Electrochemistry Communications 11 (2009) 1034–1037

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Electrochemistry Communications

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Electrodeposition of large size gold nanoparticles on indium tin oxide glassand application as refractive index sensor

Yuan Wang, Jiajia Deng, Junwei Di *, Yifeng TuDepartment of Chemistry, Soochow University, Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 February 2009Received in revised form 28 February 2009Accepted 3 March 2009Available online 10 March 2009

Keywords:Gold nanoparticlesIndium tin oxideElectrochemical depositionRefractive indexSensor

1388-2481/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.elecom.2009.03.005

* Corresponding author. Tel.: +86 512 65880354; faE-mail addresses: djw@suda.edu.cn, jwdi@public1.

Large size gold nanoparticles (GNPs) were directly deposited onto the indium tin oxide (ITO) glass surfaceby cyclic voltammetric method. The GNPs on ITO substrate were characterized by means of scanningelectron microscopy (SEM), UV–vis spectroscopy and X-ray diffraction (XRD). The nucleation and growthsteps were controllable in the GNPs deposition procedure. The addition of chloride ion in the electrolysisaffected the size and density of GNPs on the ITO surface. The response of refractive index for variousorganic solvents was also investigated. The sensitivity of refractive index increased as GNPs becamelarger.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Noble metal nanoparticles, such as Ag and Au nanoparticles, ex-hibit rich localized surface plasmon resonance (LSPR) property,which originate from the collective oscillation of conduction elec-trons in response to an optical excitation [1,2]. The frequencies andintensities of LSPR are known to be sensitive to the dielectric prop-erties of the medium, and in particular, to the refractive index (RI)of mater close to the particle surface [1–7]. This has led to thedevelopment of noble metal nanoparticles as biosensors [8–12],which is based on the measurement of the changes of LSPR proper-ties due to small changes in RI of the surrounding medium or thatoccurred during the binding of the external analyte at the surfaceof noble metal nanoparticles.

The LSPR of noble metal nanoparticles is affected not only bythe dielectric properties of the surrounding media but also by thesize, shape and charge of the nanoparticles [1,6,13]. According tothe Mie theory which is the simplest theoretical approach availablefor modeling the optical properties of nanoparticles, the position ofthe LSPR bands of the metal nanoparticles is highly sensitive totheir size [1]. The sensitivities of the peak shift to the RI changeof the surrounding medium will increase when the size of the no-ble metal nanoparticles become larger. Therefore, the sensorsbased on the larger metal nanoparticles are more sensitive.

However, even to small size nanoparticles, one must employsurfactants which provide both miscibility and sufficient steric

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x: +86 512 65880089.sz.js.cn (J. Di).

repulsion for countering long-range van der Waals attraction toproduce stable nanoparticle dispersions in nonaqueous media.When the size of nanoparticles is significantly larger than 10 nm,it is more difficult to control over dispersion stability and particleaggregation in organic solvents [14–17]. Therefore, it is necessaryto immobilize the nanoparticles on a substrate for better stabilityof noble metal nanoparticles and that provides consistent and sta-ble sensor response. In the most case, a clean glass substrate wasimmersed in an organosilane solution for functionalization andthen dipped in metal nanoparticle suspension for thin-film forma-tion [18,19]. However, the use of definite organic binding agent toimmobilize the metal nanoparticles onto the glass substrate is notfavored because it should be restricted in sensor applications dueto the affinity of the absorbed organic moieties toward differentsolvent, which can affect the RI of the surrounding medium [11].

A seed-mediated growth method can attach gold nanoparticles(GNPs) onto an indium tin oxide (ITO) substrate without using or-ganic linkers [20,21]. However, the procedure involved two steps:the attachment of small seed particles (4 nm) onto the ITO surfacethrough physisorption and the growth of them. This is somewhatcomplex and time-consuming. In our previous study, we have suc-ceeded in direct electrodeposition of GNPs onto ITO substrate[22,23]. In this paper, we report a simple approach to directly de-posit large size GNPs on ITO surface without any organic binder. Alarge GNPs deposited on ITO glass which is inserted in a quartzcuvette is used as a RI sensor for various organic solvents. To thebest of our knowledge, this is the first report on the directly elec-trodeposition of large GNPs on organically unfunctionalized glassfor the utilization as a RI sensor.

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Fig. 1. Representative SEM images of GNPs on ITO surface at average size of (A) 35, (B) 90 and (C) 130 nm. (D) UV–vis absorption spectra of the GNPs deposited on ITO surface.

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Y. Wang et al. / Electrochemistry Communications 11 (2009) 1034–1037 1035

2. Experimental

2.1. Materials and apparatus

Hydrogen tetrachloroaurate (HAuCl4) was products of GuoyaoChemical Regent Co. Ltd. ITO glass (1.1 mm thickness, 100 X resis-tances) was purchased from Suzhou NSG Electronics Co. Ltd. (Suz-hou, China). The buffer solution (pH 2.0) was prepared by Na2HPO4

and citric acid. All other chemicals were analytical grade. Twice-distilled water was used throughout the experiments.

The electrodeposition experiments were performed on aCHI830B electrochemical workstation (Shanghai Chenhua Appara-tus Corporation, China) in conjunction with a conventional three-electrode system. The working electrode was an ITO sheet, aplatinum wire and a saturated calomel electrode served as theauxiliary and reference electrodes, respectively. The adsorptionspectrum was taken out by the use of a TU-1810 spectrophotome-ter (Beijing, China). Scanning electron microscopy (SEM) was

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Fig. 2. XRD patterns of different GNPs deposited on ITO surface at average size of(a) 35, (b) 90 and (c) 130 nm.

obtained with an S-4700 scanning electron microanalyzer (Hitachi,Japan). X-ray diffraction (XRD) analysis was carried out by X‘Pert-Pro MPD (Holand).

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Fig. 3. (A) Extinction spectra of 130 nm GNPs in different solvent. (a) Air; (b) water;(c) ether; (d) acetone; (e) ethanol; (f) hexane; (g) cyclohexane; (h) benzene; (i)carbon tetrachloride; (j) trichloromethane; (k) carbon disulfide. (B) Dependence ofthe kmax shift on the refractive index for different size GNPs. (j) 35, (d) 90 and (N)130 nm.

1036 Y. Wang et al. / Electrochemistry Communications 11 (2009) 1034–1037

2.2. Electrodeposition of GNPs on ITO glass

The ITO glass was washed with dilute ammonia (1:20, v/v), eth-anol, and distilled water for 10 min consequently in an ultrasonicbath. Then, it was immersed into the electrolyte which contained0.9 mL 1 mM HAuCl4 solution, 0 or 0.05 M KCl, and 2.5 mL 0.1 Mbuffer solution (pH 2.0) in a total of 5.0 mL. Cyclic voltammogramwas performed in quiescent solution which was deaerated bynitrogen gas for at least 10 min and a nitrogen atmosphere waskept over the solutions during the electrodeposition experiments.The electrolysis was kept at 30 ± 2�C. The parameters of cyclicvoltammograms were the potential range from 0.3 V to �0.5 Vfor 20 cycles at a potential scan rate 0.05 V/s. The average sizesof gold nanoparticles were 90 nm and 130 nm, respectively.

For comparison, the gold nanoparticles with about 35 nmdiameter were also electrodeposited on ITO glass surface by similarprocess. The composition of electrolyte was 0.1 mL 1 mM HAuCl4

in the buffer solution. The potential was applied from 0.3 V to�0.85 V for 80 cycles at a scan rate 0.05 V/s.

2.3. Measurements of UV–vis spectroscopy and RI of solvents

The UV–vis absorption spectra of gold nanoparticles on ITO sur-face were recorded from 400 to 800 nm against bare ITO glass asreference. To measure the RI of different solvents, 3 mL of individ-ual solvent was taken out in a quartz cuvette whose inner wall wasinserted a GNPs/ITO glass, and the absorption spectrum was re-corded against the reference. Note that the cuvette and GNPs/ITOglass were washed completely and then dried every time, aftereach measurement of spectrum in the presence of a solvent.

3. Results and discussion

3.1. Electrodeposition of GNPs on ITO glass and their characterization

We deposited gold nanoparticles in the range of 35–130 nmonto ITO surface by cyclic voltammetric method. The typical SEMimages and UV–vis spectra of the GNPs are shown in Fig. 1. Ascan be seen, the quasi-spherical GNPs have been successfullyimmobilized on the substrate with a quite symmetric distribution.It is well known that preparation of nanosized particles depositiononto a substrate involves two basic processes: nucleation and par-ticles growth [24,25]. In order to obtain the definite size nanopar-ticles, it is necessary to well control the nucleation and growthsteps. We found it is more controllable in cyclic voltammogram[22,26] than potential step mode [27]. The low potential appliedin cyclic voltammogram is very important parameter. It was diffi-cult to deposit GNPs on ITO surface when the low potential was po-sitive than �0.4 V. At low potential �0.5 V, a nucleation with asufficient large overpotential was achieved to effectively seed thesurface with nuclei at first cycle voltammogram, then the growthof nanoparticles proceeded at next cycles. This may be that the po-tential applied is not sufficient for nucleation but enough for parti-cle growth at the most of the time in potential scan process. Eventhough a few other nuclei are seeded at next cycle, they are dis-solved completely in the anodic process. This is also demonstratedby the test of different cycles. The density of GNPs on ITO surfacewas almost not change while the particle size was about 70, 90,and 110 nm at 10, 20, and 40 cycles, respectively. Since the numberof the nuclei is limit, the large GNPs were grown on the ITO surface(Fig. 1B). When the low potential was negative than �0.6 V, theprocesses of nucleation and particle growth took place simulta-neously at every cycle. The size and density of GNPs on the ITOsurface depended on both of potential range and cycles. The den-sity of GNPs on ITO surface was also markedly increased (Fig. 1A).

The concentration of addition KCl was another factor whichinfluenced the size and density of GNPs deposited on ITO surface.When the concentration of KCl was increasing from 0 to 1 M, thedensity of GNPs on ITO surface was decreased while the size ofGNPs was increased. Fig. 1C shows the SEM image of GNPs depos-ited with addition of 0.05 M KCl. The average size of GNPs wasabout 130 nm. The maximum of surface plasmon absorption spec-tra (kmax) was red-shift with the particle size increasing (Fig. 1D),which is consistent with the Mie theory [1]. This may provide a fac-ile method to achieve low surface coverage of GNPs on ITO surface[28].

The XRD pattern of the GNPs electrodeposited on ITO glass isshown in Fig. 2. An overwhelmingly intensive diffraction peakwas located at 2h = 38.1� in the samples which corresponds tothe (111) lattice plane of the gold [11,29]. In contrast, the intensityof the peak at 2h = 44.4� corresponding to (200) lattice plane isweak in the samples.

3.2. Response of RI of organic solvents using different size GNP’s

The three different sizes of GNPs deposited on ITO glass wereused to investigate the response of different organic solvents.Fig. 3A shows the extinction spectra of 130 nm GNPs/ITO glass im-mersed in various solvents. Fig. 3B plots the value of kmax shift(kmax(solvent)�kmax(air)) of the surface plasmon resonance foreach GNPs/ITO sheet as a function of the RI of the surround med-ium. We have plotted the data as a function of peak wavelengthshift from the air value in order to normalize the data and bettercompare the slopes for different size GNPs. Within this range ofthe RI, the data points for different size GNPs can be fitted reason-ably well to a straight line. The RI of the GNPs for average diameterof 35, 90 and 130 nm is 45, 137 and 187 nm/RIU, respectively. Thisdemonstrated that the refractive index sensor is of the highersensitivity based on the larger GNPs. On the other hand, sincethe GNPs are deposited directly onto the ITO surface, they are verystable and cannot be leached out by polar or nonpolar solvents.

4. Conclusion

The large sizes of GNPs have been directly deposited on ITOsubstrate by electrochemical method. The GNPs/ITO glasses wereapplied to development of RI sensors for variety of organicsolvents. The index sensitivity of 130 nm GNPs was 187 nm/RIU.The proposed method is simple, easy and sensitive. Moreover, itmay be used to design label-free biosensor based on the LSPR ofGNPs for measurement of DNA and other biochemicals.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 20675055), the science foundation ofthe education bureau of Jiangsu Province (07KJB150101) and pre-research Project of Soochow University.

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