Electrical Control of Plasmon Resonance of Gold Nanoparticles Using

Apr 15, 2009 - Phone: (+81)-44-549-2423. ... A large shift of the localized surface plasmon resonance (LSPR) spectrum of gold nanoparticles was attain...
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J. Phys. Chem. C 2009, 113, 8484–8490

Electrical Control of Plasmon Resonance of Gold Nanoparticles Using Electrochemical Oxidation Takashi Miyazaki,* Ray Hasegawa, Hajime Yamaguchi, Haruhi Oh-oka, Hitoshi Nagato, Isao Amemiya, and Shuichi Uchikoga Electronic Imaging Laboratory, Corporate Research & DeVelopment Center, Toshiba Corporation, 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan ReceiVed: February 16, 2009; ReVised Manuscript ReceiVed: March 26, 2009

A large shift of the localized surface plasmon resonance (LSPR) spectrum of gold nanoparticles was attained by electrochemical oxidation of the nanoparticle surface. This oxidation occurred in the cell, which consisted of a pair of indium tin oxide (ITO) electrodes and water medium between the electrodes. On one side of the ITO electrode, the gold nanoparticles were adsorbed. With the application of a voltage of 5 V to the cell, a spectrum shift as large as 68 nm was obtained. Though the spectrum shift has already been observed by changing liquid crystal (LC) orientation surrounding gold nanoparticles, the size of the shift was not large (11 nm). That was because the variation of the effective refractive index of LC was rather small. Our large shift due to electrochemical oxidation resulted from the large refractive index of Au-O. The electrochemical oxidation was confirmed by XPS analysis of the gold nanoparticles with the LSPR spectrum shift. Other possible mechanisms of the shift such as charge localization, aggregation, and adsorption of charged materials proved to have no effect via SEM measurement and so on. This large shift of the resonance spectrum can be expected to lead to further development of spatial light modulators for next-generation optical communications and displays. 1. Introduction

P)

Surface plasmon resonance (SPR) is a charge-density oscillation that may exist at the interface of two media with dielectric constants of opposite signs, for instance, a metal and a dielectric. Many industrial applications such as light emitting devices1 and molecular sensors2-4 have been proposed and actively developed. These applications are referred to as plasmonics. Another possible application is considered to be spatial light modulators. Localized surface plasmon resonance (LSPR) excited on a nanoparticle surface results in wavelength-selective absorption with extremely large molar extinction coefficients.5 In fact, LSPR has been utilized as an optical filter for the coloring of stained glass since early times. If the resonance wavelength can be electrically modulated over a wide range, spatial light modulators with a large tunable range will be realized and attractive applications such as reflective displays and optical communications can be expected. Moreover, the condition of incident angle is not severe and no polarizers are required for LSPR excitation, which is also favorable to practical use. On the basis of the theoretical formula of resonance wavelength, what is required for a large LSPR spectrum shift is considered below. The resonance wavelength can be expressed theoretically on a quasielectrostatic model without solving the Maxwell equation. In this case, nanoparticles are much smaller than the wavelength of light and localized surface plasmon means induction of electric polarization by a vibrating electric field of incident light. The electric polarization P is obtained from the theory of dielectrics in the electrostatic field.6 * Towhomcorrespondenceshouldbeaddressed.E-mail:takashi5.miyazaki@ toshiba.co.jp. Phone: (+81)-44-549-2423. Fax: (+81)-44-520-1255.

ε-1 E 4π + N(ε - 1) 0

(1)

Here, the surrounding medium is the vacuum and ε is the dielectric function of the nanoparticle. N is the depolarization coefficient and depends on particle shape, for example N ) 4π/3 for a sphere. When the dielectric constant of the surrounding medium is ε1, the formula of polarization P is given by substituting ε/ε1 into ε in formula 1. According to the Drude model, the dielectric function ε is frequency dependent as shown below.

ε)1-

ωp2

(2)

ω2

ωp is the plasma frequency. Then, polarization P is expressed as shown below here:

P)

ω2 - ωp2 - ε1ω2 (4πε1 + N - Nε1)ω2 - Nωp2

(3)

When the denominator of formula 3 is equal to zero, polarization P becomes infinite and absorbance of incident light is at the maximum. This state is LSPR. Therefore, the theoretical expression of resonance wavelength of the localized surface plasmon is described as

λ)

(

2πc (4π - N)n2 + N ωp N

)

1⁄2

(4)

Here, n is the refractive index of the surrounding medium, and c is the velocity of light in vacuum. From formula 4, the resonance wavelength is a function of n, N, and ωp. Electrical control of the resonance wavelength becomes possible if these parameters can be electrically changed. To obtain a large

10.1021/jp901408w CCC: $40.75  2009 American Chemical Society Published on Web 04/15/2009

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Figure 1. (a) Schematic image of the cell structure. (b) Experimental setup for spectrum measurement.

spectrum shift, these three parameters are considered below. A. Variation of ωp. The plasma frequency ωp is given by

ωp )

(

4πNee2 m*

)

1⁄2

(5)

Here, Ne is density of the electron, e is the elementary charge, and m* is the effective mass of the electron. The plasma frequency ωp can be changed by charge localization at gold nanoparticles. When the electron/hole is injected into nanoparticles by an external electric field, the density of the electron Ne increase/decrease, which results in a spectrum shift of LSPR to the blue/the red from formulas 4 and 5.7,8 An actual observation of such a spectrum shift as large as about 10 nm has been reported elsewhere.8 However, it is unlikely that a large shift of the LSPR spectrum occurs because the localized charge is discharged by tunneling through the electric double layer formed at the solution (medium)/nanoparticle interface. B. Variation of N. The depolarization coefficient N depends on the nanoparticle shape. The LSPR spectrum moves if gold nanoparticles are deformed. However, deformation of a gold nanoparticle by an electric field is unlikely. In this case, aggregation of nanoparticles may be responsible for changing N. When nanoparticles aggregate, neighboring plasmons couple with one another and many new resonance peaks with higher order are generated. Experimental spectra acquired in aggregation are explained as a superposition of all resonance peaks described above. From the point of view that metal surface causing LSPR expands from a single nanoparticle to all nanoparticles involved in aggregation, we regard aggregation as a kind of deformation in a broad sense. The LSPR spectrum is broadened and moves to the red in aggregation.9-11 However, it is unlikely to move the LSPR spectrum reversibly because of difficulty in redispersing aggregated nanoparticles, while a large spectrum shift can be expected. C. Variation of n. An important feature of LSPR is the dependence of resonance wavelength on the refractive index of matter close to the nanoparticle surface, which led to the development of LSPR biosensors.12-14 There are three methods to change n: exchange of the surrounding medium, deposition of chemical products, and adsorption of charged material by using electrophoresis. It is difficult to exchange the medium inside the cell in the case of application to light modulators. The charged material corresponded to organic acid ions dissolved from the nanoparticle surface in this case. The effect of these organic ions is discussed in section 4. The refractive index above the nanoparticle surface also can be electrically changed

by deposition of electrochemical products. Electrochemical oxidation of gold nanoparticles is easy in spite of the chemical inertness of bulk gold.15 The gold oxide is so unstable16 that it begins to decompose on suspension of voltage application, which results in reversibility of the spectrum shift. We tried to move the resonance spectrum by electrochemical oxidation of gold nanoparticles. Electrical tuning of the resonance spectrum with LSPR in LC medium sandwiched between two parallel plate electrodes has already been reported.17,18 Kossyrev et al. reported an LSPR spectrum shift of 11 nm due to the change of refractive index above the gold nanodisk surface by controlling the orientation of LC molecules electrically.17 However, it is not easy to realize a large shift of the LSPR spectrum by using the LC medium because the variation of the effective refractive index of LC is rather small, whereas the refractive index of Au-O is very large (1.7-3.3).19 In view of this, electrochemical deposition of the gold oxide is an effective method to induce a large change of the refractive index. In our study, we obtained a six times larger shift of the LSPR spectrum from gold nanoparticles than in the case of the LC medium and found that the shift was voltage-controllable and nearly reversible. It was confirmed by XPS analysis that the mechanism of the spectrum shift was the refractive index change due to electrochemical oxidation of gold nanoparticles. Other possible mechanisms such as charge localization, aggregation of nanoparticles, and adsorption of charged material were also investigated and proved to have no effect on the shift. 2. Experimental Methods A. Preparation of Cell Structure. Schematic images of cell structure and experimental setup for the measurement of LSPR spectra are shown in Figure 1. The shift of the LSPR spectrum due to voltage application was measured with a cell structure as shown in Figure 1a. The Indium tin oxide (ITO) electrode formed on a glass substrate was used as the bottom electrode. The bottom substrate was first sonicated in acetone for 10 min. Three kinds of dispersion liquids containing gold nanoparticles with diameters of 40, 60, and 80 nm were used, which were purchased from Tanaka Kikinzoku Kogyo Co. (Au citric acid colloid-SC type). Gold nanoparticles in the dispersion liquid were stabilized by a coating of organic acid such as citric acid to prevent aggregation.20 The dispersion liquid of gold nanoparticles was dropped on the bottom electrode with a micropipet. The dispersion liquid was successively evaporated in air for about 12 h. The typical procedure to remove excess nanopar-

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Figure 2. (a) Spectrum data measured in air (n ) 1), water (n ) 1.33), ethylene glycol (n ) 1.43), and LC (n ) 1.6). (b) Relationship between the peak shift and refractive index acquired from Figure 2. (a) Plot data in the graph are fitted by the solid line. (c-f) Dark field images measured in air (n ) 1), water (n ) 1.33), ethylene glycol (n ) 1.43), and LC (n ) 1.6).

ticles adsorbed on the electrode was ultrasonic cleaning in deionized water for 10 s. The opposite electrode was an ITO film formed on the glass substrate. The opposite substrate was also sonicated in acetone for 10 min before use. Each metal wire was electrically connected with the ITO film at the edge of the bottom substrate and the opposite substrate, respectively. The gap distance was controlled by adding a 0.7 mm thick glass spacer. Deionized water was injected between the substrates. The electrical connection parts with the metal wire were separated from this water medium. B. LSPR Spectrum. Light extinction at resonance wavelength is mainly due to the occurrence of scattered light for the metal nanoparticles whose diameter was several tens of nanometers. Therefore, we measured the intensity of scattered light from the cell as the LSPR spectrum. A microscope (BX 50: Olympus Optical Co.) with a dark-field condenser was used for the measurement of the LSPR spectrum as shown in Figure 1b. The scattered light was collected through object lens (20 magnifications) and the dark-field image was acquired. A portion of the collected light was extracted via the optical fiber for spectral analysis. The analysis was conducted with a spectrometer (PMA 11: Hamamatsu Photonics Co.). A halogen lamp was used as the light source of the microscope, whose spectrum intensity was not uniform in the range of measured wavelength. To eliminate this influence, law data from the spectrometer were corrected by normalizing with a light source spectrum at each wavelength. The spectrum shift was measured with the application of external voltage to the electrodes through the wire. C. Cyclic Voltammogram. To investigate the occurrence of electrochemical reaction on gold nanoparticles in the cell with the application of voltage, cyclic voltammograms (CVs) were measured for the same structure as the above cell. The bottom electrode with the nanoparticle was employed as the working electrode. The opposite electrode was employed as the counter electrode. The reference electrode was not used. In a standard CV measurement, the reference electrode was required. However, in order to prove the occurrence of electrochemical reaction in our cell with application of an electric voltage, CV measurement should be conducted for the same structure as our cell. Since the electrochemical reaction on the working electrode is caused by potential drop at the interface between the working electrode and the solvent, the absolute potential where the electrochemical reaction occurs cannot be evaluated from the applied voltage. However, the occurrence of the electrochemical reaction can be inspected by measuring a CV of the cell because

the occurrence induces an increase of the current between the counter and the working electrode. The terminal of the reference electrode was electrically connected to the wire of the counter electrode and an electric voltage was applied between the working electrode and the counter electrode. Organic acid ions dissolved into the water medium from the nanoparticle surface operated as an electrolyte. The sweep voltage was defined as the electrical potential of the working electrode to the counter electrode and the scan rate was 0.1 V/s. The area of the working electrode was 1 cm2. 3. Results Figure 2. shows spectra data and dark-field images of the cell modified with gold nanoparticles (40 nm φ) at various refractive indexes without voltage application. The surrounding refractive index was changed by using various media such as air (n ) 1), water (n ) 1.33), ethylene glycol (n ) 1.43), and LC (4-cyano-4′-n-pentylbihenyl:5CB) (n ) 1.6). The purpose of these measurements was to confirm that the signal from the spectrometer was not due to stray light but to scattered light from gold nanoparticles. The peak top was at 534 nm and the color of the image was dark green in air. Here, it should be noted that the actual color of the cell could not be perceived owing to the insufficient amount of adsorbed nanoparticles. As shown in Figure 2, the wavelength of the peak top became larger (red shift) and the color of the scattered light changed from dark green to orange gradually as the refractive index of the medium increased. It is confirmed that LSPR can be detected with the measurement setup shown in Figure 1b from these results. It can be explained that the peak shift shown in Figure 2a is due to an increase of the n value in formula 4, which agrees well with other reports.21-28 The relationship between the resonance wavelength and the surrounding refractive index is shown in Figure 2b. From the slope of the regression line in Figure 2b, the sensitivity for the refractive index change was estimated to be 40 nm/RIU. This value is utilized in section 4. The LSPR spectrum of the cell with gold nanoparticles, whose diameter was 40 nm, was measured with application of voltage. For this cell, excess adsorbed particles after evaporation of the dispersion liquid were removed by rinsing in deionized water. The polarity sign of the applied voltage was defined as positive when the ITO electrode with gold nanoparticles was connected to the plus electrode of the voltage source with the opposite electrode connected to the ground. The applied voltage was

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Figure 3. (a) Spectrum shift under a DC voltage of +5 V. The peak shift of 68 nm was observed and the peak shift was nearly reversible. (b) A timing chart to explain the measurement procedure. The time points at which the LSPR measurements were performed are specified by upward arrows.

Figure 5. Change of the spectrum shift over time, which was investigated for XPS and SEM measurement. The cell was under a voltage of +5 V for 2 h to move the spectrum to the red followed by immediate drying of water in the cell by N2 blow. The red shift of the peak top was maintained in air for at least 280 min. Figure 4. CV curves of bare ITO (a) and the modified ITO electrode with gold nanoparticles (b). The oxidation peak and the reduction peak were observed in part b.

changed from 0 V (initial) to +5 V, and then returned to 0 V (the second). The result is shown in Figure 3a. The LSPR spectrum was measured in 10 min after the applied voltage was changed, except for the spectrum described as “5 V within 3 sec” in Figure 3a, which was measured within 3 s after the application of +5 V voltage. The time points at which the LSPR measurement was performed are specified in the timing chart shown as Figure 3b. The bias duration time in Figure 3b was 682 s because it took 82 s for one spectrum measurement. The resonance spectrum was moved to the red by applying positive voltage. In Figure 3a, the peak top moved from 545.6 to 613.6 nm with application of +5 V voltage. Moreover, this spectrum shift was nearly reversible. The spectrum started going back to the blue gradually after returning to 0 V. To determine whether the above red shift was caused by electrochemical oxidation of gold nanoparticles, we measured CV curves and analyzed the surface composition of gold nanoparticles by XPS. Figure 4b shows a CV curve of the above cell and Figure 4a shows that of a cell without adsorption of gold nanoparticles. There exist an oxidation peak at +2 V and a reduction peak at -0.6 V only in Figure 4b. This indicates

that voltage application caused the electrochemical reaction at gold nanoparticles. The surface composition of gold nanoparticles whose resonance spectrum was moved to the red was analyzed by XPS to clarify what electrochemical reaction occurred with the application of the voltage. For this purpose, a sample for XPS analysis was prepared as described below. The voltage of +5 V was applied for 2 h to move the spectrum to the red. Then, the cell was opened followed by immediate drying of water by N2 blow. The spectrum change of the above sample over time is shown in Figure 5. The LSPR spectra before opening of the cell, such as at 0 V in air, then in water, and then after +5 V for 2 h in water, are also shown. The peak top of the LSPR spectrum at 0 V in air was 527 nm and was moved to 547 nm by injecting water medium into the cell. Application of +5 V for 2 h made the peak top shift to 598 nm. After opening the cell, the peak top moved back to 571 nm owing to the refractive index change from water to air, and then it was maintained for at least 280 min. On the basis of this result, XPS analysis was measured within 280 min after water evaporation. Figure 6 shows Au-4f core level spectra. The XPS spectrum of gold nanoparticles without voltage application is also shown in Figure 6 for comparison. Here, the raw data were corrected for charging effect by using the binding energy of the C1S peak on

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Figure 6. XPS spectra of gold nanoparticles with red shift of the peak top (+5 V: gray dotted line) and without voltage application (0 V: black solid line). The spectrum for the shifted nanoparticles was deconvoluted to four subspectra with binding energy corresponding to Au0 and Au-O, whereas only two subspectra corresponding to Au0 were detected for nanoparticles without voltage application.

hydrocarbon (284.8 eV). The XPS spectrum was deconvoluted to four subspectra with a binding energy of 87.6 and 83.9 eV corresponding to Au0 and 89.3 and 85.6 eV to Au-O for nanoparticles whose resonance spectrum was moved to the red, whereas only two subspectra corresponding to Au0 were detected for nanoparticles without voltage application. From Figure 6, it is demonstrated that the red shift of the spectrum was caused by electrochemical oxidation of gold nanoparticles. Since gold oxide is an unstable material,16 the slow blue shift after returning to 0 V shown in Figures 3 and 7 was probably due to decomposition of gold oxide. The resonance wavelength could be tuned by the applied voltage as shown in Figure 7. Figure 7a shows time-variation of the resonance spectrum with the applied voltage of +5 V. No spectrum shift was observed from 3 min and beyond. Figure 7b shows spectrum curves under various voltages. The applied voltage was increased from 0 V to +5 V in increments of 1 V and returned to 0 V again. At each voltage step, the cell was held for 2 min with application of voltage at the beginning, the spectrum curve was measured, and then voltage bias was changed. It took 82 s for the spectrum measurement. The spectrum began to move at 2 V. This value was the same as the sweep voltage of the oxidation peak in Figure 4, which was consistent with the result that the electrochemical oxidation of gold nanoparticles moved the resonance spectrum. The resonance spectrum moved to the red consecutively as the voltage was increased. The spectrum shift depends not on duration but on the value of the applied voltage in these results. This controllability of the shift by electrical voltage demonstrates the possibility of applying this method to electrical devices such as spatial light modulators. Whereas Kossyrev et al. reported a spectrum shift of 11 nm using LC as the surrounding medium for the purpose of application to electric devices,17 our results were about six times larger at maximum than theirs. 4. Discussion As described in section 1, the resonance wavelength depends on three parameters: surrounding refractive index n, depolarization coefficient N, and plasma frequency ωp. Plasma frequency ωp can be changed by electrical charge localization at gold nanoparticles. If this localization had occurred in the cell, the spectrum should have moved to the blue by applying negative voltage and to the red by applying positive voltage. Panels a and b of Figure 8 show results of spectrum measurement with application of positive and negative voltage, respectively. The diameter of gold nanoparticles was

Figure 7. (a) Time variation of the resonance spectrum under DC voltage of +5 V. No spectrum shift was observed from 3 min afterward. (b) Spectra under various DC voltages. The DC voltage was increased from 0 V (initial) to +5 V in increments of 1 V and returned to 0 V (second). At each voltage step, the cell was held for 2 min with application of voltage at the beginning, the spectrum curve was measured, and then voltage bias was changed. It took 82 s for the spectrum measurement. LSPR spectra moved to the red consecutively as the DC voltage was increased.

80 nm and the maximum intensity of spectrum curves was normalized for clarification of the peak shift. The red shift of 25 nm was observed with application of a positive voltage, whereas there was no shift of the peak top with application of a negative voltage as shown in Figure 8. Therefore, it can be concluded that variation of ωp due to charge localization was not the reason for the wavelength shift of our cells. If gold nanoparticles had not directly touched the ITO electrode, this charge localization should have occurred via electron tunneling through the electrical double layer formed on the gold nanoparticle surface.8 Then, gold nanoparticles were electrically contacted with the ITO electrode and the electrical potential of gold nanoparticles was equal to that of the ITO electrode when voltage was applied. Experimental data such as Figure 3a and Figure 7a show that the shift of the LSPR spectrum was reversible. Therefore, the depolarization coefficient N was unchanged in this case because redispersion of aggregated nanoparticles is impossible. The nonaggregation of gold nanoparticles was supported by SEM results. Figure 9b shows the SEM image of gold nanoparticles

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Figure 8. Resonance spectra under positive and negative voltage bias: (a) +5 and (b) -5 V. The red shift of 25 nm was observed under positive DC voltage, whereas there was no shift of the peak top under negative DC voltage. The maximum intensity of spectrum curves was normalized for clarification of the peak shift.

Figure 9. SEM image of gold nanoparticles without voltage application (a) and with red shift of the peak top (b). There was no difference in the lateral density of nanoparticles.

(40 nm φ) whose LSPR spectrum was moved to the red. The sample for SEM measurement was prepared with the same procedure as the sample for XPS analysis. A voltage of +5 V was applied to the cell for 2 h, and then the cell was opened followed by immediate drying of the medium in the cell by N2 blow. SEM measurement was conducted within 280 min after N2 blow. Figure 9a shows the SEM image of gold nanoparticles without applying voltage. Comparing panels b and a in Figure 9, the lateral density of nanoparticles was almost equal. In addition, the electrical contact of the nanoparticle with the ITO electrode implies that the nanoparticle was fixed on the ITO electrode, which also supports the nonaggregation of gold nanoparticles. Next, variation of refractive index n due to adsorption of charged material to the gold nanoparticle surface is inspected. Since the cell was composed of ITO electrodes, gold nanoparticles, and deionized water injected as medium, organic acid ions such as citric acid ion dissolved from the nanoparticle surface corresponded to charged material in this case. These organic acid ions possibly caused the spectrum shift because it has been reported that adsorption of organic and inorganic negative ions on the nanoparticle surface caused the spectrum shift.29-31 We investigated the effect of concentration of citric acid on the spectrum shift. According to liquid chromatography analysis, the concentration of citric acid in medium was 0.02 µg/mL when deionized water was injected into the cell. If this organic ion had contributed to the spectrum shift, the shift should have become larger with an increase in their concentration to as much as the above value in deionized water before injection. The peak top shift was measured with application of +5 V voltage when the citric acid solvent of 0.5 µg/mL was used as the medium. The voltage was applied for 10 min before

measurement. However, the peak shift did not increase. Therefore, it is clear that the organic acid ions caused no spectrum shift. The binding energy of Au-O in Figure 6 was 85.6 eV, which is similar to the value of Au2O3 (85.9 eV) reported by NIST.32 On the other hand, gold oxide was composed of two kinds of Au2O3 for electrochemical oxidation of polycrystalline gold film. One is R-oxide with a refractive index of 3.3, which is mainly Au2O3 mixed with AuO. The other is β-oxide with a refractive index of 1.7-1.82, which is hydrated Au2O3.19 Then it is thought that similar gold oxide was also generated on gold nanoparticles of our cell. Gold nanoparticles adsorbed on the ITO electrode were estimated to have a refractive sensitivity of 40 nm/RIU from linear fitting of four plots in Figure 2b. By using this sensitivity, the peak shift for generation of fully thick R-oxide is calculated to be 80 nm, and 15-20 nm for fully thick β-oxide, whereas experimental results were 40-68 nm. Then it can be considered that experimentally obtained gold oxide was composed of only R-oxide or a mixture of R-oxide and β-oxide. Figure 10 shows the dependence of the peak shift on particle size. All plots in Figure 10 were measured with application of +5 V voltage. The waiting time with voltage application was 10 min before measurement. In spite of the same voltage condition, the peak shift became larger for smaller nanoparticles. The dependence on particle size shown in Figure 10 seemed to be due to the increase of surface area to volume ratio. The ratio of surface area to volume increases with decreasing particle size, which makes a small nanoparticle chemically active and more gold oxide was expected to be deposited on a small nanoparticle. It was reported that small gold nanoparticles contained a greater fraction of gold oxide than large gold nanoparticles after exposure to oxygen plasma.33 Another possible reason for the

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Figure 10. Dependence of the peak shift on particle size. The resonance spectrum of small nanoparticles moved to the red more than that of large nanoparticles.

dependence on particle size was stabilization of gold oxide by the ITO electrode.15 This size-dependent shift is in line with the fact that the ratio of the Au-ITO interface area to the whole particle surface decreases with increasing particle size. On the basis of this assumption, a larger peak shift can be expected for gold nanodisks whose ratio of Au-ITO area to the whole surface area is higher than that of spherical particles. 5. Conclusion In this paper, we observed a spectrum shift of 68 nm at maximum by applying a voltage of +5 V on the cell with gold nanoparticles adsorbed on the ITO electrode. The spectrum shift was brought by the refractive index change due to electrochemical oxidation of the gold nanoparticle. The LSPR spectrum could be moved consecutively to the red by increasing the applied positive voltage and the shift was nearly reversible. Other factors such as charge localization, aggregation, and adsorption of charged material did not contribute to the shift. References and Notes (1) Okamoto, K.; Niki, I.; Scherer, A.; Narukawa, Y.; Mukai, T.; Kawakami, Y. Appl. Phys. Lett. 2005, 87, 071102. (2) Toyama, S.; Aoki, K.; Kato, S. Sens. Actuators, B 2005, 108, 903. (3) Kang, X.; Jin, Y.; Chen, G.; Dong, S. Langmuir 2002, 18, 1713.

Miyazaki et al. (4) Ratiman, O. A.; Katz, E.; Buckmann, A. F.; Willner, I. J. Am. Chem. Soc. 2002, 124, 6487. (5) Hainfeld, J. F.; Powell, R. D.; Hacker, G. W. Nanotechnology; Niemeyer, C. M., Mirkin, C. A., Eds.; Wiley: Weinheim, Germany, 2004; p 353. (6) Kittel, C. Introduction to solid state physics, 6th ed.; John Wiley & Sons Inc.: New York, 1986; p 361. (7) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1999, 15, 866. (8) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773. (9) Zhao, S. Y.; Lei, S. B.; Chen, S. H.; Ma, H. Y.; Wang, S. Y. Colloid Polym. Sci. 2000, 278, 682. (10) Su, K. H.; Wei, Q. H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 1087. (11) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557. (12) Haes, A. J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem B 2004, 108, 109. (13) Hirsh, L. R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. L. Anal. Chem. 2003, 75, 2377. (14) Riboh, J. C.; Haes, A. J.; McFarland, A. D.; Yonzon, C. R.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 1772. (15) Diao, P.; Zhang, D. F.; Guo, M.; Zhang, Q. J. Catal. 2007, 250, 247. (16) Tsai, H. C.; Hu, E.; Perng, K.; Chen, M. K.; Wu, J. C.; Chang, Y. S. Surf. Sci. Lett. 2003, 537, L447. (17) Kossyrev, P. A.; Yin, A.; Cloutier, S. G.; Cardimona, D. A.; Huang, D.; Alsing, P. M.; Xu, J. M. Nano Lett. 2005, 5, 1978. (18) Muller, J.; Sonnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T. A.; Feldmann, J. Appl. Phys. Lett. 2002, 81, 171. (19) Xia, S. J.; Briss, V. I. J. Electroanal. Chem. 2001, 500, 562. (20) Turkevich, J. Gold Bull. 1985, 18, 86. (21) Miller, M. M.; Lazarides, A. A. J. Opt. A: Pure Appl. Opt. 2006, 8, s239. (22) Miller, M. M.; Lazarides, A. A. J. Phys. Chem. B 2005, 109, 21556. (23) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (24) Sherry, L. J.; Jin, R.; Mirkin, C. A.; Schatz, G. C.; Duyne, R. P. V. Nano Lett. 2006, 6, 2060. (25) Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 485. (26) Ni, W.; Chen, H.; Kou, X.; Yeung, M. H.; Wang, J. J. Phys. Chem. C 2008, 112, 8105. (27) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057. (28) Hanarp, P.; Kall, M.; Sutherland, D. S. J. Phys. Chem. B 2003, 107, 5768. (29) Ali, A. H.; Foss, C. A., Jr. J. Electrochem. Soc. 1999, 146, 628. (30) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (31) Mulvaney, P. Langmuir 1996, 12, 788. (32) http://srdata.nist.gov/xps/selEnergyType.aspx. (33) Cuenya, B. R.; Baeck, S. H.; Jaramillo, T. F.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 12928.

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