Anomalous Electroreflectance and Absorption Spectra of Viologen

Takamasa Sagara*, Naoyuki Kato, Ayumi Toyota, and Naotoshi Nakashima. “Organization and Function”, PRESTO, JST and Department of Materials Science...
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Langmuir 2002, 18, 6995-7001

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Anomalous Electroreflectance and Absorption Spectra of Viologen Radical Cation in Close Proximity of Gold Nanoparticles at Electrified Interfaces Takamasa Sagara,*,†,‡ Naoyuki Kato,‡ Ayumi Toyota,‡ and Naotoshi Nakashima‡ “Organization and Function”, PRESTO, JST and Department of Materials Science, Graduate School of Science and Technology, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan Received June 3, 2002. In Final Form: July 3, 2002 Electroreflectance (ER) and transmission-absorption spectral features of viologen cation radical moieties in close proximity of Au nanoparticles were investigated at electrified interfaces. ER spectra were measured at a polycrystalline Au electrode coated with a thiol-functionalized alkyl viologen (viologen-thiol: VT) monolayer on which Au nanoparticles (11 nm diameter) are immobilized. Transmission-absorption spectra were measured for VT adsorbed on Au nanoparticles immobilized on a mercaptosiloxane-modified ITO electrode. At the Au electrode, an anomalous ER spectrum was observed around the formal potential of viologen, while an ER signal due to the charging-discharging of the Au particles was observed at more positive potentials. At the ITO electrode, both potential modulated absorption spectra and difference absorption ones with respect to potential showed the anomaly as the same as the ER spectrum at the Au electrode. The anomaly is that the absorption spectrum is not a simple sum of the spectra of Au nanoparticles and viologen cation radicals, suggesting a strong interaction between viologen cation radical moieties and Au particles under plasmon excitation of the particles.

1. Introduction Design and fabrication of two- or three-dimensional architectures with nanometer-sized metal particles at electrified interfaces have been extensively studied.1,2 Metal nanoparticles, exhibiting unique functions such as quantum dots, atto-farad level capacitors, and optical signal enhancers, are realized as fascinating building blocks of a nanometer-scale electronic and optical devices. At electrified interfaces (viz. solid/liquid junction), organized thin films assembled with metal nanoparticles may exhibit novel functions based on their optical, electrochemical, and catalytic activities. To evaluate such functions, it is of importance to establish dynamic in situ methods to characterize the chemical microenvironment around the particles under plasmon excitation as a function of electrode potential, E. Elucidation of the particle-chromophore interaction should be one of the keys to the design of photo- and spectroelectrochemically functional thin films assembled with metal nanoparticles and to fabricate a sensing unit of the microenvironment in the vicinity of the particles. Although a variety of phenomena in spectroscopic measurements in relation to the interaction have been reported,3-16 the essential origin

has not been fully understood quantitatively. Such phenomena include, for example, electromagnetic enhancement of fluorescence12,13 and near-field14 spectral signals in the presence of metal particles. Metal nanoparticles can be immobilized on an electrode surface using an alkanethiol monolayer with terminal groups strongly attractive to the particles. As well, negatively charged Au particles can be electrostatically immobilized on a positively charged underlayer.1 Immobilization of metal nanoparticles on an electrode surface through an intervenient organic monolayer containing a tethered chromophore may provide a well-suited particle layers to the study of particle-chromophore interaction under plasmon excitation at electrified interfaces. Another strategy can be irreversible adsorption of a chromophorecontaining thiol derivative on the surfaces of preimmobilized nanoparticles on an electrode. The aim of this paper is to measure the light absorption characteristics of a chromophore in close proximity of Au nanoparticles at an electrified interface as a function of E. We use viologen as a redox-active chromophore. Because electrochemical oxidation-reduction reversibly converts the viologen moiety between a colorless oxidized form (dication: V2+) and a blue reduced form (radical mono-

†

“Organization and Function”, PRESTO, JST. Department of Materials Science, Graduate School of Science and Technology, Nagasaki University. * To whom correspondence should be addressed: e-mail [email protected]; Fax +81-95-843-7271. ‡

(1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 18 and references therein. (2) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27 and references therein. (3) Wang, D.-S.; Kerker, M. Phys. Rev. B 1982, 25, 2433. (4) Kano, H.; Kawata, S. Appl. Opt. 1994, 33, 5166. (5) Jensen, T.; Kelly, L.; Lazarides, A.; Schaz, G. C. J. Cluster Sci. 1999, 10, 295. (6) Kolomenskii, A. A.; Gershon, P. D.; Schuessler, H. A. Appl. Opt. 2000, 39, 3314. (7) Leitner, A.; Lippitsch, M. E.; Aussenegg, F. R. In Surface Studies with Lasers; Aussenegg, F. R., Lippitsch, M. E., Leitner, A., Eds.; Springer-Verlag: Berlin, 1983; p 90.

(8) Wang, S.; Boussaad, S.; Tao, N. J. Rev. Sci. Instrum. 2001, 72, 3055. (9) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578. (10) Linden, S.; Kuhl, J.; Giessen, H. Phys. Rev. Lett. 2001, 86, 4688. (11) Sato, T.; Tsugawa, F.; Tomita, T.; Kawasaki, M. Chem. Lett. 2001, 402. (12) Liebermann, T.; Knoll, W.; Sluka, P.; Herrmann, R. Colloid Surf. A: Physicochem. Eng. Aspects 2000, 169, 337. (13) Liebermann, T.; Knoll, W. Colloid Surf. A: Physicochem. Eng. Aspects 2000, 171, 115. (14) Benrezzak, S.; Adam, P. M.; Bijeon, J. L.; Royer, P. Surf. Sci. 2001, 491, 195. (15) Pettinger, B.; Picardi, G.; Schuster, R.; Ertl, G. Electrochemistry (Tokyo) 2000, 68, 942. (16) Xu, H.; Aizpurva, J.; Ka¨ll, M.; Apell, P. Phys. Rev. E 2000, 62, 4318.

10.1021/la026026g CCC: $22.00 © 2002 American Chemical Society Published on Web 08/09/2002

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cation: V•+), the switching of particle-chromophore interaction can be highlighted. We use two different in situ UV-visible (UV-vis) spectroelectrochemical methods: electroreflectance (ER) and transmission-absorption spectroscopies respectively at Au disk and optically transparent ITO electrodes to describe the spectral features. The ER signal represents ac reflectance change of UV-vis light in response to ac wave potential modulation.17 The ER spectrum represents the difference of reflectance spectra with respect to E. By the use of modulation technique, small and reversible changes of the optical properties at the interface in response to the potential change can be sensitively detected. Plasmon absorption of the metal particles can be used in ER spectroscopy as a probe to track the state of the particles at the interface as demonstrated in our previous paper.18 Since the absorption coefficient of monolayer-level V•+ is extremely small compared to that of a gold nanoparticle in UV-vis wavelength range,19 we need highly sensitive spectroelectrochemical methods coupled with potential modulation. This work demonstrates, for the first time, an anomalous absorption spectrum of V•+ on Au particle at both electrodes, most likely indicative of the emergence of strong electrooptical interaction between the particle and V•+ moiety. 2. Experimental Section 2.1. Materials. Water was purified through a Milli-Q Plus Ultrapure water system coupled with an Elix-5 kit (Millipore Co.) Its resistivity was over 18 MΩ cm. Citrate-stabilized Au nanoparticles (TEM diameter, 11 ( 1.7 nm; plasmon absorption maximum of the colloidal solution, λmax ) 517.5 nm) were prepared by the reduction of tetrachloroaurate ion by trisodium citrate as described in our previous paper.18 N-Pentyl-N′-(11-mercaptoundecyl)-4,4′-bipyridinium bis(hexafluorophosphate) (viologenthiol: VT) was synthesized in our previous work.20 All the other chemicals were of reagent grade and were used as received. 2.2. Preparation of Modified Au Electrode. A polycrystalline Au disk electrode (geometrical surface area: 0.02 cm2) was polished to a mirror finish by the use of suspensions of alumina powders. The electrode was immersed in acetonitrile solution of 1 mM VT + 0.1 M tetraethylammonium bromide (Et4N+Br-) for a period of 24 h to produce a VT-monolayer modified electrode. To immobilize Au nanoparticles, the electrode was subsequently immersed in the Au colloidal solution (24 nM) until saturated immobilization (typically for 1.5 h). The electrode was then rinsed well with water. The same modification was conducted on a Au thin film with enriched (111) facets (Auro Sheet (111)HS, Tanaka Noble Metals Co.) and subjected to atomic force microscope (AFM) observation. AFM images enabled us to verify the immobilization without the presence of threedimensional aggregation, though individual particle images looked diffuse probably because the particles are mobile when scanned by the AFM tip. Immobilization was also confirmed by quartz crystal microbalance (QCM) measurements of the weight gain. 2.3. Preparation of Modified ITO Electrode. An ITO electrode (surface resistivity, 10 Ω/sq, supplied from Sony Co.) of a surface area of 1.40-1.54 cm2 was cleaned by immersing in concentrated H2SO4 + water (v/v ) 1:1) for 30 s and rinsed with a copious amount of water and then ethanol. Then, the electrode was immersed in an ethanolic solution of 0.1 M 3-(triethoxysilyl)1-propanethiol for 10 min. This adsorption procedure was used in reference to ref 21. After it was rinsed well with ethanol and (17) Sagara, T. Recent Res. Dev. Phys. Chem. 1998, 2, 159. (18) Sagara, T.; Kato, N.; Nakashima, N. J. Phys. Chem. B 2002, 106, 1205. (19) Probably because of this reason, the spectral anomaly has not been so far noticed by experiments at electrified interfaces. (20) Sagara, T.; Tsuruta, H.; Nakashima, N. J. Electroanal. Chem. 2001, 500, 255. (21) Daron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313.

Sagara et al. subsequently with water, this mercaptosiloxane-modified electrode was immersed in the Au colloidal solution (24 nM) for 40 min. After it was rinsed with water, the Au particles deposited on the rear glass surface were completely wiped off. The electrode was then dried in a stream of Ar gas (>99.998%) and subsequently immersed in acetonitrile solution of 1 mM VT for 12-22 h to adsorb VT on the Au particles. The electrode was rinsed well with acetonitrile and subsequently with water. 2.4. Electrochemical Measurements. All the electrochemical measurements were conducted using a Ag/AgCl (saturated KCl) reference electrode and a coiled Au wire counter electrode in an Ar gas atmosphere at 24 ( 1 °C. All the potentials are cited with respect to this reference electrode. 2.5. Electroreflectance Measurements for Au Electrode. The Au electrode was settled in a cylindrical quartz cell filled with deaerated 0.1 M KBr solution. The potential modulation used for the ER measurements is described as

E ) Edc + Eac ) Edc + Re[∆Eac exp(jωt)]

(1)

where Edc and Eac are respectively the dc and ac potentials, ∆Eac is the amplitude of the potential modulation, j ) x-1, ω ) 2πf where f is the modulation frequency, and t is the time. The instruments and procedures used for the ER measurements were the same as described elsewhere.17 Under the potential modulation, monochromatic incident light was irradiated to the electrode surface. The ac reflectance signal from the photomultiplier (Hamamatsu R928) was monitored by a lock-in amplifier (EG&G model 5210). The reflectance signal from the photomultiplier was monitored simultaneously by an A/D converter and was timeaveraged to obtain the dc reflectance signal. In both the real part (in-phase component with respect to Eac) and the imaginary part (90° out-of-phase component), the ac reflectance signal was normalized by dc reflectance to obtain ER signals, ∆R/R. Note that if the reflectance change takes place following Eac without any delay, the imaginary part of ∆R/R should be zero. The delay due to the nonzero cell time constant for the double-layer charging and/or the sluggish electron-transfer process may produce the imaginary part. 2.6. Potential-Controlled Transmission-Absorption Spectral Measurements. The modified ITO electrode was vertically inserted in a quartz cuvette of a light path length of 10.0 mm. The electrode surface was perpendicular to the incident light. The cuvette was filled with 0.1 M phosphate buffer solution (pH 7.0, prepared from potassium salts), in which reference and counter electrodes were settled so as not to intermit the light path. A UV-vis-NIR spectrophotometer (V-570, JASCO) equipped with an integration sphere (ISN-470, JASCO) was employed to measure the transmission-absorption spectrum at a wavelength scan rate of 200 nm min-1 under potential control. 2.7. Potential-Modulated Transmission-Absorption Spectral Measurements. The quartz cuvette as described above was settled in the optics of the ER instrument so that the monochromatic light was irradiated to an ITO electrode perpendicularly. The transmitted light was focused to the photomultiplier. Under the potential modulation expressed by eq 1, the signal from the photomultiplier representing the transmitted light intensity was subjected to phase-sensitive detection by lockin amplification. In line with the ER signal, potential-modulated absorbance signal was designated as ∆I/I, where I stands for the transmitted light intensity (The meaning of ∆I/I will be described in section 3.2 in more detail.) We abbreviate this measurement technique as PMTA spectroscopy in this paper.

3. Results 3.1. Electroreflectance Study at Au Electrode. The structure of the modified Au electrode was schematically depicted in Figure 1a (presentation not in correct scale). The saturated amount of Au particles immobilized on a VT-monolayer was found by QCM measurement to be 30% of the 2D close-packed full coverage, while it was 10% on an aminoalkanethiol monolayer.18 The positive charge of VT-monolayer may effectively compensate the electrostatic repulsion between negatively charged Au particles, re-

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Figure 1. Schematic depiction of the structures of the interfaces at modified electrodes: (a) Au disk electrode; (b) ITO electrode. Presentations are not in correct scale. Counteranions of viologen moieties are omitted in these schemes.

Figure 2. Cyclic voltammograms at a sweep rate of 50 mV s-1 in 0.1 M KBr solution at 23 °C. Trace a: Au electrode modified with a VT monolayer. Trace b: Au electrode modified with a VT monolayer on which Au particles were immobilized.

sulting in the deposition of more particles. This also indicates that Au particles can see the viologen moieties at least through electrostatic interaction. The VT monolayer modified Au electrode showed a redox reaction of V2+/V•+ couple in the absence of Au particles at a formal potential of E0′ ) -340 mV with a coverage of Γ ) 4.2 × 10-10 mol cm-2 (Figure 2, trace a). The coverage was obtained by integrating faradaic current of the peak of cyclic voltammogram (CV) followed by dividing by the electrode area for the one-electron-transfer process. The electrochemistry of this electrode was the same as described in our previous paper.20 Trace b in Figure 2 represents the CV measured after immobilization of Au particles, showing the redox wave at E0′ ) -345 ( 10 mV. The peak width and separation were nearly the same as trace a, indicating the potential felt by the viologen moiety is not considerably affected by the presence of Au particle overlayer. In other words, the potential of Au particles changes with the change of E without difference. The value of estimated Γ was ca. 85% of trace a in Figure 2. This rate was minimum among four

Figure 3. Electroreflectance spectra measured at f ) 14 Hz with nonpolarized incident light at an incident angle of 45° in 0.1 M KBr solution. The thick line is the real part, while the thin line is the imaginary part. Peak wavelengths of real part spectra were typed in the figures. (a) VT-monolayer-modified Au electrode with immobilized Au particle at Edc ) 100 mV and ∆Eac ) 99 mV. (b) VT-monolayer-modified Au electrode before Au particle immobilization at Edc ) -340 mV and ∆Eac ) 57 mV. (c) VT-monolayer-modified Au electrode with immobilized Au particles at Edc ) -340 mV and ∆Eac ) 57 mV.

experiments: in other runs, it was 90-95%. The doublelayer charging current at more positive potentials than -0.2 V for trace b was 2.5 times greater than trace a, indicating that the surfaces of immobilized Au particles are charged-discharged with the change of E. At positive potentials, the adsorption-desorption process of Br- on Au particle surfaces may be an additional contributor to the increased capacitive current. Figure 3 shows the collection of three typical ER spectra at f ) 14 Hz. The ER spectrum herein is the plot of both real and imaginary parts of the ER signal as a function of the wavelength of the incident light at a given set of Edc, ∆Eac, and f. Figure 3a was measured Edc . E0′ after the immobilization of the Au particles. At this Edc, viologen is fully in colorless oxidized state (V2+). It exhibited a positive-going ER band around the plasmon absorption maximum and a negative-going band in the longer wavelength region. This spectrum is interpreted as a difference absorption spectrum of Au particles between

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at more negative and more positive potentials with respect to Edc. Appearance of the positive-going ER plasmon absorption band indicates that the absorption is strengthen with moving E to more negative. This response is originated from the potential-dependent chargingdischarging of the Au particles as discussed in our previous paper.18 Briefly, the increase in excess electrons on the particle at more negative potential results in the increase in the plasma frequency. Consequently, the plasmon absorption band becomes sharper and shifts to shorter wavelength. In the present interface, in fact, the more negative Edc (but still less negative than E0′), the shorter the wavelength of plasmon peak maximum in the ER spectrum. When the same Au particles were immobilized on an aminoalkanethiol monolayer at a Au electrode,18 the positive-going peak wavelength of the ER signal at Edc ) 0.10 V was 7 nm longer than the λmax in colloidal solution. In the present case in Figure 3a, the peak wavelength at the same potential (526 nm) was 8 nm longer than the λmax in colloidal solution, supporting that identical shift of the absorption band takes place. Additionally, we measured the electrochemistry of [Ru(NH3)6]2+/3+ in the solution phase. The resulting CV was a simple sum of electrochemically reversible redox wave of [Ru(NH3)6]2+/3+ at E0′ ) -0.14 V and trace b in Figure 2. Viologen moieties did not act as electron-transfer mediators. This fact supports good electronic communication between Au particles and Au electrode substrate. Figure 3b is the ER spectrum at E0′ measured before the immobilization of the Au particles, representing the difference absorption spectrum between V2+ and V•+ (the absorption spectrum of V•+ from which that of V2+ is subtracted).20,22 The ER bands at 408 and 551 nm are due to the absorption of the V•+ dimer. What we can predict as the ER spectrum around E0′ in the presence of immobilized Au particles is a weighted simple sum of the spectra of Figure 3a and b. Because viologen redox and Au particle charging-discharging are not necessarily synchronized, these two processes may show different phase of the ac reflectance signal with respect to Eac. As far as the light absorption processes of Au particles and V•+ moieties are independent, we can take a weighted-simple-sum approach. That is, the predicted spectrum, ERSpred, is expressed as

ERSpred ) pERSV + qERSC

(2)

where p and q are constants that may be either positive or negative values, representing the fractional factors, and ERSV and ERSC stand respectively for the ER spectra due to redox reaction of viologen moieties and chargingdischarging of Au particles. Strictly speaking, the reflection spectrum is a complex nonlinear function of the optical constants at the interface in the context of Fresnel optical theory. Therefore, eq 2 can be applied as an approximation only when three ERSs experimentally represent difference absorption spectral features and are independent of each other. We should keep it in mind that this point should be rationalized by experiments. Surprisingly, the experimental ER spectrum around E0′ (Figure 3c) was far different from the above prediction. In the wavelength region of 490-580 nm, Figure 3a and b exhibited positive-going signal, while Figure 3c exhibited a bipolar structure. Apparently, the spectrum is not a weighted simple sum of the spectra of Figure 3a and b, especially in the wavelength region where both V•+ (22) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823.

Figure 4. Electroreflectance voltammograms at three different wavelengths of the incident light at f ) 14 Hz and ∆Eac ) 57 mV. Potential sweep rate was -2 mV s-1. Because the imaginary part always showed a factored mirror image of the real part with respect to the zero line, only the real part are shown herein. Wavelength: a, 515 nm; b, 576 nm; c, 636 nm.

and Au particle possess absorption bands. On the other hand, in the wavelength region of 300-480 nm, the spectrum looks similar to the predicted one. ER spectral measurements at E0′ with various f up to 1 kHz revealed that the ER signal always consists of a single component in the wavelength region of 350-900 nm. This was also confirmed by the use of the internal phase shifting technique23,24 as well as by the fact that the spectral structures of real and imaginary parts were identical at any f if the sign of one of them was inverted and factored by a constant. The ER response cannot be deconvoluted into two or more components. Another important fact was that the ER spectrum measured at the normal incidence was the same in spectral structure as that of 45° incidence. Additionally, the ER spectral structure was independent of the polarization type of the incident light at oblique incidence. These experimental facts indicate that the spectrum directly represents the absorption spectral difference, giving the rationale behind the use of eq 2 as a guide of spectral interpretation. The potential dependence of the ER signal at several wavelengths was measured after Au particle immobilization. Typical ER voltammograms (ERVs: the plot of ER signal as a function of Edc at a given set of ∆Eac, f, and wavelength) under linear potential scan to cathodic direction are shown in Figure 4. Two wavelengths (515 and 636 nm) in Figure 4 almost correspond to positivegoing peaks in Figure 3c, while the other (576 nm) does to negative-going one. The sign and intensity of the obtained ERV signal around E0′ were consistent to the spectral signal in Figure 3c. The ERVs exhibited peaks around E0′ of viologen, indicating that the absorption spectral change takes place upon reduction of the viologen moiety from colorless V2+ to colored V•+. Additionally, the signals at all three wavelengths at E > -0.2 V indicated occurrence of the charging-discharging process of Au particles in line with Figure 3a. 3.2. Transmission-Absorption Study at ITO Electrode. The structure of the modified ITO electrode was (23) Sagara, T.; Wang, H.-X.; Niki, K. J. Electroanal. Chem. 1994, 364, 285. (24) Sagara, T.; Kawamura, H.; Ezoe, K.; Nakashima, N. J. Electroanal. Chem. 1998, 445, 171.

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schematically shown in Figure 1. The transmissionabsorption measurements are expected to shed more direct light onto the light absorption properties of the interface of our interest. Prior to adsorption of VT, absorption spectra of the mercaptosiloxane-modified ITO electrode at various potentials and PMTA spectra were recorded. Difference absorption spectra between two constant potentials with various combinations were consistent with the real part of PMTA spectra. A sharper, stronger plasmon absorption peak with blue shift with changing the electrode potential to negative direction was clearly observed, in line with the potential-dependent reflection spectra at Au electrode described in the previous section. It is hereby worthwhile to note the relationship between the difference absorption spectrum with respect to E and the PMTA spectrum. The intensity of transmitted light intensity at a given potential E, I(E), is expressed as I(E) ) I0 exp[-2.303A(E)], where I0 is the intensity of the incident light and A(E) is the apparent absorbance of the cell at a potential of E. If A(E) is much smaller than unity, we can write approximately as I(E) ) I0[1 - 2.303A(E)]. Therefore,

∆I/I(real part) ≈ 2.303I0[A(Eneg) - A(Epos)]/I0 ) 2.303[A(Eneg) - A(Epos)] (3) where Eneg < Edc and Epos > Edc within the range of the potential modulation.25 After adsorption of VT, the electrode showed a redox reaction of V2+/V•+ couple at a formal potential of E0′ ) -468 ( 15 mV. A typical CV is shown in Figure 5. The peak current was proportional to potential sweep rate, ensuring that the voltammetric response is due to the redox process of adsorbed VT. The amount of electroactive VT deposited by 12 and 22 h adsorption were obtained from CV peak charge respectively as being 4 and 9 × 10-11 mol cm-2. Using the amount of Au particles estimated from the plasmon absorption peak height (vide infra), the number of molecules on a particle was calculated to be 74 and 190 molecules particle-1, respectively. We will not go into details about the minor difference of VT electrochemistry between Au and ITO electrodes, because this is beyond our scope of the present paper. It is worth noting the fact that only firmly confined VT molecules are the contributors to the voltammetric response. We examined the presence of nonchemically bound viologen in the surface film by immersing the electrode before adsorption of VT in solutions of some viologens such as methyl viologen, heptyl viologen, and aminopropyl viologen instead of VT. After rinsing, no redox wave was observed. Viologens without thiol group cannot be incorporated in the siloxane layer. It is known that viologens can be self-adsorbed on a bare ITO electrode surface.26 Because the ITO surface was fully covered with siloxane layer, the redox response of viologen adsorbed directly on the ITO surface was not observed. When using VT, viologen moieties exist surely in close proximity of Au nanoparticles through a Au-S linkage. Lines a and b in Figure 6 show typical absorption spectra of steady state at two different constant potentials for a fully modified ITO electrode. At 0.0 V where viologen is (25) Equation 3 corresponds to the case that the process accompanied by the change of A(E) is perfectly follow the potential modulation without any delay. Otherwise, we need to multiply a factor less than unity depending on the kinetics of the process. But this never changes the spectral structure of ∆I/I. That is, the PMTA spectrum is always proportional to A(Eneg) - A(Epos). (26) Obeng, Y. S.; Founta, A.; Bard, A. J. New J. Chem. 1992, 16, 121.

Figure 5. Cyclic voltammogram at an ITO electrode in 0.1 M phosphate buffer solution (pH 7.0) for an ITO electrode modified with 3-(triethoxysilyl)-1-propanethiol on which Au nanoparticles were immobilized (ca. 2.8 × 1011 particles cm-2) and subsequently VT was adsorbed. Sweep rate in mV s-1: a, 100; b, 70; c, 50.

in oxidized form, λmax was at 540 nm. Using the plasmon peak absorbance and the absorption coefficient of the Au particle obtained in its colloidal solution, the amount of Au particles were estimated as being ca. 30% of the 2D close-packed particle layer, provided that the absorption coefficient at the peak is identical to that in solution. Line a is nearly the same as that before adsorption of VT except for the red shift of λmax by 10 nm by the adsorption. This red shift may be due to the decrease of mean refractive index of the surrounding and partial electron withdrawal effect of thiol sulfur. Another peak of unknown origin was observed around 400 nm in lines a and b in Figure 6. Adsorption of VT made the position of plasmon absorption band shift to red, while tendency and magnitude of the potential dependent shift were the same as those before the adsorption. When the potential was changed from 0.0 to -0.45 V (≈E0′ of viologen) and subsequently -0.7 V (where viologen is fully reduced to V•+), slight changes of the spectra were seen except for the blue shift of plasmon band. To extract the spectral change, difference absorption spectra were calculated. The difference spectrum of Abs(-0.7 V) - Abs(0.0 V) was shown by line c in Figure 6 (“Abs” means an absorption spectrum). This difference spectrum should reflect the difference of the oxidation state of viologen. In addition to a gradual monotonic decrease of the absorbance in shorter wavelengths, a few positive- and negative-going peaks were seen. This spectral structure was reproducible, though it is difficult to conclude which are really the peaks. For example, one may see a positive-going peak around 630 nm and a negative-going one around 540 nm, but no one can judge the direction of the peak around 400 nm. Using the difference absorption spectra, explicit recognition of the spectral change needs over 300 mV difference of the potential, on one hand. On the other hand,

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Figure 6. Potential controlled steady absorption spectra (lines a and b) and difference absorption spectrum (line c) with perpendicular incident in 0.1 M phosphate buffer solution for the same ITO electrode as Figure 5 (ca. 2.8 × 1011 particles cm-2 and 190 VT molecules particle-1). Lines a and b were measured respectively at 0.0 and -0.7 V. Subtraction of line a from b gave line c. Small stepwise noises in absorption spectra (for example, at 520 nm) are due to filter exchange in the spectrophotometer. It was confirmed that these noises never affect the difference absorption spectra.

a PMTA spectrum is capable of seeing the spectral change within the potential range of ca. x2∆Eac, which can be made much smaller than 300 mV. Therefore, we mainly use more sensitive PMTA to describe the potential dependence with double check in light of the difference of the constant potential spectra. Figure 7 shows PMTA spectra after VT adsorption at two different Edc. At Edc ) 0.0 V (Figure 7a), a positivegoing band of the real part was observed at 477 nm. At the same time, negative ER response insensitive to the wavelength was underlaid for the real part in whole wavelength range. In the region of 400-700 nm, the difference spectrum may correspond to a blue shift of the plasmon band. At -452 mV ()E0′), two positive-going peaks at 614 and 403 nm as well as two negative-going peaks at 325 and 540 nm were observed in the real part (Figure 7b). Three peaks (614, 540, and 403 nm) corresponded to the peaks in the difference absorption spectrum in Figure 6c. Using Figure 3b as a guide, the PMTA signal due to viologen redox can be predicted to show a broad positivegoing band in the 500-650 nm region. The Au nanoparticle itself may additionally show a positive-going band around 480 nm (Figure 7a). Therefore, it is clear that the negativegoing band of 460-560 nm of the real part (Figure 7b) cannot be explained by the simple-sum approach. The positive-going band at the 614 nm peak cannot be explained, either. In accord with the ER measurements at a Au electrode, the PMTA response of in Figure 7b was

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Figure 7. Potential-modulated transmission-absorption (PMTA) spectra measured at f ) 14 Hz and ∆Eac ) 99 mV with perpendicular incident in 0.1 M phosphate buffer solution for the same modified ITO electrode as Figures 5 and 6. Peak wavelengths of real part spectra were typed in the figures. Thick line is the real part, while thin line is the imaginary part. a, Edc ) 0.0 V; b, Edc ) -0.452 V.

found to be of a single component. We should conclude that the absorption spectrum at near-to and more negative than E0′ is the results of mutual coupling between plasmon and chromophore absorptions, suggesting the presence of strong interaction between Au nanoparticles and V•+ moieties. We also measured the voltammogram of ∆I/I with superimposed linear potential scans. We observed a bellshaped peak at E0′ at both 530 and 614 nm. This indicates that the absorption spectrum of the modified ITO electrode changes greatly upon reduction of the viologen moiety. 4. Discussion At Au and ITO electrodes, ER and difference absorption as well as PMTA spectra are not a simple sum of the spectra of Au particles and viologen moieties, respectively. Principally, the reflection spectral difference is not necessarily the same curve shape as the absorption spectral difference. The reflection spectrum should be represented in the context of Fresnel optical theory, and thus the reflectance cannot be represented by a linear function of absorbance, as noted in section 3.1. However, in the present systems, ER results themselves indicate that the ER spectrum can approximately be regarded as the absorption difference. Moreover, despite the electrode modification structure difference, the transmission-absorption at an ITO electrode gives rise to identical features to those of ER at a Au disk electrode. The results at ITO electrodes directly show the absorption difference. Taken together, we may conclude that the absorptions of Au nanoparticles and viologen radical cations are not independent of but

Anomalous Spectra of Viologen/Au Particle

strongly coupled with each other. In this sense, we call the obtained feature as an anomalous spectrum. The results confirm also that the anomaly may be independent of the electrode substrate. The anomaly is seen for both electrostatic (physical) and covalent (chemical) attachment of viologen moieties to Au nanoparticles. There are several reports on the absorption spectra for Au particles in the close proximity of viologen moieties,27-31 though the absorption spectra of viologen when viologen is reduced to V•+ have never been given.19 It is known that under plasmon excitation an enhanced electromagnetic field is produced around the metal nanoparticle surfaces. The enhanced electromagnetic field near the particle has been theoretically quantified,3-6 and its effect on spectroscopic signals, including those at the SPR configuration, has been examined in absorption,4,6-11 fluorescence,3,7,12,13 and near-field14 spectroscopies, aside from surfaceenhanced Raman spectroscopy. Additionally, STM tipenhanced Raman scattering has recently been reported.15 However, it is unlikely that the spectra obtained in this work can be explained solely by the electromagnetic effect, since spectral structures of viologens near the Au particles are far different from the solution spectrum. Although we cannot speculate a quantitative scenario that explains the anomalous spectrum at present, it may be worthwhile to discuss possible origins based on the following two hypotheses. Hypothesis 1. The redox process of viologen moiety and the charging-discharging process of the Au particles are exactly synchronized with each other. In fact, the ER response was observed as a single component. Then, one should see the ER and PMTA spectra as the sum of the redox response and the sign-inverted charging-discharging response with a great extent of red shift (more than 50 nm!) of the plasmon band. The sign inversion is probable only if the particle is more negatively charged when E was moved to less negative. This may be the case that the Au nanoparticle acts as a huge counteranion for the viologen moieties. However, Figure 3a and the redox response [Ru(NH3)6]2+/3+ point self-explanatorily to a good (27) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. Adv. Mater. 1999, 11, 737. (28) Shipway, A. N.; Willner, I. Chem. Commun. 2001, 2035. (29) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. (30) Lahav, M.; Shipway, A. N.; Willner, I.; Nielsen, M. B.; Stoddart, J. F. J. Electroanal. Chem. 2000, 482, 217. (31) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J. Katz, E.; Willner, I.; Du¨rr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480.

Langmuir, Vol. 18, No. 18, 2002 7001

electronic communication between electrode substrate and the Au particle. Additionally, such a sign inversion should have resulted in a negative differential capacitance that was not observed in CV. Furthermore, such a large red shift cannot be accounted for without drastic change in medium dielectric function and/or electron ejection from the particle.32 We should deny this hypothesis. Hypothesis 2. Strong electronic coupling between the V•+ moiety and Au nanoparticle alters the absorption spectrum. Gittins et al. have recently reported electronic coupling between V•+ and Au particle based on the measurements using scanning tunneling spectroscopy.33 They found a shorter inverse decay length for electron transfer between V•+ and Au particle than between V2+ and Au particle. Resonance of the LUMO of viologen with the particle may produce a new electronic transition when illuminated at the plasmon band. This mechanism may be similar to the chemical mechanism in surface-enhanced Raman scattering spectroscopy.34 A CT-like new transition appears obvious in the present system. The single component feature of both ER and PMTA spectra is consistent with this hypothesis. Although we presently have no definitive theoretical supports to this hypothesis, it can be a tentative guiding model for our further research. To examine the mechanism more explicitly, we are currently seeking after the generality of this anomaly with other dyes experimentally.35 We also need to test the phenomenon using a redox buffer in the absence of a conductive surface in order for the influence of the conductive electrode surface upon the electromagnetic effect to be excluded. Acknowledgment. This work is a part of the project of PRESTO, JST (Japan), of T.S. Partial financial support by Grant-in-Aid of Research from the Ministry of Education, Culture, Sport, Science and Technology, Japan, to T.S. is acknowledged. The authors are thankful to Mr. H. Tsuruta for his indispensable technical assistance. LA026026G (32) Kreibig, U.; Bour, G.; Hilger, A.; Gartz, M. Phys. Status Solidi A 1999, 175, 351. (33) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature (London) 2000, 408, 67. (34) Pettinger, B. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; p 285. (35) To date, we really found a similar anomaly with methylene blue. The results will be reported elsewhere.