Electrochemical Seed-Mediated Growth of Surface-Enhanced Raman

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Electrochemical seed-mediated growth of SERS-active Au (111)-like nanoparticles on indium tin oxide electrodes Jun-Gang Wang, Xiao-Wei Cao, Ling Li, Tian Li, and Rong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400633k • Publication Date (Web): 25 Jun 2013 Downloaded from http://pubs.acs.org on June 27, 2013

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Electrochemical Seed-Mediated Growth of SERS-Active Au

2

(111)-Like Nanoparticles on Indium Tin Oxide Electrodes

3

Jungang Wang, Xiaowei Cao*, Ling Li, Tian Li, Rong Wang

4

Department of Chemistry, Shanghai Normal University, Shanghai, 200234, P. R. China

5

Abstract

6

In this article, a novel electrochemical seed-mediated method has been proposed for the

7

fabrication of surface enhanced Raman scattering (SERS) active gold nanoparticle films

8

deposited on indium tin oxide electrodes. Firstly, due to the fact that a high overpotential and a

9

short time of potentiostatic transient are applied in the nucleation stage, the high controllability

10

with the density and the size of gold nanoparticles on an indium tin oxide (ITO) electrode are

11

realized. Secondly, due to the fact that AuCl4‒ ions will be preferentially reduced and deposited

12

on the previously formed Au seeds in a subsequent cyclic voltammetrical process, we achieve

13

the size-controlled gold nanoparticles which are enriched in Au (111) facet orientation (ca.

14

80.7%). With the growth of the nanopaticles, the gap between the adjacent nanoparticles

15

gradually decreases and thus plenty of ‘hot spots’ are formed on the substrate for SERS

16

applications. Thirdly, by using 4-Mercaptobenzoic acid as a probe molecule, the enhancement

17

factor of the as-prepared substrate is estimated as ~1.3×106, and its reproducibility and stability

18

for SERS measurements are also evaluated. These works reveal its great potential in routine

19

SERS applications for microanalysis. The protocol for the fabrication of such a SERS substrate

20

is facile and cost-effective, without introducing any template or surfactant.

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Keywords: electrodeposition, Au (111)-like nanoparticles, O2 reduction reaction, surface

22

enhanced Raman scattering 1

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1. Introduction

2

Surface-enhanced Raman scattering (SERS) technique has been proven to be a powerful tool

3

in obtaining vibrational information about molecules on or near a nanoscale metallic surface and

4

it has been a potential technique for non-destructive, sensitive detection of chemical and

5

biological molecules.1-5 For the super sensitivity of the method, it has the capacity to investigate

6

samples in a single molecular level.6 Having been served as a molecular spectroscopic technique,

7

SERS possesses many excellent merits including high sensitivity and negligible interference

8

from water. In addition, compared to infrared absorption spectroscopy, it does not require any

9

special sample preparation procedure, and this facilitates its effective applications in biomedical

10

sensing, environmental monitoring and analytical sciences.7-9 At present, the widely accepted

11

theory about the SERS mechanism involves electromagnetic (EM) enhancement and chemical

12

(charge transfer or CT) enhancement.10-15

13

There are numerous studies on the fabrication of noble metal nanostructures as SERS active

14

substrates.16-19 The quantitative applications of SERS are still confined by the fabrication of

15

sensitive, reproducible, and stable SERS active substrates. It is critical and difficult to control the

16

parameters for the fabrication of SERS active substrate that influencing the size, shape,

17

morphology of nanoparticles and the gap between them. How to optimize these parameters to 2


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produce high uniformity and reproducibility SERS active substrates with controllable metallic

2

nanostructures will be a main topic as for the improvement of SERS performance. The common

3

methods include wet-chemical reduction to produce gold and silver colloidal suspensions,20

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electrochemical roughing,21 and physical vapor deposition.22 In recent years, the nanosphere

5

lithography coupled to film over nanospheres (NSL-FON)4,5 and the electron-beam lithography

6

(EBL)23,24 have been also adopted to fabricate high SERS active substrate with high

7

reproducibility. However, the barriers such as high cost, necessity for advanced equipments, and

8

time-consuming procedures will restrict the popularisation and application of these methods in

9

the future. Nevertheless, there are a lot of other interesting methods such as layer-by-layer (LBL)

10

assembly, nanoparticle self-assembly, and electrochemical deposition that have been investigated

11

to fabricate high stable and sensitive SERS substrates.7,25-27 Utilizing electrochemical deposition

12

to fabricate metallic nanostructures is attractive for its versatility, facility, low cost, facile and

13

simple preparation, and easy accessibility. Several strategies have been proposed to fabricate

14

metallic nanoparticles on conductive substrates, such as utilization of a template, or employing

15

double potential pulses, or employing a cyclic voltammetry scan.28,29,30 However, no paper about

16

electrochemical deposition of gold nanoparticles by potential step followed by cyclic

17

voltammetry has been found available on known publications.

3



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Herein, we demonstrate a novel electrochemical seed-mediated method for the fabrication of

2

SERS active Au (111)-like nanoparticle array on an ITO electrode. This method improves

3

effectively the monodispersion of AuNPs in comparison with the Frens’ method, and it is a

4

valuable tool to control conveniently the growth of surface-confined AuNPs.31 By using the

5

potentiostatic transient technique in the nucleation stage, i.e. a high overpotential is applied to a

6

bare ITO electrode for a short time; a great deal of gold seeds will be formed on the ITO

7

electrode with high monodispersity and homogeneous morphology. Then, due to the property of

8

preferential deposition and reduction of AuCl4− ions on the existing Au seeds, the size of the

9

AuNPs will consequently become large and the desired gaps between adjacent nanoparticles will

10

be obtained by utilizing a subsequent cyclic voltammetry with an adequate deposition time. It is

11

convenient for us to control reasonably the AuNPs gaps and size (expressed as a statistically

12

averaged value), which is essential for the emergence of ‘hot spots’ and is also critical for a high

13

SERS activity. In our conditions, we fabricate the gold nanoparticles array with controlled

14

particle size and density, tunable LSPR property, and enrichment in Au (111) facet orientation

15

(ca. 80.7%). The as-prepared substrate shows relatively high sensitivity and stability in SERS

16

measurements. Moreover, the procedure is simple and low cost without using any linker

4


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molecules, or templates, or surfactants. These characteristics endow it with the tremendous

2

potential for routine SERS applications and LSPR sensing.

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2. Experimental Section

4

2.1.

Materials

5

ITO-coated glass (sheet resistance 20-30 Ω sp-1.; 1 mm thickness; size, 7 mm × 4 mm) was

6

purchased from Shenzhen Laibao Technology Co. Ltd (Shenzhen China). Hydrogen

7

tetrachloroaurate (HAuCl4) and sodium perchlorate (NaClO4) are purchased from Sinopharm

8

Chemical Reagent Co., Ltd (Shanghai China). 4-Mercaptobenzoic acid (p-MBA) was purchased

9

from Sigma-Aldrich Co. All reagents were analytical grade and used as received without any

10

further purification. Ultrapure water, filtered by a Milli-Q reagent water system at a resistivity

11

of >18 MΩ·cm, was used throughout the experiments. The ITO-coated glass was cleansed by

12

ethanol first, and then it was washed successively in acetone, isopropanol, pure water for at least

13

20 min with sonication. Last, it was dipped in a mixture of pure water, hydrogen peroxide and

14

ammonium hydroxide (volume ratio, 5:1:1) and boiling it for at least 30 minutes.22 It can be used

15

in the following procedure after drying with nitrogen gas.

16

2.2.

17

2.2.1. Electrochemistry

Instrumentation

18

All electrochemical experiments were performed with a CHI750B electrochemical

19

workstation (CH Instruments Inc.) in a conventional one-compartment cell in conjunction with a

20

standard three-electrode system. Experiments were carried out with NaClO4 as the supporting 5



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electrolyte, and if necessary the solutions were purged with nitrogen 20 min before

2

electrochemical experiments. Moreover, a nitrogen atmosphere was kept over the solutions.

3

2.2.2. UV-vis Spectrophotometer

4

UV-vis absorption spectra were recorded by using a 760-CRT double beam UV-vis

5

spectrophotometer (Shanghai Precision and Scientific Instrument Co. Ltd., China) and using a

6

quartz cuvette of 1cm width. The recorded wavelength range was from 350 to 800 nm against

7

bare ITO-coated glass as the reference.

8

2.2.3. Field Emission Scanning Electron Microscope

9

Field emission scanning electron microscopy (FESEM) images were obtained by using

10

Hitachi S-4800 Field emission scanning electron microscopy (Hitachi, Ltd., Japan) at

11

accelerating voltage 15.00 KV and using an in-lens ion annular electron detector.

12

2.2.4. X-Ray Diffraction

13

X-ray diffraction (XRD) analysis was carried out by D-MAX2000 X-ray powder

14

diffractometer (Rigaku Co., Japan) with a Cu anticathode (40 kV, 30 mA). The range of the

15

data are between 10° and 90°, with a step angel of 0.01°

16

2.2.5. Raman Spectroscopic Apparatus

and a scan rate of 0.15° min-1.

17

Raman spectroscopic measurements were performed by using a confocal microscopic Raman

18

system (SuperLabRam II, Dilor, France). A liquid nitrogen-cooled 1024×800 pixels

19

charge-coupled device was used as a detector and an exciting line of 632.8 nm was supplied by a

20

He-Ne laser with power ca. 5 mW. A 50× long-working-length objective was used for focusing 6


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the laser spot onto the substrates surface. The slit and pinhole are set at 100 and 1000 µm,

2

respectively. Each spectrum was measured three times and the acquisition time was 15 s. The

3

Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer.

4

2.3.

5

2.3.1.

Electrode Modification Au Nucleation

6

A saturated calomel reference electrode (SCE) was used as the reference electrode and a Pt

7

foil is used as the counter electrode. The ITO slides (geometry area ca. 0.28 cm-2, the skimmed

8

tape was wrapped around the slides leaving a certain area), which were obtained according to the

9

above method, serve as the working electrode for all electrochemical experiments. All the

10

potentials were reported in this article with respect to the SCE. The deposition solution contains

11

0.1 mM HAuCl4 and 0.1 M NaClO4. After being applied a potential step from +0.89 V to –0.8 V

12

(vs. SCE), the ITO electrode showed a great change as to the open circuit potential which varies

13

from 0.74 V to 0.89 V. It was demonstrated that the Au seeds had been formed on the surface of

14

the electrode. In fact, its subsequent cyclic voltammogram and the FE-SEM images could

15

provide strong evidence for the nucleation of AuNPs on the bare ITO electrode under such a

16

circumstance.

17

2.3.2. Au NPs Growth

18

The as-prepared AuNPs array deposited on an ITO electrode (signified as AuNPs/ITO

19

electrode) was used as the working electrode for the growth of AuNPs in the same

20

electrochemical cell. The growth of AuNPs was performed by cyclic voltammetric mode at room

21

temperature 25 ± 2 ℃. The potential range from +0.3 to ‒0.04 V was applied for different 7



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cycles at 50 mV/s. Afterward, the working electrode was removed from the deposition solution

2

and rinsed copiously with ultrapure water. Then the as-prepared AuNPs/ITO electrode was dried

3

under a gentle stream of nitrogen before its utilization. Otherwise, it will be stored in H2O at 4 ℃

4

when it is not being immediately used.

5

2.3.3. Preparation of SERS-Active Substrates

6

The AuNPs/ITO electrodes with different deposition cycles n (n=0, 50, 150, 300, 500) and a

7

bare ITO electrode were immersed into 1 mM 4-mercaptobenzoic acid (p-MBA) ethanol solution

8

for 24 hr. Residual or excess of p-MBA was removed by rinsing the electrodes with ethanol for

9

three times, and then with copious amounts of ultrapure water. Before SERS measurement, the

10

electrodes were dried under a gentle stream of nitrogen.

11

3.

12

Results and Dicussion 3.1.

Electrochemical Deposition of AuNPs on ITO

13

Our aims here were focused on controlling the size, density, and crystal orientation of the

14

AuNPs electrodeposited on a bare ITO electrode to fabricate high quality SERS-active substrate.

15

The factors like the overpotential, the concentration of electroactive species and the

16

electrodeposition time (nucleation time and growth time) had great influence on the

17

characteristics of the electrodeposit such as granularity, thickness, preferential crystallographic

18

orientation.32 For the sake of simplicity, our experiment was carried out in 0.1 M NaClO4 + 0.1

19

mM HAuCl4 which was significantly lower than those used in previous studies, and the seeding

20

overpotential only used one value (the potential is stepped from OCP 740 mV to 890 mV vs.

21

SCE).33 The effect of seeding time and growing time on the geometry and crystallography of 8


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resultant AuNPs would be investigated in this article. The electrochemical deposition method

2

used here included two steps, as schematically shown in schematic S1 (Shown in Supporting

3

Information). First, a short nucleation pulse of high cathodic polarization (En) was applied on a

4

bare ITO electrode. Then, the resultant AuNPs/ITO electrode, in which the AuNPs formed in

5

first step were regarded as gold seeds for further growth in the following step, was carried out in

6

a specific potential range (Eg1~Eg2) with relatively low overpotential and different voltammetric

7

cycles. This procedure was essentially different from the traditional double pulse method in

8

which a much longer growth pulse at low overvoltage followed after a nucleation pulse.34,35 It

9

was obvious that as for the growth of AuNPs, a variable driving force was used here instead of

10

one constant driving force used in the traditional double pulse procedure. This change would be

11

verified to produce great influence on the structure of AuNPs deposited on ITO electrodes.

12

Figure 1a illustrated a cyclic voltammogram (CV) of a bare ITO electrode measured from 1.4

13

to -0.2 V at 50 mV/s in a solution of 0.1 mM HAuCl4 in 0.1 M NaClO4. We observed a reduction

14

peak of AuCl4- at around +0.35 V on the first cyclovoltammetric scan. The corresponding

15

reaction occurring on the surface of a bare ITO electrode was shown here36:

16

AuCl4- + 3e-

Au0 + 4Cl-

17

Due to H2 evolution by reduction of water, there was a second cathodic peak at around ca.

18

-0.2 V. In the reverse scan, there was a cathodic current flowed until ca. 830 mV which

19

represented the reduction of AuCl4- to Au0 and the reductive current was lower than the current

20

in the first scan. Due to the nucleation and growth of the nuclei, it was clear that the reduction of

21

AuCl4- occurred at about 780 mV on the forward scan and remained up until 830 mV in the

22

subsequent scan. Beyond that, in the subsequent second scan the reduction peak potential was

E0= 0.994 V vs. NHE

(1)

9



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approximately 670 mV and it would bring great effect on the further reduction of AuCl4- on the

2

Au seeds formed on the bare ITO electrode. As for a bare ITO electrode, it was necessary for the

3

formation of gold nuclei on it to overcome an energy barrier by applying a relatively high

4

overpotential. Once the inoculating crystal was formed, the formation of Au seeds would

5

catalyze the reduction of AuCl4- and make it thermodynamically more facile, thus the reduction

6

would occur at a more positive potential (830 mV). Therefore, a characteristic ‘nucleation loop’

7

which signified metal nucleation and growth could be observed in the first scan.

8

Figure 1b represented the cyclic voltammograms for the ITO electrode modified with Au

9

seeds (signified as Au-seeds/ITO electrode) in a solution of 0.1 mM HAuCl4 + 0.1 M NaClO4.

10

There was no an anodic current ‘crossover’ in the cyclic voltammogram due to the presence of

11

Au seeds. Moreover, the reduction peak potential (730 mV) for it shifted along the positive

12

direction by ca. 380 mV in comparison with that for a bare ITO electrode. This behavior was

13

very similar to that for a fluorine-doped tin oxide (FDTO) electrode modified with an

14

3-aminopropyldimethoxysilane (ADMMS) monolayer.37 The remarkable change of the reduction

15

peak potential could be ascribed to the existence of small Au nuclei. The presence of Au seeds

16

reduced the activation energy of the reduction of AuCl4- due to the fact that the Au seeds possess

17

prodigious surface-to-volume ratio and relatively high surface energy. Once the Au nuclei were

18

formed on a bare ITO electrode, the growth of AuNPs immediately proceeded on the surface of

19

these Au nuclei. We could further take full advantage of this effect to fabricate the AuNPs on an

20

Au seeds/ITO electrode with controlled size and distribution in a low overpotential range (-0.04

21

~ +0.3 V) since the depletion layer around Au seeds could be reduced by lowering the applied

22

potential and the growth rate of AuNPs, which would eliminate diffusion zone coupling

23

effectively and produce higher size homogeneity and better controllability.38 10


30

a

15

Start point Loop

0 -15 -30

Scan 2 -45

Scan 1 ∆E

-60 -0.4

1

Current Density / (µ A cm-2)

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Current Density / (µ µA cm-2)

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0.0

0.4

50

b Start point

25 0 -25

Scan 1 -50

Scan 2 -75

-100

0.8

Potential vs. SCE / V

1.2

1.6

-0.4

0.0

0.4

0.8

1.2

1.6


2 3 4

Figure 1. Cylic voltammograms for gold electrodeposition on a bare ITO electrode (a) and Au seeds/ITO electrode (b). Potential was scanned with the first cyclic (solid line, black) and the second cyclic (dash line, red) at a scan rate of 50 mV/s in 0.1 mM HAuCl4 in 0.1 M NaClO4.

5

A single nucleation pulse was applied to form Au seeds on a bare ITO electrode. Figure S1

6

(Supporting Information) represented a current transient recorded for electrodeposition of AuNPs

7

by applying a potential step from 0.89 to -0.8 V vs. SCE. There was no initial sharp high current

8

appearing that was caused by the charging of double electric layer. A current decay was observed

9

which was linear with t−1/2, and this indicated that a planar diffusion regime arising due to the

10

overlapping of growing hemispherical diffusion layers which provided mass transport for

11

nanocrystal growth in the process of gold nucleation.32 The diffusion coefficient could be

12

obtained from the slop of Q versus t1/2 plot according to Cottrell equation:39

13

Q=2nFACoD1/2π-1/2t1/2

14

Where Co=0.1 mM, A=0.28 cm2 and n=3.

(2)

15

In our experimental conditions, the calculated diffusion coefficient for Au electrodeposition

16

on a bare ITO electrode under -0.8 V was 2.59×10-4 cm2/s and it was 0.67 times as high as that

17

for a polydopamine (Pdop)/ITO electrode (1.65×10-4 cm2/s).33 The results demonstrated that the

11



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electrodeposition under the potential of -0.8 V was more facile than on a biopolymer coated ITO

2

electrode. No apparent humps could be observed in Figure S1, and it could be concluded that the

3

growth of Au nuclei was relatively rapid. Because the rising portion of the peak was not

4

experimentally accessible under our conditions, estimation of the nucleation mechanism (i.e.

5

instantaneous or progressive) was not possible from current transient.37,40 It was reasonable to

6

infer that the electrochemical reduction process of AuCl4- described here undergoes an

7

instantaneous nucleation process on bare ITO electrodes since there was no noteworthy charging

8

phase.37,41,42 The so formed Au seeds would achieve higher size monodispersity than progressive

9

nucleation which would lead to the coupling of the diffusion layer of the adjacent AuNPs and the

10

inhomogeneity of the particle size. Figure S2a illustrated the influence that different scan rates

11

had on the reduction peak of AuCl4- in 0.1 M NaClO4 +0.1 mM HAuCl4. It could be observed

12

the reduction peak of AuCl4- shifted along the negative direction with increasing of the reductive

13

current intensity. A linear relationship was shown when plotting the anodic peak currents as a

14

function of the square roots of scan rates (Figure S2b), which indicated diffusion-controlled mass

15

transport in all cases. It was more interesting that as for the AuNPs growth, the cyclic

16

voltammetric process resulted in the predomination of the Au (111) facet (Figure 5) which was

12


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different from the previously reported electrochemical preparation methods but was similar to

2

the hydroxylamine seeding method.43,44

3

3.2.

FE-SEM Characterization of Au NPs-Modified ITO Electrodes

4

To get more information about morphology of AuNPs on ITO electrodes, their visual

5

representation were obtained with FE-SEM. Figure S3 (Supporting Information) and Figure 2

6

contained representative images depicting respectively the effect of seeding time (Figure S3) or

7

growing time (Figure 2) on the electrodeposition morphology of AuNPs. Gold nanoparticles had

8

been deposited on the surface of ITO electrodes using electrochemical seed-mediated growth

9

method. This approach involved the electrochemical deposition of nanoscale Au seeds on a bare

10

ITO electrode and further growth of the Au seeds by electrochemical deposition rather than by

11

hydroxylamine, which was different from the method adopted by Raj.45 Figure S2a showed

12

typical FE-SEM images of Au seeds formed by electrodeposition from 0.1 mM HAuCl4 in 0.1 M

13

NaClO4 at the potential of -800 mV over a period of 10 s. The dark, grainy background

14

corresponded to the bare ITO glass and the bright, somewhat rough and spherical features

15

corresponded to the AuNPs deposited on the ITO electrode. As could be seen, the Au

16

nanoparticles with a quite symmetric distribution were appeared on the ITO electrode and the Au

17

seeds with hemispheric morphology had the size distribution between 3 and 19 nm with a

18

standard deviation 2 nm. When the seeding time reached to 80 s, there were large particles

19

formed on the ITO electrode and they were randomly distributed on the ITO surface. The size

20

distribution of the AuNPs (Relative Standard Deviation, RSD 31 %) was larger than that in the

21

10 s (RSD 24 %). We considered that this might be related to the coupling of the diffusion zones 13



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Page 14 of 38

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of the adjacent particles and the formation of new nucleation sites throughout the deposition

2

process, which lead to appearance of some new seeds and their consequent growth. Forster and

3

Zamborini et al. had obtained same conclusion from their works.36,41 Due to the high density of

4

the Au seeds (Figure S2), the diffusion shielding effect of the nanoelectrode ensembles probably

5

occurred which would result in an inhomogeneous distribution of AuCl4- around the small

6

AuNPs surfaces in subsequent electrochemical reactions.46 Though adjusting the duration of the

7

electrodeposition could control the size of the gold nanoparticles, the long seeding time would

8

lead to the heterogeneity of particle size. Compton’ group came to the conclusion that an

9

increase in the deposition time would result in the enhancement of the average particle size of Au

10

nanoparticles, but their corresponding RSD would also increase with prolonging of the

11

deposition time.28 Meanwhile, it was very important to slack the effect of non-uniform

12

distribution of AuCl4- around AuNPs in the diffusion process. Therefore, we selected a relatively

13

short duration (10 s) as the seeding time in this work for the purpose of obtaining high density

14

and size monodispersity.

15

Zamborini and co-workers had discussed the different deposition potentials of AuCl4- on an

16

ITO electrode.36 With increasing electrode potential from -0.2 to 0.8 V, the size of the AuNPs

17

increased and the density of the AuNPs decreased. The electrochemical deposition of metal went

18

through the nucleation of metal clusters and subsequent growth.36,38 High overpotential could

19

activate a bare ITO electrode and alleviate its significant surface roughness to achieve narrow

20

particles distributions.47 At a low overpotential relative to the thermodynamically favored

21

potential for AuNPs deposition, the energy was not sufficient for the rapid AuNPs nucleation all

22

over the surface and the energy was significant relative to the electron energy (governed by

23

potential).36 In our condition, as expected, the AuNPs with high density (1800 particles/µm2) and 14


Page 15 of 38

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1

low particle size distribution were obtained at high overpotential (-0.8 V) (Figure S2a). The

2

particle size distribution was not especially large (RSD 24 %) and the mean particle size was ca.

3

9 nm. It was significant that the Au seeds with acceptably narrow size distribution had great

4

influence on the growth process.41 When the nucleation event occurred, the nucleation sites

5

catalyzed further AuCl4‒ ions reduction, leading to their preferential growth over the formation

6

of new nucleation events on the bare ITO electrode by using the slow growth approach.34,36,41 It

7

had same effect on the growth of Au nanocrystals relative to the chemical seeding method using

8

hydroxylamine.42-44,48 For the preferential growth of the Au seeds, we could control the size and

9

density of the Au seeds in the seeding process by using different overpotentials and the changes

10

of the conditions of the seeding process would have great influence on the electrochemical

11

growth process. In order to fabricate SERS-active substrate, a relatively high overpotential (-0.8

12

V) were chosen to produce high particle density and size monodispersity for the high

13

overpotential nucleation pulse was favorable to instantaneous nucleation. As shown in Figure 2,

14

after the seeding process, the size of the Au seeds would become large and the distance between

15

the adjacent AuNPs would decrease which could produce tremendous local electromagnetic field

16

enhancement when the gap was smaller than 10 nm.49,50 They would contribute to the excellent

17

SERS-active performance of the substrates.

18

Seed-mediated growth approach using electrochemical method was applied for the growth of

19

monodisperse Au nanoparticles. Figure 2 showed a series of FE-SEM images to estimate the

20

effect of the growing time on the AuNPs. It was clearly seen that particle density decreased;

21

meanwhile, the average size of the AuNPs was reached to 50 nm after 500 cycles. We noted that

22

the size of the AuNPs depended on the electrodeposition cycles. The average size of the AuNPs

23

increased from about 9 ± 2 nm to 50 ± 8 nm. We could adjust the number of electrodeposition 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

1

cycles to control the size of the AuNPs on the ITO electrode. It was worth while mentioning that

2

the distance between the adjacent AuNPs became shorter after different deposition cycles and it

3

was less than 10nm after 500 cycles (in Figure 2a). For the short distance between AuNPs, it was

4

expected that under such a circumstance abundance of ‘hot spots’ would be provided which

5

could produce high performance of SERS effect. When the gap between adjacent Au

6

nanoparticles reached to sub-10 nm, which had been demonstrated that in this range, the

7

substrate modified with noble metal nanoparticles would produce a lot of “hot spots” which

8

would result in interacting among the surface plasmon resonances though dipole-dipole EM

9

interactions.72 Though the distance between AuNPs after 50 cycles approached to 10 nm, the size

10

of the nanoparticles only reached to 16 nm which could not contribute to the high performance of

11

SERS. On the basis of FE-SEM analysis, the average diameters of the AuNPs on the ITO

12

electrode were 16 ± 3, 18 ± 4, 32 ± 6, 50 ± 9 nm, respectively, for electrodeposition of 50, 150,

13

300, 500 cycles. The relative standard deviations of the average AuNPs diameter did not show

14

statistically significant differences (17, 19, 18, 17 % for deposition cycles of 50, 150, 300, 500

15

cycles, respectively). However, there was significant improvement of standard deviations of the

16

average AuNPs diameter compared to that for the Au seeds (RSD 24 %). The particle density

17

decreased with increasing of the deposition cycles (Figure 3a). It could be due to (1) interparticle

18

diffusion coupling that occurred with higher density of AuNPs34,36,38,46 and (2) the Ostwald

19

electrochemical mechanism that might be involved in our experiment to explain the change of

20

the particle density.51 There was a concentration gradient due to the size disparities and it

21

allowed the diffusion of AuCl4- toward large AuNPs. Larger Au particle accepted three electrons,

22

for the reduction of AuCl4- ions onto its surface. It would accept three electrons from a

23

neighboring smaller AuNPs through the conducting substrate to reestablish electrical equilibrium. 16


Page 17 of 38

1

The Ostwald electrochemical coarsening leaded to the particle density decrease due to the large

2

particles scavenged smaller ones. However, we did not observe the broadening of AuNPs size

3

distribution with increasing of deposition cycles. In our conditions, when the cycles of

4

electrodeposition reached ca. 200, there was no significant variation of the density of the AuNPs

5

on the ITO substrate (Figure 3a). Furthermore, we inferred that the system of the deposition was

6

kinetically and thermodynamically biased toward the formation of large single crystals, for the

7

different deposition rate of AuCl4- at polycrystalline and single crystal surface. Brus and Ohsaka

8

et al. got the same conclusion in their previous works.43,51 The characterization of the lattice

9

plane of the AuNPs/ITO electrode obtained here was shown in Figure S4 (Supporting

10

Information). The results from FE-SEM image analysis for electrochemically deposited gold

11

nanocrystals on ITO electrode were shown in Table 1. 140 mean=50nm σdia.=8.5nm

Count

120 RSD dia.=17% 100 Density=225 particles µm-2 80 60 40 20 0

0

10

20

30

40

50

60

70

80

Particle Diameter / nm

12

mean=32nm σdia.=5.9nm

140 120

RSDdia.=18% Density=230 particles µm-2

100

Count

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0

13

0

10

20

30

40

50

60

70

80


17




140 120

RSDdia.=19% Density=750 particles µm-2

Count

100 80 60 40 20 0

0

10

20

30

40

50

60

70

80


1 160


140

RSDdia.=18%

120

Densitry=1250 particles µm-2

Count

100 80 60 40 20 0

0

10

2

2000

9 10

30

(a)

1600 1200 800 400 0

0

150

300

Cycles

6 7 8

30

40

50

60

70

80

Figure 2. Scanning electron micrographs and particles size histograms for Au NPs on ITO glass at deposition potential of -800 mV and a deposition duration of 10s, followed by cyclic voltammetry, a: 500 cycles b: 300 cycles; c: 150 cycles; d: 50 cycles; at potential range of +0.3 to -0.04 V.

Nanogap distance / nm

3 4 5

20


Density / (particles / µ m2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

450

(b)

25 20 15 10 5 0 0

100

200

300

Cycles

400

500

Figure 3. (a) The AuNPs density dependence of the cycles of electrodeposition; (b) the nanogap distance between AuNPs with the different deposited cycles. (300 nanogaps are analyzed).

3.3.

XRD Characterization and Electrochemical Characterization of AuNPs-Modified ITO Electrodes

18


Page 19 of 38

Au(111)

200

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 ∗ ∗

100

c

50

b a

0 0

20

40

60

80

100

2−Theta (degrees)

1 2 3 4 5

Figure 4. XRD patterns of bare ITO (a), AuNPs seed/ITO (b) prepared from a solution of 0.1 M NaClO4 +0.1 mM HAuCl4 via applying a potential step from +0.89 V to –0.8 V (vs. SCE) , and AuNPs/ITO (c) electrodeposited by cyclic voltammmetry from +0.3 to -0.04 V(vs. SCE) for 500 cycles at scan rate of 0.05 V s-1 based on the AuNPs seed/ITO in 0.1 M NaClO4 +0.1 mM HAuCl4 (peaks of ITO are marked with asterisks).

6

The crystalline structures of the AuNPs on ITO electrode were further investigated by XRD.

7

XRD patterns of the AuNPs/ITO electrodes described above were illustrated in Figure 4. The

8

peaks around 30.2º and 35.2º were assigned to ITO (222) and ITO (400) facet (reference to the

9

JCPDS file 06-0416), respectively.

42,52

The peak at 38.2º was ascribed to the Au (111) facet

10

(reference to the JCPDS file 04-0416).44,53 It was interesting that there was only Au (111) crystal

11

facet in the AuNPs/ITO, and the pattern of Au (200), Au (220), and Au (311) were so weak that

12

we could not recognize them in the Figure 4. The higher intensity of the peak at 2θ =38.2º

13

indicated the higher population of Au (111) facet in the AuNPs/ITO electrode. In all the cases,

14

the ITO electrode was used as the substrate for the electrochemical deposition of AuNPs and it

15

should be noted that since the path length of the X-ray photons is of the order of micrometer, the 19



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Page 20 of 38

1

X-ray beam might penetrate the substrate in the case.53 There were clear patterns of ITO.

2

Furthermore, it could be observed in the FE-SEM that nanoarray of AuNPs were formed on the

3

ITO substrate instead of a compact Au film. The AuNPs deposited on the ITO electrode showed

4

an Au (111) facet preferential orientation that could be related to the electrochemical deposition

5

potential range used in the fabrication of the nanoarray. In fact, Ohsaka’ group had ever utilized

6

a chemical seeding approach followed by an electrochemical means to fabricate Au electrode

7

with a high surface area (ca. 7.5×10-3 cm2) and enriched with Au (111) facet (ca. 60%).53 Dong’s

8

group had introduced a simple approach for preparation of Au (111) single-crystal

9

nanoisland-arrayed ensembles which was based on fine colloidal Au monolayer-directed seeding

10

growth.44 The common characteristic of these methods lied in the employment of colloidal gold

11

surface-catalyzed reduction of Au3+ by hydroxylamine. In our condition, we used an

12

electrochemical seeding approach rather than hydroxylamine seeding followed by an

13

electrochemical mean. Nevertheless, it seemed that they had the same effect on the growth of the

14

Au nanoparticles. In the potential range of deposition from +0.3 to -0.04 V after 500 cycles, we

15

found the Au nanoparticales predominantly oriented in the Au (111) crystal facet (Figure 4). The

16

XRD patterns of Au-seed/ITO were very weak and almost absent. It might be that for applying

17

high overpotential in deposition, though there were numerous Au nuclei formed on the ITO

20


Page 21 of 38

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1

substrate, the size of Au seeds only reaches ca. 9 nm, resulting in the absence of the information

2

of the XRD patterns of such a Au-seed/ITO.

3

To evaluate the quality of the AuNPs on the ITO electrode, the electrochemical

4

characterization of the as-prepared substrates that were measured in a deoxygenated (i.e.,

5

N2-saturated) 0.5 M H2SO4 solution at a potential scan rate of 0.1 V s-1 was shown in Figure S4

6

(Supporting Information). The CV of the AuNPs/ITO showed the peaks were similar to the Au

7

(111) single-crystal in the 0.5 M H2SO4 and were different from the peaks that observed at the

8

polycrystalline Au electrode in the positive region (1.1∼1.5 V, Figure S4), indicating that the

9

fabrication protocol herein could populate the Au surface to grow with Au (111) facet.54

10

In order to get more information of the crystallographic orientation of the Au nanoparticles on

11

the ITO glass for deposition 500 cycles under potential 0.3 to -0.04 V, the reductive thiol

12

desorption experiments were undertaken (Figure S5). Different single-crystalline domain

13

exhibits different binding strengths toward an attached self-assembled thiol. For instance, for a

14

short-chain thiol species like cysteine, a multiple reductive desorption pattern was recorded for

15

the reductive desorption from a polycrystalline Au electrode.43, 55 The cathodic peak observed at

16

ca. -0.75 V corresponding to desorption of L-cysteine from Au (111) facet and a broad cathodic

17

peak at ca. -0.95 V∼-1.0 V for desorption of L-cysteine from the Au (100) facet and Au (110)

21



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Page 22 of 38

1

facet were also recognized. In the AuNPs/ITO electrode, Au nanoparticles enriched with ca.

2

80.7 % Au (111) facet were founded.

A

a B

d

b

e f g

c

h

50µ µA cm-2

-0.8 -0.6 -0.4 -0.2 0.0

0.2


-2 250µ µA cm -0.8 -0.6 -0.4 -0.2 0.0

0.2


3 4 5 6 7

Figure 5. CVs obtained at: (A) :(a) AuNPs/ITO (500 cycles); (b) polycrystalline Au electrodes; (c) bare ITO electrode; (B) (d) Au seed/ITO electrode; (e) AuNPs/ITO (50 cycles); (f) AuNPs/ITO (150 cycles); (g) AuNPs/ITO (300 cycles); (h) AuNPs/ITO electrode (500 cycles) in (a) N2- and (b, c, d, e, f, g and h ) O2-saturated 0.5 M KOH solution at a scan rate of 0.1 V s-1. (Note: spectra are stacked for clarity)

8

Moreover, the O2 reduction reaction (ORR) could reflect the surface state of the Au

9

electrode and this reaction exhibited a strong relationship with the crystallographic orientation of

10

the Au electrode.56-58 It had been shown that ORR proceeds via a quasi-reversible pathway at the

11

Au (111) electrodes in alkaline media. Hence, the ORR would furnish further evidence for the

12

Au (111) orientation-enriched AuNPs/ITO electrode without any additive during the

13

electrodeposition process.59 Figure 5 showed a typical set of CVs for the ORR at a bare ITO

14

(curve a), polycrystalline Au (curve b), Au-seed/ITO (curve d), AuNPs/ITO (50, 150, 300, 500 22


Page 23 of 38

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1

cycles; curve d, e, f, g, h) electrodes in O2-saturated 0.5 M KOH solution. The curve-a showed

2

the corresponding response of the AuNPs/ITO electrode (500 cycles) in N2-saturated 0.5 M KOH

3

solution and there was no any peak appearing in the cycles and there was a cathodic peak at -0.8

4

V for the reduction of the O2 at the bare ITO electrode in curve-c. It was clear from curve-b that

5

the ORR proceeded irreversibly at the polycrystalline Au electrode where occurred four-electron

6

reduction of O2 for the ratio of the Au(100) was relatively high in the poly-Au electrode, while a

7

quasi-reversible two-elecron behavior was observed for the ORR at the Au-seed/ITO or the

8

AuNPs/ITO electrodes (50, 150, 300, 500 cycles). Ohsaka and co-workers had demonstrated that

9

(1) the Au (111) was the least active basal plane, whereas Au (100) was shown to be the most

10

active and the catalysis performance to the reduction of O2 of these crystallographic planes was

11

in the order Au (111) < Au (110) < Au (100); (2) the ORR was completely irreversible at Au

12

(100) electrode and a quasi-reversible behavior was undergone at Au (111) and Au (110)

13

electrodes.53,56,60-62 The cathodic peak at the Au (111) was related to the catalysis of the O2 to

14

hydrogen peroxide. Nevertheless, the anodic peak was correspond to the reoxidation of hydrogen

15

peroxide (HO2- in 0.5 M KOH) (which was confirmed by the fact that the quasi-reversible

16

reduction pathway of O2 is not affected by the presence of the SOD, while in the case of the

17

presence of catalase, a significant increase of the cathodic peak current was observed and the

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

1

anodic peak current was severely suppressed)56. Interestingly, there was the anodic peak at 0 V

2

and that was to say that the ratio of the Au (111) in the Au-seeds/ITO electrode was higher than

3

that in the natural polycrystalline Au electrode. With the increasing of the electrodeposition

4

cycles, it was seen clear from the curve-B that there was a gradual positive shift of the cathodic

5

peak potential and small increase in the peak current. The ratio of the cathodic to anodic peak

6

was higher at AuNPs (50, 150, 300, 500 cycles) compared to that at single Au (111) electrode,

7

which could be considered as the consequence of the partial involvement of the other facets of

8

Au in ORR.53 In Figure S5, there was 19.3% Au (100) and Au (110) facet in the AuNPs/ITO

9

electrode (500 cycles). The peak potential for the ORR at AuNPs/ITO (500 cycles) appeared at

10

ca.-0.45 V different with that at the bare ITO electrode in the same medium (ca. -0.8 V). Thus, it

11

could be consider that the bare ITO substrate did not participate in reducing the O2 to HO2-. The

12

cathodic peak shifted from ca.-0.7 to ca.-0.45 V (Au seeds/ITO -0.7 V, 50 -0.65 V, 150 -0.50 V,

13

300 -0.48 V and 500 cycles -0.45 V) in curve-B. It could be concluded that this might be

14

attributed to the enrichment of the Au (100) facets in the AuNPs, which leaded to the positive

15

shift of the cathodic peak.60

16

gradually increasing of the real active surface of the electrode with increasing in

17

electrodeposition cycles (Table 1).

The higher of the cathodic peak current might be produced by the

24


Page 25 of 38

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1

We could estimate the real surface area of the electrodeposited Au particles on ITO glass by

2

calculating the amount of charge consumed during the reduction of the gold oxide monolayer

3

(Figure S6). A reported value of 400 µC cm-2 was adopted in these calculations.28,63 The results

4

were presented in the Table 1. The real surface area (500 cycles) was higher than those of the

5

AuNPs/ITO electrodes fabricated by Compton’s group and Ohsaka’s group.28,53

6 7

Table 1. Effect of Nucleating and Growing Time on the Mean Diameter and Density of Gold Nanoparticles Electrodeposited on ITO Glass En a /mV -800 -800 -800 -800 -800 -800 a

tnb /s 10 80 10 10 10 10

Range of Eg/mV ---40~300 -40~300 -40~300 -40~300

Cycles of Total surface depositio area of gold

Mean Density diameter/ (particles

n

naoparticle/cm2

nm

/µm2)

--50 150 300 500

0.26 -0.87 1.3 2.9 3.1

9±2 13±4 16±3 18±4 32±6 50±9

1800±200 1700±150 1250±100 750±60 230±50 225±30

RSD EF (%) (105) 24 31 17 18 19 18

0.31 0.54 1.8 5.3 9.5 13

The potential for the formation of Au seeds on the indium tin oxide electrode. bThe time for the formation

of Au seeds on the indium tin oxide electrode.

8 9

3.5.

Optical Properties of the AuNPs-Modified ITO Electrodes

10

Figure 6A showed the UV-visible absorption spectroscopy of the Au nanoparticles deposited

11

on the ITO substrate. The AuNPs on the ITO substrate with nanometer-scale dimensions

12

absorbed light in the visible and/or near-infrared region due to localized plasmon resonance,

13

which was light-induced collective oscillation of conduction electrons at the metal surface, and

14

that the resonance wavelength depended on particle size, shape, and local dielectric 25



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Page 26 of 38

1

environment.64,65 The absorbance peaks of the UV-vis spectra corresponding to the localized

2

surface plasmon resonance band were observed to increase linearly with the number of cycles

3

(Figure 6B). Figure 6C showed the corresponding photographs of the AuNPs/ITO with different

4

deposition cycles. The electrochemically formed AuNPs on the ITO substrate with spherical

5

shape and high monodispersity possessed especially interesting optical properties.42 Depending

6

on the growing time, the interfaces of the ITO electrode exhibited different colors ranging from

7

colorless (Au seed) to red (50 cycles) and blue-purple (500 cycles) (Figure 6C). After

8

electrochemical deposition of 500 cycles, one broad absorption peak was observed at about 625

9

nm and the LSPR signals were not very intense. With increasing the deposition cycles, we could

10

observe the obvious red shift of the peak of the absorbance. It should be related to that with

11

increasing of the growing time, the size of the AuNPs became larger and the distance between

12

the adjacent Au nanoparticles diminished which would produce the enormous local

13

electromagnetic enhancement. It demonstrated that our method had the capacity to adjust the

14

location of the LSPR peaks of the SERS-active substrates by changing the number of the

15

electrodeposition cycles. The substrate’s performance for Raman scattering enhancement should

16

be related to the excitation of surface plasmon within the nanostructure.66 We used 632 nm laser

17

as excitation line to evaluate the SERS performance of the AuNPs/ITO substrates and found that

18

the substrate with electrodeposition 500 cycles showed the best SERS performance (Figure 7).

19

The wavelength of the absorption maximum for the AuNPs/ITO after 500 cycles was ca. 625 nm.

20

The λmax of the absorption spectrum was close to the excitation line (632 nm) of SERS

21

spectroscopic measurement, this would further facilitate EM enhancement. It had been shown

22

that when the λmax corresponded to the excitation wavelength, it would produce the largest SERS

23

enhancement.66 26


Page 27 of 38

A

B

640

1.2

a

λmax=503.6+0.227n R=0.964

λmax / nm

Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

0.9

b c d e f g

0.6

0.3

1

400

600

Wavelength / nm

800

560

520

0

100

200

300

400

500

Number of cycles / n

2 3 4 5 6 7 8 9 10

Figure 7. (A) UV-vis spectra of the AuNPs/ITO, a: 500 cycles; b: 300 cycles; c: 200 cycles; d: 150 cycles; e: 70 cycles; f: 50 cycles; g: seed/ITO (0 cycle); at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry. (Note: spectra are stacked for clarity) (B) The absorbance peak of the UV-vis spectra versus the number of cycles of deposition (statistics based on one measurement for three samples). (C) Photographs taken on a white paper sheet of the samples prepared by electrochemical seed-mediated growth. a: bare ITO; b: seeds; c: 50 cycles d: 70 cycles; e: 100 cycles; f:150 cycles; g: 200 cycles; h: 300 cycles; i: 500 cycles; at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry.

3.6.

SERS Application of the AuNPs-Modified ITO Electrodes

11

The AuNPs/ITO with controlled nanostructure made them attractive for use as SERS-active

12

substrates. 4-Mercaptobenzoic acid was chosen as the SERS probing molecule because it was

13

well studied and its thiol group was easily cleaved to form a metal-S bond when adsorbed on a

14

metal surface.1,67 SERS spectra of p-MBA adsorbed on the AuNPs/ITO electrodes with different

15

electrodeposition cycles were shown in Figure 8. We could observe the SERS signal became

16

larger with increasing in the electrodeposition cycles. It was noted that AuNPs/ITO with 500

17

cycles (Figure 8f) gave the strongest Raman signals compared with the others in Figure 8. There

18

were no characteristic p-MBA vibrational signals for the bare ITO substrate. However, as the 27



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Page 28 of 38

1

growing time of AuNPs increased, the characteristic vibrational modes of p-MBA began to

2

appear and increase in their intensities. We could observe the ring breathing and axial

3

deformation modes at 1075 and 1585 cm-1, the bending of the CH groups on the ring at 1178

4

cm-1, and the stretching associated with the carboxylate group at 1283 cm-1.1,68,69 We chose the

5

intensity of the CC ring-breathing mode (1074 cm-1) to calculate the enhancement factor (EF) of

6

the AuNPs/ITO substrates. The EF value for AuNPs/ITO (500 cycles) was calculated to be about

7

1.26×106 (The calculation method had been shown in Supporting Information, Figure S7). It was

8

lower than the p-MBA SAMs which served as the linker molecules in the sandwich structure

9

between gold substrates and immobilized gold nanocubes.1 The calculated enhancement factors

10

apparently increased with increasing the growing time (Table 1). The difference of the metal

11

nanostructures contributed to the variation of the Raman spectra. For the increase of the growing

12

time, the size of the Au nanoparticles became larger and began to form the junctions between the

13

adjacent Au nanoparticles to increase the SERS signals and enhancement factor, which could

14

attribute to electromagnetic (EM) enhancement mechanism. For the frequency of excitation light

15

source corresponding to that of the localized surface plasmon resonance in the AuNPs, it would

16

produce extremely intense local electromagnetic fields in the gaps between adjacent Au NPs (hot

17

spots). When the interparticle spacing was in the sub-10 nm range, it could provide large SERS

18

enhancement in the junction geometry.70 We concluded that the strong SERS-active effect for the

19

AuNPs/ITO (500 cycles) could attribute to their particular geometry (the size of Au NPs and the

20

spacing between the adjacent Au NPs).

21

To obtain the information of the uniformity, we successively measured 30 spots randomly on

22

the AuNPs/ITO substrate (Figure 9a). Obviously, the Raman spectra of pMBA were enhanced

23

greatly at each point, indicating excellent reproducibility of the AuNPs/ITO substrate. 28


Page 29 of 38

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1

Meanwhile, we still found that the fluctuation of the Raman signals could be attributed to the

2

variation of the nanostructures over the as-prepared substrate. It demonstrated that the substrate

3

was not complete uniform and existed spatial non-uniformity of the nanostructure, which was

4

regarded as the dominating factor affecting the degree of enhancement of Raman scattering. In

5

our condition, the ratio of the 1074 and 1588cm-1 band with different points did not have

6

significant variation and the standard deviation only reached to 0.047 indicated that the pMPA

7

had the same adsorption geometry over the substrate’s surface (Figure 9b) and the SERS

8

intensity deviation of the 1075cm-1 band of the 30 spectra was shown in Figure 9c and the

9

standard

deviation

only reached

to

0.092.

These

results

demonstrated

the

good

10

SERS-performance and relative uniformity of the substrate across the AuNPs/ITO substrate. It

11

was sufficient to provide reproducible SERS signals that made the as-prepared substrate possess

12

great potential in quantitative SERS analysis application. It should be mentioned that the stability

13

of the AuNPs/ITO substrate had been tested and the substrate stability could be retained for at

14

least 3 weeks without markedly attenuation of the SERS signals (Figure 9d). The statistical data

15

of the successively measured SERS spectra of 30 spots randomly of 1mM pMBA on the

16

AuNPs/ITO substrate (500 cycles) after three weeks had been shown in Figure S8.

29



400cps

ν(C-C) ring ν(COO-) δ(CH)

500 cycles

Bare ITO f e d c b a

500

1 2 3

1000

1500

2000

Raman shift / cm-1

Figure 8. SERS spectra taken at 632.8 nm of the p-MBA on the AuNPs/ITO: a: bare ITO; b: Au seed/ITO; c: 50 cycles; d: 150 cycles; e: 300 cycles; f: 500 cycles; at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry.

b

a

tn um be r

I1074 / I1588

0.8

30

25

20

800

1200

1600

2000

Raman shift / cm-1

Sc a

400

0.6 0.4 0.2

n

5

po in

15 10

0.0 0

5

35000

10

15

20

30

c

d

30000 25000 20000

20

10000

10

5

10

15

20

25

Scan point number

30

400

800

1200

1600

Raman shift / cm-1

2000

Sc an

5000

po in t

15

nu

15000

m be r

30

25

5

5 6 7 8 9 10 11 12

25

Scan point number

4 I1074 / Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

Figure 9. The uniformity of the SERS signals on the AuNPs array-based substrate. (a) Successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles). (b) The SERS intensity distribution of the ratio of the 1074 and 1588 cm-1 band. The black line represents the average ratio of the 30 spectra and the standard deviation is 0.047. The blue lines show the standard deviation of the ratio. (c) The SERS intensity distribution of the 1074 cm-1 band. The red bands represent the deviation of the average intensity of the 30 spectra. (d) Successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles) after three weeks.

30


Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


4.

Conclusions

2

This work demonstrates an economical, feasible yet robust method for electrochemical

3

deposition gold nanoparticles on indium tin oxide (ITO) glass as SERS active structure. To

4

thoroughly consider the nucleation and the growth of the nanoparticles, the controlled particle

5

size, distribution, the distance between the adjacent gold nanoparticles which produce numerous

6

‘hot spots’ on the substrate. The nanoparticles are predominantly oriented in the Au (111) crystal

7

plane, showing a good face-selective adsorption of molecular with the thiol moiety. For the

8

as-prepared SERS active substrate with high reproducibility, stability, and cost advantage, this

9

electrochemical seed-mediated method has great potential to be commercialized and the

10

substrate could serve as the commercially portable SERS active substrate for on-site

11

environmental monitoring and other sensing applications in the near future. Through using the

12

different growth potential range to adjust the gold crystallographic orientation as SRES active

13

substrate is under study.

14

Associated Content

15

Supporting Information

16 17 18 19

Electrochemical characteristics, calculation of Enhancement Factor data and the statistical data of the successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles) after three weeks. This material is available free of charge via the Internet at http://pubs.acs.org. 31



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Page 32 of 38

1

Author Information

2

Corresponding Author

3

*E-mail: [email protected].

4

Notes

5

The authors declare no competing financial interest.

6

Acknowledgements

7

Financial support from Chinese National Foundation of Natural Science Research

8 9

(No.20775049, 20973114) and the Program of Shanghai Normal University (DXL122) is gratefully acknowledged. We also thank Mr. H.Jin for FE-SEM measurements.

10 11

References

12 13

(1) Sisco, P. N.; Murphy, C. J. Surface-Coverage Dependence of Surface-Enhanced Raman Scattering from Gold Nanocubes on Self-Assembled Monolayers of Analyte. J. Phys. Chem. A

14 15 16

2009, 113, 3973-3978. (2) Jun, B.-H.; Kim, G.; Noh, M. S.; Kang, H.; Kim, Y.-K.; Cho, M.-H.; Jeong, D. H.; Lee, Y.-S. Surface-Enhanced Raman Scattering-Active Nanostructures and Strategies for Bioassays.

17 18 19

Nanomedicine 2011, 6, 1463-1480. (3) Kudelski, A. Analytical Applications of Raman Spectroscopy. Talanta 2008, 76, 1-8. (4) Zhang, X.; Shah, N. C.; Van Duyne, R. P. Sensitive and Selective Chem/Bio Sensing Based

20 21

on Surface-Enhanced Raman Spectroscopy (SERS). Vib. Spectrosc. 2006, 42, 2-8. (5) Haynes, C. L.; McFarland, A. D.; Duyne, R. P. V. Surface-Enhanced Raman Spectroscopy.

22 23

Anal. Chem. 2005, 77, 338 A-346 A. (6) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by

24 25 26

Surface-Enhanced Raman Scattering. Science 1997, 275, 1102-1106. (7) Zhai, W.-L.; Li, D.-W.; Qu, L.-L.; Fossey, J. S.; Long, Y.-T. Multiple Depositions of Ag Nanoparticles on Chemically Modified Agarose Films for Surface-Enhanced Raman

27

Spectroscopy. Nanoscale 2012, 4, 137-142. 32


Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

(8) He, L.; Lamont, E.; Veeregowda, B.; Sreevatsan, S.; Haynes, C. L.; Diez-Gonzalez, F.;

2

Labuza, T. P. Aptamer-Based Surface-Enhanced Raman Scattering Detection of Ricin in Liquid

3 4 5

Foods. Chem. Sci. 2011, 2, 1579-1582. (9) Yang, L.-b.; Chen, G.-y.; Wang, J.; Wang, T.-t.; Li, M.-q.; Liu, J.-h. Sunlight-Induced Formation of Silver-Gold Bimetallic Nanostructures on DNA Template for Highly Active

6

Surface Enhanced Raman Scattering Substrates and Application in TNT/Tumor Marker

7 8

Detection. J. Mater. Chem. 2009, 19, 6849-6856. (10) Osawa, M.; Ikeda, M. Surface-Enhanced Infrared Absorption of p-Nitrobenzoic Acid

9

Deposited on Silver Island Films: Contributions of Electromagnetic and Chemical Mechanisms.

10 11

J. Phys. Chem. 1991, 95, 9914-9919. (11) Xu, H.; Aizpurua, J.; Käll, M.; Apell, P. Electromagnetic Contributions to Single-Molecule

12 13 14 15 16

Sensitivity in Surface-Enhanced Raman Scattering. Phys. Rev. E 2000, 62, 4318-4324. (12) Schatz, G. C.; Van Duyne, R. P. In Handbook of Vibrational Spectroscopy; John Wiley & Sons, Ltd: 2006. (13) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. Charge Transfer Resonance Raman Process in Surface-Enhanced Raman Scattering From p-Aminothiophenol Adsorbed on Silver:

17 18 19

Herzberg-Teller Contribution. J. Phys. Chem. 1994, 98, 12702-12707. (14) Il'ichev, Y. V.; Kühnle, W.; Zachariasse, K. A. Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the

20 21 22

Size of the Amino and Benzonitrile Subunits by Alkyl Substituents. J. Phys. Chem. A 1998, 102, 5670-5680. (15) Park, W.-H.; Kim, Z. H. Charge Transfer Enhancement in the SERS of a Single Molecule.

23 24 25

Nano Lett. 2010, 10, 4040-4048. (16) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; WuDe, Y. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature

26 27

2010, 464, 392-395. (17) Kahraman, M.; Aydin, Ö.; Çulha, M. Size Effect of 3D Aggregates Assembled from Silver

28 29

Nanoparticles on Surface-Enhanced Raman Scattering. Chemphyschem 2009, 10, 537-542. (18) Lee, S. J.; Guan, Z.; Xu, H.; Moskovits, M. Surface-Enhanced Raman Spectroscopy and

30 31

Nanogeometry:‒ The Plasmonic Origin of SERS. J. Phys. Chem. C 2007, 111, 17985-17988. (19) Alvarez-Puebla, R. A.; Ross, D. J.; Nazri, G. A.; Aroca, R. F. Surface-Enhanced Raman

32 33

Scattering on Nanoshells with Tunable Surface Plasmon Resonance. Langmuir 2005, 21, 10504-10508.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

1

(20) Kim, K.; Kim, K. L.; Shin, K. S. Selective Detection of Aqueous Nitrite Ions by

2 3

Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol on Au. Analyst 2012, 137, 3836-3840.

4

(21) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a

5 6

Silver Electrode. Chem. Phys. Lett. 1974, 26, 163-166. (22) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. Wavelength-Scanned

7 8

Surface-Enhanced Raman Excitation Spectroscopy. J. Phys. Chem. B 2005, 109, 11279-11285. (23) Félidj, N.; Aubard, J.; Lévi, G.; Krenn, J. R.; Salerno, M.; Schider, G.; Lamprecht, B.;

9

Leitner, A.; Aussenegg, F. R. Controlling the Optical Response of Regular Arrays of gold

10 11 12

Particles for Surface-Enhanced Raman Scattering. Phys. Rev. B 2002, 65, 075419. (24) Ditlbacher, H.; Felidj, N.; Krenn, J. R.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Electromagnetic Interaction of Fluorophores with Designed Two-Dimensional Silver

13 14 15

Nanoparticle arrays. Appl. Phys. B-Lasers O. 2001, 73, 373-377. (25) He, P.; Liu, H.; Li, Z.; Liu, Y.; Xu, X.; Li, J. Electrochemical Deposition of Silver in Room-Temperature Ionic Liquids and Its Surface-Enhanced Raman Scattering Effect. Langmuir

16 17 18

2004, 20, 10260-10267. (26) Pazos-Perez, N.; Ni, W.; Schweikart, A.; Alvarez-Puebla, R. A.; Fery, A.; Liz-Marzan, L. M. Highly Uniform SERS Substrates Formed by Wrinkle-Confined Drying of Gold Colloids.

19 20 21 22

Chem. Sci. 2010, 1, 174-178. (27) Abalde-Cela, S.; Ho, S.; Rodríguez-González, B.; Correa-Duarte, M. A.; Álvarez-Puebla, R. A.; Liz-Marzán, L. M.; Kotov, N. A. Loading of Exponentially Grown LBL Films with Silver Nanoparticles and Their Application to Generalized SERS Detection. Angew. Chem., Int. Ed.

23 24

2009, 121, 5430-5433. (28) Dai, X.; Compton, R. G. Direct Electrodeposition of Gold Nanoparticles onto Indium Tin

25 26 27 28

Oxide Film Coated Glass: Application to the Detection of Arsenic(III). Anal. Sci. 2006, 22, 567-570. (29) Wang, Y.; Deng, J.; Di, J.; Tu, Y. Electrodeposition of Large Size Gold Nanoparticles on Indium Tin Oxide Glass and Application as Refractive Index Sensor. Electrochem. Commun.

29 30

2009, 11, 1034-1037. (30) Liu, H.; Penner, R. M. Size-Selective Electrodeposition of Mesoscale Metal Particles in the

31 32

Uncoupled Limit. J. Phys. Chem. B 2000, 104, 9131-9139. (31) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse

33 34

Gold Solutions. Nat. Phys. Sci. 1973, 241, 20-22. (32) El-Deab, M. S. On the Preferential Crystallographic Orientation of Au Nanoparticles:

35

Effect of Electrodeposition Time. Electrochim. Acta 2009, 54, 3720-3725. 34


Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

(33) Ye, W.; Wang, D.; Zhang, H.; Zhou, F.; Liu, W. Electrochemical Growth of Flowerlike

2

Gold Nanoparticles on Polydopamine Modified ITO Glass for SERS Application. Electrochim.

3 4 5

Acta 2010, 55, 2004-2009. (34) Ueda, M.; Dietz, H.; Anders, A.; Kneppe, H.; Meixner, A.; Plieth, W. Double-Pulse Technique as an Electrochemical Tool for Controlling the Preparation of Metallic Nanoparticles.

6 7

Electrochim. Acta 2002, 48, 377-386. (35) Sandmann, G.; Dietz, H.; Plieth, W. Preparation of Silver Nanoparticles on ITO Surfaces

8 9 10

by a Double-Pulse Method. J. Electroanal. Chem. 2000, 491, 78-86. (36) Ivanova, O. S.; Zamborini, F. P. Electrochemical Size Discrimination of Gold Nanoparticles Attached to Glass/Indium−Tin-Oxide Electrodes by Oxidation in

11 12

Bromide-Containing Electrolyte. Anal. Chem. 2010, 82, 5844-5850. (37) Finot, M. O.; Braybrook, G. D.; McDermott, M. T. Characterization of Clectrochemically

13 14 15 16

Deposited Gold Nanocrystals on Glassy Carbon Electrodes. J. Electroanal. Chem. 1999, 466, 234-241. (38) Fransaer, J. L.; Penner, R. M. Brownian Dynamics Simulation of the Growth of Metal Nanocrystal Ensembles on Electrode Surfaces from Solution. I. Instantaneous Nucleation and

17 18 19 20

Diffusion-Controlled Growth. J. Phys. Chem. B 1999, 103, 7643-7653. (39) A.J. Bard, L. R. F. Electrochemical Methods Foundamentals and Application John wiley and Sons Inc: New York, 2001. (40) Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B. Electrochemical Nucleation:

21 22 23 24

Part I. General Considerations. J. Electroanal. Chem. Interfacial Electrochem. 1982, 138, 225-239. (41) Sheridan, E.; Hjelm, J.; Forster, R. J. Electrodeposition of Gold Nanoparticles on Fluorine-Doped Tin Oxide: Control of Particle Density and Size Distribution. J. Electroanal.

25 26 27

Chem. 2007, 608, 1-7. (42) Praig, V. G.; Piret, G.; Manesse, M.; Castel, X.; Boukherroub, R.; Szunerits, S. Seed-Mediated Electrochemical Growth of Gold Nanostructures on Indium Tin Oxide Thin

28 29 30

Films. Electrochim. Acta 2008, 53, 7838-7844. (43) Rahman, M. R.; Saleh, F. S.; Okajima, T.; Ohsaka, T. pH Dependence of the Size and Crystallographic Orientation of the Gold Nanoparticles Prepared by Seed-Mediated Growth.

31 32

Langmuir 2011, 27, 5126-5135. (44) Jin, Y.; Dong, S. Fabrication of Au(111) Single-Crystal Nanoisland-Arrayed Electrode

33 34

Ensembles by Template-Directed Seeding Growth. Chem. Commun. 2002, 1780-1781. (45) Das, A. K.; Raj, C. R. Facile Growth of Flower-Like Au Nanocrystals and Electroanalysis

35

of Biomolecules. J. Electroanal. Chem. 2010, 638, 189-194. 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

1

(46) Guo, S.; Wang, L.; Wang, E. Templateless, Surfactantless, Simple Electrochemical Route

2

to Rapid Synthesis of Diameter-Controlled 3D Flowerlike Gold Microstructure with "Clean"

3 4 5

Surface. Chem. Commun. 2007, 3163-3165. (47) Scharifker, B. R.; Mostany, J. Three-Dimensional Nucleation with Diffusion Controlled Growth: Part I. Number Density of Active Sites and Nucleation Rates Per Site. J. Electroanal.

6 7 8

Chem. Interfacial Electrochem. 1984, 177, 13-23. (48) Zhang, J.; Kambayashi, M.; Oyama, M. A Novel Electrode Surface Fabricated by Directly Attaching Gold Nanospheres and Nanorods onto Indium Tin Oxide Substrate with a Seed

9 10

Mediated Growth Process. Electrochem. Commun. 2004, 6, 683-688. (49) Alvarez-Puebla, R. A.; Liz-Marzan, L. M. Traps and Cages for Universal SERS Detection.

11 12 13

Chem. Soc. Rev. 2012, 41, 43-51. (50) Chen, T.; Wang, H.; Chen, G.; Wang, Y.; Feng, Y.; Teo, W. S.; Wu, T.; Chen, H. Hotspot-Induced Transformation of Surface-Enhanced Raman Scattering Fingerprints. ACS

14 15

Nano 2010, 4, 3087-3094. (51) Redmond, P. L.; Hallock, A. J.; Brus, L. E. Electrochemical Ostwald Ripening of Colloidal

16 17 18

Ag Particles on Conductive Substrates. Nano Lett. 2004, 5, 131-135. (52) Huang, X.-J.; Yarimaga, O.; Kim, J.-H.; Choi, Y.-K. Substrate Surface Roughness-Dependent 3-D Complex Nanoarchitectures of Gold Particles from Directed

19 20

Electrodeposition. J. Mater. Chem. 2009, 19, 478-483. (53) Chowdhury, A.-N.; Alam, M. T.; Okajima, T.; Ohsaka, T. Fabrication of Au(111) Facet

21 22

Enriched Electrode on Glassy Carbon. J. Electroanal. Chem. 2009, 634, 35-41. (54) Hamelin, A. Cyclic Voltammetry at Gold Single-Crystal Surfaces. Part 1. Behaviour at

23 24 25

Low-Index Faces. J. Electroanal. Chem. 1996, 407, 1-11. (55) Sakai, N.; Fujiwara, Y.; Arai, M.; Yu, K.; Tatsuma, T. Electrodeposition of Gold Nanoparticles on ITO: Control of Morphology and Plasmon Resonance-Based Absorption and

26 27

Scattering. J. Electroanal. Chem. 2009, 628, 7-15. (56) El-Deab, M. S.; Arihara, K.; Ohsaka, T. Fabrication of Au(111)-Like Polycrystalline Gold

28 29 30 31

Electrodes and Their Applications to Oxygen Reduction. J. Electrochem. Soc. 2004, 151, E213-E218. (57) El-Deab, M. S.; Ohsaka, T. Quasi-Reversible Two-Electron Reduction of Oxygen at Gold Electrodes Modified with a Self-Assembled Submonolayer of Cysteine. Electrochem. Commun.

32 33

2003, 5, 214-219. (58) Solla-Gullon, J.; Vidal-Iglesias, F. J.; Feliu, J. M. Shape Dependent Electrocatalysis. Annu.

34

Rep. Prog. Chem., Sect. C: Phys. Chem., 2011, 107, 263-297. 36


Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

(59) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Oxygen Reduction at Au Nanoparticles

2 3

Electrodeposited on Different Carbon Substrates. Electrochim. Acta 2006, 52, 1792-1798. (60) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Size and Crystallographic Orientation Controls

4 5

of Gold Nanoparticles Electrodeposited on GC Electrodes. J. Electrochem. Soc. 2005, 152, C1-C6.

6

(61) El- Deab, M. S.; Ohsaka, T. An Extraordinary Electrocatalytic Reduction of Oxygen on

7 8

Gold Nanoparticles-Electrodeposited Gold Electrodes. Electrochem. Commun. 2002, 4, 288-292. (62) S‒trbac, S.; Adžić, R. R. The Influence of pH on Reaction Pathways for O2 Reduction on

9 10

the Au(100) Face. Electrochim. Acta 1996, 41, 2903-2908. (63) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Elementary Steps

11

of Electrochemical Oxidation of Single-Crystal Planes of Au Part II. A Chemical and Structural

12 13 14

Basis of Oxidation of the (111) Plane. J. Electroanal. Chem. Interfacial Electrochem. 1987, 228, 429-453. (64) Tian, Y.; Liu, H.; Zhao, G.; Tatsuma, T. Shape-Controlled Electrodeposition of Gold

15 16

Nanostructures. J. Phys. Chem. B 2006, 110, 23478-23481. (65) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev.

17 18

2011, 111, 3828-3857. (66) Addison, C. J.; Brolo, A. G. Nanoparticle-Containing Structures as a Substrate for

19 20 21

Surface-Enhanced Raman Scattering. Langmuir 2006, 22, 8696-8702. (67) Hui, W.; Jing, L.; Yuling, W.; Erkang, W. Silver Nanoparticles Coated with Adenine: Preparation, Self-Assembly and Application in Surface-Enhanced Raman Scattering.

22 23

Nanotechnology 2007, 18, 175610. (68) Michota, A.; Bukowska, J. Surface-Enhanced Raman Scattering (SERS) of

24 25

4-Mercaptobenzoic acid on Silver and Gold Substrates. J. Raman. Spectrosc. 2003, 34, 21-25. (69) Bishnoi, S. W.; Rozell, C. J.; Levin, C. S.; Gheith, M. K.; Johnson, B. R.; Johnson, D. H.;

26 27

Halas, N. J. All-Optical Nanoscale pH Meter. Nano Lett. 2006, 6, 1687-1692. (70) Wang, H.; Levin, C. S.; Halas, N. J. Nanosphere Arrays with Controlled Sub-10-nm Gaps

28 29 30 31 32 33 34

as Surface-Enhanced Raman Spectroscopy Substrates. J. Am. Chem. Soc. 2005, 127, 14992-14993.

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1 2

Page 38 of 38

Text and graphics for Table of Contents:

3 4

Herein, a novel electrochemical seed-mediated method for the fabrication of Au (111) like

5

nanoparticles on indium tin oxide thin electrode was demonstrated. This method improves the

6

monodispersion of Au nanoparticles in comparison with the Frens’ method and it is a valuable

7

tool to control the growth of surface-confined Au nanoparticles. Utilizing the decrease of the gap

8

between the adjacent Au nanoparticles to produce numerous ‘hot spots’ in the process of

9

electrodeposition, the as-prepared substrate has the promising potential in route SERS

10

applications for the high uniformity and sensitivity of the SERS signal.

38


Electrochemical Seed-Mediated Growth of Surface-Enhanced Raman

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