Electrode Reaction of Methylene Blue at an Alkanethiol-Modified Gold

Aug 21, 1996 - Abstract. The electrode reaction of methylene blue (MB) at a polycrystalline gold electrode modified with a self-assembled monolayer of...
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Langmuir 1996, 12, 4253-4259

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Electrode Reaction of Methylene Blue at an Alkanethiol-Modified Gold Electrode As Characterized by Electroreflectance Spectroscopy Takamasa Sagara,* Hirofumi Kawamura, and Naotoshi Nakashima Department of Applied Chemistry, Nagasaki University, Bunkyo, Nagasaki 852, Japan Received December 11, 1995X The electrode reaction of methylene blue (MB) at a polycrystalline gold electrode modified with a selfassembled monolayer of n-dodecanethiol was analyzed by using voltammetric and electroreflectance (ER) techniques. The results for the gold electrode modified with a dodecanethiol monolayer were compared with the data obtained at a bare gold electrode and a gold electrode modified with butanethiol, thiophene, or sulfur. The ER spectrum of MB at the gold electrode modified with dodecanethiol monolayer was in accord with the difference absorption spectrum between reduced and oxidized forms of MB in the solution. The ER spectrum of MB at the bare gold electrode had a different structure. We concluded that MB molecules were partitioned into the dodecanethiol monolayer interior on the gold electrode surface. The MB molecules, however, were not in direct contact with the gold surface when the electron transfer takes place.

Introduction Thiol, disulfide, or silane compounds bearing long hydrocarbon chains adsorb on metal electrode surfaces spontaneously to form compact monolayers, so-called selfassembled monolayers. Self-assembled monolayers provide us with many means of fabrication and regulation of the interfacial microenvironment at electrode/solution interfaces. The monolayer of an alkanethiol immobilized on a gold electrode exhibits the ability of blocking access of the redox species from the solution phase to the electrode surface.1-8 It is known that the blocking ability of an n-alkanethiol film depends on the properties of the electroactive species in the solution phase, on the length of the alkyl chain, on the coverage of the film, on the electrode substrate, on the adsorption procedure, and on the solvent used in the electrochemical measurement. The blocking abilities of the alkanethiol monolayers against hexacyanoferrate or other metal complexes have been studied in detail, and the capability to control the thickness of the electron-tunneling barrier by changing the hydrocarbon chain length has been demonstrated for uniform films. For example, Miller and his colleagues4,9 showed that the kinetic parameters of fast electron transfer reactions involving soluble metal complexes can be measured at a gold electrode modified with ω-hydroxy alkanethiols of various chain lengths. In contrast, the rate of these reactions at film free electrodes is usually controlled by mass transfer, and hence the potential dependence of the reaction rate can hardly be determined. The redox reaction of organic redox species at the alkanethiol monolayer-modified electrode has been also * Author to whom correspondence should be sent. Fax: +81 958 49 4999. Telephone: +81 958 47 1111 Ext. 2747. E-mail: sagara@ net.nagasaki-u.ac.jp. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Hockett, L. A.; Creager, S. E. Langmuir 1995, 11, 2321. (2) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854. (3) Chidsey, C. E. D. Science 1991, 251, 919. (4) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (5) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (6) Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Dastner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989, 5, 35. (7) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (8) Porter, M. D.; Bright, T.; Allara, D.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (9) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657.

S0743-7463(95)01530-7 CCC: $12.00

tested. The monolayer of n-dodecanethiol on a gold electrode does not completely block the electrode reactions of viologen, ferrocene, and other organic redox dyes in aqueous solutions.10 Groat and Creager examined the electrode reaction of various redox species, ranging from neutral to charged organic compounds, at an alkanethiolcoated gold electrode in propylene carbonate.11 They suggested that the molecular charge is a determining factor in the electron transfer kinetics rather than the molecular size. Finklea and his colleagues have pointed out that the pinholes at an octadecanethiol monolayer play an important role for the redox reaction of (trimethylamino)methylferrocene in an aqueous solution.12 They also studied the electrode reactions of surfactant bipyridinium dications at a gold electrode modified with octadecanethiol.13 Creager and his colleagues used the electrode reaction of (hydroxymethyl)ferrocene as a probe of the defectiveness of the alkanethiol monolayer on various gold substrates.2 The electrochemical behaviors of various thiols with covalently tethered redox groups have also been studied in detail from the viewpoints of microenvironment and spatial potential profile in the alkanethiol layer or permeability of ions and solvent across the film.1,14 However, the mechanism of the electrode reaction of the soluble redox species which is hardly blocked at the alkanethiol-coated electrode has not been well evaluated to our knowledge. It is important for the molecular-level design of the electrode/solution interface to clarify the role played by the hydrocarbon chain assembly of the alkanethiol monolayer. It may act not only as a barrier but also as an ultrathin medium in the interfacial redox reaction. In the present work, we would like to answer the question: Where does the redox species, the electrode reaction of which is not blocked by the alkanethiol monolayer, undergo the electron transfer with the gold electrode, at the (10) Honjyo, K.; Sagara, T.; Nakashima, N. Polym. Repr. Jpn. 1994, 43, 2297. (11) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668. (12) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatini, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (13) Finklea, H. O.; Fedyk, J.; Schwab, J. In Electrochemical Surface SciencesMolecular Phenomena at Electrode Surfaces; Soriaga, M. P., Ed.; American Chemical Society: Washington DC, 1988; p 431. (14) Rowe, C. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500.

© 1996 American Chemical Society

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monolayer/solution interface, within the monolayer, or in touch with the gold surface? We analyzed the electrode reaction of methylene blue, which is not blocked by the alkanethiol films, by using potential-modulated UV-vis reflectance spectroscopy (electroreflectance, ER). It is known that, if a chromophore of a redox dye is not in contact with the electrode surface, the ER spectrum represents a difference between absorption spectra for oxidized and reduced forms of the dye in the solution determined by transmission measurements.15-17 On the other hand, if the chromophore is in direct contact with the electrode surface, the ER spectrum is not always the same structure as the difference absorption spectrum.17-23 Therefore, we expect that the ER spectral structure is sensitive to the position of the reacting species within the alkanethiol monolayer and that the ER method enables us to highlight the state of the reacting methylene blue molecule. The results of the present ER study of the methylene blue redox reaction at a gold electrode modified with an n-dodecanethiol monolayer showed that most of the methylene blue molecules are not in direct contact with gold surface, though methylene blue molecules are partitioned into the alkanethiol layer from the solution phase. Experimental Section Methylene blue (3,7-bis(dimethylamino)phenothiazin-5-ium chloride, MB), purchased from Junsei Chemicals Co., was recrystallized from ethanol three times. Alkanethiols, purchased from Tokyo Kasei Kogyo Co., were used as received. Water was purified through an Ultra-pure water system Milli-Q Plus (Millipore Co.). Its resistivity was over 18 MΩ cm. All other chemicals were of reagent grade. Phosphate buffer solution (100 mM, pH 7.0) was used for all measurements as a base solution. For the electrochemical and spectroelectrochemical measurements, a quartz cell with an optically flat quartz window was used. The reference electrode was an Ag/AgCl electrode in saturated KCl solution, to which all potentials were referenced. The counter electrode was a gold wire. The polycrystalline gold (Au) disk electrode (geometrical electrode area, 2.01 mm2; BAS; product number, 11-2014) was polished to a mirror finish with an alumina slurry of 1, 0.3, and subsequently 0.05 µm and sonicated in pure water to remove the embedded alumina particles. The electrode was then subjected to the oxidation-reduction cycles (ORCs) between -0.07 and 1.6 V in 0.01 M HClO4 solution until the well-known cyclic voltammogram of a clean polycrystalline Au electrode24 was obtained. The real electrode area was calculated by dividing the charge of the peak of reduction of Au oxide on the ORC voltammogram by 293 µC cm-2.25,26 The current density, doublelayer capacitance, and amount of adsorbed species were calculated using the real electrode area. For the modification of the electrode surface with alkanethiol, the Au electrode was immersed in a freshly-prepared 10 mM alkanethiol solution in absolute ethanol for 24 h at 25 °C. Then, the modified electrode was rinsed well

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Figure 1. Cyclic voltammogram of a bare Au electrode in 4.5 µM MB solution measured 19 h after immersion in the solution. Sweep rate in mVs-1: (a) 200; (b) 100; (c) 50; (d) 20. with clean ethanol and subsequently with the buffer solution and was immediately placed in the cell filled with the buffer solution. Thiophene was immobilized by immersing the Au electrode in 10 mM thiophene aqueous solution for 60 min at 25 °C. Sulfur modification was performed according to the procedure reported by Arvia and his colleagues.27 The quality of the alkanethiol-modified Au electrode was diagnosed by the value of double-layer charging current in the cyclic voltammogram measured in the buffer solution as well as by the ability of blocking the electrode reaction of hexacyanoferrate(III). In the case of modification with n-dodecanethiol (C12SH), the electrode with a double-layer capacitance (Cd) greater than 5.0 µF cm-2 was discarded. The C12SH-modified Au electrodes used in this work exhibited excellent ability in blocking the electrode reaction of hexacyanoferrate(III): the cyclic voltammogram in the potential range between -600 and 600 mV was superimposable onto that in the base solution. All the measurements were conducted in a nitrogen atmosphere and at room temperature (22 ( 3 °C). The instrumentation for the ER measurements has been described elsewhere.20,21 The incident angle of nonpolarized monochromatic light was fixed to be 45°. The potential modulation waveform was a sine wave, the amplitude of which (∆Eac) is reported as a root mean square value. In the ER spectral measurement, both the real part of the ER response (the in-phase component of ∆R/R with respect to the potential modulation) and the imaginary part (90° out-ofphase component) were monitored simultaneously during the wavelength scan, where ∆R/R is the ac reflectance divided by the time-averaged reflectance. The standard ac reference signal used for the phase-sensitive detection of ER response by a lockin amplifier (EG&G PARC, model 5210) was the potential modulation, unless otherwise stated. The ER spectrum was also measured with a phase-shift procedure when necessary. The details of the phase-shift procedure were described elsewhere.18,19,28 The voltammetric and ER data presented in this paper were typical and were reproduced in at least three to four experiments. If the reproducibility was poor, the deviation will be mentioned in the following sections.

Results (15) Hinnen, C.; Parsons, R.; Niki, K. J. Electroanal. Chem. 1983, 147, 329. (16) Sagara, T.; Murakami, H.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 3190. (17) Sagara, T.; Niki, K. Langmuir 1993, 9, 831. (18) Imabayashi, S.; Nakamura, T.; Sagara, T.; Niki, K. Denki Kagaku 1994, 62, 526. (19) Imabayashi, S.; Nakamura, T.; Sagara, T.; Niki, K. J. Electroanal. Chem. 1994, 378, 103. (20) Sagara, T.; Takeuchi, S.; Kumazaki, K.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525. (21) Sagara, T.; Sato, H.; Niki, K. Bunseki Kagaku 1991, 40, 641. (22) Sagara, T.; Iizuka, J.; Niki, K. Langmuir 1992, 8, 1018. (23) Kim, S.; Scherson, D. A. Anal. Chem. 1992, 64, 3091. (24) Hinnen, C.; Nguyen van Huong, C.; Rousseau, A.; Dalbera, J. P. J. Electroanal. Chem. 1979, 95, 131. (25) Krysinski, P.; Chamberlain, R. V.; Majda, M. Langmuir 1994, 10, 4286. (26) Kessler, T.; Castro Lura, A. M.; Triaca, V. E.; Arvia, A. J. J. Appl. Electrochem. 1986, 16, 693.

Electrode Reaction of MB at a Bare Au Electrode. The cyclic voltammogram at a bare Au electrode in 4.5 µM MB solution is shown in Figure 1. We recorded the cyclic voltammograms every 15 min for 6 h after immersion of the electrode in the MB solution. The electrode was left at an open circuit potential during the time that intervened between potential scans. We observed little increase of the peak current (ip) with time (typically a 10% increase within 6 h) and no shift of formal potential. The value of ip in Figure 1 was proportional to the sweep rate (v) in the range between 10 and 5000 mV s-1, (27) Lezna, R. O.; de Tacconi, N. R.; Hahn, F.; Arvia, A. J. J. Electroanal. Chem. 1991, 306, 259. (28) Sagara, T.; Wang, H.-X.; Niki, K. J. Electroanal. Chem. 1994, 364, 285.

Methylene Blue at an Alkanethiol-Modified Gold Electrode

Figure 2. ER spectrum of a bare Au electrode in 4.5 µM MB solution (solid line, real part; broken line, imaginary part) and difference absorption spectrum of MB solution (absorption spectrum of MB°red, from which that of MBox was subtracted). Parameters for the ER measurement: dc potential (Edc), -190 mV; potential-modulation frequency (f), 14 Hz; ∆Eac, 80 mV.

indicating that the voltammetric response is due to the MB confined on the electrode surface. Note that when the MB concentration exceeded 100 µM, the diffusioncontrolled current appeared dominant over the redox current due to adsorbed MB. The formal potential (E°′) obtained as the midpoint potential between cathodic and anodic peak potentials was -192 (( 3) mV (average of three experiments including Figure 1), which is about 35 mV more positive than the E°′ value of MB in the solution phase. The saturated amount of adsorbed MB (Γ) in 4.5 µM MB solution was calculated from the charge of the cathodic voltammetric peak area assuming a two-electron redox reaction

MB+ox + 2e- + H+ f MB°red

(1)

as being 3.2 ((0.6) × 10-11 mol cm-2, which corresponds to less than 1/3 monolayer coverage of MB. Note that the anodic peak was smaller and broader than the cathodic peak in the voltammogram. The ER spectrum measured in the MB solution at 14 Hz is shown in Figure 2. The spectral structure was independent of modulation frequency (f). The ER spectrum displayed a bipolar feature. The wavelengths of the positive peak maximum, the zero ER response, and the negative peak maximum were, respectively, 491, 575, and 681 nm (deviation among three experiments was less than (4 nm). The ratio of the magnitude of the real part ER response at the positive peak against that at the negative peak was 0.44 ((0.03). If only the light-absorption process is responsible for the ER response, the ER spectrum structure will be in accord with the difference absorption spectrum (dotted line in Figure 2) obtained by subtracting the absorption spectrum of the oxidized form of MB from that of the reduced form. The spectral structure will never be bipolar, because one counterpart of the redox couple, i.e. the reduced form of MB, which is the so-called leuco-MB, is colorless. The bipolar ER spectral structure in Figure 2 is markedly different from the difference absorption spectrum. The ER response therefore results from the reflection at the Au/MB-adsorption layer/solution threephase system but not solely from the light-absorption process. The main contributor to the ER response is surface-confined MB and the ER response arises from the MB molecules in direct contact with the Au electrode surface. When a bare Au electrode was immersed in an aqueous solution free of redox species, we observed the ER response of Au, which represented a broad negative ER band (λmax

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Figure 3. Cyclic voltammogram of a Au electrode transferred from 4.5 µM MB solution into buffer solution free of MB measured 3 h after the transfer. Sweep rate in mVs-1: (a) 200; (b) 100; (c) 50; (d) 20.

Figure 4. ER spectrum of a Au electrode transferred from 4.5 µM MB solution into buffer solution free of MB measured 3 h after the transfer: solid line, real part; broken line, imaginary part. Parameters for the ER measurement: Edc, -185 mV; f, 14 Hz; ∆Eac, 100 mV.

∼ 500 nm) on the real part, in accordance with reported data.15,29 However, the response of Au was not seen in Figure 2. Although we conducted a phase-shift ER measurement (vide infra) for the electrode system of Figure 2, the response of Au was not discriminated. This fact implies that the density of electrons at the Au surface does not change significantly upon potential modulation when MB is present. This fact also supports the direct contact of MB molecules on the Au electrode surface. The Au electrode used in the measurement of Figure 2 was then transferred into MB-free buffer solution after rinsing the electrode thoroughly with the buffer solution. The cyclic voltammogram thus measured is shown in Figure 3. The voltammetric response of MB was observed at E°′ ) -191 ((8) mV (average of five experiments including Figure 3). Methylene blue still remained irreversibly adsorbed on the Au surface after 3 h. The value of ip of the cathodic peak in Figure 3 was proportional to v in the range between 700 and 5000 mV s-1. The anodic peak appeared much broader than the cathodic peak. The value of Γ ranged from 1.2 × 10-11 mol cm-2 (immediately after transfer) to 5.2 × 10-12 mol cm-2 (after 3 h). Figure 4 shows the ER spectrum of the Au electrode with adsorbed MB measured in the buffer solution free of MB. The ratio of the magnitude of the ER response at the positive peak (436 nm) against the that of negative peak (680 nm) was 0.74, being much greater than the ratio measured for MB solution (Figure 2). Other differences in the spectral structure of Figure 4 compared to Figure 2 are (i) the peak around 650 nm is structureless and much broader and (ii) the wavelength of the crossing (29) McIntyre, J. D. E. In Advances in Electrochemistry and Electrochemical Engineering; John Wiley and Sons: New York, 1973; Vol. 9, p 61.

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Figure 5. Cyclic voltammogram of a Au electrode modified with C12SH in 4.0 µM MB solution measured 5 h after immersion. Sweep rate in mV s-1: (a) 200; (b) 100, (c) 50, (d) 20.

of the spectral curve with the zero line is shorter. These results connote that the state of the MB adsorption layer remaining on the electrode after transfer into MB-free solution is not identical with that before transfer. After the transfer from the MB solution, only strongly-adsorbed MB is present on the electrode surface with a low density and thus minimal lateral interaction. We also measured the cyclic voltammogram (not shown here) in 1 mM Fe(CN)63- + 1 mM Fe(CN)64- solution for the Au electrode on which MB was adsorbed. The voltammetric response was indistinguishable from that at a bare Au electrode around the formal potential of hexacyanoferrate(II/III). This fact reveals that the submonolayer amount of MB confined to the electrode surface does not inhibit the electrode reaction of hexacyanoferrate at all. Electrode Reaction of MB at a C12SH-Modified Au Electrode. The cyclic voltammogram at a C12SH-modified Au electrode in 4.09 µM MB solution is shown in Figure 5. The voltammetric response due to the redox reaction of MB was observed as oxidation and reduction peaks. The curve shape was quite different from the cyclic voltammogram of a ferrocene derivative reported by Finklea and his colleagues at an octadecanethiol (C18SH)modified electrode.12 That is, the electrode reaction of MB does not take place at the pinholes of the alkanethiol layer, as opposed to the electrode reaction studied by Finklea and his colleagues. The curve shape was definitely different from that for the case of the array of microelectrodes.30,31 The values of ip for both anodic and cathodic peaks increased with time after immersion of the electrode in MB solution. The rate of the current increase depended on the experimental procedure: the current increased more rapidly when the electrode potential was kept at -600 mV during the time that intervened between potential scans than when it was kept at the open-circuit potential or +200 mV. In the latter case, the ip value 5 h after immersion was about 2.5 times as much as was observed immediately after immersion. At pH 7.0, the oxidized form of MB is charged +1, while the reduced form is neutral (see eq 1). Therefore, the above-mentioned difference in the rate of current increase suggests that the neutral, reduced form of MB penetrates more easily (30) Amatore, C.; Saveant, J. M.; Tessier, D. J. J. Electroanal. Chem. 1983, 147, 39. (31) Armstrong, F. A.; Bond, A. M.; Hill, H. A. O.; Psalti, I. S. M.; Zoski, C. G. J. Phys. Chem. 1989, 93, 6485.

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into the alkanethiol layer than the charged, oxidized form. In fact, the oxidized form of MB is insoluble in n-dodecane, while the reduced form is soluble. Figure 5 was measured 5 h after immersion, and ip was almost saturated. The E°′ value was -167 mV, regardless of the time passed after immersion. The slightly more positive value of E°′ compared to the bare gold electrode suggests either that the alkanethiol film stabilizes the reduced form of MB relative to the oxidized form or that a change in the ion spatial distribution causes the change in the potential distribution at the electrode/solution interface.14 As opposed to the case of the bare Au electrode in MB solution (Figure 1), the anodic peak was sharper and higher than the cathodic peak in Figure 5. The formation of a conductive multilayer film of accumulated MB reduction product on a bare or a sulfur-modified gold electrode surface is well-known.32-35 The reoxidation of the film gives rise to a sharp and enhanced anodic peak in the cyclic voltammogram. The formation of the solidstate film depends on the concentration of MB, on the supporting electrolyte, and on the pH. In order to clarify whether or not the sharper anodic peak in Figure 5 is due to the film formation, we examined the dependence of the anodic peak on the negative vertex potential (Env) of the potential sweep and on the holding time (th) at Env. It was found that the height and width of the anodic peak are almost independent of Env (< -0.4 V) and th (10 s to 10 min). This fact reveals that under the present experimental conditions, the formation of the solid-state film of MB reduction product does not take place at the C12SHmodified gold electrode. In the measurement of Figure 5, the plots of ip versus v or v1/2 were not linear in the sweep rate range between 10 and 5000 mV s-1. At 200 mV s-1, the anodic ip value was 3.26 µA cm-2. The theoretical ip value calculated for the same concentration of MB assuming that the reaction is reversible (i.e. totally diffusion-controlled) is 1.24 µA cm-2. In this calculation, a diffusion coefficient of 8.5 × 10-6 cm2 s-1 (ref 36) was used. The observed ip value was nearly three times as large as the theoretical value for a reversible electrode reaction of soluble MB. However, the observed ip value was smaller than the value calculated for the case of the reversible reaction of monolayeradsorbed MB. These facts indicate that the electrode reaction occurring at the C12SH-modified Au electrode in MB solution involves the redox reaction of MB accumulated (but not forming a solid-state film) in the vicinity of the electrode surface. It is also worthwhile to note that the peak separation between anodic and cathodic peaks became smaller along with an increase in the peak height. When the electrode was transferred into a more dilute MB solution from a 4.0 µM solution, the peak height became smaller and at the same time the peak separation became greater. It is likely that the rate of the electron transfer reaction is a function of the extent of accumulation of MB in the film (i.e. intermolecular electron transfer between accumulated MB is enhanced at higher concentration) or that the rate of ingress/egress of MB into/from the film is a function of MB concentration. Since the kinetics of these processes might depend on the oxidation state of MB, the rate of (32) Svetlicˇicˇ, V.; Clavilier, J.; Zˇ utic´, V.; Chevalet, J. J. Electroanal. Chem. 1991, 312, 205. (33) Clavilier, J.; Svetlicˇicˇ, V.; Zˇ utic´, V.; Busˇcˇicˇ, B.; Chevalet, J. J. Electroanal. Chem. 1988, 250, 427. (34) Zˇ utic´, V.; Svetlicˇicˇ, V.; Clavilier, J.; Chevalet, J. J. Electroanal. Chem. 1987, 219, 183. (35) Svetlicˇicˇ, V.; Zˇ utic´, V.; Clavilier, J.; Chevalet, J. J. Electroanal. Chem. 1985, 195, 307. (36) Unwin, P. R.; Bard, A. J. Anal. Chem. 1992, 64, 113.

Methylene Blue at an Alkanethiol-Modified Gold Electrode

Figure 6. ER spectrum of a Au electrode modified with C12SH in 4.0 µM MB solution measured 5 h after immersion in the MB solution. solid line, real part; broken line, imaginary part; dotted line, difference absorption spectrum of MB. Parameters for the ER measurement: Edc, -165 mV; f, 14 Hz; ∆Eac, 100 mV.

electrode reaction would differ between anodic and cathodic reactions, which may explain the asymmetry of the peak shape mentioned above. ER spectra of the C12SH-modified Au electrode in 4.0 µM MB buffer solution are shown in Figure 6. The imaginary part was greater than the real part, as opposed to the case in Figure 2. Two ER spectral bands were clearly seen on the ER spectrum regardless of the modulation frequency: one at 620 nm as a shoulder and the other at 669 nm as a peak. The former band corresponds to MBox dimer absorption, and the latter, to MBox monomer, though the wavelengths of the ER spectral peaks were about 4 nm longer than the absorption maxima of MBox in the solution. If the ER spectra in Figures 2 and 6 are compared with the difference absorption spectrum of MB solution, the spectrum in Figure 6 is more in accord with the difference absorption spectrum. The spectral structure corresponding to monomer and dimer absorption is not seen in Figure 2, while it is explicitly seen in Figure 6. The magnitude of the ER response in the range 400-550 nm relative to the magnitude in the range 650-700 nm is much smaller in Figure 6 than in Figure 2. These differences in the spectral features will be discussed in the following sections in more detail. The phase shift technique18,19,28 was used to investigate whether the ER response in Figure 6 consisted of multicomponents. If there is more than one species reacting simultaneously, the ER response measured is the sum of several ER responses; in other words, the measured ER response has multiple components. If each component represents an ER spectral structure distinguishable from the other components, the ER spectrum measured by adjusting the phase to one of the components gives rise to a nonzero imaginary part of the ER spectrum.28 On the other hand, if the ER response is of a single component, the imaginary part of the ER spectrum with the phase adjustment at any wavelength always appears to be zero. For the Au electrode modified with C12SH in 4.5 µM MB solution, phase adjustment in the lock-in detection of the ER signal was made at the wavelength of maximal ER response (669 nm), so as to make the imaginary part response at this wavelength to be zero. The ER spectrum was measured under this phase adjustment condition. At the modulation frequency of 3, 14, 47, or 170 Hz, the imaginary part of the phase-shifted spectrum is almost zero in the range 400-800 nm. The fact that phase-shifted ER spectra at more than one frequency exhibited a near zero imaginary part response indicates that the ER response is of a single component. The result of phase-shift measurements, together with the frequency independence of the ER spectral structure up to 170 Hz, demonstrates that the ER response

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represents the redox reaction of MB accumulated in the immediate vicinity of the C12SH film. At higher frequencies such as 170 Hz, the contribution from diffusing species in the solution phase to the ER response is minimal, because the reaction of the diffusing species in the solution phase cannot follow the potential modulation at higher frequencies.17 The ER spectra of the C12SH-modified Au electrode in MB solution as well as in buffer solution free of MB did not involve an ER response associated with the Au substrate. This fact implies that, at the Au surface covered with C12SH, the surface electron density does not change significantly in response to potential modulation because of the chemisorption of thiol molecules. The Au electrode was then transferred into MB-free buffer solution after the electrode was rinsed thoroughly with the buffer solution. The voltammetric response of MB remaining on the C12SH-modified Au electrode surface was observed at E°′ ) -192 mV. Immediately after the transfer of the electrode, the ip value was less than about 1/3 of the value in the MB solution immediately before the transfer. The peak height decreased slowly with time, though the voltammetric response was still observable at 220 h after the transfer. The peak separation became greater with time: it exceeded 150 mV (at 200 mV s-1) at 2 h after the transfer in the cyclic voltammetry measurement. The plots of ip versus v or v1/2 were not linear. The ER response of the C12SH-modified Au electrode after the transfer from MB solution into the buffer solution was so weak that sensitive measurement was unfortunately impossible. We also measured the cyclic voltammogram in 1 mM Fe(CN)63- + 1 mM Fe(CN)64- solution for the Au electrode mentioned above. The voltammetric response due to the direct electron transfer between the electrode and hexacyanoferrate was completely blocked, the same as a newly prepared C12SH modified electrode, whereas the reduction current of Fe(CN)63- mediated by MB was observed in the potential region more negative than the formal potential of MB. We also recorded a single-scan voltammogram of the electrode in 0.01 M HClO4 solution with a positive limit of +1.50 V. Oxidation or reduction of the Au surface was not observed at all. These facts reveal that the C12SH layer still retained its blocking ability and that the adsorbed alkanethiol molecules were not displaced by MB. ER Spectra of MB at a Thiophene-, Sulfur-, or n-Butanethiol-Modified Au Electrode. The comparison of the ER spectrum at a C12SH-modified Au electrode with the ER spectra at Au electrodes modified with smaller sulfur compounds may be useful for discussing the ER spectral structure in more detail. The Au electrodes with three different modifications were subjected to ER spectral measurements in MB solution. n-Butanethiol (C4SH) forms an adsorption monolayer, much thinner and probably less ordered than the C12SH film. Thiophene is known to form a compact monolayer on a Au electrode surface.37 It is also known that MB strongly adsorbs on the inorganic sulfur-modified Au electrode.27 ER spectra measured in MB solution at the three different modified Au electrodes are shown in Figure 7. For all three modified electrodes, the spectral structure was unchanged when the potential-modulation frequency was increased. At higher frequencies, the contribution from the diffusing species to the ER response is negligible.17 Therefore, the main contributor to the ER spectra in Figure 7 is not the diffusing species in the solution phase. The ER spectral structure of a thiophene-modified Au electrode closely resembles that of the C4SH-modified (37) Qu, X.-G.; Lu, T.-H.; Dong, S.-J.; Zhou, C.-L.; Cotton, T. M. Bioelectrochem. Bioenerg. 1994, 34, 153.

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Figure 7. ER spectra at a C4SH-modified Au electrode (part A, in 3.9 µM solution; real part is shown by solid line), a thiophene-modified Au electrode (part A, in 3.6 µM MB solution; real part is shown by dotted line), and a sulfur-modified Au electrode (part B, in 3.2 µM MB solution; both real and imaginary parts are shown, respectively, by solid and broken lines). Edc, at formal potentials; f, 14 Hz; ∆Eac, 100 mV.

electrode (Figure 7A). On the other hand, the ER spectral structure of a sulfur-modified electrode (Figure 7B) is markedly different from the spectra in Figure 7A. We classify the spectra in Figure 7 into two types according to the spectral features: the spectra of Figure 7A are designated as type A spectra and the spectrum of Figure 7B is designated as a type B spectrum. The differences between type A and B spectra can be summarized as follows: (i)the type A spectrum is more in accord with the difference absorption spectrum of MB solution. (ii) the type A spectrum shows sharp negative bands corresponding to the absorptions of MB monomer and dimer. These two bands are well separated. On the other hand, the negative band of a Type B spectrum is broad and structureless. (iii) The type B spectrum shows an enhanced positive band at wavelengths shorter than 550 nm, relative to the negative band. The type B spectrum clearly represents a bipolar feature. According to the spectral features, the ER spectrum at a C12SH-modified electrode in MB solution (Figure 6) can be classified as a type A spectrum, while the ER spectrum at a bare Au electrode (Figure 2) is a type B spectrum. In addition, we conducted a film-transfer experiment. The modified electrodes used for the ER measurements in MB solution (Figure 7) were transferred into buffer solution free of MB, and ER spectra were measured. The spectral feature of a sulfur-modified electrode was unchanged upon transfer. For both thiophene- and C4SHmodified electrodes, the monomer-dimer structure of the negative ER band became slightly less clear. However, the classification mentioned above still holds after transfer. Discussion and Summary ER Spectral Structure at a C12SH-Modified Au Electrode. Our chief concern is the origin of the difference between types A and B spectra. Three possible reasons for the difference can be cited.

Sagara et al.

1. The spectral structure depends on the distance between the Au surface and the position of reacting MB. The spectral structure is especially sensitive to whether or not MB is in direct contact with the Au surface. At the Au electrode modified with thiophene, C4SH, or C12SH, the MB chromophore is not in direct contact with the Au surface and is not in the immediate vicinity of the Au surface. In contrast, at the bare or the sulfur-modified Au electrode, the chromophore is in direct contact with the Au surface. II. The adsorption of MB on a sulfur-modified or bare Au electrode is associated with the change in chemical structure of MB due to the strong interaction of MB with a sulfur-covered or bare Au surface. The adsorption induces a change in the electronic structure of MB, resulting in a difference in the ER spectrum, because the ER spectrum is in principle the electronic spectrum. III. The dynamic exchange of surface MB with MB in the solution phase takes place in response to the potential modulation at electrodes which produce type A spectra. It cannot occur at the sulfur-modified or bare electrode due to the strong chemisorption of MB. Reason II above is of course closely associated with reason I. As long as the adsorption of MB has a chemisorption nature, the electronic interaction between the molecular orbital of MB and the electronic state of Au cannot be ignored. Unless the chromophore is in direct contact with substrate surface, the interaction may be insignificant. In the film transfer experiments for thiophene-, sulfur-, or C4SH-modified Au electrodes, the effect of the exchange of MB (reason III) was minimized. The classification of the ER spectra still holds after film transfer. Therefore, reason III can be ruled out. Reason II was previously suggested by Lezna and his colleagues at the sulfurmodified Au electrode.27 Sulfur modification of Au greatly affects the electrode reaction of MB. However, a considerable difference between the spectral feature of a sulfurmodified Au electrode and a bare Au electrode was not observed in the present work. The shift of formal potential was as small as -53 mV upon sulfur modification. Therefore, it is possible that the change in chemical structure of MB upon adsorption contributes to the ER spectral features of bare and sulfur-modified Au electrodes to nearly the same extent. However, because the reduced form of MB is colorless, the positive part of the ER spectrum of type B in the wavelength region around 500 nm cannot be understood only by reason II. In conclusion, it is most likely that the distance between reacting MB and the Au surface is a determining factor of the ER spectral structure. If the distance is nearly zero, the ER spectrum is no longer the difference absorption spectrum but is a difference reflection spectrum at the electrode/ film/solution three-phase system. In the electrode systems which produce a type B spectrum, the chromophores are in direct contact with the electrode surface and thus the ER spectral structure is different from the difference absorption spectrum. The additional effect on the spectral feature of the electronic structural change of the interface due to direct contact (reason II) is also probable. At the C12SH-modified electrode, the spectral structure is classified as a type A spectrum even at high frequencies, at which the effect of reason III is eliminated. Thus, we can conclude that most of the reacting MB molecules at the C12SH-modified electrode are not in direct contact with the gold surface. Reaction Mechanism of MB at an AlkanethiolModified Au Electrode. At the Au electrode modified with a C12SH-monolayer in MB solution, MB in the solution phase is incorporated (penetrated) into the

Methylene Blue at an Alkanethiol-Modified Gold Electrode

alkanethiol layer. An increase in the voltammetric response with time is due to the accumulation of MB in the film. The incorporation of the reduced form, which is neutral, is a faster process than that of the oxidized form, which is monocationic. The sweep rate dependence of the cyclic voltammogram indicates that MB is not in a firmly fixed position in the alkanethiol layer. The MB in the film is mobile and is able to exchange with the MB in the solution phase. In this sense, the alkanethiol layer may act as a very thin, hydrophobic supporting medium for MB. The phase-shift experiment in the ER spectral measurement shows that the state of MB incorporated into the film is homogeneous. The incorporation of MB into the C12SH monolayer may be understood as the partitioning of MB. It is known that organic solutes are partitioned into a supported n-alkyl hydrocarbon monolayer in HPLC.38 Partition chromatography (reversedphase liquid chromatography) is in fact widely used in industries for separation of dyes. The MB partitioned into the alkanethiol film is not in direct contact with the Au surface. The alkanethiol film still retains the ability of blocking the electrode reaction of hexacyanoferrate. The alkanethiol adsorbed on the Au electrode surface is not displaced by MB. Concluding Remarks The probe redox species commonly used for characterization of the structure and quality of the monolayers of thiols with long alkyl chains are metal complexes such as (38) Dorsey, J. G.; Dill, K. A. Chem. Rev. 1989, 89, 331.

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hexacyanoferrate or organic redox species. If one uses hexacyanoferrate, for example, one can regard the film thickness as the tunneling barrier for electron transfer as long as the film is uniform and defectless. Hexacyanoferrate may also be useful to see the presence of pinholes, the sizes of which are much larger than the size of the complex ion, because hexacyanoferrate cannot gain access to the monolayer interior. On the other hand, the alkanethiol layer acts as an ultrathin medium for organic dyes, though the direct contact of the molecule with the electrode surface is prevented. A dye molecule which can be partitioned into the hydrocarbon chain phase would be useful as a spectroelectrochemical probe to see the microenvironment of the film interior. In the present work, we compare the ER spectral feature at several sulfur-compound-modified electrodes. The results show that the spectral structure is very sensitive to whether or not the chromophore is in direct contact with the electrode surface. In order to clarify the interfacial structure directly from the ER spectral structure, the establishment of an interpretation protocol for the ER spectrum is strongly called for. We are now attempting the theoretical simulation of the ER spectrum. Acknowledgment. This work is supported by a Grantin-Aid from the Ministry of Education, Science, Sports, and Culture of Japan to N.N. and T.S. and by financial support from Toray Science Foundation to N.N. Discussions with Dr. J. Lipkowski are also gratefully acknowledged. LA951530P