Redox Properties of the Ferricyanide Ion on Electrodes Coated with

Nov 29, 2005 - The combination of synthetic polymers including poly(allylamine hydrochloride) ... Natan, M. L. Langmuir 1999, 15, 844. (c) Mamedov, A...
0 downloads 0 Views 95KB Size
Download PDF
2870

Langmuir 2006, 22, 2870-2875

Redox Properties of the Ferricyanide Ion on Electrodes Coated with Layer-by-Layer Thin Films Composed of Polysaccharide and Poly(allylamine) Takio Noguchi and Jun-ichi Anzai* Graduate School of Pharmaceutical Sciences, Tohoku UniVersity, Aramaki, Aoba-ku, Sendai 980-8578, Japan ReceiVed NoVember 29, 2005. In Final Form: January 24, 2006 Polyelectrolyte multilayer thin films were prepared by an alternate deposition of poly(allylamine hydrochloride) (PAH) and anionic polysaccharides {carboxymethylcellulose (CMC) and alginic acid (AGA)} on the surface of a gold (Au) disk electrode, and the binding of ferricyanide [Fe(CN)6]3- and hexaammine ruthenium ions [Ru(NH3)6]3+ to the films was evaluated. Poly(acrylic acid) (PAA) was also employed as a reference polyanion bearing carboxylate side chains. A quartz-crystal microbalance study showed that PAH-CMC and PAH-AGA multilayer films grow exponentially as the number of depositions increases. The thicknesses of five bilayers of (PAH-CMC)5 and (PAHAGA)5 films were estimated to be 150 ( 20 and 90 ( 15 nm, respectively, in the dry state. The PAH/polysaccharide multilayer film-coated Au electrodes exhibited a redox response to the [Fe(CN)6]3- ion dissolved in solution, irrespective of the sign of the surface charge of the film, suggesting the high permeability of the films to the [Fe(CN)6]3- ion. In contrast, the PAH-PAA film-coated Au electrodes exhibited a redox response only when the outermost surface of the film was covered with a positively charged PAH layer. However, the permeation of the [Ru(NH3)6]3+ cation was severely suppressed for all of the multilayer films. It was possible to confine the [Fe(CN)6]3- ion in the films by immersing the film-coated electrodes in a 1 mM [Fe(CN)6]3- solution for 15 min. Thus, the [Fe(CN)6]3--confined electrodes exhibited a cyclic voltammetric response in the [Fe(CN)6]3- ion-free buffer solution. The loading of the [Fe(CN)6]3- ion in the films was higher when the surface charge of the film was positive and increased with increasing film thickness. It was also found that the [Fe(CN)6]3- ion confined in the films serves as an electrocatalyst that oxidizes ascorbic acid in solution.

1. Introduction It has been established that an alternate deposition of anionic and cationic polyelectrolytes on a solid surface affords multilayer thin films.1 The materials used for preparing polyelectrolyte multilayer (PEM) films include synthetic polymers,2 proteins,3 DNA,4 and nanoparticles.5 PEM films have been prepared by taking advantages of not only the electrostatic force of attractions of polyelectrolytes but also hydrogen bonding and biological affinity.6 Typical applications of the PEM films include surface coatings for optical and electrochemical sensors,7 selective membranes for separation and purification,8 construction of protein nanoarchitectures,9 encapsulation and controlled release,10 and stimuli-sensitive systems.11 PEM films are divided into two * To whom correspondence should be addressed. E-mail: junanzai@ mail.pharm.tohoku.ac.jp. (1) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Makromol. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232. (2) (a) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823. (b) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (3) (a) Lvov, Y.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (b) Disawal, S.; Qiu, J.; Elmore, B. B.; Lvov, Y. M. Colloids Surf., B 2003, 32, 145. (c) Zhang, J.; Senger, B.; Vautier, D.; Picart, C.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Biomaterials 2005, 26, 3353. (4) (a) Sukhorukov, G. B.; Montrel, M. M.; Petkov, A. I.; Shabarchina, L. I.; Sukhorukov, B. I. Biosens. Bioelectron. 1996, 11, 913. (b) Lang, J.; Liu, M. J. Phys. Chem. 1999, 103, 11393. (c) Chen, X.; Lang, J.; Liu, M. Thin Solid Films 2002, 409, 227. (d) Trimaille, T.; Pichot, C.; Delair, T. Colloids Surf., B 2003, 221, 39. (5) (a) Liu, Y.; Wang, Y.; Claus, R. O. Chem. Phys. Lett. 1998, 298, 315. (b) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. L. Langmuir 1999, 15, 844. (c) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (6) (a) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (b) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (c) Wang, L.; Cui, S.; Wang, Z.; Zhang, X. Langmuir 2000, 16, 10490. (d) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Thin Solid Films 1996, 284-285, 797. (e) Rao, S. V.; Anderson, K. W.; Bachas, L. G. Biotech. Bioeng. 1999, 65, 389.

categories based on their deposition behavior: multilayer films whose thickness and mass loading increase linearly with an increasing number of layers and other films in which the thickness and mass loading increase exponentially. The combination of synthetic polymers including poly(allylamine hydrochloride) (PAH), poly(ethyleneimine) (PEI), poly(dimethyldiallylammonium chloride) (PDDA), poly(styrene sulfonate) (PSS), and poly(vinyl sulfate) (PVS) usually affords linearly growing PEM films, whereas polypeptides such as poly(lysine) (PLL) and poly(7) (a) Lee, S. H.; Kumar, J.; Tripathy, S. K. Langmuir 2000, 16, 10482. (b) McShane, M. J.; Brown, J. Q.; Guice, K. B.; Lvov, Y. M. J. Nanosci. Nanotechnol. 2002, 2, 1. (c) Liu, A.; Anzai, J. Langmuir 2003, 19, 4043. (d) Chen, T.; Friedman, K. A.; Lei, I.; Heller, A. Anal. Chem. 2000, 72, 3757. (e) Hoshi, T.; Saiki, H.; Kuwazawa, S.; Tsuchiya, C.; Chen, Q.; Anzai, J. Anal. Chem. 2001, 73, 5310. (f) Noguchi, T.; Hoshi, T.; Anzai, J. Sens. Lett. 2005, 3, 164. (8) (a) Katayama, H.; Ishihama, Y.; Asakawa, N. Anal. Chem. 1998, 70, 2254. (b) Meier-Haack, J.; Lenk, W.; Lehmann, D.; Lunkwiz, K. J. Membr. Sci. 2001, 184, 223. (c) Krasemann, L.; Toutianoush, A.; Tieke, B. J. Membr. Sci. 2001, 181, 221. (9) (a) Morpurgo, M.; Hofstetter, H.; Bayer, E. A.; Wilchek, M. J. Am. Chem. Soc. 1998, 120, 12734. (b) Hoshi, T.; Saiki, H.; Anzai, J. Biosens. Bioelectron. 2000, 15, 623. (c) Anzai, J.; Kobayashi, Y. Langmuir 2000, 16, 2851. (d) Anzai, J.; Hoshi, T.; Nakamura, N. Langmuir 2000, 16, 6306. (e) Hoshi, T.; Akase, S.; Anzai, J. Langmuir 2002, 18, 7024. (f) Calvo, E. J.; Danilowicz, C.; Lagier, C. M.; Manrique, J.; Otero, M. Biosens. Bioelectron. 2004, 19, 1219. (10) (a) Schuer, C.; Caruso, F. Macromol. Rapid Commun. 2000, 21, 750. (b) Lvov, Y.; Caruso, F. Anal. Chem. 2001, 73, 4212. (c) Qiu, X.; Leporatti, S.; Donath, E.; Mowald, H. Langmuir 2001, 17, 5375. (d) Ai, H.; Jones, S. A.; de Villiers, M. M.; Lvov, Y. M. J. Controlled Release 2003, 86, 59. (e) Antipof, A. A.; Sulhorukov, G. B. AdV. Colloid Interface Sci. 2004, 111, 49. (f) Lu, Z.; Shutava, T.; Sahiner, N.; John, V.; Lvov, Y. Chem. Lett. 2005, 34, 1536. (11) (a) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (b) Schuer, C.; Caruso, F. Biomacromolecules 2001, 2, 921. (c) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (d) Chinnayelka, S.; McShane, J. M. J. Fluoresc. 2004, 14, 585. (e) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammond, P. T. Langmuir 2005, 21, 1603. (f) Sato, K.; Imoto, Y.; Sugama, J.; Seki, S.; Inoue, H.; Odagiri, T.; Hoshi, T.; Anzai, J. Langmuir 2005, 21, 797. (g) Inoue, H.; Sato, K.; Anzai, J. Biomacromolecules 2005, 6, 27. (h) Inoue, H.; Anzai, J. Langmuir 2005, 21, 8354. (i) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 37.

10.1021/la053226u CCC: $33.50 © 2006 American Chemical Society Published on Web 02/16/2006

Redox Properties of the Ferricyanide Ion

(glutamic acid) (PGA) afford exponentially growing films.12 Hyaluronic acid (HA), a negatively charged polysaccharide, also gives the latter film.13 It has been demonstrated that in exponentially growing PLLHA multilayer films PLL chains diffuse in and out of the films during preparation but HA does not.14 Thus, the diffusion of one of the used polyelectrolytes in and out of the film is the origin of the exponential growth of the polypeptide and polysaccharide multilayer films. In view of the fact that polyelectrolyte chains can penetrate the interior of film, it is reasonable to assume that this type of PEM film is loosely packed and rather porous, as compared to the densely packed and ordered structure of linearly growing films. Therefore, this type of PEM film may show different properties with respect to binding or penetration of ions and molecules from those of linearly growing PEM films. In fact, Barrett et al. studied physical properties of PAH-HA and PLL-HA films such as swelling, surface wettability, and loading and release of charged dyes and found that these properties significantly depend on the pH conditions.15 The interactions between [Fe(CN)6]3-/4- ions and exponentially growing PEM films have recently been studied, and it was found that [Fe(CN)6]3-/4- ions penetrate the films even when the films contain negative charges on the surface.16 This is a clear contrast with respect to the permeability of linearly growing PEM films, in which [Fe(CN)6]3-/4- anions cannot penetrate the films when the surface is covered with negatively charged polyelectrolytes such as PVS and PSS.17 In addition, the [Fe(CN)6]3-/4- ions confined in the exponentially growing films were found to be exchanged with PGA chains dissolved in the solution.18 It is still interesting to evaluate the ion-binding properties of exponentially growing PEM films composed of other polyelectrolytes because the structure and properties of PEM films would significantly depend on the materials used. Therefore, we have studied the binding properties of polysaccharide PEM films containing carboxymethycellulose (CMC) and alginic acid (AGA). In fact, these PEM films have been found to grow exponentially. We are interested in the electrochemical properties of [Fe(CN)6]3-/4- ions confined in these films because of their electrocatalytic ability to act as electron-transfer mediators for biosensors. In this context, PEM films composed of ferrocenemodified poly(amine)s and osmium (Os) complex-appended poly(vinylpyridine) have been used to construct mediator-type biosensors.19 In these systems, the redox mediators are attached to the polymer chains to immobilize the redox center in the PEM films because the redox compounds are often water-soluble. In this situation, it would be advantageous if redox ions such as [Fe(CN)6]3- and [Ru(NH3)6]3+ ions could be immobilized in PEM films and used as an electron mediator for biosensing. (12) (a) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414. (b) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. C. Langmuir 2003, 19, 440. (13) Picart, G.; Ladam, G.; Senger, B.; Voegel, J. C.; Schaaf, P.; Cuisinier, F. J. G.; Gergely, C. J. Chem. Phys. 2001, 115, 1086. (14) Picart, C.; Mutterer, J.; Rechert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (15) Burke, S. E.; Barrett, C. J. Macromolecules 2004, 37, 5375. (16) Hubsch, E.; Fleith, G.; Fatisson, J.; Labbe, P.; Voegel, J. C.; Schaaf, P.; Ball, V. Langmuir 2005, 21, 3664. (17) (a) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (b) PardoTissar, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110. (18) Ball, V.; Hubsch, E.; Schweiss, R.; Voegel, J. C.; Schaaf, P.; Knoll, W. Langmuir 2005, 21, 8526. (19) (a) Hodak, J.; Etchenique, R.; Calvo, E. J.; Singhal, K.; Bartlett, P. N. Langmuir 1997, 13, 2716. (b) Li, W.; Wang, Z.; Sun, C.; Xian, M.; Zhao, M. Anal. Chim. Acta 2000, 418, 225. (c) Narvaez, A.; Sucrez, G.; Popescu, I. C.; Katakis, I.; Dominguez, E. Biosens. Bioelectron. 2000, 15, 43. (d) Liu, A.; Ehara, M.; Takahashi, S.; Hoshi, T.; Anzai, J. Electrochemistry 2003, 71, 509. (e) Rusling, J. F.; Forster, R. J. J. Colloid Interface Sci. 2003, 262, 1. (f) Liu, A.; Anzai, J. Anal. Bioanal. Chem. 2004, 380, 98.

Langmuir, Vol. 22, No. 6, 2006 2871

However, a systematic study of the immobilization of redox ions in PEM films and their use in electrocatalysis has not been reported. The present article reports the redox properties of [Fe(CN)6]3ions confined in PAH-CMC and PAH-AGA PEM films, as well as in PAH-PAA films, as a function of the number of layers in the films. The electrocatalytic properties of the [Fe(CN)6]3- ion-confined PEM films in the electrochemical oxidation of ascorbic acid are also discussed. 2. Experimental Section Materials. An aqueous solution (20%) of poly(allylamine) hydrochloride {PAH; average molecular weight (MW) ∼10 000} was purchased from the Nittobo Co. (Tokyo, Japan). Poly(acrylic acid) (PAA; MW ∼90 000) was obtained from the Aldrich Chemical Co. (Milwaukee, WI). Sodium carboxymethylcellulose (CMC; MW ∼245 000) and sodium alginate (AGA) were obtained from the Tokyo Kasei Co. (Tokyo, Japan). All of the polyanions used (CMC, AGA, and PAA) contain carboxylic acid residues in the polymer chains. Sodium 3-mercapto-1-propanesulfonate (MPS) was purchased from the Tokyo Kasei Co. All other reagents were of the highest grade available and were used without further purification. All solutions were prepared in Milli-Q water. Apparatus. A quartz-crystal microbalance (QCM) (QCA 917 system, Seiko EG & G, Tokyo, Japan) was employed for the gravimetric analysis of the polyelectrolyte multilayer films. A 9 MHz AT-cut quartz resonator coated with a thin Au layer (0.2 cm2) was used as a probe in which the adsorption of 1 ng of a substance induces a -0.91 Hz change in the resonance frequency. All electrochemical measurements were carried out using an electrochemical analyzer (ALS, model 660B). Preparation of PEM Film-Coated Electrodes.The PEM films were prepared on the surface of a gold (Au) disk electrode (3 mm diameter). The surface of the Au electrode was polished thoroughly using alumina powder and rinsed in distilled water before use. The polished Au electrode was further treated electrochemically in a 0.5 M H2SO4 solution by scanning the potential from -0.2 to 1.5 V at a scan rate of 0.1 V s-1 for 15 min. The cleaned Au electrode thus prepared was dipped into a freshly prepared aqueous MPS solution (5 mM) overnight to form a self-assembled monolayer of MPS on the Au surface. After this treatment, the surface of the Au electrode should be negatively charged because of -SO3- residues on the MPS monolayer. The negatively charged surface of the electrode was further modified with the PEM film by dipping it alternately in 0.5 mg mL-1 PAH and 0.5 mg mL-1 CMC, AGA, or PAA solutions (10 mM tris-HCl buffer containing 150 mM NaCl, pH 7.4) for 30 min with an intermediate 5 min rinse in the buffer. The multilayer films were prepared by repeating the above procedure. Gravimetric Analysis of the Deposition of PEMs.The Au-coated quartz resonator was thoroughly rinsed with water before use. The surface of the quartz resonator was cleaned electrochemically in 0.5 M H2SO4 in a similar manner as in the case of cleaning a Au disk electrode. The surface of the Au-coated quartz resonator was first modified with the MPS monolayer, and then PEM films were deposited in a similar manner on the MPS-modified Au surface of the resonator. The PEM films were deposited on both surfaces of the quartz resonator (i.e., the total area of the Au-coated surface is 0.4 cm2), and the film-deposited probe was dried in air after each deposition until the frequency showed a steady-state value with which to estimate the weight of the film. Electrochemical Measurements. The electrochemical response of PEM film-coated electrodes was measured in a glass cell using a PEM film-modified electrode as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl electrode (3.3 M KCl) as the reference electrode. All measurements were performed in 10 mM tris-HCl buffer, pH 7.4, containing 150 mM NaCl at room temperature (∼20 °C).

2872 Langmuir, Vol. 22, No. 6, 2006

Noguchi and Anzai

Figure 1. QCM frequency changes for the formation of PAHCMC (a) and PAH-AGA multilayer films (b). The odd layer numbers show the deposition of PAH, and the even numbers correspond to the deposition of CMC or AGA.

3. Results and Discussion Deposition Behavior of PAH-CMC and PAH-AGA Multilayer Films. Gravimetric analysis was carried out using QCM to monitor the formation of PAH-CMC and PAH-AGA multilayer films. For this purpose, the PEM films were deposited on the MPS-modified Au-coated quartz resonator, and the resonance frequency (F) was recorded after drying in air to evaluate the weight of the PEM films in the dry state. Figure 1 plots the decrease in the resonance frequency (-∆F) observed upon depositing PAH-CMC (a) and PAH-AGA multilayer films (b) as a function of the number of depositions. For both films, F decreased as the number of depositions increased, suggesting that the multilayer films were successfully formed on the quartz resonator. The film formation relies on the electrostatic force of attraction between the positive charges on the PAH chains and negatively charged carboxylate ions in CMC and AGA because the films were prepared in pH 7.4 media in which both amino residues in PAH and carboxylate side chains in CMC and AGA are charged. The data show that both films grow exponentially as a function of the number of depositions. The exponential growing behavior was more significant for the PAH-CMC film than for the PAH-AGA film. The thickness of the films in the dry state is estimated to be 150 ( 20 nm for the (PAH-CMC)5 film and 90 ( 15 nm for the (PAH-AGA)5 film from the QCM data, assuming that the density of the polyelectrolyte films is ∼1.2 g cm-3.20 Electrochemical Response of PEM Film-Coated Electrodes. The electrochemical response of PEM film-coated electrodes to the [Fe(CN)6]3- ion was evaluated. We prepared Au electrodes modified with PEM films whose thicknesses were systematically varied, and their cyclic voltammograms (CVs) were measured in a 1 mM [Fe(CN)6]3- solution (10 mM tris-HCl buffer, pH 7.4, containing 150 mM NaCl). Figure 2 shows CVs of the [Fe(CN)6]3- ion on the Au electrodes coated with (PAH-CMC)1 and (PAH-CMC)1PAH (a), (PAH-AGA)1 and (PAHAGA)1PAH (b), and (PAH-PAA)1 and (PAH-PAA)1PAH films (c). All CVs were recorded after the electrodes had been immersed in the [Fe(CN)6]3- solution for ca. 15 min because the amplitude of the redox peaks increased with time during the first several minutes. For (PAH-CMC)1, (PAH-CMC)1PAH, (PAHAGA)1, and (PAH-AGA)1PAH film-coated electrodes, welldefined CVs were observed at 0.2-0.3 V and were associated with the [Fe(CN)6]3-/4- redox couple. The size and shape of the CVs are nearly identical to one another. However, for the electrodes coated with the (PAH-PAA)1 and (PAH-PAA)1PAH (20) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337.

Figure 2. Cyclic voltammograms of 1 mM K3Fe(CN)6 on the PEM film-coated Au electrodes: (a) (PAH-CMC)1 (solid line) and (PAHCMC)1PAH films (broken line), (b) (PAH-AGA)1 (solid line) and (PAH-AGA)1PAH films (broken line), and (c) (PAH-PAA)1 (solid line) and (PAH-PAA)1PAH film (broken line). The electrolyte solution was 10 mM tris-HCl buffer, pH 7.4, containing 150 mM NaCl. The scan rate was 50 mV s-1.

films, the redox response to the [Fe(CN)6]3- ion was severely suppressed when the negatively charged PAA layer was located on the outermost surface of the film because of an electrostatic force of repulsion, whereas a reversible CV was observed when the outermost surface was covered with positively charged PAH. A similar result has been reported for the electrodes coated with a multilayer film of PAH and poly(styrene sulfonate) (PSS), in which the response was suppressed when the surface was covered with PSS.17 Thus, the data in Figure 3 show that the [Fe(CN)6]3ion penetrates the (PAH-CMC)1, (PAH-CMC)1PAH, (PAHAGA)1, and (PAH-AGA)1PAH films irrespective of the sign of the electric charges on the outermost surface of the film. To evaluate the effects of the thickness of the films on the redox properties, we have recorded CVs on the electrodes coated with (PAH-CMC)n and (PAH-CMC)nPAH films, where n ) 2, 3, 4, and 5 (Figure 3). For all electrodes, the redox peaks were clearly observed in the [Fe(CN)6]3- solution. The separation between the oxidation and reduction peaks, ∆Ep, was 67-73 and 67-80 mV for the (PAH-CMC)n and (PAH-CMC)nPAH film-coated electrodes, respectively, showing that the [Fe(CN)6]3ion penetrates the PEM films and diffuse smoothly even if the surfaces of the films are covered with negatively charged CMCs. An interesting feature is found in the CVs for the electrodes coated with the thicker films. In Figure 3d, for example, the shape of the CVs for the (PAH-CMC)5 and (PAH-CMC)5PAH film-coated electrodes deviated from that of the reversible CV observed for thinner films, suggesting that another redox couple may be contained in the region of 0.3-0.4 V in addition to the redox peak at 0.2-0.3 V that was observed for thinner filmcoated electrodes. The origin of the second redox couple will be discussed later. A similar trend in CVs was observed for (PAH-

Redox Properties of the Ferricyanide Ion

Langmuir, Vol. 22, No. 6, 2006 2873

Figure 3. Cyclic voltammograms of 1 mM K3Fe(CN)6 on the Au electrodes coated with (PAH-CMC)n (solid line) and (PAHCMC)nPAH multilayer films (broken line). The electrolyte solution was 10 mM tris-HCl buffer, pH 7.4, containing 150 mM NaCl. The scan rate was 50 mV s-1.

Figure 4. Cyclic voltammograms of 1 mM K3Fe(CN)6 on the Au electrodes coated with (PAH-PAA)n (solid line) and (PAHPAA)nPAH multilayer films (broken line). The electrolyte solution was 10 mM tris-HCl buffer, pH 7.4, containing 150 mM NaCl. The scan rate was 50 mV s-1.

AGA)n and (PAH-AGA)nPAH film-coated electrodes (data not shown). It has been reported that [Fe(CN)6]3-/4- ions are permeable for exponentially growing PEM films composed of PAH and poly(L-glutamic acid) (PGA) when the surface was covered with both PAH and PGA.16 In the PAH-PGA filmcoated electrodes, however, the amplitudes of the redox peaks did not change even after the deposition of thicker films, and in fact, reversible CVs were observed from 0.2-0.3 V on the (PGAPAH)10 film-coated electrode. (A thickness-dependent CV was not reported.) This is a clear contrast to the redox behavior observed on the PAH-CMC and PAH-AGA film-coated electrodes, in which CVs depended on the thickness of the films. Different behavior was observed for the electrodes coated with (PAH-PAA)n and (PAH-PAA)nPAH films (Figure 4). The redox response was almost completely suppressed when the outermost surface was covered with negatively charged PAA, which is in line with the reported results.17 However, the electrodes coated with PAH-terminated films exhibited a redox response to the [Fe(CN)6]3- ion. When the film was thinner (n ) 2 and 3), the redox peaks were observed at 0.2-0.3 V, whereas the peaks were shifted in the positive direction for thicker filmcoated electrodes (n ) 4 and 5). In addition, the shape of the CVs on the thicker film-coated electrodes deviated from that of the reversible CV, which is probably due to the suppressed diffusion in the thicker films. The data reported in Figures 3 and 4 strongly suggest that two kinds of mechanisms are involved in the redox reaction of the [Fe(CN)6]3- ion in the PEM film. It is reasonable to assume that

the redox peaks observed in the 0.2-0.3 V region may not be observed in the buffer solution containing no [Fe(CN)6]3- ion because the redox peaks probably originate from the diffusing [Fe(CN)6]3- ion in the films. To confirm this assumption, the PEM film-coated electrodes, which had been immersed in 1 mM [Fe(CN)6]3- solution for 15 min, were thoroughly rinsed for 10 min, and then CVs were measured in the [Fe(CN)6]3- ion-free buffer solution. Figure 5 illustrates the CVs of the electrodes in the buffer solution. For the (PAH-CMC)5 and (PAHCMC)5PAH film-coated electrodes, a redox couple was still clearly observed at 0.30-0.35 V, whereas no peak was found in the region of less-positive potential. The oxidation and reduction peaks in the CVs were located at 0.319 and 0.313 V for the (PAH-CMC)5 film-coated electrode and at 0.342 and 0.332 V for the (PAH-CMC)5PAH film-coated electrode. These data explicitly show that the redox waves at 0.2-0.3 V observed in the [Fe(CN)6]3- solution originate from the diffusing [Fe(CN)6]3ion in the films. However, the [Fe(CN)6]3- ion confined in the film may be responsible for the redox waves observed at 0.300.35 V. The ∆Ep of these redox couples is less than 10 mV, which strongly supports the view that the redox reaction originates from the surface-confined species. The (PAH-AGA)5 and (PAH-AGA)5PAH film-coated electrodes also exhibited redox responses originating from the confined [Fe(CN)6]3- ion; ∆Ep values were 29 and 19 mV, respectively. The (PAH-PAA)5PAH film-coated electrode afforded a similar CV whose ∆Ep was 13 mV, whereas no peak was found for the (PAH-PAA)5 film-

2874 Langmuir, Vol. 22, No. 6, 2006

Figure 5. Cyclic voltammograms of the PEM film-coated Au electrodes in 10 mM tris-HCl buffer containing 150 mM NaCl, pH 7.4. The electrode was coated with (a) (PAH-CMC)5 (solid line) and (PAH- CMC)5PAH films (broken line), (b) (PAH-AGA)5 (solid line) and (PAH-AGA)5PAH films (broken line), and (c) (PAH-PAA)5 (solid line) and (PAH-PAA)5PAH films (broken line). The scan rate was 50 mV s-1.

coated electrode. This is reasonable because no [Fe(CN)6]3- ion penetrates the PAA-terminated film even in the [Fe(CN)6]3solution. Thus, these data clearly show that the [Fe(CN)6]3- ion is confined in the PEM films. Figure 6 plots the anodic peak current, ipa, of the CVs recorded in the buffer solution as a function of the number of layers in the PEM film. The odd layer numbers show the films terminated with PAH, and the even numbers correspond to the films whose surfaces are covered with CMC, AGA, or PAA. For the PAHterminated films, ipa increased as the number of layers increased, which is probably due to an increasing number of [Fe(CN)6]3ions confined in the thicker films, which in turn implies that the [Fe(CN)6]3- ion is bound to whole layers from inner to outer layers of the film. It is noted here that the ipa for the CMC- and AGA-terminated film-coated electrodes is always lower than that of the corresponding PAH-terminated film-coated electrodes. The effects of surface charges on the ipa values may be ascribed to two possible reasons. One plausible reason is that the number of [Fe(CN)6]3- ions confined in the PAH-terminated film is higher than that in the films terminated with the polyanions. The negative charges on the surface of the films may hinder the penetration of the [Fe(CN)6]3- ion into the films to some extent. However, the CV data is not rationalized only from the different loading of the [Fe(CN)6]3- ion in the films because ipa and peak potentials also depend on the sign of the surface charge. In fact, the redox potentials for the PAH-terminated film-coated electrodes are always ca. 20 mV more positive than those for the electrodes coated with CMC- or AGA-terminated films. Another possible explanation relates to the electrostatic effects of the surface charges on the redox reactions in the films. In this context, we have previously reported that the redox current and formal potential of ferrocene-modified PEM film-coated electrodes significantly

Noguchi and Anzai

Figure 6. Anodic peak current of the [Fe(CN)6]3- ion-immobilized PEM film-coated electrodes as a function of the number of layers in the film. The odd layer numbers show the films terminated with PAH, and the even numbers correspond to the films whose surface is covered with CMC, AGA, or PAA. The scan rate was 50 mV s-1.

Figure 7. Cyclic voltammograms of ascorbic acid on the MPSmodified Au electrode (a), the (PAH-CMC)5PAH film-coated Au electrode (b), and the [Fe(CN)6]3- ion-immobilized (PAHCMC)5PAH film-coated Au electrode (c). The scan rate was 50 mV s-1. Sample solutions consisted of 0, 1, 5, and 10 mM ascorbic acid in 10 mM tris-HCl buffer containing 150 mM NaCl, pH 7.4.

depend on the sign of the surface charges of the film even if the loading of the ferrocene moiety in the film is constant.7c It is likely that both mechanisms are involved in the PAH-CMC and PAH-AGA films.

Redox Properties of the Ferricyanide Ion

The stability of the [Fe(CN)6]3- ion confined in the film was evaluated by monitoring the CV response. Electrodes coated with [Fe(CN)6]3- ion-confined (PAH-CMC)5 and (PAHCMC)5PAH films were immersed in a 150 mM NaCl solution (pH 7.0), and the CV was occasionally recorded. During the first 4 to 5 h, ca. 30% of the total number of [Fe(CN)6]3- ions were released out of the film, and after 1 day, ca. 50% of the total number of [Fe(CN)6]3- ions remained in the film. Thereafter, the release of [Fe(CN)6]3- ions from the film was slow. We have also tried to immobilize the hexaammine ruthenium(III) ion [Ru(NH3)6]3+ in the polysaccharide PEM films in a similar manner. However, [Ru(NH3)6]3+ ion could not be confined in the PAH/polysaccharide films or in the PAH-PAA multilayer films. These results suggest that positive binding sites may exist in the films for binding the [Fe(CN)6]3- ion. Judging from the fact that the [Ru(NH3)6]3+ ion cannot be confined in the films, the positive sites on the PAH chains may be the origin of the driving force to confine the [Fe(CN)6]3- ion in the films. Electrocatalytic Oxidation of Ascorbic Acid on the [Fe(CN)6]3- Ion-Confined Electrode. Redox-active PEM filmcoated electrodes have widely been used for electrocatalysis in enzymatic and nonenzymatic determinations of biologically important compounds.19 Therefore, it is interesting to study the catalytic ability of the [Fe(CN)6]3- ion confined in the PEM films. Figure 7 shows CVs of ascorbic acid on the PEM film-free Au electrode (a), the (PAH-CMC)5PAH film-coated electrode without the [Fe(CN)6]3- ion (b), and the [Fe(CN)6]3- ion-confined (PAH-CMC)5PAH film-coated electrode (c). The PEM filmfree Au electrode exhibited a response associated with an irreversible oxidation of ascorbic acid in the potential range of 0.5 V or higher. Basically the same redox response was observed on the [Fe(CN)6]3- ion-free (PAH-CMC)5PAH film-coated electrode, though the oxidation current was slightly suppressed, which is probably due to the diffusion barrier of the film. In

Langmuir, Vol. 22, No. 6, 2006 2875

contrast, for the [Fe(CN)6]3- ion-confined (PAH-CMC)5PAH film-coated electrode, electrocatalytic waves were observed in the potential range of 0.4 V or lower, originating from the oxidation of ascorbic acid by [Fe(CN)6]3- ions in the PEM film (eq 1) and subsequent electrooxidation of the resulting [Fe(CN)6]4ion at the electrode (eq 2). Thus, the [Fe(CN)6]3- ion-confined (PAH-CMC)5PAH film-coated electrode can successfully be used for the electrocatalytic oxidation of ascorbic acid in solution.

ascorbic acid + 2[Fe(CN)6]3- f dehydroascorbic acid + 2[Fe(CN)6]4- (1) [Fe(CN)6]4- f [Fe(CN)6]3- + e-

(2)

4. Conclusions PAH/polysaccharide multilayer films prepared from CMC or AGA were found to grow exponentially upon an alternate deposition with PAH. The Au electrodes coated with the PAHCMC and PAH-AGA multilayer films exhibited a redox response in the [Fe(CN)6]3- solution irrespective of the sign of the electric charge on the surface of the film, which originates from the smooth diffusion of the [Fe(CN)6]3- ion, which is probably due to the loose packing of polyelectrolyte chains in the films. It was found that the [Fe(CN)6]3- ion is confined in the films, the loading being dependent on the thickness of the film. The confined [Fe(CN)6]3- ion can be used as an electrocatalyst for oxidizing ascorbic acid in solution. The [Fe(CN)6]3- ion-confined PEM film-coated electrodes may be useful for developing mediatortype chemical sensors and biosensors. From the viewpoint of the biosensor applications of the films, the use of polysaccharides as a material is beneficial because of its biocompatibility and low cost. LA053226U