Electrochimica Acta 47 (2002) 1713–1719

www.elsevier.com/locate/electacta

Electrostatic incorporation of alizarin red S intopoly[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium] films

Takahiro Yamaguchi *, Yuji Yokono, Kohshin Takahashi, Teruhisa KomuraDepartment of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa Uni�ersity, 40-20 Kodatsuno 2-chome,

Kanazawa 920-8667, Japan

Received 1 October 2001; received in revised form 3 January 2002

Abstract

Poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium]chloride films on glassy carbon electrodes greatly increased the voltammetricpeak currents of alizarin red S (ARS), which is an anthraquinone derivative. We propose that the adsorption sites of ARS in thepolymer channel be divided into two different classes: the hydrophobic interfacial zone and the hydrated zone. The theoreticalcurve based on our proposed model well fits the experimental data points for the relationship between the ARS concentrationsin the polymer film and in the immersing solution. The electrocatalytic effect produced by the polymer film is attributed to aneffective extraction of ARS from very dilute solution (the partition coefficients on hydrophobic interfacial zones, K1 is 2.5×107,and that on hydrated zones, K2 is 6.3×104 in 0.2 M H2SO4). A K1/K2 value of 400 means that the ion-exchange ability of ARSin the hydrophobic interfacial zone is much larger than that in the hydrated zone. Although anthraquinone-2,6-disulfonate andacid violet were strongly bound to the PMPP film, ARS can not been fixed to the polymer because of a weak binding force. Usingour proposed model, the ion-exchange ability of PMPP film is controlled by both of electrostatic and hydrophobic interactions,but the adsorption ability changes with steric hindrance between the anionic species and polycationic films. © 2002 ElsevierScience Ltd. All rights reserved.

Keywords: Poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium]chloride film; Alizarin red S; Electrocatalytic effect; Loaded ionomer; Hydrophobicinteraction

1. Introduction

Conjugated polymers, such as polyaniline,polypyrrole and polythiophen are electrochemicallyswitched between conducting and insulating states witha substantial change in optical and chemical properties[1]. Such polymers have received considerable attentionbecause of possible applications in organic batteries,microelectronic devices and electrocatalysis. Electroac-tive polymer films deposited on electrodes generallyprovide very dense redox sites and can mediate electrontransfer to a solution species. The selective response ofthis kind of polymer towards dissolved ions has madethem useful as an important class of ion and molecularsensors [2].

The oxidation state of these electroactive polymerscoated on the electrode surfaces permit ionic species to

move into or out of the polymer films in order tomaintain their electroneutrality. This can be utilized forcontrolling the release or binding of a particular ionfrom or to a polymer film [3–6]. To achieve thispurpose, one can use the following electrochemicalreactions that cause a substantial change in the interac-tion between the ionic species and the polymer: achange in the oxidation state of electroactive polymerand change in the charge of the redox ions incorporatedinto the polyionic polymer material [7]. Although alarge number of studies have been made on redoxswitching in conducting polymer films [8,9], there islimited information available on the binding force ofcounterions to ionomers.

The anion-exchange properties of the oxidized formof polypyrrole have often been used to immobilizecatalytic species on electrode surfaces: metallopor-phyrins [10], phthalocyanines [11] and ferricyanide [7]were incorporated in the polymer films as counter-ions.On the other hand, the binding capacity of polypyrrole

* Corresponding author. Fax: +81-76-234-4800.E-mail address: t-yamagu@t.kanazawa-u.ac.jp (T. Yamaguchi).

0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.PII: S 0 0 13 -4686 (02 )00011 -7

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–17191714

disappears when it is reduced. There has been consider-able interest in preparing functionalized polypyrrolefilms [12]. If the conductive polypyrrole backbone facil-itates rapid electron transport to all areas of the film,the functionalized polypyrroles are attractive in themediation of electron transfer and the electrocatalysisof slow electrochemical reactions. Pyrrole monomerswith alkylammonium or pyridinium substituents haveelectrolytically produced polypyrrole films having im-proved anion-exchange properties [13,14]. Mao andPickup [15] have prepared a polypyrrole film with an-ion-exchange properties by polymerizing quaternized3-(pyrrol-1-ylmethyl) pyridine and showed that thispolycationic film can incorporate large amounts ofhexacyanoferrate (II). The high concentration of per-manent cationic sites in the film is expected to increaseits permeability to ions. This increase should lead tomore rapid ion transport through the film, which is aphysical property required for chemically modified elec-trodes. We have recently studied the incorporation oforganic anions into the polycationic film and foundthat the voltammetric currents of indigodisulfonate[4,16] and anthraquinone-2,6-disulfonate [4,17] aregreatly enhanced. However, few papers have been pub-lished on the binding force of organic ions to theion-exchange polymer.

In this paper, we report the large electrocatalyticaction of a poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridi-nium]chloride film to alizarin red S (ARS), and describethe characteristics of the binding interaction betweenthe incorporating ion and polymer.

2. Experimental

2.1. Materials

1-Methyl-3-(pyrrol-1-ylmethyl)pyridinium perchlo-rate (MPP) was prepared from 3-(pyrrol-1-ylmethyl)-pyridine (Aldrich) and methyl iodide according to theliterature procedure [15], followed by substitution of I−

with ClO4−. The prepared compound was characterized

by 1H NMR, IR spectroscopy and elemental analysis[18]. Acetonitrile (Kanto Chemical, GR) was distilledover calcium hydride immediately before use, and tetra-butylammonium perchlorate (Kanto) was recrystallized

from ethanol and dried under vacuum at 70 °C. ARS(Wako) and all other chemicals were analytical gradecommercial materials and used as received.

2.2. Electrode modification

The films of poly-1-methyl-3-(pyrrol-1-ylmethyl)pyri-dinium (Fig. 1 (PMPP)) perchlorate were electrolyti-cally deposited on glassy carbon (GC, area=0.07 cm2)at a current density of 0.4 mA cm−2 from acetonitrilesolutions containing 0.05 M MPP and 0.1 M tetrabuty-lammonium perchlorate. The polymer films were grownup to a polymerization charge density of 48 mC cm−2

for many measurements. For similar films, a linearrelationship with a slope of 0.15 C cm−2 �m−1 hasbeen observed between the polymerization charge den-sity and film thickness [15]. In the present work, thisrelationship was used for estimating the film thickness;the above polymerization charge density corresponds toa film thickness of 0.32 �m. The PMPP films loadedwith ARS were prepared by immersing polymer-coatedelectrodes in 0.2 M H2SO4 aqueous solutions contain-ing ARS for 10 h, followed by washing with water.Then, ARS-loaded film electrodes were used for electro-chemical measurements in the pure 0.2 M H2SO4

solution.

2.3. Electrochemical measurements

All electrolyte solutions for electrochemical measure-ments were prepared with doubly-distilled water andpurged with nitrogen gas before and during the mea-surements. Electrochemical experiments were carriedout in a two-compartment, three-electrode glass cell atroom temperature. A large Pt gauze (�10 cm2) and anAg/AgCl/3.3 M KCl electrode were used as counterand reference electrode, respectively. All electrode po-tentials are referred to the Ag/AgCl electrode. Cyclicvoltammograms were obtained by a Hokuto DenkoHA-501 potentiostat/galvanostat coupled with HB-104function generator and recorded on a Yokogawa 3025X-Y recorder.

3. Results and discussion

N-substituted polypyrroles possess a more positiveredox potential and a lower degree of doping levelcompared with that of polypyrrole [19]. In aqueousmedia, PMPP films obtained were irreversibly oxidizedat around 0.8 V on the first positive scan, which led toa virtual disappearance of electroactivity. This oxida-tion wave was caused by monomer and oligomer re-mained in the film. Therefore, the film going through afew cycle (0–0.9 V) produced no faradaic currents inthe potential range where ARS showed a pair of redoxFig. 1. Chemical structure of PMPP.

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–1719 1715

Scheme 1. The redox reaction of ARS.

a 0.2 M H2SO4 solution containing 20 �M ARS afterimmersing in the solution for 1 h. The cathodic peakcurrent of ARS at the PMPP-modified electrode was 30times as large as that at a bare GC electrode. As PMPPis electrochemically inactive and non-conducting in thepotential range of +0.3 to −0.75 V, this result indi-cates that ARS is concentrated selectively in the PMPPfilm in spite of the existence of supporting electrolyte.Fig. 3 shows the dependence of the cathodic peakcurrent at a PMPP-modified electrode on the immer-sion time. The cathodic peak current was proportionalto the immersion time up to 3 h and then saturatedover 6 h. When a similar measurement was done foranthraquinone-2, 6-disulfonate, the cathodic peak cur-rent was saturated over 20 h. Therefore, the rate ofincorporation of ARS is faster than that ofanthraquinone-2,6-disulfonate.

After having been equilibrated with 0.2 M H2SO4

solution containing (0–100 �M) ARS for 10 h, thePMPP film was washed with water and transferred to apure 0.2 M H2SO4 solution. Fig. 4 shows the cyclicvoltammograms of ARS-loaded film electrodes in anARS-free 0.2 M H2SO4 solution. The peak current ofARS increased with ARS concentration (Cs) in thesoaking solution. When Cs was equal to 100 �M, atailing of the voltammetric wave occurred clearly,which indicates the diffusion-like propagation of thecharge. Fig. 5 shows the dependence of the cathodicpeak current on Cs. Two points are noteworthy. First,the cathodic peak current increased rapidly up to 0.1�M. Second, it increased more slowly as the concentra-

Fig. 2. Cyclic voltammograms of a PMPP-modified GC electrode(— ) and bare GC electrode (---) in a 0.2 M H2SO4 solution contain-ing 20 �M ARS. Scan rate, 50 mV s−1; film thickness, 0.32 �m. TheARS-loaded films were prepared by immersing PMPP-modified elec-trode in the same solution for 1 h.

Fig. 3. Changes in cathodic peak current with time. Solution, 10 �MARS in 0.2 M H2SO4 (�), 10 �M anthraquinone 2,6-disulfonate in0.2 M H2SO4 (�). Scan rate, 100 mV s−1; film thickness, 0.32 �m.

Fig. 4. Cyclic voltammograms at an ARS-loaded film electrode in apure 0.2 M H2SO4 solution. Scan rate, 10 mV s−1; film thickness,0.32 �m. The ARS-loaded film were prepared by immersing a PMPP-modified electrode in a 0.2 M H2SO4 solution containing 1 �M (-·-·-),10 �M (— ), and 100 �M (---) ARS for 10 h.

waves (Scheme 1, its formal potential= −0.197 V in0.2 M H2SO4). Fig. 2 shows representative cyclicvoltammograms for a PMPP-modified GC electrode in

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–17191716

Fig. 5. Cs dependence of ipc at an ARS-loaded film electrode in a pure0.2 M H2SO4 solution. Scan rate, 10 mV s−1. After having beenequilibrated with 0.2 M H2SO4 solution containing ARS of differentconcentrations for 10 h, the film was transferred to the supportingelectrolyte solution.

We evaluated the ARS concentration in the PMPPfilm from the voltammetric charge (Q) consumed inreducing all the redox ions within the film. The chargeQ was determined from the area under linear sweepvoltammograms measured at a scan rate of 1 mV s−1.At Cs=100 �M, a tailing of the wave occurred. Thistailing indicated either the appearance of diffusion-likecharge transport or an increased contribution fromcapacitive charging. Then that Q was included in esti-mated error of �10%. The ARS concentration (Cf)within the film in equilibrium with the immersing solu-tion was calculated from:

Cf=�/d=Q/2FAd (1)

where � is the surface coverage, F is the Faradayconstant, A is the electrode area, and d is the filmthickness.

Fig. 7 shows the relationship between Cf and Cs.Although the Cf–Cs curve is similar to the ipc versus Cs

curve (as shown in Fig. 5), Cf increased slowly up toCs=100 �M after an early rapid rise. Cf showed aconstant value in the Cs range from 0.5 to 2 �M, abovewhich it increased again. Such a dependence of Cf on Cs

is similar to the Brunauer–Emmett–Teller adsorptionisotherm, but this isotherm can not be applied to ourdata because of hardly acceptable assumptions for theisotherm. Therefore, we assumed that the adsorptionsites of ARS in the polymer channel are divided intotwo different classes: the hydrophobic interfacial zonewhere the adsorption is controlled by both of electro-static and hydrophobic interactions, and the hydratedzone where the adsorption is controlled only by electro-static interaction (as shown in Fig. 8). Since cationicPMPP binds anionic ARS probably by electrostatic

Fig. 6. Variations in �Ep with Cs. Scan rate, 10 mV s−1 (�), 1 mVs−1 (�). The experimental condition were the same as that in Fig. 4.

Fig. 7. Relationship with Cf and Cs.

tion was increased from 0.1 to 50 �M, above which itdecreased. The peak-to-peak separation (�Ep) (mea-sured at potential scan rates of 1 and 10 mV s−1) isplotted against Cs in Fig. 6. Since the solution resis-tance between the working and reference electrodes wasequal to about 40 � as measured by the impedancetechnique, a negligible IR drop of 0.9 mV is expectedwhen the peak current at 10 mV s−1 is 23.6 �A.Interestingly, the values of �Ep decreased up to 10 �Mand then increased to 120 mV (at 10 mV s−1) and 50mV (at 1 mV s−1) at Cs=100 �M. The increase of �Ep

indicates the decrease of the apparent electrode reactionrate of ARS.

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–1719 1717

Fig. 8. Model for a polymer channel consisting of two adsorptionzones: the hydrophobic interfacial zone and the hydrated zone.

ARS−+PMPP+ClO4−�ARS−·PMPP++ClO4

− (2)

where PMPP+ represents cationic pyridinium groups ofthe polymer. Only the adsorption in the hydrophobicinterfacial zone was analyzed by the Langmuir adsorp-tion isotherm, and the other adsorption was analyzedby using the partition coefficient (K2) of the anionbetween a PMPP film and 0.2 M H2SO4 solution.According to the Langmuir adsorption isotherm, therelationship between the ARS concentration (Cf1) in thehydrophobic interfacial zone and Cs is given by:

Cf1=K1CsCf1, max/(1+K1Cs) (3)

K1 is the partition coefficient of ARS between thehydrophobic interfacial zones and 0.2 M H2SO4 solu-tion. Using Eq. (3) and the concentration (Cf2=K2Cs)of ARS adsorbed irregularly on hydrated zones, we canexpress Cf as:

Cf=Cf1+Cf2=K1CsCf1, max/(1+K1Cs)+K2Cs (4)

As shown in Fig. 7(b), the value of Cf is likely to besaturated at Cs=0.5–2 �M. Since Cf1 became a con-stant (Cf1, max) at Cs�2 �M, we obtained K2=6.3×104 from the slope of the Cf versus Cs plot at Cs�5�M. The Cf1 value was calculated by subtracting Cf2

from Cf. Rearranging Eq. (3), we plotted Cs/Cf1 againstCs in Fig. 9. The plot gave a straight line; this slope andintercept yielded K1=2.5×107 and Cf1, max=0.24 M.The ion-exchange capacity of PMPP has been found tobe 5.6 M for monovalent anions by Mao and Pickup[15]. If the binding force between ARS and PMPP isonly electrostatic interaction, Eq. (4) is given by:

Cf=6.0×106Cs/(2.5×107Cs+1)+6.3×104Cs

(if Cs�84 �M, then Cf=5.6) (5)

A solid line in Fig. 10 shows a theoretical curvecalculated from Eq. (5). The curve fits the experimentaldata points well. A Cf value of 6.1�0.5 M estimatedexperimentally at Cs=100 �M is slightly higher than5.6 M, which is the pyridinium group concentration inPMPP. It indicates the possibility that the dominantbinding force of ARS to the PMPP film is not onlyelectrostatic interaction but also hydrophobic interac-tion peculiar to organic molecules [4,16–18]. A K1/K2

value of 400 means that the ion-exchange ability ofARS in the hydrophobic interfacial zone is much largerthan that in the hydrated zone.

We evaluated the concentration of ARS in the PMPPfilm by recording the cyclic voltammogram of ARS-loaded film in a pure supporting electrolyte solution.After the PMPP film has been equilibrated with 0.2 MH2SO4 solution containing 20 �M ARS, its cyclicvoltammogram was measured in a pure 0.2 M H2SO4

solution. Fig. 11 shows the cathodic peak current ipc/ipc(t=0) of cyclic voltammograms measured as a func-tion of time. A rapid decrease in ipc with time indicates

Fig. 9. Plot of Cs/Cf1 against Cs. Cf1 denotes the ARS concentrationin the hydrophobic interfacial zone.

Fig. 10. Cf plotted against Cs. A dotted line shows the Cf versus Cs

curve calculated from equation 6.

binding, the adsorption equilibria both in the hydro-phobic interfacial zone and in the hydrated zone can beinterpreted in terms of ion exchange reaction (2).

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–17191718

that ARS is rapidly released from PMPP. On the otherhand, for anthraquinone-2,6-disulfonate and acid violet(Fig. 12), ipc did not decrease after the loaded films wereexposed to a pure 0.2 M H2SO4 for 6 h. Althoughanthraquinone-2,6-disulfonate and acid violet arestrongly bound to the PMPP film, ARS can not beenfixed to the polymer because of a weak binding forcebetween ARS and the PMPP film. Therefore, when theloaded film was exposed to an ARS-free 0.2 M H2SO4

solution, ARS in the film is exchanged by SO42−, and

released from the PMPP film. Interestingly, this de-crease (ipc/ipc(t=0)=0.25 at 7 h) is nearly equal to thevalue of Cf1, max/Cf observed upon immersing the film ina 0.2 M H2SO4 solution containing 20 �M ARS. Wesuggest that ARS released rapidly from the film origi-nates from the ion irregularly adsorbed on hydratedzones in the film channel, and the ion adsorbed onhydrophobic interfacial zones is strongly bound to thepolymer, likewise anthraquinone-2-sulfonate, an-thraquinone-2,6-disulfonate and acid violet.

From a previous study of anthraquinone derivativesand indigo derivatives [4], the order of partition coeffi-

cients estimated demonstrated that they were not deter-mined solely by the magnitudes of negative charges ofanion. We proposed that the dominant binding oforganic anions to the polymer film was due to electro-static and hydrophobic interactions. Nevertheless, thedifference in adsorption phenomenon between ARSand anthraquinone derivative is not explicable by usingthose interactions. As anthraquinone-2-sulfonate with acharge number of 1- was also strongly bound to thepolymer in the previous study [4]. We pay attention tothe position of anionic functional groups in molecularstructure. Two distant sulfonate groups of an-thraquinone-2,6-disulfonate are fixed strongly to thepyridinium groups of the polymer by electrostatic inter-action. However, in the case of ARS, steric hindranceby 1- and 2-hydroxyl groups adjacent to the 3-sulfonategroup has weakened the electrostatic binding forcebetween ARS and the polymer. The adsorption of ARSin the hydrated zone is controlled only by electrostaticinteraction. Thus, the adsorption ability of ARS in thehydrated zone is greatly inferior to that of ARS onhydrophobic interfacial zones. In addition, Fig. 11shows ipc/ipc(t=0) versus t plots obtained at different pHvalues of ARS-free 0.2 M Na2SO4 solutions. The ipc/ipc(t=0) at t=6 h indicated a larger value as the solutionpH increased. Especially, for a thick ARS-loaded film(d=0.32 �m), the ipc/ipc(t=0) versus t relation at pH 7was clearly different from that at pH 1. These resultsindicates that deprotonation of the 1-hydroxyl group ofARS at high pH values [20] raised the electrostaticbinding force between ARS and PMPP film. Neverthe-less, we consider that the binding interaction of twosuch adjacent anionic groups with the polymer pyri-dinium group is weaker than that of anthraquinone-2,6-disulfonate having two distant sulfate groups.

Thus, if the adsorption state of ARS in the film isdivided two classes, we can interpret the dependence of�Ep on Cs (Fig. 6) as below. The value of �Ep shows arough estimate of the apparent electrode reaction rate.This reaction rate contains electron transfer rate to/from the electrode and electron transport rate in thefilm. In the Cs range from 1 to 10 �M, where Cf is closeon Cf1, max, ARS-loaded electrodes behave as a re-versible monolayer-adsorption system (�Ep=0) at slowscan rates. Since the charge transfer resistance betweenARS and the electrode is negligible, the apparent elec-trode reaction rate of ARS in the film depend on twoelectron transport processes; electron self-exchange be-tween neighboring redox centers, and the physical dif-fusion of incorporated redox species. In the range ofCs�10 �M, ARS incorporated in the film mainlyadsorb on hydrophobic interfacial zones, and the elec-tron transport process in the film is controlled byelectron self-exchange between the redox speciesstrongly adsorbed. Therefore, the distance between

Fig. 11. Decreases in ipc/ipc (t=0) with time. Film thickness: (a) 0.08�m; (b) 0.32 �m. The solution pH: (�) 1, (�) 5, and (�) 7. Scanrate=100 mV s−1. After having been equilibrated with a 0.2 MH2SO4 solution containing 20 �M ARS, a PMPP-coated electrodewas transferred to the ARS-free solution.

Fig. 12. The molecular structure of anthraquinone-2,6-disulfonate (A)and acid violet (B).

T. Yamaguchi et al. / Electrochimica Acta 47 (2002) 1713–1719 1719

neighboring adsorbed redox species became longer asCf decreases from Cf1, max to 0 (Cs�1 �M), and thus�Ep increases and the apparent electrode reaction ratedecreases. On the other hand, at much higher concen-trations than Cf1, max, �Ep increase gradually in spite ofa decrease in the distance between neighboring redoxcenters. This electron transport process probably dis-turb the diffusion of ARS weakly adsorbed on hydratedzones; the diffusion occur at ARS reduction in the filmand at the film and solution interface. Thus, ARS-loaded electrodes behave as a diffusion-controlled sys-tem in the range of Cs�10 �M.

4. Conclusions

An increased voltammetric current at the PMPPfilm-modified electrode is attributed to large partitioncoefficients of ARS between the polymer and the solu-tion in our proposed model, K1=2.5×107, and K2=6.3×104. Our explanations of the results obtained areshown below:1. The cationic PMPP film incorporates large amounts

of anionic ARS as well as anthraquinone-2,6-disul-fonate and indigodisulfonate, probably by electro-static binding.

2. The adsorption site of ARS in PMPP film aredivided into two classes: the hydrophobic interfacialzone and the hydrated zone in the polymer channel.Although ARS adsorbed on hydrophobic interfacialzones is strongly bound to the polymer, the ionadsorbed on hydrated zones is weakly bound.

3. The ion-exchange ability of PMPP is dominated byboth of electrostatic and hydrophobic interactions,but its adsorption ability changes with both electro-

static interaction and steric hindrance between an-ionic species and polycationic films.

References

[1] A.F. Diaz, M.T. Nguyen, M. Leclerc, in: I. Rubinstein (Ed.),Physical Electrochemistry, Marcel Dekker, New York, 1995, p.555.

[2] M.E.G. Lyons, in: M.E.G. Lyons (Ed.), Electroactive PolymerElectrochemistry, Part 1, Plenum Press, New York, 1994, p. 237.

[3] C.D. Paulse, P.G. Pickup, J. Phys. Chem. 92 (1988) 7002.[4] T. Komura, Y. Mori, T. Yamaguchi, K. Takahashi, Elec-

trochem. Acta 42 (1997) 985.[5] B. Zinger, L.L. Miller, J. Am. Chem. Soc. 106 (1984) 6861.[6] M.W. Espenscheid, C.R. Martin, J. Electroanal. Chem. 188

(1985) 73.[7] L.L. Miller, B. Zinger, Q.-X. Zhou, J. Am. Chem. Soc. 109

(1987) 2267.[8] M.E.G. Lyons, in: M.E.G. Lyons (Ed.), Electroactive Polymer

Electrochemistry, Part 1, Plenum Press, New York, 1994, p. 1.[9] A.J. Heeger, in: T.A. Skotheim (Ed.), Handbook of Conducting

Polymers, vol. 1, Marcel Dekker, New York, 1986, p. 729.[10] A. Deronzier, R. Devaux, D. Limosin, J.M. Latour, J. Elec-

troanal. Chem. 324 (1992) 325.[11] C.S. Choi, H. Tachikawa, J. Am. Chem. Soc. 112 (1990) 1757.[12] A. Deronzier, M. Essakalli, J. Phys. Chem. 95 (1991) 1737.[13] H. Mao, P.G. Pickup, J. Phys. Chem. 96 (1992) 5604.[14] S. Cosnier, A. Deronzier, J.C. Moutet, J.F. Roland, J. Elec-

troanal. Chem. 271 (1989) 69.[15] H. Mao, P.G. Pickup, J. Electroanal. Chem. 265 (1989) 127.[16] T. Komura, T. Kobayashi, T. Yamaguchi, K. Takahashi, J.

Electroanal. Chem. 454 (1998) 145.[17] T. Komura, Y. Yokono, T. Yamaguchi, K. Takahashi, J. Elec-

troanal. Chem. 478 (1999) 9.[18] T. Komura, Y. Mori, T. Yamaguchi, K. Takahashi, Elec-

trochem. Acta 42 (1997) 985.[19] A. Diaz, J. Castillo, K. Kanazawa, J. Logan, M. Salmon, O.

Fajardo, J. Electroanal. Chem. 133 (1982) 233.[20] J.-N. Li, J. Zhang, P.-H. Deng, J.-J. Fei, Analyst 126 (2001)

2032.