Nanocomposite multilayer films containing Dawson-type polyoxometalate and cationic phthalocyanine: Fabrication, characterization and bifunctional electrocatalytic properties

Thin Solid Films 515 (2007) 5490 – 5497 www.elsevier.com/locate/tsf

Nanocomposite multilayer films containing Dawson-type polyoxometalate and cationic phthalocyanine: Fabrication, characterization and bifunctional electrocatalytic properties Yana Jin, Lin Xu ⁎, Liande Zhu, Wenjia An, Guanggang Gao Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun 130024, P.R. China Received 11 January 2006; received in revised form 22 November 2006; accepted 6 December 2006 Available online 19 December 2006

Abstract Water-soluble cationic phthalocyanine — Alcian blue (AB) and 2:18 tungstophosphate anions (P2W18) were alternately deposited on a -aminopropyltriethoxysilane-modified indium tin oxide-coated glass electrodes or quartz substrate through a layer-by-layer method. The resulting organic-inorganic hybrid films were characterized by ultraviolet-visible absorption spectra, cyclic voltammetry and X-ray photoelectron spectra. A stepwise and regular deposition of AB and P2W18 in both pure water and 0.01 M poly(styrenesulfonate) (PSS) were investigated. Differences in linear growth were observed between pure water and 0.01 M PSS-based solutions. The voltammetric curves show well-defined anodic and cathodic peaks, which is indicative of both electrocatalytic oxidation and reduction of nitrite. © 2007 Elsevier B.V. All rights reserved. Keywords: Multilayer films; Polyoxometalate; Alcian blue; Bifunctional electrocatalysis

1. Introduction Highly ordered, ultrathin assemblies with molecularly controllable surface properties are well-suited for application as electrocatalysts [1]. A challenge is to develop bifunctional electrocatalysts, particularly systems that can catalyze both reductions and oxidations [2–5]. A more general approach is to use a bifunctional composite instead of a single chemical species to serve this purpose. For example, Kulesza and Faulkner [5] have reported systems including Pt deposited in a tungsten oxide film and a mixture of an osmium complex with a polyoxometalate (POM) film. Recently, Cheng and Cox et al. [1] have reported that nanocomposite film of a ruthenium metallodendrimer and a Dawson-type polyoxometalate shows different electrocatalytic activity when different chemicals were used as the outest layer. POMs, which exhibit a remarkably rich redox chemistry with stable redox states and multiple electron transfer steps, have become attractive candidates for catalyst, energy storage and

⁎ Corresponding author. Tel./fax: +86 431 5099668. E-mail address: [email protected] (L. Xu). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.175

materials science. One of the most important properties of POM anions is their ability to accept various numbers of electrons giving rise to mixed-valency species. This property has made these compounds very useful in electrode modification and electrocatalytic research [6,7]. In general, there are three main methods for modifying this kind of species on electrode surfaces: electrochemical deposition [7], adsorption [8,9] and immobilization of heteropolyanions as dopants in a conduction polymer matrix [10–13]. The layer-by-layer (LBL) assembly, initially developed for pairs of oppositely charged polyelectrolytes, has recently been applied to the preparation of inorganic materials such as thin films of metalloporphyrin [14], metal chelate [15] and nanoparticle [16]. Ichinose et al. have prepared electrostatic layer-by-layer self-assembly films containing isopolymolybdate (NH4)4Mo8O26 [17]. Kurth and co-workers [18,19] have also prepared polyelectrolyte multilayer films containing molybdate (IV) POM clusters. Their results display that the LBL technique can be applied in fabricating multilayer films of anionic POMs and cationic polyelectrolyte. POMs are extremely versatile inorganic building blocks for the construction of functional materials [20]. Dong et al. have prepared POMs-containing multilayer

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films on a 4-aminobenzoic acid derivatization glassy carbon electrode by the LBL method, with metalloporphyrins as counterions [21]. The as-prepared multilayer films exhibit remarkable electrocatalytic activity for hydrogen evolution reaction at much more positive potentials in acid media and can also be used as promising dissolved-oxygen sensors. Nitrite ion is commonly used as an additive in some foods [22] and as a corrosion inhibitor [23]. Detection of nitrite by electrochemical methods can provide a rapid way of determination or can also be coupled to high performance liquid chromatography techniques [24,25]. It is well known that direct electroreduction of nitrite ion requires a large overpotential at most electrode surfaces [26]. Various transition metal complexes have been used as enzymatic models for nitrite reduction, including iron chelate, iron or cobalt porphyrin, and cobalt cyclam [27–32]. Moreover it has been shown that a large variety of POMs of the Keggin- and Dawson-type such as 2:18 tungstophosphate anions (P2W18) are also efficient in the electrocatalytic reduction of nitrite and can be employed as enzymatic models [33,34]. Phthalocyanines have been proven suitable materials for photovoltaic, electrochemical, and gas sensing applications [35]. Metallic phthalocyanines, in particular, have been exploited as catalysts for a variety of oxidation–reduction reactions. Caro C.A. et al. [36] have investigated the oxidation of nitrite on a vitreous carbon electrode modified with cobalt phthalocyanine. Many techniques have been developed to assemble phthalocyanines as thin films [37]. Thermal evaporation is the most widely used technique but it has its disadvantages in terms of economic feasibility for device fabrication at the industrial scale. Phthalocyanine derivatives with proper ionic groups can be fabricated

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using LBL deposition [38] technique. However, only a few studies of electrochemical properties of phthalocyanine LBL deposited films have been reported until now, and the control of composition and thickness in the LBL method is still a challenge. Because P2W18 and phthalocyanine have the character of electrochemical reductions and oxidations, respectively, the multilayer electrode can be predicted to provide heterogeneous bifunctional electrocatalysis. Their cooperative interaction will be possible to produce unprecedented function. In this work we fabricate the multilayer film of cationic copper(II) phthalocyanine–Alcian Blue(AB) and P2W18 anion by the LBL deposition technique. A stepwise and regular deposition of AB and P2W18 in both pure water and 0.01 M poly (styrenesulfonate) (PSS)were observed. The voltammetric curves with well-defined anodic and cationic peaks are indicative of electrocatalytic oxidation and reduction of nitrite, showing a bifunctional electrocatalysis of the composite films. 2. Experimental section 2.1. Materials 3-aminopropyltriethoxysilane (APS), Poly(styrenesulfonate) (PSS MW = 70,000), and poly(allylaminehydrochloride) (PAH MW = 70,000) were purchased from Aldrich and were used without further treatment. Dawson type polyoxometalate K6(P2W18O62)·14H2O (P2W18) were prepared according to literature method and identified by UV–Vis adsorption spectra and cyclic voltammetry [39,40]. Cationic phthalocyanine Alcian Blue–tetrakis(methyl pyridinium) chloride(AB) were purchased from Chroma and rinsed with acetone prior to use. Their

Fig. 1. Structure of 3-aminopropyltrimethoxysilane (1), poly(allylamine hydrochloride) (2), and poly(styrenesulfonate) (3), P2W18 (4), Alcian-blue(5).

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chemical structures are shown respectively in Fig. 1. The water used in all experiments was deionized to a resistivity of 18 MΩ. 2.2. Multilayer preparation Quartz substrates and indium tin oxide-coated glass electrodes (ITO)-coated glass were used for the film fabrication of selfassembly. The substrates were cleaned by immersion in the “piranha solution” containing three parts H2O2 (30% aqueous solution) and seven parts oil of vitriol (H2SO4) at 80 °C for 40 min (caution: piranha is a strong oxidizer that should be handled with care and not stored in closed containers) and rinsed with copious deionized water. They were then dried completely and submerged into freshly distilled toluene containing 0.5 wt.% of APS for functionalization, resulting in a NH2-modified surface. Finally, the substrates were stored in 0.01 M HCl solution overnight. Thus the hydrophilicity of the surface for the substrates changed. Prior to deposition of AB and P2W18, two layers of polyelectrolyte (PSS/PAH) were deposited to ensure an evenly charged surface and increase the surface charge density of the substrate. The silanized substrated were immersed in PSS solution of 10− 2 M for 20 min, rinsed with water, and dried under a nitrogen stream. Then the PSS coated substrates were exposed to a PAH solution of 10− 2 M (containing 1 M NaCl; pH = 2–4) for 20 min. Multilayers were then built up by alternating deposition of P2W18 (3 mM; pH = 2) and AB (0.5 mg/mL) until the desired layer number was reached. This was either done in PSS (10− 2 M)-based P2W18 solution or in pure water P2W18 solution. Water rinse and nitrogen drying steps were performed after each adsorption step. 2.3. Physical measurements UV–vis absorption spectra of quartz- and ITO-supported films were recorded on a 756CRT UV-visible spectraphotometer. X-ray photoelectron spectra (XPS) were measured on a silicon wafer using an ESCALAB MK II Surface Analysis System (including X-ray photoelectron spectrometer) with

Fig. 3. (a) UV–Vis spectra of (P2W18–AB)n film with n = 5, 9, 13, 17 respectively; (b) UV–Vis spectra of (AB–P2W18/PSS)n film with n = 1, 3, 5, 7, 9, 11, 13 (increase from bottom to top)(on both sides) on the precursor filmmodified quartz substrate.

aluminum Kα (1486.6 eV) as X-ray source. A concentric hemispherical analyser (CHA) was used with 50 eV pass energy, and the XPS analysis depth was 3–5 nm. The cyclic voltammetry were carried out at ambient temperature (25 °C) in sodium acetate (NaAc)/acetic acid (HAc) buffer solutions. pH value measurements were performed on a Model-Df 808 digital pH/ion meter. The pH buffer solutions were prepared by changing the relative amount of 0.2 M NaAc, glacial acetate acid and 0.1 M NaOH solutions with deionized water. ITO electrodes coated with the self-assembled films were placed in solution with a platinum wire as counter electrode and the Ag/ AgCl (3 mol/L KCl) as reference electrode. 3. Results and discussion 3.1. Ultraviolet–visible absorption spectra for monitoring film growth

Fig. 2. UV–Vis spectra of 0.5 mg/mL AB solution.

The phthalocyanine has characteristic spectral absorption in the UV–Vis region, Fig. 2 shows the UV–Vis absorption spectra of Alcian blue(AB) in aqueous solution. The absorbance at about 342 nm (B-band) in the UV region arises from the deeper π-levels-LUMO transition. The absorption of Q-band have two peaks at approximately 678 nm and 630 nm which are due to monomer and aggregate of AB, respectively, is also typical absorption of phthalocyanine materials. The band edge has been assigned to a π–π⁎ transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of the phthalocyanine ring through extended Huckel calculations by Schaffer et al. [41]. A shoulders peak at about 630 nm arise from face-to-face aggregates because of extensive coupling between the π-electron systems of adjacent rings. The P2W18 has a characteristic absorption–sharp peak at 210 nm and

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Fig. 4. Plots of the absorbance values for quartz-supported multilayer films at 340 nm [(P2W18–AB)n and (AB–P2W18/PSS)n] and 690 nm of [(P2W18–AB)n] as a function of n.

weak at 310 nm (the figure is not shown), which are due to the terminal oxygen and bridge oxygen to tungsten charge transfer transitions, respectively. Fig. 3(a) shows the UV–Vis absorption spectra of (P2W18– AB)n multilayers assembled on a precursor film on quartz substrates (on both sides). The substrates were pre-deposited with two layers of polyelectrolyte prior to deposition of AB and P2W18 to ensure maximum surface charge density and to provide equal surface conditions within each type of substrate. As shown in Fig. 3(a), these films all exhibit the characteristic absorption of the AB cation in the UV region with characteristic bands at 340 nm, 630 nm and 690 nm when comparing to the AB solution (Fig. 2), which confirms the incorporation of AB into the composite films. The characteristic band at about 340 nm is due to the combination of the bands for AB and P2W18 at 342 nm and 310 nm respectively. At 340 nm, there is an increase of optical density at both P2W18 and AB adsorption steps. At 690 nm there is no absorbance for P2W18, and the film optical density increased only at the AB adsorption steps. Thus, the incremental increase is smaller at this longer wavelength. There is a ca. 10 nm red shift for the bands at 690 nm, however, when compared to that of the solution absorption spectrum of AB solution (0.5 mg/mL) (Fig. 2) at 680 nm. This may be related to the electrostatic interaction between the P2W18 and AB. It is also found that the Q bands at 690 nm and 630 nm are broadened and weakened, indicating the occurrence of molecular aggregation of the compound in the films [42]. Fig. 4 displays the plots of the absorbance values for quartzsupported (P2W18–AB)n multilayer films with n at 690 nm(a) and 340 nm(b) as a function of the number of (P2W18–AB)n bilayers. From the inset of Fig. 4(a), we can see that the plots of the absorbance at 340 nm and 690 nm for the deposited LBL films vs. the layer number of LBL film appears in a straight line, suggesting that P2W18–(AB) are constantly incorporated in the multilayer. As shown in Fig. 4(c), however, in comparison with the film (AB–P2W18/PSS)n which was produced from prior mixing of

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P2W18 with polyelectrolyte PSS, smaller amount of AB or P2W18 was incorporated in each bilayer and worse linearity is observed. Such difference could be observed from the different denseness of surface color for the two films of (AB–P2W18)n and (AB–P2W18/ PSS)n. This is because that there are some difficulties in immobilization of small molecules and direct assembly of non-flexible components. The reason for this case is maybe that electrostatic attraction cannot be maximized among globular and plane plateshaped species [43]. To solve this problem, we mixed the solution of P2W18 with PSS first, and the mixed solution was then used for the purpose of alternate assembly with AB. The activity increase may be ascribed to de-aggregation for P2W18. And what's more P2W18 and PSS were co-adsorbed with physical entanglement of P2W18 in the PSS layer for some extent [44]. The films (AB– PSS)n have also been fabricated and good growth linearity and stability was observed, which indicated better electrostatic interaction between small non-flexible molecules and flexible polyelectrolyte molecules. It was believed that the good electrostatic interaction between PSS and AB should be responsible for the stable co-adsorption of P2W18. 3.2. Cyclic voltammetry for P2W18 solution and (P2W18/AB)n film Cyclic voltammetry (CV) of an aqueous 1 mM P2W18 solution (in 0.1 M Hac–NaAc buffer at pH = 3.2), using a bare ITO-coated glass electrode, (immersion area 1.0 × 1.0 cm2 ) consists of four clearly potential peaks A1 = 102.5 mV, A2 = − 77.5 mV, A3 = − 452.2 mV, A4 = − 718.2 mV during the

Fig. 5. Cyclic voltammograms of 1 mM P2W18 in solution (line a, ITO-coated glass as working electrode) and a (P2W18–AB)15-modified ITO electrode in Hac–NaAc buffer solution (line b) (platinum coil as the counter electrode and Ag/AgCl/KCl (3 mol/L) as reference electrode with scan rate = 50 mV/s).

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cathodic peaks V(c1) = − 424.0 mV, V(c2) = − 720.4 mV and V(c3) = − 1000.0 mV during the cathodic sweep and three anodic peaks V(a1) = −46.7 mV, V (a2) = −550.6 mV and V(a3) = −821.0 mV during the anodic sweep (Fig. 5(b)), a1 and a2 are the counter parts of c1 and c2 which have a little difference when comparing to the peaks of 10− 3 M P2W18 in solution. Both the first and second pairs of peaks, a1/c1, a2/c2 correspond to two 2e−/2H+ redox process. However, in comparison with the first and second pair of peaks of the P2W18 solution, we could only observed one pair of broad peaks for (P2W18–AB)15 multilayer films. This may result from the possible interaction between P2W18 and AB. Additionally, the protonation plays an important role in the charge compensation of the self-assembly films [47]. When protons are difficult to incorporate into the reduction process in a self-assembled film, the neighboring two cathodic peaks will be combined into one. Similarly, deprotonation of reduced species also depends on a more positive potential, which may make the adjacent two anodic peaks one. Such a merging phenomenon is more obvious for thicker multilayer films and at higher scan rates. The CV curve of P2W18 in solution and the multilayer film demonstrates that the properties of P2W18 are fully maintained in the multilayer film. Additionally, smaller peak separation for the a2/c2 and a3/c3 couples and sharper peak current occurs for the (P2W18–AB)n modified ITO electrode in Hac–NaAc buffer (pH = 3.0) than that at bare ITO-coated glass electrode in the same electrolyte, namely two-electron, two6− proton waves are easier to appear for P2W18O62 modified electrode under the same conditions. This may be attributed to the modified electrode similar to aprotic organic environment for 6− P2W18O62 [48].

Fig. 6. (a) Cyclic voltammograms of 1 mM P2W18 in solution at different scan rates (the variable “v” denotes the scan rate; from inner to outer: 25, 50, 100, 200, 300 mV/s) (b) the dependence of the second anodic peak current on scan rates. The inset shows the relationship plot of the second anodic peak current with square root of the scan rate (the v1 / 2 denotes the square root of the scan rate).

anodic sweep and four potential peaks C1 = − 30.5 mV and C2 = − 203.2 mV, C3 = − 688.0 mV C4 = − 1007.6 mV during the cathodic sweep (Fig. 5(a)). A1, A2, A3, A4 are the anodic counterparts of C1, C2, C3 and C4. The C1/A1 and C2/A2 pairs of peaks correspond to 1e− /1H+ , other two pairs of peaks C3/ A3 and C4/A4 correspond to 2e− /2H+ . In addition, the peak values are somewhat different to those reported in the literature [45,46] because of different electrode and experimental parameters used. Cyclic voltammograms in the potential range +0.3 V to − 1.1 V of the (P2W18–AB)15-modified ITO electrode (immersion area 1.0 × 1.6 cm2) in Hac–NaAc buffer solution with pH = 3.2, using a platinum wire as the counter electrode and Ag/ AgCl/KCl (3 mol/L) as reference electrode, displays three

Fig. 7. Reduction and oxidation of nitrite at a bifunctional (P2W18–AB)15 film electrode in Hac–NaAc buffer containing (a) 0 mM, (b)10 mM (c) 16 mM, (d) 25 mM nitrite. Scan rate 50 mV/s.

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Fig. 6(a) shows the cyclic voltammograms of the (P2W18– AB)15 film restricted to the tungsten wave at different scan rates in the Hac–NaAc buffer (pH = 3.0). When the scan rate was varied from 20 to 300 mV/s, the potentials changed gradually: the cathodic peak positions shifted to the negative direction and the corresponding anodic peak positions shifted to the positive direction with increasing scan rate. Moreover the increasing ΔEp (potential peak separations) was observed as the thickness of the film grew, which can be ascribed mainly to the increment of the obstruction of ions and electrons transfer shuttling in the thick film. The plots of the second peak current versus scan rates are shown in Fig. 6(b). At scan rates lower than 100 mV/s, the anodic currents (Ipa) were proportional to the scan rate suggesting that redox process is surface-confined; however, at scan rates higher than 100 mV/s, Ipa were proportional to square root of the scan rate, which indicates that the redox process is diffusion-controlled. 3.3. Electrocatalytic activities on the reduction of HNO2 and IO3− of the multilayer film (AB–P2W18)n As is known, the electroreduction of nitrite requires a large overpotential, and no response is observed in the range +1.2 to − 1.1 V in 0.1 M NaAc–HAc buffer (pH = 3.0) containing 5 mM NO2- with bare ITO electrode. But when (P2W18–AB)15 modified ITO electrode was used as the working electrode, with the addition of nitrite, the reduction peak currents increase while the corresponding oxidation peak currents decrease, suggesting that nitrite reduction is mediated by especially fourelectro-reduced species. The catalytic current increases linearly with increasing concentration of nitrite (see Fig. 7). Since it is known that NO2− disproportionates to form NO and NO3− in acid solutions: HNO2 →Hþ þ NO−2 ð1ÞpKa ¼ 3:3 at 18-C 3HNO2 →HNO3 þ 2NO þ H2 O ð2Þ A large amount of NO substituted for NO2- participates in the reaction like other catalytic processes. So the reduction mechanism involving proton transfer is as follows (taking the 8− 2e− reduced product — P2W18O62 as representation): þ 6− P2 W18 O8− 62 þ NO þ mH →P2 W18 O62 þ productsðcontainingNÞ

At the same time, the electrocatalytic activity of the adsorbed AB towards nitrite oxidation is observed according to Fig. 8, in terms of a negative shift of the peak potential and an increase of the current intensity. The same multilayer films electrode also exhibits electrocatalytic response toward the IO3− reduction (the figure is not shown). 3.4. X-ray photoelectron spectra(XPS) of (P2W18–AB)n (n = 4) films

Fig. 8. XPS spectra of [PSS/PAH/(P2W18/AB)4] film (a) Cu2p (b) N1s (c) W4f.

To identify the elemental composition of the multilayer films, we measured the XPS of the (P2W18–AB)n (n = 4) films. Although the XPS measurement gives only a semiquantitative elemental composition, the presence of C, O, N, Cu, P, and W

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elements in the film is confirmed and the percentage of these elements in this films were 12.95%, 18.84%, 3.17%, 1.19%, 1.05% and 59.05% respectively. Accordingly the molar ratio of P to W is also approximately established to be 1:9.6, which is close to the expected ratio of 2:18 in tungstophosphate anions used here. The XPS of the (P2W18–AB)n films (n = 4) are shown in Fig. 8. We observe that the peak intensities of the W4f, Cu2p and N1s levels (at 35.2 eV, 935.5 eV and 401.2 eV, respectively) increase with the increasing number of (P2W18–AB)n bilayers. The observed variation of the relevant peak intensities with successive deposition is clearly indicative of the constant film growth as a result of LBL deposition, in agreement with the UV–Vis spectra. 4. Conclusions This article demonstrates the preparation of hybrid multilayer films consisting of water-soluble cationic copper (II) phthalocyanine AB and Dawson-type P2W18. The P2W18containing multilayer films exhibit both electrocatalytic oxidation and electrocatalytic reduction of nitrite in acid media. The method we used is very simple and allows for easy preparation. A stepwise and regular deposition of Alcian Blue and P2W18 in both pure water and 0.01 M PSS were observed. The film (AB– P2W18/PSS)n obtained from 0.01 M PSS-based solutions showed greater amount of AB or POM incorporated each bilayer and almost the same CV activity as the film (AB– P2W18) n. Additionally, the stability was investigated by measuring voltammetric currents of (AB–P2W18/PSS)n. A negligible decrease was observed after storage in air for up to 1 month. Thus, (AB–P2W18/PSS)n multilayers formed by the LBL method were bifunctional catalytic systems with the stability needed for practical application. Scheme 1

Scheme 1. Schematic representation of the internal layer structure of a [PSS/ PAH(AB–P2W18/PSS)n] multilayer film self-assembled on a substrate. Note that this drawing is an oversimplification of the actual layer structure.

Acknowledgements The authors are thankful for the financial supports from the National Natural Science Foundation of China (Grant No. 20371010), the state Key Laboratory for Structural Chemistry of Unstable and Stable Species in Peking University(No. 03– 12), and the Natural Science Foundation of Jilin Technology Office of China(No. 20030512–1). References [1] L. Cheng, J.A. Cox, Chem. Mater. 14 (2002) 6. [2] N.M. Markovic, P.N. Ross, J. Electrochem. Soc. 141 (1994) 2590. [3] C.K. Lee, K.A. Striebel, F.R. McLarnon, E.J. Cairns, J.Electrochem. Soc. 144 (1997) 3801. [4] P.J. Kulesza, L.R. Faulkner, J. Electroanal. Chem. 259 (1989) 81. [5] P.J. Kulesza, G. Roslonek, L.R. Faulkner, J. Electroanal. Chem. 280 (1990) 233. [6] X.L. Wang, Q. Zhang, Z.B. Han, E.B. Wang, Y.Q. Guo, C.W. Hu, J. Electroanal. Chem. 563 (2004) 221. [7] B. Keita, D. Bouaziz, L. Nadjo, A. Deronzier, J. Electroanal. Chem. 279 (1990) 187. [8] S. Dong, B. Wang, Electrochim. Acta 37 (1992) 11. [9] B. Wang, S. Dong, J. Electroanal. Chem. 328 (1992) 245. [10] S. Dong, M. Liu, J. Electroanal. Chem. 372 (1994) 95. [11] M. Liu, S. Dong, Electrochim. Acta 40 (1995) 197. [12] S. Dong, L. Cheng, X. Zhang, Electrochim. Acta 43 (1998) 563. [13] G. Bidan, E.M. Genies, M.J. Lapkowski, Electroanal. Chem. 251 (1998) 297. [14] K. Araki, M.J. Wagner, M.S. Wrighton, Langmuir 12 (1996) 5393. [15] S.L. Clark, E.S. Handy, M.F. Rubner, P.T. Hammond, Adv. Mater. 11 (1999) 1031. [16] N.A. Kotov, I. Dekany, J.H. Fendler, J. Phys. Chem. 99 (1995) 13065. [17] I. Ichinose, H. Tagawa, S. Mizuki, Y. Lvov, T. Kunikake, Langmuir 14 (1998) 187. [18] F. Caruso, D.G. Kurth, D. Volkmer, M.J. Koop, A. Muller, Langmuir 14 (1998) 3462. [19] D.G. Kurth, D. Volkmer, M. Ruttorf, B. Richter, A. Muller, Chem. Mater. 12 (2000) 2829. [20] J.S. Zhang, L. Xu, Y. Cui, W.X. Cao, Z. Li, Mater. Chem. Phys. 90 (2005) 47. [21] Y. Shen, J.Y. Liu, J.G. Jiang, B.F. Liu, S.J. Dong, J. Phys. Chem. B 107 (2003) 9744. [22] A. Alonso, B. Etxaniz, M.D. Martinez, Food Addit. Contam. 9 (1992) 111. [23] N. Sparatu, T.N. Rao, D.A. Tryk, A. Fujishima, J. Electrochem. Soc. 148 (2001) E112. [24] M.J. Moorcroft, J. Davis, R.J. Commpton, Talanta 54 (2001) 785. [25] V.D. Matteo, E. Esposito, J. Chromatogr. A 789 (1997) 213. [26] L. Ruhlmann, G. Genet. J. Electroanal. Chem. 568 (2004) 315. [27] K. Oyura, H. Ishikawa, J. Chem. Soc. Faraday Trans. 180 (1984) 2243. [28] M.H. Barley, K.J. Taheuch, T.J. Meyer, J. Am. Chem. Soc. 108 (1986) 5876. [29] M.H. Barley, M.R. Rhodes, T.J. Meyer, Inorg. Chem. 26 (1987) 1746. [30] J.N. Younathan, K.S. Wood, T.J. Meyer, Inorg. Chem. 31 (1992) 3280. [31] S.H. Chang, Y.O. Su, Inorg. Chem. 33 (1994) 5847. [32] I. Tansguchi, N. Nakoshima, K. Matsushita, K. Ysukouchi, J Electroanal. Chem. 224 (1987) 199. [33] B. Keita, L. Nadjo, R. Contant, M. Fournier, G. Herve, French Patent (CNRS) 89/1,728; 10 February 1989. [34] B. Keita, L. Nadjo, R. Contant, M. Fournier, G. Herve, Eur. Patent, (CNRS), Appl. EP 382, 644. [35] V. Zucolotto, M. Ferreira, M.R. Cordeiro, C.J.L. Constantino, D.T. Balogh, A.R. Zanatta, W.C. Moreira, O.N. Oliveira Jr., J. Phys. Chem. B 107 (2003) 3733. [36] C.A. Caro, F. Bedioui, J.H. Zagal, Electrochim. Acta 47 (2002) 1489. [37] J. Locklin, K. Shinbo, K. Onishi, F. Kaneko, Z. Bao, R.C. Advincula, Chem. Mater. 15 (2003) 1404. [38] Y.M. Lvov, G.N. Kamau, K.L. Zhou, J.F. Rusing, J. Colloid Interface Sci. 212 (1999) 570.

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