Electropolymerization of Cationic Amphiphilic Pyrrole Derivatives on

Jan 1, 1994 - Liliane Coche-Gu&ente,+ Alain Deronzier,*tt Bruno Galland,? Jean-Claude Moutet,*p+ Pierre Labb6,rJ Gilbert Reverdy,r Yves Chevalier,l an...
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Langmuir 1994,10,602-610

602

Electropolymerization of Cationic Amphiphilic Pyrrole Derivatives on Electrodes. Evidence for Environmental Effects on Redox Potentials of Trapped Anions Liliane Coche-Gu&ente,+ Alain Deronzier,*tt Bruno Galland,? Jean-Claude Moutet,*p+Pierre Labb6,rJ Gilbert Reverdy,r Yves Chevalier,l and Jamal Amhrar' Laboratoire d'Electrochimie Organique et de Photochimie Rldox, URA CNRS 1210, Universitl Joseph Fourier, BP 53X,38041 Grenoble CBdex, France, Laboratoire de Chimie Mollculaire et Environnement, ESIGEC, UniversitB de Savoie, Campus Savoie Technolac, 73376 Le Bourget du Lac, France, and Laboratoire des MatBriaux Organiques h PropriBtls SpBcifiques, UPR 9031 CNRS, BP 24,69390 Vernaison, France Received August 12,1993. In Final Form: November 15,1999 Amphiphilic(pyrrolylalky1)ammoniummonomers, differing in the size of their cationic polar headgroup have been synthesized and studied. Surfactant properties of these compounds -N+(C,H,,+1)3 ( n = 1-4,6), have been examined. The monomer with n = 1 forms micelles in solution. All the other monomers do not lead to the formation of organized assemblies. However, they disperse in pure water upon sonication and lead to the formation of milky dispersions, from which monomer films are coated on an electrode surface and subsequently electropolymerized in an aqueous electrolyte. This process produces stable, electroactive cationic fiims which could incorporate bulky electroactive anions. The permeabilities and anion-exchangeproperties of these poly(amphiphi1icpyrrole) films depend on the headgroup size of the monomer. Incorporation of electroactive anions could also be accompliahed in ooe 8tep;by electropolymerizationof monomer-anion mixtures coated on an electrode surface. With the anthraquhoneaulfonate anion, peak splitting of the cyclic voltammetry curve of the modified electrode studied in an aqueous electrolyte suggests a regular surface arrangement.

Introduction Over the past decade, the preparation of highly organized monolayers or multilayers at the electrode-solution interface has generated strong interest. These systems allow precise control of the molecular architecture at electrode surfaces. Potential applications concem various areas such as electrocatalysis, electroanalysis, display technology, energy conversion, or macromolecular electronics. Amphiphilic molecules are commonly used to build interfacial organized structures. The most common methods include transfer of Langmuir-Blodgett (LB)f i i to e1ectrodes,l4 casting lipids onto electrodes from organic solvents,S16 or self-assembling surfactants at electrode solution interfaces.7-9 In addition, it is highly desirable that these supramolecular assemblies maintain their structure and organization for long periods of time and in a variety of chemical environments. One possibility consists of polymerizing such organized structures at the electrode-solution interface. For example, it has been demonstrated that the LB technique can be used to process electrically conductive polymers into stable, multilayered thin films, having original electronic properties, by chem-

* To whom correspondence should be addressed. + Universitb Joseph Fourier. 1 Universitb de

Savoie.

8 Present address: Univarsit.6 Joseph Fourier.

Organiques B PropribtBs Sp6cifiques. Abstract published in Aduance ACS Abstracts, January 1,1994. (1)Menning, R. Discuss. Faraday SOC.1974,58, 261. (2) Daifuku, H.; Aoaki, K.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. 1982,140, 179. (3) Fujikira, M.; Poosittaak, S. J. Electroanal. Chem. 1986, 199, 69. (4) Zhang, X.; Bard, A. J. Am. Chem. SOC.1989,111,8098. ( 5 ) Tanaka, K.; Tamamushi, R. J.Electroanal. Chem. 1987,236,305. ( 6 ) Rusling, J. F.; Zhang, H. Langmuir 1991, 7, 1791. (7) Facci, J. S. Langmuir 1987, 3, 525. (8)Donohue, J. J.; Buttry, D. A. Langmuir 1989, 5, 671. (9) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. I Laboratoire des Matbriaux

i d 0 or electrochemical'l polymerization of amphiphilic pyrrole derivatives. Very recently, it has been shown that a lamellar conducting homopolymer could be obtained by self-assembly of an electropolymerizable monomer with high molecular anisotropy.12 Following our preliminary report on the unusual electropolymerization behavior of the amphiphilic pyrrole 2 in aqueous electrolyte and the anion-exchange properties of the resulting polymeric films,13 we present here an extensive study on a series of amphiphilic (pyrrolylalky1)ammonium monomers (1-5) differing in the size of their ammonium groups. X-ray diffraction, polarized optical microscopy,surface tension, and fluorescencespectroscopy measurements have been performed to characterize pure monomers and their organization in aqueous media. Only monomer 1 self-assembles in water, giving rise to micelle formation. In contrast monomers 2-5 are only poorly soluble in water and do not lead to self-organized supramolecular aggregates. Organic and inorganic anions have been incorporated in poly(1-5) films during the polymerization step or in preformed polymer films,in order to compare their anionexchange properties and their permeabilities. The electrochemical features of poly(2-5) films containing anthraquinonesulfonate in aqueous electrolyte suggest an organized structure on the electrode surface. (10) (a) Hong, K.; Rubner, M. F. Thin Solid Films 1990,62,187. (b) Rahman, A. K. M.; Samuelson, L.; Minehan, D.; Clough, S.; Tripathy, S.; Inagaki,T.; Yang, X. Q.; Skotheim,T. A.;Okamoto, Y. Synth. Met. 1989, 28,237. (c) Barraud, A. Thin Solid Film 1989,175,73. (d) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Mater. 1990,2,82. (11) Iyoda, T.; Ando, M.; Kaneko, T.; Ohtani, A.; Shiidzu, T.;Honda, M.Tetrahedron Lett. 1986,27, 5633. (12) Collard, D. M.; Fox, M. A. J. Am. Chem. SOC. 1991, 113,9414. (13) Coche-GuBrente,L.;Deronzier, A.; Galland, B.; LabW, P.; Moutet J.-C.; Reverdy, G. J. Chem. SOC.,Chem. Commun. 1991, 386.

0743-7463/94/2410-0602$04.50/0 0 1994 American Chemical Society

Electropolymerization of Cationic Pyrrole Derivatives

6

6

1 : R = CH3

1

6

2: R = CHz-CH3

6

1

8

4: R = CH2-( CH2)2- CH3

1

1

3: R = CH2- CHz- CH3 6

7

8

S : R = CH2-( CH2)4- CH3

Experimental Section Instrumentation, Electrolytes, and Chemicals. The electrochemicalequipmentand purification of tetraethylammonium perchlorate (TEAP)have been described previ0us1y.l~Lithium perchlorate (GSF Smith), tetramethylammonium tetrafluoroborate (Fluka),and acetonitrile (Rathburn,HPLC grade S) were used asreceived. Aqueouselectrolyteswerepreparedfrom Analar reagents and water doubly distilled in R quartz apparatus. Potassium hexacyanoferrate(III),2,2'-azinobis(3-ethylbenzothiazoline-6-sulfoaicacid) disodium salt (ABTS), riboflavin, 12bromododecanol, and trimethylamine were comwerciallyavailable and wed as received. Triethylambe, tripropylamine, tributylamine, and trihexylamine were distilled under vacuum prior to use. Potassiumoctacyanomo1ybdabW)wassynthesized according to a known procedure.16 All potentials in CHsCN electrolyteare referred to the Ag/Ag+ (10-2 M) in 0.1 M TEAP + CHaCN reference electrode and those in aqueous electrolyteto the saturatedcalomel reference electrode (SCE). Platinum and glassy carbon disks (5-mm diameter)were polished with 1-wmdiamond paste. All electrochemicalstudies were" under an argonatmosphere. Amphiphilicpyrroleswere ultrasonicallydispersedin water using a Branson 200-W generator coupled to a "Cup-Horn" transducer. Surface tension measurements were carried out at 50 1 O C by the du Nouy ring method with a Lauda tensiometer as described in ref 16. Experimental surface tension values were corrected for the weight of pulled liquid according to Harkins and 30rdan.l' Electrical conductivity measutementa were performed at 50 f 1OC with a Tacussel CD810 conductivity meter equippedwith an XE15O electrode. Fluorescenceequipmentand experimental procedures have been described elsewhere.18 Synthesis of 12-Pyrrol-1-yldodecan-1-01. Pyrrolylgotassium (2 g, 21 mmol) and 12-bromododecano1(2.8g, 10.5 mmol) were refluxed for 30 min in a mixture of anhydrous dimethyl sulfoxide (5 mL) and tetrahydrofuran (20 mL). The resulting solution was diluted with water and extracted with cH2C12. The CHzCIl solution was dried over NaB04, and the solvent was removed by rotary evaporation. The desired pyrrolyldodecanol was purified by chromatography on a silica column eluted with a 3 1 n-heptane-Eh0 mixture: yield 2.5 g (95%);lH NMR (200 MHz, CDCls) 6 (ppm) 1.38 (m, 20 H), 3.62 (t,J = 6.5 Hz, 2 H), 3.84 (t,J = 7.2 Hz, 2 H), 6.11 (t,J = 2 Hz, 2 H), 6.23 (t, J = 2 Hz, 2 H). Synthesis of 12-Pyrrol-1-yldodecylpTolueaesulfonate. A solution of 12-pyrrol-1-yldodecan-1-01 (2.1 g, 8.4 mmol) and tosyl chloride (1.6 g, 8.4 mmol) in 25 mL of anhydrous pyridine was stirred at 5 O C for 8 h. The mixture was poured into water and extracted with EkO. The organic phase was washed four times with a 5 % HCl aqueous solution and then with water and dried over Na2SO4. After the solvent was removed by a rotary evaporator,the desired product was obtained as an orange solid (yield 1.87 g, 55%) and used without further purification. Synthesis of Monomers 1-5. As briefly described for monomer 2,13 12-pyrrol-1-yldodecylp-toluenesulfonate was refluxed for 12-40 h in ethanol in the presence of a 3-6-fbld excess of the desired (C,Hh+l)sN amine. The solvent and excess amine (n = 1-3) were removed under vacuum. Higher amines

*

~~

~~

~

(14) Coenier, S.; Deronzier, A.; Moutet, J . 4 . J. Mol. Catal. 1988,45, 381. (16) Furman, N. H.; Miller, C. 0. Znorg. Synth. 1980,3,160. (16) Chevalier, Y.; Storet, Y.; Pourchet, S.; Le Perchec, P. Langmuir 1991, 7,848. (17) Harkins, W.D.;Jordan, H.F.J. Am. Chem. SOC.1930,52,1751. (18) Brahimi, B.; Labb6, P.; Reverdy, G. Langmuir 1992, 8 , 1908.

Langmuir, Vol. 10, No. 2, 1994 603 (n = 4 and 6) were extracted with n-heptane. Tosylatesalts were dissolved in 1:l wafer + methanol and passed through an ionexchange column (Amberlib IRA 93) in BF4- form. After the solvent was removed, 1 was obtained as a white powder. Monomere 2-6 were obtained as brown yellow oils. They were charact&& by 1HNMR(200MHz)and fast atom bombardment (FAB) mass spectrometry. (12-Pyrrol-l-yldodecyl)trimethyIam"Tetra rate (1): lH NMR (CDCU 6 (ppm) 1.27 (m, 16 H, H-81, 1.62 (m, 4 H, H-2, H-4), 3.05 (s,9 H, HS), 3.16 (m, 2 H, H-5),3.73 (t,J=7.15Hz, 2 H, H-l),6.00 (t,J =2Hz,2H, H-B,fl), 6.52 (t,J = 2 H, H-a,a'); MS (FAB)m/e (positivemode),C+, 293. Data for (12Byrrol-1-y1dodecyl)triethyla"onium Tetrafluoroborate (2): lH NMR (CDCls) 6 (ppm) 1.30 (m, 27 H, H-3, H-4, H-8), 1.68 (m, 2 H, H-2),3.08 (m, 2 H, H-5), 3.25 (4, J = 7.27 Hz,2 H, H-6), 3.80 (t,J = 7.15 Hz, 2 H, H-l), 6.06 (t, J =2 Hz, 2 H, H-&@'),6.58 (t,J =2 Hz, 2 H, H-a,a'); MS (FAB) m/e (positive mode), C+, 335. Data for (12-Pyrrel-l-yldodecy1)tripropylam" Tetrafluoroborate (3): lH NMR (CDCla) 6 (ppm) 0.87 (m, 9 H, H-8), 1.13 (m, 18H, H-3, H-4), 1.60 (m, 8 H, H-2, H-7), 2.95 (m, 6 H, H-6),3.32(m, 2 H, H-5),3.69 (t,J = 7.15 Hz, 2 H, H-l),6.00 (t, J = 2 Hz,2 H, H-B,B'), 6.52 (t, J = 2 Hz, 2 H, H-a,a'); MS (FAB) m/e (positive mode), C+,377. Data for (12-Pyrrol-1-yldodecy1)tributyla"onium Tetrafluombrats (4): lH NMR (CDCls) 6 (ppm) 0.96 (m, 9 H, H-8) 4 H,H-3, H-4, H-7),3.12 (m, 8 H, H-6, H-5),3.83 2 H, H-l), 6.10 (t, J 2 Hz, 2 H, H-j3,fl),6.21 (t,J (t, J. H, H-a,a'); MS (FAB) m/e (positive mode), C+, 419. Data fer (12-Pyrrol-1-y1dodecyl)trihexyla"onium Tetrafluoroborate (5): lH NMR (CDCk, 6 (ppm) 0.74 (m, 9 H, H-8), 1.20 (m, 36 H, H-3, H-4, H-7),1.56(m, 8 H, H-2, H-7),3.00 H-6), 3.73 (t, J = 7.15 Hz,2 H, H-l), 6.00 (t,J = (m, 8 H, H-5, 2 Hz, 2 H, H-@,@), 6.52 (t,J = 2 Hz, 2 H, H-a,a'); MS (FAB)m/e (poeitive mode), C+,503.

Results and Discussion Self-organization Properties of Monomers 1-5. Observation of the dry or wet monomers 2-5 with an optical microscope equipped with two crossed polars did not give any evidence of organized system formation from room temperature to 80 "C. This result was further confirmed by the absence afBragg diffraction lines in low-angle X-ray diffraction measurements. The monomers 2-5 are poorly soluble in water at room temperature. In the case of monomer 2, which is the less hydrophobic, turbidity measurements indicate that its solubility is less than 8 X 10-6 M in pure water. Sonication at 50 "C initially disperses monomer 2 into a turbid dispersion which then becomes optically transparent with a slight tinge. This dispersion remains stable even at room temperature. Since polarizing optical microscopy and X-ray diffraction experiments did not show the formation of any liquid crystalline phase, this dispersion phenomenon does not result from vesicle formation. Light scattering experiments on monomer 2 dispersion indicate the presence of particles with a mean diameter of 205 nm. The stability of this dispersion could be explained by the fact that, at the particle-water interface, the amphiphilic monomers 2 are expected to point their ionic headgroups toward the water phase. Diffusion of the BFd- counterions into the bulk aqueous phase leads to a positively charged interface, providing an efficient interparticle repulsion which prevents their aggregation. The more hydrophobic monomers 3-5 can be less easily dispersed and lead to the formation of milky dispersions upon sonication. The stability of these dispersions is limited, but sufficient to allow their use for electrode coating (vide infra). In contrast monomer 1 leads to the formation of thermotropic and lyotropic liquid crystalline phases. Figure 1shows the polarized microscopy views of

Coche-Gubrente et al.

604 Langmuir, Vol. 10,No. 2,1994

n l ioL4

1 0 ' 3

10'2

14-1

Concentration (mol/l)

B

C

Figure 1. Optical texture (crossedpolars)for the differentphases of dry monomer 1: (A) room temperature, (B) 80 "C. (C)Optical microscope (crossed polars) penetration scan (lateral hydration) for monomer l/water at 80 "C.

1 in the dry state recorded at different temperatures. A t room temperature, monomer 1 is crystalline in the solid state (Figure 1A). At 80 "C a liquid crystalline phase (unidentified) appears (Figure lB), as demonstrated by the classical appearance of characteristic optical defects.lg Then an isotropic phase appears at 88 "C which corresponds to the liquid state of monomer 1. Figure 1C shows that, upon a lateral hydration at 80 "C, the anisotropic liquid crystalline phase dissolves in water to give a new isotropic phase: a diluted micelle solution or a more concentrated isotropic cubic phase. Micelle formation in an aqueous solution of 1was further confirmedby surface tension and conductivity measurements along with fluorescence experiments using the hydrophobic pyrene probe. The Krafft point was determined at the critical micelle concentration (cmc) by visual observation: crystals of the tensioactive compound were slowly heated in pure water until complete dissolution was achieved. The Krafft temperature was 44 f 2 "C. (19) Rosevear, F. B. J. Am. Oil Chem. SOC.1954,31,628.

Figure 2. Surface tension as a function of monomer 1 concentration (in pure water).

The cmc was then determined at 50 O C by surface tension and electrical conductivity measurements. In spite of the presence of a pyrrole at the hydrophobic chain end, both surface tension and conductivity data (Figures 2 and 3) look like those of classical (methyl-terminated) surfactants.20 Thus, micelle formation is expected with this compound, and the cmc value can be obtained from these data. Micelle formation was unambiguously confirmed by using the hydrophobic pyrene fluorescent probe.2' The ratio of the intensities of the first and the third vibronic bands of the pyrene fluorescence spectrum (I/III ratio) is known to be sensitive to the polarity of the environment of the pyrene molecule.21 Figure 4 shows the fluorescence spectrum of an aqueous solution of pyrene (5 X M) at 45 "C in the presence of 5 X 10-4 M (spectrum A) and 4 X le3M (spectrum B) monomer 1. In the absence of micelles (spectrum A), pyrene molecules experience an aqueous polar environment and the I/III ratio of 1.66 is the same as the one recorded for pyrene solubilized in pure water. At a concentration of 4 X 10-3 M (spectrum B) the I/III ratio of 1.24 indicates that pyrene molecules experience a less polar environment which is in good agreement with their solubilization inside the lipophilic hydrocarbon core of micelles of monomer 1. It is striking to observe that lowering the temperature below the Krafft temperature leads to the disruption of micelles and to the precipitation of monomer 1. Then pyrene molecules experience again the polar aqueous environment as demonstrated by a I/III ratio of 1.66 recorded at this temperature (Figure 4, spectrum C). From surface tension data (Figure 2) the cmc is 5.6 X M. The interfacial area a0 per surfactant molecule is calculated from the Gibbs equation:

The present value of a0 is quite large (160 A2/molecule) as compared to those of usual n-alkyltrimethylammonium cationic surfactants.22 The electrical condutivity data can be plotted in two ways: conductance against surfactant concentration ( K vs (20) (a) Mukerjee, P.; Mysels, K. J.; Critical micelle concentrations of aqueous surfactant systems;National StandardReference Data Series NSRDS-NBS36;National Bureau of Standards: Washington, DC, 1971. (b) Lindman, B. Wennerstriim, H. Top. Curr. Chem. 1980, 87, 1. (c) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990,53, 279. (21) Kalyamasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977, 99,2039. (22) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binka, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowaki, A. J. Phys. Chem. 1992,96,1383.

Langmuir, Vol. 10,No. 2, 1994 605

Electropolymerization of Cationic Pyrrole Derivatives

0

5 10 Concentration (mmol/l)

15

0

.05

.lo

.15

(moV21-''')

Figure 3. (A, left) Conductance against monomer 1 concentration (in pure water). (B, right) Molar conductance against square root of monomer 1 concentration (in pure water).

Figure 4. Fluorescence spectrum (hx = 334 nm) of an aqueous solution of pyrene (5 X le7mol L-1) in the presence of (A) monomer 1 (5 X lo-' M) at 45 OC, (B)monomer 1 (4 X 10.3 M) at 45 "C, and (C) monomer 1 (4 X 10-9 at 25 "C.

C) or molar conductance against the square root of

effects are the manifestation of the same phenomenon. Indeed, a large cy value allows strong electrostatic interactions between polar headgroups, and thus a large interfacial area per molecule ao. The consequence is also that those micelles are rather loose aggregates with an ill-defined cmc, and thus with a cmc value which strongly depends on the measurement methods. On this basis, it can be expected that the micelle aggregationnumber would be low and that extensive water-alkyl chain contact would occur. The common origin of these effects is not clear, but one can speculate that the hydrophilic pyrrole adsorbs at the water-alkyl chain interface, constraining the alkyl chain to fold as a loop. These results compare well with those of bolaform surfactants (surfactants with headgroups at both ends of a hydrophobic polymethylene arm)28-29 which also have large cy and aovalues. Chemical relaxation measurements on bolaform surfactants have indeed shown that micelles were looser aggregates than with classical surfactants having a single alkyl chain and a single polar head.3m2 The unusual BF4- counterion may nevertheless also have some influence in such phenomena. Surface Modification with Polymeric Films. On a platinum or glassy carbon electrode, cyclicvoltammograms for 2 mM monomers in CH3CN-TEAP (0.1 M) exhibit the regular irreversible oxidation peak (E, = 1.1 V) of N-substituted pyrroles.33 Films of poly(1-5) could be grown on the electrode surface by controlled potential oxidation at 0.85 V. The resulting modified electrodes exhibit the quasi-reversible redox peak system (Ell2 = 0.2-0.3V vs Ag/10-2 M Ag+ in CH3CN electrolyte, 0.6-0.7 V vs SCE in aqueous electrolyte) typical of N-substituted polypyrroles.33 Apparent surface concentrations of pyrrole and thus of alkylammonium units ( r N + ) were calculated from the integrated charge under the polypyrrole oxidation wave, assuming that one in three pyrrole units is oxidized.33 Thin films with r N + of 2 X 1W mol cm-2 were obtained with electropolymerization yields around 40 5%.

concentration (Avs C1I2).mAlthough it had been reported that the latter plot provides slightly higher cmc values than the former one,2Oa both kinds of plots (Figure 3) gave the same cmc value of 8.3 X 10-3 M within experimental accuracy. This value is larger than that found from surface tension measurements. It is known that the conductivity method usually gives larger cmc values than the surface tension method,mabut the difference is quite large in the present study. In the K vs C plot, the ratio of the slopes above and below the cmc provides an estimate of the fraction of free BF4- counter ion^,^^^^^ cy = 0.53. This is a large value as compared to literature values (0.2-0.4)for (26) Menger, F. M.; Wrenn, S. J . Phys. Chem. 1974, 78, 1387. usual n-alkyltrimethylammoniumcationic ~ u r f a c t a n t s . ~ ~ ~ ~(27) Ikeda, K.; Yasuda, M.; Ishikawa, M.; Esumi, K.; Meguro, K.; All these experimental data, the discrepancy between Binana-Limbele, W.; Zana, R.; Colloid Polym. Sci. 1989, 267, 825. (28) Ikeda, K.; Ishikawa, M.; Yasuda, M.; Esumi, K.; Meguro, K.; cmc values as determined by surface tension and conBinana-Limbele, W.; Zana, R. Bull. Chem. SOC.Jpn. 1989,62, 1032. ductivity methods, the large interfacial area per molecule, (29) Yasuda, M.; Ikeda, K.; Esumi, K.; Meguro, K. Bull. Chem. SOC. and the large fraction of free counterions, show that this Jpn. 1976,80, 2651. (30) Yiv, S.; Kale, K. M.; Lang, J.; Zana, R. J . Phys. Chem. 1976, 80, surfactant is different from n-alkyl surfactants. These (23) Evans, H. C. J . Chem. SOC.1956, 579. (24) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (25) Sepulveda, L.; CortBe, J. J. Phys. Chem. 1985,89,5322.

2651. (31) Yiv, S.; Zana, R. J . Colloid Interface Sci. 1980, 77, 449. (32) h a , R.; Yiv, S.; Kale, K. M. J . Colloid Interface Sci. 1980, 77, 456. (33) Deronzier, A.; Moutet, J.-C. Acc. Chem. Res. 1989,22, 249.

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/

Figure 5. Electropolymerization of (A) 2 (0.07 pmol deposited on a 5-mm Pt disk electrode)and (B)5 (0.07 pmol deposited on a 5-mm C disk electrode)by repeated potential scanning in 0.1 M LiC104-HZ0. (C) Modified electrodeprepared as in (A) and transferred ta clean electrolyte after 40 cycles. rN+ = 3.7 x 10-8 mol cm-2; sweep rate Y = 50 mV s-l. Monomers 2-5 which are poorly soluble in aqueous electrolytesa could be electropolymerizedusing an original and monomer-saving procedure described thereafter. A drop (10-40 pL) of a stock solution of monomer (1-5 mM) in acetonitrile, or of an optically transparent dispersion in water of monomer (2-4 mM) obtained by sonication, is laid on the electrode surface (5-mm-diameter Pt or C disk). Water is removed either by heating the electrode surface by means of a 500-W tungsten lamp or under vacuum to form an adsorbed layer of monomer. Then, the electrode is soaked in a 0.1 M LiClO4 aqueous electrolyte and the oxidative electropolymerization of the monomeric film is conducted at 0.76 V vs SCE until zero current is reached. Polymeric films can also be grown by repetitively cycling over the -0.2 to +0.8 V potential range (Figure 5) until the obtention of a steady-state voltammogram. Cyclic voltammetry curves of the resulting films show the characteristic reversible oxidation wave (E112= 0.60 V; see Figure 5C for example) of the polypyrrole redox system, whatever the polymerization procedure. Apparent surface concentrations rN+ of (pyrrolylalky1)ammonium mol cm-2 could be easily units from 10-8 to 1.5 X (34) For example,from turbidity measurements the solubilityof 2 was e M in pure water and 2 X l e M in estimated to be less than 8 X l LiClO, (0.1 M) aqueous electrolyte, from ref 13.

Coche-GuBrente et al. obtained, according to the concentration of monomer in the stuck CHBCNsolution or aqueous suspension. Films of larger size, with rN+ up to 4.6 X lo-' mol cm-2, have been elaborated by repeating the monomer deposition procedure. On the analogy of the related nonamphiphilic poly[(pyrrolylalkyl)ammoniuml poly(6) films,36poly(%) films with rN+of lO-'mol cm-2have an estimated thickness of 0.2 pm. We have no information a t the moment on how the monomeric units are arranged in the polymeric phase. However, the surface occupied by one monomeric unit can be roughly estimated at 100-200 A2. As a matter of fact, it is known that the surface area occupied by a trimethylammonium headgroup in a densely packed bilayer of a long-chain alkylammonium surfactant is 50 A2.36 Taking into account the larger size of an alkyltriethylammonium headgroup, the surface area occupied in this packed configuration can be estimated at 100 A2. On the other hand, from a molecular model the surface area of monomer 2 lying flat on a surface can be estimated to be nearly 200 A2. Thus, a poly(2) film with rN+ of lO-' mol cm-2 would correspond to 600-1200 monolayers. We found that the yield of deposited polymer, Le., the mole ratio of pyrrolylammonium units in the polymer to deposited monomer, depends on the solvent used for the deposition process. As a matter of fact, part of the monomeric layer is lost by dissolution in the electrolyte before ita complete electropolymerization. For thin poly(2) films (rN+ I mol cm-2),the polymerization yield is higher when a CH3CN solution is used (yield80%) rather than an aqueous dispersion (yield 36 5% ). This observation can be explained by the larger solubility in aqueous electrolyte of monomeric layers deposited from ultrasonically dispersed 2 in water. One can reasonably postulate that the aggregates formed in aqueous media are more or less preserved in the dried monomeric layer,37leading to a large release of (pyrrolylalky1)ammonium units in aqueous electrolyte before their polymerizationtakes place. Figure 5 shows that in these experimental conditions 2 polymerizes faster than 5. After 14 cycles between -0.4 and +0.8 V,the anodic peak current of the poly(2) wave (20 pA) is twice that measured for the poly(5) film (9 PA). Monomers 3 and 4 present the same behavior as 5. The better polymerization ability of monomer 2 coatings could be due to their higher permeability to aqueous electrolytes (see below). In addition, two distinct redox systems are clearly seen on the cyclic voltammetry curves, especially on the first polymerization scans. This behavior is not clearly understood at the present time. Similar phenomena have already been observed in substituted polypyrrole38and p0lythiophene3~films. They have been shown to be dependent on the lipophilic properties of the supporting electrolyte ions, which could be responsible for two different doping processes by cation or anion exchange. In our case, any difference in peak splitting has been observed by using LiC104 or (CHs)dNBF4 supporting electrolytes. A reasonable explanation could be the existence of two different polypyrrole redox couples, (35) De Gregori, I.; Carrier, M.; Deronzier, A,; Moutet, J.4.; Bedioui, F.; Devynck, J. J. Chem. SOC.,Faraday Trans. 1992,88,1567. (36) Pashley,R. M.; McGuiggan, P. M.; Horn, R. G.;Ninham, B. W. Collozd Interface Sci. 1988,126, 569. (37) Experiments conducted in organic media with enzyme molecules trapped in poly(2) f i e have demonstrated that some water ia retained in the polymeric film, and thus in the monomeric film: Coche-GuBrente, L.; Ccanier, S.; Innocent, C.; Labb6, P. To be publihed. (38) (a) Kuwabata, S.; Yoneyama, H.; Tamura, H. Bull. Chem. SOC. Jpn. 1984,57,2247. (b) Delabouglise, D.; Garnier, F. Adu. Mater. 1990, 2,91. ( c ) Delabouglise, D.; Roncali, J.; Lemaire, M.; Garnier, F. J. Chem. SOC.,Chem. Commun. 1989,475. (39) Roncali, J.; Garreau, R.; Delabouglise, D.; Gamier, F.; Lemaire, M. J. Chem. SOC.,Chem. Commun. 1989, 679.

Electropolymerization of Cationic Pyrrole Derivatives

Langmuir, Vol. 10, No. 2, 1994 607

n V

Figure 7. First five cycles recorded in 20 mM LiClOrHzO for a C/poly(t) electrode (rw = 3.9 x 10-8 mol cm-2) after soaking in a solution of 1 mM &Mo(CN)a in HzO, v = 50 mV s-l.

Figure 6. Cyclic voltammogram recorded in phosphate buffer containing0.3mM riboflavin on a 5-mmC disk electrodemodified by thin fiis (rN+= 1.2 x 10-8mol cm-2) of (A) poly (11, (B)poly (2), (C) poly (3), poly (41, and poly(9, v = 50 mV 8-l. due to the presence of two distinct arrangements of the polymeric chains. Monomer 1 could not be polymerized under these unusual conditions, since ita coatings on Pt or C electrodes quickly dissolved when soaked in an aqueous electrolyte. Attempts a t the electropolymerization of micellar solutions of 1 were also unsuccessful, since the addition of a supporting electrolyte leads to the precipitation of 1. Permeability of Poly( 1-5) Films in Aqueous Media. The permeability of the different polymers has been investigated in aqueous electrolytes by studying the electrochemical response of a neutral molecule on carbon electrodes modified with thin films (rN+ = 1.2 X le7 mol cm-2). Riboflavin was chosen as a test molecule. Cyclic voltammograms recorded in phosphate buffer containing 0.3 mM riboflavin are shown in Figure 6. Well-behaved waves were observed for poly( 1) (Figure 6A) and poly(2) (Figure 6B) films. On the contrary, poor responses were obtained with poly(3-5) modified electrodes, as shown for example (Figure 6C) for a poly(4) film. These last three polymers seem to have a more hydrophobiccharacter than poly(1) and poly(2) films. This hydrophobicity can be related to the size of the cationic head of these amphiphilic molecules. Anion-Exchange Properties of Poly(2) Films. The incorporation of anionic species in these polycationic materials has been largely studied using poly(2) films. As already demonstrated for nonamphiphilic poly( [(pyrrolylalkyl)ammoniuml coatings,%@inorganic and organic ions such as hexacyanoferrate (1111, octacyanomolybdate (IV),and 2,2’-azinobis(3-ethylbenzothiamlint+6-sulfonate) (abbreviated as ABTS) anions, as their sodium,potassium, or tetrabutylammonium salta, were readily extracted from diluted ( 1mM) aqueous or organicsolutions and retained in poly(2) films. In order to obtain voltammograms free

-

(401 (a) Cosnier, S.;Deronzier, A.; Moutet, J.-C.; Roland, J. F. J. ElectroaMI. Chem. 1989, 271, 69. (b) Moutet, J.-C.; Pickett, C. J. J. Chem. Soc., Chem. Commun. 1989,188. (c)Keita, B.; Bouazig, D.; Nadjo, L.; Deronzier,A. J. Electroanol. Chem. 1990,279,187. (d) De Oliveira, I. M. F.; Moutet, J.4.; Hamar-Thibault,S . J.Mater. Chem. 1992,2,167.

of the additional electrochemical response of the polypyrrole skeleton, their electroactivity has been previously destroyed41by repeated potential scans between -0.2 and +1.4 V in acetonitrile electrolyte. Pt/poly(2) and C/poly(2) modified electrodes in which [Fe(CN)#- and [Mo(CN)s]” anions have been incorporated were thoroughly rinsed with water and transferred to clean LiClO4-HzO electrolyte. Despite the use of a low concentration of supporting electrolyte (20 mM) to limit the release of these anions from the polymeric layer following an ion exchange with clod- anions, repetitive scans in the anodic region result in a continuousdecrease of the intensity of the cyclic voltammetry waves of the trapped anions ([Fe(cN)#+-, E l p = 0.20 V; [Mo(CN)81P/”, E1p = 0.58 V; Figure 7). The potentials of these reversible redox couples are in good agreement with the half-wave potentials measured in the solution phase at an uncoated electrode (0.21 and 0.53 V, respectively). Incorporation ratios were defined as 3 times the ratio r[Fe(CN&rN+ and 4 times the ratio r[Mo(CN)8]k/rN+, taking into account that the trianionic ferricyanide or the tetraanionic octacyanomolybdatehas to bind to three or four monocationic alkylammonium groups, respectively. The apparent surface concentrations of ferricyanide and octacyanomolybdate species were determined from the charges measured at a low scan rate (usually 5-10 mV 8-l) on the first scan under the FelI1/=and reduction waves. In a thin film of r N + = 6 X mol cm-2, the incorporation of [Mo(CN)8l6 is higher (67%) than that of [Fe(CN)@-(48%). Alargeorganicanionsuchas ABTS could be efficiently trapped and retained in poly(2) films. A cyclic voltammogramof the resulting modified electrode transferred to CH3CN containing 20 mM LiC104 shows (Figure 8A) a reversible oxidation wave for the ABTS/ ABTS’+ redox system ( E t p = 0.44 V). The oxidation potential for trapped ABTS is ca. 240 mV less positive than that measured in CH3CN electrolyte at an uncoated electrode (Elp = 0.205 mV). Electrostatic interactions could be responsible for this potential shift. The incorporation ratio in a film of rN+= 5.9 X 104mol cm+eached 35%, taking into account that the dianionic ABTS molecule must be bound to two alkylammonium units. The modified electrode is stable in CH&N electrolyte, since there is no decrease in the peak size of the cyclic voltammetry curve after 80 scans. One-Step Elaboration of Poly(2) Electroactive Films. The incorporation of an electroactive anion could also be accomplished in one step, by the electropolymerization of an aqueous monomer-anion mixture spread and dried on the electrode surface. As an example, in Figure 8B is depicted the cyclic voltammogram for a C/poly(2ABTS) modified electrode obtained upon controlled-

lv

(41) Cosnier, S.; Deronzier, A.; Roland, J.-F. J. Electrounol. Chem. 1990,190, 133.

608 Langmuir, Vol. 10, No. 2, 1994

-

Figure 8. Cyclic voltammogram of C/poly(t-ABTS) electrode in CH&N + 20 mM LiC104, v = 50 mV s-l: (A) preformed poly(2) f i (r 5.9 X 10-8molcm-2) soaked in a 1 mM ABTS solution in CH&N and transferred to clean electrolyte [(a) 1st scan, (b) 80thscan], (B)repetitive scanswith a modified electrodeprepared by polymerization of a coating containing0.26 pmol of 2 and 0.23 pmol of ABTS.

Coche-Gubrente et al.

A

Figure 10. Incorporationof anthraquinonesulfonatein poly(2) and poly(6) films by repeated scans: (A) poly(2) film (r* = 5.2 X 10-8 mol cm-2) prepared in an aqueous LiClO4 electrolyteand cycled in 1 mM A& in H2O + 20m M LiC104,(El) same experiment as in (A) with a poly(6) film (rN+ = 7.3 X 10-8 mol (C) poly(2) (rN+= 5.7 x 10-8mol cm-2)prepared in an aqueous Et,,NBF4 electrolyteand cycled in 1 mM A& in H2O + 20 mM E4NBF4, v = 50 mV 8-1.

Figure 9. (a) Cyclic voltammetry curves for a C/poly(L-AQ) modified electrodeprepared as described in the text and cycled in H20 20 mM LiClO4 at pH (A) 11.2 and (B) 1.65 (s = 2 PA). (b) electrochemical response of a 2 mM AQ solution in the same electrolytes (s = 10 PA).

+

potential polymerization of a coating containing 0.26 pmol of 2 and 0.23 pmol of ABTS sodium salt. The electropolymerization appeared less efficient than with 2 alone. Less monomer was polymerized, and its polymerization required longer times. This could be due to the quenching effect of the basic sulfonate groups on the pyrrole polymerization. The cyclic voltammogram of the poly(%-ABTS) electrode prepared in this way is identical to that of the modified electrode obtained by soaking a preformed poly(2) film in a solution of ABTS (Figure 8A). The stability of the cyclic voltammetry curve upon repetitive scanning in the anodic region demonstrates that ABTS anions are irreversibly trapped in the polymeric matrix. Entrapment of Anthraquinone-2-sulfonateAnions in Poly( 1-5) Films. The one-step procedure described above was applied to the immobilization of the anthraquinone-2-sulfonate (AQ) ion. In Figure 9A (curve a) is represented the cyclic voltammetry curve for a C/poly(2-AQ) modified electrode prepared from 0.19 pmol of 2 and 0.22 pmol of AQ sodium salt and transferred to an aqueous LiC104 electrolyte (pH 11). The high stability of

this modified electrode is demonstrated by the stability of the cyclic voltammogram upon repetitive scans. As already noted for the polymerization of 2-ABTS mixtures, the polymerization currents with 2-AQ coatings were very weak, as compared with those measured during the polymerization of pure 2 layers. The elaboration of poly(2-AQ) filmsrequired long polymerizationtimes (more than 2 h), while the polymerization of a pure 2 layer was accomplished in less than half an hour. It is noteworthy that the bielectronic AQ/AQH2 reversible reduction system gives two distinct redox couples at -0.63and -0.76 V, while in homogeneous solution a regular AQ/AQH2 system is characterized by single oxidation and reduction peaks (Figure 9A, curve b). Peak splitting was also observed when a C/poly(2) electrode was soaked and repeatedly cycled in an aqueous electrolyte containing 103 M AQ (Figure 10A). The gradual growth of the size of the AQ/AQH2 waves to reach steady-state peak currents proves its incorporation in the preformed film. Peak splitting is observed from the first scan, whatever the film deposition process, Le., electrodeposition from CH&N electrolyte solutions of 2 or electropolymerization in an aqueous electrolyte of a preadsorbed monomer film. However, if the modified electrode is transferred to clean aqueous electrolyte and cycled over the AQJAQH2 redox system, a fast decay of ita electrochemical response is observed, due to a quick release of AQ from the polymer film. Comparison with the behavior of the modified electrode prepared by polymerization of a 2-AQ mixture (Figure 9) demonstrates that the one-step procedure allows the elaboration of stable systems. In the latter case, the anion is not simply retained by electrostatic binding but really caged in the polymeric matrix, as a consequence of the formation of the polymeric backbone around the bulky

Langmuir, Vol. 10, No.2, 1994 609

Electropolymerization of Cationic Pyrrole Derivatives

AQ anions. It should be noted that no peak splitting has been found for AQ incorporated in a preformed poly(6) film,- by repeated scans in an AQ aqueous solution (Figure 1OB). Obviously, the specific properties of poly(2) films are due to the long C n chain linking the pyrrole group to the triethylammonium head.

V 6

We have first checked that this unusual behavior was not due to specific interactions. Such interactions are known to take place between lithium cations and reduced quinones in organic electrolytic solutions.42 A C/poly(%) electrode was prepared in an aqueous solution containing Me&JBF4as supporting electrolyte. The incorporation of AQ by repeated scans was also carried out in an electrolyte (MerNBF4 + H2O) free of lithium cations (Figure 1OC). The same peak splitting as in the experiment realized in LiC104 electrolyte (Figure 10A) was observed. However, the polymer prepared in MeaBF4 aqueous electrolyte is more permeable, as demonstrated by the faster and larger incorporation of AQ is this film. This different permeability is probably due to the size of the cations of the supporting electrolyte used for the electropolymerization. Although the most important factor in improving the permeability of electrosynthesized polypyrroles is obviously the size of electrolyte anion in the electropolymerization solution,a it has been demonstrated that the cation size must also be considered43 since polymeric films are known to retain some electrolyte in their structure. Perchlorate and tetrafluoroborate anions having similar sizes, the more bulky Me4N+ cation must be responsible for the improved permeability of poly($) films, as compared with those elaborated in the presence of the smaller Li+ cation. The influence of the pH on the cyclic voltammetry wave form has also been investigated. It is noteworthy that the

Figure 11. Cyclic voltammogram for a C/poly(%-AQ)modified electrode prepared by electropolymerizationin LiClO, aqueous electrolyte of a coating containing 0.16 pmol of 2 and 0.14 pmol of AQ, dried and transferred in CHsCN + 20 mM MedNBF4, v = 50 mV s-I.

/V

1 1

1 1

+2e, +2H+

AQ AQH, peak system remains identical at high (Figure 9A, curve a) and low (Figure 9B, curve a) pH, while a large difference is observed in homogeneous solution (Figure 9, curves b) due to the protonation of AQ at pH 5 4 . So, peak splitting cannot be attributed to an increase of the pH in the polymer film, caused by the release of protons during the polymerization. The polymeric matrix protects entrapped molecules against changes of the electrolyte pH. In contrast, no peak splitting was found when the modified electrode was transferred and studied in dry DMSO or CH&N electrolyte. In organic solvents, the cyclic voltammogram presents (Figure 11) the regular features for the redox response of a quinone in an organic electrolyte: two well-behaved, largely separated and reversible one-electron waves corresponding to the formation of AQ'- and AQ" species. Finally, oxidative electropolymerization in aqueous LiC104 electrolyte of a 2-AQ coating by cycling over an extended potential area including the cathodic region demonstrates (Figure 12) that the double peak phenomenon appears only in the polymerized layer. Peak splitting is not seen on the first scans (curve a, 2nd scan, for example),but appears clearly (42)Eggins, B. R. J. Chem. Soc., Chem. Commun. 1969, 1267. (43) Cosnier, S.; Deronzier, A.; Roland,J.-F. J. Electroanol. Chem. 1991, 310, 71.

II , ] I

v Figure 12. Oxidative electropolymerization in HzO + 20 mM LiClOl of a coating containing 0.20 pmol of 2 and 0.24 pmol of AQ deposited on a C disk electrode by repeated scans over the -1.2 to +0.8 V range: (a) 2nd cycle, s = 4 pA, (b) 18th cycle, s = 1pA, v = 50 mV s-I.

when the polymer is formed (curve b, 18th scan), despite a large release of AQ molecules during the electropolymerization. The only hypothesis which can be propoundedto explain peak splitting observed in aqueous electrolyte is the existence of a regular surface arrangement in the polymer film, giving two distinct environments for the AQ species. Double peak voltammograms have already been reported in organized surface microstructures, such as mono- and multilayered films transferred by the Langmuir-Blodgett method4 and self-assembled electroactive amphiphilee,& or with a poly(vinylferr0cene) film containing liquid crystalline dopants.& Specific interactions were postulated to be responsible for the formation of discrete species having different redox potentials. For example, in ad(44) Lee, C. W.; Bard,A. J. J.Electroanal. Chem. 1988,239,441;Chem. Phys. Lett. 1990, 170, 57. (45) (a) Lin, M. D.; Leidner, C. R. J. Chem. SOC.,Chem. Commun. 1990, 383. (b) De Long, H.C.; Buttry, D.A. Lagmuir 1992, 8, 2491. (46) Mariani, R. D.; Abruna, H. D.Electrochim. Acta 1987,32,319.

610 Langmuir, Vol. 10, No. 2, 1994 A

Figure 13. (A) Incorporation of AQ in a poly(1) film (r = 2.3 X 1 P mol cm-2)by repeated scans in 1 mM AQ in HzO + 20 mM LiC104, 1st to 15th scans. (B) Modified electrode transferred and cycled in clean electrolyte, Y = 50 mV s-l.

Coche-GuBrente et al.

due to ita larger hydrophilic character. The incorporation ratio reached 87 % , which is better than in a poly(2) film with a similar thickness (3595 ). However, continuous cyclingof the modified electrode in an AQ-free electrolyte results in a quick release of dopant anions (Figure 13B). On the other hand, the poor permeability of poly(%), poly(l), and poly(5) films in aqueous electrolytes is responsible for a very weak incorporation of AQ. For example, an incorporation ratio of 2% was measured in a preformed poly(%)film (rN+ = 4.8 X 10-8 mol cm-2). One-step elaboration of a poly(3-AQ) modified electrode also results in a poor retention of electroactive anions, despite a fair electropolymerization of the 3-AQ layer. This is probably due to a large release of AQ anions from the hydrophobic film during the growth of the polymer. Very weak incorporations were also observed in poly(4) and poly(5) films. It should be noted that the cyclic voltammetry curve for the one-step-synthesized C/poly(3-AQ) modified electrode is also characterized by adouble wave, demonstrating the existence of an organization similar to that in poly(%-AQ)films.

sorbed films containing viologen g r o ~ p s ~ 9 ~their 6 b oneelectron-reduced forms are known to interact strongly in highly congested assemblies to produce new cyclic voltammetry waves with formal potentials related to the equilibrium constant of the resulting T complex. However, such interactions have not been described for the reduced forms of AQ. On the other hand, peak splitting observed for [Ru(bpy)312+(bpy = 2,2'-bipyridine) incorporated in a polystyrenesulfonate film have been interpreted on the basis of the existence of two distinct, interconvertible interaction sites or of two distinct configurations of the cationic complexes with respect to the sulfonate groups and the polymericbackbone!' Finally, a theoretical model based on a statistical mechanical description of surfacebound monomolecular films containing redox species suggests that one-electron-transfer peak splitting occurs when the surface arrangement is regular.48 The incorporation of AQ has also been studied in the other poly(amphiphi1icpyrrole) films. Since poly(1) films cannot be elaborated by electropolymerization of preadsorbed films of 1,AQ has been incorporated in preformed films deposited by electropolymerization of 1 in CHsCN electrolyte. Poly( 1-AQ) modified electrodes do not exhibit a double-peaked wave (Figure 13). The fast incorporation of AQ in poly(1) as compared to that observed in poly(%) films has to be noticed. A steady-state voltammogram is obtained from the first scan recorded immediately after the C/poly(l) modified electrode has been soaked in the AQ solution (Figure 13A). This is probably the result of the higher permeability of poly( 1)to aqueous electrolytes,

Conclusion We have demonstrated that oxidative electropolymerization of amphiphilic (pyrrolylalky1)ammonium monomers in aqueous electrolytes provides stable anionexchange coatings on platinum or glassy carbon electrodes that can incorporate large amounts of multivalent anions. While monomer 1,which contains a trimethylammonium group, leads to the formation of micelles in water, monomer 2 which differs from 1by the presence of a larger cationic head, Le., a triethylammonium group, does not give any evidence of the formation of any organized system. However, it is noteworthy that the redox behavior of anthraquinonesulfonate incorporated in poly(2) films is characterized by a double peak phenomenon, suggesting that the films have regular arrangement. Poly(3-5) films have similar properties, but differ from poly(2) by a higher hydrophobic character, due to the increase of the size of their trialkylammonium head. Preformed poly(1) and poly(%)films, and to less extent poly(3-5) films, exhibit anion-exchange properties. In contrast with the more soluble surfactant 1, monomers 2-5 have the attractive feature of polymerizing in aqueous electrolyte when preadsorbed on an electrode surface, even when associated with bulky redox anions. We have very recently taken advantage of these unique properties to elaborate enzymatic layers which have applications as b i o s e n s ~ r sAs .~~ a matter of fact, the electropolymerization of amphiphilic pyrrole-enzyme mixtures in aqueous electrolytes leads to the efficient and controlled immobilization of enzyme molecules onto an electrode surface, avoiding their denaturation.

(47) Majda, M.; Faulkner, L. R. J. Electroanal. Chem. 1984,169,77. (48) Matsuda, H.; Aoki, K.; Tokuda, K. J. Electroanal. Chem. 1987, 217, 1, 15.

(49) (a) Coder, S.; Innocent, C. J. Electroanal. Chem. 1992,328,361. (b) Coche-Gdrente, L.; Cosnier, S.;Innocent, C.; Mailley, P.; Moutet, J.-C.; Morelis, R. M.; Leca, B.; Coulet, P. R. Electroanalysis 1993,5,647.