of Cd and Hg. Nevertheless, the charged heavy metal species
acted as typical acceptors with the study, revealing how replacing a carbon
atom with a heavy metal adatom modifies the geometric structure of GQDs
and the changes in their vibrational and electronic properties. This research
work suggested the route toward optically detecting toxic heavy metals using
GQD-based sensing.
Ivan et al. [85] reported the artificial vacancy-type defects created in GQDs
which were utilized for the reaction of the active sites with Cd, Hg, and Pb.
The researchers employed density functional theory and time-dependent
density functional theory methods to predict the effect of the vacancy-based
complexes to elaborate the ability of GQDs to bind to heavy metals. Monovacancy and trivacancy defects were very active in reacting with heavy metals
in contrast to the even-numbered vacancy and defect-free GQDs. Meanwhile
the interaction between GQDs and Pb is controlled by the charge transfer,
which showed that the Pb atoms could bind more strongly close to the vacancy
defects than the other heavy metals (Cd and Hg).
In 2013, the detection of Hg2þ.for the first time by employing GQDs was
reported. They synthesized the GQDs by carbonizing the citric acid and the
solution was made in the alkaline solution. Various metal ions were tested with
this solution, and Hg2þ.entirely quenched the emission. This quenching fluorescence property aroused because of the Hg2þ.adsorption on GQDs surface.
The detection limit of Hg2þ.was detected to be 3360 nM [86].
Later, GQDs was prepared through ultrasonic method utilized by Hg2þ.to
quench the fluorescence of GQDs [87]. As the transfer of electron occurred by
the adding of Hg2þ, it caused nonradiative electron-hole annihilation. The
synthesized GQDs augmented with carboxylate groups revealed high affinity
for Hg2þ. The detection limit of Hg2þ was detected to be 100 nM under a
linear range between 0.8 and 9 mM.
In 2015, the investigation on the optical detection was further advanced by
Li et al. [88]. The research group prepared the GQDs through the citric acid
pyrolysis and utilized them to detect Hg2þ. The GQDs fluorescence was
quenched by the Hg2þ through the mechanism of charge transfer. The limit of
detection of Hg2þ.was detected to be 0.439 nM under a linear range
(1e50 nM) concentration of Hg2þ. A recent research work using the electrochemiluminescence sensor effectively attained the lowest detection limit
(¼0.00.248 nM) for Hg2þ. First, citric acid underwent pyrolysis to produce
GQDs as a by-product. Then, an integration of DNA (single stranded) through
the sulfhydryl and amino groups at each end of the gold nanoparticle and
GQDs, respectively was carried out. The significant role of gold nanoparticle
was to enhance the signal amplification by increasing the GQDs load. The
sensor showed a wide linear range of 0.01e100 nM [89].The various research
work done upon the Hg2þ detection by using different GQDs-based optical
sensors is given in Table 9.1.
Graphene quantum dots for heavy metal detection and removal Chapter | 9
169
As discussed earlier, exposure to Pb2þ albeit at minute concentration has
harmful effects on the central nervous system, reproductive, and human health.
One study reported the effects of Pb2þ.exposure to guinea pig, which showed
the increment in heart beat frequency [106]. Qi et al. used GQDs for the first
time for the detection of Pb2þ in 2013. This study used 3,9-dithia-6monoazaundecane functionalized GQDs and tryptophan for the fluorescent
detection of Pb2þ. Electrostatic interaction of the Pb2þ produced a rigid
structure. The carboxylate group of tryptophan and S atom on 3,9-dithia-6monoazaundecane functionalized GQDs surface, which resulted in the
improvement of the fluorescent. This occurred by the energy-transfer interactions between 3,9-dithia-6-monoazaundecane functionalized GQDs and
tryptophan. However, this investigation reported the lowest detection limit
(0.009 nM) so far, with a linear range of 0.01e1 nM [107].
Dong et al. [108] developed a coreactant (GQDs and L-cysteine) electroluminescence system for the Pb2þ.sensing. According to this investigation, the
electroluminescence signals were developed by dissolved oxygen, oxidation of
L-cysteine, and the reduction of GQDs. The L-cysteine oxidation checked free
radicals RSO , RSO2 , and RSO3 and a quenched the electroluminescence
signal confirmed their formation. A linear dependence between the
Pb2þ.concentration and the quenched ratio was attained in the range of o
0.10e10 mM with a limit of detection of 70 nM [108].
Later, graphene oxide and the GQDs-aptamer conjugate were utilized to
detect Pb2þ. Graphene oxide acted as a quenching agent and electron acceptor,
while GQDs acted as a fluorophore. The graphite powder was used to synthesize both graphene oxide and GQDs. Further, GQDs was reduced by using
sodium borohydride to produce rGQDs. The GQDs was accumulated on the
graphene oxide surface via electrostatic force and p-p stacking, that quenched
the fluorescence. The complexation of rGQDs and Pb2þ caused the fluorescence with a linear relation between Pb2þconcentration and fluorescence intensity. The detection limit was evaluated to be 0.6 nM in a wide range of
9.9e435 nM [109].
Another research work utilized the combination of gold nanoparticles and
GQDs via the pairing reaction between GQDs and gold nanoparticles both
altered with DNAs. Thus, the fluorescence resonance energy transfer between
gold nanoparticles and GQDs caused fluorescence quenched, which can be
recovered by adding Pb2þ.The presence of Pb2þstimulates the catalytic activity to cut the DNAs linker in order to detach GQDs and gold nanoparticles,
which detect the Pb2þdue to the fluorescence recovery. This fabricated sensor
exhibited a detection limit (¼16.7 nM) with a wide linear range of Pb2þ
(0.05e4 mM) [110].
Another investigation employed transferring fluorescence resonance energy in self-assembled multilayers to detect Pb2þ. The authors built the sensor
by employing glutathione-functionalized GQDs as the energy donor, graphene
oxide as energy acceptor, and poly(diallydimethylammonium) chloride and G5
170 Graphene Quantum Dots
(G-rich DNA) strand as the linker. Pb2þ being present in specific amounts,
causes G5 DNA to fold into quadruplex, thus shortening the distance between
graphene oxide and glutathione-functionalized GQDs, hence causing the
enhancement in the energy transfer between graphene oxide and glutathionefunctionalized GQDs with the quenching of fluorescence to recognize
Pb2þ.The value of detection limit was found to be 2.2 nM within broad range
of concentration (2.4e11.5 nM) [111].
The GQDs doped by diethyl dithiocarbamate was synthesized by citric acid
and diethyl dithiocarbamate pyrolysis. For the detection of Pb2þ., the doped
GQDs was used as resonance light scattering probe. The intensity of resonance
light scattering linearly increased, confirming the presence of Pb2þ with a
concentration range of 4.83e48.3 nM. The limit of detection was found to be
3.86 nM [112].The selectivity of this sensor was enhanced by the addition of
tartaric acid solution used to mask the Pb2þ. Table 9.2 mentioned below
showed different combination of GQDs for the Pb2þ. ion detection.
A hydrogel adsorbent was prepared by employing in situ polymerization of
N-vinyl imidazole (VI), two different cross-linkers and appropriate quantity of
nitrogen-doped GQDs [115]. This nitrogen-doped GQDs hydrogel was used to
remove Cd2þ, Ni2þand Cr6þ ions form water. The adsorption of these heavy
metal ions was analyzed at pH 1.0, 7.0, and 9.0 and optimum removal efficiencies of Cd2þ, Ni2þ, and Cr6þ ions were found to be 75%, 94.6%, and
70.9%, respectively at pH 7.0. The kinetics and adsorption isotherm were also
analyzed to calculate the adsorption behavior of the nitrogen-doped GQDsbased hydrogel. The maximum adsorption capacity of nitrogen-doped GQDs
hydrogel was found to be 5000, 5000, and 370 mg g1 for Cd2þ, Ni2þ, and
Cr6þ ions, respectively.
Another research work has reported a nanoadsorbent (magnetic) made of
NiFe2O4/hydroxyapatite (HAP)/GQDs to remove Cd2þ ions from aqueous
solutions [116]. The outcome of the study revealed that equilibrium time was
10 min for the adsorption of the Cd2þ ions onto the NiFe2O4/HAP/GQDs
adsorbent. The removal of Cd2þ ions was studied at pH 6.0 with maximum
absorption capacity of 344.83 mg g1 at 25 C. This study showed that the
NiFe2O4/HAP/GQDs nanoadsorbent was effective for the removal of Cd2þ
ions from aqueous solutions. The possible adsorption mechanism of Cd2þ ions
on NiFe2O4/HAP/GQDs nanoadsorbent is shown in Fig. 9.5.
For the removal of Hg2þ ions the synthesis of multifunctional nanocomposite (Fe3O4@SiO2@GQDs) was carried out [100]. Here, the GQDs
were covalently loaded on the silica-coated magnetite nanospheres surface.
This nanocomposite exhibited a strong fluorescence, which was quenched by
Hg2þ ions. The GQDs in the nanocomposite have high specific surface area
and enough binding sites, which resulted in a good adsorption capacity of
68 mg g1 for Hg2þ ions. This nanocomposite material was recycled with
EDTA and repeatedly utilized for Hg2þ ions detection and removal from
wastewater.
Optical method
Linear range
Detection
limit (nM)
References
Pyrolysis
Resonance light
scattering
4.83e48.3 nM
3.86
[112]
Anthracite coal
Electrochemical
oxidation
FP
1e20 mM
750
[113]
Glutathionefunctionalized GQDs
Citric acid/
Glutathione
Pyrolysis
Fluorescence
resonance energy
transfer
2.4e11.5 nM
2.2
[111]
GQDs and gold
nanoparticles
Purchased
e
Fluorescence
resonance energy
transfer
0.05e4 mM
16.7
[110]
S-doped GQDs
Pyrene/1,3,6trinitropyrene
Hydrothermal
FP
0.1e140.0 mM
30
[114]
rGQDs
Graphite powder
Oxidation/
reduction
Fluorescence
9.9e435 nM
0.6
[109]
GQDs/L-Cysteine
Carbon black
Chemical
oxidation
Electroluminisence
100e1000 nM
70
[108]
GQDs- 3,9-dithia-6monoazaundecane
GO/3,9-dithia-6monoazaundecane
Hydrothermal
Fluorescent probe
0.01e1 nM
0.009
[107]
Variety of GQDs
Starting materials
Diethyl
dithiocarbamatedoped GQDs
Citric acid/Diethyl
ddithiocarbamate
N, P, S co-doped
GQDs
Synthesis
method
Graphene quantum dots for heavy metal detection and removal Chapter | 9
TABLE 9.2 The various GQDs-based sensors for the detection of Pb2þ.
171
172 Graphene Quantum Dots
FIGURE 9.5 Cd2þ adsorption mechanism on NiFe2O4/hydroxyapatite/GQDs nanoadsorbent.
In a study, a simple and commercial nanosorbent made of GQDs coated on
quartz sand were prepared for the removal of Hg2þ and Pb2þ [84]. This study
showed the improved adsorption performance of the GQDs coated on quartz
sand in comparison to the quartz sand, owing to the large surface area of the
GQDs. The study revealed the influence of particle size of GQDs on the heavy
metal removal efficiency. GQDs obtained from carbonization process for
30 min showed the highest removal percentage. The maximum adsorption
capacity of this nanosorbent for Hg2þ and Pb2þ were evaluated to be 24.65 and
24.92 mg g1, respectively. This study provided a cheap, stable, and effective
nanosorbent (GQDs coated on quartz sand), which could be utilized for industrial wastewater treatment.
This study on the development of a Förster resonance energy transfere
based optical sensing system for the detection of heavy metals. The sensing
system is made of GQDs as donor and carbon nanodots as an acceptor. These
optical sensor works when fluorescent carbon nanodots are in the range of
Förster resonance energy-transfer distance, the donor GQDs is quenched by
the transfer of energy to acceptor carbon dots. The Förster resonance energytransfer signals were reduced upon the addition of heavy metals like As5þ and
Hg2þ. The mechanism of Förster resonance energy transfer on the molecular
interactions between GQDs and C-dots are shown in Fig. 9.6.
At present, heavy metal contamination has still been a critical issue.
Beyond a certain limit, the presence of heavy metals adversely affects the
Graphene quantum dots for heavy metal detection and removal Chapter | 9
173
FIGURE 9.6 Schematic diagram of (a) Förster resonance energy-transfer mechanism and
(b) spectral overlap of Förster resonance energy-transfer pair. Adapted with permission
Mohammad-Rezaei R, Jaymand M. Graphene quantum dots coated on quartz sand as efficient and
low-cost adsorbent for removal of Hg2þ and Pb2þ from aqueous solutions. Environ Prog Sustain
Energy 2019;38:S24eS31. https://doi.org/10.1002/ep.12911. Elsevier.
human health and environment. For this reason, an immediate solution is
required to sort out this problem. An immediate detection of these heavy
metals below their hazardous limit is required. Now, it is the immense need to
develop a sensor with high selectivity and sensitivity to detect these heavy
metals at very low concentration. That’s why GQDs have been used commonly
to detect the heavy metal ions. The combination of surface plasmon resonance
and GQDs and their hybrid materials has the potential to detect heavy metal
ions frequently. The portable heavy metal ion detection can be developed by
using various GQDs fluorescence technique allowing high specificity and
sensitivity. However, quantum dots fluorescence based heavy metal detection
is still in progress.
9.4 Conclusions
This chapter explained the background, development of the synthesis methods,
and various applications of the GQDs. Several methods for the fabrication of
GQDs have been discussed with their various unique properties which can be
utilized for many applications. Since graphene been discovered, finds limited
application due to zero band gap. However, the development of GQDs has
turned out to be an excellent way to utilize various applications of graphene.
Unquestionably, vital research efforts have been carried out for the synthesis
and employability of GQDs. As discussed in this chapter, the GQDs materials
gain recognition as essential materials whose functions can be harnessed in
174 Graphene Quantum Dots
different areas such as the optical, medical, and energy fields. Though, the
further investigations are mandatory for some of the applications of GQDs
where the working mechanism is not precise. However, the utilization of
GQDs in various applications grabbed colossal attention among researchers
owing to its several outstanding benefits. This chapter has provided an outlook
of synthesis and functionalization routes with various ways of the GQDs.
Further, the identification and treatment of toxic transition metals (Pb2þand
Hg2þ) by employing GQDs are explained with the reporting of detection limit
with certain linear range of concentration.
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Chapter 10
Graphene quantum dots for
clean energy solutions
Waris1, Abdul Hakeem Anwer2 and Mohammad Zain Khan1
1
Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Aligarh
Muslim University, Aligarh, Uttar Pradesh, India; 2School of Mechanical Engineering, Yeungnam
University, Gyeongsan, Gyeongaan, Republic of Korea
10.1 Introduction
One of the world’s biggest challenges is to meet the rising demand for
renewable energy in an environmentally friendly and sustainable way, especially in developing countries with rapidly increasing populations and living
standards. The main requirement is to include clean energy solutions in this
respect [1]. Energy is a fundamental part of a developing country’s economy.
The utilization of resources worldwide was approximately 520 quintillion
BTUs (British Thermal Units) in 2010 and is expected to shoot up by 56%
(820 quintillion BTU) in 2040. 78% of the world’s energy consumption is
derived from natural resources such as coal, oil, and natural gas, while only
19% of existing energy comes from renewable sources [2,3]. As natural
(nonrenewable) energy sources are being used up continuously, it is also
harming our environment by petroleum products. Decreasing renewable energy sources and growing energy demand suggest that we should act honestly
and decisively to establish completely clean, sustainable, progressive, and
feasible sources of energy. A large number of scientific experts in the world are
looking for a source of sustainable substitutions to fulfill the energy demand of
the world. Renewable energy can be generated by different resources such as
wind, solar, geothermal, waste energy, etc. The biggest worry may not be the
exhausting petroleum products but the rising global pollution that deviates
from the atmospheric balance which could affect humans and other living
beings [4]. In this sense, the growth of renewable, sustainable and clean energy
sources has become a subject of urgency [5]. According to the studies, energy
consumption around the world has increased by 92% from 1975 to 2020.
Unfortunately, only 10% of this consumption energy is being derived from
renewable energy sources [6].
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00004-2
Copyright © 2023 Elsevier Ltd. All rights reserved.
183
184 Graphene Quantum Dots
Energy and the environment are now critical subjects around the world.
The use of clean energies, including solar and wind energy, attracts growing
attention. However, owing to the wind and solar energy fluctuations, energy
storage, and conversion is of particular significance in the effective use of
clean energy. Supercapacitors are considered promising ways to store
renewable energy [7]. The solar cell directly converting a solar photon into
electricity through its photoelectric effect or photochemical effect [5]. However, the performance of the available solar cells, supercapacitors, and batteries is unlikely to meet the demands for enormous storing energy. To address
this problem, it is urgent to pursue new carbon-based materials with improved
efficiency for energy storage and conversion devices [7].
Research toward sustainable, clean and environmentally friendly renewable resources to develop carbon-based materials used in energy storage and
conversion devices has been prompted by the challenges of environmental
pollution degradation [8]. The excellence of carbon materials such as quantum
dots (QDs), carbon quantum dots (CQDs), and graphene quantum dots (GQDs)
have gained significant interest in several potential fields, like broad surface
area, controllable structure, conductivity, high durability and low toxicity, etc.
[9e11]. These materials have a very promising and remarkable potential in the
area of energy storage and conversion systems, and electrode materials [12].
The development of carbon-based nanomaterials with a new generation and
forming a more sustainable energy materials industry can be seen as a potential precursor. Recently, biowaste-derived carbon materials have revealed
potential applications in the area of energy storage and conversion. However,
further work is needed to commercialize carbon materials derived from
biomass for adequate performance, operation, and productivity [13].
Due to their chemical inertness and lower photobleaching and low toxicity,
carbon-based QDs (such as GQDs) have several favorable conditions over
noncarbon QDs. They can be formed from biomass for instance. Carbon-based
QDs have been studied as supercapacitors, batteries, water splitting devices,
LEDs, solar cells, biosensors, and catalysts in recent years and have also been
mixed in optoelectronic devices with noncarbon dots [14].
The latest research progresses on the preparation and applications of GQDs
are covered in this chapter. Acid etching [15], hydrothermal methods [16],
ultrasonication [15], electrochemical exfoliation [17], microwave-assisted
hydrothermal [18], and carbonization [15] were discussed. Furthermore, the
potential applications of GQDs, such as energy storage devices (supercapacitors, batteries) and conversion devices (solar cells), are thoroughly
discussed [19]. The analysis demonstrates that GQDs outperform conventional
semiconductors and will be employed in a variety of innovative materials and
clean energy storage and conversion devices Scheme 10.1.
Graphene quantum dots for clean energy solutions Chapter | 10
185
SCHEME 10.1 GQDs in electrochemical clean energy devices. All the devices shown in the
image can benefit from the improved performance afforded by GQDs electrodes. Reproduced with
the permission of Kumar R, Sahoo S, Joanni E, Singh RK, Maegawa K, Tan WK, et al. Heteroatom
doped graphene engineering for energy storage and conversion. Mater Today 2020;39:47e65.
10.1.1 Challenges of clean energy
Environmental changes, energy protection, and monetary solidity are inseparable related [21]. Without understanding and moving against this fact, the
eager targets that were set to reach all of the difficulties can never be fully
understood. The world will be facing an overwhelming test in the coming
century to monitor the world’s financial growth flexibly with constant and
moderate resources, without creating insupportable disruption to the world’s
atmosphere due to fossil fuel use. The challenge involves two key proportions
[22,23]:
l
In the coming decades, global energy will remain dominated by fossil
fuels, mainly with the increase in energy demand in developing
countries: The International Energy Agency’s (IEA) latest policy scenario,
which anticipates that recent government promises will be thoroughly
implemented, will allow global demand for primary energy to increase by
one-third between 2010 and 2035 with 90% of the non-OECD growth. The
percentage of fossil fuels in global primary energy consumption has fallen
186 Graphene Quantum Dots
l
around 81%e75% in 2035. Renewables energy increased from 13% of the
mix today to 18% in 2035.
Energy destitution in nonindustrial nations stays a worldwide challenge in near future: Indeed, even with quickly developing economies in
some nonindustrial countries, the insights are still stunning. Today, more
than 20% of the world’s population need access to electricity, and 40% of
the world’s population depends on traditional biomass cooking methods.
It’s extended that the difficulty will endure and even develop in the more
drawn-out term: 1.2 billion individuals need admittance to power in 2030,
and the quantity of individuals depending on the conventional utilization of
biomass for cooking ascends to 2.8 billion out of 2030. The far and wide
utilization of biomass prompts deforestation and the natural atmospheric
effect of dark carbon, which is a significant part of a worldwide temperature alteration. Much more terrible, the family air contamination from the
biomass utilization in inefficient ovens would prompt over 1.5 million
unexpected losses every year in 2030 more regrettable even than unexpected losses from jungle fever, tuberculosis. Many attentions have been
drawn to the environmental concerns of renewable and sustainable energy
research and commercialization, and high-energy and high-power storage
technologies are imperative for the rapidly growing energy demand.
10.1.2 Clean energy solution
The energy generated by employing renewable energy implies that the environment is not polluted. Sustainable energy sources that do not have a natural
duty may also be applied to spending money that cannot be supplanted or
seriously damage the environment so that citizens in the future have to take
care of today’s problems. Who doesn’t need clean or green energy? Yet,
conjure up a scenario in which this cost somewhat more. We live in a period
where all that we use and all we collaborate with inside our everyday lives
must be clean. Therefore, it is worthy to state that perfect energy arrangements, including supercapacitors, batteries, photovoltaic devices, and lighttransmitting diodes, must be clean too. As to the clean energy portfolio, we
have to make it under these clean energy arrangements [24]. With the created
front line innovations and computerized reasoning applications, we have to
change the game-plan in managing energy matters, covering the whole energy
range under these classifications, energy basics and ideas, energy materials,
energy transformation, and energy the board. With the worldwide energy
emergency and environmental change concerns, it is turning out to be
increasingly clearer that we have to change the course of action and change
from traditional techniques, approaches, frameworks, answers for a spotless
energy portfolio where the practicality arrangements are focused on. Changing
to clean energy arrangements doesn’t imply that we can disregard the ideas
Graphene quantum dots for clean energy solutions Chapter | 10
187
and basics which should be treated as a structure that can’t remain without
columns. Energy arrangements can’t make do without ideas and essentials
[24]. Through a source-system-service approach, renewable energy systems
provide promising solutions that require explicit tasks to be refined to provide
a complete and sustainable base, as shown in Fig. 10.1. It is crucial to select a
perfect wellspring of energy, to begin with.
There are surely a few standards to hold up under as a top priority, for
instance, wealth, attainable neighborhood quality, moderateness, trustworthiness, security, natural effects, and so on. Most of the promising options appear
to be renewables. Next, it is basic to look at framework misfortunes and
proficiencies on top of choosing a perfect clean energy source. By and large, a
system can be explored through the following important steps:
l
l
l
l
For minimum degradation and the maximum number of usable outputs,
process upgrade.
Device integration with enhanced outcomes for extrareliable service.
Multigeneration by using the same power input to maximize the number of
valuable items.
Growing performance by defining and modifying loss causes.
When it comes to the service stage, which can be called an application
step, it is equally necessary to mitigate losses, irreversibility, waste, and so on,
and to recover valuable resources such as heat from carbon-based materials
(such as CDs, CQDs, GQDs, etc.) [1]. Clean energy systems are needed to
resolve global energy concerns without adversely affecting the climate, the
economy, and human capital in the future as well as sustainability. Clean
energy strategies are intended to achieve the accompanying basic objectives
for better sustainability [24].
FIGURE 10.1 The source-System-Service path to sustainability. Reproduced with the permission
of Dincer I, Acar C. Smart energy solutions with hydrogen options. Int J Hydrogen Energy
2018;43:8579e8599, copyright 2018, Elsevier.
188 Graphene Quantum Dots
10.2 Theoretical background
10.2.1 Quantum dots background and creation of graphene QDs
Since the 19th century, carbon has been widely explored and has continuously
expanded in studies as one of the most abundant elements. Fig. 10.2 shows the
complete development of carbon content. In 1859 Brodie synthesized graphite
oxide in several risky experiments to determine the molecular weight of
graphite and coined the name “Graphon.” Kohlschutter and Haenni did not
define the properties of graphite oxide at that time until 1918. Bernal
continued work in 1924 with single-crystal division measurements on the
structural properties of graphite oxide. As for advances in characterization
technology, Wallace first studied graphite electronic properties and suggested
the graphene principle in 1947 [25]. One year later, a few layers of graphite
were successfully identified by transmission electron microscopy (TEM) by
Ruess and Vogt. In 1957, a new process for preparing graphite oxide was
developed by Hummers and Offeman, which was more productive and environmentally friendly as compared to Brodie’s method. Boehm introduced
graphene in 1962 and after that, the theoretical study was done by David
DiVincenzo and Mele in 1984 using a massless Dirac equation, which was a
very unconvincing thing during that time. Smalley discovered a soccer-shaped
FIGURE 10.2 Development of carbon materials [25].
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189
fullerene during the production of carbon materials in 1985. It is the first
carbon allotropy to be used by scientists around the world. In 1987 Mouras
introduced ’graphene’ as a single graphite sheet, although theoretical calculations left the stable graphite sheets inexistent. The other carbon allotropy was
discovered by Lijima 1991, identified as carbon nanotubes (CNTs). More than
a decade later, A. K. Geim and K. S. Novoslov performed a very outstanding
graphene experiment using scotch tape technology in 2004 at Manchester
University, leading to monolayer graphene. This extraordinary work has been
open a wide range of 2D material science and offers a new technology for
different applications. Geim and Novoslov were awarded the Nobel Prize in
2010 for this remarkable experimental work on graphene. The second time
Novel prized was awarded to researchers for working on the carbon materials.
The various form of the carbon materials, such as 3D (graphite), 2D (graphene), 1D (nanotubes), and 0D (quantum dots), have been successfully
explored till now.
After the discovery of graphene, the scientific community has explored the
properties and the application of graphene. However, during the study of
graphene, scientists have observed many problems of graphene, like no
banding in the band and less absorptivity, etc. To reduce this drawback, the
modification in structural properties of the graphene was subsequently
investigated.
Ponomarenko was successfully developed the GQDs in 2008 inspired by
Xu et al. experiment work on carbon dots (CDs) in 2004. The GQDs consist of
graphene lattice in the dot, which has a size of 100 nm and 10 layers of
graphene, due to which GQDs and CDs have a different structure [26]. The
structure of CDs is typically quasi-spherical carbon nanoparticles (NPs) with a
size of approximately 10 nm [26]. Due to the quantum confinement effect in
the GQDs, Pan et al. observed fluorescent properties of the GQDs in 2010 [27].
The doping in the GQDs was first introduced by Zhao and their colleagues in
2012 by using it as a dopant in GQDs to change their physical and chemical
properties [28]. Moreover, GQDs have larger solubility in comparison to CNTs
due to the wide edge effect, this can be changed by using different functional
groups, on the other hand, CNTs have a limitation due to 1D characteristics.
The 0D GQDs have been developed by changing 2D materials but have a
very large Bohr radius of excitement [29]. These results indicated quantum
confinement and wide edge effects in the GQDs, and the crystal edge changed
the electron distribution by reducing the structural size at the nanometer scale.
Additionally, GQDs possessed a bandgap but graphene has zero bandgaps. The
band gaps in GQDs have been demonstrated by theoretical studies and also
experimentally confirmed by optical and electrical measurements. The semiconductor or insulator GQDs can be transformed by semiconducting graphene.
The expansion of graphene optical absorption improved energy range since the
bandgap is opened in GQDs. GQDs are often chemically and physically
distinguished by their special edge and quantum captivity effects compared to
190 Graphene Quantum Dots
other carbon materials, such as graphene, CDs, and CNTs, etc. In the field of
nanomaterials, the manufacturing methods typically affect numerous physicochemical properties of nanomaterials. In recent years, there are many
techniques have been developed to synthesized GQDs, based on the precursor
material, these techniques are classified into two major sections, (1) top-down
and (2) bottom-up [25]. The edge and quantum confinement effect are the
fundamental properties of the GQDs. These fundamental properties of the
GQDs provided remarkable physical and chemical properties, such as solubility, surface grafting, nontoxicity, and biocompatibility [25].
This chapter covers the different uses of these fascinating materials. We
will focus on optoelectronics and energy application and introduce us briefly
to other applications. Also, we provide a perspective for GQDs, with possible
applications and trends in growth. With numerous outstanding reviews
focusing on various aspects of GQDs including their synthesis and environmental applications, we expect this segment will be a valuable insight into and
inspire new thinking and more study on the current state of research on optoelectronic devices clean energy systems in GODs.
10.2.2 The outlooks of graphene quantum dots
The study of GQDs is still in its early stages and the researcher must focus on a
variety of other issues, as shown in Fig. 10.3. However, there are numerous
FIGURE 10.3
In line with the latest study, the challenges of GQDs [25].
Graphene quantum dots for clean energy solutions Chapter | 10
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significant favorable circumstances and potential uses of GQDs which is
deciding further analysis to explore the properties of the GQDs material and
achieved the requirements of the application. As a result, GQDs studies have
been ongoing to resolve the five crucial issues as mentioned in Fig. 10.3. It is
necessary to produced GQDs at a large scale with minimal cost to meet the
business prerequisites. However, the production yield of GQDs by using
present techniques is significantly poor (<10% in general), so new strategies
should be used, such as the photograph Fenton answer strategy the best return
of 45% [30] and simple production method using coals, which can increase the
product yield. Furthermore, the observed quantum yield (QYs) of GQDs is
typically between 2% and 22.9% [30], which is significantly lower than that of
standard semiconductor QDs. Therefore, low QYs is the biggest challenge.
The present study demonstrated that the quantum yields of GQDs could be
modified by surface engineering, for example, the preparation of metal-carbon
composites. Moreover, the electrical and optical properties like bandgap [31],
fiuorescence [32] and other properties are dependent on the dimension of the
GQDs. The precision in managing the dimension of GQDs, as well as other
stages of production techniques, are the most important variables in resolving
these problems. GQDs have shown different exceptional marvels, for example,
upconversion photoluminescence (PL) [33]. Although the current systems
were for the most part obtained by the optical practices prompting befuddling
results because of the diverse readiness conditions, thus there is an absence of
comprehension of the specific PL component.
Subsequently, a few types of standard estimations combined with hypothetical examinations should be completed to more readily comprehend the PL
marvel. Until this point, most of the announced PL shades of GQDs were gone
from blue to yellow [30]. The restricted phantom inclusion of GQDs is
restricting its use in the optoelectronic field. Extending the otherworldly inclusion of GQDs to every noticeable frequency and near-infrared (NIR) is a
significant territory of examination later on. A few groups have been reported
NIR PL spectra in the GQDs by using nitrogen (N) as a dopant in the GQDs
[34].
When the above issues are overcoming more efficiently and costeffectively for nanomaterials in the future. This chapter discusses the
already established synthesis techniques for GQDs in Section 10.3. In Section
10.4, the physicochemical properties such as electronics and doping synthesized its nanocomposite with other promising materials. Some GQDs related
applications will be reviewed in Section 10.5. Last, the conclusion will be
presented.
10.3 Methods for the synthesis of GQDs
A good class of monodisperse GQDs have been synthesized by extensive attempts. Numerous techniques are very important for the synthesis of GQDs
192 Graphene Quantum Dots
from different materials, including biomass waste. The GQDs synthesis process can be divided into two main groups: one is bottom-up and the other is
top-down (Fig. 10.4). In previous techniques mentioned that the GQDs
structures can be synthesized by the exfoliation of 3D materials, 2D sheets,
and 1D graphic materials such as carbon nanotube and fiber, etc. using scotch
tape. Therefore, these techniques are capable of obtaining strong graphitic
behavior. Some techniques such as electrochemical exfoliation [35], ultrasonication, acid etching [36] and hydrothermal synthesis [27] etc., are examples of top-down techniques this method initiates through the defects made
by especially epoxy groups, which promote fragmentation. That’s why it
would help gain a large yield of GQDs along their edge, consisting of a large
number of eCOOH and eOH functional groups. The solubility is enhanced by
these high-density functional groups, which existed on their edges. On the
other side, the GQDs also prepared by chemical synthesis using molecular
precursors, where we can regulate the moieties of graphene. These bottom-up
techniques include precursor microwave synthesis [37], and carbonization etc.
Apart from this, the bottom-up techniques can optimize the shape and the size
of the GQDs results in reduced nonsize selection problems, which is present in
the top-down techniques. But the main disadvantage of these techniques is that
it involves very tough experimental environment and selection of the suitable
molecular precursor [38].
10.3.1 Top-down approach
10.3.1.1 Acid etching
This process is used for the exfoliation of GQDs from large precursors such as
carbon black, activated carbon, carbon fires, CNTs, graphene oxide (GO), and
coal by using strong acid (HNO3). The acidic solutions negatively oxidized
groups from GQDs (Fig. 10.4a), improve their properties by making GQD
surface hydrophilic and develop the defective spots. The highly rich defective
places influence the surface area of the GQD and enhance its performance.
However, the complete removal of oxidizing agents is a major challenge,
whereas this technique is appropriate for large solution production [15].
10.3.1.2 Electrochemical (EC) exfoliation
The electrochemical (EC) exfoliation technique is a streamlined one-step
procedure that consistently produces a high yield of GQDs by electrochemical slashing from CNTs [35], reduced graphene oxide (rGO) films,
chemical vapor deposition (CVD)-grown graphene [18], and graphite rods
[17]. Here, the consumption of O and OH radicals formed by anodized
oxidation of water acts as electrochemical scissions begin at edge spots and
accelerate at defect sites of release GQDs (Fig. 10.4b).
Graphene quantum dots for clean energy solutions Chapter | 10
193
FIGURE 10.4 Two strategies for synthesizing GQDs: are “top-down” for larger molecules and
“bottom-up” for smaller molecules. Schematic illustration of GQDs fabrication by (a) acid etching
(b) electrochemical exfoliation (c), hydrothermal [10], (d) ultrasonication, and (e) carbonization.
Reproduced with the permission of Shen J, Zhu Y, Yang X, Li C. Graphene quantum dots: emergent
nano lights for bioimaging, sensors, catalysis and photovoltaic devices. Chem Commun
2012;48:3686e3699; Ye R, Xiang C, Lin J, Peng Z, Huang K, Yan Z, et al. Coal as an abundant
source of graphene quantum dots. Nat Commun 2013;4:1e7, copyright 2019, Elsevier.
194 Graphene Quantum Dots
FIGURE 10.4 cont’d
10.3.1.3 Hydrothermal and solvothermal
Hydrothermal technology is a promising technique to prepare GQDs from
carbon-based starting materials with strong oxidizers such as H2SO4 HNO3,
H2O2, which are used to break apart carbon NPs into GQDs. The advantage of
the hydrothermal synthesis of GQDs is that the particle size of the GQDS may
be controlled by using different hydrothermal temperatures. To put it another
way, when the hydrothermal temperature increased, the size of the GQDs
particle reduced [39]. For example, Tian et al. used a hydrothermal process to
prepare GQDs with GO as a carbon source and H2O2 as a reagent. At high
temperatures, H2O2 is dissociated into OH radicals, which then thermally
split graphite sheet into fragments as the hydrothermal reaction activate. The
results of the experiments indicate that graphite disintegrates at higher temperatures [40]. Pan et al. has discovered a hydrothermal path for the development of blue light GQDS. In concise terms, through controlled oxidation in
an ultrasonic combination containing H2SO4 and HNO3, the graphene layers
were chopped into tiny pieces. The oxidized tiny graphene sheet was subsequently reduced in a Teflon walled autoclave at a high temperature under
hydrothermal conditions (Fig. 10.4c). Using quinine sulfate as a reference, the
GQDs produced had an average diameter of 9.6 nm containing one to three
layers of graphene and a QY of 6.9% [27]. Shen et al. synthesized GQDs by a
single-step hydrothermal method utilizing polyethylene glycol (PEG). As
initial materials, a GO sheet and PEG was utilized. The monodisperse GQDsPEG resulting from this process had a regular diameter range of 5e25 nm
[10]. In comparison to bare GQDs, the prepared GQDs-PEG showed good
luminescent capabilities and the PL QY of 360 nm emission was around 28%
using rhodamine B (RhB) as reference [16].
10.3.1.4 Ultrasonication
Ultrasonication technique prepares GQDs in a low-cost and eco-friendly
manner by simply cutting bulk precursors using mechanical force. Graphene
sheets can be chopped into GQDs with ultrasonic treatment after several purification steps with the assistance of solvothermal methods as shown in
Graphene quantum dots for clean energy solutions Chapter | 10
195
(Fig. 10.4d) [41]. Additionally, graphene sheets can be oxidized under acidic
conditions with the help of ultrasonication [42]. Furthermore, multiple steps
are required for this approach to produce uniform GQDs, including oxidizing
in acidic solution and solvothermal or microwave treatment [15]. Zhuo et al.
demonstrated GQDs produced from an ultrasonic technique by oxidizing
graphene in concentrated HNO3 and H2SO4 solution at room temperature for
12 h. After that, the solution mixture was subjected to an ultrasonication for
12 h using an ultrasonic device. The resulting mixture was then calcined at
350 C for 20 min to remove the concentrated HNO3 and H2SO4 [42]. It is
important to note that the materials as acquired are redispersed in water. The
next thing to filter the obtained black suspension was a 0.22 mm microporous
membrane for obtaining a brown filter solution. Finally, the solution for GQDs
has been further dialyzed [43].
10.3.2 Bottom-up approach
10.3.2.1 Carbonization
Carbonization, also known as pyrolysis, is the process of condensing organic
molecules by heating them over their melting temperatures, which promotes
nucleation and contribute to the formation of GQDs (Fig. 10.4e) [10]. This
technique is easy and low-cost, has a large-scale capability, and permit a
natural legacy of heteroatoms from precursors such as glycerol, L-glutamic
acid, ascorbic acid, citric acid, and coffee beans. Besides, plasma-assisted and
microwave incorporated carbonization measures were additionally utilized
other than a simple combustion method for prepared GQDs [15].
10.3.2.2 Microwave-assisted hydrothermal (MAH) method
GQDs are usually produced via the hydrothermal technique, which takes a
lengthy period. Therefore, a quick method was first developed by assisting
with microwave, i.e., the microwave-assisted hydrothermal (MAH) method
[44]. MAH is used to synthesize GQDs using a microwave, which has the
advantages of both microwave and hydrothermal procedures. It was developed
by Lau’s team to make water-soluble GQDs using glucose as a precursor. The
microwave heating, which results in rapid heating, simultaneous, and uniform
contributes to the homogeneous distribution of QDs [38]. When compared to
other QDs the reported emission energy of the GQDs was 4.1 eV, which was
the highest energy emission at the relatively short emission wavelength [45].
In 2012, Tang et al. explained this for the first time with the GQD, which was
operating a 197 nm beam of light. The GQDs can be produced by employing
the glucose precursors using the MAH method in Fig. 10.5a.
Fig. 10.5b showed a way to develop GQDs with different useful functional
groups with no reagents on surface passivation or inorganic chemicals [34].
Dehydration of the glucose molecules results in nucleation crystals and
196 Graphene Quantum Dots
FIGURE 10.5 (a) Schematic diagram of the synthesis of GQDs by hydrothermal microwaveassisted (MAH) technique and (b) The GQDs formation process with functional groups. Adapted from Eda G, Lin YY, Mattevi C, Yamaguchi H, Chen HA, Chen IS, et al. Blue photoluminescence
from chemically derived graphene oxide. Adv Mater 2010;22:505e509. Copyright, 2012 ACS
Publication.
chemically active functional groups attached to the surface of the GQDs. It
pyrolyzed glucose molecules before converting them into GQDs. Monodisperse GQDs were synthesized using GO precursors under radiant energy in
another study, with limitations applied at 200 C for 5 min. Then the pH was
then neutralized with Na2CO3 (sodium carbonate), centrifuged, and the supernatant was then separated as GQDs. GQDs can also be coupled by varying
the solvent and level of doping [16].
10.3.3 Green approach
Many researchers have continued to introduce and enhance new production
methods for GQDs in addition to the many synthesis methods listed in the
above section. To achieve those characteristics and features, the synthesis of
GQDs was very important. All the processes in the above section are an
effective attempt to establish environmentally friendly, low-toxic alternatives
for quantum semiconductor QDs. The real challenge is to eliminate by-
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products such as some acids and inorganic salts even though has a high quality.
Moreover, using these methods in the manufacture of high-quality synthesized
materials is nearly challenging to implement. GQDs are environmentally safe,
nontoxic and photosensitive compared to QD semiconductors and organic
dyes. GQDs are synthesized using green synthesis methods using different
types of carbon sources such as peels, fruit extracts, food waste, algal flowering, and bacteria [46e48].
When compare to the conventional synthesis, the green chemistry approaches have high morphological variance and UV absorption alternatives to
traditional synthesis. GQDs was synthesized by Teymourinia et al. with the use
of maize powder as a precursor [48]. RhB photodegradation has been screened
under UV light radiation by synthesized GQDs. GQDs/TiO2 shows excellent
photocatalytic behavior for RhB degradation compared to conventional metal
oxide (TiO2). The degradation rate is around 53% in 80 min. The temperature
and reaction time are controlled by different color emissions under UV light
and normal light [48]. Furthermore, Chen et al. produced GQDs using natural
polymer starch and efficient and environmentally friendly hydrothermal
techniques. GQDs, water, and carbide precipitate were produced, with GQDs
ranging in size from 2.25 to 3.50 nm in diameter [49]. Anooj et al. (2019)
investigated the physical and chemical properties of GQDs made from Nutmeg
seed utilizing green and hydrothermal procedures at 150e200 C for 6e10 h.
The GQDs that resulted exhibited high optical absorption in the UV range of
TABLE 10.1 Summary of GQDs preparation through green chemistry.
S.
No
Precursors
Methods
Parameters
References
1.
Cabbage, deionized
water
Hydrothermal
Heating at140 C for
5h
[51]
2.
Pyridine-2,6dicarboxylic acid,
salicylic acid, double
deionized water
UV
irradiation
Heating at 50 C for
2 h under UV light
and then cooled to
5 C
[47]
3.
Corn fine powder,
deionized water
Hydrothermal
Heating at180 C for
8 h and centrifuge at
14,000 r.p.m
[48]
4.
Coconut husk,
deionized water
Hydrothermal
Heating at 200 C for
3 h and centrifuge at
15,000 r.p.m
[52]
5.
Mango leaves
(Mangifera indica),
ethyl alcohol
Heated under 900 W,
centrifuge at 8000
r.p.m, dried at 65 C
[46]
198 Graphene Quantum Dots
260e320 nm [50]. Table 10.1 lists the many precursors, methods, and
experimental parameters used in the green chemistry production of GQDs. The
main advantage of using these precursors is that they are safe, easy to use, and
almost often non-toxic. Using green chemical technology, biomass carbon
supply and low reaction temperature with marked fluorescence proofs
depending on the surface functionality can carbonize and work [16].
10.4 Physicochemical properties
10.4.1 Electronic properties
The most interesting properties of the semiconductor NPs as a function of size
are the enormous bandgap variety. The electron wave function was confined to
its size when the dimension is reduced from the 3D bulk structure to 0D QDs.
Therefore, the energy states of the electron are limited and also the sizedependent because the motion of the electron in the 0D was restricted. The
radius of the bulk Bohr-excitation is greater than the QD size, the boundary
potential is very high results in the wave function of the holes and the electrons
are constrained. Under such conditions, the system can have discrete energies
level. According to the Brus equation, the exciting energy for the first two
energy states will be larger than the bandgap, which depends on the size of the
QDs [53]. A previous theoretical study on the quantum confinement effect in
GQDs indicate a reasonable change in the position of the chaotic neutrino
(Dirac) billiard for mass fewer fermions by varying the dimension 10e40 nm
of GQDs. This change was observed in the position of chaotic neutrino (Dirac)
billiard due to charge impurity, short-range scatters and edges roughness,
which produced disorder in the structures. However, Guttinger et al. applying
magnetic fields perpendicular to the graphene plane to experiment studied the
energy states of GQDs and performed some transport measurements on GQDs
[54]. In this study, the main aim is that applying the perpendicular magnetic
field results in excited state spectra in the near vicinity of the charge neutrality
point and signatures of the electron-hole crossover. An 80 nm long and 50 nm
wide dot coulomb blockade resonance was detected around the transport
distance ranging from hole to electron transport at all gate voltages.
10.4.2 Doping
Heteroatom doping is a much easier way of producing electronic materials.
GQDs sensitivity or reactivity to specific environments based on multiple
heteroatomic doping (like F, N, Se, S, etc.) allows and can be used in multidisciplinary and many different areas. The improvement in the intensity of
photoluminescence and the wavelength of emission is one of the intriguing
benefits of doping. Moreover, the doping of nitrogen is the most favorable and
popular doping technique due to similar in carbon size. Because the radius of
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the carbon is about 70 p.m. and nitrogen is about 56 p.m. [55]. The previous
theoretically study observed by nitrogen doping produce a shift in the Fermi
level position at the Brillouin zone apex, where the carbon-bearing positive
charge is adjacent to nitrogen [56]. Doped GQDs have a remarkable contribution in sensing applications by utilizing such extraordinary properties.
Latest studies suggest that N-doped GQDs with functional groups abundant in
oxygen can be used to selectively detect Fe (III) ions [57]. Some other groups
have also published the synthesis of fluorine (F), nitrogen (N), sulfur
(S) codoped GQDs in the presence of anionic liquid using a microwaveassisted technique [55]. Selenium-doped GQDs were produced in another
research and shown to be an ultrasensitive redox fluorescent switch for OH and
glutathione (GSH) detection. It was attributed to the reversible oxidation of the
groups CeSe and SeeSe. The electronic configuration of the doped graphene
moieties has produced a clearer image of the doping environment simultaneously as theoretical experiments exploring the bond relation between the
carbon atom and dopants. Bottom-up methods by Li et al. also synthesized
nitrogen doped GQDs (N-GQDs) [58]. The synthesized QDs demonstrated an
improved catalytic behavior of the oxygen reduction reaction (ORR). A new
doped graphene analogue, C3N, known as hole-free graphene extension, was
seen in a recent study [59]. The synthesized QDs demonstrated an improved
catalytic behavior of the oxygen reduction reaction (ORR). A new doped
graphene analogue, C3N, known as hole-free graphene extension, was seen in
a recent study [59]. However, the crucial problem yet to be discussed is the
specific regulation of the doping percentage for tuning the intrinsic properties.
The Wu group used improved molecular melting procedures to describe the
first application of highly GQDs codoped with oxygen and nitrogen in MSCs,
with oxygen and nitrogen concentrations as high as 17.8% and 21.3%,
respectively [60].
10.5 Applications of GQDs in energy storage and
conversion devices
As population growth continues and resources are cut, the quest for fresh,
renewable energy for the future is a pressing problem. Apart from biomass,
multiple types of precursors have been used (such as carbon fiber, graphene,
and CNTs) to prepare GQDs for various applications. GQDs from biomass
have similar properties as seen in the synthesis section to those derived from
precursors with graphene. GQDs with much high conductivity was therefore
implemented in energy applications regardless of the predecessor being used
[17,61,62]. GQDs have shown promising energy applications, such as supercapacitors [63], lithium-ion batteries [64], solar cells are summarized in detail.
200 Graphene Quantum Dots
10.5.1 Supercapacitors
There are two main factors that possibly contributed mainly to the progress of
renewable energy storage devices and technologies, one is a large amount of
energy consumption and the other is a continuous growth of technology [65].
The storage and conversion of electrical energy systems subsequently became
an appealing option and thus an emerging area of research in the academic and
industrial areas. An electrochemical energy storage system (EESS) has
capable of the production of electricity from the chemical [66]. EESS has
attracted considerable interest due to high charging rates and long-life expectancy [67]. The supercapacitor has the essential method of the EESS,
which is capable to resolve the current situation and the key sources of
electricity. This type of supercapacitor also known as an electrochemical
capacitor which provides a fast charging-discharging process and offers a
high-power density as well as long-term reliability for cycling. This technology enables them to be one of the best performers in the field of portable
electronics, electric cars, and backup power systems for EESS materials [68].
Along with capacitance and cycling stability, supercapacitors electrochemical
efficiency depends on the composition and structure of their probe materials.
As the energy storage media, carbon materials, transition metal oxide, and
have been developed until now [69]. Most recently, many researchers have
performed on the EESS properties of GQDs and their possible applications as
electrode properties. The GQDs can make it possible to develop advanced
energy storage materials since (i) the 0D structure of the conjugated carbon
skeleton is very elongated to produce complicated and conductive structures,
(ii) the improved edge assemblies and functional groups can provide large
quantities of active energy storage locations and (iii) good chemical reactivity
and migration properties enable fast assembly or processing of these locations
[66].
In supercapacitors, especially in conducting polymers (CPs), polymers also
play a crucial role as compared to metals due to their unique properties like
easy preparation technique, durability, high electron moment and redox action.
The excellent properties of CPs and of GQDs both enhanced the demand of the
supercapacitors. Jin et al., have recently been synthesized a very good
conductive polyaniline (PANI) by incorporating of GQDs to analyze their
electrochemical characterization [70]. The modifications in the electrochemical characteristics of GQDs@PANI depend on the amount of GQDs
incorporated into the PANI. GQDs@PANI is used as a potential electrode in
the supercapacitor due to their maximum specific capacitance [70]. Syed et al.
(2018), was also fabricated PVA-GQD/PEDOT nanocomposite as a potential
electrode for the supercapacitor due to their high current and fast reaction. In
GQDs, the surface area is very large therefore these unique properties were
higher in the PVA-GQD/PEDOT as compared to the PEDOT, which helped to
enhanced the charge accumulation and storage capacity. It is observed in the
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PVA-GQD/PEDOT nanocomposite that electrolyte ions are diffused into the
surface of the potential electrode with increasing the scan rates by analyzing
the CV curve. Therefore, at a high scan rate, the ions are not capable to reach
the potential electrode because the specific capacitance (Csp) value is very low.
Moreover, it is also found that the stability in the PVA-GQD/PEDOT nanocomposite is much higher in comparison to PEDOT [71].
GQDs have been recently produced for high-efficiency supercapacitors by
Mohammad et al. (2017) and found it has a 5.5% capacity higher than naked
carbon fiber. They show a maximum degree of holding rate of 97% following
5000 cycles [63]. Among all graphene-based materials, GQDs have excellent
performance (97.8% stability after 5000 cycles [63] and Csp: 1044 F ge1 at
1 A ge1) [72]. The rich ion-connecting edges, wide, particular surface areas,
abundantly available edges, flaws, functional groups and GQDs mobility sites
ensure improved capacity [13].
10.5.2 Lithium-ion batteries
Carbon emission and increase demand for fossil fuels are the main concern of
the scientific community. To resolve the above issues the researcher more
focused on renewable energy sources (such as solar cells, Photoelectrochemical and thermoelectric), which is very important for the
improvement of sustainable energy technology and renewable energy sources
in human society. The development of electrochemical energy storage (EES)
devices has drawn various researchers’ interest for this purpose. Reliable EES,
such as supercapacitors and batteries, are important components that make it
possible to build these energy systems [73].
The lithium-ion (Li-ion) batteries are more indisputably the important
solution for other energy storage devices as compared to the currently used
other renewal energy sources such as lead-acid, nickel-metal-hydride (Nie
MeH), nickel-cadmium (NieCd) and supercapacitors. The Li-ion batteries
have many qualities such as good performance, lightweight and very good
stability. These qualities made lithium-ion (Li-ion) batteries is the most useful
and reliable renewable energy source and it also reduced the demand for energy in the world. In recent years, advanced and powerful technologies have
been developed in the field of batteries and fabricated different types of batteries, such as sodium-aluminum, and aqueous metal-ion batteries, and still,
research is going on to improve their total performance for potential realistic
applications [73,74]. A Li-ion battery typically contains five important parts,
which are the anode, cathode, electrolyte, exterior case, and closing components. Many types of Li-containing cathode materials have been studied as of
now, such as lithium manganese, Li-ion phosphate, lithium cobalt oxide, CPs,
V2O5 and FeS2, etc. The anode in the batteries are commonly fabricated by
metallic lithium, graphitic carbon, hard carbon, synthetic graphite, and
silicone-based products, which are easily accessible in the marketplace [74].
202 Graphene Quantum Dots
Solutions of electrolytes such as LiClO4, LiPF6, LiCF3SO3, and LiAsF6 are
used. Li-ion batteries have been generally used to provide power in many
transportable electrical devices due to their zero-corban emission and greater
energy capacity. However, in Li-ion batteries, certain harmful heavy metals
and chemical electrolytes are often used, which may cause major environmental contamination, such as cobalt and flammable organic solvents [73,75].
Li-ion rechargeable batteries are generally used in daily life very frequently
these days and can perform a very significant function in energy generation
applications, compact electrical structures in the current scenario. Researchers
have experienced the difficulties of generating high specific power and high
energy density that are inadequate to meet increased energy demand technologies such as electric cars and grid energy storage. Several scientists are
trying to find a better battery technology revolutionizing the existing systems,
to address these issues [75]. The unique features of graphene materials gained
significant interest, which greatly eliminates other EESS electrical material
replacements, such as Li-ion batteries and, supercapacitors devices, etc. [76].
The quantum confinement effect of the GQDs produces a restricted bandgap in
the substance that internally alters the electron conductivity. Lithium diffusivity is caused by quantum confinement, which affects the battery’s electrochemical stability and long-life cycling [77]. GQDs are fascinatingly expanded
to their top with oxygen functional units, which have also been noted for
special features properties like excitation e and no bandgap. Also, due to its
small scale, GQDs are supposed to be able to protect the target material
adequately. All of the GQDs have been designed to be a cover medium for
energy storage systems [78]. The addition of GQDs increases the electron
transport process through the electrolyte, resulting in enhancing the efficiency
of the Li-ion batteries [64]. The primary reason for employing GQDs as a thin
layer on the electrode which the development of the wide surface area between
the active material and the electrolyte for ion transfer for ultrafast storage and
energy release [79]. Li-ion batteries show superior high-rate performance and
cycling reliability with the above characteristics of composite coated GQDs
and other metal as an electrode. Chao et al., tested GQDs-coated a VO2
content as a Li-ion batteries electrode with improved electrochemical behavior
[64]. The usage of GQDs in batteries has reportedly resulted to increased
battery performance. Such performance improvements with GQDs are
importantly highly cost-effective. Consequently, applying GQDs in batteries is
possible and commercially viable to provide superior energy storage [80].
10.5.3 Solar cells
A solar cell is an electrical system that transforms light energy into electricity
directly via a photovoltaic effect. The solar cell performance can be improved
by the incorporation of GQDs into advance solar cell materials. GQDs is very
good at the extraction of the charge carriers generated by the light. In addition,
Graphene quantum dots for clean energy solutions Chapter | 10
203
GQDs is also helping to upconversion of light due to their tunable bandgap
properties and it is very low-cost materials for solar cell application. GQDs are
very useful and important materials for improving efficiency and reduce the
cost of the different solar cells such as dye-sensitized solar cells (DSSCs) [81],
inorganic and organic solar cells [82] and perovskite solar cells [83]. GQDs are
widely used for photovoltaic devices as a sensitizer, a hole electron transfer
surface or an active layer additive and have high electron mobility, broad
specific surface area and an excellent ability to absorb and convert visible light
from ultraviolet light [80].
10.5.3.1 Dye-sensitized solar cell (DSSC)
A DSSC is a kind of solar cell (SCs) that is developed as a substitute for
standard SCs in moderate photovoltaic (PV) devices with appealing photoelectric conversion efficiency (PCE). As shown in Fig. 10.6, the DSSC’s
2þ
þ3
þ
universality structure contains an I
3 /I , Co /Co , and more recently Cu /
2þ
Cu filler between the electrodes. A dye-sensitized photoanode (TiO2) is
used, as well as a platinum counterelectrode are used [84]. GQDs attractive
properties produce excellent results in a variety of fields, particularly in energy
storage applications. Furthermore, the multiple electrons are generated by a
single photon by the use of GQDs, whereas dye cannot. In this way, the GQDs
can produce multiple electrons to infuse in the dye-sensitized photoanode
(TiO2) as compared to dye, thus upgrading the Jsc and DSSCs. As a result,
GQDs play a role comparable to that of the dye in DSSCs.
Therefore, the excellent properties of GQDs can be converted into DSSCs
which was be used for enhancing the activity of the photoelectrode, resulting
FIGURE 10.6 Mechanism of the common design of DSSC. Adapted from Di Carlo G, Biroli AO,
Tessore F, Caramori S, Pizzotti M. b-Substituted ZnII porphyrins as dyes for DSSC: a possible
approach to photovoltaic windows. Coord Chem Rev 2018;358:153e177, copyright 2018,
Elsevier.
204 Graphene Quantum Dots
in to increase in DSSC efficiency. Here, GQDs are used as a co-sensitizer
photoelectrode material along with TiO2 [84]. Fang et al. (2014) produced
DSSCs using GQDs adsorbed on TiO2 as an electrode material. The results
showed that by incorporating GQDs, the photoelectrodes properties could be
significantly enhanced while the quantity of dye used could be condensed [85].
In another investigation, Mihalache et al. have created the TiO2-GQD cell
composite as an electrode sensitizer material made up of N3Ru dye, where an
enhancement in PCE was attained [86].
Kundu et al. (2012) demonstrated an improved PCE of 11.7% and a fill
factor of 71% for DSSC with an electrode surface of 0.16 cm2 after changing
the TiO2 photoanode with selective-size N, S, F-codoped GQDs (NSF-GQDs)
with a PL QY of 70%. There has been an upward increase in the level of
Fermi, which may lead to better performance and the potential to inhibit the
transmission of rear electrons from TiO2 [87]. This work shows that
the introduction of GQDs, controlled in size and hetero-nuclear, can improve
the efficacy of DSSCs and enable new optoelectronic applications.
10.6 Summary and perspective
GQDs have significant applications in different energy fields, such as energy
storage and conversion, because of their special structure and intrinsic properties. There has been less and less research on supercapacitors using moderate
GQDs as electrodes in recent years. Instead, with GQDs as conductive agents,
pseudocapacitive supercapacitor studies have gradually increased. However,
there is a decrease in the conductivity of GQDs by their doping and modification. Some heteroatoms can be doped to have pseudocapacitive properties
for GQDs. The application of GQDs in nanomaterials improves charging
carriers extracting ability to increase the power of the SC due to its special
optoelectronic characteristics, field luminescence improvement, and bandgap
modulation properties. GQDs are also widely used for photosensitive applications as biosensors, photon layer transport, or surface additives. GQDs can
effectively reduce Liþ or Naþ battery growth, increase ion diffusion and
electron transfer rates, increase electricity interface level and improve electrical efficiency. Since there is no bandgap in bulk graphene, there are significant limitations in other applications. GQDs have quantum confinement
and edge effects compared to bulk graphene, so in GQDs bandgaps modify
carrier behavior and expand their applications in a light-emitting diode. GQD
based energy converters may absorb, convert or transform heat into electricity
through light, humidity, pressure, and other energy types like electricity.
While in recent years, GQDs and their implementations have made great
strides, multiple issues remain unsolved. In summary, the first problem is that
the definite dimensions of GQDs is not specified, which has a significant effect
on the following analysis of GQDs. Relatively minor variations will be
amplified for observational studies, even if the gap is only a few nanometers. A
Graphene quantum dots for clean energy solutions Chapter | 10
205
very wide size range of GQDs causes a lack of size specifications. The
physicochemical properties of QDs of graphene are also unclear in this regard.
Another issue that needs to be tackled urgently is the industrial development
and green synthesis of GQDs. The failure to commercial production has
created another hurdle to increase the production efficiency of GQDs, which
has a significant effect on its practical implementation. However, over the
years, the development of GQDs by green chemistry has gained more interest.
The green synthesis of GQDs is progressing through lower affluence, nontoxic,
and reduced energy consumption, but there is still demand further progress in
these directions. In addition, the merger of the friendly environmental chemical processes and inexpensive commercial production has adequate growth
potential. We assume that GQDs-based research in these directions would
show tremendous potential as well as considerable advantages in future.
Acknowledgments
The authors thankfully acknowledged the Aligarh Muslim University Aligarh for providing
the necessary manpower and a conducive research environment to prepare this scientific
piece of work. Authors are also obliged to the University Grants Commission, New Delhi,
India for financial support.
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Chapter 11
Graphene quantum dots for
optical application
Rameez Ahmad Aftab1, Aftab Aslam Parwaz Khan2, 3, Mohd Ayaz4,
Mohammad Nazim5 and Abdullah M. Asiri2, 3
1
Department of Chemical Engineering, Zakir Hussain College of Engineering and Technology,
Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 2Chemistry Department, Faculty of
Science, King Abdulaziz University, Jeddah, Saudi Arabia; 3Centre of Excellence for Advanced
Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia; 4Ministry of Higher
Education, Applied Biotechnology Department, Sur College of Applied Sciences, Sur, Oman;
5
Department of Chemical Engineering, Kumoh National Institute of Technology, Gumi-si,
Gyeongbuk-do, Republic of Korea
11.1 Introduction
There are numerous uses for carbon in the universe. Different allotropes of
carbon exhibit completely different properties, depending on the arrangement
of adjacent carbon atoms [1,2]. It has been a focus of research in the field of
nanotechnology since the discovery of graphene in the early 1980s due to its
extraordinary electronic [3], mechanical [4], and optical properties [5]. A
major research focuses since it was discovered, graphene’s remarkable properties have led to a plethora of research efforts. In addition to these fascinating
properties, graphene has several negative properties, such as no bandgaps, poor
absorption, and aggregation. In optoelectronic devices and semiconductors
graphene’s zero bandgap is a constraint imposed by its half-metallic nature [6].
To resolve this disadvantage, cutting graphene into nanometer-sized pieces or
reducing graphene’s dimensionality is a very promising approach. Essentially,
a quantum dot is a crystal of semiconductor material containing free charge
carriers that are quantum constrained in all three spatial directions. Quantum
dots are several nanometers in size. Quantum dots have electronic properties
that are in between bulk structure and an individual molecule, which is the size
and shape of the quantum dots.
Ponomarenko and Geim in 2008 developed GQDs based on work done by
Xu et al. on carbon dots (CDs) [7,8]. There has been much interest in new
classes of materials in recent years such as CDs and GQDs mainly due to their
unique optical characteristics.GQDs are carbon nanoparticles less than 100 nm
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00008-X
Copyright © 2023 Elsevier Ltd. All rights reserved.
211
212 Graphene Quantum Dots
in diameter and less than 10 layers thick. CDs are smaller than 10 nm in
diameter and smaller than 10 nm in thickness [9]. Due to the lattice structure
within the dots of graphene quantum dots (GQDs), this is possible. It was Pan
et al. who discovered that GQDs have extraordinary fluorescent properties
(strong blue emission) [10]. A hydrothermal route is used to fabricate GQDs.
A zigzag site with a triplet ground state is responsible for this strong fluorescence. The have attracted a great deal of interest over the past couple of
decades due to their wide variety of applications, including optoelectronics,
solar cells, light-emitting diodes (LEDs), and bioimaging [11]. Quantum dots’
tunable optical and electrical properties can be used for a variety of applications [12]. Despite their intrinsic toxicity and stability, colloidal QDs have
limited applications due to their inherent toxicity. Zhao et al. [13] examined
how doping altered the properties of GQDs with the addition of nitrogen.
It has been established experimentally and theoretically that GQDs, unlike
graphene, have nonzero bandgaps. A new class of zero-dimensional materials
were recently identified among all graphene derivatives: GQDs, whose dimensions are typically less than 20 nm [14,15].Ponomarenko et al. report that
these nanostructures exhibit unique quantum confinement and edge effects,
which enhances their possibility for use in nanoscale optoelectronics [16].
They differ from other carbon allotropes in both chemical and physical
properties due to quantum confinement. In addition, GQD investigations are
still in the primeval stage, and they need light shed on the various aspects. A
further investigation of GQDs is necessary to enhance their properties in order
to meet desirable application demands.Graphene is the preferred material for
GQDs larger than 22 nm, and Hummers’ method generally reduces a GQD to
one to three graphene layers [17,18]. According to the research findings, the
average height of GQDs increases with increasing length. Polygonal GQDs
have armchair edges, but circular and elliptical GQDs have zigzag edges. In
addition, Raman spectroscopy and high resolution transition electron microscopy (HRTEM) have shown that GQDs have the same crystalline structure as
graphene [19,20].
In general, GQDs with a zigzag edge and a size 7e8 nm show a metallic
nature.Energy gaps are ranged from 0 to 3 eV for GQDs with armchair edges,
where L1 is the length of the hexagonal edges [21], quantum confinement
effect corresponds with this. GQDs in their optical properties exhibit intense
peaking in the UV region between w4.6e6.2 eV and a tail shifting into the
visible region between w2.1 [10]. The first peak is a result of p / p*
conversion of C]C bonds, while the second peak is a result of n / p*
conversion of bonds of C]O [10]. GQD size increases from 5 to 35 nm,
shifting the absorption peak fromw6.2 to w4.6 eV because of its properties is
similar to those of graphene due to the quantum confinement effect [18]. Thus,
the determining factors for emission wavelength remain a matter of discussion.
A variety of parameters can influence the luminescent properties of GQDs,
including shape, size, excitation wavelength, functional groups, pH, and
Graphene quantum dots for optical application Chapter | 11
213
solvent [22e25]. This quantum dot’s main feature is its strong PL. Quantum
dots have been proposed to have PL due to band gap transitions corresponding
to conjugated p-domains and the size of the quantum dots may influence
these. Thus, the UV irradiation of quantum dots under differing conditions can
present different colors of quantum dots. There are also defects associated with
graphene structure such as the surface edge effect and defects associated with
graphene structure that greatly affect the property of PL.
However, PL’s origins remain unclear. As the GQDs size increases from
w17 to w18 nm, the photoluminescence peak energy decreases, which is
compatible with the quantum confinement effect. The size of GQDs and their
edge-state can then be controlled to improve PL emissions through electronic
shifts [18]. Hopefully, our current chapter will provide some new information
about GQDs and its derivatives that will help with exploring a wide range of
optical applications of GQDs. Thus, by controlling size and edge-state of
GQDs, the electronic shifts can be improved to generate strong PL emissions.
As a result, we anticipate providing a broader overview of GQDs and their
derivatives in terms of potential optical applications in photodetector applications [26,27]. There is a large variety of GQD optical applications to be
explored using LED, photocatalysis, etc. [28e35]. Synthetic routes such as
“top-down” and “bottom-up” approaches have been studied so far to fabricate
GQDs. Through a variety of methods, top-down approaches are used to synthesize and fabricate zero-dimensional GQDs derived from carbon source
materials. There are several methods that are used for fabricating components
with electron beam lithography, chemical oxidation, solvothermal, microwaveassisted hydrothermal, microfluidization, and electrochemical [16,36,37].
Smaller benzene derivatives are combined to form larger GQDs through the
bottom-up approach. A number of different procedures are presented in
Fig. 11.1., and these are categorized into two groups: (1) top-down and (2)
bottom-up.
GQDs were first fabricated using electron beam lithography, one of the topdown approaches. Due to the need for expensive equipment, this technique has
not gained widespread use. The 0D family of carbon materials was nonetheless
opened up by electron beam lithography. A relatively new and scarce method
is used to produce GQDs of different shapes. This method is called metalcatalyzed method. To produce triangular and hexagonal shapes of GQDs,
fullerene is used as a precursor with ruthenium metal catalyst [38]. Although
this is not an ordinary method due to the presence of metal catalysts and the
unusual raw material configuration, it is still rare. Comparing the top-down
method to the bottom-up method, the bottom-up method produces a higher
yield. A bottom-up approach can also be used to alter their physical and
chemical properties as well as increase the selection of precursors.
11.2 Functionalization of graphene quantum dots
The utilization applications of pure GQDs are limited by several restrictions
are generally considered to be able to perform to their best abilities when
214 Graphene Quantum Dots
FIGURE 11.1 Preparation methods of graphene quantum dots.
functionalized. GQDs can have their properties modified by functionalization.
They can have their electronic, chemical, optical properties modified, enabling
them to be applied in a wide range of applications. There are a variety of ways
of functionalizing a material, including heteroatom doping, polymer or inorganic material composites, and regulating the shape and size of GQDs
Fig. 11.2 [39]. With the development of more functional materials, many
optical, chemical, and electronic properties are available to them, which enables them to be utilized in various applications. Fig. 11.2 below shows how
the system works and adding heteroatoms to inorganic compounds changing
the size or shape of polymers or materials. It is possible to consider GQDs as
functionalization. There are several main attributes. The study of carbon
nanotubes continues to be a hot-topic due to the possibility of GQDs being
functionalized via those material nano systems. A description will also be
provided of how doping GQDs changes their basic properties. In the doping of
GQDs to date, K, Na, B, N, P, N, S, Se, F, and Cl have been used as a heteroatom Fig. 11.2. As a result of codoping these atoms, GQDs can be
enhanced in certain ways. Electronegativity and other properties of the dopant
atoms could produce the changed properties. In addition to functionalizing
GQDs with organic and inorganic composites for novel applications, they are
also being explored for overcoming their limitations. To pair the characteristics of GQDs with the advantages of organic materials [40,41], organic materials offer various factors such as easy film formation, high carrier mobility,
etc. To improve the properties of GQDs for opening new application fields,
GQDs are incorporated with inorganic materials such as ZnO, TiO2, Au
Graphene quantum dots for optical application Chapter | 11
215
FIGURE 11.2 Schematic of the functionalization of GQDs through different routes and
enhancement of its properties [39]. Copyright 2018. Reproduced with permission from Elsevier.
nanoparticles, and SnO2 [42e44]. To improve the properties of GQDs for
opening new application fields, GQDs are incorporated with inorganic materials such as ZnO, TiO2, Au nanoparticles, and SnO2. A number of methods for
designing size and shape dependent bandgaps have been developed experimentally, theoretically, and experimentally [45e47]. GQDs are anticipated to
be used in this manner in the future.
11.3 Applications of graphene quantum dots
GQDs are potential candidates in diverse areas such as medical, optical, and
energy as shown in Fig. 11.3 [47,48]. There has been much research dedicated
to tailoring their properties for various applications, including pharmaceutical
delivery, LEDs, solar cells, and optical, etc. [49e52]. This chapter discusses
GQDs and their derivatives that will help us explore a wide range of optical
applications.
11.3.1 Optical applications
Various optodevices are currently being developed due to the outstanding
properties of GQDs, such as upconversion, intense PL, and modification of the
energy band gap (Eg) through altering the device’s shape and size [53]. For
national defense, space exploration, etc., photodetectors and photosensors are
key components. Because GQDs can easily be customized with unique
216 Graphene Quantum Dots
FIGURE 11.3 Applications of GQDs in different fields [39]. Copyright 2018. Reproduced with
permission from Elsevier.
properties, they have become a focus of attention in improving photodetectors.
Research has been conducted on GQDs composed with silicon nanowires [54],
ZnO nanorods and P3HT for their use in photodetectors GQDs are characterized by novel properties that make them attractive for LEDs [55,56]. GQDs
can be used in two ways to boost LED light power and wavelength, one is to
cover the LED with GQDs, and the other is to make the entire structure of the
LEDs from GQDs [57,58]. Besides being able to change the color and intensity of light emitted, LEDs made from GQDs also have another feature.
Due to their superior electron mobility, long electron-hole pairs life, and
upconversion performance, GQDs belong in the carbon family and can be used
for photocatalysis. There has been an increase in absorption intensity for
visible light with GQDs made from materials like cadmium sulfide (CdS),
titanium dioxide (TiO2), and zinc oxide nanowires [59,60]. Different opto
devices have been developed using GQDs because of their unique optical
properties, such as photodetectors, LEDs, and photocatalysis.
GQDs have certain special characteristics, such as a strong photoluminescence, upconversion, and the ability to tune their energy gap by controlling their size and shape, which make the technology different from
conventional quantum dots like ZnO QDs and TiO2 QDs. The GQDs were
functionalized using different techniques and showed excellent performance in
related applications. In the following sections, we will discuss some of the
applications of GQDs in the optical field. Photodetectors/photosensors are
devices that detect light or electromagnetic energy [61] following the main
categories of optoelectrical detectors, LEDs, and photocatalysis. As a result,
they are a crucial part of national security, real-time monitoring, space
exploration, and other activities. These features make them very easy to
customize and their optical properties make them unique.
Graphene quantum dots for optical application Chapter | 11
217
The use of GQDs in nanomaterials photodetectors has been studied by
several research groups. Here are a few of them: composite GQDs, such as
silicon nanowires or ZnO nanowires, made from traditional materials, such as
silicon, ZnO, and P3HT. The use of these compounds in photodetectors has
been studied [62e64]. A hybrid composite of GQD and a 2D material, such as
graphene, has also been made using GQD such as MoS2 WSe2 and graphene,
etc. [65e70]. There are few instances of pure GQDs being used in active
layers. Despite the fact that some groups have attempted them because located
mostly in the ultraviolet range, GQDs are characterized by absorption peaks.
This is why a photodetector would typically respond to wavelengths in the
short range, as indicated model of DUV light in Fig. 11.4. It led to the
development of pure GQD photodetectors [71]. A recent study by Tang and
coworkers [72] used pure GQDs doped with nitrogen to create a photodetector
that compensated for the spectrum from UV to NIR. A Tang-Lau method is
one of these novel techniques.
In luminescence, light is produced from materials when they are excitation
by an external source [53,61,73]. It is of crucial importance to develop
illumination-grade lighting technologies, as lighting accounts for a significant
portion of global energy consumption [74]. LEDs have used GQDs because of
their excellent optical properties. The use of GQDs in LEDs can be divided
into two approaches. To modify the light intensity and wavelength of LED
lighting, a first approach is to coat them with GQDs (Fig. 11.5a) [75e78]. The
second method is to use GQDs within LED structures as the emitting layer
(Fig. 11.5b) [58,79]. Frequently, GQDs are coated onto LED surfaces as
phosphors, resulting in color changes (Fig. 11.5c) [80].
In addition, there is the possibility to use GQDs as active layers by forming
composites. To improve LED performance, LEDs are combined with other
emitting materials. The intensity of LEDs irradiation can be controlled by
FIGURE 11.4 Schematic illustration of the pure GQDs photodetector with asymmetric layer in
the photodetector [71]. Copyright 2015. Reproduced with permission from American Chemical
Society.
218 Graphene Quantum Dots
FIGURE 11.5 GQD applications in LEDs illustrated by illustrations and results. (a) GQDs
covering on LEDs; (b) GQDs as emitting layer; (c) Fluorescence images of single color GQDs of
the GQD phosphors/PDMS composites with 365 nm irradiation [80]. Copyright 2017. Reproduced
with permission form John Wiley and Sons.
changing the content of GQDs in the composites [81]. GQDs can also be
functionalized with different functional group agents to tune the color of LEDs
based on a functional group agent [82e84].
Basically, photocatalysis involves accelerating a chemical reaction by light,
accompanied by a catalyst. The use of this technology in renewable energy
sources has attracted considerable attention. Considering this fact, water
splitting into hydrogen and oxygen can be used to produce clean energy
[85e89]. To split water into H2 and O2, photocatalytic materials that are
sensitive to visible light must be present [90]. In addition to low-cost, scalable,
and sustainable materials, another important factor in the design of such an
application is the availability of photocatalysts. We have investigated the
possibility of using carbon materials for cost-effective hydrogen production as
photocatalysts since they are low-cost, abundant, and environmentally
friendly. GQDs stand out among carbon materials since they possess many
outstanding properties that make them ideal candidates for photocatalysis
applications. In GQDs, the bandgap can be tailored using 0D semiconductors.
The size, shape, and surface chemistry of the edge can be varied. Further, their
surface area is large. Easily transports photogenerated charge carriers due to its
high electron mobility. Electron-hole pair lifetimes and upconversion behavior
will be extended. GQD composites as photocatalysts in different forms with
other inorganic materials. The synthesis and study of nanoparticles, nanobelts,
and nanowires has been carried out [91e95]. Researchers, Yang and colleagues, have proposed a mechanism for hybridizing titania dioxide (TiO2)
Graphene quantum dots for optical application Chapter | 11
219
with GQDs [60]. The band gap diagram of S,N-GQD/TiO2 composites that
show the possible mechanism for photocatalytic H2 evolution done by Li et al.
[96]. GQDs were attached to the surfaces of CdS nanoparticles, and the
composites were fabricated. Lei and colleagues explained the mechanism of
how this enhanced the intensity of absorption of visible light [59].As photocatalysts, these composites of GQDs and ZnO NW were fabricated under the
irradiation of the sun [95]. Furthermore, g-C3N4 composites of GQDs and
GQDs also exhibit upconversion behavior [59].
An easy physical method for the research of C3N4 graphene coated with S,
N codoped GQDs significantly improved the photocatalytic activity to RhB
degradation UV light irradiation, GQDs/C3N4 are equivalent to their counterparts in pristine C3N4, Consequently, electrons and holes are efficiently
separated in the interface when they are photogenerated [35].Under visible light
irradiation, it was found that GQDs/mpg-C3N4 composite demonstrated high
photocatalytic activity of RhB degradation and the removal of toxic tetracycline
hydrochloride pollutants. As a result of the in situ generation of a large quantity
of superoxide radical active species, GQDs/mpg-C3N4 nanocomposite exhibit
highly promising photocatalytic properties (Fig. 11.6aec) [97]. Interestingly,
Hao et al. reported mesoporous Bi2MoO6 modified with sube3 nm GQDs
(Fig. 11.7aec). In the reaction systems such as percentage of OH [98], the
photodegradation environment showed broad-spectrum photodegradation activities that were possible through improved electron-hole separation efficiency
and the effective in situ production of highly active species.
In comparison to nanowires, nanobelts have a higher surface area, therefore
they can be loaded with more GQDs to enhance the photocatalytic performance. Rhodamine B photodegradation reaction using ZnS nanobelts coated
with GQDs done by Jang et al. [91] in Fig. 11.8.
Optical applications have taken advantage of the unique properties of
GQDs. GQDs are being studied in a continuing effort to understand their
properties and to develop new functionality. In the near future, many innovative and new optical applications are expected. Therefore, there is potential
for the development of GQDs to revolutionize optical, electronics, imaging,
and for biomedical applications research.
FIGURE 11.6 (a) TEM and (b) GQDs/MPG-C3N4 nanocomposites exhibit photocatalytic performance under visible light. (c) Nanocomposite of GQDs/mpg-C3N4 and its photocatalytic
mechanism. Reproduced with permission Liu J, Xu H, Xu Y, Song Y, Lian J, Zhao Y, Wang L, Huang
L, Ji H, Li H. Appl Catal B Environ 2017;207:429e437. Copyright 2017 Elsevier Publishing Group.
220 Graphene Quantum Dots
FIGURE 11.7 (a) SEM (b) Under visible light, GQDs-BM nanocomposites perform photo
catalytically and (c) GQDs-BM nanocomposites photocatalyzed according to the above schematic
illustration. Reproduced with permission Hao Y, Dong X, Wang X, Zhai S, Ma H, Zhang X. J Mater
Chem 2016;4:8298e8307. Copyright 2016 Royal Chemical Society.
FIGURE 11.8 An organic dye over a G/ZnS nanocomposite decomposes under light irradiation
as shown in this schematic. Reproduced with permission Ham S, Kim Y, Park MJ, Hong BH, Jang
DJ. RSC Adv 2016;6:24115e24120. Copyright 2016 Royal Chemical Society.
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Chapter 12
Graphene quantum dots and
their role in environmental
sustainability
Kiran Jeet
Electron Microscopy and Nanoscience Laboratory, Punjab Agricultural University, Ludhiana,
Punjab, India
12.1 Introduction
Carbon compounds constitute the foundation of all known life on Earth, and
possess several allotropes other than diamond and graphite. The other allotropes of carbon such as carbon nanotubes (CNTs), fullerenes, graphene, and
graphene-based nanostructures have unparalleled physical and chemical
properties such as high durability, magnificent corrosion resistance, stability
under extreme environment, and outstanding electrical and thermal conduction
that makes them an interesting material to explore for widely varying applications. In the extensive family of carbon allotropes, carbon-based quantum
dots are the recent members being added to list and have been explored and
utilized widely mainly because of their powerful and tunable fluorescence
emission characteristic. Fig. 12.1 illustrates the structure of graphene quantum
dots (GQDs) along with graphene and graphene oxide.
Carbon-based quantum dots include GQDs and carbon quantum dots
(CQDs), which have sizes less than 10 nm. GQDs are bits of graphene that are
restricted to zero dimension. The GQDs and graphene, as such, have similar
constitutions (that of C, O, and H), surface moiety (carbonyl, carboxyl, hydroxyl and epoxy), and crystalline nature. CQDs are spherical nanoparticles
amorphous to nanocrystalline core having sp2 hybridized carbon and graphene
oxide sheets fused together in a fashion similar to diamondlike sp3 hybridized
carbon [1]. GQDs have gained tremendous attention in recent years due to
their fascinating optical, electrical and optoelectrical properties arising due to
its unique structure. GQDs are better over semiconductor quantum dots in
respect to their high solubility, chemical inactivity, facile modification and
high resistance to photobleaching [2]. Due to chemical and physical stability,
Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00011-X
Copyright © 2023 Elsevier Ltd. All rights reserved.
227
228 Graphene Quantum Dots
FIGURE 12.1 Schematic illustration of structure of (a) Graphene, (b) Graphene Oxide and (c)
Graphene Quantum Dots.
nontoxicity, and biological inertness, GQDs have started gaining intersect
throughout the globe by the academicians as well as industrial corridor/sector.
Understanding the structure of GQDs is an important step for further
exploring its potential in varied sectors of science and technology. GQDs are
known to be small fragments of graphene but it is quite interesting to know
that the restriction of graphene such as their zero bandgap and low absorptivity
are among the major reason for discovery and origin of GQD. To reduce flaws
of graphene, investigations were performed on its structural modification
which further leads to formation of GQDs.
The basic structure of graphene involves a thin single layer of hexagonally
arranged carbon atoms arranged having length of few microns [3]. GQDs can
be obtained from graphene sheet by constant fragmenting the graphene sheet
till it reaches a size lower than 20 nm [4]. GQDs are mostly observed to be
circular and elliptical in shape having size in the range 1e20 nm. Presence of
band gap owing to quantum confinement and edge effects makes GQDs
different from that of graphene which doesn’t shows any band gap [5,6]. Just
like graphene, GQDs are also sp2 hybridized carbon structure and are crystalline in nature [6,7]. Due to existence of quantum confinement, surface defects and zigzag/armchair edges, the GQDs possesses the fascinating of
fluorescence [6]. According to the theoretical predictions and experimental
works, GQDs exhibit tunable fluorescence properties due to tunable band gap
and that makes them interesting material over semiconductor quantum dots
(SQDs) [5,8].
The manufacturing of GQDs was first described by Ponomarenko et al. [6]
in year 2008. The unique fluorescent property of GQDs was discovered in
2010 by Pan et al. [4] and after that the further modification in its properties
were initiated by many researchers to make them useable in diversified areas.
Graphene quantum dots and their role in environmental Chapter | 12
229
The first-ever modification in the properties of GQDs was performed by Zhao
et al. [9] by doping it with nitrogen. Modification of GQDs by mean of adding
functional groups at the edges has made it more soluble over other allotropes
of carbon such as carbon nanotubes.
There are numerous methods for production of GQDs under the broad
classification of top down, bottom-up and waste/biomass derived synthesis
approaches. Fig. 12.2 illustrate the various methods for production of GQDs.
Top-down approach for synthesis of GQDs involves direct cutting of precursor
such as such as CNTs, carbon fibers, carbon black, graphene, graphene oxide,
graphite powder and coal source into quantum size material following varied
synthesis approaches [4,5,10e14]. Under the bottom-up technique, graphenelike smaller polycyclic aromatic hydrocarbons (PAHs) such as benzene,
hexaperi-hexabenzocoronene, glucose, and fullerene [15e18] are used as
precursors employing chemical route to get converted into GQDs.
Most of the methods discussed involved expensive instrumentation and
raw/precursor materials like CNTs, graphene, graphene oxides, and their
byproducts. Also the cost of production of these materials employing above
mentioned techniques comes out to very high in comparison to the product
yield. The better alternative to curtail the cost of production is the use of the
biomass/agricultural waste. Agricultural waste stands out as a natural,
economical and viable carbon source for mass production of GQDs.
In addition to getting the cheaper source for production of smart materials,
use of agricultural waste (as shown in Fig. 12.3) also offers a solution to get
clean soil, air, and water that gets contaminated due to wrong dumping of
agricultural waste. The agricultural wastes like sugarcane biomass, fruit peels,
rice straw, wheat straw and pineapple leaves are usually burned by farmer
community in most of the regions and thus causes environmental pollution.
Therefore, idea of using agricultural waste for production of smart materials is
FIGURE 12.2
Schematic illustration of various methods for synthesis of GQDs.
230 Graphene Quantum Dots
FIGURE 12.3 Agricultural sources for synthesis of carbon-based quantum dots.
like hitting two birds with one stone. It not only offers cheap source of precursor for production of material but also helps in getting cleaner environment.
This book chapter highlights the possibility of recycling agricultural waste
into smart innovative carbon-based nanomaterial such as GQDs, and their upcoming utilization in field of sensors; energy conversion and storage; and
photocatalytic degradation of contaminants.
12.2 Synthesis of biomass derived graphene quantum dot
Taking biomass as precursor for production of GQDs, numerous techniques
such as hydrothermal heating, carbonization, plasma induced pyrolysis and
microwave heating of waste are available.
In the bottom-up approach, sp2 carbon domains are synthesized from
organic molecules via intermolecular coupling and carbonation process. These
techniques are time consuming and complicated but results in manageable size
and structure of products. On the other hand, top-down approach results in
high yield of the product and involves fragmenting precursor (generally any
sp2 based carbon structure) either through oxidative cutting/chemical etching
or through mechanical shear [4]. Fig. 12.4, illustrates the production of GQDs
under top down and bottom-up approaches. Except for the difficulty in
selecting the appropriate precursor, these production techniques are considered
to be simplified, less time consuming and low budget techniques. The precursors employed under these approaches are restricted to those with large area
of sp2 carbon domains, such as graphene, carbon fiber, carbon black, etc. [4,5].
Sugarcane is the most frequently used sources of biomass energy
throughout the world. Bagasse and cane trash are two types of biomass residues produced from sugarcane. Bagasse is the fibrous residue obtained after
Graphene quantum dots and their role in environmental Chapter | 12
231
FIGURE 12.4 Schematic illustration of top down and bottom-up approaches for synthesis of
GQDs.
milling of the cane and is considered to be one of the prominent solid agricultural waste residues. Their abundance, low cost, biodegradability and
recyclability make them useful for large scale production of carbon nanostructures. Cellulose, hemicellulose, pentosans, lignin, sugars, wax, and minerals are main contents of bagasse [19]. Chemical composition of bagasse
contents is shown in Table 12.1, which indicates that sugarcane bagasse is rich
in cellulose and hemicellulose content which is necessary for the formation of
carbon-based quantum dots.
Recent research on use of sugarcane bagasse as precursor for production of
GQDs has demonstrated a simple, systematic and inexpensive method with
eco-friendly approach for efficient recycling of agricultural waste taking into
consideration the important aspect of environmental sustainability [20]. It was
reported that for the production of GQDs, first the graphene oxide was synthesized from sugarcane bagasse, which was further used as a precursor for
synthesis of GQDs (Shown in Fig. 12.5).
TABLE 12.1 Chemical composition of sugarcane bagasse.
Constituent
(%)
Sugarcane
bagasse
Holocellulose
Cellulose
Hemicelluloses
Klasson
lignin
50e84
32e55
27e32
19e25
232 Graphene Quantum Dots
FIGURE 12.5 Schematic illustration of preparation of graphene oxide from sugarcane bagasse.
Study reported the use of two varying precursors like graphite flakes and
sugarcane bagasse for production of graphite oxide. Employing oxidized
shearing method (as shown in Fig. 12.6), and using KMnO4/H2SO4, two
different kinds of GQDs were synthesized. High resolution imaging analysis of
GQD derived from sugarcane bagasse showed mono-dispersed nature of
quantum dots along with their nearly spherical shape and higher yield in
comparison to GQDs obtained from graphite as precursor. Diameter distribution of GQDs derived from sugarcane bagasse was reported in the range
5e11 nm.
FIGURE 12.6
oxide.
Schematic illustration of synthesis of graphene quantum dots from graphene
Graphene quantum dots and their role in environmental Chapter | 12
233
The PL quantum yield of GQDs derived from sugarcane bagasse was
observed to be twice than that of the GQDs prepared from graphite flakes. The
probable reasons for difference in PL behavior for variant quantum dots could
be variance in surface functionalities, defects and sizes of the manufactured
quantum dots (Fig. 12.7).
Similarly other agricultural waste/biomass such as rice husk, neem leaves,
coffee grounds, rice grains, neem leaf extract, fenugreek leaf extract and
carbon fibers has been reported as the precursor for production of GQDs.
Different precursors for production of GQDs along with production approach
and obtained sizes of GQDs are illustrated in Table 12.2 [20e26].
To obtain the desired properties of GQDs many prior/postsynthesis techniques have been investigated in the past few years. The photoluminescence
(PL) property of GQDs is strongly based on its size, shape, surface modification and heteroatom doping [27e30]. Due to edge effects, different shapes
of the GQDs such as hexagonal, parallelogram, triangular, trapezoid obtained
under different annealing temperature will show different PL property [27,30].
Surface modification such as surface oxidation, polymer passivation, and
chemical moieties attachment also greatly alters the PL properties of GQDs
[31,32]. The addition of heteroatoms through precursor during the production
of GQDs has emerged out to as one of the most promising technique to fine
tune the PL property as well as the quantum yield of the GQDs [29]. Freedom
of varying the morphology and functionality of GQDs to fine tune their
properties provides ample of opportunity to explore their application in widely
varying field. Among the varied applications, role of GQDs in environment
sustainability is one of the aspects that need special and urgent attention.
12.3 Applications of GQDs with special attention to
environment sustainability
The material synthesis and deciphering its properties only become interesting
if that could be further utilized in developing applications having potential in
improving the livelihood of all living beings around the globe. It strongly
FIGURE 12.7 Fluorescence Micrograph of (a) graphite derived GQDs and (b) sugarcane
bagasse-derived GQD under UV filter.
234 Graphene Quantum Dots
TABLE 12.2 Illustration of GDQs synthesized from different biomass/
Agricultural waste under varying production approaches.
Size
(nm)
Precursor
Preparation approach
Reference
Neem leaves
Pyrolysis, hydrothermal
treatment
5e6
Suryawanshi
et al. [21]
Fenugreek leaf
extract
Pyrolysis, hydrothermal
treatment
3e10
Roy et al. [22]
Neem leaf
extract
Pyrolysis, hydrothermal
treatment
2e8
Roy et al. [22]
Rice husk
Hydrothermal process
3e6 nm
Wang et al. [23]
Coffee grounds
Hydrothermal treatment
1.88
Wang et al. [24]
Rice grains
Pyrolysis
2e6.5
Kalita et al. [25]
Wood charcoal
Electrochemical oxidation
3e6
Nirala et al. [26]
Sugarcane
bagasse
Chemical oxidation and
exfoliation process
5e11
Himani et al.
[20]
includes applications that should not have adverse effects on the environment.
In order to address the present-day environment challenges and future human
needs, the following text is dedicated to the discussion of applications of
GQDs in environment sustainability as function of providing clean air, water,
soil and energy source. The environment related applications are broadly
divided into two heads: sensing/detection and remediation of contaminants and
energy-related application for future energy solution.
12.3.1 Sensing/detection
Sensor is a device/module that can detect some events or changes in its
environment or the quantitative detection of some material in the environment
by transforming the response of detected event/material/molecular specie into
an electronic signal to be exhibited on some human readable device
(Fig. 12.8). Owing to the distinct property of fixed band gap of GQDs and
good conducting material due to high electron motion with fast moving reaction within them, makes them best candidate for sensing applications.
Based on the properties, the GQD-based sensors can be classified as
photoluminescence, electrochemiluminescence, gas, humidity, and electrochemical sensors. A lot of research has been reported for use of these sensors
for detection of contaminants in water, air, and soil and food products but for
Graphene quantum dots and their role in environmental Chapter | 12
235
FIGURE 12.8 Applications of GQDs in sensor mechanism.
the sake of understanding the concept, only a few examples are quoted in the
following sections.
12.3.1.1 Photoluminescence sensor
GQD-based photoluminescence sensors are based on variation in optical
properties of GQDs arises due to adsorption of the ionic or molecular species
on their surfaces which further changes their bandgap resulting in increase/
decrease of PL intensities [33]. These types of measurements can be well
sensed and measured, and can produce sensible information to design a sensor.
The very first GQD-based PL sensor was studied by Wang et al. [34] for
detection of Fe3þ ions. They illustrated that the luminescence of GQDs can be
selectively quenched by Fe3þ ion via shift of charge phenomenon. To upgrade
the selectivity, sensitivity and specificity of sensors toward sensing of heavy
metal ions, biomaterials and organic molecular species, GQDs can be surface
functionalized/doped [35]. To study the comparison of selectivity of GQDs
and functionalized GQDs, work has been documented for numerous metal ions
such as mercury, copper, silver, iron, etc. [36]. A lot of work in this area has
already been reported by many researchers, but for the sake of understanding
the concept, only few examples are quoted here in this chapter.
The method of fluorescence intensity quenching effect of GQDs is
commonly employed method for metal ion detection. On similar principal, Jin
et al. [36] demonstrated the use of GQDs for detecting the presence of mercury
ions in the campus lake water. Dong et al. synthesized a precise, sensitive and
more or less a green PL sensor for detection of chlorine in water [37]. Their
236 Graphene Quantum Dots
results added an advancement in the knowledge that GQDs based sensors can
be employed for detection of both cationic and anion ionic species. Similarly,
many other researchers have reported the use of GQDs as a sensing material
for detection of heavy metal ions in water.
12.3.1.2 Electrochemiluminescence
Electrochemiluminescence (ECL), a robust technique, combines electrochemistry and chemiluminescence, resulting in electro generated chemiluminescence [38]. By and large, it depends on the emission of light from an
excited state produced in the development of electron transfer processes in the
middle of the radical cations and anions of a luminophore. Application of
potential to electrodes can change the electrochemical energy into irradiative
energy [39].
During the electrochemical reaction, the bright luminescence signal was
found to be observed from the excited state of the ECL luminophore produced
by the electrode [40]. The most striking benefit of ECL is that it doesn’t
require the usage of external light sources.
ECL activities of GQDs provide them a potential to be utilized as sensor
for detection of some harmful metal ions. A study done by Chen et al. [41]
reported development of GQDs/peroxydisulfate (GQD/S2O2_
8 ) based ECL
sensor to detect Cr (VI) ions. Similarly, to detect presence of Cr (VI) in spiked
river water, numerous other sensors were also developed. On the basis of their
prior studies reported, Chen et al. [42] also proposed another ECL sensing
technique using NGQDs/chitosan film for detection of nitroaniline (NA),
which was not only highly selective and sensitive but also convenient to use.
12.3.1.3 Gas sensor
Gas sensing is one of the major inventions that has significant role in environmental remediation particularly in the area of monitoring as well as
capturing the gases emitting from varied sources. From the last few decades,
increasing greenhouse gas emissions and global warming issues has aggravated to much extent owing to presence of volatile organic compounds (VOCs)
that used for varied domestic and industrial applications. GQDs have emerged
as better alternative for fabrication of gas sensor over other 2D material due to
presence of large number of edge atoms and surface atoms allowing better
adsorption capability over other 2D materials [43]. Chen et al. [44] reported
production of two different kinds of GQDs (neutral and acidic) based gas
sensors for detection of NH3 gas. It was concluded from their study that
different types of GQDs based gas sensors provides different electrical
response even if the concentration of gas molecule is same. Not only the
GQDs types but the sensing results were observed to be greatly affected by the
operating conditions such as pH and humidity.
Graphene quantum dots and their role in environmental Chapter | 12
237
12.3.1.4 Humidity sensor
Humidity is a significant parameter and one of the important physical conditions that stimulate everything in living beings. Constant monitoring of
humidity is crucial in many areas such as agriculture; for accessing physical
conditions of soil and soil sickness, bundling industry, semiconductor fabrication, industries for preparing nourishment products, therapeutic industry,
structural building industries, electronic based industries, cooling frameworks
and state of art laboratories [45e48]. Selection of appropriate material for
fabrication of humidity sensor is much significant as well as difficult. There
are number of material such as polymers, metal oxides (MOs), carbon-based
materials and their composites that are among the common choice for
sensor fabrication. The choice of the material for fabrication of sensor should
qualify condition of being highly sensitive over the entire selected range of
relative humidity (RH) and should undergo noticeable shift/change in electrical parameters/signals for even a minor change in humidity.
Recently, studies were performed to explore the potential of graphenebased materials toward detection of humidity. Adsorption of water and
change in the values of capacitance and resistance were the indicators of
detection mechanism. It was noted that the presence of oxygen-containing
molecules at the surface of material plays a significant role in sensing
mechanism. But their excessive presence may cause the material to act as an
electrical insulator, as in the case of GO. GQDs are the class of materials that
possesses all the characteristics that are necessarily required for synthesizing
humidity sensors along with an added advantage of tunable bandgap [49e51].
Another edge of using GQDs over other carbon-based material is their high
sensitivity toward different environmental humidity conditions [52]. It has
been observed that the conductivity of GQDs decreases under photon illumination due to adsorption of oxygen and water molecules from the surface of
GQDs when exposed to ambient temperature. This phenomenon is known as
negative photoconductivity (NPC) and can be acknowledged as the processes
of sequential surface adsorbates-induced carrier trapping and photoelectronic
detrapping.
Another study reported by Hosseini et al. demonstrated the synthesis of
GQD-based highly sensitive and flexible humidity sensors offering optimized
properties such as great response and selectivity (approx. 390 for RH change
of 99%), broad detection range (1%e100% RH), and the best of all is, short
response and recovery times (12 and 43 s) [48]. The sensing ability of GQDs
based sensors at lower RH was demonstrated in the study reported by Alizadeh
et al. [45]. They developed the method of controlling the carbonization of
citric acid to achieve better sensibility even at lower RH values.
12.3.1.5 Electrochemical sensor
The speedily growing urban society and industrial sector have resulted in
dispersal of many effluents that has majorly affected the air, soil and water
238 Graphene Quantum Dots
bodies. The major contaminants that are commonly observed from the urban
and industrial effluent are natural and industrial organic and inorganic pollutants. Before their complete mitigation, their detection in terms of quality
and quantity is an important parameter to study. Among the common methods,
electrochemical sensors are considered as one of the efficient tools for
detection of toxic chemicals reagents from the environment.
Electrochemical sensors are the class of chemical sensors that are based on
phenomenon of converting chemical reactions of the analytes on electrode into
electrical signals thus providing current information about the surrounding
environment [53]. Electrochemical sensors are sorted as following types;
Potentiometric, conductometry and amperometric or voltammetry based on the
measurement of the electrical signal [54].
For sensor development, nanomaterials with layered structures are
considered to play a significant role due to their unique properties. The layered
nanomaterials forms a stable composites with different polymers that noticeably enhances the overall performance of sensors [55,56]. Tan et al. [57]
demonstrated the synthesis of an electrochemical sensor, where electrodes of
sensor were developed with the composites of polypyrrole/GQDs for sensing
Bisphenol A (BPA) in aqueous medium. They employed differential pulse
voltammetry (DPV) that was exhibiting linear response in the range
0.01e50 mM and detection limits up to0.04 mM. Another study by Wen et al.
[58] reported the synthesis of GQDs based sensors for detection of a small
traces (i.e., 3 1010 M) of Cu þ ions. They amalgamated O3/H2O2/ultrasound for development of purified GQDs for detection of trace elements.
12.3.2 Role of GQDs for future energy solutions
Rapidly growing population coupled with industrialization has resulted in
severe shortage of energy resources. The advancement in technology resulting
in depletion of fossil fuel resources and deterioration of environment have
generated a serious question mark to the scientific community and policy
makers to look for an alternative source. It has become the most urgent quest
throughout the globe to search advanced renewable energy resources for fulfilling the present as well as future energy needs. Owing to this, many researchers have fascinated toward development of efficient energy storage
(EES) devices. It is most expected that the storage system should offer high
efficacy, extended cycling life, low weight, high energy density, and good
durability to deal with the energy demand. The requirement of the above said
energy structures can be fulfilled through storage batteries and supercapacitor
in the domain of efficient EES systems [59]. Due to its abundance and unique
properties, carbon is determined as an excellent candidate for applications
related to energy sector. Among the carbon family, GQDs are emerging as
strong contender for many energy-related application due to their tunable
properties. The extraordinary properties of GQDs, namely immensely high
Graphene quantum dots and their role in environmental Chapter | 12
239
aspect ratio, high electrical conductivity, tunable luminescence, low chemical
impedance, and tunable bandgap [60e64] have exhibited potential in
numerous energy-related applications, such as supercapacitors [65,66], batteries [67,68], and photovoltaic devices (Fig. 12.9) [62,69].
12.3.2.1 Supercapacitor
The GQDs are emerging as a strong contender for fabrication of the advanced
and improved energy-storage devices such as supercapacitor because of the
following reasons. First and the most important is the 0D carbon skeleton
structure which is highly appropriate for building any complex and conductive
architectures, Second is presence of enhanced edge structure and available
functional groups at the peripheries of the dots providing active sites’ energystorage applications and last and the most important is good chemical reactivity and migration property that allows their easy assemblage [70,71].
Appreciable research has been performed in this area but for the sake of
discussing the concept, only few examples are quoted here.
Fabrication of GQDs based microsupercapacitor on the interdigital Au
finger microelectrode following electrophoretic deposition technique was reported by Liu et al. [66]. They reported high performance of supercapacitor
with specific capacitance of 534.7 mF cm2 at current density of 15 mA cm2
with a power density of 7.5 mW cm2 and an energy density of 0.074 mW h
cm2 possessing long cycling life (97.8% stability after 5000 cycles). Because
of the unique advantage of being flexible in nature, high electron attraction,
easy synthesis, and active redox behavior, conductive polymers (CPs) are
considered as important building block for fabrication of supercapacitor. The
activity of supercapacitors gets enhanced with the unique combination of
FIGURE 12.9
Applications of GQDs in energy storage and conversion.
240 Graphene Quantum Dots
GQDs and CPs. Recently, a research has reported on the electrical properties
of composite synthesized with conductive polyaniline (PANi) and GQDs [72].
The electrochemical properties of GQDs@PANi were reported to be affected
by the mixing ratio of GQDs in PANi and have reported specific capacitance
3632.0 F g_1 for a composite that was further utilized for fabrication of
supercapacitor electrodes.
12.3.2.2 Batteries
In addition to supercapacitors, lithium-ion (Li ion) batteries also bags in the
top few position in the list of energy-storage devices. Due to their lightweight,
high energy and good performance characteristics, they are expected to provide better solution for future energy needs and can also address the existing
energy demands. In the recent past, efforts are constantly being made by many
research groups to overall improve the performance of Li ion batteries. Among
various class of materials, allotropes of carbon especially GQDs have gained
significant recognition due to their extraordinary properties that can appreciably minimize the use of other materials for electrode materials as well as
can be added in electrolytes for enhancing the transport and electrical properties of the batteries [67,73]. The merit of using GQDs as an electrode coating
material is the improvement in the transport of ions between electrolyte and
active material due to enhancement in the surface area of the electrode
resulting in quick energy storage and release [74].
The ever first utilization of GQDs in Li ion batteries was reported by Chao
et al. [67]. They fabricated cathode material for Li ion batteries by utilizing
GQD-coated VO2-nanobelt arrays on 3-D graphene foam. The overall performance of GQDs-anchored VO2 electrode was reported to be much better
than that of VO2 based electrodes in terms of specific capacitance (i.e.,
421 mA h g1), coulombic efficiency (i.e., 99%), and stability retention (94%
capacity after 1500 cycles). Similarly, many researchers have documented
significant improvement in performance of Li ion batteries by using GQDs.
Following factors are considered to be responsible for improved performance
of GQDs based Li ion batteries. Foremost, the GQDs deposited electrodes
offers higher surface area that leads to improved metal ion storage. Second, the
high conductivity of GQDs significantly improves the conductivity of the
electrolyte as well as charge collection efficiency of the electrodes. It has
noted that the use of GQDs in batteries not only enhances their performance
but also makes their fabrication cost effective. Therefore, it is quite appropriate
to remark that the application of GQDs in fabrication of batteries is practical as
well as commercially viable thought.
12.3.2.3 Photovoltaic devices/solar cells
Keeping the present scenario in consideration, every single day, new and
modernized devices are entering the technology sector. With rapid increase in
Graphene quantum dots and their role in environmental Chapter | 12
241
number of devices, the demand of energy consumption is also increasing at
alarming rate. It has become most important to explore options in renewable
energy resources. Solar energy is one of the renewable energy resources that is
not only green economical alternative but also offers the conversion of sunlight
to power/electricity without creating contamination and ecological harm. The
photovoltaic materials and devices work on the principal of photoelectric effect for translation of sunlight to electricity [75].
Generally, the function of any electronic device is strongly dependents
upon the material used for its fabrication. Due to attractive physicochemical
and electronic properties, GQDs can improve the effectiveness of catalytic
reactions in energy-conversion applications/devices. The basic characteristics
of GQDs, such as the down conversion, strong fluorescence, intensive absorption at UV range and easily functionalization had led to development solar
cells of varied kind having appreciably enhanced performances. Hybrid of Si
and GQDs has emerged out as a leading material for fabrication of solar cells
giving output power conversion efficiency of 16.55% by harnessing the
property of GQDs, i.e., strong absorption at UV range [76,77]. Besides metals,
the loading/mixing of GQDs with ZnO [78e83], TiO2 [84e86] and some
classical solar materials [87e93] were reported to be equally efficient having
ability of enhancing the power conversion efficiency of the device by 1.26%,
7.19%, and 35% respectively. Due to ease of functionalization, organic materials are also finding great potential in GQDs based composite materials for
fabrication of solar cell [94e96].
12.3.3 Catalytic applications
Photocatalysis is one of the processes that is gaining considerable attention
now a days due to its application in renewable energy resources. Photocatalysis is speeding up of chemical reaction in the presence of a catalyst when
exposed to appropriate light source. One of the applications of photocatalysis
is the generation of energy by splitting of water into H2 and O2 using sunlight
[97e101]. Materials which are sensitive to visible light of electromagnetic
spectrum are capable of splitting water into H2 and O2 and are considered as
critical materials in photocatalytic water-splitting phenomenon [102]. The
criteria for selection of these materials are the use of only those materials
which are scalable, sustainable and have low cost. Carbon-based materials
qualifies all these features and possesses photocatalysis property for costeffective production of hydrogen production. Due to outstanding properties
such as a tuneable band gap, enhanced surface area, fast electron mobility and
ability to improve lifetime of electron-hole pairs and up conversion, GQDs are
ranked as the best material among the carbon allotropes for applications based
on phenomenon of photocatalysis [103].
One of the studies fabricated nitrogen-doped GQDs with nitrogen atom
embedded in the frame of GQDs and oxygen groups present at crystal surfaces
242 Graphene Quantum Dots
to discuss the photo catalytic property of GQDs [102]. Increase in efficiency
by 12.8% for production of H2 was observed on combining NGQDs with Pt as
cocatalyst [104]. Another study reported enhancement in efficiency for H2
production by 29% on doping Pt-deposited GQDs with both nitrogen and
sulfur [105].
Considerable work has already been initiated in recent past to understand
the photocatalytic behavior of GQDs to explore their potential in application
such as photocatalytic hydrogen evolution and CO2 reduction, electrocatalytic
oxygen reduction, water splitting and CO2 reduction, as well as photoelectron
catalysis [106]. In addition to this, GQDs are also used in detection of various
ions as well as chemical groups. This is possible due to intrinsic structure of
GQDs which further allows them to exhibits different selective quenching
phenomena [107] and thus makes their use compatible in varied types of
sensors [108e111].
While demonstrating the role of GQDs in detection of Fe3þ ions, Wang
et al. discussed the trend indicating variation in the fluorescence intensity of
GQDs with variation in concentration of Fe3þ ions, which further observed to
be dependent upon the complexation between Fe3þ ions and phenolic hydroxyl group of GQDs [112]. Taking the advantage of electrochemical fluorescence emission property of GQDs, Li.et al. developed sensors for detection
of cd2þ ions [113]. Fan et al. developed technology for detection of trinitrotoluene (TNT) in solution phase by utilizing the phenomenon of quenching of
fluorescence of GQDs by absorption of TNT through p-p bonding on GQDs
surface. It was noted that the fluorescence resonance energy transfer occurs
between TNT and GQDs through molecular dipole-dipole interaction [108].
Due to heavy load of harmful dyes and other poisonous effluents in ground
and surface water bodies situated near industrial plants, it becomes the foremost necessity to treat them. GQDs exhibit an outstanding ability to break
harmful chemical dyes present in water. The modification of GQDs with
graphene increases surface area, surface charge density, selective adsorption
and high dispersibility. Also, it is the number of rings in the molecule that
decides the equilibrium adsorption performance irrespective of the nature of
dye. The decolorization of methylene blue is reported to increase by 85% with
NGQDs doped into TiO2 system rather than using commonly employed pure
titanium dioxide which exhibits photocatalytic action under visible light. It is
the transfer of electron form NGQDs to TiO2 that forms oxygen and hydroxyl
free radicals which further aided the oxidation of dye [114]. Another hybrid of
GQDs with TiO2 i.e., impregnation of vertically aligned TiO2 with GQDs is
observed to be better over their pristine states in photo decolorization of
methylene blue dye [115]. Another study reported on degradation of methylene blue dye had used nanocomposite of zinc porphyrin functionalized graphene quantum dots (GQDs/ZnPor). Addition of GQDs to nanocomposite not
only increase the surface area, charge density and effectiveness but also
Graphene quantum dots and their role in environmental Chapter | 12
243
enhances the electron conductivity of the photogenerated carriers thereby
increasing the photocatalytic activity of the whole complex [116].
Nie et al. synthesized composite of GQDs with manganese and nitrogen
codoped mesoporous TiO2 for photodegradation of organic pollutants (such as
p-nitrophenol, diethyl phthalate and ciprofloxacin) along with photocatalytic
production of hydrogen. They used density functional theory (DFT) based
simulations to discuss the photodegradation of pollutants [117].
In the similar way, numerous works has been reported in literature indicating significant role of GQDs in removing varied types and classes of pollutants through their photocatalytic degradation mechanism.
12.4 Summary
To address the role of GQDs in environmental sustainability, a greener
approach for synthesis of GQDs, along with their applications lying only in the
domain of environment sustainability, is discussed here. The production of
GQDs from the agricultural source is one of the cheapest and most scalable
approaches. It not only provides economical benefits but also provides a solution for remediation of environmental pollution. Owing to their outstanding
properties, GQDs have a bright scope in sensors as well as energy-storage and
energy-conversion devices. Discussion on varied classes of GQD-based sensors for detection of organic, inorganic contaminants, and poisonous gases in
environment are included. Humidity sensors that play vital role in activities
ranging from agricultural to any hi-tech technology are also included and
discussed in the applications of GQD-based sensors. Energy-related applications of GQDs based on energy storage and energy conversion were discussed
in the chapter.
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Index
Note: Page numbers followed by “f ” indicate figures and “t” indicate tables.
A
Acid etching, 192
AFM. See Atomic force microscopy (AFM)
Agricultural wastes, 229e230, 234t
recycling of, 231
Allotropes, 1, 101e102, 211
Amperometric immunosensors, 103e104,
104f
Anodic aluminum oxide (AAO), 6e11
Antibiotics, 67e68
Antimicrobial reagents, 72e73, 73f
Antiseptics, 68
Atomic force microscopy (AFM), 33e34
B
Bacteria, 67e68
drug-resistant, 121e124
sensing, 69e76
Bandgap, 48e49, 116e117, 211
energy, 84
Batteries, 240
BBB. See Blood-brain barrier (BBB)
BeereLambert law, 28e29
Biodegradable polymers, 90
Biodistribution, 94e95, 96f, 96t
Bioimaging, 106e107
Biokinetics, 92, 93f
Biomass, 230e233, 233f, 234t
bottom-up approach, 230, 231f
sugarcane bagasse, 230e231, 231t
Biomedical sciences, 102e105
immunological assay, 102e105
amperometric immunosensors, 103e104,
104f
electrochemical immunosensors,
102e103, 103fe104f
Biomedicine, 83
Blood-brain barrier (BBB), 83
Bohr exciton radius, 1
Boron atom, 38e39
Bottom-up approaches, 53e55, 135f,
160e162
biomass, 230
carbonization, 195
fullerene, decomposition of, 54e55
hydrothermal method, 161
using microwave, 161
metal-catalyzed method, 162
microwave-assisted hydrothermal (MAH)
method, 195e196, 196f
precursors pyrolysis, 53, 54f
soft-template method, 161
step-by-step synthetic route, 53e54
Brodie’s method, 188e189
C
Carbon dots (CDs), 60e61, 116, 211e212
antimicrobial property of, 70
antiviral mechanisms of, 75t
for combating bacteria, 70e72, 71f
for combating virus, 73e74
structure of, 189
Carbonization, 195
Carbon materials, 188f
Carbon nanomaterials, 1
Carbon nanotubes (CNTs), 2, 101e102,
188e189
Carbon quantum dots (CQDs), 227e228
Carboxylated graphene quantum dots, 94e95,
96f, 96t
Catalytic applications, 241e243
Cavitation phenomenon, 87e88
Centers for Disease Control and Prevention
(CDC), 67
Chemical oxidation method, 87
Chemical vapor deposition (CVD), 33
Chitosan, 90
Citric acid, 135
Clean energy
challenges of, 185e186
solution, 186e187, 187f
CNTs. See Carbon nanotubes (CNTs)
Codoped, 40
Computational PL theory, 16e17, 16f
251
252 Index
Conducting polymers (CPs), 200e201
Contaminated water, 47e48
Coronavirus infection, 74
CQDSpds, 70
Cytotoxicity assay, 19e22, 20t
fluorescent GQDs, 21e22
GQDs vs. CdTe and CdS semiconductor
QDs, 20, 20t, 21f
GQDs vs. C60 QDs, 20e21
GQDs vs. surface-passivated GQDs, 22
D
D band, 29e30
2D bands, 30, 31f
Density-functional theory (DFT), 16, 36e37
Dimethylformamide (DMF), 52
Dimethyl sulfoxide (DMSO), 52
Disinfection/disinfectants, 68, 121e124
Doping, 37e41, 198e199
double heteroatoms, 40e41, 41f
single heteroatom, 38e39, 39f
Double heteroatom doping, 40e41, 41f
DOXGQDMSNs, 69
Doxorubicin (DOX), 76e77, 105
in alveolar macrophages, 90
Drinking water supply system, 121e124
Drug delivery, 74e75
graphene quantum dots (GQDs), 105e106,
105fe106f
methods of, 89e95
carboxylated graphene quantum dots,
biodistribution and toxicology of,
94e95, 96f, 96t
fluorescent graphene quantum dots
application, 90e92, 92fe93f
foliate, 90
long-term biodistribution, 92e94, 93f
nanoparticles, 89f
proteins and peptides, 89e90
Drug-resistant bacteria, 121e124
Dye molecules, 68
Dye-sensitized solar cells (DSSCs), 202e204,
203f
E
Efficient energy storage, 238e239
Electrochemical energy storage (EES), 201
Electrochemical energy storage system
(EESS), 200
Electrochemical (EC) exfoliation, 192
Electrochemical immunosensors, 102e103,
103fe104f
Electrochemical sensor, 237e238
nanomaterials, 238
types, 238
Electrochemiluminescence (ECL), 236
Electron beam lithography method, 163,
164te165t
Electrons, 84, 114
Electrostatic Coulomb force, 84
Emerging pollutants (EPs), 118e119
Environmental pollution, 184
Environmental sustainability, 231, 233e243
future energy solutions, 238e241
sensing/detection, 234e238, 235f
electrochemical, 237e238
electrochemiluminescence (ECL), 236
gas, 236
humidity, 237
photoluminescence, 235e236
Escherichia coli (E. coli), 57e58, 70e71
Exfoliation, 52e53
Exhausted leftovers, 133e134
F
Fabrication, 3, 4f, 9f
Field emission scanning electron microscope
(FESEM), 4, 9f
Fluorescence imaging, 107
Fluorescence resonance emergency
transference (FRET), 102e103
Fluorescent graphene quantum dots
application, 90e92, 92fe93f
Foliate, 90
Fourier transform infrared spectrometer
(FTIR), 34, 35f, 35t
Fullerenes, 68
decomposition of, 54e55
G
Gas sensor, 236
G band, 29e30
Graphene oxide (GO), 85
graphene quantum dots from, 232f
structure of, 2, 228f
sugarcane bagasse, 232f
Graphene quantum dots (GQDs), 2e3, 5te6t,
48, 101e102
absorption peak of, 85
agricultural sources for, 230f
Index
applications of, 4f, 88e89, 117, 134, 159f,
165f, 216f
in biomedical sciences, 102e105
energy-related, 165e166
environment sustainability, 233e243
heavy metal detection and removal,
166e173, 171t, 172fe173f
lithium-ion batteries, 201e202
medical, 163
optical, 163e165, 215e219, 217fe218f,
220f
solar cells, 202e204
supercapacitors, 200e201
for bacterial sensing, 69e76
bandgap of, 116e117
bioimaging, 106e107
biomedicine, 83
carbon-based resources, 113e114
carbon composition, 157e158
carbon materials, 86
characterization, 28e36, 216
microscopy, 33e34
optical, 28e32
surface state, 34e36
defined, 134
doping, 198e199
in electrochemical clean energy devices,
185
electronic properties, 198
energy gaps, 212e213
features of, 7te8t
functionalization of, 213e215, 215f
GQD-DMA, 149e150, 149f
heavy metals reduction, 57e60
inherent effects, 159f
live cells real-time molecular tracking,
76e78
manufacturing of, 228e229
mechanism, 60e61, 60f
membrane filter, 125e126
microbial and heavy metal load reduction,
121e125
MO and MB, degradation of, 147, 147f
nanomedicine, 83
New Fuchsin (NF) dye, 148, 148f
for organic pollutants degradation,
117e121, 122te123t
outlooks of, 190e191, 190f
preparation of, 160f, 214f
production of, 229
properties of, 3e22, 28, 159f
cytotoxicity assay, 19e22
253
morphological and structural
elucidation, 4, 9f
nitrogen (N)- doping, 12e13, 12f
optical analysis, 13e19
photoelectrochemical (PEC) cell, 19
surface-enhanced Raman scattering
(SERS), 6e12
quantum confinement, 85
quantum size of, 27e28
rhodamine-B RhB, photodegradation of,
148, 149f
structure of, 228, 228f
surface modification of, 28, 36e41
synthesis of, 7te8t, 50e55, 86e88,
160e163, 191e198, 193f
biomass, 230e233
bottom-up approaches, 53e55, 160e162,
195e196
CdS/GQDs using g-C3N4 nanosheet, 138
chemical oxidation method, 87
citric acid coated with iron codoped
TiO2, 135
graphene oxide (GO), 138e139, 140t
green approach, 196e198, 197t
ground coffee, 136
hydrothermal method, 87
lignin, 137
metal free N dopped carbon quantum
dots, 138
methods for, 229f
N, S codoped commercial TiO2/GQDs,
137e138
pyrocatechol, 134e135
rice husk, 136e137
spent tea, 136
top-down approaches, 50e53, 162e163,
192e195
ultrasound assisted method, 87e88
toxicity of, 107, 108f
water disinfection, 57e60
for water treatment, 55e56, 57t
ZnO-GQD, photocatalytic activity of,
144e147, 147f
Graphene sheets (GSs), 2
synthesis of, 3
Green approach, 196e198, 197t
Ground coffee, 136
GSs. See Graphene sheets (GSs)
H
Heavy metals
detection and removal, 166e173, 173f
254 Index
Heavy metals (Continued )
Cd2+ adsorption mechanism, 172f
electrochemical and electronic analysis,
166e167
fluorescence resonance energy, 169e170
gold nanoparticles, 169
hydrogel adsorbent, 170
sensors for, 171t
load reduction, 57e60, 121e125
Heteroatom doping, 198e199
Highest occupied molecular orbital (HOMO),
36e37
Highly oriented pyrolytic graphite (HOPG), 1
Holes, 84
Human cervical carcinoma (HeLa) cells, 76
Humidity sensor, 237
Hyaluronic acid (HA), 75e76
Hydrothermal method, 50e52, 51f, 87, 161,
194
using microwave, 161
I
Immunological assay, 102e105
amperometric immunosensors, 103e104,
104f
electrochemical immunosensors, 102e103,
103fe104f
optical immunosensors, 104e105
piezoelectric immunosensors, 104e105
Industrialization, 115e116
Interferons (IFNs), 73e74
International Energy Agency’s (IEA),
185e186
L
Laser scanning confocal microscope
(LSCM), 19e20
Lignin, 137
Liquid exfoliation method, 162
Lithium-ion batteries, 201e202
Lithography process, 52
Live cells real-time molecular tracking,
76e78
Long-term biodistribution, 92e94, 93f
Lowest unoccupied molecular orbital
(LUMO), 14e15, 36e37
M
Membrane filter, 125e126
Metal-catalyzed method, 162
Metal nanoparticles, 68, 133e134
Micropollutants (MPs), 118e119
Microscopy characterization, 33e34
atomic force microscopy (AFM), 33e34
transmission electron microscopy (TEM),
33
Microwave-assisted hydrothermal (MAH)
method, 195e196, 196f
Microwave treatment, 113e114
Mining industries, 115e116
MnOx quantum dots, 60e61
Multidrug resistant (MDR), 67
N
Nanomaterials, 92e97, 238
features, 113
Nanomedicine, 83
Nano metal oxides, 133e134
Nanoscience, 78
biokinetics in, 92
Nanotechnology, 1, 157, 211
Nanotomy, 52e53
N-doped GQDs (NGQDs), 118
Near-infrared (NIR), 69
Negative photoconductivity (NPC), 237
New Fuchsin (NF) dye, 148, 148f
Nitrogen, 38
doping, 12e13, 12f
O
One-dimensional (1D) carbon nanotubes,
101e102
Optical analysis, 13e19
computational PL theory, 16e17, 16f
pH-dependent properties, 13e15, 14fe15f
temperature-dependent PL, 17e19, 18f
up-conversion PL emission, 17, 17f
Optical applications, 215e219, 219fe220f
LEDs, 218f
luminescence, 217
photocatalysis, 218e219
photodetectors, 217, 217f
Optical characterizations, 28e32
photoluminescence (PL), 30e32, 32f
Raman spectroscopy, 29e30, 30fe31f
UV-Vis spectroscopy, 28e29
Optical immunosensors, 104e105
Organic pollutants, 139e144, 143f
glyphosate, 142e143
NF dye solution, 143
nitrogen doped, 144
Index
particle size distribution, 141f
photocatalytic efficiency, 145t
photodegradation, 142f, 146f
RB5, 139e142
rhodamine B-hyaluronate solution, 146f
Organic pollutants degradation, 117e121,
122te123t
Oxygen reduction reaction (ORR), 40e41,
198e199
P
Photobleaching, 184, 227e228
Photocatalysis, 241
Photodynamic inactivation (PDI), 68
Photodynamic treatment (PDT), 77e78
Photoelectrochemical (PEC) cells, 19, 19f, 38
Photoluminescence (PL), 30e32, 32f
sensor, 235e236
Photosensitizers, 68
Photovoltaic devices, 240e241
Physiologically based pharmacokinetic
(PBPK) models, 94
Piezoelectric immunosensors, 104e105
Planck constant, 84
Pollutants, 115e116
nonbiodegradable, 133e134
organic, removal of, 139e144
photodegradation of, 243
Polyamines, 70
Polyaniline (PANI), 200e201
Polycyclic aromatic hydrocarbons (PAHs),
53e54, 229
Polyethylene glycol (PEG), 194
Polyethyleneimine (PEI), 107
Precursors pyrolysis, 53, 54f
Pyrocatechol, 134e135
Pyrolysis, 136, 195
Q
Quantum confinement effect, 84
Quantum dots (QDs)
applications, 83
background and creation of, 188e190,
188f
bandgap of, 48e49
in biodegradable polymers, 90
chemical and physical characteristics of, 84
confinement effect, 48e49
255
distinctive luminescent characteristics,
48e49
electric properties, 48e49
optical characteristics of, 83
optical properties, 48e49
Quantum yields (QYs), 190e191
R
Raman intensity, 29e30
Raman spectroscopy, 29e30, 30fe31f,
41e42, 52, 74
Reactive oxygen species (ROS), 68
Reduced graphene oxide (rGO), 50e52
carbon dots (C-dots), 60e61
structure of, 2
Renewable energy, 183
Resonant Raman spectroscopy (RRS), 4
Room temperature (RT), 1, 2t
RRS. See Resonant Raman spectroscopy
(RRS)
S
SARS CoV-2, 74
Scanning tunneling microscopy (STM),
54e55
Semiconductor quantum dots (SQDs), 228
Semiconductors, 68
Sensing/detection, 234e238, 235f
electrochemical, 237e238
electrochemiluminescence (ECL), 236
gas, 236
humidity, 237
photoluminescence, 235e236
SERS. See Surface-enhanced Raman
scattering (SERS)
Single heteroatom doping, 38e39, 39f
Soft-template method, 161
Solar cells, 184, 240e241
dye-sensitized solar cell (DSSC), 203e204,
203f
Solar energy, 184
Solvothermal technology, 52, 194
Staphylococcus aureus (S. aureus), 57e58,
71e72
Sugarcane bagasse, 230e231, 231t
graphene oxide (GO), 232f
Supercapacitors, 184, 200e201,
239e240
256 Index
Surface-enhanced Raman scattering (SERS),
6e12, 10f
GQD microbowls, 11
GQD-NTs, 11
Surface modifications, 36e41
doping, 37e41
double heteroatoms, 40e41, 41f
single heteroatom, 38e39, 39f
tunable through size, 36e37, 37f
Surface plasmon resonance (SPR), 104e105
Surface state characterization, 34e36
Fourier transform infrared spectrometer
(FTIR), 34, 35f, 35t
X-ray photoelectron spectroscopy (XPS),
34e36, 36f
Sustainable energy, 186e187
T
TEM. See Transmission electron microscopy
(TEM)
Top-down approaches, 50e53, 135f,
162e163
acid etching, 192
electrochemical (EC) exfoliation, 192
electrochemical method, 53
electron beam lithography method, 163,
164te165t
hydrothermal process, 50e52, 51f, 194
liquid exfoliation method, 162
lithography process, 52
nanotomy, 52e53
solvothermal method, 52, 194
ultrasonication, 194e195
Transmission electron microscopy (TEM),
32f, 33, 188e189
1,3,5-Triamino-2,4,6-trinitrobenzene
(TATB), 161
Trypsin, 88
U
Ultrasonication, 194e195
Ultrasonic exfoliation synthesis method, 56
Ultrasound assisted method, 87e88
United Nations (UN), 114e115
United Nations World Water Development
Report (2020), 47e48
UV disinfection, 48
UV radiation, 48
UV-Vis spectroscopy, 28e29
V
Vitamin C, 71e72
Volatile organic compounds (VOCs), 236
W
Wastewater treatment, 113, 133e134
Water consumption demand, 47e48
Water disinfection, 57e60
Water pollution
impact on life, 115e116
pollutants, 133e134
Water protection, 114e115
Water treatment, 57t
conventional techniques, 48
impurities, 55e56
limit of detection (LOD), 56
ultrasonic exfoliation synthesis method, 56
Wind energy, 184
World Health Organization (WHO), 47e48,
166
World Water Development Report (WWDR),
114e115
Worldwide water crisis, 114e116
water pollution and impact on life,
115e116
Wound pathogen disinfection, 74e76
X
X-ray diffraction (XRD), 4, 9f
X-ray photoelectron spectroscopy (XPS),
34e36, 36f
Z
Zero-dimensional C molecules, 68
Zero-dimensional (0D) fullerenes, 101e102