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Stabilization of the assemblies of short alkyl chain asymmetric viologens on alkanethiol-coated electrodes
Electrochimica Acta 45 (1999) 1127 – 1133 www.elsevier.nl/locate/electacta
Stabilization of the assemblies of short alkyl chain asymmetric viologens on alkanethiol-coated electrodes Swamidoss A. John, Takeo Ohsaka * Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8502, Japan Received 10 August 1999
Abstract Self-assembled monolayers (SAMs) of n-alkanethiol (CH3(CH2)n SH, n=3 – 17)-coated electrodes have been used to stabilize the assemblies of N-ethyl-N%-tetradecylviologen (1) and N-ethyl-N%-dodecylviologen (2) on the electrode surface. The redox behavior of 1 and 2 on the bare Au electrode was unstable because of the weak adsorption of their assemblies on the Au electrode surface. On the other hand, both 1 and 2 are strongly confined and compact in the SAM domain of alkanethiol and showed a stable redox response. The compactness of the assemblies of 1 and 2 on alkanethiol-coated electrodes can be understood from its blocking effect on the redox reaction of [Ru(NH3)6]2 + /3 + and EQCM measurements. EQCM measurements demonstrated that the ingress of water molecules into the assembly of 1 and 2 was markedly suppressed at the assembly of 1 and 2 bound to alkanethiol-coated electrodes. Furthermore, the redox behavior of 1 and 2 on the bare and alkanethiols-coated electrodes was also studied in the presence of − − weakly hydrated anions of ClO− 4 and PF6 . For the ClO4 ion, an additional oxidation peak was observed for the first reduction of 1 and 2 along with a usual redox wave. While, in the presence of PF− 6 ions, the electrochemical behavior was largely dependent on the alkyl chain length of thiol. Two redox waves were observed on short chain thiol (n= 3)-coated electrode whereas a single redox wave was observed on the electrodes coated with alkanethiols of n=5, 7 and 9. An irreversible redox response was observed on the longer alkyl chain (n\ 9) thiol-coated electrodes. The observed irreversible response has been attributed to the effective blocking of water molecules at the assembly of 1 and 2 on the long chain alkanethiol-coated electrodes in the presence of weakly hydrated PF-6 ions. At these electrodes, 1 and 2 may form an insoluble salt with PF− 6 ion and are electroinactive. When the same electrodes were − − transferred to solutions of Cl− or ClO− or ClO− 4 ions, the PF6 ions could be readily exchanged with Cl 4 ions and 1 and 2 became electroactive. Such a situation may not arise at the assembly of 1 and 2 bound to short chain thiol (nB 9) coated-electrodes because of the free flow of water molecules into the disordered monolayers of short chain thiols. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Asymmetric viologen; n-Alkanethiols; Voltammetry; EQCM
1. Introduction
* Corresponding author. Tel.: +81-45-924-5404; fax: +8145-924-5489. E-mail address: [email protected] (T. Ohsaka)
Recently, we and others have reported the electrochemical behavior of asymmetric N-alkyl-N%-methyl (or ethyl) viologens with alkyl chains, the number (m) of carbons of which is 12 to 18, on Au and GC electrode surfaces [1 – 5]. It has been found that the viologens
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with alkyl chains of m=16 and 18 are relatively stable on the electrode surface and their redox reactions are very fast. However, the viologens of mB 16 have a weak tendency to self-assemble on the electrode surface and they are immediately lost from the electrode surface after transfer to the electrolyte solution [1,3,4]. Stabilization and excellent redox characteristics of such viologens on the electrode surface are primarily required in developing potential applications of the viologen-modified electrodes, for example, they could be very useful for electrical communication with dissolved redox enzymes having negative reduction potentials (e.g., nitrate reductase [6], cytochrome-c [7] and ferredoxins [8]). In this work, we used n-alkanethiols to stabilize the redox reaction of short chain viologens such as N-ethyl-N%-tetradecylviologen (1) and N-ethylN%-dodecylviologen (2) on the electrode surface. Recently two reports have been published concerning the co-adsorption of electroinactive long chain alkaenthiols to stabilize the viologen moieties covalently attached to the electrode surface [9,10]. In this paper, we wish to report the stable redox behavior of 1 and 2 bound to the self-assembled monolayers (SAMs) of n-alkanethiols (CH3(CH2)n SH, n=3–17) on an Au electrode. In addition, we also report the electrochemical behavior of 1 and 2 in the presence of weakly hydrated ions of − ClO− 4 and PF6 .
2. Experimental The polycrystalline Au electrode (1 or 2 mm diameter) surface was polished to a mirror finish with fine emery paper (Sankyo Rikagaku, Japan) and then aqueous slurries of successively finer alumina powder (1 and 0.06 mm) on a polishing microcloth, sonicated for 10 min in water. The Au electrode was then electropolished by potential cycling in 0.05 M H2SO4 in the potential range of −0.2 to +1.5 V at the potential scan rate of 100 mV s − 1 for 20 min or until the CV characteristic for a clean Au electrode was obtained. Such an electrode was considered as a bare Au electrode. The real surface area was calculated from the charge required to reduce the surface oxide layer using the previously established formula, 0.43 mC cm − 2 [11]. The surface roughness was calculated as the ratio of the real surface area to the geometric area. The surface roughness of the electrode was found to be 1.2. The asymmetric viologen (bromide salt), N-ethyl-N’tetradecylviologen (1) and N-ethyl-N%-dodecylviologen (2) were synthesized and characterized by the reported
procedure [4]. Octadecanethiol (Aldrich) and other alkanethiols (Tokyo Kasei Kogyo and Wako Chemicals) were used as received. The SAMs of 1 and 2 were prepared by immersing the clean Au electrode in 100 – 200 mM aqueous solution of 1 or 2 for 30 – 90 min. Then the electrode was gently rinsed with water and used for electrochemical experiments. The alkanethiol monolayers were formed by immersing the Au electrode into a 1 – 10 mM ethanol solution of the respective alkanethiol for 1 – 6 h. The electrode was then washed with ethanol, water and then dried. The electrode was then immersed in an aqueous solution of 1 or 2 for 30 – 90 min and rinsed with water and transferred to the supporting electrolyte for electrochemical measurements. All electrochemical experiments were performed at room temperature using a standard three-electrode, two compartment configuration with a Au working electrode, a spiral platinum counter electrode and a NaClsaturated Ag/AgCl reference electrode. The cyclic voltammetric experiments were carried out with a computer-controlled electrochemical system (BAS 100 B/ W). The surface coverage (G/mol cm − 2) was estimated by graphically integrating the cyclic voltammograms recorded at various scan rates. All solutions were thoroughly deoxygenated by purging with nitrogen gas, and during the electrochemical experiments a nitrogen atmosphere was maintained above the solution. The EQCM principle and the instrumentation have been well established [12]. AT-cut quartz crystals of nominal fundamental frequency of 6 MHz were used in the present study. Signal averaging was employed to allow for the facile detection of frequency changes as small as 0.1 Hz. The Au quartz crystals were purchased from Hokuto Denko (Japan). The electrochemically active area of the Au electrode was 1.33 cm2. The surface roughness of the electrode was determined to be 1.2 by the gold oxide method [11]. Since both the electrochemistry and EQCM measurements were done on the same electrode, the Gs reported in the EQCM measurements do not take this roughness into account so as to allow direct comparison of G and frequency changes (which are influenced identically by surface roughness). The signal from the oscillator was sent to a Hokuto Denko, ECQCM Controller HQ-101B. A potentiostat (PS-07, Toho Tech.) and an X-Y-Y% recorder (Graphtech) were used to record the EQCM response.
3. Results and discussion
3.1. Stability of 1 and 2 on bare and SAMs of alkanethiol-coated Au electrodes Fig. 1 shows the CVs obtained for the SAM of 1 (G 1.8× 10 − 10 mol cm − 2) on the Au electrode in the
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presence of 0.1 M of KCl at different time intervals. It shows a broad redox wave at −0.44 V corresponding to the one-electron reduction of 1, to the radical cation and its oxidation to 1. The CVs recorded after 5 and 10
Fig. 1. CVs obtained for the SAM of 1 on the bare Au electrode in 0.1 M KCl. Voltammogram (a) was recorded after the electrode was immersed in the electrolyte solution, and voltammograms (b) and (c) were recorded after 5 and 10 min immersion, respectively. Scan rate = 0.2 V s − 1.
Fig. 2. CVs obtained for the assembly of 1 on the BT-coated electrode in 0.1 M KCl at scan rates of 1–10 V s − 1 (G= 3.5× 10 − 10 mol cm − 2). Inset: plot of cathodic peak current versus the scan rate.
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min immersion in 0.1 M KCl showed a decrease in the G of 1 (Fig. 1(b,c)). The decrease in the G of 1 can be understood from the weak adsorption of 1 on the electrode surface because of the presence of a short alkyl tail in 1 when compared to the viologens containing a long alkyl tail [1 – 5]. Similar electrochemical behavior was also observed for the assembly of 2 on the Au electrode. Fig. 2 shows the CVs obtained for the assembly of 1 on butanethiol (BT, n= 3)-coated electrode at scan rates of 1 – 10 V s − 1. Several significant features were observed for the assembly of 1 on the BT-coated electrode. It shows a broad redox wave at −0.40 V with a peak separation of 10 mV at a scan rate of 1 V s − 1. The CV behavior is more symmetric and sharper when compared to the assembly of 1 on the bare Au electrode (see Fig. 1). In addition, the double layer capacitance (Cdl) calculated for the assembly of 1 on the BT-coated electrode is smaller than that calculated for the assembly of 1 on the bare Au electrode, e.g. the Cdl values were ca. 9 and 27 mF cm − 2 at 0 V for the former and the latter, respectively. The observed small Cdl indicates that the assembly of 1 is more compact on the BTcoated electrode when compared to the bare Au electrode. Moreover, the G value (G = 3.5×10 − 10 mol cm − 2) calculated for the assembly of 1 on the BTcoated electrode is higher than that on the Au electrodes. Most of the assembly of 1 was lost from the Au electrode surface during the rinsing with water (G for the assembly of 1 on the Au electrode is 1.8× 10 − 10 mol cm − 2 (Fig. 1)). However, the assembly of 1 was more stable on the BT-coated electrode even during rinsing and thus it showed a higher G. The small potential difference between the cathodic and anodic peak potentials (10 – 25 mV) and the linearity of the plot of peak current versus scan rate (the inset of Fig. 2) clearly indicate that the redox response of 1 on the BT-coated electrode is due to the surface-confined species [13]. The observed fast reversible redox reaction of 1 even at a high scan rate of 10 V s − 1 indicates that the redox moiety is assembled close to the electrode surface and not to the exterior of the alkanethiol monolayer [10]. The assembly of 1 on the BT-coated electrode was highly stable, as readily seen from the comparison of the CVs obtained before and after the electrolysis at a series of potential scans of 1 – 10 V s − 1 (Fig. 3): a 15 mV positive shift in the redox peak potential was observed, but the G was unchanged. Similar CV behavior was also observed for the assembly of 2 on the BT-coated electrode in 0.1 M KCl. The stability of the assemblies of 1 and 2 on the alkanethiol-coated electrodes is also understood from their close packing on the alkanethiol-coated electrodes. This was examined by their blocking effect on the electrochemical behavior of diffusing solution-species. The CVs obtained for Ru(NH3)6Cl3 on the clean Au
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[Ru(NH3)6]3 + is significantly affected (Fig. 4(b)). The shape of the CV indicates that the reduction of [Ru(NH3)6]3 + is mediated by 2 on the DDT-coated electrode [14]. The observed blocking effect for [Ru(NH3)6]3 + by the assembly of 2 on the DDT-coated electrode demonstrates the close packing of the assembly of 2 on the DDT-coated Au electrode. Similar experiments were also conducted at the assembly of 2 on the bare Au electrode. In this case, the redox reaction of [Ru(NH3)6]3 + was unaffected (not shown). This indicates that the assembly of 2 is less compact on the bare Au electrode than on the alkanethiol-coated electrode.
3.2. Electrochemical beha6ior of 1 and 2 on bare and alkanethiol-coated Au electrodes in the presence of − hydrophobic anions of ClO− 4 and PF 6
Fig. 3. CVs obtained for the assembly of 1 on the BT-coated electrode in 0.1 M KCl (a) before and (b) after recording the CVs of Fig. 2. Scan rate = 0.2 V s − 1.
Fig. 5 shows the CV and EQCM curve obtained for the assembly of 2 (3.7×10 − 10 mol cm − 2) on the Au electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Since the assembly of 2 was less stable on the Au electrode, the experiment was carried out in the presence of 30 mM of 2 in solution. The assembly of 2 shows one reduction peak and two sharp oxidation peaks for the first redox reaction of 2, whereas only a single broad redox wave was observed for the assembly of 2 under identical conditions except for Cl− as electrolyte anion (data not shown). In addition, the value for G calculated in the presence of ClO− 4 is two times higher than that calculated in the presence of Cl−. The solubility of 2 is comparatively low in the presence of
Fig. 4. CVs obtained for (a) the bare Au electrode and (b) the assembly of 2 on the DDT-coated Au electrode in 0.1 M KCl solution containing 0.5 mM Ru(NH3)6Cl3 and (c) the assembly of 2 on the DDT-coated electrode in 0.1 M KCl. Scan rate=0.2 V s − 1.
electrode and the assembly of 2 bound to the dodecanethiol (DDT, n=11)-coated Au electrode (G= 2.5× 10 − 10 mol cm − 2) are shown in Fig. 4. A well-defined diffusion-controlled redox reaction for Ru(NH3)26 + /3 + was observed on the bare Au electrode (Fig. 4(a)). However, on the DDT-coated electrode to which the assembly of 2 was bound, the diffusion of
Fig. 5. CV and EQCM curve obtained for the assembly of 2 on the Au quartz electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Scan rate = 0.025 V s − 1. G= 3.7 ×10 − 10 mol cm − 2.
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Fig. 6. CV and EQCM curves obtained for the assembly of 2 on the ODT-coated Au quartz electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Scan rate= 0.1 V s − 1. G= 2.0× 10 − 10 mol cm − 2. − and therefore ClO− 4 when compared to that in Cl − ClO4 effectively anchors the assembly of 2 on the electrode surface, which led to a high value for G in the presence of ClO− 4 [5]. Unfortunately, we were unable to carry out the similar experiment in the presence of PF− 6 because of the precipitation of 2 with PF− ions in 6 solution. In this case, for 2 at a concentration of 30 mM, the electrochemical response of the molecule diffusing to the electrode surface from the bulk of the solution is negligible. Recently, two pairs of redox peaks for the first redox reaction of long alkyl chain viologen, C18MV2 + , have been reported on the GC electrode in the presence of ClO− 4 ion, but only one pair in Cl− [5]. In this case, it has been proposed that ClO− 4 significantly increases the order of the film compared with Cl− and splits the redox wave [5]. Similar ordered arrangement is also expected at high values of G for 2 in the presence of ClO− 4 . A total frequency change of 14 Hz was observed for the first reduction of the assembly of 2, which corresponds to a loss of mass of 928 g mol − 1 from the electrode surface. The EQCM curve reveals that mass is lost during the reduction of 2 and this mass loss is reversed during the reoxidation (Fig. 5). These frequency changes have been previously interpreted as indicative of anion loss (or gain) from the monolayer, which is a consequence to the injection of electrons [12]. The mass changes were calculated by using the Sauerbrey equation (see Eq. (1)) [15].
Df= − Cfm
(1)
where Df is the frequency change, m is the mass per cm2 of the assembly of 2 and Cf is the proportionality constant. In the calculation of the amount of water transported per anion, it is assumed that injection of each electron into the monolayer to reduce one 2 causes the loss of one anionic charge (one anion for the monovalent anions). We found that ca. 42 water molecules are lost from the monolayer for one ClO− 4 ion lost during the reduction and this compositional change is reversible on the CV time scale. Donohue and
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Buttry have studied the CV and EQCM behavior of surfactants of ferrocene with chain lengths of C12 and C14 on the Au electrode [16]. They found that the oxidized form of the ferrocene surfactant was desorbed from the electrode surface, which was inferred from the EQCM measurements. It was calculated that only 30% of the mass were regained during the reverse scan. On the other hand, the mass lost in the reduction, in the present case, was completely regained in the reoxidation (Fig. 5). Thus the adsorption of 2 on the Au electrode surface is strong and 2 did not desorb from the electrode surface into the 30 mM solution of 2. Recently, we have studied the EQCM behavior of C18EV2 + on the Au electrode surface in the presence of different supporting electrolytes [1]. According to this, the loss of water molecules from the assembly of 2 was more than four times that from the C18EV2 + assembly in the presence of ClO− 4 . The observed loss of a higher number of water molecules from the assembly of 2 shows that 2 forms a more loosely organized assembly on the electrode surface than C18EV2 + . In the case of Cl−, more than ca. 50 water molecules are lost from the assembly of 2 during the reduction. We have also studied the CV and EQCM behavior of 1 in the presence of Cl− and ClO− 4 . It was found that ca. 34 and 42 water molecules are lost from the monolayer in the − presence of ClO− 4 and Cl , respectively. CV and EQCM curves obtained for the assembly of 2 (G =2.0× 10 − 10 mol cm − 2) on the octadecanethiol (ODT)-coated electrode (prepared under the same conditions as on the Au electrode) in 0.1 M NaClO4 are shown in Fig. 6. On the ODT-coated electrode, the assembly of 2 shows only a single redox peak, in contrast to the split anodic peak on the Au electrode. The double layer capacitance (Cdl) calculated for the assembly of 2 on the ODT-coated electrode is less than that calculated at the assembly of 2 on the Au electrode, indicating that the assembly of 2 is more compact on the ODT-coated electrode than on the Au electrode. Since the monolayer of ODT is highly compact and dense, the adsorption of 2 into the ODTcoated electrode is less and showed a less G than that (3.7× 10 − 10 mol cm − 2) for the assembly of 2 on the Au electrode. A total frequency change of 2 Hz was observed for the assembly of 2 on the ODT-coated electrode, which corresponds to a loss of mass of 245 g mol − 1 from the electrode surface. Thus, approximately eight water molecules are lost from the monolayer for one ClO− 4 ion lost during the reduction and this compositional change is reversible on the CV time scale. The observed loss of a much lower number of water molecules from the assembly of 2 on the ODT-coated electrode, compared with that on the Au electrode indicates that the assembly of 2 on the ODT-coated electrode is more compact than on the Au electrode. In the presence of Cl−, approximately 20 water molecules
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are lost from the assembly of 2 on the ODT-coated electrode upon the reduction. On the other hand, six and 17 water molecules are lost from the assembly of 1 on the ODT-coated electrode during the reduction in − the presence of ClO− 4 and Cl , respectively. Fig. 7(a) shows the CVs obtained for the assembly of 2 on the BT-coated electrode in the presence of 0.1 M NH4PF6. In the reduction process, two cathodic peaks were observed at −0.50 and −0.53 V. In the oxidation process two peaks were also observed at − 0.46 and −0.43 V. On the other hand, the assembly of 2 showed a single redox wave in 0.1 M KCl and a single redox wave with an additional oxidation peak in 0.1 M NaClO4. The observed electrochemical behavior was stable even when the electrode was immersed in 0.1 M NH4PF6 for an extended period of time (e.g. 2 h). A very sharp redox peak was observed for the assembly of 2 on the hexanethiol (HT)-coated electrode in the presence of 0.1 M NH4PF6 (vide infra). However, in the case of octanethiol (OT)- and decanethiol (DT)-coated electrodes only a single redox wave was observed, and an irreversible response was obtained for the electrodes coated with alkanethiols of n=11–17. The CV obtained for the assembly of 2 on the dodecanethiol (DDT)-coated electrode in 0.1 M NH4PF6 is shown in Fig. 7(b). It shows an irreversible redox response, in contrast to the multiple redox behavior observed on the BT-coated electrode. When the electrode used in NH4PF6 solution was transferred to 0.1 M KCl, a clear reversible redox response was observed (Fig. 7(c)). The observed irreversible response of 2 in PF− 6 solution may
Fig. 7. CVs obtained for the assembly of 2 on (a) BT- and (b) DDT-coated electrodes in 0.1 M NH4PF6 and (c) on the same electrode as used in (b) after transferal to 0.1 M KCl. Scan rate= 0.1 V s − 1.
be attributed to the formation of an electroinactive, insoluble salt of 2 with PF− 6 on the electrode surface. It is well-known that n-alkanethiols of n\9 form a more densely packed crystalline-like assembly on Au electrodes than alkanethiols of n B9 [17]. Thus at the assembly of 2 on the DDT-coated electrode, unlike on the BT-coated electrode, water molecules are effectively blocked in the presence of PF− 6 ions. On transferring the same electrode in Cl− solution, PF− 6 ions of the insoluble salt were exchanged with Cl− ions and the assembly of 2 became electroactive. Similar electrochemical behavior was also observed for the assembly of 1 in the presence of PF− 6 ions.
3.3. Electrochemical stability of alkanethiol monolayers The potential window for the n-alkanethiols and functionalized alkanethiol monolayers on different electrode surfaces has been studied by several groups [18 – 20]. Cheng and Brajter-Toth [18] reported that n-hexanethiol monolayers were stable between −0.8 and 1.0 V versus saturated calomel electrode (SCE) even at acidic pH. On the other hand, based on a series of continuous potential cycling experiments at different potential windows, Beulen et al. [20] have shown that the monolayers of decanethiol and functionalized alkanethiols are stable between −0.8 and 0.4 V versus mercurous sulfate reference electrode (MSE) ( − 0.4 – 0.8 V versus Ag/AgCl). We have studied the stability of n-alkanethiol monolayers in the potential region of 0 to −0.65 V, where the redox reactions of 1 and 2 occur. The continuous CVs obtained for the assembly of 2 on the n-HT-coated Au electrode in 0.1 M NH4PF6 after
Fig. 8. Continuous CVs obtained for the assembly of 2 on HT-coated electrode in 0.1 M NH4PF6 after (a) 10, (b) 20, (c) 40 and (d) 50 cycles. Scan rate=0.1 V s − 1.
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10, 20 40 and 50 cycles are shown in Fig. 8. When the CV was continuously recorded, both the reduction and oxidation peak currents of 2 increased up to 20 cycles and after which no increase in the peak currents was observed. In addition, the double layer capacitance (Cdl), e.g. at 0 V remains unchanged even after 50 continuous potential cycling, which indicates that the monolayers of n-HT are highly stable in the potential region of 0 to −0.65 V. Similarly n-BT (n= 3)-monolayers are also highly stable after a series of potential scans between 0 and −0.65 V (Fig. 3).
4. Conclusions The present work demonstrates that alkanethiolcoated electrodes can effectively stabilize the short alkyl chain asymmetric viologens of 1 and 2 by their unique property of blocking water molecules, that is, the viologens are strongly confined in the SAM domain of alkanethiols on electrodes. The assemblies of 1 and 2 are less organized on the Au electrode and thus water molecules freely enter them, resulting in their desorption from the electrode surface. On the other hand, the assemblies of 1 and 2 are more compact on the alkanethiol-coated Au electrodes than on the bare Au electrode which is inferred from their blocking effect on the redox reaction of solution-species [Ru(NH3)6]2 + /3 + and EQCM measurements. The electrochemical behavior of 1 and 2 has been significantly affected by the presence of PF− when compared to ClO− 6 4 . In the − presence of PF6 ions, the assemblies of both 1 and 2 on the short alkyl chain thiol-coated electrode (e.g., BTcoated electrode) showed two pairs of redox peaks for the first redox reaction, whereas in the presence of ClO− 4 only an additional oxidation peak was observed in addition to the single redox wave. The observed behavior has been explained by the different interac− tions of ClO− 4 and PF6 with 1 and 2.
Acknowledgements The present work was financially supported by Grant-in Aids for Scientific Research in Priority Areas,
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‘New Polymers and Their Nano-Organized Systems’ (No.277/10126219), ‘Exploratory Research’ (No.09875207) and ‘Scientific Research (A)’ (No.10305064) from the Ministry of Education, Sciences, Sports and Culture, Japan (Monbusho). We thank Professor K. Tokuda for helpful discussions and comments. S.A.J. thanks the Monbusho for the fellowship. References [1] S.A. John, T. Okajima, T. Ohsaka, J. Electroanal. Chem. 466 (1999) 67. [2] C.-W. Lee, A.J. Bard, J. Electroanal. Chem. 239 (1988) 441. [3] C.A. Widrig, M. Majda, Langmuir 5 (1989) 689. [4] M. Gomez, J. Li, A.E. Kaifer, Langmuir 7 (1991) 1797. [5] C.M. Judkins, E.W. Bohnnan, A.K. Herbig, J.A. Powers, D.A. van Galen, J. Electroanal. Chem. 451 (1998) 39. [6] R.B. Mellor, J. Ronnengerg, W.H. Campbell, S. Diekmann, Nature 355 (1992) 717. [7] W.R. Heineman, T. Kuwana, C.R. Hartzell, Biochem. Biophys. Res. Commun. 50 (1973) 892. [8] S. Cosnier, C. Innocent, Y. Jouanneau, Anal. Chem. 66 (1994) 3198. [9] B. Lee, Langmuir 8 (1992) 2491. [10] E. Katz, N. Itzhak, I. Willner, Langmuir 9 (1993) 1392. [11] E. Gileadi, K.E. Eisner, J. Penciner, Interfacial Electrochemistry — An Experimental Approach, AddisonWesley, Reading, MA, 1975. [12] D.A. Buttry, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 17, Marcel Dekker, New York, 1991, pp. 1 – 85. [13] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980 (Chapter 12). [14] S.E. Creager, D.M. Collard, M.A. Fox, Langmuir 6 (1990) 1617. [15] G.Z. Sauerbrey, Z. Phys. Chem. 155 (1959) 206. [16] J.J. Donohue, D.A. Buttry, Langmuir 5 (1989) 671. [17] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [18] Q. Cheng, A. Brajter-Toth, Anal. Chem. 67 (1995) 2767. [19] M.H. Schoenfisch, J.E. Pemberton, Langmuir 15 (1999) 509. [20] M.W.J. Beulen, M.I. Kastenberg, F.C.J.M. van Veggel, D.N. Reinhoudt, Langmuir 14 (1998) 7463.
Stabilization of the assemblies of short alkyl chain asymmetric viologens on alkanethiol-coated electrodes Swamidoss A. John, Takeo Ohsaka * Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8502, Japan Received 10 August 1999
Abstract Self-assembled monolayers (SAMs) of n-alkanethiol (CH3(CH2)n SH, n=3 – 17)-coated electrodes have been used to stabilize the assemblies of N-ethyl-N%-tetradecylviologen (1) and N-ethyl-N%-dodecylviologen (2) on the electrode surface. The redox behavior of 1 and 2 on the bare Au electrode was unstable because of the weak adsorption of their assemblies on the Au electrode surface. On the other hand, both 1 and 2 are strongly confined and compact in the SAM domain of alkanethiol and showed a stable redox response. The compactness of the assemblies of 1 and 2 on alkanethiol-coated electrodes can be understood from its blocking effect on the redox reaction of [Ru(NH3)6]2 + /3 + and EQCM measurements. EQCM measurements demonstrated that the ingress of water molecules into the assembly of 1 and 2 was markedly suppressed at the assembly of 1 and 2 bound to alkanethiol-coated electrodes. Furthermore, the redox behavior of 1 and 2 on the bare and alkanethiols-coated electrodes was also studied in the presence of − − weakly hydrated anions of ClO− 4 and PF6 . For the ClO4 ion, an additional oxidation peak was observed for the first reduction of 1 and 2 along with a usual redox wave. While, in the presence of PF− 6 ions, the electrochemical behavior was largely dependent on the alkyl chain length of thiol. Two redox waves were observed on short chain thiol (n= 3)-coated electrode whereas a single redox wave was observed on the electrodes coated with alkanethiols of n=5, 7 and 9. An irreversible redox response was observed on the longer alkyl chain (n\ 9) thiol-coated electrodes. The observed irreversible response has been attributed to the effective blocking of water molecules at the assembly of 1 and 2 on the long chain alkanethiol-coated electrodes in the presence of weakly hydrated PF-6 ions. At these electrodes, 1 and 2 may form an insoluble salt with PF− 6 ion and are electroinactive. When the same electrodes were − − transferred to solutions of Cl− or ClO− or ClO− 4 ions, the PF6 ions could be readily exchanged with Cl 4 ions and 1 and 2 became electroactive. Such a situation may not arise at the assembly of 1 and 2 bound to short chain thiol (nB 9) coated-electrodes because of the free flow of water molecules into the disordered monolayers of short chain thiols. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Asymmetric viologen; n-Alkanethiols; Voltammetry; EQCM
1. Introduction
* Corresponding author. Tel.: +81-45-924-5404; fax: +8145-924-5489. E-mail address: [email protected] (T. Ohsaka)
Recently, we and others have reported the electrochemical behavior of asymmetric N-alkyl-N%-methyl (or ethyl) viologens with alkyl chains, the number (m) of carbons of which is 12 to 18, on Au and GC electrode surfaces [1 – 5]. It has been found that the viologens
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with alkyl chains of m=16 and 18 are relatively stable on the electrode surface and their redox reactions are very fast. However, the viologens of mB 16 have a weak tendency to self-assemble on the electrode surface and they are immediately lost from the electrode surface after transfer to the electrolyte solution [1,3,4]. Stabilization and excellent redox characteristics of such viologens on the electrode surface are primarily required in developing potential applications of the viologen-modified electrodes, for example, they could be very useful for electrical communication with dissolved redox enzymes having negative reduction potentials (e.g., nitrate reductase [6], cytochrome-c [7] and ferredoxins [8]). In this work, we used n-alkanethiols to stabilize the redox reaction of short chain viologens such as N-ethyl-N%-tetradecylviologen (1) and N-ethylN%-dodecylviologen (2) on the electrode surface. Recently two reports have been published concerning the co-adsorption of electroinactive long chain alkaenthiols to stabilize the viologen moieties covalently attached to the electrode surface [9,10]. In this paper, we wish to report the stable redox behavior of 1 and 2 bound to the self-assembled monolayers (SAMs) of n-alkanethiols (CH3(CH2)n SH, n=3–17) on an Au electrode. In addition, we also report the electrochemical behavior of 1 and 2 in the presence of weakly hydrated ions of − ClO− 4 and PF6 .
2. Experimental The polycrystalline Au electrode (1 or 2 mm diameter) surface was polished to a mirror finish with fine emery paper (Sankyo Rikagaku, Japan) and then aqueous slurries of successively finer alumina powder (1 and 0.06 mm) on a polishing microcloth, sonicated for 10 min in water. The Au electrode was then electropolished by potential cycling in 0.05 M H2SO4 in the potential range of −0.2 to +1.5 V at the potential scan rate of 100 mV s − 1 for 20 min or until the CV characteristic for a clean Au electrode was obtained. Such an electrode was considered as a bare Au electrode. The real surface area was calculated from the charge required to reduce the surface oxide layer using the previously established formula, 0.43 mC cm − 2 [11]. The surface roughness was calculated as the ratio of the real surface area to the geometric area. The surface roughness of the electrode was found to be 1.2. The asymmetric viologen (bromide salt), N-ethyl-N’tetradecylviologen (1) and N-ethyl-N%-dodecylviologen (2) were synthesized and characterized by the reported
procedure [4]. Octadecanethiol (Aldrich) and other alkanethiols (Tokyo Kasei Kogyo and Wako Chemicals) were used as received. The SAMs of 1 and 2 were prepared by immersing the clean Au electrode in 100 – 200 mM aqueous solution of 1 or 2 for 30 – 90 min. Then the electrode was gently rinsed with water and used for electrochemical experiments. The alkanethiol monolayers were formed by immersing the Au electrode into a 1 – 10 mM ethanol solution of the respective alkanethiol for 1 – 6 h. The electrode was then washed with ethanol, water and then dried. The electrode was then immersed in an aqueous solution of 1 or 2 for 30 – 90 min and rinsed with water and transferred to the supporting electrolyte for electrochemical measurements. All electrochemical experiments were performed at room temperature using a standard three-electrode, two compartment configuration with a Au working electrode, a spiral platinum counter electrode and a NaClsaturated Ag/AgCl reference electrode. The cyclic voltammetric experiments were carried out with a computer-controlled electrochemical system (BAS 100 B/ W). The surface coverage (G/mol cm − 2) was estimated by graphically integrating the cyclic voltammograms recorded at various scan rates. All solutions were thoroughly deoxygenated by purging with nitrogen gas, and during the electrochemical experiments a nitrogen atmosphere was maintained above the solution. The EQCM principle and the instrumentation have been well established [12]. AT-cut quartz crystals of nominal fundamental frequency of 6 MHz were used in the present study. Signal averaging was employed to allow for the facile detection of frequency changes as small as 0.1 Hz. The Au quartz crystals were purchased from Hokuto Denko (Japan). The electrochemically active area of the Au electrode was 1.33 cm2. The surface roughness of the electrode was determined to be 1.2 by the gold oxide method [11]. Since both the electrochemistry and EQCM measurements were done on the same electrode, the Gs reported in the EQCM measurements do not take this roughness into account so as to allow direct comparison of G and frequency changes (which are influenced identically by surface roughness). The signal from the oscillator was sent to a Hokuto Denko, ECQCM Controller HQ-101B. A potentiostat (PS-07, Toho Tech.) and an X-Y-Y% recorder (Graphtech) were used to record the EQCM response.
3. Results and discussion
3.1. Stability of 1 and 2 on bare and SAMs of alkanethiol-coated Au electrodes Fig. 1 shows the CVs obtained for the SAM of 1 (G 1.8× 10 − 10 mol cm − 2) on the Au electrode in the
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presence of 0.1 M of KCl at different time intervals. It shows a broad redox wave at −0.44 V corresponding to the one-electron reduction of 1, to the radical cation and its oxidation to 1. The CVs recorded after 5 and 10
Fig. 1. CVs obtained for the SAM of 1 on the bare Au electrode in 0.1 M KCl. Voltammogram (a) was recorded after the electrode was immersed in the electrolyte solution, and voltammograms (b) and (c) were recorded after 5 and 10 min immersion, respectively. Scan rate = 0.2 V s − 1.
Fig. 2. CVs obtained for the assembly of 1 on the BT-coated electrode in 0.1 M KCl at scan rates of 1–10 V s − 1 (G= 3.5× 10 − 10 mol cm − 2). Inset: plot of cathodic peak current versus the scan rate.
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min immersion in 0.1 M KCl showed a decrease in the G of 1 (Fig. 1(b,c)). The decrease in the G of 1 can be understood from the weak adsorption of 1 on the electrode surface because of the presence of a short alkyl tail in 1 when compared to the viologens containing a long alkyl tail [1 – 5]. Similar electrochemical behavior was also observed for the assembly of 2 on the Au electrode. Fig. 2 shows the CVs obtained for the assembly of 1 on butanethiol (BT, n= 3)-coated electrode at scan rates of 1 – 10 V s − 1. Several significant features were observed for the assembly of 1 on the BT-coated electrode. It shows a broad redox wave at −0.40 V with a peak separation of 10 mV at a scan rate of 1 V s − 1. The CV behavior is more symmetric and sharper when compared to the assembly of 1 on the bare Au electrode (see Fig. 1). In addition, the double layer capacitance (Cdl) calculated for the assembly of 1 on the BT-coated electrode is smaller than that calculated for the assembly of 1 on the bare Au electrode, e.g. the Cdl values were ca. 9 and 27 mF cm − 2 at 0 V for the former and the latter, respectively. The observed small Cdl indicates that the assembly of 1 is more compact on the BTcoated electrode when compared to the bare Au electrode. Moreover, the G value (G = 3.5×10 − 10 mol cm − 2) calculated for the assembly of 1 on the BTcoated electrode is higher than that on the Au electrodes. Most of the assembly of 1 was lost from the Au electrode surface during the rinsing with water (G for the assembly of 1 on the Au electrode is 1.8× 10 − 10 mol cm − 2 (Fig. 1)). However, the assembly of 1 was more stable on the BT-coated electrode even during rinsing and thus it showed a higher G. The small potential difference between the cathodic and anodic peak potentials (10 – 25 mV) and the linearity of the plot of peak current versus scan rate (the inset of Fig. 2) clearly indicate that the redox response of 1 on the BT-coated electrode is due to the surface-confined species [13]. The observed fast reversible redox reaction of 1 even at a high scan rate of 10 V s − 1 indicates that the redox moiety is assembled close to the electrode surface and not to the exterior of the alkanethiol monolayer [10]. The assembly of 1 on the BT-coated electrode was highly stable, as readily seen from the comparison of the CVs obtained before and after the electrolysis at a series of potential scans of 1 – 10 V s − 1 (Fig. 3): a 15 mV positive shift in the redox peak potential was observed, but the G was unchanged. Similar CV behavior was also observed for the assembly of 2 on the BT-coated electrode in 0.1 M KCl. The stability of the assemblies of 1 and 2 on the alkanethiol-coated electrodes is also understood from their close packing on the alkanethiol-coated electrodes. This was examined by their blocking effect on the electrochemical behavior of diffusing solution-species. The CVs obtained for Ru(NH3)6Cl3 on the clean Au
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[Ru(NH3)6]3 + is significantly affected (Fig. 4(b)). The shape of the CV indicates that the reduction of [Ru(NH3)6]3 + is mediated by 2 on the DDT-coated electrode [14]. The observed blocking effect for [Ru(NH3)6]3 + by the assembly of 2 on the DDT-coated electrode demonstrates the close packing of the assembly of 2 on the DDT-coated Au electrode. Similar experiments were also conducted at the assembly of 2 on the bare Au electrode. In this case, the redox reaction of [Ru(NH3)6]3 + was unaffected (not shown). This indicates that the assembly of 2 is less compact on the bare Au electrode than on the alkanethiol-coated electrode.
3.2. Electrochemical beha6ior of 1 and 2 on bare and alkanethiol-coated Au electrodes in the presence of − hydrophobic anions of ClO− 4 and PF 6
Fig. 3. CVs obtained for the assembly of 1 on the BT-coated electrode in 0.1 M KCl (a) before and (b) after recording the CVs of Fig. 2. Scan rate = 0.2 V s − 1.
Fig. 5 shows the CV and EQCM curve obtained for the assembly of 2 (3.7×10 − 10 mol cm − 2) on the Au electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Since the assembly of 2 was less stable on the Au electrode, the experiment was carried out in the presence of 30 mM of 2 in solution. The assembly of 2 shows one reduction peak and two sharp oxidation peaks for the first redox reaction of 2, whereas only a single broad redox wave was observed for the assembly of 2 under identical conditions except for Cl− as electrolyte anion (data not shown). In addition, the value for G calculated in the presence of ClO− 4 is two times higher than that calculated in the presence of Cl−. The solubility of 2 is comparatively low in the presence of
Fig. 4. CVs obtained for (a) the bare Au electrode and (b) the assembly of 2 on the DDT-coated Au electrode in 0.1 M KCl solution containing 0.5 mM Ru(NH3)6Cl3 and (c) the assembly of 2 on the DDT-coated electrode in 0.1 M KCl. Scan rate=0.2 V s − 1.
electrode and the assembly of 2 bound to the dodecanethiol (DDT, n=11)-coated Au electrode (G= 2.5× 10 − 10 mol cm − 2) are shown in Fig. 4. A well-defined diffusion-controlled redox reaction for Ru(NH3)26 + /3 + was observed on the bare Au electrode (Fig. 4(a)). However, on the DDT-coated electrode to which the assembly of 2 was bound, the diffusion of
Fig. 5. CV and EQCM curve obtained for the assembly of 2 on the Au quartz electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Scan rate = 0.025 V s − 1. G= 3.7 ×10 − 10 mol cm − 2.
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Fig. 6. CV and EQCM curves obtained for the assembly of 2 on the ODT-coated Au quartz electrode in 0.1 M NaClO4 solution containing 30 mM of 2. Scan rate= 0.1 V s − 1. G= 2.0× 10 − 10 mol cm − 2. − and therefore ClO− 4 when compared to that in Cl − ClO4 effectively anchors the assembly of 2 on the electrode surface, which led to a high value for G in the presence of ClO− 4 [5]. Unfortunately, we were unable to carry out the similar experiment in the presence of PF− 6 because of the precipitation of 2 with PF− ions in 6 solution. In this case, for 2 at a concentration of 30 mM, the electrochemical response of the molecule diffusing to the electrode surface from the bulk of the solution is negligible. Recently, two pairs of redox peaks for the first redox reaction of long alkyl chain viologen, C18MV2 + , have been reported on the GC electrode in the presence of ClO− 4 ion, but only one pair in Cl− [5]. In this case, it has been proposed that ClO− 4 significantly increases the order of the film compared with Cl− and splits the redox wave [5]. Similar ordered arrangement is also expected at high values of G for 2 in the presence of ClO− 4 . A total frequency change of 14 Hz was observed for the first reduction of the assembly of 2, which corresponds to a loss of mass of 928 g mol − 1 from the electrode surface. The EQCM curve reveals that mass is lost during the reduction of 2 and this mass loss is reversed during the reoxidation (Fig. 5). These frequency changes have been previously interpreted as indicative of anion loss (or gain) from the monolayer, which is a consequence to the injection of electrons [12]. The mass changes were calculated by using the Sauerbrey equation (see Eq. (1)) [15].
Df= − Cfm
(1)
where Df is the frequency change, m is the mass per cm2 of the assembly of 2 and Cf is the proportionality constant. In the calculation of the amount of water transported per anion, it is assumed that injection of each electron into the monolayer to reduce one 2 causes the loss of one anionic charge (one anion for the monovalent anions). We found that ca. 42 water molecules are lost from the monolayer for one ClO− 4 ion lost during the reduction and this compositional change is reversible on the CV time scale. Donohue and
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Buttry have studied the CV and EQCM behavior of surfactants of ferrocene with chain lengths of C12 and C14 on the Au electrode [16]. They found that the oxidized form of the ferrocene surfactant was desorbed from the electrode surface, which was inferred from the EQCM measurements. It was calculated that only 30% of the mass were regained during the reverse scan. On the other hand, the mass lost in the reduction, in the present case, was completely regained in the reoxidation (Fig. 5). Thus the adsorption of 2 on the Au electrode surface is strong and 2 did not desorb from the electrode surface into the 30 mM solution of 2. Recently, we have studied the EQCM behavior of C18EV2 + on the Au electrode surface in the presence of different supporting electrolytes [1]. According to this, the loss of water molecules from the assembly of 2 was more than four times that from the C18EV2 + assembly in the presence of ClO− 4 . The observed loss of a higher number of water molecules from the assembly of 2 shows that 2 forms a more loosely organized assembly on the electrode surface than C18EV2 + . In the case of Cl−, more than ca. 50 water molecules are lost from the assembly of 2 during the reduction. We have also studied the CV and EQCM behavior of 1 in the presence of Cl− and ClO− 4 . It was found that ca. 34 and 42 water molecules are lost from the monolayer in the − presence of ClO− 4 and Cl , respectively. CV and EQCM curves obtained for the assembly of 2 (G =2.0× 10 − 10 mol cm − 2) on the octadecanethiol (ODT)-coated electrode (prepared under the same conditions as on the Au electrode) in 0.1 M NaClO4 are shown in Fig. 6. On the ODT-coated electrode, the assembly of 2 shows only a single redox peak, in contrast to the split anodic peak on the Au electrode. The double layer capacitance (Cdl) calculated for the assembly of 2 on the ODT-coated electrode is less than that calculated at the assembly of 2 on the Au electrode, indicating that the assembly of 2 is more compact on the ODT-coated electrode than on the Au electrode. Since the monolayer of ODT is highly compact and dense, the adsorption of 2 into the ODTcoated electrode is less and showed a less G than that (3.7× 10 − 10 mol cm − 2) for the assembly of 2 on the Au electrode. A total frequency change of 2 Hz was observed for the assembly of 2 on the ODT-coated electrode, which corresponds to a loss of mass of 245 g mol − 1 from the electrode surface. Thus, approximately eight water molecules are lost from the monolayer for one ClO− 4 ion lost during the reduction and this compositional change is reversible on the CV time scale. The observed loss of a much lower number of water molecules from the assembly of 2 on the ODT-coated electrode, compared with that on the Au electrode indicates that the assembly of 2 on the ODT-coated electrode is more compact than on the Au electrode. In the presence of Cl−, approximately 20 water molecules
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are lost from the assembly of 2 on the ODT-coated electrode upon the reduction. On the other hand, six and 17 water molecules are lost from the assembly of 1 on the ODT-coated electrode during the reduction in − the presence of ClO− 4 and Cl , respectively. Fig. 7(a) shows the CVs obtained for the assembly of 2 on the BT-coated electrode in the presence of 0.1 M NH4PF6. In the reduction process, two cathodic peaks were observed at −0.50 and −0.53 V. In the oxidation process two peaks were also observed at − 0.46 and −0.43 V. On the other hand, the assembly of 2 showed a single redox wave in 0.1 M KCl and a single redox wave with an additional oxidation peak in 0.1 M NaClO4. The observed electrochemical behavior was stable even when the electrode was immersed in 0.1 M NH4PF6 for an extended period of time (e.g. 2 h). A very sharp redox peak was observed for the assembly of 2 on the hexanethiol (HT)-coated electrode in the presence of 0.1 M NH4PF6 (vide infra). However, in the case of octanethiol (OT)- and decanethiol (DT)-coated electrodes only a single redox wave was observed, and an irreversible response was obtained for the electrodes coated with alkanethiols of n=11–17. The CV obtained for the assembly of 2 on the dodecanethiol (DDT)-coated electrode in 0.1 M NH4PF6 is shown in Fig. 7(b). It shows an irreversible redox response, in contrast to the multiple redox behavior observed on the BT-coated electrode. When the electrode used in NH4PF6 solution was transferred to 0.1 M KCl, a clear reversible redox response was observed (Fig. 7(c)). The observed irreversible response of 2 in PF− 6 solution may
Fig. 7. CVs obtained for the assembly of 2 on (a) BT- and (b) DDT-coated electrodes in 0.1 M NH4PF6 and (c) on the same electrode as used in (b) after transferal to 0.1 M KCl. Scan rate= 0.1 V s − 1.
be attributed to the formation of an electroinactive, insoluble salt of 2 with PF− 6 on the electrode surface. It is well-known that n-alkanethiols of n\9 form a more densely packed crystalline-like assembly on Au electrodes than alkanethiols of n B9 [17]. Thus at the assembly of 2 on the DDT-coated electrode, unlike on the BT-coated electrode, water molecules are effectively blocked in the presence of PF− 6 ions. On transferring the same electrode in Cl− solution, PF− 6 ions of the insoluble salt were exchanged with Cl− ions and the assembly of 2 became electroactive. Similar electrochemical behavior was also observed for the assembly of 1 in the presence of PF− 6 ions.
3.3. Electrochemical stability of alkanethiol monolayers The potential window for the n-alkanethiols and functionalized alkanethiol monolayers on different electrode surfaces has been studied by several groups [18 – 20]. Cheng and Brajter-Toth [18] reported that n-hexanethiol monolayers were stable between −0.8 and 1.0 V versus saturated calomel electrode (SCE) even at acidic pH. On the other hand, based on a series of continuous potential cycling experiments at different potential windows, Beulen et al. [20] have shown that the monolayers of decanethiol and functionalized alkanethiols are stable between −0.8 and 0.4 V versus mercurous sulfate reference electrode (MSE) ( − 0.4 – 0.8 V versus Ag/AgCl). We have studied the stability of n-alkanethiol monolayers in the potential region of 0 to −0.65 V, where the redox reactions of 1 and 2 occur. The continuous CVs obtained for the assembly of 2 on the n-HT-coated Au electrode in 0.1 M NH4PF6 after
Fig. 8. Continuous CVs obtained for the assembly of 2 on HT-coated electrode in 0.1 M NH4PF6 after (a) 10, (b) 20, (c) 40 and (d) 50 cycles. Scan rate=0.1 V s − 1.
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10, 20 40 and 50 cycles are shown in Fig. 8. When the CV was continuously recorded, both the reduction and oxidation peak currents of 2 increased up to 20 cycles and after which no increase in the peak currents was observed. In addition, the double layer capacitance (Cdl), e.g. at 0 V remains unchanged even after 50 continuous potential cycling, which indicates that the monolayers of n-HT are highly stable in the potential region of 0 to −0.65 V. Similarly n-BT (n= 3)-monolayers are also highly stable after a series of potential scans between 0 and −0.65 V (Fig. 3).
4. Conclusions The present work demonstrates that alkanethiolcoated electrodes can effectively stabilize the short alkyl chain asymmetric viologens of 1 and 2 by their unique property of blocking water molecules, that is, the viologens are strongly confined in the SAM domain of alkanethiols on electrodes. The assemblies of 1 and 2 are less organized on the Au electrode and thus water molecules freely enter them, resulting in their desorption from the electrode surface. On the other hand, the assemblies of 1 and 2 are more compact on the alkanethiol-coated Au electrodes than on the bare Au electrode which is inferred from their blocking effect on the redox reaction of solution-species [Ru(NH3)6]2 + /3 + and EQCM measurements. The electrochemical behavior of 1 and 2 has been significantly affected by the presence of PF− when compared to ClO− 6 4 . In the − presence of PF6 ions, the assemblies of both 1 and 2 on the short alkyl chain thiol-coated electrode (e.g., BTcoated electrode) showed two pairs of redox peaks for the first redox reaction, whereas in the presence of ClO− 4 only an additional oxidation peak was observed in addition to the single redox wave. The observed behavior has been explained by the different interac− tions of ClO− 4 and PF6 with 1 and 2.
Acknowledgements The present work was financially supported by Grant-in Aids for Scientific Research in Priority Areas,
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‘New Polymers and Their Nano-Organized Systems’ (No.277/10126219), ‘Exploratory Research’ (No.09875207) and ‘Scientific Research (A)’ (No.10305064) from the Ministry of Education, Sciences, Sports and Culture, Japan (Monbusho). We thank Professor K. Tokuda for helpful discussions and comments. S.A.J. thanks the Monbusho for the fellowship. References [1] S.A. John, T. Okajima, T. Ohsaka, J. Electroanal. Chem. 466 (1999) 67. [2] C.-W. Lee, A.J. Bard, J. Electroanal. Chem. 239 (1988) 441. [3] C.A. Widrig, M. Majda, Langmuir 5 (1989) 689. [4] M. Gomez, J. Li, A.E. Kaifer, Langmuir 7 (1991) 1797. [5] C.M. Judkins, E.W. Bohnnan, A.K. Herbig, J.A. Powers, D.A. van Galen, J. Electroanal. Chem. 451 (1998) 39. [6] R.B. Mellor, J. Ronnengerg, W.H. Campbell, S. Diekmann, Nature 355 (1992) 717. [7] W.R. Heineman, T. Kuwana, C.R. Hartzell, Biochem. Biophys. Res. Commun. 50 (1973) 892. [8] S. Cosnier, C. Innocent, Y. Jouanneau, Anal. Chem. 66 (1994) 3198. [9] B. Lee, Langmuir 8 (1992) 2491. [10] E. Katz, N. Itzhak, I. Willner, Langmuir 9 (1993) 1392. [11] E. Gileadi, K.E. Eisner, J. Penciner, Interfacial Electrochemistry — An Experimental Approach, AddisonWesley, Reading, MA, 1975. [12] D.A. Buttry, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 17, Marcel Dekker, New York, 1991, pp. 1 – 85. [13] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980 (Chapter 12). [14] S.E. Creager, D.M. Collard, M.A. Fox, Langmuir 6 (1990) 1617. [15] G.Z. Sauerbrey, Z. Phys. Chem. 155 (1959) 206. [16] J.J. Donohue, D.A. Buttry, Langmuir 5 (1989) 671. [17] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [18] Q. Cheng, A. Brajter-Toth, Anal. Chem. 67 (1995) 2767. [19] M.H. Schoenfisch, J.E. Pemberton, Langmuir 15 (1999) 509. [20] M.W.J. Beulen, M.I. Kastenberg, F.C.J.M. van Veggel, D.N. Reinhoudt, Langmuir 14 (1998) 7463.