ELECTROCHEMICAL DETECTION OF HYDROQUINONE USING GLASSY
CARBON ELECTRODE MODIFIED WITH POLYANILINE DECORATED WITH
NITROGEN DOPED GRAPHENE OXIDE
By
Allen Sedze
(R133434V)
Submitted in partial fulfilment of the requirements for the degree of
Bachelor of Science Honours in Chemical Technology Department of Chemical Technology
in the
Faculty of Science and Technology at the
Midlands State University
Supervisor: Professor Mambo Moyo
May 2017
DEDICATION
This project is dedicated to my loving parents Mr. and Mrs. Sedze for their unconditional and
lasting love and prayers. I express my sincere gratitude to them and my loving sister Martha T.
and brother Tendai D. Thank you so much for your unwavering support.
1
ACKNOWLEDGEMENTS
First and foremost, I offer my sincerest gratitude to the Almighty God. I give Him all the glory,
honour and adoration for seeing me through this journey, giving the strength and wisdom
through the Holy Spirit and letting me know that it is never too late to get a degree. I owe my
deepest and immense gratitude to my supervisor Professor .M. Moyo whom God has blessed me
with and has chosen to be my supervisor, mentor and instructor, for his unwavering guidance
throughout the journey and making me to believe in myself that I can do it. I will forever be
grateful for your kindness. Goethe said, “Treat people as if they were what they ought to be and
you help them to become what they are capable of being.” You have treated me as though I were
capable of reaching the stars. Because of you, I have. ‘You are Awesome indeed’. To my friends,
colleagues and loved ones in the Lab cascading from top to bottom Mrs. M. Gonzo down to
Fuyana Vezubuhle and other lab technician students, you are all gratefully acknowledged. All
members of the Chemistry Department, Midlands State University, I say thank you for your
cordial working relationship.
Thank you, Lord Jesus.
2
ABSTRACT
A simple sensitive low-cost electrochemical sensor for the detection and quantitative
determination of hydroquinone.
Nitrogen doped graphene oxide decorated with Polyaniline
(NGO/PANI) was synthesized by a chemical method, then casted on a surface of glassy carbon
electrode to form NGO/PANI-GCE. Electrochemical impedance spectroscopy and Bode plots
were used to characterize the synthesized modifiers, NGO NPs, PANI NPs, NGO / PANI NPs
and to probe the chemical linkage between NGO and PANI. Cyclic voltammetry, linear sweep,
chronoamperometry and differential pulse voltammetry where used to assess the electro catalytic
efficiency of the nanocomposite electrode towards the oxidation of hydroquinone. The electrode
surface area after modification was 0.13 cm2, almost twice as large that of the bare glassy carbon
electrode, indicating that the modified electrode displayed an improved surface area for electro
catalysis. The modified electrode had a surface coverage of 1.31 x 10
constant, k was 3.9 x 10 6 M
-1
-12
mol cm-2. The rate
s-1, fast electron transfer during oxidation. The limit of detection
for hydroquinone was 1.03569 x 10-7 M and the limit of quantification was 3.4531 x 10-7 M. The
electrode exhibited good reproducibility with lower oxidation potential at 0.8 V and high
sensitivity towards hydroquinone.
3
DECLARATION
I, Allen Sedze., hereby declare that I am the sole author of this dissertation. I authorize Midlands
State University to lend this dissertation to other institutions or individuals for the purpose of
scholarly research.
Signature ………………………………
Date ………………………………….
4
APPROVAL
This dissertation entitled “Electrochemical detection uof hydroquinone using glassy carbon
electrode modified with polyaniline decorated with nitrogen doped graphene oxide” by Allen
Sedze meets the regulations governing the award of the degree of Chemical Technology of the
Midlands State University, and is approved for its contribution to knowledge and literal
presentation.
Supervisor ………………………………………
Date …………………………………………….
5
Table of Contents
DEDICATION
i
ACKNOWLEDGEMENTS
ii
ABSTRACT
iii
DECLARATION
iv
APPROVAL
v
Chapter 1
0
1.0 Introduction
0
1.1 Background
0
1.2 Aims of the Research
2
1.3 Objectives
2
1.4 Problem Statement
3
1.5 Justification
4
CHAPTER 2
6
LITERATURE REVIEW
6
2.0 Summary
6
2.1 Hydroquinone
6
2.12 Reduction-oxidation equilibria
7
Graphene Oxide (GO)
7
2.1.2 Graphene oxide preparation using modified Hummer’s method
9
6
2.1.4 Properties of Graphene oxide
10
2.1.5 Chemical reactivity of graphene oxide
10
2.1.6 Nitrogen doped Graphene oxide
10
2.2 Polyaniline
11
2.2.1 Structure and oxidation state of Polyaniline
12
2.2.2 Conductivity in polyaniline
13
2.2.3 Synthesis of polyaniline
13
2.2.4 Doping in polyaniline
14
2.2.5 Polymer nanocomposites
14
2.2.6 Methods of preparation of polymer nanocomposite
15
2.2.7 Amalgamation/ Solution Mixing of Nitrogen doped graphene oxide and Polyaniline 15
2.3 Electrochemical Synergism
15
2.3.1 Electrode Modification
16
2.3.2 Characterisation of synthesized composite
17
Infrared spectroscopy, IR
17
Electrochemistry
18
2.4 Voltammetry
19
2.4.1 Instrumentation
19
2.4.2 The potentiostat
19
2.4.3 Electrodes
20
7
2.4.4 The reference electrode (RE)
20
2.4.5 Counter electrode (CE)
20
2.4.6 Working electrode (WE)
21
2.4.7 Cyclic voltammetry
21
2.4.8 Illustration of a typical cyclic sweep voltammetry.
21
Differential Pulse Voltammetry
25
Linear sweep voltammetry
25
Chronoamperometry
26
Electrochemical impedance spectroscopy
27
Bode Plots
27
2.5 Electro-catalysis
27
2.5.1 Chemically modified electrode
29
CHAPTER 3
31
EXPERIMENTAL
31
Introduction
31
3.1 Equipment
31
3.2 Chemicals and reagents
32
3.2.1 Synthesis procedures
32
Synthesis of Polyaniline
32
Synthesis of Polyaniline nanoparticles
33
8
3.2.2 Graphene oxide
33
3.2.3 Doping Nitrogen on Graphene oxide and modification with polyaniline
35
3.3 Characterization of the chemical modifiers and the composite
35
3.3.1 Fourier Transfer Infrared Spectroscopy (FTIR)
35
3.3.2 UV -Vis Spectroscopy
36
3.3.3 Electrode modification
36
3.4 Hydroquinone analysis
36
3.5 Electrochemical characterization
37
3.5.1 Electrochemical behavior of modifiers in 1 mM [Fe (CN)36]3-/4 solution
37
3.5.2 Electrochemical impedance spectroscopy and bode plots
37
3.5.3 Scan rate studies in in 1mM [Fe (CN)6]3-/4 solution
37
3.6 Optimisation of parameters
37
3.6.1 Effect of pH
37
3.6.2 Comparative studies in pH 4 phosphate buffer solution
38
3.6.3 Scan rate studies in pH 4 phosphate buffer solution
38
3.7 Kinetic studies
38
3.8 Langmuir adsorption isotherm studies
38
3.9 Catalytic rate constant
39
3.10 Differential pulse voltammetry
39
3.11 Stability studies
40
9
3.12 Reproducibility studies
40
Effect of interference
40
CHAPTER 4
41
RESULTS AND DISCUSSION
41
Introduction
41
4.1 NGO / PANI Np FTIR
42
4.2 Electrochemical characterisations
45
Voltammetric studies in 1.0 mM [Fe (CN)6]3-/4- in 1M KCl
45
4.3 Electrochemical impedance spectroscopy
47
Bode plots in 1.0 mM Fe(CN)6 3–/4– aqueous solution using 0.1 M KCl as the supporting
electrolyte.
50
4.4 Surface area determination
51
4.5 Optimization of parameters
53
4.5.1 Effect of pH
53
4.6 Comparative study in pH 4 buffer
54
4.7 Scan rate studies in pH 4 phosphate buffer solution without an analyte
55
4.8 Comparative study in 1 mM hydroquinone in pH 4 PBS 4
55
4.9 Electrochemical impedance Spectroscopy in 1 mM Hydroquinone in pH 4 PBS
58
4.10 Bode Plots in 1mM hydroquinone
59
4.11 Kinetic studies of hydroquinone
60
10
Tafel slopes
61
Mechanism of hydroquinone oxidation with NGO / PANI-GCE
62
Linear Sweep Studies
64
Langmuir adsorption isotherm plot for NGO/PANI-GCE
65
4.16 Chronoamperometric Studies
66
Chronoamperograms for different Hydroquinone concentrations
67
Plots of 𝐼𝑐𝑎𝑡𝐼𝑏𝑢𝑓 vs. time
68
Plot of slopes2 vs. [Hydroquinone]
68
DPV for NGO/PANI -GCE
70
Stability
71
Reproducibility
72
Interference Studies
72
CHAPTER 5
73
5.0 Introduction
73
5.1 Conclusion
74
5,2 Recommendations
74
References
75
APPENDICES
85
APPENDIX A: MATERIALS
85
List A: Apparatus used in synthesis and characterisation
85
11
Table A1; Reagents and chemicals
85
Table A2 : Instrumentation
86
Treatment of Glassware
86
APPENDIX B: Working electrodes
87
B1: Working electrodes used in this study
87
APPENDIX C: Calculation
87
C1: Effective Surface area
87
Limit of Detection and Limit of Quantification
88
Excel Linest function
88
ABBREVIATIONS
12
EIS - Electrochemical impedance spectroscopy
CA- Chronoamperometry
CV- cyclic voltammetry
NGO NPs – Nitrogen doped Graphene oxide nanoparticles
PANI NPs – Polyaniline nanoparticles
NGO/ PANI NPs– Nitrogen doped graphene oxide modified with polyaniline nanoparticles
NGO-GCE – Nitrogen doped graphene oxide glassy carbon electrode
PANI-GCE – Polyaniline modified glassy carbon electrode
NGO/PANI-GCE – Glassy carbon electrode modified with Polyaniline decorated with nitrogen
doped graphene oxide
DPV- Differential pulse voltammetry
DMSO- dimethylsulphoxide
FTIR- Fourier Transfer Infrared
GCE- Glassy carbon electrode
LSV- Linear sweep voltammetry
LOD- Limit of detection
LOQ- Limit of quantification
ICP – Intrinsically conducting polymer
PANI NP s- polyaniline
LIST OF FIGURES
13
FIG 1: CHEMICAL STRUCTURE OF HYDROQUINONE…………………………………………….1
Fig 2.1: Figure Schematic diagram of structure of graphene oxide (GO)………………………...8
Fig 2.3: GO synthesis ……………………………………………………………………………..9
Fig 2.3: GO synthesis ……………………………………………………………………………12
FIG 2.4 TYPICAL FT-IR SPECTRA OF GRAPHENE OXIDE……………………………………..18
Typical cyclic voltammogram for a reversible reaction………………………………………..24
Fig 3.1 Mechanism of polymerization of aniline monomer to polyaniline……………………..33
Fig 3.1: Synthesis of graphene oxide nanoparticles……………………………………………34
Fig 4.1 FT-IR Spectrum of (a) GO , (b) , PANI , (c) NGO- PANI Nps………………………41
Fig 4.1 FT-IR Spectrum of (a) GO , (b) , PANI , (c) NGO- PANI Nps………………………42
Fig 4.1.3 Polyaniline FTIR……………………………………………………………………43
Fig 4.1.3 UV- visible spectra of graphene oxide……………………………………………..44
Figure 4.1.4 Pani UV-Vis……………………………………………………………………44
Fig 4.2: Cyclic voltammograms for: a) bare GCE b) NGO- GCE c) PANIi-GCE d) NGO/PANIGCE in 1 mM [Fe (CN)6]3-/4- solution. Scan rate = 100 mV/s……………………………..45
Fig 4.3 Nyquist plots obtained for a) bare GCE b) NGO-GCE c) PANI -GCE d) and e)
NGO/PANI -GCE in 1mM [Fe(CN)6] solution in 1M of KCl.
Inset is the Randles circuit model
used in fitting data……………………………………………………………………………….46
14
Fig 4.4 Bode (phase angle versus log f) plots obtained for i) bare GCE ii) NGO-GCE iii) PANI
-GCE iv) and v) NGO/PANI -GCE in 1mM [Fe(CN)6] solution in 1M of KCl ………………..50
Fig 4.3 Effect of scan rate on peak potentials and currents (a) 50 mV/s, (b) 100 mV/s, (c) 150
mV/s, (d) 200 mV/s, (e) 250 mV/s, (f) 300 mV/s, (g) 350 mV/s, on NGO/PANI-GCE. Inset: Plot
of Ipa, Ipc versus √ν……………………………………………………………………………..52
Fig 4.5 Cyclic Voltammograms for NGO/PANI-GCE in (i) pH 3, (ii) pH 4, (iii) pH 5, (iv) pH 6,
(v) pH 7 phosphate buffer solution containing 1 mM Hydroquinone Inset: plot of Ipa vs pH. Scan
rate = 100 mV/s………………………………………………………………………………….53
Fig 4.6: Cyclic Voltammograms for i) bare GCE ii) NGO-GCE iii) PANI-GCE and vi)
NGO/PANI/-GCE, in phosphate buffer pH 4. Scan rate = 100 mV/s…………………………...54
Fig 4.7: Voltamograms of NGO/PANI-GCE in phosphate buffer ph 4 with increasing scan rate at
(a) 50 mV/s, (b) 100 mV/s, (c) 150 mV/ s, (d) 200mV/s, (e) 250 mV/s, (f) 300 mV/s (g) 350
mV/s……………………………………………………………………………………………...55
Fig 4.8: Voltammograms of (a) Bare GCE b) NGO-GCE c) PANI-GCE d) NGO/PANI-GCE in
1mM Hydroquinone pH 4 phosphate buffers……………………………………………………56
Fig 4.9 Nyquist plots obtained for (i) Bare GCE, (ii) NGO-GCE, (iii) PANI- GCE, and (iv)
NGO/PANI-GCE in 1 mM Hydroquinone in pH 4 phosphate buffer…………………………58
Figure 4.10 : Bode (phase angle versus log f) plots obtained for (i) bare GCE, (ii) NGO-GCE,
(iii) PANI-GCE and (iv) NGO / PANI-GCE in 1mM Hydroquinone (pH4)……………………59
15
Fig 4.11: Effect of scan rate on peak potentials and currents a) 50 mV/s, b) 100 mV/s, c) 150
mV/s, d) 200 mV/s, e) 250 mV/s, f) 300 mV/s, g) 350 mV/s NGO/PANI-GCE for Hydroquinone
oxidation. [Hydroquinone] = 1mM. Inset: plot of Ipa vs √v……………………………………..61
Fig 4.1.2 : Plot of potential versus log scan rate in 1mM Hydroquinon………………………..62
Fig 4.13: Plot of peak potential against pH for the detection of 1mM of Hydroquinone on
NGO/PANI-GCE………………………………………………………………………………...63
Fig 4.14 Linear sweep voltammograms (a) 10 μM, (b) 20 μM, (c) 30 μM (d) 40 μM (e) 50 μM
and 60 μM of phosphate concentrations in pH 4 PBS. Inset plot of Ipa vs.
[Hydroquinone]. ………………………………………………………………………………...64
Fig 4.15 Langmuir adsorption isotherm plot for NGO/PANI-GCE in a) 10 µM, (b) 20 µM, (c)
30 µM (d) 40 µM (e) 50 µM and 60 µM of Hydroquinone concentrations in pH 4 PBS.
Oxidation currents employed.……………………………………………………………………65
Fig 4.16 Chronoamperograms for different Hydroquinone concentrations. In PBS pH 4, a) 20
μM, b) c) 40 μM ,d) 60 μM , e) 80 μM and f) 100 μM Inset Ipa vs [Hydroquinone]………….67
Fig
4.17
:
Plots
of
𝐼𝑐𝑎𝑡
𝐼𝑏𝑢𝑓
vs.
time
(s)
Fig
4.19
Plot
of
slopes2
vs.
[Hydroquinone]………………….
Figure 4.17 : Plot of slopes2 vs. [Hydroquinone]………………………………………………69
Figure 4.21 ; DPV for NGO/PANI -GCE in: a) 0.2 μM, b) 0.4 μM, c) 0.6 μM, d) 0.8 μM, e) 1
μM, f) 1.2 μM, g) 1.4 μM, h) 1.6 µM. Inset: Plot of Ipa vs [Hydroquinone ].
…………………………………………………………………………………………70
16
Figure 4.22 continuous cyclic voltammetric evolutions for 1 mM Hydroquinone generated on
GCE
modified
with
NGO/PANI.
Scan
rate
=
100
mV/s.
pH
4
PBS……………………………………………............................................................................71
Fig 4.23: Differential pulse voltammograms for three repetitions in 1mM Hydroquinone in pH 4
PBS solution at NGO-PANI/GCE. Scan rate = 100 mV/s at potential 0.4 to 1.0 V……………..72
Fig 4.24 for (a) Hydrogen peroxide , (b) Mixed analytes, (c) hydroquinone (d) phenol…….73
17
Chapter 1
1.0 Introduction
This chapter serves to highlight the background of the research and other techniques that were
used in the determination of hydroquinone. The aims, objectives, problem statement and
justification are of the study are also discussed.
1.1 Background
Hydroquinone is an active ingredient in personal care products, food antioxidants, anthraquinone
dyes, black white film developers and polymerization inhibitors [1]. It is also used in cosmetics
and medical preparations. Dermal exposure is a result of the use of cosmetic and medical
products containing hydroquinone, such as skin lighteners. Absorption via the skin is slower but
may be more rapid with carriers such as alcohols. Hydroquinone , benzene 1.4 diol, Fig 1.1 is
metabolized to p-benzoquinone and other oxidized products, and is detoxified by conjugation to
monoglucuronide, monosulfate and mercapturic derivatives [2]. The excretion of hydroquinone
and its metabolites is rapid and occurs primarily via the urine. Due to its physicochemical
properties, hydroquinone will be distributed mainly to the water compartment when released into
the environment.
FIGURE 1: CHEMICAL STRUCTURE OF HYDROQUINONE
1
Different approaches such as spectrophotometry, high performance liquid chromatography,
capillary electrochromatography, gas chromatography, fluorescence quenching and electro
chemiluminescence, have been used to determine the concentration of hydroquinone [3].
It should be however be noted that the practical application of these methods is restricted by
many disadvantages which include, formidable instrumentation, lengthy analysis time and
tedious process [4]. As a conventional method, electrochemical analysis has played an important
role in analytical chemistry due to the fast response, low instrument cost and feasibility for
on-site measurement [4]. Much effort has been paid to develop the versatile electrodes to
determine hydroquinone [4]. Typical examples are the modified electrodes, and the materials to
decorate the surface adopt electrospun carbon nano fibers , L-cysteine oxide/gold , poly
(thionine)
[5], carbon nanotube-ionic liquid composite [5], Pt-MnO2 composite particle, graphitic
mesoporous carbon , 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) , copper
hexacyanoferrate and platinum film [5]. Gold nanoparticles (AuNPs), due to its large surface
area, high conductivity and electro catalytic capability, have been employed to increase the
detection limit in electrochemical analysis [5]. In this research, a nitrogen doped graphene oxide
decorated with polyaniline modified glassy carbon electrode (NGO / PANI -GCE) was fabricated
and employed as the working electrode for hydroquinone determination. The combination of
nitrogen doped graphene oxide nanoparticles (NGO NP) and polyaniline nanoparticles (PANI
NP) firmly increased the electron transfer rate and the active area of the electrode. The
experimental results indicated that the as-modified electrode exhibited excellent electrochemical
catalytic activities towards the redox of hydroquinone.
2
1.2 Aims of the Research
⮚ Development of an electrochemical sensor for hydroquinone using glassy carbon
electrode decorated with polyaniline and nitrogen doped graphene oxide.
1.3 Objectives
⮚ To synthesise nitrogen doped graphene oxide nanoparticles (NGO NPs) with polyaniline
nanoparticles (PANI NPs).
⮚ To modify the glassy carbon electrode with nitrogen doped graphene oxide nanoparticles
(NGO NPs), polyaniline nanoparticles (PANI NPs), and nitrogen doped graphene oxide
nanoparticles decorated with polyaniline nanoparticles (NGO / PANI NPs).
⮚ To characterise the electrode modifiers, NGO Np, PANI Np and NGO/ PANI Np using
FTIR, UV-Vis spectroscopy, Cyclic Voltammetry, and electrochemical impedance
spectroscopy.
⮚ To determine the surface coverage of the modified electrode NGO / PANI - GCE by
performing scan rate studies in 1mM Ferricyanide solution.
⮚ To carry out pH studies in phosphate buffer solution for the optimisation of pH for the
analysis of hydroquinone using NGO / PANI – GCE electrode.
⮚ To carry out comparative studies on modified electrodes a) bare GCE, b) NGO-GCE, c)
PANI - GCE, d) NGO / PANI-GCE in pH 4 phosphate buffer solution.
⮚ To evaluate the effect of pH, scan rate and concentration on peak currents and peak
potentials.
⮚ To determine the electrode transfer kinetics, Tafel slopes, Langmuir adsorption isotherm,
catalytic rate constants and relative catalytic effects of NGO / PANI - GCE.
3
⮚ To determine the limit of detection of hydroquinone in pH 4 phosphate buffer solution by
using differential pulse voltammetry and chronoamperometry.
⮚ To study the effect of interferences in the presence of hydroquinone on the NGO / PANI –
GCE.
⮚ To carry out reproducibility and stability studies on the NGO /PANI - GCE towards the
oxidation of hydroquinone in pH 4 phosphate buffer solution.
1.4 Problem Statement
Hydroquinone is an organic pollutant which when absorbed through the skin and mucous
membranes can cause damage to the lungs, liver, kidney and genitor urinary tract in the animals
and human [5]. It is toxic to humans in concentrations that exceed 2 parts per million and it has
relatively low difficult degradability, hence the industrial and clinical implications associated
with it are a cause for concern [1]. This has sparked a growing need for safe detection procedures
[1]. At present, the main determination methods are chromatography spectrophotometry and
electrochemical methods [1]. Among all these techniques, spectrophotometry suffers from easy
interference by related compounds and needs complicated pre-separation steps, which are
unsuitable for multicomponent analysis [1]. Chromatography has potential for the detection of
hydroquinone. It however also requires time-consuming sample pretreatment steps and
expensive instruments [1]. Due to their merits of simple operation, prompt analysis and no need
for complicated sample pretreatment, electrochemical methods have been developed as one of
the most efficient alternatives in environmental analysis[1]. However, the direct determination of
hydroquinone at conventional electrodes such as glassy carbon electrodes is difficult because the
similar chemical structures lead to the overlap of their redox peaks [1]. In order to overcome this
problem, many materials such as nano materials , transition metal compounds conducting
4
polymers , are widely applied to modify the electrodes. It is important to note that due to the
exquisiteness and expensive nature of these instruments, they are not easily accessible. These
expensive instruments are also associated with tedious complex time consuming and expensive
sample pretreatment which requires qualified expert technicians to operate them. They have high
detection limits and less sensitivity. Despite the effectiveness of conventional methods, they
however have limitations; in that, they are very costly, time consuming, laboratory borne, they
need a lot of skill in the operation and sometime suffer from low detection limit [6].
Furthermore, a large amount of sample volume and solvent are needed in separation and
extraction procedures [6].
1.5 Justification
Formulations of personal care products that contain hydroquinone exposes a huge part of the
populace to irritant defects caused by hydroquinone, hence it is important to come up with a
user-friendly technique to detect it promptly on site [6].
The concern about the human health risks associated with the exposure to mutagenic, teratogenic
and carcinogenic hydroquinone in the environment has brought about the interest in the
development of a suitable method of analysis. The method should be cheaper and easier to use in
identifying and quantifying [3]. Due to the problem of low detection limit of hydroquinone, an
electro- chemical sensor, a super nanocomposite that is able to detect them at very minute
concentration is made in this investigation.
Presently, instrumental methods of analysis
involving chromatographic (TLC, GC, HPLC), spectroscopic (UV-Vis, IR, MS) or coupled
techniques (GC-MS), are heavily relied upon for environmental analysis.
5
Electrode modification improves the glassy carbon electrode s’ electro catalysis properties by
lowering its potentials and increasing current and surface area towards determination of
hydroquinone [7]. Polyaniline is an intrinsically conducting polymer that contains delocalised
electrons and it confers its conducting properties on to the glassy carbon together with nitrogen
to produce a synergistic super conducting electrode [7]. It has excellent environmental stability,
ease of synthesis, low cost, high electrical properties (as a result of their delocalized π-π
conjugated systems),
Electroanalytical methods have low cost of instrumental requirements and better speeds of
analysis. Electrode modification improves the glassy carbon electrode through lowering the
potential and increasing the current and surface area hence the modifiers were combined together
in order to reduce the over potential of the bare glassy carbon electrode and also to increase
sensitivity towards the detection of hydroquinone [8].
6
CHAPTER 2
LITERATURE REVIEW
2.0 Summary
This chapter presents a literature review comprising of an outline on hydroquinone,
nanocomposite based on polyaniline and nitrogen doped graphene oxide and their method of
synthesis. Also, a background of the characterization techniques used are discussed as well as all
of the theory of electrochemistry.
2.1 Hydroquinone
Hydroquinone (1,4-benzenediol; C6H4(OH)2) is a white crystalline substance when pure, with a
melting point of 173-174 °C [9]. The specific gravity is 1.332 at 15 °C, and the vapour pressure
is 2.4 x 10-3 Pa (1.8 x 10-5 mmHg) at 25 °C [9]. It is highly soluble in water (70 g/litre at 25 °C)
and the log n-octanol/water partition coefficient is 0.59 [3]. With respect to organic solvents, the
solubility varies from 57% in ethanol to less than 0.1% in benzene [3]. Hydroquinone is
combustible when preheated [4]. It is a reducing agent
which is reversibly oxidized to its
semiquinone and quinone. Hydroquinone (1,4-benzenediol; C6H4(OH)2) is a white crystalline
substance when pure, with a melting point of 173-174 °C [4]. The specific gravity is 1.332 at 15
°C, and the vapour pressure is 2.4 x 10-3 Pa (1.8 x 10-5 mmHg) at 25 °C [4]. It is highly soluble
in water (70 g/litre at 25 °C) and the log n-octanol/water
respect to organic solvents, the
partition coefficient is 0.59 [4]. With
solubility varies from 57% in ethanol to less than 0.1% in
benzene [4]. Hydroquinone is combustible when preheated. It is a reducing agent which is
reversibly oxidized to its semiquinone and quinone [4].
7
2.12 Reduction-oxidation equilibria
Hydroquinone undergoes reversible redox changes which can involve a variety of pathways
and redox couples. Each redox couple has an electrochemical potential dependent upon the
degree of protonation and electron reduction [4].
quinone + 2 H+ + 2 e-
Hydroquinone
(2.1)
Hydroquinone / Quinone redox couple implies the transfer of two electrons and two protons [3].
The rate of this process is determined by many factors among which electric conductivity of the
polymer layer [3].
Graphene Oxide (GO)
Carbon nanostructures have been widely been harnessed in numerous applications as a result of
their excellent properties [10] . Carbon based nanomaterials such as carbon nanotube (CNT),
fullerene and graphene have gained considerable interest due to their potential use as
multifunctional materials and wide range of applications in different kind of devices [10].
Nano-science and nanotechnology primarily deal with the synthesis, characterization,
exploration, and exploitation of nanomaterials. Graphene is the world’s thinnest, strongest, and
stiffest material, as well as being an excellent conductor of heat and electricity. It is the basic
building block of other important carbon allotropes [10].
Graphene is a single atomic layer of sp2 carbon atoms and as a single or one-atom-thick,
two-dimensional crystal, graphene has been greatly considered as basic building block for all sp2
graphitic materials including fullerenes, carbon nanotubes, and graphite [10]. It can be wrapped
8
up into zero dimensional (0D) fullerenes, rolled into one dimensional (1D) nanotubes or stacked
into three dimensional (3D) , Fig 2.1 [11].
The two-dimensional structure of graphene enables it to exhibit several unique properties which
differ from other carbon nanomaterials. These include a high specific surface area of over
2,600m2/g [11], excellent electronic properties, current density, superior thermal conductivity ,
high mechanical and tensile strength [12], optical transmittance [13] and fascinating transport
phenomena such as quantum Hall Effect [13]. The oxidation of graphene using strong oxidizing
agents produces a hydrophilic graphene oxide material [14]. The aromatic lattice of graphene of
graphene is disturbed by epoxides, alcohols, ketones and carboxylic groups [15]. In graphene
oxide, the carbon layers in graphite are interspaced with oxygen molecules [13].
Lattice
disruption is shown by the increase in interlayer spacing from 0.335 nm for graphite to more than
0.625 nm for graphene oxide Its structure has oxygen containing groups such as hydroxyl,
carbonyl groups, carboxylic and epoxy group [16], Fig 2.1
9
FIGURE 2.1: FIGURE SCHEMATIC DIAGRAM OF STRUCTURE OF GRAPHENE OXIDE (GO)
Graphene oxide gained more attention than graphene because it is easy to functionalize, easily
labelled with fluorescent probe and is compatible with most biomolecules and can be
encooperated into a conducting polymer matrix [16]. Graphene oxide has hydrophilic groups
which expand the film property of modified electrodes [16]. Doping nitrogen on graphene oxide
improves the process ability and sensing performances [17]. Graphene oxide can be dispersed in
aqueous solutions [13].
2.1.2 Graphene oxide preparation using modified Hummer’s method
This method uses a combination of potassium permanganate and sulphuric acid Diamanganese
heptoxide as the active specie.
KMnO4 + 3H2S04
MnO3 + MnO-4
K+ MnO3 + H3O +3HSO-4
MNO7
(2.2)
(2.3)
The reaction of potassium permanganate with sulphuric acid produces a dark red oil. The
bimetallic heptoxide is much more reactive than its monometallic tetra oxide counterpart, an is
10
known to detonate when heated to temperatures greater than 55 OC or when exposed to organic
compounds [18], Fig 2.3 is a diagram of synthesis of GO from graphite flakes.
FIG 2.3: GO SYNTHESIS
2.1.4 Properties of Graphene oxide
Graphene oxide is a versatile compound that is able to be optimised and functionalised to suit
different properties like hygroscopic, dispersibility and toxicity [10], the exact same sense why
it is successfully doped with nitrogen. Excellent electrochemical performances are achieved as a
result of electronic structure modification of graphene oxide, which promotes intrinsic electron
transfer between the host substrate and electroactive materials.
2.1.5 Chemical reactivity of graphene oxide
Graphene oxide is an insulator due to its disrupted sp2 bonding network, however its electrical
conductivity can be conjured up by restoring the π – network. This is done through reduction
especially on the epoxide group [20]. Doping nitrogen on graphene oxide is done through ring
opening of the epoxy group under appropriate conditions. In covalent functionalization,
substituted amines are introduced through this route
[20].
Microscopic and very minute
molecules can be grafted on the surface of graphene oxide via a grafting on to approach [23].
11
Pre-formed poly vinyl alcohol, (PVA) is attached to carboxylic acid groups of graphene oxide
platelets via ester linkages [20].
2.1.6 Nitrogen doped Graphene oxide
Nitrogen-doping increases the conductivity of graphene oxide by raising the Fermi level towards
the conduction band compared with pristine graphene [8]. Nitrogen-doped graphene oxide
nanocomposite (N-RGO) is prepared via a facile method, in which ethylene glycol and ammonia
are used as the reducing agent and nitrogen precursor [8].
2.2 Polyaniline
Polyaniline is among the family of intrinsically conducting polymers (ICP), Fig 2.3 [2]. It has
been extensively studied owing to the good environmental stability, ease of synthesis, low cost,
high electrical conductivity, and mechanical flexibility exhibited by the polymer [25]. It has
unique multiple oxidation states as well as acid /base doped/ de doping response which has made
it an excellent for acid/base chemical vapor sensor [26]. All these properties have prompted its
potential application in various devices such as super capacitors, sensor, electrochromic,
actuators [2]. Other application includes its use for making electromagnetic shielding , antistatic
coating and flexible electrodes [2]
12
Name of polymer
Structure
Conductivity
Type of
(S cm-1)
doping
Poly(acetylene)
200 -1000
n, p
Polythiophene
10- 100
p
13
Polyaniline
5- 200
n, p
FIGURE 2.3; INTRINSICALLY CONDUCTING POLYMERS
2.2.1 Structure and oxidation state of Polyaniline
Emeraldine form of polyaniline is often referred to as base (EB), is neutral, if doped, ( the imine
nitrogen has been protonated by an acid.) it is called emeraldine salt (ES), termed as protonation
[25]. Emeraldine base is best considered as the most useful form of polyaniline , apart from been
highly stable at room temperature, the doped form, which is emeraldine salt (ES),is highly
electrically conducting [27]. The two other forms of polyaniline, leucomeraldine and
pernigraniline are poor conductors, even in their doped state [28].
This transformation of
polyaniline to different forms with different colours, apart from the oxidation state, showed part
of the uniqueness in its properties compared to other polymers [2]. The emeraldine salt (green
colour) obtained as the product from the polymerisation of aniline in acidic medium (electrolyte)
[2]. Conductivity of polyaniline has provided its application in some devices such as
supercapacitors electrochromic [29].
2.2.2 Conductivity in polyaniline
Conducting polymers (CPs) exhibit small perturbations at their surfaces and generate strong
changes in their electroactivity which can be probed by amperometry or potentiometry [31].
Conducting polymers offer a modifying conducting surface that they is easy to functionalize
with additional chemical functions [32]. The emeraldine form of polyaniline provides the
opportunity to be protonated by doping to form a conducting form of emeraldine [2]. Reported
14
studies revealed that the degree of doping can actually determines the rate of conductivity,
studies also reported that the highest conductivity of polyaniline can be attained at 50% degree of
doping, which is associated with the emeraldine salt of polyaniline [33], and the conductivity
level and variation in their order of magnitude to be 10-2 S cm-1 for undoped emeraldine and up
to 103 S cm-1 for doped emeraldine [2].
2.2.3 Synthesis of polyaniline
The method of synthesis commonly adopted to prepare polyaniline is electrochemical and
chemical oxidative polymerisation method. The chemical oxidative polymerisation method of
synthesis, involving the presence of an oxidising agent in acidic medium [31].
2.2.4 Doping in polyaniline
The protonated emeraldine, produced during the oxidative polymerisation of aniline in aqueous
acid, is electrically conducting due to the presence of the cation radical in its structure [11].
These positive charged units on the structure are then balanced by negative charged counterion
(conjugated base) from any dopant such as chloride or sulphate ions which are usually inserted
during electrochemical polymerisation and as well referred to as protonic acid doping [31].
However, the concentration of dopant from studies reported to have profound effect on the
morphology, conductivity and electro-catalytic activity of polyaniline and the polarisation
method [31].
2.2.5 Polymer nanocomposites
Polymer nano-composites (PNC) consist of a polymer or copolymer having nanoparticles or
nano-fillers dispersed in the polymer matrix [11]. These may be of different shape platelets,
fibres, spheroids), but one dimension must at least be in the range of 1–50 nm [34]. These
15
systems require orientation of the dispersed phase, controlled mixing/compounding, stabilization
of the achieved dispersion, and the compounding strategies for all multi-phase systems, including
PNC [35]. However, nanomaterial of conducting polymers is of special interest as their
properties are considerably different from the properties of corresponding macroscopic materials
[34]. Usually, a change in surface properties is observed when a conducting polymer is
surrounded by a dopant, either nanomaterial or bulky substances such as nanotubes,
nanoclusters, nanoparticle nano-cystals quantum dot, graphene oxide [36]. This however, forces
the polymer backbones to interact with the molecules [32]. A nanocomposite with different
morphology is formed and which is capable of solubilising the conducting polymer [11].
Nanomaterial of polyaniline can effectively provide a controlled electrochemical catalysis,
orientation and solubilisation, as a result of the formation of micelles and obviously enhancing
the quality of the polymer which has given them potential in application such as sensors [11].
Conducting polymer nanomaterial for a sensor construction has gained a lot of recognition owing
to the large surface area dependent[31]. The yield strength and hardness of microstructure
polycrystalline materials typically increases with decrease in size particles [30].
2.2.6 Methods of preparation of polymer nanocomposite
Methods used in the preparation of polymer nanocomposite include the sol−gel process [5] and
monomer/polymer grafting to clay layers [11]. Others include melt method, solution mixing
and in-situ intercalative polymerisation method [11].
2.2.7 Amalgamation/ Solution Mixing of Nitrogen doped graphene oxide and Polyaniline
Nitrogen doped graphene oxide and polyaniline are dispersed in DMF and vigorously shaked
overnight until NGO sobubilises and fully dispersed into the polyaniline (PANI) matrix [11].
16
2.3 Electrochemical Synergism
The synergistic effect of a nitrogen-doped graphene oxide and the intrinsically conducting
polymer polyaniline are explored and harnessed in a facile novel method that forms an exquisite
nanocomposite electrochemical sensor [37]. Graphene oxide is doped using ethylene glycol as
the reducing agent and ammonia water as nitrogen precursor [23]. The addition of
nitrogen-doped graphene results in a high contact area between the electrolyte / electrode, which
facilitates transportation of the electrolyte ion and electron into the inner region of the electrode
[20]. Nitrogen-doping significantly improves the conductivity of graphene oxide, which is
valuable in promoting electron transfer in electrochemical reactions [24]. Specifically, this in situ
synthesis method assures the uniform distribution of polyaniline surfaces of NGO [38]. The
NGO/PANI- nanocomposite exhibits not only high capacity, but also excellent rate capability
[29].
2.3.1 Electrode Modification
Electrode Modification is whereby layers of electro active species are attached on to the
electrode surface to enhance electron transfer reactions occurring on the surface [39]. The
surface takes on the properties of the attached species [39].
Bare electrodes require large oxidation potential and so chemically modified electrodes (CMEs)
reduce over potentials [40]. Glassy carbon electrodes, (GCE) is modified by intrinsically
conducting polymer, (ICP) polyaniline and nitrogen doped graphene oxide, (NGO) in a novel
facile method [41]. It results in improved electro-catalysis and freedom from the surface fouling
effects. Alternatively, electrodes can be modified to prevent undesirable reactions from
competing kinetically with the desired electrode process [39].
17
Studies carried out in the past involving the use of carbon nanotubes for electrode modification
unearthed striking electro catalytic properties [2]. Its integration on the electrode surface
produced lower over potentials [42].
Polymer modified electrodes have the ability to catalyse electrode reactions and can
significantly improve the electro-catalytic properties of the substrates, decrease the over
potential, decrease the potential difference (ΔE), increase the reaction rate and improve the
stability of the electrode response [5]. These electro-catalytic processes, proceeding at modified
electrodes, present
Carbon nanotubes are famed for their high sensitivity, a result of their unusual nanostructure, the
reason why they are used for analyte detection [2]. The use of graphene oxide has attracted
many researchers because of its sp3 hybridized material which exhibits a very larger surface area,
many functional groups and it allows smooth electron mobility on its planar, making it an
excellent modifier coupled with nitrogen-doping that also increases its conductivity by raising
the Fermi level towards the conduction band compared with prisitin graphene [19].
2.3.2 Characterisation of synthesized composite
Infrared spectroscopy, IR
Infrared spectroscopy is an important technique that provides easy way to identify the presence
of certain functional groups in a molecule [43]. Also, the unique collection of absorption bands
can be used to confirm the identity of a pure compound or to sense the presence of specific
impurities [29]. In infrared spectroscopy, IR radiation is passed through a sample where some of
the radiation is absorbed by the sample and some of it is passed through (transmittance) [11].
The resulting spectrum represents the molecular absorption and transmission, producing a
molecular fingerprint of the sample [44]. Similar to a fingerprint, two unique molecular
18
structures cannot produce the same infrared spectrum. As a result, infrared spectroscopy can
result in a positive identification (qualitative analysis) of every diverse kind of material [11].
Additionally, the size of the peaks in the spectrum is a direct indication of the amount of
material present [37]. This makes infrared spectroscopy a useful technique for several types of
analysis in terms of identifying an unknown material, determining the quality or consistency of a
sample and the amount of component in a mixture [4]. A typical specta is illustrated in fig 2.4
below.
FIGURE 2.4 TYPICAL FT-IR SPECTRA OF GRAPHENE OXIDE.
Electrochemistry
Electrochemistry is the study of production of electricity from energy released during
spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous
chemical transformations [45]. It involves the transfer of electrons between electrodes and
19
reactant molecules in a solution producing electric power [45]. The electrochemical techniques
used involve a three-electrode system and the cell is monitored by a combination of a computer
and a potentiostat / galvanostat [46]. A chemical reaction is brought about by application of
excess voltage from an external power and the resultant current flow is measured to obtain the
characteristic of the analyte under study [47].
2.4 Voltammetry
Voltammetry is a class of electrochemical method used in analytical chemistry and various
industrial processes whereby, information about an analyte is obtained by measuring the current
against the potential [6].
In a voltammetry technique, the common characteristic involves the application of a potential (E)
to an electrode and the monitoring of the resulting current (I) flowing through the
electrochemical cell [48]. The applied potential in many cases is varied or the current monitored
over a period of time (T) [6]. Voltammetry techniques can be described as some function of E, I
and T. Therefore, voltammetry can be generally defined as the investigation of three-dimensional
space relating to potential (E), current (I) and time (T). At least two electrodes are required to
conduct an experiment in voltammetry. The working electrode (WE), reference electrode (RE)
and usually a counter electrode (CE) are the types of electrodes commonly used in voltammetry
technique [49].
2.4.1 Instrumentation
The basic component of a modern electroanalytical technique for voltammetry comprises of the
following; a potentiometer, a computer and the electrochemical cell. The types of the electrodes
are, working, reference and counter electrodes [50].
20
2.4.2 The potentiostat
The working principle of a potentiostat actually depends on its connection to the electrochemical
cell. It controls the potential of the counter electrode (CE) against the working electrode (WE)
so that the potential difference between the reference electrodes (RE) is distinct, and agrees with
the value specified by the user [51]. Therefore, in potentiostatic / galvanostatic mode, the current
flow between the working electrode and the current is controlled [52]. The potential difference
between the reference electrode and working, and the current flowing between the current and
working electrode are constantly monitored [52].
2.4.3 Electrodes
Generally, an electrode provides the interface across which a charge can be transferred or where
the effect of the charge can be felt like the working electrode (WE), which is where the action of
interest takes place [52]. The electrode is an utmost important part of the system. The shape, size
and style of modification on the surface, all depends on the application process [52].
2.4.4 The reference electrode (RE)
The reference electrode is an electrode with a steady and recognizable electrode potential [6]. It
is used as point of reference to control and measure the potential of the other electrodes [53]. The
saturated calomel electrodes (SCE), silver/silver chloride (Ag/AgCl) electrode are the most
commonly used reference electrode for aqueous solutions [52].
2.4.5 Counter electrode (CE)
The counter electrode (also known as auxiliary electrode) is usually made of an inert material of
platinum (Pt) or metallic foil, gold (Au), graphite or sometimes glassy carbon may be used [52].
Counter electrode does not usually take part in the electrochemical reaction, but the total surface
21
act as source of electron so that current can flow between the working electrode and counter
electrode which make it not to be isolated from the reaction [52]. The surface area must however
be higher than that of the working electrode so that it will not be a limiting factor in the kinetics
of the process under investigation [11].
2.4.6 Working electrode (WE)
The working electrode is where the reaction or transfer of interest is taking place [48]. At an
appropriate potential, oxidation or reduction of a substance on the surface of the working
electrode will bring about a mass transport of a new material along with current been produced.
The commonly used materials for working electrodes are glassy carbon (GC), platinum (Pt), gold
(Au). Others include small mercury drop and film electrodes. The size and shape also varies and
depends on application. The quality of an ideal working electrode includes a wide potential
range, low resistance as well as a surface that is reproducible [11].
2.4.7 Cyclic voltammetry
The cyclic voltammetry is the most extensively used technique for acquiring qualitative
information about electrochemical reactions [11]. It has the ability to rapidly provide
considerable information on the thermodynamics of redox processes and the kinetics of
heterogeneous electron-transfer reactions as well as the coupled chemical reactions or adsorption
processes [6].
The current potential curve usually shows a peak at a potential where the oxidation or reduced
reaction occurs whilst the height of the peak current could be used for the quantification of the
concentration of the oxidation or reduction species [11] .
22
2.4.8 Illustration of a typical cyclic sweep voltammetry.
In simple analysis, the surface is started at a particular voltage with respect to a reference
half-cell such as calomel or Ag/AgCl, the electrode voltage then changes to a higher or lower
voltage at a linear rate and finally changed back to the original value at the same linear rate [45].
When the surface becomes sufficiently negative or positive, a solution species might gain
electrons from the surface or transfer electrons to the surface resulting in a measurable current in
the electrode circuitry [45]. However, some important information can also be investigated from
the parameters, this include whether the electrochemical process displayed by the sample is
reversible, irreversible or quasi –reversible, and also give close information on how fast the
electron process is, relative to other processes such as diffusion. For example, if the electron
transfer is fast relative to the diffusion of electroactive species from the bulk solution at the
surface of the electrode, the reaction is said to be electrochemically reversible, and the peak
separation (ΔEp) is given by equation;
ΔE = Epa- Epc = 2,303RT/nF
(2.4)
Where ΔEp is the peak separation (V), Ep,a is the anodic peak potential (V), Ep,c is the cathodic
peak potential (V), n represent the number of electrons and F the Faraday constant (96 486 C
mol-1), R is the gas constant (8.314 J mol-1 K-1) and T is the absolute temperature of the system
(298 K) [11]. Therefore, number of electrons (n) involved in the electrochemical process can be
estimated from the above equation [8]. Therefore, for a redox couple that is reversible, the cyclic
voltammograms has been studied to have the following defined characteristics;
The expression;
ΔE = Epa -Epc = 59/n mV
(2.5)
Determines the voltage separation between the current peaks.
23
The ratio of the peak currents is always equal to one:
Ipa/Ipc =1
(2.6)
The formal potential Eo' for a reversible redox couple is easily determined as the average of the
two peak potentials
: Eo' = (Epa + Epc)/2
(2.7)
The peak currents are usually proportional to the square root of the scan rate
(Ip is proportional to ʋ½)
Quantitative information regarding analyte concentration, that specifies the anodic peak current,
(Ip,a) and cathodic peak current (Ip,c) in terms of analyte concentration, C [11]. The scan rate
(mV s-1) of the process is varied, ( how rapidly the electroactive specie is diffusing through the
solution to and from the surface of the working electrode), is obtained from the voltammograms
using Randles- Sevcik equation [51],
Ip = -0.4463nF [nF/ RT]1/2 T*D ½ vA
(2.8)
Where, n is number of electrons appearing in the half-reaction for the redox couple, ʋ is the rate
at which potential is swept, F is the Faradays constant (96485 C mol-1), A, the electrode area
(cm2), R represent the universal gas constant (8.314 J mol-1 K-1), T is the absolute temperature
(K), D is the diffusion coefficient analyte (cm2 sec-1) and Γ⃰ represents the surface
concentration of the adsorbed species on the modified electrode [54].
If temperature is assumed to be 25 0C (298.15 K), the equation can be written as [51],
Ip =2.69 ×105 n 3/2 AD ½ CO
2.8.1
24
An illustration of a cyclic voltammetry (CV) for the reduction of 1 mM ferricyanide in 0.1 M
KCl at glassy carbon electrode, scan rate; 100 mV s-1 is shown in in Fig 2.5 [11] . The peak
shape of the oxidative and the reverse reductive current versus electrode potential is typical of an
electrode reaction in which the rate is governed by diffusion of the electroactive species to a
planar electrode surface. (Ip,a and Ip,c) are the anodic and cathodic peak respectively [51].
Typical cyclic voltammogram for a reversible reaction
Plotting the peak current against the square root of the scan rate, the slope of the linear plot can
be used to estimate the diffusion coefficient according to the Randles-Sevcik equation shown
above [55]. At this instance, the log of peak current versus the log of scan rate gives a linear plot
whose slope distinguishes between diffusion controlled peaks, adsorption peaks or even a
mixture of the two [55]. For diffusion peak, a plot of the Log ip versus log ʋ is linear with a slope
of 0.5 and a slope of 1 for an adsorption peak [11] . Intermediate values of the slope are
25
sometimes observed, and suggested to be a “mixed” diffusion-adsorption peak [11]. The surface
concentration of the adsorbed material is estimated by the use of the Brown-Anson model
equation;
Ip = n 2 F 2T*Av / 4RT
2.8.2
Where Ip, n, F, A, R, v, T and Γ⃰ (the surface concentration of the adsorbed species of the
electrode modifier) are all expressed in the equations above [11]. It is important to note that, in a
sensor operation, some parameter such as potential scanning rate, operating temperature and
diffusivity of the reactant are very necessary [56]. The peak current may be used to quantify the
concentration of the reactant of interest provided, that the effect of concentration on the
diffusivity is negligible [4].
Differential Pulse Voltammetry
The sensitivity of cyclic voltammetry is considerably low hence the aim of pulsed voltammetric
methods is to lower detection limits [57]. The relationship between current and concentration
gives quantitative information about the system studied and this can be clearly seen if diffusion
controls the current magnitude [11]. In differential pulse voltammetry (DPV) fixed voltage
pulses are superimposed on a linear potential ramp and are applied to the working electrode at a
period of selected time just before the potential pulse drops [57]. The current is measured twice
which at a point just before the pulse is being imposed and then again just before the pulse
begins to drop [57]. In DPV when a potential pulse is applied to an electrode the capacitive
current flows proportionally to it but decays exponentially with time [57]. The magnitude of the
faradaic current decreases as an exponential function versus (t)1/2 [58].
26
Linear sweep voltammetry
Linear sweep voltammetry is a voltammetric method where the current at a working electrode is
measured while the potential between the working electrode and a reference electrode is swept
linearly in time [59]. Oxidation or reduction of species is registered as a peak or trough in the
current signal at the potential at which the species begins to be oxidized or reduced [57]. Linear
sweep voltammetry can identify unknown species and determine the concentration of solutions
[59]. E1/2 can be used to identify the unknown species while the height of the limiting current can
determine the concentration. The sensitivity of current changes vs. voltage can be increased by
increasing the scan rate [57]. Higher potentials per second result in more oxidation/reduction of a
species at the surface of the working electrode [58].
Chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working
electrode is stepped and the resulting current from faradaic processes occurring at the electrode
(caused by the potential step) is monitored as a function of time [57]. Limited information about
the identity of the electrolyzed species can be obtained from the ratio of the peak oxidation
current versus the peak reduction current [57]. However, as with all pulsed techniques,
chronoamperometry generates high charging currents, which decay exponentially with time as
any RC circuit [60].
Chronoamperometry is a very powerful method for the quantitative analysis of a nucleation
process [60]. This useful technique leads to obtain the initial information about nucleation and
growth mechanism in a studied system [5]. Additionally, the amount of charge for deposition
(dissolution) can be determined [60]. Also, this method can be applied for the determination of a
nucleation rate constant and an adsorption isotherm [4]. With the chronoamperometry, the
27
current is measured versus time as a response to a (sequence of) potential pulse [60]. The
recorded current can be analyzed and its nature can be identified from the variations with time
[60].
Electrochemical impedance spectroscopy
EIS is a perturbative characterization of the dynamics of an electrochemical process [61].
Electrochemical impedance is the response of an electrochemical system (cell) to an applied
potential [60]. The frequency dependence of this impedance can reveal underlying chemical
processes [61]. It is regarded as a derivative of linear sweep voltammetry or staircase
voltammetry, with a series of regular voltage pulses overlaid on the potential linear sweep or stair
steps [61]. The current is measured instantaneously before each potential change, and the current
difference is plotted as a function of potential [62]. The Nyquist plots obtained in
electrochemical impedance spectroscopy comprise of a straight line and a semi-circle [63]. The
diameter of the semicircle corresponds to the charge transfer resistance and diffusion controlled
process respectively [61]. The straight line portion represents the Warburg impedance which
takes into account the frequency dependence on diffusion transportation to the electrode surface
[64].
Bode Plots
The nature of the Bode plots confirms the structural differences between the bare electrode and
modified electrodes [62]. Bode plots, shows a well- defined symmetrical peak for the bare
electrode at particular angle and corresponding frequency representing the relaxation process at
the electrode solution interface [65]. This relaxation process shifts to different phase angles and
28
frequencies on modification of the electrode [62]. Theseshifts indicate that the reactions are now
occurring at a modified surface rather than the bare electrode [65].
2.5 Electro-catalysis
Electro-catalysis can be defined as a process whereby, an electro-catalyst participate as a catalyst
in electrochemical reaction [56]. The electro-catalyst can be a specific form of catalyst that
function at the surface of the electrode or may be the surface of the electrode itself [42]. The
catalyst materials modify and increase the rate of chemical reaction without being consumed in
the process. The process can be heterogeneous, which involve chemical reaction occurring at the
surface of the electrode and electron transfer usually takes place at the interface, leading to a new
product formation (in catalytic electro-synthesis), or homogeneous reaction which is a sequence
of reactions that involve a catalyst in the same phase as the reactants [23].
However, a homogeneous catalyst is most commonly co-dissolved in a solvent with the reactants
(coordinating complex or enzymes) [66]. The ability to catalyse some reaction is one of the most
outstanding properties of conducting polymers [42]. The kinetics of electrode processes of some
solution specie is able to be improved by a thin layer of a conducting polymer, deposited onto the
surface of a substrate electrode [45].
These electro-catalytic processes, proceeding at a conducting polymer electrode is presently
yielding application in various fields of electrochemistry [54]. Three processes are considered to
be happening during the electro-catalytic conversion of solution species at conducting polymer
modified electrode [55]. Firstly, heterogeneous electrons transfer between the electrode and a
conducting polymer layer, and electron transfer within the polymer film [48].
This process is normally accompanied by the movement of charge compensating anions and
solvent molecules within the conducting polymer film [48]. Many factors determine the rate of
29
this process and which include, electric conductivity of polymer layer, electron self-exchange
rate between the chains/or clusters of polymer, and anion movement within polymer films also
seems to be greatly significant [48]. Secondly, the diffusion of solution species to the reaction
zone, where the electro-catalytic conversion occurs [48]. Unlike simple electrode reaction, this
process can be more complicated in cases where the electro-calytic conversion occurs within the
polymer film [48]. In that case, the diffusion of species within the film, in addition to the
possible electrostatic interaction of this specie with the polymer film should be taken into
account [67].
In the third process, a chemical (heterogeneous) reaction takes place between solution species
and conducting polymer [67].
Electro-catalysis takes place when the transfer of electrons between the hydroquinone and NGO /
PANI- GCE surface. In this regard, the catalyst on the modified electrode becomes oxidised in
the electrolyte and then interacts with the analyte which are mostly in their reduced state. There
is electron process taking place which leads to the oxidation of the analyte and simultaneous
reduction of the catalyst [69].
2.5.1 Chemically modified electrode
Chemically modified electrode (CMEs) involves an approach to electrode system design that
finds application in electrochemical investigation [69]. It includes the relationship of
heterogeneous electron transfer and chemical reactivity of electrode surface chemistry, electron
and electrostatic phenomena at electrode surface, as well as ionic transport phenomena in
polymers [70].
A chemically modified electrode allows a thin film of selected conducting material to be
bounded on its surface for the purpose of improving the chemical, electrochemical, optical,
30
electrical, transfer of electron and other desirable properties of the film in a rational chemically
designed manner [64]. Therefore, the significant drive for modifying electrode surface is
electro-catalysis of the electrode reaction of an analytical desired substrate. It is often observed
that redox reaction of an analyte on a naked electrode surface using voltammetry techniques such
as cyclic voltammetry or square wave voltammetry are slow, so that oxidation or reduction
occurs at a potential that is greatly higher than positive or negative, respectively, than the
expected thermodynamic potential [18].
Such situation can be sorted by accelerating the desired reaction with an immobilized mediator
catalyst [64]. The mediator is attached on the surface of the electrode to function as facilitator for
the charge transfer between the analyte and electrode [64]. The oxidised form of the mediator
catalyst is rapidly reduced, and then, its reduced form reacts with the analyte specie in solution
[64]. It can then be indicated that the electron transfer occurred between the mediator and the
electrode and not between electrode and the analyte, directly. From the illustrated equation
below, a mediator can be represented by M and the analyte, A,
Mox + ne
Mred + A ox
Mred
2.8.3
Mox + A red
2.84
31
CHAPTER 3
EXPERIMENTAL
3.0 Introduction
The chapter gives information on materials and different experimental procedures employed for
the preparation of an electrochemical sensor based on nitrogen doped graphene oxide modified
with polyaniline. Electrochemical characterization of hydroquinone was evaluated using cyclic
voltammetry, impedance spectroscopy, bode plots, linear sweep, differential pulse voltammetry
and chronoamperometry. Parameters like pH and scan rate were investigated to evaluate the
applicability of the NGO / PANI – GCE in the electro catalysis of hydroquinone.
3.1 Equipment
F.T.I.R. Spectra were obtained using Thermo scientific Model equipped Model with OMNIC
software. UV – Vis spectroscopy was used for characterization. Sonicator model KQ- 20B was
used for agitation of samples. All electrochemical experiments were performed using Auto lab
potentiostat PG STAT302N equipped with NOVA version 1.10 software and encompassed with a
three electrochemical cell made up of glassy carbon electrode, platinum wire counter and a
Ag|AgCl reference electrode. Samples were dried in tan oven drier. Weighing of samples was
done using OHAUSE analytical balance. pH measurement was done using the pH meter, PHS
-3C model.
32
3.2 Chemicals and reagents
Chemicals used in this study were of analytical grade and were used without further purification.
Potassium ferrocyanide and potassium Ferricyanide, aniline (C6H5NH2), ammonium persulphate,
ammonium chloride (NH4Cl), sodium hydroxide(NaOH), dimethyformamide (DMF), potassium
bromide (KBr), hydrochloric acid (HCl, 37%), hydrogen peroxide (H2O2, 35 wt% water
solution), sulphuric acid (H2SO4), sodium nitrite (NaNO2), potassium permanganate (KMnO4),
hydroquinone,
di-
potassium
orthophosphate (KH2PO4),
orthophosphate,
(K2HPO4) and potassium
di-hydrogen
Polishing pads and alumina micro powder (1.0, 0.3 and 0.05 μm
alumina slurries) were used for polishing the glassy carbon electrode (GCE).
3.2.1 Synthesis procedures
Synthesis of Polyaniline
Aniline was first distilled to remove impurities. The aniline (0.2 M) was dissolved in 10 mL of 1
M HCl aqueous solution. While maintaining vigorous stirring at room temperature ammonium
peroxydisulfate (APS 0.05 M) was dissolved in 10 mL of 1 M HCl to form an oxidant solution
[25]. The oxidant solution was then quickly poured into the aniline solution at room temperature
followed by immediately magnetic stirring for 2hours. Polymerization can be observed when
green color of PANI emeraldine salt became visible. The stirring was stopped after 2hours and
left undisturbed to react for 24hours. After which the precipitated polymer was collected by
filtration and repetitively washed with water ethanol and hexane until the filtrate became
colorless. Polyaniline sample was collected having been dried at 60○C, Fig 3.1 [25]. Oxidation
of aniline hydrochloride with ammonium persulfate to yield polyaniline (emeraldine)
hydrochloride salt [27].
33
Synthesis of Polyaniline nanoparticles
Fig 3.1 Assumed possible mechanism of polymerization of aniline monomer to polyaniline
3.2.2 Graphene oxide
Graphene oxide nanoparticles were synthesized using the improved modified Hummers’ method,
a slightly modified method of the original hammers’ method in Fig 3.2
34
Fig 3.1: Synthesis of graphene oxide nanoparticles
Graphite flakes (2 g) and NaNO3 (2 g) were mixed in 90 mL of H2SO4 (98%) in a 1000 ml
volumetric flask kept under at ice bath (0-5.C) with continuous stirring. 2. The mixture was
stirred for 4 hours at this temperature and potassium permanganate (12 g) was added to the
suspension very slowly. The rate of addition was carefully controlled to keep the reaction
temperature lower than 15 oC. The mixture is diluted with very slow addition of 184 ml water
and kept under stirring for 2 hours [20].
The ice bath was then removed, and the mixture was stirred at 35°C for 2 hrs. The above
mixture is kept in a reflux system at 98°C for 10-15 minutes. After 10 minutes, the temperature
was changed to 30°C which gave a brown colored solution. Again after 10 min, it was changed
to 25°C, and maintained the temperature for 2 hrs. The solution was finally treated with 40 ml
H2O2 by which color changed to bright yellow. An aliquot of 200 ml of water is taken in two
35
separate beakers and equal amount of solution prepared is added and stirred for 1 hr. It was kept
without stirring for 4 hours, where the particles settled at the bottom and remaining water is
poured to filter. The resulting mixture was washed repeatedly by centrifugation with 10% HCl
and then with deionized (DI) water several times until it formed a gel like substance (pHneutral). After centrifugation the gel like substance is vacuum dried at 60°C for more than 6 hrs
to GO powder [71].
3.2.3 Doping Nitrogen on Graphene oxide and modification with polyaniline
A mass of fifty milligrams of GO NPs was added to 60 mL of ethylene glycol under sonication
for 4 hours to obtain a well-dispersed GO NPs suspension [72]. An aliquot 10 mL of ammonia
solution was added and vigorously stirred for 10 minutes, polyaniline in DMF solution (0.75 mL)
was also added under vigorous stirring for 2 hours at the room temperature, and then heated at
95◦C for 7 h. The precipitate was isolated by filtration then washed with ethanol and deionized
water. It was then dried at 80◦C overnight. Highly crystalline N-GO / PANI NPs composite was
obtained [71].
3.3 Characterization of the chemical modifiers and the composite
3.3.1 Fourier Transfer Infrared Spectroscopy (FTIR)
All the synthesized compounds GO NPs, PANI NPs and NGO/PANI NPs were characterized by
FTIR. A mass of 0.0100 g of the respective samples were mixed in a mortar and pestle and
combined with 1g KBr to form a pellet and this was characterized within the range 400 - 4000
cm-1.
36
3.3.2 UV -Vis Spectroscopy
The synthesized nanoparticles, (GO NPs), and (PANI NPs), (0.005g) were dissolved in 10 mL
dimethyformamide (DMF) and experimental wavelength were scanned between 200 to 700 nm
[11].
3.3.3 Electrode modification
The BCGE was polished with alumina paste on Buehler Felt pads and ultrasonically cleaned in
ethanol for about 5 min. The electrode was further cleaned in water for about 5 min then finally
rinsed with distilled water to ensure there are no impurities or interferences, and then dried in the
sun. A mass of 0.1 g of the prepared conjugates was dissolved in a volume of 5 mL of DMSO
and these solutions were sonicated for 30 min. The cleaned BCGE was modified using the drop
and dry method using NGO, PANI and NGO / PANI in 1ml DMF. The electrode was dipped in
solution of the modifiers in DMSO [73].
3.3. 0 Buffer solution prepaparation
18.000 g/mol dipotassium hydrogen phosphate and 14.000 g/mol potassium hihydrogen
phosphate was taken and made up to 200 ml using distilled water to make the buffer. The
designated pH was reacted by adjusting
the pH using sodium hydroxide
( 0.1 M) and
hydrochloric acid (0.1 M) [74].
3.4 Hydroquinone analysis
A volume of 1mM of Hydroquinone solution was prepared by 0.150 g diluted to 1000ml with
phosphate buffers in the pH range between 3 to 8.
37
3.5 Electrochemical characterization
3.5.1 Electrochemical behavior of modifiers in 1 mM [Fe (CN)36]3-/4 solution
Cyclic voltammetry was used for the investigation of electron transfer kinetics for the bare GCE,
NGO-GCE, PANI-GCE and NGO/PANI-GCE. The study was carried out in 1 Mm [Fe (CN)6] 3-/4
solution at a scan rate of 100 mV/s from -0.4 to 0.6 V.
3.5.2 Electrochemical impedance spectroscopy and bode plots
Electrochemical impedance spectroscopy and bode plots were carried out in 1 mM [Fe (CN)6]3-/4
solution for bare GCE, NGO-GCE, PANI-GCE and NGO/PANI-GCE in order to confirm the
electron transfer resistance of the electrodes in correspondence to results obtained in cyclic
voltammetry. Bode plots and electrochemical impedance spectroscopy occurs at the same time
during analysis but in separate windows.
3.5.3 Scan rate studies in in 1mM [Fe (CN)6]3-/4 solution
Scan rate studies were performed in 1 mM [Fe (CN)6]3-/4 solution by using NGO/PANI-GCE
electrode in order to determine the surface coverage of the modified electrode. Scan rate range
was from the range of 100 to 350 mV/s from -0.4 to 0.6V.
3.6 Optimisation of parameters
3.6.1 Effect of pH
The effect of pH was studied in the range from pH 3 to 8. Adjusting of the buffer solution was
carried out with dilute concentration of 0.1 M NaOH and 0.1 M HCl. The studies were carried
out by preparing 5 volumetric flasks (100ml each) with different pH phosphate buffer solution
containing 1 mM of hydroquinone. The studies were carried out in cyclic voltammetry by using
NGO/PANI-GCE electrode. The studies were performed by scanning the electrode from 0.0 to
0.9V at a fixed scan rate of 100 mV/s.
38
3.6.2 Comparative studies in pH 4 phosphate buffer solution
Comparative studies were performed in pH 4 phosphate buffer solution containing 1mM
hydroquinone. The studies were carried out by using modified electrodes a) Bare GCE,
NGO-GCE, PANI-GCE and NGO/PANI-GCE in order to confirm the reduction in potentials and
increase in anodic peak currents on the oxidation of hydroquinone. Cyclic voltammetry was used
for this study. The studies were performed by scanning each electrode from 0.0 to 0.9V at a fixed
scan rate of 100 mV/s.
3.6.3 Scan rate studies in pH 4 phosphate buffer solution
Scan rate studies were carried out in-order to determine the effective surface coverage of the
modified electrode NGO/PANI-GCE. There studies were done in pH 4 phosphate buffer solution
by using NGO/PANI-GCE modified electrode. Cyclic voltammetry was used for this study. The
studies were performed by scanning NGO/PANI-GCE electrode from 0.0 to 0.9V at a fixed scan
rate ranging from 100 to 350 mV/s.
3.7 Kinetic studies
Kinetic studies were carried out in 1mM hydroquinone solution in pH 4 phosphate buffer. The
studies were performed in cyclic voltammetry by using NGO/PANI- GCE modified electrode.
The studies were performed by scanning NGO/PANI-GCE electrode from 0.0 to 0.9V at a fixed
scan rate ranging from 100 to 350 mV/s.
3.8 Langmuir adsorption isotherm studies
Linear sweep studies were carried out in order to determine the Langmuir adsorption isotherm of
hydroquinone on the surface of NGO/PANI-GCE. The study was performed by using working
standard solutions of hydroquinone 10 µM, 20 µM, 30 µM, 40 µM, 50 µM and 60 µM prepared
39
from serial dilution of 1mM hydroquinone in phosphate buffer solution pH 4. The working
standard solution were prepared in 100ml volumetric flask and diluted to the mark with pH 4
phosphate buffer solution. The response behavior was observed on NGO/PANI-GCE. The
studies were performed by stirring the electrode for 10 minutes and scanning the electrode from
0.0 to 0.9V at a fixed scan rate of 100mmV/s .
3.9 Catalytic rate constant
Chronoamperometric studies were performed in order to determine catalytic rate constant was of
hydroquinone on the surface of NGO/PANI-GCE. The studies were performed by preparing
working standard solutions of 20 µM, 40 µM, 60 µM, 80 µM and 100 µM form 1mM stock
solution of hydroquinone in pH 4 phosphate buffer solutions. The working standard solution
were prepared in 50ml volumetric flask and diluted to the mark with pH 4 phosphate buffer
solution. The prepared working standard solutions were filled in the electrochemical cell
respectively and the behavior was observed on NGO-PANI/GCE. The studies were performed by
scanning the electrode at different potentials 0.00V, 0.8 and 0.8. The electrode was scanned for
25 seconds at fixed scan rate of 100mV/s.
3.10 Differential pulse voltammetry
Differential pulse voltammetry is a sensitive technique that was used to determine the limit of
detection and limit of quantification. The study was carried out by preparing working standard
solution of 0.2µM, 0.4µM, 0.6µM, 0.8µM, 1.0µM, 1.2µM, 1.4µM and 1.6µM from 1mM stock
solution of hydroquinone in pH 4 phosphate buffer. The working standards were prepared in
100ml volumetric from flask and diluted to mark with pH 4 phosphate buffer solutions. The
prepared 100ml solution of working standards was placed in the electrochemical cell
respectively. Before analysis each solution was purged with nitrogen for 10minutes prior to
40
analysis and then the behavior was observed on NGO/PANI-GCE electrode. Parameters: initial
potential 0.00V, final potential 0.90V, step potential 0.00500V, modulation amplitude 0.0500V,
initial time 0.5s and scan rate 0.100 V/s.
3.11 Stability studies
Stability studies was performed on NGO/PANI-GCE in 1mM hydroquinone solution
phosphate buffer pH 4 by performing 20 continuous
in
cycles by using cyclic voltammetry
technique at 0.1V/s scan rate at 0.2 to 1.0V.
3.12 Reproducibility studies
NGO / PANI - GCE was analyzed in a solution containing 1mM of Hydroquinone solution by
employing the DPV technique. Before and after each analysis the electrode was washed using
distilled water in order to remove adhered particles at a scan rate of 0.01 V/s. potential of 0.2 to
1.2 V and amplitude of 0.05 V for a period of about 3 times.
Effect of interference
Interference studies were investigated using DPV technique. 1 mM of hydroquinone was
detected in the presence of hydrogen peroxide and phenol which are assumed to interfere with
detection of hydroquinone. Equimolar solutions of interfering analytes were prepared in the same
matrix with hydroquinone in phosphate buffer solution pH 4. The mixture was analyzed from 0
to 1.2 V potential and amplitude of 0.05 V.
41
CHAPTER 4
RESULTS AND DISCUSSION
Introduction
The chapter highlights all the results and findings obtained from the research. The obtained
results are discussed in detail and compared to work published by other researchers.
4.1 F.T.I.R characterisation
42
Fig 4.1 FT-IR Spectrum of (a) GO , (b) , PANI , (c) NGO- PANI Nps
The FTIR spectrum shows the stacked spectra of graphene oxide, polyaniline and nitrogen
doped graphene oxide decorated with polyaniline (NGO/PANI NPs)
4.1 NGO / PANI Np FTIR
NGO / PANI Np spectra at (a) shows the presence of the quinoid ring and the benzoid ring shifts
to 1599.29 cm-1 and 1492 cm-1, respectively. These could be attributed to the inco-operation of
the polyaniline matrix into the graphene oxide lattice.
Further the C-N stretching vibrations
which appeared at 1143.61 cm-1 in PANI where shifted to 1599.29 cm-1 in NGO/PANI. The C-H
stretching in polyaniline shifted from 755.70. to 617.37 cm-1. Shift of bands were a result of
formation of the Graphene oxide-nitrogen complex.
Figure 4.1.2 Graphene oxide FTIR
Graphene oxide was characterized by FT-IR. Spectra were obtained from 4000-400 cm-1 region
[9]. According to the literature of Graphite, no distinct peaks are detected in graphite however
43
graphene oxide (GO) showed collections of peaks corresponding to the following functional
groups: C=O (cm-1), aromatic C=O (1646 cm-1), carboxyl C-O (1406 cm-1), epoxy, alkoxy C-O
(1000 cm-1), hydroxyl O-H (3500 cm-1). The appearance of the collection of the functional
peaks gave an indication that the original extended conjugated orbital system of the graphite
powder was destroyed during the oxidation process and oxygen containing functional groups has
been inserted into the carbon skeleton according to literature.
Figure 4.1.3 Polyaniline FTIR
The FTIR spectrum of PANI, shows the characteristic at 1658 .79. and 1464.61 cm-1 are
assigned to the C=C stretching of quinoid ring and benzenoid rings respectively. The
characteristics peaks at 1202.60 and 1143.61 cm-1 are attributed to the C-N stretching vibration
of the secondary aromatic amine group and aromatic C-H in plane bending vibration respectively
[47]. The peak at 765.70 cm-1 represent C-H the small peak at 606.26 cm-1 represented
para-distributed aromatic rings indicating polymer formation.
44
Fig 4.1.3 UV- visible spectra of graphene oxide
A maximum absorption peak at around 245 nm is observed for GO in the UV visible spectra,
which corresponds to the π-π⃰ transition of aromatic C-C bonds. Literature highlights that the
absorption peak is well around 230 nm hence the obtained absorption peak is well in range.
Figure 4.1.4 Pani UV-Vis
45
Literature highlights the presence of two bands at 590–690 (π*←n transitions –quinone-imine
groups- exciton transition of the quinoid rings), and 290–390nm
(π*← π transitions) , to π-π* transition of the benzenoid rings and the other due to charge
transfer excitations of the quinoid structure which are characteristic of emeraldine base [27] .
4.2 Electrochemical characterisations
Voltammetric studies in 1.0 mM [Fe (CN)6]3-/4- in 1M KCl
Figure 4.2: Cyclic voltammograms for: a) bare GCE b) NGO- GCE c) PANIi-GCE d)
NGO/PANI- GCE in 1 mM [Fe (CN)6]3-/4- solution. Scan rate = 100 mV/s
Cyclic voltammetry was used for the investigation of electron transfer kinetics for the bare GCE,
NGO-GCE, PANI-GCE and NGO-PANI-GCE. The study was carried out in 1 mM [Fe (CN)6]
solution at a scan rate of 100 mV/s from -0.6 to 0.8 V.
46
ΔEp values of working electrodes
Electrode
ΔEp values / V
BGCE
0.40
NGO-GCE
0.19
PANI-GCE
0.18
NGO- PANI -GCE
0.16
Electrodes gave peak potential differences with the following trend; NGO/PANI-GCE <
PANI-GCE< NGO-GCE< BGCE. The NGO/PANI-GCE exhibited the best electron transfer
kinetics as compared, better than the other electrodes. The lower Ep for NGO/PANI-GCE
compared to the rest of the modifiers electrode is possibly due to the linkages between the NGO
and PANI as well as their good alignment on the electrode surface which makes, electron
exchange between the redox probe and electrode modifier much faster confirming its improved
electron transfer compared with the rest of the modified electrodes.
The electron transfer kinetics of the modifiers on the surface of glass carbon were also confirmed
by the increase in anodic peak current the order of electron transfer is therefore Bare -GCE (𝐼𝑃𝑎:
88 ×10-5 A) > NGO-GCE (𝐼𝑃𝑎: 126 ×10-5 A)> PANI-GCE (𝐼𝑃𝑎: 170 ×10-5 A) > NGO-PANI-GCE
(𝐼𝑃𝑎: 332 ×10-5 A). The increase in peak current for NGO/PANI-GCE as compared to other
electrodes is due to improved flow of pie electrons on the surface of nitrogen doped graphene
oxide. A high surface area as well as good conductivity possessed by NGO/PANI-GCE can also
47
be attributed to the catalytic properties of nitrogen doped graphene as a result of its higher
surface-to-volume ratio which allows accessibility surface to polymer matrix [24], hence a more
enhancement compared to PANI –GCE.
The large Ep for NGO-GCE implies that on its own, it has poor electron transfer properties,
however when coupled with polyaniline, its catalytic properties are activated as evidenced by the
pronounced decrease in Ep separation. This is explained in terms of the electron donating nature
of PANI which reduce the redox potentials. It should also be equally noted that NGO has better
electron kinetics than BGCE, a property attributed to the nitrogen for pumping electrons in the
GO lattice. The amide linkage formed between NGO and PANI facilitates the easy flow of
electrons to and from the linked NGO molecule. Improved electron transfer kinetics is also a
result of the nanostructured sizes of the conjugates which enables them to provide a large surface
area for the transfer of electrons. NGO/PANI-GCE showed better electron transfer properties.
4.3 Electrochemical impedance spectroscopy
Electrochemical impedances spectroscopy (EIS) measurements in [Fe(CN)6]3-/4 solution using 0.1
M KCl as the supporting electrolyte were investigated to assess the electron transfer properties of
the modified glassy carbon electrode.
48
Fig 4.3 Nyquist plots obtained for a) bare GCE b) NGO-GCE c) PANI -GCE d) and e)
NGO/PANI -GCE in 1mM [Fe(CN)6] solution in 1M of KCl.
Inset is the Randles circuit
model used in fitting data.
Fig 4.3 shows a Nyquist plots for bare-GCE, NGO-GCE, PANI-GCE and NGO/PANI -GCE. The
Nyquist plot data was fitted using Randles equivalent region (electron-transfer properties) and
low frequency region (electrolyte diffusion controlled properties). The region of interest to this
work is the high frequency region which is the kinetically controlled region and will give us the
kinetics of the modified bare glassy carbon electrode. Kinetically controlled region in the
Nyquist plot is of high importance since it shows the semi-circle which is related to the charge
–transfer resistance (RCT). The smaller the RCT, the more conducting the electrode or the
materials used to modify the electrode [75]. The RCT value for a bare electrode surface was 113.9
kΩ cm-2 and this value was 86.5 kΩcm-2 on NGO-GCE, 65.4 kΩcm-2 PANI-GCE and 58.2
kΩcm-2 on NGO-PANI-GCE. The Rct value was the smallest at NGO-PANI-GCE compared to
49
all other electrodes. The decrease in Rct values at NGO-PANI-GCE is attributed to (i) PANI and
NGO being a good conductor of electrons.
Table 4.2 Charge transfer resistance values of modified electrodes
R c t ( kΩ cm-2)
Electrode
BARE GCE
113.9
NGO-GCE
86.5
PANI-GCE
65.4
NGO/PANI-GCE
58.2
50
Bode plots in 1.0 mM Fe(CN)6 3–/4– aqueous solution using 0.1 M KCl as the supporting
electrolyte.
Figure 4.4 Bode (phase angle versus log f) plots obtained for i) bare GCE ii) NGO-GCE iii)
PANI -GCE iv) and v) NGO/PANI -GCE in 1mM [Fe(CN)6] solution in 1M of KCl
Bode plots were used to obtain frequency related information, which cannot be obtained from
their Nyquist plots. The nature of the Bode plots confirmed the structural differences of the GCE
modified electrodes and the bare GCE [40]. The bare GCE showed unsymmetrical peak with a
maximum phase angle value of 53o corresponding to the relaxation process of the GCE /
hydroquinone interface. After modification of GC electrode with NGO, PANI, and NGO/PANI,
all the peaks shifted towards lower frequencies for the relaxation processes of the modifier –GCE
/ hydroquinone interfaces, with the PANI spectrum showing a broad band which stretches from
lower frequencies to higher frequencies. The relaxation process of the NGO/PANI-GCE /
Hydroquinone was at a phase angle of 67O. Changes in phase angle and frequencies confirmed
51
that the oxidation of hydroquinone was taking place at modified platform rather than on the bare
GCE surface. Both the Nyquist and the Bode plots confirmed the poor electron transfer kinetics
for the NGO-GCE for hydroquinone.
Such a trend was also observed from the cyclic voltammetry of hydroquinone on different
electrode where NGO/PANI-GCEs showed reduced potentials and improved currents. At the
frequency region of the impedance under study the charge transfer Rct, decreased for the
electrode modifiers due to facilitation of the electron transfer which is an indication that films
form high electron conduction pathways between the electrode and electrolyte / analyte [76].
The phase angles values for all electrode surfaces studied in this work are less than the ideal 90○
for a true capacitor. The bare GCE had a phase angle at 53○ corresponding to the GCE/
hydroquinone interface.
4.4 Surface area determination
The surface area of NGO/PANI-NPs GCE was determined in 1 mM K3[Fe(CN)6]3-/4 by applying
the Randles –Sevcik Equation [77].
Ip= (2.69 × 105) n3/2D1/2CAeffv1/2
(4.0)
where ip the peak current, n is equal to the number of electrons transferred at the surface of the
electrode
Fe(CN)63- + e-
Fe(CN)64-
(4.1)
As a result , n is equal to 1, D is the diffusion coefficient of the analyte in solution 7.6 × 10-6
cm2/s-1 and C is the solution concentration in (mol/cm-3),Aeff is the effective surface area and v is
the scan rate (V/s-1). Based on this, ip is proportional to v1/2 and produces a linear plot with slope
m given by the equation
52
m = (2.69 × 105)n3/2 Aeff D1/2C
(4.2)
Figure 4.3 shows the obtained voltammograms at different scan rates from (a) 50 mV/s, (b) 100
mV/s, (c) 150 mV/s, (d) 200 mV/s, (e) 250 mV/s, (f) 300 mV/s and (g) 350 mV/s. The plots of
peak current, 𝐼𝑃 for both anodic and versus square root of scan rate (𝑣
1/2
) (Figure 4.2 insert)
2
were linear (𝑅 = 0. 9984 and 0.9902) , signifying a diffusion –controlled redox process [78].
Fig 4.3 Effect of scan rate on peak potentials and currents (a) 50 mV/s, (b) 100 mV/s, (c)
150 mV/s, (d) 200 mV/s, (e) 250 mV/s, (f) 300 mV/s, (g) 350 mV/s, on NGO/PANI-GCE.
Inset: Plot of Ipa, Ipc versus √ν.
The NGO/PANI-GCE had an effective surface are of 0.13 cm2 comparing with known surface
area of bare electrode of 0.0712 cm2 [8]. This indicates that the modified electrode displayed a
large surface area for electro catalysis.
53
4.5 Optimization of parameters
4.5.1 Effect of pH
pH is one of the factors affecting oxidation of hydroquinone.
Fig 4.5 Cyclic Voltammograms for NGO/PANI-GCE in (i) pH 3, (ii) pH 4, (iii) pH 5, (iv) pH
6, (v) pH 7 phosphate buffer solution containing 1 mM Hydroquinone Inset: plot of Ipa vs
pH. Scan rate = 100 mV/s.
pH studies where carried out to probe the best pH for oxidation of hydroquinone in the range
between pH three up to eight. The best peak current maximum response was obtained at pH four.
The plot of pH against current showed that as the pH changed from pH 4 to lower acidic values,
54
the peak current would decrease. Under basic pH, the peak current was very low and almost
insignificant [6].
4.6 Comparative study in pH 4 buffer
Voltammograms of the bare and modified electrodes i) bare GCE) NGO-GCE ii) PANI-GCE iii)
NGO/PANI-GCE vi were obtained by scanning the electrodes in pH 4 phosphate buffer solution
at a scan rate 100 mV/s.
Fig 4.6: Cyclic Voltammograms for i) bare GCE ii) NGO-GCE iii) PANI-GCE and vi)
NGO/PANI/-GCE, in phosphate buffer pH 4. Scan rate = 100 mV/s.
The performance of the electrodes was evaluated in pH buffer four without an analyte and as
expected there were no peaks but however, NGO/PANI-GCE rather showed excellent catalysis
with lower initial oxidation potential better than the other electrodes as shown by the graph.
55
4.7 Scan rate studies in pH 4 phosphate buffer solution without an analyte
Fig 4.7: Voltamograms of NGO/PANI-GCE in phosphate buffer ph 4 with increasing scan
rate at (a) 50 mV/s, (b) 100 mV/s, (c) 150 mV/ s, (d) 200mV/s, (e) 250 mV/s, (f) 300 mV/s (g)
350 mV/s
4.8 Comparative study in 1 mM hydroquinone in pH 4 PBS 4
Comparative studies in 1mM hydroquinone was carried out for all electrodes Bare-GCE,
NGO-GCE, PANI-GCE and NGO/PANI - GCE in order to observe the reduction in potential and
increase in anodic peak current on the electro catalysis of hydroquinone.
56
Fig 4.8: Voltammograms of (a) Bare GCE b) NGO-GCE c) PANI-GCE d) NGO/PANI-GCE
in 1mM Hydroquinone pH 4 phosphate buffers.
The voltammograms indicate that current is increasing in the order, BGCE< NGO-GCE<
PANI-GCE< NGO/PANI-GCE. The oxidation of hydroquinone initially occurs at a lower
potential on the NGO/PANI-GCE electrode. NGO/PANI-GCE also shows a gradual definite peak
rise unlike the other electrodes showing peaks which are less pronounced and defined. The
pronounced peak rise in NGO/PANI-GCE is attributed to the synergistic effect of nitrogen doped
graphene oxide with intrinsically conducting polymer Polyaniline. Peak
currents, Initial
Oxidation potential and peak oxidation potential for all probes, in 1mM hydroquinone.
57
Table 4.3
Electrode
Initial Oxidation
Peak Oxidation
Current
Potential
Potential, E/V
(A)
BCGE
0.790
0.81
5 × 10-4
NGO-GCE
0.734
0.54
2.94 × 10-4
PANI-GCE
0.695
0.81
8.23× 10-4
NGO/PANI-GCE
0.625
0.82
1× 10-3
The values in the table above show that there is an increase in the peak current from BGCE to
NGO/PANI-GCE. The distinctive peak current can be attributed to the spectacular synergistic
conductivity between NGO and PANI. Graphene oxide of NGO provided a large surface area for
electro catalysis together with the projection of different functional groups that project in the
composite [15]. The peak and initial oxidation potential of NGO/PANI-GCE for the oxidation of
hydroquinone showed that it is the best electrode as compared to other electrodes used in the past
researches.
Previous researches for the electrochemical determination of hydroquinone were done using
hydroxyapatite modified glassy carbon electrode and the oxidation potential was 0.92 V
58
Comparing with previous studies, NGO/PANI-GCE is the best electrode for the determination of
hydroquinone.
4.9 Electrochemical impedance Spectroscopy in 1 mM Hydroquinone in pH 4 PBS
A three-electrode electrochemical impendence spectroscopy (EIS) was used to probe the redox
and structural features of the different working electrodes (WE). Fig 4.9 shows the Nyquist plots
obtained in 1 mM hydroquinone. The bare GCE, NGO–GCE, PANI-GCE and NGO/PANI-GCE
displayed identical semi-circular Nyquist plots with straight line portion at lower frequencies.
Fig 4.9 Nyquist plots obtained for (i) Bare GCE, (ii) NGO-GCE, (iii) PANI- GCE, and (iv)
NGO/PANI-GCE in 1 mM Hydroquinone in pH 4 phosphate buffer.
The electrochemical behavior of the four electrodes was further investigated using the
electrochemical impedance spectroscopy (EIS) in 1.0 mM hydroquinone. The Nyquist plot in the
59
Figure above of the four electrodes comprised of semicircles and straight-line portions. The
diameter of the semicircle corresponds to the charge transfer resistance and diffusion controlled
process respectively [6]. The straight-line portion represents the Warburg impedance which takes
into account the frequency dependence on diffusion transportation to the electrode surface [6].
The representative circuit for the Nyquist plots shown by insert, where Rs, CPE, Rct and W
represents the solution resistance, a phase element, the charge transfer resistance and the
Warburg impedance respectively. The order of decrease of electron transfer efficiencies is
BCGE<NGO-GCE<PANI-GCE< NGO/PANI-GCE. This trend correlates with the one in cyclic
voltammetry study where NGO/PANI-GCE showed the best reduced potentials and the best
improved currents.
4.10 Bode Plots in 1mM hydroquinone
Figure 4.10 Bode (phase angle versus log f) plots obtained for (i) bare GCE, (ii) NGO-GCE,
(iii) PANI-GCE and (iv) NGO / PANI-GCE in 1mM Hydroquinone (pH4).
60
A plot above shows frequency which cannot be obtained from the nyquist plot. Surfaces with
the phase angle shifted towards lower frequencies in the bode plot further confirms better
catalytic activity of efficiency; hence the Bode plot further confirms better catalytic activity of
the modified surface towards the oxidation of Hydroquinone.
Electrode
Phase angle
Log Frequency
BCGE
66.9
3.4
NGO-GCE
65.9
2.1
PANI-GCE
52.6
2.0
NGO/PANI-GCE
51
1.9
4.11 Kinetic studies of hydroquinone
The effect of varying scan rate of 1 mM hydroquinone at NGO/PANI-GCE was studied and
increase in peak current and shifts in peak potentials towards more positive values were observed
for the conjugate as the scan rate increases from 100 to 350 mV/s. This is an indication of
irreversibility of the redox reaction. The plot of current against square root of scan rate showed a
linear relationship, with a correlation value of 0.995 showing that the electro-catalytic oxidation
of hydroquinone is diffusion controlled Fig 4.11
61
Fig 4.11: Effect of scan rate on peak potentials and currents a) 50 mV/s, b) 100 mV/s, c) 150
mV/s, d) 200 mV/s, e) 250 mV/s, f) 300 mV/s, g) 350 mV/s NGO/PANI-GCE for
Hydroquinone oxidation. [Hydroquinone] = 1mM. Inset: plot of Ipa vs √v.
Tafel slopes
Fig 4.1.2 shows a linear plot of peak potential, Ep versus log v. An increase in scan rate from 100
to 450 mV/s resulted in the hydroquinone oxidation peak shifting towards positive potentials,
indicating the chemical irreversibility of the electro-catalytic oxidation process. The relationship
between peak potential and scan rate for an irreversible diffusion process is graphically shown
below.
62
Fig 4.1.2 : Plot of potential versus log scan rate in 1mM Hydroquinone
Plots of Ep versus log v gave a linear relationship as represented by the Eq. Tafel slope of 326
mV decade
-1
(2 × slope of plot was obtained for hydroquinone. Tafel slopes of this magnitude
have no kinetic meaning but could indicate a passivated phenomenon occurring on the electrode
surface. Tafel slopes much greater than the normal 30-120 mV decade-1 for a one electron rate
determining step have been observed and have been either to chemical reaction coupled to
electrochemical steps or to substrate catalyst interactions in a reaction intermediate.
Mechanism of hydroquinone oxidation with NGO / PANI-GCE
The number of electrons involved in hydroquinone oxidation at the NGO/PANI -GCE surface
was obtained from a plot of oxidation against pH of hydroquinone solution
63
Fig 4.13: Plot of peak potential against pH for the detection of 1mM of Hydroquinone on
NGO/PANI-GCE.
From the plot above, it was observed that the peak potential decreased gradually with increasing
pH values, with a slope of 58 m V/pH. The slope is indicating that the process is a two-electron
transfer process.
64
Linear Sweep Studies
Linear sweep voltammetry (LSV) was done to show adsorption behaviour of
NGO/PANI-GCE
Fig 4.14 Linear sweep voltammograms (a) 10 μM, (b) 20 μM, (c) 30 μM (d) 40 μM (e) 50
μM and 60 μM of phosphate concentrations in pH 4 PBS. Inset plot of Ipa vs.
[Hydroquinone].
65
Langmuir adsorption isotherm plot for NGO/PANI-GCE
Langmuir adsorption isotherm plot for NGO/PANI-GCE in a) 10 µM, (b) 20 µM, (c) 30 µM (d)
40 µM (e) 50 µM and 60 µM of Hydroquinone concentrations in pH 4 PBS. Oxidation currents
employed.
Fig 4.15 Langmuir adsorption isotherm plot for NGO/PANI-GCE in a) 10 µM, (b) 20 µM,
(c) 30 µM (d) 40 µM (e) 50 µM and 60 µM of Hydroquinone concentrations in pH 4 PBS.
Oxidation currents employed.
Fig 4.15 shows LSV plots obtained after keeping the electrode in a stirred solution for 10
minutes to allow for adsorption. Applying the Langmuir adsorption theory (Eq 4.8) a plot of the
ratio of hydroquinone concentration to catalytic current against concentration of hydroquinone
66
gave a linear plot which is which can be interpreted as an adsorption controlled electrochemical
process.
[𝐻𝑦𝑑𝑟𝑜𝑞𝑢𝑖𝑛𝑜𝑛𝑒]
𝐼𝑐𝑎𝑡
=
1
ß𝐼𝑚𝑎𝑥
+
[𝐻𝑦𝑑𝑟𝑜𝑞𝑢𝑖𝑛𝑜𝑛𝑒]
𝐼𝑐𝑎𝑡
(4.8)
where ß is the adsorption equilibrium constant, 𝐼𝑚𝑎𝑥 is the maximum current and 𝐼𝑐𝑎𝑡 is the
catalytic current. From the slope and the intercept of Fig 4.16 the adsorption equilibrium constant
3
ß was established to be 1. 4 ×10 𝑀
−1
. Using equation 4.9 which relates Gibbs free energy
○
○
change due to adsorption (Δ𝐺 ) to the adsorption equilibrium constant ß, (Δ𝐺 ) was found to be
-10.42kJ. This value is in comparable to those reported elsewhere for high Tafel slopes [79].
○
(4.9)
Δ𝐺 =− 𝑅𝑇 𝑙𝑛 ß
Where R is the molar gas constant and T is room temperature
4.16 Chronoamperometric Studies
Catalytic rate constants are measures of how fast redox processes proceed at the electrode /
analyte interface. The rate of oxidation of hydroquinone was evaluated on NGO/PANI-GCE
through Chronoamperometric studies.
67
Chronoamperograms for different Hydroquinone concentrations
Fig 4.16 Chronoamperograms for different Hydroquinone concentrations. In PBS pH 4, a)
20 μM, b) c) 40 μM ,d) 60 μM , e) 80 μM and f) 100 μM Inset Ipa vs [Hydroquinone]
68
Plots of
𝐼𝑐𝑎𝑡
𝐼𝑏𝑢𝑓
vs. time
Fig 4.17 Plots of
𝐼𝑐𝑎𝑡
𝐼𝑏𝑢𝑓
vs. time (s) Fig 4.19 Plot of slopes2 vs. [Hydroquinone]The rate
constant for the detection of hydroquinone was calculated using the equation
(kCO t) ½π
Plot of slopes2 vs. [Hydroquinone]
69
𝐼𝑐𝑎𝑡
𝐼𝑏𝑢𝑓
= π1/2
Figure 4.17 Plot of slopes2 vs. [Hydroquinone]
The rate constant for the detection of hydroquinone was calculated using the equation:
𝐼𝑐𝑎𝑡
𝐼𝑏𝑢𝑓
1
= π 2 (𝑘𝐶𝑂𝑡)
1/2
(4.10)
Where Icat and Ibuf are currents in the presence and in the absence of Hydroquinone, k is the
catalytic constant (M-1s-1) for the hydroquinone oxidation and t is the time in seconds’. Fig 4.19.
Shows the linear relationship for the Icat and Ibuf vs t1/2 plots for different hydroquinone
concentrations obtained from the Chronoamperograms. Fig 4.20 showed the linear relationship
for the slope2 vs. [Hydroquinone]. The slope of Figure 4.20 is equal to πk and this gives a k value
of 3.9 ×10 -7M-1s-1. The value of k obtained is large. The larger the k value, the faster the rate of
oxidation at the modified electrode. This showed that NGO/PANI-GCE was the best electrode
for fast detection of hydroquinone. Figure 4.18 insert shows the plot of peak current against
concentration. A linear relationship was observed from the plot.
70
Differential Pulse Voltammetry is a very sensitive technique that was used to determine limit of
detection using NGO-PANI-GCE in different concentrations of hydroquinone.
DPV for NGO/PANI -GCE
Figure 4.21 DPV for NGO/PANI -GCE in: a) 0.2 μM, b) 0.4 μM, c) 0.6 μM, d) 0.8 μM, e) 1
μM, f) 1.2 μM, g) 1.4 μM, h) 1.6 µM. Inset: Plot of Ipa vs [Hydroquinone ].
Figure 4.21 inset shows the plot of peak current against concentration. A linear relationship was
−5
observed from the plot. A linear regression equation was obtained as 𝑖𝑝 = 88. 6𝑥 + 1 ×10
and R2 = 0.9926 where 𝑥 is the concentration of the analyte. The limit of detection is equivalent
to 3σ/𝑠 where σis the standard deviation of the intercept and s is the slope of the calibration
curve. LOD was found to be 1.03569 × 10-7. The LOQ (10σ/𝑠)was found to be 3.4531 × 10 -7 M.
In a previous study on the voltammetric determination of hydroquinone in cosmetics the LOD
71
was found to be 2.3× 10-5 µM, comparing with our obtained results, the NGO/PANI-CE is a
better electrode in the detection of hydroquinone.
Stability
Figure 4.22 continuous cyclic voltammetric evolutions for 1 mM Hydroquinone generated
on GCE modified with NGO/PANI. Scan rate = 100 mV/s. pH 4 PBS
The peak current is initially high and it gradually decreases as the number of cycles continues
with the highest decrease between the first and the second scan and the drop in current eventually
becomes very small showing the passivation of the electrode. The rate at which current drops is a
measure of resistance to passivation of the electrode towards the analyte [5]. There is highest
increase in the oxidation potential of the first and second scan. The sharp increases eventually
become small. The oxidation potential of the first scan is lower than the second scan but it as the
cycle continues it becomes uniform, and constant showing passivation of the electrode. This
electrode fouling could be attributed to the polymeric film that is formed by Hydroquinone
radicals on the electrode surface [6].
72
Reproducibility
The modified surface area provided better reproducibility as indicated by the slight decrease in
the peak current after washing the electrode two different times with ethanol with relative
standard deviation of 2.58 % and 4.22 %, which is below 5 % and this indicated good
repeatability of the modified electrode [75].
Fig 4.23: Differential pulse voltammograms for three repetitions in 1mM Hydroquinone in
pH 4 PBS solution at NGO-PANI/GCE. Scan rate = 100 mV/s at potential 0.4 to 1.0 V
Interference Studies
The interference study was done using DPV on NGO / PANI - GCE and the concentration of the
interfering compounds was carried in a 1:1:1 ratio of hydroquinone: hydrogen peroxide : phenol
Peaks were recorded for the hydroquinone alone, hydrogen and phenol alone then finally the
three analytes were mixed in a 1:1:1 ratio and DPV voltammograms were recorded, Figure 4.24.
73
Fig 4.24 for (a) Hydrogen peroxide , (b) Mixed analytes, (c) hydroquinone (d) phenol
The effect of the interfering analytes was determined using DPV and it was observed that there is
relevantly no significant change in the peak currents and peak potentials of the hydroquinone.
The relative percentage response of the sensor calculated using the formulae:
𝑅 % = [ 1 − (𝐼 𝑎𝑛𝑎𝑙𝑦𝑡𝑒 + 𝐼𝑖𝑛𝑡𝑒𝑟÷𝑖𝑛𝑡𝑒𝑟) ] × 100
Where I analyte and I interferents are the peak currents hydroquinone and for the interfering
analytes respectively. The RSD % above 10 % shows that the compound interferes with the
analyte [8]. All the interferences showed to have no effect as they had interference effect of less
than 10 %, demonstrating an excellent tolerance to interference of the modified electrodes.
74
CHAPTER 5
5.0 Introduction
The chapter concludes the work done in the research and suggests recommendations with
reference to future study.
5.1 Conclusion
A new electrochemical sensor based on GCE modified with NGO/PANI composite was
successfully prepared, characterized and utilized for electrochemical determination of
hydroquinone. The PANI nanoparticles showed enhancement on the sensitivity of the modified
electrode considerably by increasing the conductivity and effective electroactive surface area of
the electrode. The electrochemical conductivity and adsorption ability of the added layer was
used as a sensing platform for hydroquinone. The NGO/PANI-GCE showed a wider linear
range and a lower detection limit.
NGO/PANI – GCE can be used to lower glassy carbon electrode over potentials in the
determination of hydroquinone.
The electrode surface area after modification is 0.13 cm 2.The rate constant, k is 3.9 × 10
-7
showing fast and excellent electron transfer during oxidation. LOD determined is 1.03569 × 10 -7
M and LOQ determined is 3.45 × 10 -7 M. The NGO / PAN I - GCE showed good stability
reproducibility and high sensitivity towards hydroquinone without suffering from due
interferences from hydrogen peroxide and phenol.
75
5,2 Recommendations
I recommend further studies on the the effects of doping graphene oxide with other dopants like
boron and sulphur in the same matrix with other intrinsically conducting polymers like
poly(acetylene) or polythiophene. I also recommend the use of nitrogen doped graphene oxide
modified with polyaniline in the modification of glassy carbon electrode for analysis of different
compound as well as further studies.
76
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APPENDICES
APPENDIX A: MATERIALS
List A: Apparatus used in synthesis and characterisation
Erlenmeyer flask, Buchner funnel, distillation flask, measuring cylinders(25ml,50ml,100ml)
,volumetric flasks(50ml,100ml,250ml,500ml, 1000ml), micropipette, beakers, spatula, pestle
,mortar, wash bottles, conical flasks, petri dishes, burettes, filter papers, vials
Table A1; Reagents and chemicals
Name
Chemical formulae
Manufacturer
Concentratio
n
Distilled water
H2O
MSU Lab
Hydrochloric acid
HCL
Sigma aldrich
32 %
Potassium
K3[ Fe (CN)] 6
ACE
10M
Sodium hydroxide
NaOH
Skylabs
0.1M
Sulphuric acid
H2SO4
ACE
0.1M
Aniline
C6H5NH2
Unichem
0.2 M
Hydrogen peroxide
H2O2
ACE
0.1 M
Sodium nitrite
Na3 NO
ferricyanide
87
Ammonium
NH4S2O3
Skylabs
0.05
persulphate
Table A2 : Instrumentation
Name
Model
Manufacturer
Use in lab
Analytical balance
JJ224BC
G&G
Weighing
pH meter
Az -8601
Thermoscientif
pH
measurement
Sonicator
KQ -250B
China Corp
Agitation
Potentiost
PG stat 302N
Autolab
Electroanalysis
FTIR
Nicolet 6700
Thermoscientic
Characterisatio
n
UV -vis
UV 1700
Shimadzu
Absorbance
measurement
Treatment of Glassware
Laboratory detergents were used to wash glassware after which they were rinsed with distilled
water to remove contaminants and impurities. This minimizes interferences that during electro analyses.
88
APPENDIX B: Working electrodes
B1: Working electrodes used in this study
Electrode modifier
Method of modification
Bare Glassy Carbon Electrode
Electrode designation
BCGE
Nitrogen doped graphene oxide
Drop and dry
NGO-GCE
polyaniline
Drop and dry
PANI -GCE
Nitrogen doped graphene oxide
Drop and dry
NGO/PANI-GCE
decorated with polyaniline
89
APPENDIX C: Calculation
C1: Effective Surface area
Constant R = 8.314 Jmol-1, T = 273 K 5
Randle-sevick equation
5
3
1
𝑚 = (2. 69 ×10) 𝑛 2 𝐴𝑒𝑓𝑓 𝐷2 𝐶2
Ip = (2.69 x 105) n3/2AD1/2V1/2C
−6
D= 7.6 × 10-6cm2/s, C= 1×10
−5
cm2, m=1×10
2
and Aeff = 0. 13 𝑐𝑚
Limit of Detection and Limit of Quantification
Excel Linest function
Slope
Intercept
Standard error of the slope
Standard error of the intercept
R2
Standard error in y
LOD = 1.035569 × 10-7 M
LOQ = 3.4531 × 10 -7 M
90