Langmuir 2003, 19, 4043-4046
Ferrocene-Containing Polyelectrolyte Multilayer Films: Effects of Electrochemically Inactive Surface Layers on the Redox Properties Aihua Liu and Jun-ichi Anzai* Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan Received December 18, 2002. In Final Form: February 18, 2003
1. Introduction Since Decher and co-workers first introduced a layerby-layer deposition technique to build up a polyelectrolyte multilayer (PEM) film, much attention has been devoted to the development of functional PEM films.1 This approach has been successfully extended to the preparation of composite films containing nucleic acids,2 proteins,3 dyes,4 virus,5 and so forth. Recently, redox-active PEM films have also been developed using polyelectrolytes containing redox sites such as viologen,6 poly(thiophene),7 ferrocene (Fc),8 and osmium bipyridyl complex (Os-bpy).9 It has been reported that the redox-active PEM film-coated electrodes exhibit an electrochemical response depending on the architecture and the number of layers (or the loading of the redox polymer) in the film. For example, Hodak et al. found that the redox property of Fc-modified poly(allylamine) (Fc-PAH) PEM films depended significantly on the number of layers.8 We have also prepared a series of PEM films by a layer-by-layer deposition of Fc-PAH and polyanionic poly(vinyl sulfate) (PVS) on the surface of a gold (Au) electrode and studied their redox properties. A cyclic voltammetric study revealed that the redox * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Decher, G.; Hong, J.-H. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (b) Decher, G. Science (Washington, DC) 1997, 277, 1232. (2) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (3) (a) Decher, G.; Lehr, B.; Loack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677. (b) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (c) Willner, I.; Lion-Dagan, M.; Marx-Tibbon, S.; Katz, E. J. Am. Chem. Soc. 1995, 117, 6581. (d) Riklin, A.; Willner, I. Anal. Chem. 1995, 67, 4118. (e) Shoham, B.; Migron, Y.; Riklin, A.; Willner, I.; Tartakovsky, B. Biosens. Bioelectron. 1995, 10, 341. (f) Willner, I.; Katz, E.; Willner, B. Electroanalysis 1997, 9, 965. (g) Zu, X.; Lu, Z.; Zhang, Z.; Schenkman, J. B.; Rusling, J. F. Langmuir 1999, 15, 7372. (h) Anzai, J.; Hoshi, T.; Nakamura, N. Langmuir 2000, 16, 6306. (i) Anzai, J.; Akase, S. Macromol. Biosci. 2002, 2, 361. (4) (a) Cooper, T.; Campbell, A.; Crane, R. Langmuir 1995, 11, 2713. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2244. (c) Yoo, D.; Wu, A.; Lee, J.; Rubner, M. F. Synth. Met. 1997, 85, 1425. (d) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193. (e) Tadeschi, C.; Caruso, F.; Mu¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (f) Suzuki, I.; Ishizaki, T.; Hoshi, T.; Anzai, J. Macromolecules 2002, 35, 577. (g) Suzuki, I.; Ishizaki, T.; Inoue, H.; Anzai, J. Macromolecules 2002, 35, 6470. (5) Lvov, Y.; Haas, J.; Decher, G.; Mu¨hwald, H.; Mikhailov, A.; Mtchedlishivily, B.; Morgunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (6) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (7) Lukkari, J.; Salome`ki, M.; Viinikanoja, A.; C¸ a˚ritalo, T.; Paukkunen, J.; Kochanova, N.; Kankare, J. J. Am. Chem. Soc. 2001, 123, 6083. (8) Hodak, J.; Etchenique, R.; Calvo, E. J. Langmuir 1997, 13, 2708. (9) (a) Narvaez, A.; Suc¸ rez, G.; Catalin, I.; Popescu, C.; Katakis, I.; Domı´nguez, E. Biosens. Bioelectron. 2000, 15, 43. (b) Zichen, W.-L.; Wang, Z.; Sun, C.; Xian, M.; Zhao, M. Anal. Chim. Acta 2000, 418, 225. (c) Calvo, E. J.; Pietrasanta, L.; Wolosiuk, A.; Danilowsz, C. Anal. Chem. 2001, 73, 1161. (d) Calvo, E. J.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 8490.
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response of the films is enhanced with increasing Fc content in the polymer chains and increasing number of layers.10 On the other hand, it may also be interesting to evaluate the effects of electrochemically inactive layers built in the Fc PEM film on the redox properties. To evaluate this, an Au electrode was first covered with redox-active Fc-PAH/ PVS multilayers, on which PEM films composed of electrochemically inert layers were deposited. We report here that the redox behavior of the Fc-PAH/PVS film is sensitive to the polarity of the electric charges on the outermost surface of the inert film; the redox current of Fc-PAH/PVS film-coated electrodes increased upon deposition of polycation on the surface and decreased by depositing polyanion. The redox potentials also shifted positively and negatively upon adding cationic and anionic polymers on the surface, respectively. The effects of pH and ionic strength on these phenomena were also studied systematically. To our knowledge, this is the first report describing the effects of electrochemically inert layers on the redox properties of PEM films. 2. Experimental Section Materials. An aqueous solution (20%) of poly(allylamine) [PAH; average molecular weight (MW) 10 000] and a 30% aqueous solution of poly(ethylenimine) (PEI; MW 60 000-80 000) were purchased from Nittobo Co. (Tokyo, Japan) and Nakalai Tesque Co. (Kyoto, Japan). Poly(potassium vinyl sulfate) (PVS; MW 242 000) and poly(dimethyldiallylammonium chloride) (PDDA; MW 100 000-200 000) were obtained from Aldrich Chemical Co. (Milwaukee, WI) and Nakalai Tesque Co. (Kyoto, Japan), respectively. Sodium 3-mercapto-1-propanesulfonate (MPS) was obtained from Tokyo Kasei Co. Ferrocene-appended poly(allylamine) (Fc-PAH) was synthesized as reported.8,10 The content of Fc residues in the Fc-PAH was 4 mol % (molar ratio of Fc to amine groups), as determined by UV-visible absorption at 251 nm using a molar extinction coefficient of 4.5 × 103 M-1 cm-1 for (ferrocenylmethyl)dimethylamine as a model compound in water. The chemical structure of Fc-PAH is illustrated in Figure 1. All the materials were of the highest grade available and were used as received. All solutions were prepared in Milli-Q water. Pretreatment of Au Disk Electrode. A commercially available polycrystalline Au disk electrode (diameter: 1.6 mm) was used. Prior to the Au electrode being coated with multilayer films, the surface was first polished with aqueous slurries of successively finer alumina paste and then sonicated thoroughly in water. A clean surface of the electrodes was obtained by a potential sweep from -0.2 to 1.7 V (vs Ag/AgCl) in 0.5 M H2SO4 at a scan rate of 10 V s-1 for 15 min. Assembly of PEM Films on an MPS-Modified Au Electrode. The Au electrode was dipped in a freshly prepared aqueous MPS solution (10 mM) overnight; then it was washed thoroughly with water. After this procedure, the Au surface should be negatively charged. The negatively charged surface was modified with PEM film by dipping it in a 2 mg mL-1 Fc-PAH solution and in a 2 mg mL-1 PVS solution for 15 min, with an intermediate 5 min rinse in pure water. Five or ten bilayers of (Fc-PAH/PVS) film were thus deposited by repeating the above procedure. The thickness of each Fc-PAH/PVS layer is estimated to be ∼4.2 nm from a quartz crystal microbalance study.11 The (Fc-PAH/PVS)5 or (Fc-PAH/PVS)10 film-covered electrode was then immersed in a 2 mg mL-1 PAH solution for 15 min and, after rinsing for 5 min in pure water, in a 2 mg mL-1 PVS solution for 15 min, successively. All the polymeric materials were dissolved in a solution containing 10 mM 2-hydroxyethylpiperidine-N′-2ethanesulfonic acid and 100 mM NaCl (pH 5.0). (10) Liu, A.; Kashiwagi, Y.; Anzai, J. Electroanalysis, in press. (11) The thickness of the film was estimated by assuming the density of the film to be ∼1.2 g cm-3 according to the literature: Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf. 1999, 146, 337.
10.1021/la0209740 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/22/2003
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Langmuir, Vol. 19, No. 9, 2003
Notes
Figure 1. Chemical structure of Fc-PAH. Electrochemical Measurements. Electrochemical response was measured in a glass cell using a PEM-modified Au electrode as working electrode, a platinum wire as counter electrode, and a Ag/AgCl electrode (3.3 M KCl) as reference electrode. All experiments were performed at room temperature (∼20 °C). Preparation of Multilayer Films on a Quartz Slide. The film was prepared on a quartz slide to estimate the loading of the Fc moiety in the film by means of UV-visible absorption spectroscopy. The surface of a quartz slide (50 × 10 × 1 mm3) was cleaned with a mixture of sulfuric acid and chromic acid. The quartz slide was immersed in dichlorodimethylsilane (10% solution in toluene) overnight at room temperature to make the surface hydrophobic and sonicated successively in toluene, acetone, and distilled water before use. The Fc-PAH/PVS multilayer film was deposited on the slide precoated with a PEI/ PVS film in a similar manner. The UV-visible absorption spectrum was measured using a Shimadzu UV3100 spectrophotometer (Kyoto, Japan). Gravimetric Measurement with a QCM. A quartz crystal microbalance (QCM) (QCA 917 system, Seiko EG&G, Tokyo, Japan) was employed to monitor the formation of multilayer films. A 9 MHz At-cut quartz resonator coated with a thin platinum layer was used as a probe, on which the adsorption of 1 ng of substance induces a -0.91 Hz change in the resonance frequency.12 The surface of the quartz resonator was rinsed thoroughly with water before use. PEM films were deposited on the quartz resonator in a similar manner as in the formation of PEM films on an Au electrode. The probe was dried in air until the frequency showed a steady-state value. All data were obtained in air for the dry films.
3. Results and Discussion Redox Properties of Fc-PAH/PVS Films. Redoxactive PEM films such as Fc-PAH/protein, (Os-bpy)-PAH/ PVS, and (Os-bpy)-poly(vinylpyridine)/protein were prepared on the surface of an electrode, and their redox properties have been studied in relation to the layered structure of the films.8,9 The redox PEM film-modified electrodes exhibited two characteristic features in the cyclic voltammogram (CV): (1) the peak current increased with increasing number of redox layers in the film, and (2) the peak current and redox potential varied systematically depending on the polarity of the electric charge on the outermost layer of the films. We checked here whether this is also the case for our Fc-PAH/PVS films, and we found that the Fc-PAH/PVS film-coated electrodes exhibited basically the same trend in the CV as that for the reported films. In fact, the peak current for the (FcPAH/PVS)n-1Fc-PAH films, in which the outermost surface was covered with polycationic Fc-PAH, was always higher than those for (Fc-PAH/PVS)n films which were terminated with polyanionic PVS, although the contents of Fc-PAH in the films were identical to each other. The peak current increased with increasing number of Fc-PAH layers for both the (Fc-PAH/PVS)n and (Fc-PAH/PVS)n-1Fc-PAH films. On the other hand, the redox potential shifted positively upon addition of an Fc-PAH layer, while the addition of a PVS layer induced a negative shift (data not shown). (12) Sauerbrey, G. Z. Phys. 1959, 155, 206.
Figure 2. Cyclic voltammograms of (Fc-PAH/PVS)5 (a), (FcPAH/PVS)5PAH (b), and (Fc-PAH/PVS)5(PAH/PVS) film-modified Au electrodes (c) in 10 mM acetate buffer (pH 5.0). Scan rate: 50 mV s-1.
Effects of Surface Layers on the Redox Properties. Thus far, the redox properties have been studied mainly using PEM films containing redox sites in every bilayer, in which the redox sites are distributed throughout the film.8,9 On the other hand, some researchers employed hetero-PEM films composed of a redox-inactive inner layer and an active outer layer, to evaluate insulating effects of the inner layer which is sandwiched between the redoxactive layer and the electrode.6,7 It is also interesting to evaluate the effects of redoxinactive layers deposited on the surface of (Fc-PAH/PVS)n films. In the present study, we deposited PAH and PVS layers on the surface of the (Fc-PAH/PVS)5 film-coated electrode, and the effects of the PAH and PVS layers were evaluated. Figure 2 shows CVs for Au electrodes coated with the (Fc-PAH/PVS)5, (Fc-PAH/PVS)5PAH, and (FcPAH/PVS)5(PAH/PVS) films. The (Fc-PAH/PVS)5 filmcoated electrode exhibited oxidation and reduction peaks at 0.575 and 0.488 V, respectively, which are arising from redox reactions of the Fc moiety in the film.8,10 The peak current increased significantly by the addition of a PAH layer on the surface of the (Fc-PAH/PVS)5 film, although the loading of Fc residues in the film did not change before and after adding the outer PAH layer. The loading of Fc residues was (1.35 ( 0.07) × 10-9 mol cm-2 for both the (Fc-PAH/PVS)5 and (Fc-PAH/PVS)5PAH films, which was estimated separately by means of UV absorption of the Fc residues in the films prepared on the quartz slide. The deposition of the next PVS layer induced a decrease in the peak current and broadening of the peak, suggesting the suppressed electron transfer in the film. The loading of Fc did not change upon deposition of the PVS. The higher response of the PAH-terminated film may be attributed in part to the facilitated anion transport through the film.13 The redox peak potentials also shifted positively and negatively upon depositing the polycation and polyanion, respectively. Other polycations such as PEI and PDDA also exhibited similar effects on the redox property of the (Fc-PAH/PVS)5 film, confirming a decisive role of electric charges as an origin of the effects. The effects of the PAH and PVS outer layers on the CVs of (Fc-PAH/PVS)5 and (Fc-PAH/PVS)10 film-coated electrodes are collected in Table 1. The full width at halfheight (fwhh) is slightly larger than the value expected for an ideal one-electron redox system (∼90.6 mV) for all cases, suggesting repulsive interactions between the redox (13) Pardo-Yissar, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110.
Notes
Langmuir, Vol. 19, No. 9, 2003 4045 Table 1. Cyclic Voltammetry Parameters for Fc-Containing PEM Filmsa PEM film
Epb/ V
(Fc-PAH/PVS)5 (Fc-PAH/PVS)5PAH (Fc-PAH/PVS)5PAH/PVS (Fc-PAH/PVS)10 (Fc-PAH/PVS)10PAH (Fc-PAH/PVS)10PAH/PVS
0.534 0.544 0.532 0.540 0.547 0.541
fwhhc/ 10-6Qd/ electroactive Fc V (C cm-2) in the filme (%) 0.127 0.107 0.122 0.118 0.114 0.116
51.9 80.3 40.4 125 128 101
40.0 61.5 31.2 56.1 57.8 45.2
a CV was measured in 10 mM acetate buffer (pH 5.0). Scan rate: 50 mV s-1. The average values of three preparations are listed (error in Q values: (10%). b Ep ) (Epa + Epc)/2. c The full width at half-height. d Electric charges obtained by the area under the anodic peak in CV. e These values were calculated on the basis of the Fc content in the films estimated from UV-visible absorption data (the Fc content was 1.35 × 10-9 mol cm-2 for the 5-bilayer films and 2.30 × 10-9 mol cm-2 for the 10-bilayer films).
sites.14 The local charge distributions around the Fc moiety in the film may be slightly different from each other, resulting in a dispersion of formal potentials and the rate of electron transfer. It is reasonable to assume that the charge distribution in the film can be rearranged through electrostatic force upon addition of the PAH or PVS layer. The amount of integrated redox charge (Q), estimated by the area under the peak in the CV, depends significantly on the type of polyelectrolyte adsorbed on the outermost surface. Comparison of the Q values with the total amount of Fc in the film, which was determined by UV-visible absorption, revealed that a significant portion of the Fc residues is not involved in the redox reaction under the present experimental conditions. The redox properties of a (Fc-PAH/PVS)10 film-coated electrode were also modulated by the outer PAH and PVS layers, though the effects were smaller than those on the thinner (Fc-PAH/PVS)5 film. These observations clearly suggest that the redox properties of the Fc moiety in the film strongly depend on the deposition of outer layers even if the outer layers contain no redox site. To evaluate the effects of outer (PAH/PVS)n layers on the redox properties, we deposited (PAH/PVS)n (n ) 2-5) layers further on the surface of the (Fc-PAH/PVS)5 film. The deposition of a second PAH layer induced again the enhancement of the peak current, though the peak was slightly smaller than that for the (Fc-PAH/PVS)5PAH film. The peak current was decreased again upon deposition of the second PVS layer. Figure 3a plots Q values of the film calculated from the CV as a function of the number of PAH and PVS layers. Figure 3b shows the apparent redox formal potentials (Ep) as a function of the layers. The electrostatic effects arising from the PAH and PVS may be responsible for the potential shifts. The deposition of polycationic PAH would induce instability of the oxidized form of Fc (or Fc+ ion), resulting in the positive shift of the redox potential of the film, and the addition of a PVS layer can cancel the positive charges of the PAH. Similar electrostatic effects have been reported; the redox potential of surface-confined Fc shifted under the influence of ions adsorbed in the vicinity of the Fc.15 It should be noted here that the effects are still observed even for the thicker (PAH/PVS)n films, implying that the electric charges on the outermost layer exert their effects on the (Fc-PAH/ PVS)5 film across the thicker (PAH/PVS)n layers. This is (14) (a) Bard, A.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (b) Nahir, T. M.; Bowden, E. F. J. Electroanal. Chem. 1996, 410, 9. (15) (a) Rowe, G. K.; Creger, S. E. Langmuir 1991, 7, 2307. (b) Beulen, M. W. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 503. (c) Casado, C. M.; Cuadrado, I.; Morc¸ n, M.; Alonso, B.; Garcı´a, B.; Gonzc¸ lez, B.; Losada, J. Coord. Chem. Rev. 1999, 185-186, 53.
Figure 3. Effects of the number of PAH and PVS layers on the amount of redox charges (top panel) and on the formal redox potential (bottom panel). The odd number and even number of layers mean the outermost surface is covered with PAH and PVS, respectively. The CVs were measured in 10 mM acetate buffer (pH 5.0). Scan rate: 50 mV s-1.
an unexpected result because the decay length of the electrostatic force is much thinner than the thickness of the outer (PAH/PVS)n layer;16 the thickness of the unit (PAH/PVS) layer is estimated to be ∼5.0 nm (in dry state) under the experimental conditions on the basis of the gravimetric measurements with a QCM.11 Another possible explanation is that outer PAH and PVS layers penetrate the (Fc-PAH/PVS)n layer to alter the conformation of Fc-PAH chains in the film, affecting the electron transfer with the electrode.2,17 In this context, Xie and Granick have reported that the dissociation equilibrium of poly(methacrylic acid) embedded within a PEM film depends significantly on the polarity of the strong polyelectrolyte deposited on the outermost surface of the film.18 In fact, the deposition of polycation facilitated ionization of the carboxylic acid and the original acid form was recovered upon deposition of the next polyanion layer. Such effects were observed even after five polycation/polyanion bilayers were deposited, showing that electrostatic changes can be induced far away within the film interior. The results were ascribed to an ionic solidlike property and the hydrophobic nature of the PEM film. They suggested usefulness of the system in designing a novel type of surface sensor sensitive to ions and polyions. (16) (a) Berkowitz, M. L.; Raghavan, K. In Biomembrane Electrochemistry; Blank, M., Vodyanoy, I., Eds.; American Chemical Society: Washington, DC, 1994; p 3. (b) Philpott, M. R.; Glosli, J. N. In SolidLiquid Electrochemical Interfaces; Jerkiewicz, G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; American Chemical Society: Washington, DC, 1995; p 13. (17) Lu¨sche, M.; Schmit, J.; Decher, G.; Bouwman, W. G.; Kjear, K.; Macromolecules 1998, 31, 8893. (18) Xie, A. F.; Granick, S. J. Am. Chem. Soc. 2001, 123, 3175.
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Table 2. Effects of pH and Ionic Strength on the Formal Redox Potential (Ep)a Epb/V film
pH 4.0
pH 6.0
pH 8.0
pH 10.0
pH 11.0
(Fc-PAH/PVS)5 (Fc-PAH/PVS)5PAH (Fc-PAH/PVS)5PAH/PVS
0.462 0.476 0.453
0.430 0.450 0.412
0.382 0.400 0.374
0.310 0.320 0.291
0.240 0.241 0.239
Epc/V film
0.05 M NaCl
0.1 M NaCl
0.5 M NaCl
5 M NaCl
(Fc-PAH/PVS)5 (Fc-PAH/PVS)5PAH (Fc-PAH/PVS)5PAH/PVS
0.412 0.434 0.405
0.409 0.426 0.401
0.386 0.397 0.382
0.365 0.365 0.365
a E ) (E b p pa + Epc)/2. CV was measured in 25 mM NaCl solution, whose pH was adjusted by adding HCl and NaOH. Scan rate: 50 mV s-1. c CV was measured in 10 mM Tris-HCl buffer (pH 7.3), whose ionic strength was adjusted by adding NaCl. Scan rate: 50 mV s-1. All data were average values of three preparations (error: (5 mV).
Effects of pH and Ionic Strength. The electrostatic effects may be sensitive to the pH and ionic strength of the solution because the electrostatic force can be shielded by an electrolyte on the surface and in the film depending on the pH and ionic strength. Table 2 summarizes Ep values of the modified electrodes in different pH and ionic strength conditions. For all electrodes tested, the Ep values shifted in the negative direction when the pH of the solution was changed from acidic to basic, probably due to the decrease in the amount of positive charges in the film.8,19 This is qualitatively in line with the results in Figure 2 that the Ep value was found at less positive potential for the polyanion-terminated (Fc-PAH/PVS)n and (Fc-PAH/PVS)n(PAH/PVS) films than for the (Fc-PAH/ PVS)nPAH film containing a polycationic surface layer. In other words, the decrease in positive charges (or increase in negative charges) on the surface or interior of the film induces a negative shift of the Ep values. The difference in the Ep values between the (Fc-PAH/PVS)5PAH film and (Fc-PAH/PVS)5(PAH/PVS) film did not depend on pH in the range pH 4-10, while the three kinds of films exhibited nearly identical Ep values at pH 11 (i.e., ∼240 mV). Amino groups in Fc-PAH and PAH chains in the film should exist as the neutral -NH2 form at pH 11, resulting in loss of the electrostatic effects originating from the -NH3+ ion. In fact, the Ep values at pH 11 are almost the same as the Ep values of Fc-PAH in aqueous solution. The effects of ionic strength on the surface chargeinduced potential shift are obvious; for all electrodes, the (19) Yoshikawa, Y.; Matsuoka, H.; Ise, N. Br. Polym. J. 1986, 18, 242.
Ep values shifted to the less positive potentials with increasing ionic strength. In addition, the difference in the Ep values among the three kinds of films decreased with increasing ionic strength and, in 5.0 M NaCl, the films exhibited identical Ep values to each other. These results should be originating from charge screening by NaCl in the film. 4. Conclusions It became apparent that the redox properties of Fccontaining PEM films depend on the deposition of outer layers even if the outer layers contain no redox sites. The deposition of a polycationic outer layer induced a positive shift in Ep and enhanced the Q value, while the Ep shifted negatively and the Q value was suppressed upon deposition of a polyanionic layer. Thus, the CV of (Fc-PAH/PVS) film-coated electrodes depended significantly on the polarity of the outermost layer. The electric charges on the outermost layer exerted their effects on the redox properties of (Fc-PAH/PVS)n films even across the thicker (PAH/PVS)5 layers. The combination of recognition elements and the redox films may be useful for constructing electrochemical sensors which selectively determine ionic species. Acknowledgment. This work was supported in part by a Grant-in-Aid (No. 14657568) from the Ministry of Education, Sciences, Sports, Culture and Technology of Japan. LA0209740
