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Procedia CIRP 40 (2016) 551 – 556
13th Global Conference on Sustainable Manufacturing - Decoupling Growth from Resource Use
Novel Ag/Au Nanocubes Modified the Negative/Positive Charge on the Surface and Their
Application in Surface-Enhanced Raman Scattering
Tran Thi Bich Quyena*, Bing-Joe Hwangb,c
a
Department of Chemical Engineering, College of Engineering Technology, Can Tho University, Campus II, 3/2 Street, Ninh Kieu District, Can Tho City
900000, Vietnam
b
Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
c
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
* Corresponding author. Tel.: +84-907.500.797; fax: +84-7103.831.151. E-mail address: ttbquyen@ctu.edu.vn
Abstract
Bimetallic nanostructures have received considerable attention in recent years due to their widespread use in applications such as photonics,
catalysis and surface-enhanced Raman scattering (SERS) detection. In this work, a simple and effective strategy has been developed to
synthesize Ag/Au@negative(COO-)-Ag/Au@positive(NH3+) charge on the nanocubes’ (with hollow or porous structures) surfaces, based on a
galvanic replacement reaction and co-reduction method of the corresponding ions. The prepared bimetallic Ag/Au@NH 3+-Ag/Au@COOnanocubes have been characterized by UV–vis, TEM, and XRD. Our results show that the SERS technique is able to detect R6G within wide
concentration range, i.e. 10-16 – 10-8 M, with lower limit of detection (LOD) being 10-16 M. It demonstrates that the Ag/Au@NH3+Ag/Au@COO- nanocubes have potential applications in SERS for the detection of biomolecules or dye molecules.
© 2016
2016 The
The Authors.
Authors. Published
Published by
by Elsevier
©
Elsevier B.V.
B.V. This is an open access article under the CC BY-NC-ND license
Peer-review under responsibility of the International Scientific Committee of the 13th Global Conference on Sustainable Manufacturing.
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the International Scientific Committee of the 13th Global Conference on Sustainable Manufacturing
Keywords— SERS; Ag/Au bimetallic nanocubes (Ag/Au NCBs); negative charge; positive charge; Ag/Au@NH3+ and Ag/Au@COO- NCBs; Rhodamine 6G
(R6G)
1. Introduction
In recent years, bimetallic or trimetallic nanoparticle
assemblies of noble metals (Ag, Pt, Au, Ru, and Rh) have
played an important role in catalysis. The use of a second
and/or third metal not only leads to changes in physical
properties like particle size, shape, surface morphology and
chemical properties, including catalytic activity and selectivity,
but also results in new properties and capabilities because of
the intermetal synergy [1-11]. These nanomaterials are also
useful in the fields of electronics, optics, photonics,
communications, biology, and medicine [4, 12-22]. The
following specific examples illustrate well the significant
benefits offered by multimetal nanostructures. Pt@Pd
core/shell nanoparticles exhibit outstanding performance in
polymer electrolyte membrane fuel cells and direct methanol
fuel cells [16]. The frequency of the localized surface plasmon
resonance (LSPR) can be tuned by varying the composition of
the Au-Ag alloy nanostructures [23, 24]. Bimetallic and
trimetalic (alloy) nanostructures of Ag, Au, Cu and Pt, for
example, Pt-Ag nanoparticles, Au-Ag nanoporous nanotubes,
Cu-Au nanotubes and Ag/Au/Pt trimetallic nanocages provide
a significant SERS enhancement [25-28].
The assembly of nanomaterial across extended length
scales is a key challenge to the integration of functional
nanodevices and nanomaterials. The real value of
nanotechnology is to develop advanced nanodevices of
superior function and properties. In recent years, selfassembly of nanoparticles are a rapidly developing field of
research due to the properties of superlattices can be as
different from their individual components as the physical
properties of nanoparticles are from bulk materials. Assembly
of nanomaterial is governed by the repulsive forces (i.e, steric
forces and electrostatic repulsion between ligands of like
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the International Scientific Committee of the 13th Global Conference on Sustainable Manufacturing
doi:10.1016/j.procir.2016.01.132
552
Tran Thi Bich Quyen and Bing-Joe Hwang / Procedia CIRP 40 (2016) 551 – 556
charge) and the balance of attractive forces (i.e, electrostatic
attraction between negative charge and positive charge
ligands, covalent or hydrogen bonding, depletion forces or
dipole-dipole interactions). For example, end-by-end or sideby-side of gold nanorods can be driven by triggering attraction
between the distinct ligands attached to the long and short
facets of the nanorods in the phase solution based on selfassembly exploits site-specific interactions of chemically
heterogeneous nanoparticles.
Most of the ordered structures published recently are
nanoparticle-based with some noticeable drawbacks. Firstly, it
is difficult to build assemblage of atoms or molecules in the
form of “superparticles” with well-defined and uniform size
and shape. Secondly, the assembly of nanoparticles is difficult
to control by previously reported methods which depend
mainly on the morphology of nanoparticles. Moreover, its cost
is quite high if we use the self-assembled character between
nanoparticles via the antigen-antibody interactions. Thus, it is
also a challenging issue to assemble nanoparticles for the
enhancement of SERS signal. To overcome these challenges,
we have used a very simple and low-cost method to generate
“self-assembled” structures uniformly and much greatly
increased SERS signal intensity from substrate materials
based synthetic method of the negative charge and the positive
charge modified on the Ag/Au nanocubes’ surfaces. Normally,
Ag nanocube synthesis requires long reaction times (15–26 h)
[29]. Addition of small amounts of NaBH4 and HCl shortens
the reaction time to about 5 h. The above reducing agent
increases the rate of Ag+ reduction, leading to smooth
nucleation and formation of uniform Ag nanocube particles.
Then, Ag/Au nanocubes are prepared by the galvanic
replacement reaction. Although Au is expensive, the
formation of porous structures after alloying with Ag allows
for a significant reduction in material costs.
The SERS signal intensity can be affected by
electromagnetic enhancement and chemical enhancement, the
former being much stronger. Electromagnetic enhancement
takes place under conditions of surface plasmon excitation.
The electromagnetic field of light on the nanoparticle surface
is greatly enhanced by amplification of both the incident laser
field and the scattered Raman field through their interaction
with the surface. Noble metals such as Ag, and Au exhibit
surface plasmon excitation by incident light. Chemical
enhancement arises through electronic resonance and charge
transfer between a molecule and a metal surface; this results in
an increase in the polarizability of the molecules. Anisotropic
nanomaterials (e.g. nanocubes) have enhanced plasmon modes
due to transverse and longitudal polarizations caused by
topological differences [30-33]. Therefore, new anisotropic
Ag/Au nanomaterials was modified by the negative chagre
and the positive charge on their surfaces to further studies on
the SERS mechanism and applications are of great interest,
showing that the Ag/Au@nagative and positive charges
(Ag/Au@NH3+-Ag/Au@COO-) nanocubes exhibit a good
SERS for the detection of R6G molecules due to their good
stability to oxidation and building “hot spots” formed by many
porous structures on the material’s surface.
2. Experimental Materials and Methods
2.1. Materials
Ethylene glycol (EG), silver nitrate (AgNO3), poly(Nvinylpyrrolidone) (PVP; MW | 55,000), Rhodamine 6G (R6G;
≥99%), hydrogen tetrachloroaurate (HAuCl4.3H2O; ≥
99.99%), hydrochloric acid (HCl; 36-37%), sodium
borohydride (NaBH4), ethanol (C2H5OH: ≥ 99.5%), acetone
3-aminopropyl trimethoxysilane
(CH3COCH3: ≥99.5%),
(APS: C6H17NO3Si, 95%), and poly(ethylene glycol)
bis(carboxymethyl) ether (PEGC; 600 g mol-1) were
purchased from Acros and Sigma-Aldrich. All solutions were
prepared using deionized water from a MilliQ system.
2.2. Nanocube synthesis
2.2.1. Preparation of silver nanocubes
Silver nanocubes (Ag NCBs) were synthesized according
to a modification of a previously reported method [29].
Typically, in a 100-mL round-bottomed flask, EG (5 mL) was
heated at 140oC in an oil bath with magnetic stirring for 40
min. Next, solutions of the following reagents were added by
a micropipette in succession: 1) NaBH4 (10 mM in EG; 20
PL); 2) after 2 min, quickly, HCl (3 mM in EG; 1 mL); 3)
after another 10 min, at a rate of 45 mL/h, AgNO3 (94 mM in
EG; 3 mL) and PVP (147 mM (based on the repeating unit) in
EG; 3 mL). Upon injection of the solution of AgNO3, the
reaction mixture changed color from milky white, through
transparent light yellow and red to ochre while being heated at
140°C. After 5 h, the solution was centrifuged (10000 rpm; 15
min), the precipitated Ag NCBs were washed with acetone,
ethanol and then H2O to remove excess EG and PVP and redispersed in deionized water. The average edge length of the
Ag NCBs as prepared was ~70 nm.
2.2.2. Synthesis of Ag/Au and Ag/Au modified by negative
and positive charges on the nanocubes’ surfaces
Typically, Ag NCB suspension as prepared above (100 μL)
was dispersed in H2O (5 mL) containing PVP (1 mg/mL) with
magnetic stirring and heated to boil for 10 min. A HAuCl4
solution (1 mM in H2O, 1.0 mL) was added to the flask via
syringe pump at a rate of 45 mL/h with magnetic stirring. The
mixture was heated for another 10 min until the color was
stable. After cooling down to room temperature and
centrifugation (10000 rpm), the precipitated Ag/Au NCBs
were washed with brine to remove AgCl and then with
deionized (DI) H2O several times to remove PVP and NaCl.
The NCBs were re-dispersed in DI H2O (1.0 mL) for the
SERS study.
Next, the surface of the Ag/Au NCBs (10 mL) was
modified with amine groups by reacting of APS (40 PL, 1
mM) for 1 h at room temperature to form the positive charge
on the Ag/Au NCBs’ surface (Ag/Au@NH3+). And another
the mixture solution, PEGC (40 μL, 5 mM) was added to
Ag/Au NCBs (10 mL) to generate carboxyl binding (the
negative charge) on the surface of Ag/Au@COO- NCBs and
553
Tran Thi Bich Quyen and Bing-Joe Hwang / Procedia CIRP 40 (2016) 551 – 556
the mixed solution was kept stirring for 40 min. Non-specific
binding moieties were removed by centrifugation; every
solution the precipitate was washed several times with DI
H2O and re-dispersed in 2.0 mL DI H2O.
NCBs decreases in intensity and is blue-shifted from 721 to
797 nm after Ag/Au@NH3+ and Ag/Au@COO- solutions are
mixed together, respectively (Fig. 1(d)).
452
1.2
721
2.3. Characterization
2.4. Preparation of SERS substrates
Droplets (50 PL of a 1.18×1010 particle mL-1 solution) of
Ag/Au@NH3+-Ag/Au@COO- NCB were spread on silicon
wafers (~1 cm2). Aqueous R6G solution (10-8 M; 5 PL
droplets) was spread on the Ag/Au@NH3+-Ag/Au@COONCB surfaces and kept in the dark for 1 h at room
temperature prior to testing. For R6G quantification, samples
with concentration of 10-8, 10-10, 10-12, 10-14, 10-15, and 10-16 M
were used. SERS spectra for all samples were measured in
triplicate within 10 min over 3 different areas on the sample
focus for 10 s (at 25°C). These measurements are
reproducible with agreeable measurement errors.
Absorbance (a.u)
The UV–Vis (absorbance) spectra of particle solutions
were acquired on a Shimadzu UV-675 spectrophotometer.
Transmission electron microscopy (TEM) was performed on a
Philips Tecnai F20 G2 FEI-TEM microscope (accelerating
voltage 200 kV). Specimens were prepared by dropping on a
copper grid and drying at 60°C in an oven. The powder X-ray
diffraction (XRD) patterns of the Ag NCB and Ag/Au NCB
samples were acquired on a Rigaku Dmax-B diffractometer
with a Cu Kα source operated at 40 kV and 100 mA. A scan
rate of 0.05 deg-1 was used for 2T between 30° and 90°.
Raman measurements were performed on a Renishaw 2000
confocal Raman microscope system. A He-Ne laser operating
at O = 532 nm was used as the excitation source with a laser
power of 20 mW. All Raman spectra were obtained at 10 s
exposure time. The laser line was focused onto the sample in
backscattering geometry using a 10x objective providing
scattering area of ~0.25 mm2.
1.0
(b)
0.8
797
(d)
0.6
0.4
349
1042
(c)
(a)
0.2
0.0
400
600
800
1000
1200
Wavelength (nm)
Fig. 1. UV-vis spectra of (a) AgNCBs (~70 nm), Ag/Au NCBs with different
amounts of gold solution (1 mM): (b) 1.0 mL; (c) 1.4 mL; and (d)
Ag/Au@NH3+/Ag/Au@COO- (with gold solution of 1 mM; 1.0 mL).
Fig. 2 shows representative TEM images of Ag NCB,
Ag/Au NCB and Ag/Au@NH3+-Ag/Au@COO- NCB
samples. The image of the bimetallic NCBs reveals that the
porous nanocube structure of the former has been preserved
during the nanoalloying process, indicating a direct
replacement of Ag atoms (Ag(0)) with Au (Au(0)) upon
addition of HAuCl4. There is a clear contrast between the
interior and exterior of the NCBs in the TEM image Fig. 2(c),
indicating a hollow or porous interior structure. Fig. 2(e, f)
shows the TEM image of Ag/Au@NH3+ and Ag/Au@COONCBs after mixed together, creating “hot spots” much more
than that of Ag/Au NCBs – see Fig. 2(c, e and f) .
(b)
(a)
3. Results and Discussion
3.1. Characterization of the Ag nanocubes and Ag/Au
nanocubes
The UV-Vis spectra of Ag NCBs (70 nm in edge length)
exhibited a main, broad band with a maximum at 452 nm and
a shoulder at 349 nm. Fig. 1 shows the solution absorbance
spectra of Ag/Au NCBs with different amounts of gold
solution (1 mM): (b) 1.0 mL; (c) 1.4 mL; and (d)
Ag/Au@NH3+/Ag/Au@COO- (with gold solution of 1 mM;
1.0 mL) prepared. During the preparation of Ag/Au NCBs,
the position of the SPR peak is continuously shifted from the
visible (Ң452 nm for the Ag NCBs) to the near-infrared (Ң721
nm) when HAuCl4 solution is being added into the Ag NCBs
solution (Fig. 1(b)). Fig. 1(c) shows the typical absorption
spectra of the Ag/Au NCBs is blue-shifted from 721 to 1024
nm and gradually disappears with increasing Au content.
Beside, the absorption peak of Ag/Au@NH3+-Ag/Au@COO-
(c)
(e)
(d)
(f)
Fig. 2. TEM images of (a) Ag NCBs (~70 nm), Ag/Au NCBs with different
amounts of gold solution (1 mM): (b) 500 mL; (c) 1.0 mL; and (d) 1.4 mL,
and (e, f) Ag/Au NCBs modified by the negative charge and the positive
charge on their surfaces after two solutions of Ag/Au@NH3+(positive charge)
and Ag/Au@COO-(negative charge) was mixed together.
Tran Thi Bich Quyen and Bing-Joe Hwang / Procedia CIRP 40 (2016) 551 – 556
The X-ray diffraction (XRD) patterns of Ag NCBs and
Ag/Au NCBs are also shown in Fig. 3. The diffraction peaks
located at 38.1°, 44.3°, 64.7°, 77.56° and 81.88° can be
indexed to the (111), (200), (220), (311), and (222) planes
respectively, of the face-centered cubic (fcc) structure of Ag,
and Au (JCPDS No. 87-0720 and 04-0784, respectively). The
(200) peak of Ag NCB sample is more intense than the (111)
peak that dominates the JCPDS pattern mainly because the
cubes are joined at the {100} facets and the powder standard
is overwhelmed by the lower energy {111} facets. Moreover,
the relative diffraction intensity is different. For example, the
(200) plane intensity at a scattering angle (2T=44.1o)
decreases relative to that of the (111) plane, with increasing
the ratio of gold molar in the alloy nanocubes (nanocages).
This ratio ranges from 0.764 for pure silver nanocubes to
0.236 for pure gold nanocubes.
Next, the SERS signal intensity of R6G with a wide range
from 10-8 to 10-16 M on Ag/Au@NH3+-Ag/Au@COO- NCBs
could be effectively detected (Fig. 5). The characteristic peak
at 1663 cm-1 could still be identified even at the lower R6G
concentration of 10-16 M. These results imply that the
Ag/Au@NH3+-Ag/Au@COO- NCBs can be used for simple
and sensitive quantitative detection of biomolecules in vivo.
1663
15000 cps
1379 1524
Intensity (a.u)
554
1216
1298
637
943
773
(d)
1145
(c)
(b)
(a)
Ag/Au NCBs
Ag NCBs
600
800
1000
1200
1400
1600
-1
1800
Intensity (a.u)
Raman shift (cm )
Fig. 4. SERS spectra of (a) 100 mM R6G on the silicon substrate, 10 -8 M R6G
on (b) Ag NCBs; (c) Ag/Au NCBs; and (d) Ag/Au@negative-positive charge
nanocubes (Ag/Au@NH3+-Ag/Au@COO- NCBs), respectively.
200
1663
(A)
111
30
40
50
60
311 222
70
80
90
2T(Degree)
Fig. 3. XRD patterns of Ag NCBs and Ag/Au NCBs with amount of 1.0 mL
gold solution (1 mM).
637 773
1216
1298
943
(a)
1145
(b)
(c)
(d)
(e)
(g)
(f)
3.2. Application of the Ag/Au nanocube modified the
negative/positive charge on its surface in SERS measurements
600
800
1000
1200
1400
-1
1600
1800
Raman shift (cm )
60000
Intensity (a.u)
R6G is an excellent model analyte in SERS studies
because it is highly photostable and lacks absorption in the
near-infrared (NIR) region. Fig. 4 shows a comparison of the
SERS intensity of R6G is significantly different on various
supports of Ag NCBs, Ag/Au NCBs and Ag/Au@NH 3+Ag/Au@COO- NCBs. There was no SERS signal on a silicon
substrate (a) whereas the signal was the most intense on
Ag/Au@NH3+-Ag/Au@COO- NCBs (d), Ag/Au NCBs (c)
and Ag NCBs (b). The peak at ~637 cm-1 is assigned to the C–
H out-of-plane bend mode, the peak at ~1145 cm-1 – to the C–
H in-plane bend mode and the series of peaks at ~1379, 1524
and 1663 cm-1 – to the aromatic ring vibrational modes of
R6G – see Fig. 4(d). The SERS signal intensity on
Ag/Au@NH3+-Ag/Au@COO- NCBs was stronger than that
on the silicon substrate with an enhancement factor of SERS
activity of 6.01×109 (Fig. 4(d, a), respectively) based on
calculated method [34]. The pores and conbination to form
many “hot spots” between Ag/Au@NH3+ NCBs and
Ag/Au@COO- during the negative charge and the positive
charge on the Ag/Au nanocubes’ surfaces are expected to
contribute to the high SERS activity due to a large electric
field enhancement. Also, a significant charge transfer
interaction between the ligand and the negative/positive
charge-coated Ag/Au substrate is expected in view of the
difference in electro-negativity of Ag and Au.
1379 1524
15000 cps
Intensity (a.u)
220
20
40000
(B)
2
R = 0.95
20000
0
-17 -16 -15 -14 -13 -12 -11 -10
-9
-8
-7
Log[R6G concentration] (M)
Fig. 5. (A) Representative SERS spectra of rhodamine 6G (R6G) on
Ag/Au@NH3+-Ag/Au@COO- NCBs at concentrations of: (a) 10-8 M; (b) 10-10
M; (c) 10-12 M; (d) 10-14 M; (e) 10-15 M; (f) 10-16 M; and (g) 0 M. (B) A
logarithmic plot of R6G concentration vs. signal intensity for the band at
1663 cm-1. Each data point represents an average value from three SERS
spectra. Error bars show the standard deviations (coefficient of determination,
R2 = 0.95).
Tran Thi Bich Quyen and Bing-Joe Hwang / Procedia CIRP 40 (2016) 551 – 556
4. Conclusions
In summary, this paper describes the first simple method
for synthesis of Ag/Au porous nanocubes modified the
negative/positive charge on the surface (Ag/Au@NH3+Ag/Au@COO- NCBs). The formation of high quality porous
nanocubes results from the inter-metallic synergy in the
Ag/Au alloy which are generated via the galvanic replacement
reaction. As-prepared Ag/Au@NH3+-Ag/Au@COO- porous
nanocubes show a significant enhancement of the SERS
signal due to multiple “hot spots” built by the porous
nanocubes and the interaction of negative/positive charges.
R6G can be detected in the concentration range from 10-8 to
10-16 M, with the lower concentration to be 10-16 M. It is also
demonstrated that the Ag/Au@NH3+-Ag/Au@COO- porous
nanocubes may have potential applications in SERS because
of their good stability in air, relatively low cost, and the
convenience for building “hot spots” to significantly increase
SERS-active enhancement.
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