Electrochimica Acta 49 (2004) 4455–4460

Influence of charged tail groups of self-assembled monolayers onelectrodeposition of polyaniline

Li Niu a,b, Rose-Marie Latonena, Carita Kvarnströma,∗, Ari Ivaskaa

a Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, FIN-20500 Turku-Åbo, Finlandb State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, PR China

Received 16 March 2004; received in revised form 28 April 2004; accepted 2 May 2004

Available online 9 June 2004

Abstract

Self-assembled monolayers (SAMs) of thiols with three different tail groups,−COOH,−SO3Na, and−NH2, were used to modify theAu substrates for electrodepositing polyaniline (PANI). Electrochemical quartz crystal microbalance (EQCM) results indicated a slower rateof deposition of PANI on a SAM surface consisting of positively charged amine groups compared to polymerization on bare gold and on aSAM of carboxyl acid groups. The properties of the SAM layers are dependent on the pH value of the solutions, and are effective only atvery low pH values (pH< 2). A layer of the positively charged amino groups in acidic solution acted as a barrier for electron transfer inelectro-oxidation of aniline monomer. The positively charged SAM of amine groups also increased repulsion between the coupled anilinespecies and the electrode surface and in this way hindered electrodeposition. Modification of the surface with pre-patterned SAMs have beendemonstrated to be a convenient and practical way to fabricate selectively deposited thin films of polyaniline.© 2004 Elsevier Ltd. All rights reserved.

Keywords:Self-assembled monolayers; Selective electrodeposition; Polyaniline; Charged tail groups; Electrostatic repulsion

1. Introduction

Self-assembled monolayers (SAMs) have received con-siderable attention due to their convenient control of in-terfacial properties[1,2] for many different applications,such as corrosion prevention, improvement of adhesion,molecular recognition, sensors, and light-emitting diodes.SAM-modified surfaces can serve as templates in buildingcomplex multilayers[3] and in synthesis of materials ofwell-defined structures, i.e. special structures of conductingpolymers [4]. The influence of different SAMs, terminalgroups and type of molecular chain, on the formation, andproperties of the polymer layer has been reported in a largenumber of publications[5–26].

It has been shown that a SAM pre-adsorption on theelectrode surface can significantly increase the density ofan electrochemically grown polyaniline (PANI) film[5,6].

∗ Corresponding author. Tel.:+358 2 215 4419; fax:+358 2 215 4479.E-mail address:ckvarnst@abo.fi (C. Kvarnström).

Furthermore, it was also demonstrated that SAMs could beused to control the adhesion of PANI and other conduct-ing polymers. Adhesion of PANI increases with surface-freeenergy, which is controlled by the terminal group of theSAM, i.e. oxygen-rich hydrophilic surfaces promote strongadhesion. On the other hand, the adhesion is weak but therate of deposition of a conducting polymer is high at a hy-drophobic surface[7,8]. Selective deposition and pattern-ing of conducting polymers have been achieved with SAMsand has been reported in several papers[7,8,14,17,24–26].In previous reports, some polymerizable tail groups, suchas 4-aminothiophenol[5,6,9,10], thiophene[15], and pyr-role[18,21–23], showed a strong enhancement in the growthof polymer and a clear improvement in the morphologyof the conducting polymer films. Alkyl chains of SAMsalso show a great influence on the nucleation and growthof conducting polymers. Electrodeposition of conductingpolymers is initially inhibited by long alkyl chain SAMs[11,13,14,16,19,20,24–26]. As a result, ultra thin films ofconducting polymers can be accomplished on the defect sitesof a modified SAM[13,14,19].

0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2004.05.005

4456 L. Niu et al. / Electrochimica Acta 49 (2004) 4455–4460

A SAM surface can easily be modified by changing thepH value of the solution from its protonated form at low pHinto its corresponding base form at high pH[27–35]. Thecharge at the SAM-modified electrodes influences stronglythe interaction between the charged components in the so-lution and the electrode surface[27–30]. The structure ofthe SAM varies due to changes in the van der Waals forcesand by the electrostatic interaction between the molecules[28,29]. In this study, we report the influence of interfacialcharge at SAM-modified surfaces on electrodeposition ofPANI.

2. Experimental

2.1. Chemicals

Commercially available thiols: 3-mercaptopropionic acid(3-MPA, >99%), sodium 3-mercapto-1-propanesulfonate(3-MPS, 90%), and 2-aminoethanethiol hydrochloride(2-AET, 98%) were purchased from Aldrich Chemical Co.and used without further purification. Aniline monomer andperchloric acid were from J.T. Baker. All chemicals for phos-phate buffer solutions: tri-sodium phosphate-dodecahydrate(Merck), sodium monohydrogen phosphate (J.T. Baker),sodium dihydrogen phosphate-1-hydrate (Merck), andortho-phosphoric acid (Merck), were of analytical gradeand used as received. All solutions were prepared with ultrapure water obtained from a Maxima ultra pure water system.

2.2. Instrumentation

All Au film substrates were prepared with a Balzers SCD050 sputter coater under a vacuum of 2× 10−2 mbar. Anapproximate 15 nm underlayer of chromium was used to im-prove the mechanical stability of the gold film (ca. 300 nm)directly on the glass substrate. Electrochemical measure-ments were carried out with an Autolab instrument usinga three-electrode glass cell with a bottom hole (0.5 cm2).The working electrode (Au/glass substrate) was placed atthe bottom hole of the cell and sealed with a O-ring (0.8 cmi.d., perfluoroelastomer, Kalrez, USA). Electrochemicalquartz crystal microbalance (EQCM) measurements wereconducted with a homemade system (10 MHz quartz crys-tal, 0.227 Hz cm2/ng), which was configured with a HPfrequency counter and an ELECTROFLEX potentiostat(Hungary). Raman spectra were obtained using a RenishawRamascope spectrometer equipped with a semiconductordiode laser (Renishaw NIT 780TF, excitation wavelength780 nm). All potentials were measured versus Ag/AgCl (insaturated KCl solution) reference electrode.

2.3. Procedures

Prior to sputtering of the Au film, the glass substrates(15 mm × 26 mm) were cleaned with piranha solution,

rinsed copiously with water and let to dry completely.After Au sputtering, the Au/Cr/glass substrates were im-mersed in 10 ml of 5 mM thiol/ethanol (3-MPS in H2O)solution for 30 min. After immersion, the Au substratesmodified with a thiol monolayer were cleaned abundantlywith ethanol followed by water to remove unreactedthiol components where after the substrates were let todry. Electro-oxidation and polymerization of the anilinemonomers at the SAM-modified Au substrates were per-formed in an aqueous solution of 0.5 M perchloric acid.

3. Results and discussion

3.1. Electrochemical oxidation of aniline monomer atSAM-modified Au electrodes

In general, oxidation of solution phase aniline monomersat bare gold surface occurs at approximately 1.05 V[9]. Thepath of oxidation and deprotonation of aniline monomer inacidic media[36] is summarized and illustrated inScheme 1.

All the species at the initial stage of aniline coupling, suchas protonated aniline monomer, oxidized monomer, etc.,are positively charged. Electrostatic interaction between thecharged species in the solution and the charged electrode sur-face has a remarkable influence on the adsorption of specieson the electrode.Fig. 1 shows electrochemical oxidation of10 mM aniline at 2-AET, 3-MPA, and 3-MPS modified andbare Au electrodes in 0.5 M HClO4 solution, pH 0.4, a pHat which the aniline monomer is in the protonated form inthe solution (pK� = 4.5) [37]. Our results show that electro-chemical oxidation of protonated aniline has been blockedat the positively charged 2-AET surface. The oxidation po-tential of aniline at the 2-AET modified electrode shifts pos-itively to ca. 1.15 V (dotted curve inFig. 1). The startingpotential of aniline oxidation has also been increased from0.8, 0.82, and 0.83 V at bare, 3-MPA, and 3-MPS modifiedAu electrodes, respectively, to 0.85 V at 2-AET modified Auelectrode. It can be seen that aniline is much easier oxidizedat bare Au (1.00 V), 3-MPS/Au (1.00 V), and 3-MPA/Au

NH

H

-e

NH

*

NH

H* +N

H

H+*

-e

NH

H2+

NH

+

-e

-H+

-H+

nitrenium cation

Scheme 1. Oxidation and deprotonation of aniline monomer before cou-pling.

L. Niu et al. / Electrochimica Acta 49 (2004) 4455–4460 4457

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4

-0,5

0,0

0,5

1,0

1,5

2,0

2,5

3,0

dc

b

a

I /m

A

E /V vs.Ag|AgCl

Fig. 1. Cyclic voltammograms of oxidation of 10 mM aniline in 0.5 MHClO4 solution at bare Au (a), 3-MPA (b), 3-MPS (c), and 2-AET (d)modified Au electrodes. Scan rate: 0.1 V/s.

(1.05 V) electrodes than at 2-AET/Au (1.15 V). The poten-tial of aniline oxidation follows the order of 2-AET/Au >3-MPA/Au > 3-MPS/Au, and bare Au. At the 3-MPA/Auelectrode, the slight increase in the oxidation potential from1.00 V at bare Au surface to 1.05 V can be caused by block-ing the electron transfer by the 3-MPA layer. In addition,a higher oxidation current can be found at the 3-MPA/Auelectrode. This can be attributed to the possible interactionbetween the acidic 3-MPA SAM terminal group and the ani-line base.

Similar behavior in oxidation of aniline was also found athigher monomer concentrations in 0.5 M HClO4, and even in0.1 M HClO4 electrolyte solutions (pH 1.1) (data not shownhere). Almost no difference in oxidation of aniline at bareAu, 3-MPA, 3-MPS, and 2-AET modified electrodes can beseen between these surfaces when aniline concentration ishigher than 50 mM and/or in electrolyte solutions of higherpH (0.01 M HClO4/0.09 M NaClO4 solution, pH 2.1).

3.2. Electrodeposition of polyaniline at SAM-modified Ausurfaces

Aniline (10 mM) was electrochemically polymerized on3-MPA, 3-MPS, and 2-AET modified Au electrodes in orderto study the effect of differently charged electrode surfaceson PANI formation. For comparison, aniline was also poly-merized on the bare surface of Au. Polymerization was per-formed using a constant potential of 0.75 V during 1800 sin 0.5 M HClO4 (pH 0.4). Fig. 2 shows the cyclic voltam-mograms of the PANI films formed on the correspondingelectrodes recorded in a monomer-free electrolyte solution(0.5 M HClO4). It can be seen that the charge involved in theoxidation and reduction reaction of PANI on the 3-MPS/Auelectrode is higher than with the other films. Obviously, thenegative charge of the surface due to the 3-MPS SAM in anacidic solution facilitates the initial nucleation and growth ofPANI. When the polymerization was performed in 10 mManiline solution using a constant potential of 0.7 V (instead

-0,2 0,0 0,2 0,4 0,6 0,8 1,0-100

-50

0

50

100

I/µ

A

E/V vs. Ag|AgCl

Fig. 2. Cyclic voltammograms of PANI in 0.5 M HClO4 solution at bareAu (solid), 3-MPA (dash), 3-MPS (dash dot), and 2-AET (dot) modifiedAu electrodes. Scan rate: 0.1 V/s.

of 0.75 V) almost no PANI film was obtained at the elec-trode surfaces. On the other hand, when the concentrationof aniline was increased to 20 mM and the polymerizationwas performed at 0.75 V, almost no difference in the PANIresponse between the different SAM electrodes and bare Aucould be observed (the results not shown here).

The electrodeposition of PANI on 3-MPA and 2-AETmodified surfaces was further studied by EQCM at constantpotential (0.75 V) and the results are presented in Fig. 3a. Asignificant inhibition of PANI deposition can be seen at the2-AET modified surface (upper curve in Fig. 3a). Hardly anydecrease in frequency at the 2-AET monolayer is observedduring the initial 2000 s of deposition, and only a slight fre-quency decrease equivalent to a very slow deposition rateof PANI after 2000 s (ca. 0.05 Hz/s, 0.23 ng/(s cm2)) can beobserved.

By comparing PANI electrodeposition on bare Au and3-MPA modified surfaces, a higher deposition rate can beobserved in case of 3-MPA modification (middle curve inFig. 3a) than on bare Au surface (lower curve in Fig. 3a).After the initial deposition and growth of PANI, both thebare Au and 3-MPA surface show a uniform deposition rateof PANI, ca. 0.27 Hz/s (1.2 ng/(s cm2)) and almost no differ-ence (after 2500 s deposition) can be observed between thelayers.

It can therefore be concluded that electrostatic repulsionbetween the positively charged 2-AET monolayer and theprotonated anilinium ions plays an important role in inhibi-tion of PANI electrodeposition. This effect is only observedin growth of thin films.

In order to further elucidate the role of the electrostatic re-pulsion between a charged surface and monomer units dur-ing electrodeposition of PANI, experiments have been per-formed in solutions of different aniline monomer concentra-tions. The frequency responses in the EQCM measurementsof PANI electrodeposition in 0.5 M HClO4 solution of 10,20, and 50 mM aniline on a 2-AET monolayer are shownin Fig. 3b. In dilute aniline solutions (<10 mM, solid curve

4458 L. Niu et al. / Electrochimica Acta 49 (2004) 4455–4460

0 500 1000 1500 2000 2500 3000

-300

-200

-100

0

100

200

3-MPA

2-AET

Bare

f /H

z

Time /s

∆

(a)

(b)

(c)

0 500 1000 1500 2000 2500 3000-4

-3

-2

-1

0

50mM

20mM

10mM

∆f

/kH

z

Time /s

0 500 1000 1500 2000 2500 3000-20

-15

-10

-5

0

pH=2.1

pH=1.1

pH=0.4

∆f

/kH

z

Time /s

Fig. 3. Frequency response during potentiostatic polymerization of anilineat 0.75 V: (a) at bare Au (solid), 3-MPA (dash), and 2-AET (dot) SAMelectrodes in 10 mM aniline/0.5 M HClO4 solution (pH 0.4); (b) at 2-AETmodified electrodes in 0.5 M HClO4 solution of 10 mM (solid), 20 mM(dash), and 50 mM (dot) aniline; (c) at 2-AET electrodes in 20 mManiline/0.5 M HClO4 (pH 0.4, solid), 0.1 M HClO4 (pH 1.1, dash), and0.01 M HClO4/0.09 M NaClO4 (pH 2.1, dot) solutions.

in Fig. 3b), no frequency decrease due to electrodepositionof a PANI film is observed. The rate of deposition increasesgradually with increase in the monomer concentration. Atthe aniline concentration of 50 mM, the deposition becomesidentical on both, bare Au and 3-MPA modified Au surfaces(dotted curve in Fig. 3b). The change in the pH value, whichoriginates from the different aniline concentrations, can be

neglected (pH: 0.41 in 10 mM, 0.44 in 20 mM, and 0.5 in50 mM). This indicates that deposition of PANI based onelectrostatic interaction with the modified substrate is effi-cient and applicable only in dilute monomer solutions. Athigh monomer concentration, the coupling and adsorptionof species in the initial step in the polymerization is moredependent on the access of monomer than on the modifica-tion of the electrode surface.

Influence of pH on electrodeposition of PANI and elec-trostatic repulsion at a 2-AET modified electrode was alsostudied. Fig. 3c shows the frequency change during PANIelectrodeposition (20 mM aniline) at constant potential of0.75 V on 2-AET monolayer at different pH values (0.4, 1.1,and 2.1). Evidently, the pH value of the polymerization so-lution plays a key role in electrostatic interaction and elec-trodeposition of a PANI film. Deposition occurs rather fastat pH 2.1, dotted curve in Fig. 3c compared with depositionat lower pH values. This may originate from protonation ofthe SAM layer at lower pH values resulting in repulsion be-tween the charged amino terminal groups of the SAM andprotonated anilinium units.

3.3. Electrodeposition of polyaniline at the mixed SAMs/Ausurface

The positively charged 2-AET monolayer successfully in-hibits the electro-oxidation of protonated anilinium and theelectrodeposition of PANI due to strong electrostatic repul-sion. Electrodeposition of a PANI film was also studied at amixed 2-AET and 3-MPA SAM substrate. Scheme 2 outlinesthe general principal of the procedure concerning mixedSAM co-adsorption and selective deposition of PANI. In thiswork, however, half of the Au surface was first immersed inthe 2-AET solution, where after the whole electrode surfacewas exposed to 3-MPA solution. In this procedure, half ofthe Au surface was covered by 2-AET and the other half by3-MPA.

In general, the area-selective deposition of a conductingpolymer film using a SAM template depends principally onthe interfacial properties and the exposed molecular func-tionality of the surface, such as amphiphilicity [7,8] andreaction activity [9,10,18,21–23]. In this case, electrostaticinteraction, especially repulsion due to charged terminalgroups plays the key role in selective deposition of PANI.The competitive adsorption between adsorbed 2-AET and3-MPA should be neglectable due to this short time im-mersion in the 3-MPA solution. The 2-AET SAM changescharge of its terminal group from neutral to positive in anacidic solution as mentioned before and as indicated inScheme 2. Thus, electro-oxidation of protonated aniliniumand deposition of PANI should take place much slower onthe positively charged 2-AET regions than on the uncharged3-MPA regions, as already shown by the electrochemicaland EQCM measurements.

Electrodeposition of PANI was performed on the mixedSAM/Au surface in a 0.5 M HClO4 solution of 10 mM ani-

L. Niu et al. / Electrochimica Acta 49 (2004) 4455–4460 4459

Scheme 2. Illustration of electrodeposition of PANI on mixed 2-AET and3-MPA SAM templates.

line monomer at constant potential (E = 0.75 V). The re-sulting PANI film at such a mixed SAM substrate was thencharacterized by Raman spectroscopy, and the results areshown in Fig. 4. The spectrum a is obtained from a bareAu/glass substrate. Spectrum b is from the area coveredwith 2-AET after electropolymerization in 10 mM anilinein 0.5 M HClO4 at 0.75 V during 50 min. Both spectra arealmost identical showing that practically no deposition ofPANI has taken place at this modified surface. A remark-able difference is observed in spectrum c, which is obtainedfrom a 3-MPA area used for electropolymerization of PANI.New Raman bands can be seen at 1584 and 1166 cm−1 cor-responding to C=C stretching and C–H bending vibrationsof the emeraldine salt form of PANI, respectively [38–40].The bands at 1494, 1223, and 523 cm−1 are assigned toC–C stretching, C–N stretching, and possible amine defor-mation in the oxidized units of PANI, which are excitedat 780 nm. This implies that PANI is formed only on the3-MPA modified area. Raman bands from PANI can not beseen in the spectrum obtained from the areas of the electrodecovered with 2-AET at the initial stage of electrodeposition(<50 min). These results are consistent with our results fromthe EQCM measurements shown in Fig. 3. In addition, ex-perimental results indicate that such a selective depositionon the 3-MPA modified surface can be enhanced by stirringdue to electrostatic repulsion of positively charged anilinemonomers from the positively charged 2-AET surface.

1600 1200 800 400

c

b

a

d

Inte

nsity

(ar

b. u

nits

)

Wavenumber /cm-1

Fig. 4. Raman spectra of (a) Au/glass, (b) 2-AET/Au, and (c) 3-MPA/Ausubstrates after 50 min electrodeposition of PANI in 10 mM aniline/0.5 MHClO4 solution at 0.75 V; and (d) 2-AET/Au after 2 h electrodepositionof PANI in the same solution.

After longer deposition (2 h) in the same monomer solu-tion, bands characteristic for PANI can also be seen at the2-AET covered area of the electrode (Fig. 4, spectrum d).Raman bands originated from the oxidized units of PANIcan be observed at 1591, 1487, 1450–1420, and 1162 cm−1,which are assigned to C=C stretching, C=N stretching,C–C stretching, and C–H bending vibrations, respectively[38–40]. Furthermore, a symmetric C–N stretching vibra-tion is seen at 1220 cm−1. In addition, other bands relatedto para-disubstituted benzene ring deformation can be seenat 779 and 645 cm−1 and amine deformation at 840 and747 cm−1. It can be concluded that electrochemical oxi-dation of protonated aniline and deposition of PANI takesplace even at the positively charged 2-AET regions, butby a much slower rate than at the uncharged 3-MPA sur-face. These results are consistent with the EQCM measure-ments.

4. Conclusions

The charge of terminal groups of SAMs shows a stronginfluence on the electrodeposition of PANI. Electron transferbetween the solution and the electrode substrate is inhibiteddue to strong electrostatic repulsion between the positivelycharged amino groups and the positive redox molecule inacidic media. Electro-oxidation and protonation of anilinemonomer (aniline to anilinium in acidic solution) is alsoinhibited due to this repulsion. Electrodeposition of PANItakes place at different rates at the positively charged 2-AETcompared to the uncharged 3-MPA and negatively charged3-MPS modified substrates. The PANI film grows sloweron the 2-AET modified surface than on 3-MPA or 3-MPS.Finally, it can be concluded that thin PANI films can selec-tively be electrodeposited on patterned Au substrates withthe aid of different end groups of the thiols used for modi-fication of the surface.

4460 L. Niu et al. / Electrochimica Acta 49 (2004) 4455–4460

Acknowledgements

This work is a part of the activity of the Åbo Akademi Uni-verisity, Process Chemistry Center, nominated to a NationalCentre of Excellence (CoE) by the Academy of Finland for(2000-2005). The authors wish to gratefully acknowledgethe financial support for the joint project (between LoAC,ÅA, Finland and SKLEAC, CIAC, China) from Academyof Finland and from NSFC, China.

References

[1] A. Ulman, An Introduction to Ultrathin Organic Films fromLangmuir-Blodgett to Self-Assembly, Academic, San Diego, 1991.

[2] A. Ulman, Chem. Rev. 96 (1996) 1533.[3] G. Decher, Science 277 (1997) 1232.[4] T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds, Handbook of Con-

ducting Polymers, second ed. Marcel Dekker, New York, Basel, HongKong, 1998.

[5] I. Rubinstein, J. Rishpon, E. Sabatani, A. Redondo, S. Gottesfeld, J.Am. Chem. Soc. 112 (1990) 6135.

[6] E. Sabatani, Y. Gafni, I. Rubinstein, J. Phys. Chem. 99 (1995) 12305.[7] L.F. Rozsnyai, M.S. Wrighton, Chem. Mater. 8 (1996) 309.[8] Z. Huang, P.-C. Wang, A.G. MacDiarmid, Langmuir 13 (1997) 6480.[9] W.A. Hayes, C. Shannon, Langmuir 14 (1998) 1099.

[10] S.-J. Choi, S.-M. Park, Adv. Mater. 12 (2000) 1547.[11] M. Mazur, P. Krysinski, Thin Solid Film 396 (2001) 131.[12] D. Li, W. Ding, X. Wang, L. Lu, X. Yang, J. Mater. Sci. Lett. 20

(2001) 1925.[13] M. Mazur, P. Krysinski, J. Phys. Chem. B 106 (2002) 10349.[14] M. Mazur, P. Krysinski, B. Palys, J. Electroanal. Chem. 533 (2002)

145.[15] S. Inaoka, D.M. Collard, Langmuir 15 (1999) 3752.[16] M. Mazur, P. Krysinski, Langmuir 16 (2000) 7962.

[17] B. Bergman, T.W. Hanks, Macromolecules 33 (2000) 8035.[18] R.-K. Lo, J.E. Ritchie, J.-P. Zhou, J. Zhao, J.T. McDevitt, J. Am.

Chem. Soc. 118 (1996) 11295.[19] M. Mazur, P. Krysinski, K. Jackowska, Thin Solid Films 330 (1998)

167.[20] J. Wang, B. Zeng, C. Fang, F. He, X. Zhou, Electroanal. 11 (1999)

1345.[21] R. Georgiadis, K.A. Peterlinz, J.R. Rahn, A.W. Peterson, J.H. Grassi,

Langmuir 16 (2000) 6759.[22] C.O. Noble IV, R.L. McCarley, J. Am. Chem. Soc. 122 (2000) 6518.[23] G. Dahlgren, A. Smith, D.B. Wurm, Synth. Met. 113 (2000) 289.[24] X. Li, X. Zhang, Q. Sun, W. Lu, H. Li, J. Electroanal. Chem. 492

(2000) 23.[25] L.F. Rozsnyai, M.S. Wrighton, Langmuir 11 (1995) 3913.[26] C.B. Gorman, H.A. Biebuyck, G.M. Whitesides, Chem. Mater. 7

(1995) 526.[27] Q. Cheng, A. Brajter-Toth, Anal. Chem. 64 (1992) 1998.[28] V. Molinero, E.J. Calvo, J. Electroanal. Chem. 445 (1998) 17.[29] J. Zhao, L. Luo, X. Yang, E. Wang, S. Dong, Electroanal. 11 (1999)

1108.[30] Z. Dai, H. Ju, Phys. Chem. Chem. Phys. 3 (2001) 3769.[31] M.A. Bryant, R.M. Crooks, Langmuir 9 (1993) 385.[32] K. Hu, A.J. Bard, Langmuir 13 (1997) 5114.[33] K. Hu, A.J. Bard, Langmuir 13 (1997) 5418.[34] T. Kakiuchi, M. Iida, S. Imabayashi, K. Niki, Langmuir 16 (2000)

5397.[35] K. Nishiyama, A. Kubo, A. Ueda, I. Taniguchi, Chem. Lett. 1 (2002)

80.[36] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277.[37] R. Sfez, L. De-Zong, I. Turyan, D. Mandler, S. Yitzchaik, Langmuir

17 (2001) 2556.[38] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima, T. Kawagoe,

Macromolecules 21 (1988) 1297.[39] K. Berrada, S. Quillard, G. Louarn, S. Lefrant, Synth. Met. 69 (1995)

201.[40] M. Cochet, J.-P. Buisson, J. Wery, G. Jonusauskas, E. Faulques, S.

Lefrant, Synth. Met. 119 (2001) 389.