Quartz Crystal Microbalance and Electrochemical Studies on a

Chronocoulomograms of the SAM-modified electrodes indicated that V10SH .... Penetration of Distributed Generation with Shunt FACTS Using GA/Fuzzy Rule...
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Langmuir 2002, 18, 5804-5809

Quartz Crystal Microbalance and Electrochemical Studies on a Viologen Thiol Incorporated in Phospholipid Self-Assembled Monolayers Noriyuki Nakamura,* Hong-Xiang Huang, Dong- Jin Qian,*,† and Jun Miyake Tissue Engineering Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan Received January 22, 2002. In Final Form: April 23, 2002 Binary self-assembled monolayers (SAMs) were prepared on gold surfaces by use of a viologen, N-methylN′-(10-mercaptodecyl)-4,4′-bipyridinium bishexafluorophosphate (V10SH) and a phospholipid, dipalmitoylphosphatidylthioethanol (DPPTE). Quartz crystal microbalance (QCM) measurements indicated a similar adsorption trend for the V10SH and DPPTE assembling on the gold surface. Well reversible redox waves of V10SH and Fe(CN)63-/4- were recorded for the electrodes modified by either pure V10SH SAM or binary SAMs prepared in mixed solutions of V10SH and DPPTE with molar ratios below 1:5. Chronocoulomograms of the SAM-modified electrodes indicated that V10SH acted as an electron-transfer mediator for the Fe(CN)63-/4- ions in the electrolyte solution to “gated” access to the electrode surface across the DPPTE monolayer.

Introduction Electrochemical properties of a redox molecule incorporated in a phospholipid monolayer, bilayer, or membrane have attracted much attention in the past several decades.1 Such a study can provide a way for elucidating the mechanism by which the redox molecule mediates electron transfer across the phospholipid monolayer. The use of redox proteins is attractive,2 but large redox proteins often do not respond or they show a weak signal at conventional electrodes because getting the redox center close enough to the electrode for electron transfer is difficult.3 The synthesized compounds, on the other hand, overcome this disadvantage and allow the control of the composition and structure of the phospholipid monolayer, thus simplifying experimental conditions and facilitating the signaling out of the species directly responsible for electron mediation. Viologens are actually a kind of desirable electron mediator. Viologens (V’s) exist in three main oxidation states, namely, V2+ T V•+ T V0. These redox reactions, especially the first one (V2+ T V•+), are highly reversible and can be cycled many times without significant side reactions.4 Because of these characteristics, viologens have been extensively investigated: (i) electron-transfer mediator to aqueous ions and many proteins including cytochrome c, hydrogenase, and horseradish peroxidase;2,5 (ii) behavior of supramolecular assemblies at electrode surfaces;6 (iii) applications for electrochromic display devices;7 (iv) * Corresponding authors. Telephone: 81-298-61-3013. Fax: 81298-61-2565. † E-mail: [email protected]. (1) (a) Guidelli, R.; Aloisi, G.; Becucci, L.; Dolfi, A.; Moncelli, M. R.; Buoninsegni, F. T. J. Eletroanal. Chem. 2001, 504, 1. (b) Glazier, S. A.; Vanderah, D. J.; Plant, A. L.; Bayley, H.; Valincius, G.; Kasianowicz, J. J. Langmuir 2000, 16, 10428. (2) (a) Zhang, J.; Rosilio, V.; Goldmann, M.; Boissonnade, M.-M.; Baszkin, A. Langmuir 2000, 16, 1226. (b) Herrero, R.; Buoninsegni, F. T.; Becucci, L.; Moncelli, M. R. J. Electroanal. Chem. 1998, 445, 71. (c) Wiese, A.; Brandenburg, K.; Lindner, B.; Schromm, A. B.; Carroll, S. F.; Rietschel, E. T.; Seydel, U. Biochemistry 1997, 36, 10301. (3) (a) Lewis, N. S.; Wrighton, M. S. Science 1981, 211, 944. (b) Li, J.; Yan, J.; Deng, Q.; Cheng, G.; Dong, S. Electrochim. Acta 1997, 42, 961. (4) Bird, C.-L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49.

surface-enhanced Raman studies of adsorption at electrode surfaces;8 and (v) others.9 The purpose of the present study is to investigate the electrochemical properties for viologens incorporated in a phospholipid monolayer and behaviors of viologens as an electron mediator for ferricyanide ions in electrolyte solution. Previously, we used the Langmuir-Blodgett (LB) method to immerse an amphiphilic viologen into phospholipid monolayers at the air-water interface, where we found that viologen electrochemical properties are closely related to the molar ratios of the mixtures and the length of alkyl chains of phospholipid.10 Ferna´ndez et al. demonstrated that the viologen headgroup was located encapsulated between the headgroup of the phospholipid and the solid surface in the mixed LB films of a phospholipid and viologen,11 and that a possible reorientation of the viologen headgroup may occur in the transferred mixed LB monolayer when the LB monolayer modified electrode was immersed in the 0.3 mol/L perchlorate electrolyte solution.12 Moreover, it has been reported that the radical cation V+ can act as electron-transfer mediator from the mercury surface to the aqueous ferricyanide (5) (a) Herrero, R.; Moncelli, M. R.; Becucci, L.; Guidelli, R. J. Phys. Chem. 1997, 101, 2815. (b) Tatsumi, H.; Takagi, K.; Fujita, M.; Kano, K.; Ikeda, T. Anal. Chem. 1999, 71, 1753. (c) Qian, D. J.; Nakamura, C.; Noda, K.; Zorin, N. A.; Miyake, J. Applied Biochem. Biotechnol. 2000, 84-86, 409. (6) (a) Judkins, C. M.; Bohannan, E. W.; Herbig, A. K.; Powers, J. A.; Galen, D. A. V. J. Electroanal. Chem. 1998, 451, 39. (b) Sagara, T.; Tsuruta, H.; Nakashima, N. J. Electroanal. Chem. 2001, 500, 255. (c) John, S. A.; Kasahara, H.; Okajima, T.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 1997, 436, 267. (d) Qian, D. J.; Nakamura, C.; Miyake, J. Thin Solid Films 2000, 374, 125. (e) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (7) Cotton, T. M.; Kim, J.-H.; Uphaus, R. A. Microchem. J. 1990, 42, 44. (8) Feng, Q.; Yue, W.; Cotton, T. M. J. Phys. Chem. 1990, 94, 2082. (9) (a) Gong, M. S.; Lee, M. H.; Rhee, H. W. Sens. Actuators, B 2001, 73, 185. (b) Baker, W. S.; Lemon, B. I., III; Crooks, R. M. J. Phys. Chem. B 2001, 105, 8885. (c) Li, L. S.; Li, A. D. Q. J. Phys. Chem. B 2001, 105, 10022. (10) Qian, D. J.; Nakamura, C.; Miyake, J. Colloids Surf., A: Physicochem. Eng. Asp. 2000, 175, 93. (11) Ferna´ndez, A. J.; Ruiz, J. J.; Camacho, L.; Martı´n, M. T.; Mun˜oz, E. J. Phys. Chem. B 2000, 104, 6799. (12) Ferna´ndez, A. J.; Martı´n, M. T.; Ruiz, J. J.; Mun˜oz, E.; Camacho, L. J. Phys. Chem. B 1998, 102, 6799.

10.1021/la020070e CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002

V10SH Incorporated in DPPTE SAMs

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Figure 2. Time dependence of frequency changes of (a) V10SH and (b) DPPTE modified QCM gold resonator in 2 mmol/L ethanol/acetonitrile solution.

Figure 1. Viologen and phospholipid used in this work.

across a phospholipid monolayer,5a where ferricyanide can be adsorbed by the viologens due to an electrostatic interaction. The different redox potentials for the viologen and ferricyanide are a benefit for the studies on their electrochemical properties. In the present work, a coassembled method was used to incorporate a viologen thiol into the phospholipid layer to form a binary self-assembled monolayer (SAM). This procedure gives rise to a half bilayer with the hydrocarbon tails of phospholipid and hydrophilic viologen head (redox center) toward the aqueous solution. Both the viologen and phospholipid covalently attach to the gold surface by a thiol group. An in situ quartz crystal microbalance (QCM) measurement indicated a similar adsorption trend and frequency change for the formation of the SAMs of the viologen and phospholipid. We further discuss the electron-transfer process of the viologen incorporation and its role as an electron mediator from aqueous ferricyanide to the electrode across the monolayer. Experimental Section Materials. Dipalmitoylphosphatidylthioethanol (DPPTE, Figure 1) was purchased from Avanti Polar-Lipids Inc.; 4,4′bipyridinium, 1,10-dibromodecane, and potassium hexacyanoferrate were purchased from Wako Pure Chemical Industries Ltd. HPLC-grade ethanol and acetonitrile used as solvents were from Nacalai Tesque, Inc. All reagents were used as received and without further purification. Ultrapure water (18.3 MΩ) was prepared with a Milli-Q filtration unit of Millipore Corp. N-Methyl-N′-(10-mercaptodecyl)-4,4′-bipyridinium bishexafluorophosphate (V10SH) shown in Figure 1 was synthesized according to a literature method and checked by the measurement of 1H NMR.13 1H NMR (CD3CN, δ, ppm): 9.23-9.29 (d, 4H), 8.74-8.78 (d, 4H), 4.77-4.82 (t, 2H), 4.56 (s, 3H), 3.58 (m, 2H), 2.1 (t, 2H), 1.2-1.4 (m, 14H). Monolayer Preparation. The general procedure for monolayer preparation was to expose a pretreated gold electrode or QCM crystal to a solution of V10SH or DPPTE or V10SH-DPPTE mixtures in ethanol-acetonitrile (1:1 v/v) that was purged with Ar. The Au electrode (geometric area 2.0 mm2, from BAS Co., Ltd., USA) was polished with alumina paste; the final polish was carried out with 0.03 µm particles. After sonication in the water and ethanol, the polished Au electrode was subjected to oxidation-reduction cycles at a scan rate of 100 mV/s between 1600 and -100 mV in a 0.01 mol/L HClO4 solution until a cyclic (13) (a) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta 1998, 43, 2183. (b) Godı´nez, L. A.; Castro, R.; Kaifer, A. E. Langmuir 1996, 12, 5087.

voltammogram of the clean polycrystalline gold electrode was obtained.14 The QCM crystals were cleaned in a piranha solution (H2SO4/H2O2; 3:1) for 10 min, then washed with copious amounts of water, and finally dried and kept under Ar atmosphere. QCM Measurements. QCM measurements were carried out using AT-cut gold-coated quartz crystals with a resonant frequency of 9 MHz (5 mm diameter, Seiko EG&G, Seiko Instruments Inc.). The frequency of the QCMs was measured with a Seiko EG&G Model 917 quartz crystal analyzer. The crystals were mounted in the cell by means of O-ring seals, with only one face in contact with the solution. The frequency was recorded after immersing the crystals into the solution of 2 mmol/L V10SH or DPPTE or V10SH-DPPTE mixture at room temperature. The circuit and power supply for the QCM were insulated from external electromagnetic fields by placing them in a copper mesh Faraday cage. The measurements were done under Ar atmosphere. Electrochemical Measurements. The cyclic voltammogram (CV) was measured using a BAS 100B electrochemical analyzer (BSA Co., Ltd., USA). A Pt wire and Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively, and the modified Au electrode was used as the working electrode with 0.1 mol/L NaClO4 or 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] as the electrolyte solution. An initial potential of -200 mV was applied for 2 s, and sebsequent cyclic scans to a final potential of -800 mV were done for 10 cycles. Estimation of the peak currents and surface coverage was based on the curve of the 10th cycle; also the CV curves shown in the present work were the 10th cycle. Chronocoulomograms were measured by setting a fixed initial potential Ei and several final potentials E. The charge Q(t) following each potential jump Ei f E was recorded versus the time t elapsed from the instant of the jump for 250 ms, after which the potential was stepped back to Ei. The Ei value was set equal to -300 mV, and E was range from -350 to -650 mV for the measurements of Q(t)-t curves in the 0.1 mol/L NaClO4 electrolyte solution. The Ei was set equal to 500 mV, and E was range from 450 to -100 mV for the measurements in the 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] electrolyte solution. All electrochemical experiments were done under Ar atmosphere at room temperature.

Results and Discussion QCM Response. Figure 2 shows the frequency change (∆F) as a function of time (t) for the QCM gold resonator in the 2 mmol/L V10SH or DPPTE ethanol/acetonitrile solution. In both cases, the frequency (F) decreased quickly at first (∼40 min after immersion), and then the decrease became slower and slower. The assembling process of the V10SH and DPPTE SAMs can be finished completely in about 1.5 h. The measured ∆F for V10SH and DPPTE was (14) Hinnen, C.; Van Huong, C. N.; Rousseau, A.; Dalbara, J. P. J. Electroanal. Chem. 1979, 95, 131.

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about 60 and 45 Hz, respectively. From these ∆F values, we calculated that the mass of the adsorbed V10SH and DPPTE was about 64 and 48 ng/cm2, respectively, according to the equation ∆F ) -2F02∆m/(AFq1/2µq1/2), where F0 is the fundamental resonant frequency of 9 Hz, ∆m (g) is the mass change, A is the electrode area (0.196 cm2), Fq is the density of the quartz (2.65 g/cm3), and µq is the shear module (2.95 × 1011 dyn/cm2).15 Note that the molecular weight of V10SH and DPPTE is about 635 and 731 g/mol, respectively. Thus, the surface coverage (Γ) for V10SH and DPPTE on the gold surface is about 1.0 × 10-10 and 6.6 × 10-11 mol/cm2, respectively. Langmuir monolayers formed at the air-water interface are usually considered as closely packed monomolecular layers. It has been reported that the saturated surface coverage of viologen is 2.2 × 10-10 mol/cm2 estimated from the average molecular area (0.75 nm2) for the monolayers of viologen molecules oriented with the alkyl chains perpendicular to the electrode surface.16a The saturated surface coverage of DPPTE is estimated to be 3.7 × 10-10 mol/cm2 from the average molecular area (0.45 nm2) of the DPPTE monolayer at the air-water interface.16b Thus, the surface coverage estimated from QCM measurement is smaller than the saturated coverage. Note that the saturated coverage is calculated from the π-A isotherms at a surface pressure above 20 mN/m; the molecules are very closely packed in the Langmuir monolayers. In the present work, the SAMs are formed spontaneously on the gold surface; it is difficult for the molecules to pack so closely as in the Langmuir monolayers due to the strong steric effect, especially for the DPPTE molecules. Therefore, we suggest that the formed SAMs are loosely packed monolayers. The adsorption trend of the V10SH and DPPTE SAMs is comparable with those results reported in the literature.17,18 It has been suggested that the time for monolayers of alkanethiols fully formed is typically in the range of 10-100 min.17 Bain et al. further demonstrated that the adsorption process could be characterized by two distinct phases: a fast initial adsorption in some minutes and then a slow adsorption period lasting several hours.19 This two-step process can also be seen in Figure 2. The similarity of both the adsorption trend and ∆F value for the V10SH and DPPTE SAMs provides a possibility to co-assemble them onto the gold surface simultaneously. Probably due to this similarity, we failed to record a significant difference of the ∆F-t curves between the V10SH-DPPTE binary SAMs and pure V10SH or DPPTE SAM. Redox Properties of the V10SH SAM Modified Electrode. Figure 3 shows the CV curves for the V10SH SAM modified electrode (abbreviated as V10SH electrode) in the 0.1 mol/L NaClO4 electrolyte solution at scan rates of 20-500 mV/s. All curves indicate well reversible broad redox waves, which agree with the electrochemical properties of a lot of viologen-modified electrodes,6a-c,20 but different from others where the waves are split into two peaks.6d,e The split has been attributed to two forms (15) Seo, B. I.; Lee, H.; Chung, J. J.; Cha, S. H.; Lee, K. H.; Seo, W. J.; Cho, Y.; Park, H. B.; Kim, W. S. Thin Slid Films 1998, 327-329, 722. (16) (a) Lee, K. A. B. Langmuir 1990, 6, 709. (b) Qian, D. J.; Nakamura, C.; Miyake, J. Thin Solid Films 2001, 397, 266. (17) Kim, H. J.; Kwak, S.; Kim, Y. S.; Seo, B. I.; Kim, E. R.; Lee, H. Thin Solid Films 1998, 327-329, 191. (18) Shimazu, K.; Yag, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (19) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (20) (a) John, S. A.; Ohsaka, T. Electrochim. Acta 1999, 45, 1127. (b) Gu, N.; Dong, S. Electrochem. Commun. 2000, 2, 713.

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Figure 3. Cyclic voltammograms of the V10SH electrode in 0.1 mol/L NaClO4 solution at various scan rates: (a) 500, (b) 200, (c) 100, and (d) 20 mV/s.

of viologen redox reactions: normal V2+ T V•+ and its π-complex dimers V•+ T 0.5(V+)2.6d,e Our previous studies for various viologen LB films have revealed a couple of sharp peaks for the π-complex dimers and broad peaks for the normal V2+ T V•+.6d This means that the redox wave in Figure 3 corresponds to a reaction of V10SH2+ T V10SH•+. The cathodic (Epc) and anodic (Epa) potential peaks are centered at -480 to -500 mV and -440 to 460 mV, respectively, so the potential separation ∆Ep (∆Ep ) Epc - Epa) is about 40 mV. This ∆Ep value again agrees with the ∆Ep for the normal V2+ T V•+ reaction, and is smaller than that for the dimer reaction.6d,e Several research groups have revealed that ∆Ep is reduced when an electrode is modified by a mixed film of viologen and amphiphile, such as phospholipids.10,12 Kim et al. suggested that this is due to the separation of viologen headgroups by hydrophobic alkyl chains.21 For the V10SH electrode, which is composed of pure V10SH, the small ∆Ep might be ascribed to a loosely packed arrangement of the V10SH molecules in the SAM. This loosely packed structure may relate to the unsaturated surface coverage as described below. By integrating the area under the reduction peak,16,22 we obtained a surface coverage of the electroactive species of 7 × 10-11 mol/cm2. This value is a little smaller than the result from the QCM measurement, but the two values are comparable. Similar surface coverage was also observed from the SAMs of some other viologens on the Au electrode,12,16 and from the viologen LB films on indiumtin oxide electrode surface,6d but was smaller than the saturated coverage of 2.2 × 10-10 mol/cm2.16 The explanations for the smaller surface coverage have been suggested to be that solution ions (electrolytes) cannot easily penetrate the organic layer formed by the alkyl chain between viologen groups and electrode surface, so the charge current measured was low,6a and that not all viologen groups were electroactive due to this insulating barrier of alkyl chains.23 However, a comparison of the surface coverage from the QCM with that from the saturated coverage (as discussed above) indicates that the loosely packed arrangement of the viologen on the gold surface may be another important explanation. Redox Properties of the V10SH-DPPTE Binary SAMs. The redox behaviors of the V10SH-DPPTE binary (21) Kim, J.-H.; Lee, K. A. B.; Uphaus, R. A.; Cotton, T. M. Thin Solid Films 1992, 210/211, 825. (22) Tanigichi, T.; Fukasawa, Y.; Miyashita, T. J. Phys. Chem. B 1999, 103, 1920. (23) Oyama, N.; Ikeda, S.; Hatozaki, O.; Shimomura, M.; Mishima, K.; Nakamura, S. Bull. Chem. Soc. Jpn. 1993, 66, 1091

V10SH Incorporated in DPPTE SAMs

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Figure 4. Cyclic voltammograms of binary V10SH-DPPTE electrodes in 0.1 mol/L NaClO4 solution at scan rate 100 mV/s. Molar ratio of V10SH/DPPTE: (a) 1:0, (b) 1:5, and (c) 1:50.

Figure 5. Cyclic voltammograms of binary V10SH-DPPTE electrodes in 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] solution at scan rate 100 mV/s. Molar ratio of V10SH/DPPTE: (a) 1:0, (b) 1:5, and (c) 1:50.

SAM modified electrode (abbreviated as V10SH-DPPTE electrode) were investigated both in the 0.1 mol/L NaClO4 solution and in the 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] mixed electrolyte solution. Figure 4 shows the CV curves of the V10SH-DPPTE electrode (molar ratios 1:0, 1:5, and 1:50) in the 0.1 mol/L NaClO4 electrolyte solution at scan rate 100 mV/s. Compared with the results from the V10SH electrode, Figure 4 reveals the following features for the V10SH-DPPTE electrodes. First, both reduction and oxidation peaks of the normal reaction V10SH2+ T V10SH•+ in the binary SAMs shift to more negative potentials. Second, the current intensity becomes weaker in the binary SAMs. By integrating the area under the reduction peak of the V10SH-DPPTE electrode (1:5), the surface coverage is about 0.8 × 10-11 mol/cm2. Compared with the surface coverage of the pure V10SH electrode (7 × 10-11 mol/cm2), we can estimate that the molar fraction of V10SH is about 0.11 in the V10SH-DPPTE electrode (1:5). Although this value is a little smaller than the assumed proportion of the viologen (0.17), it is a comparable scale if we take into account the electron barrier formed by the alkyl chain of DPPTE. Third, the reductionoxidation wave is hardly measured when the molar ratio of V10SH/DPPTE increases up to 1:50. Figure 5 shows the CV curves of the V10SH and V10SH-DPPTE electrodes in the 0.1 mol/L NaClO4-0.1 mmol/L K3Fe(CN)6 electrolyte solution. The V10SH electrode shows a reversible reduction-oxidation wave of Fe(CN)63-/4-, with cathodic and anodic potentials at about 180 and 230 mV, respectively. For the V10SH-DPPTE

Figure 6. Q(t) versus t curves for the electroreduction of V10SH and V10SH-DPPTE electrodes in 0.1 mol/L NaClO4 solution. The curves were obtained by stepping the potential from a fixed initial value Ei to final values E varying from -300 to -650 mV. Molar ratio of V10SH/DPPTE: (a) 1:0, (b) 1:5, and (c) 1:50.

electrodes, the reduction peak shifts to more negative potential and the oxidation peak to more positive potential. As shown in Figure 5b, the wave peaks shift to about 120 and 280 mV for the V10SH-DPPTE electrode (1:5). A very small redox wave can be distinguished for the V10SHDPPTE electrode (1:50) with the peak position difficult to determine. The electrochemical properties above indicate that redox behaviors of V10SH and Fe(CN)63-/4- are closely related to the amount of viologens in the binary SAMs. For V10SH in the binary SAMs, decrease of the current intensity is because of the decrease of the V10SH assembled. For the Fe(CN)63-/4- complex ion, the results indicate that this ion can access the electrode or have an electron transfer to/from electrode when the molar ratios of V10SH are high; conversely, the ion cannot access or transfer electrons.

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Figure 8. Cottrell plot for V10SH and V10SH-DPPTE electrodes in (A) 0.1 mol/L NaClO4 solution and (B) 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] solution. Molar ratio of V10SH/DPPTE: (a) 1:0, (b) 1:5, and (c) 1:50. Scheme 1. Schematic Drawing of Formed SAMs and Charge-Transfer Process

Figure 7. Q(t) versus t curves for the electroreduction of V10SH and V10SH-DPPTE electrodes in 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] solution. The curves were obtained by stepping the potential from a fixed initial value Ei to final values E varying from 500 to -100 mV. Molar ratio of V10SH/DPPTE: (a) 1:0, (b) 1:5, and (c) 1:50.

The shift of redox peaks relates to the increase of resistance in the binary SAMs when DPPTE is coassembled. It has been pointed out that hydrocarbon layer hinters the access of the electrolyte to viologen groups, and the thicker the hydrocarbon layer the larger the resistance.16 A 33 mV negative shift was found for symmetrically bis-alkylated viologen LB films when the length of alkyl chain changed from 12 to 18 CH2 groups.6d Although we did not change alkyl chain of the viologen in this study, the insulating DPPTE molecules co-assembled may hinder the access of Fe(CN)63-/4- ions to viologen headgroups and lead to the peak shift. Chronocoulometry Properties of the V10SH and V10SH-DPPTE Binary SAMs. Charge-transfer process

in the V10SH and V10SH-DPPTE SAMs was investigated by the chronocoulometry method.5a,24 Figure 6 shows a series of Q(t)-t curves of the V10SH and V10SH-DPPTE electrodes in the 0.1 mol/L NaClO4 electrolyte solution. The potential jump is from an initial potential of -300 mV to several final potential E’s. The Q(t)-t curves of (24) (a) Eng, L. H.; Elmgren, M.; Komlos, P.; Nordling, M.; Lindquist, S.-E.; Neujahr, H. Y. J. Phys. Chem. 1994, 98, 7068. (b) Herrero, R.; Buoninsegni, F. T.; Becucci, L.; Moncelli, M. R. J. Electroanal. Chem. 1998, 445, 71. (c) Hui, T.-W.; Baker, M. D. J. Phys. Chem. B 2001, 105, 3204.

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Table 1. Charge Diffusion Coefficient in the SAMs SAM

electrolyte

V10SH V10SH-DPPPTE (1:5) V10SH-DPPTE (1:50) V10SH V10SH-DPPPTE (1:5) V10SH-DPPTE (1:50)

NaClO4 NaClO4 NaClO4 NaClO4-K3[Fe(CN)6] NaClO4-K3[Fe(CN)6] NaClO4-K3[Fe(CN)6]

DeC2 (×1018)

De (×109)

32 0.24 0.018 90 58 0.0049

V10SH electrode in Figure 6a can be described as follows. (i) At the less negative E values, E > -450 mV, at which the V10SH is still electroinactive, Q(t) increases very quickly in less than 5 ms because of the flow of the capacitive current that is required to charge the interface, and then attains a time-independent value.5a (ii) As E becomes progressively more negative, E ≈ Epc, Q(t) increases quickly at the initial 5 ms due to the capacitive contribution, and then more slowly due to the gradual electroreduction of the V10SH in the monolayer. (iii) With a further gradual shift of E toward negative values, E < - 600 mV, Q(t) increases quickly due to the electroreduction of the V10SH; the whole reduction process is completed within 10 ms. The Q(t)-t curves of the V10SH-DPPTE electrode show two salient features, as compared to those of the V10SH electrode (Figure 6b,c). The first one is that Q(t) increases quite slowly in the V10SH-DPPTE electrode, and this slow increase continues during the whole jump process. This means that the electroreduction rate of V10SH in the V10SH-DPPTE electrode is slower than that in the V10SH electrode. This slow electroreduction rate in the binary SAMs may be due to the increase of the resistance. The second feature is that Q(t) is smaller in the V10SH-DPPTE electrode than in the V10SH electrode. This is reasonable by taking into account a smaller amount of V10SH molecules in the binary SAMs. Figure 7 shows a series of Q(t)-t curves of the V10SH and V10SH-DPPTE electrodes in the 0.1 mol/L NaClO40.1 mmol/L K3[Fe(CN)6] mixed electrolyte solution. The potential jump is from an initial potential 500 mV to several final potential E’. For each Ei f E jump, similar Q(t)-t curves can be observed for the V10SH and V10SHDPPTE electrodes (a comparison among Figure 7a-c), with only the difference of the Q(t) values. Generally, at E < 400 mV, Q(t) increases very little after the jump due to an electroinactive Fe(CN)63- ion. At -100 mV < E < 350 mV, Q(t) increases quickly, especially at the initial 20 ms due to the reduction of Fe(CN)63-. However, Q(t) does not reach a time-independent value even after 250 ms. The results mean that ferricyanide can be reduced or oxidized by using the V10SH and V10SH-DPPTE electrodes and that this redox process is slow. Herrero et al. reported that viologens, actually their radical cations, can act as an electron-transfer mediator from Fe(CN)63-/4- dissolved in the aqueous solution to the mercury surface across a dioleoylphosphatidylcholine monolayer.5a Since our experiments on the CV and Q(t)-t measurements of Fe(CN)63-/4- were done in the potential range of 500 to

-100 mV, in which viologen cannot be reduced to its radical cation, it is difficult to attribute the electron transfer to be mediated by the reduced V10SH. Krysinski et al. suggested that the redox species, if sufficiently hydrophobic and small, could partition into the monolayer at the electrode surface, where it can be “trapped” by the hydrophobic chains and/or by the adsorption at the surface of electrodes.25 On the other hand, due to the positive charge of V10SH is and the negative charge of Fe(CN)63-/4ion, Fe(CN)63-/4- might be adsorbed by V10SH at the electrode surface. Thus, we suggest that the electron transfer between the electrode and Fe(CN)63-/4- is via the immobilized or incorporated V10SH, which acts as a mediator for Fe(CN)63-/4- ions to “gated” access to the electrode surface (Scheme 1). Figure 8 shows a Cottrell plot of the quantity of electricity Q(t) to the root of time (t) for the V10SH and V10SH-DPPTE electrodes in 0.1 mol/L NaClO4 or in the 0.1 mol/L NaClO4-0.1 mmol/L K3[Fe(CN)6] electrolyte solution. According to the Cottrell equation

Q(t) ) 2nFAC(Det)1/2/π1/2 the slope of the line equals 2nFACDe1/2/π1/2, where n is electron number, F is the Faraday constant, A is the electrode area, C is the concentration of electroactive species, and De is the “diffusion coefficient”. Except for the V10SH electrode in the 0.1 mol/L NaClO4 solution, all plots shows a linear increase of Q(t) with t1/2. Since the reduction-oxidation process for the V10SH electrode is completed within 20 ms in the 0.1 mol/L NaClO4 solution, we only focused on the initial increase of the Q(t)-t curve for the calculation of De2C (Figure 8a).26 Other values of De or De2C are calculated based on the slope of the whole curves. The De or De2C data are summarized in Table 1, which indicates that De is significantly reduced when the binary SAMs are prepared with a high molar ratio of DPPTE in the mixtures. Conclusion We have demonstrated a co-assemble method to prepare binary SAMs of a viologen thiol and a phospholipid. The immersed viologens can act as an electron-transfer mediator for ferricyanide to “gated” access to the electrode surface across the phospholipid monolayer. The heterogeneous system provides an alternative route to mimic the electron- or charge-transfer processes in the membranes. Acknowledgment. This work was supported by NEDO’s International Joint Research Grant Program. The authors thank Prof. Takamasa Sagara, Nagazaki University, for valuable assistance for the synthesis of the viologen thiol. LA020070E (25) Krysinski, P.; Brzostowska-Smolska, M.; Mazur, M. Mater. Sci. Eng., C 1999, 8-9, 551. (26) Since the concentration of the viologen in the mixed SAMs was difficult to calculate with molar ratio of viologen/DPPTE up to 1:50, we provided the DeC values instead of De.