Polyaniline films based ultramicroelectrodes sensitive to pH

Available online at www.sciencedirect.comJournal of

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Journal of Electroanalytical Chemistry 612 (2008) 53–62

ElectroanalyticalChemistry

Polyaniline films based ultramicroelectrodes sensitive to pH

Cyrine Slim, Nadia Ktari, Dushko Cakara, Frederic Kanoufi, Catherine Combellas *

Laboratoire Environnement et Chimie Analytique, UMR 7121, CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France

Received 25 June 2007; accepted 10 September 2007Available online 25 September 2007

Abstract

Poly-aniline and o-methoxyaniline films can be obtained directly on an ultramicroelectrode through the anodic electropolymerizationof the monomer through successive potential cycles. The potentiometric response depends on the nature of the anion incorporated in thecourse of the electropolymerization. At best, a linear relationship with pH is observed for 3 < pH < 9–10, with a slope of �60 or 50 mVper pH unit for polyaniline and poly(o-methoxyaniline) ultramicroelectrodes, respectively. Such modified electrodes can be used to probepH variations resulting from ion exchange in a thin Nafion� film.� 2007 Published by Elsevier B.V.

Keywords: Polyaniline; pH sensor

1. Introduction

The glass electrode is considered as the standard methodfor pH measurements, due to its numerous advantagessuch as its Nernstian response for 2 < pH < 12, its absenceof sensitivity to redox interferents, its high reproducibilityand its long lifetime. However, some drawbacks exist, suchas its lack of stability in strongly acidic or basic media, itshigh price and the difficulty to miniaturize it [1]. The latterpoint is crucial with the development of microelectrome-chanical systems (microfluidic devices, lab-on-a-chip, min-iaturized sensors, . . .). Compared to classical systems, suchmicrosystems present different advantages, such as theirhigh sensitivity, analyses reduced times and low cost. Inthis field, there is an increasing need for alternative meansto measure pH in small volumes.

Among the new sensors that are currently developed forpH detection on a micrometer scale or below, we may men-tion potentiometric metal oxide semiconductors [2,3], lightaddressable potentiometric sensors [4], ion-sensitive fieldeffect transistors (ISFET) [5], scanning probe potentiome-

0022-0728/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.jelechem.2007.09.020

* Corresponding author. Tel.: +33 1 40794608; fax: +33 1 40794425.E-mail address: [email protected] (C. Combellas).

ter [6], potentiometric hydrogel functionalized cantilever[7]. PH sensors, based on conducting polymers are alsodeveloped [8–12]; to our knowledge, there are few resultsin the literature of such systems on a micrometer [13,14]or nanometer [15,16] scale. Our objective is to contributeto this field and propose a pH sensor based on a conduct-ing polymer electropolymerized on an ultramicroelectrode(UME).

Many conducting polymers are pH sensitive since theycontain atoms that are prone to protonation such as nitro-gen. We can mention polyphenylenediamine, polyalkyle-neimines, polyaniline, . . . We have focused on polyaniline(PANI) and substituted PANIs, whose use as actuatorhas been widely described, due to the numerous advantagesof this conducting polymer [17–19]. PANIs can be easilysynthesized or electrosynthesized by aniline oxidation inacidic aqueous solutions [20–26], they are stable and solu-ble in most organic solvents and can be processed easilyfor many applications.

The chemistry of PANI is a little intricate, due to theexistence of different acidic functions and oxidation states[25–28]. There are three oxidation states, each redox couplecorresponds to a 2e� exchange. The chemical structures ofthe different forms of PANI are given in Scheme 1. The lessoxidized state, leucoemeraldine (LEB), the intermediate

mailto:[email protected]

AA

AA

N

N

N

NR

R

R

R

N

N

N

NR

R

R

R

N

N

N

NR

R

R

R

+

+

N

N

N

NR

R

R

R

++

H

N

N

N

NR

R

R

R

. .

H

HH

H H

HH

H

HH

H H

H

- -

--

[

[

[

+ 2 e-

+ 2 H+

]n

]n

]n

]n

]n

[

[

+ 2 e-

+ 4 H+

+ 2 A-

+ 2 e-

+ 2 H++ 2 e-

- 2 A-

LEB

EB

ES

P

- 2 H+

- 2 A-

Scheme 1. The different forms of polyanilines (here R = H, PANI; CH3, PMeANI; OCH3, POMeANI).

54 C. Slim et al. / Journal of Electroanalytical Chemistry 612 (2008) 53–62

state emeraldine (EB) and the most oxidized state, pernigr-aline (P) are insulating, the only conductive form is theintermediate emeraldine salt (ES) [25,29]. As for manypolyelectrolytes having pH dependent moities, the pKasof PANI depend on the nature of the polymerization con-ditions (solvent, electrolyte, . . .). However, among the vastliterature, it is generally admitted that the reduced form ofPANI, LEB, possesses amine groups that can be proton-ated at acidic pH ð–NHþ2 =–NHÞ. A pKa around or smallerthan 2.5 is proposed for the protonation of one out of thesefour amine groups [30]. The emeraldine state, EB assketched in Sheme 1, possesses 2 imine groups that are pro-tonated in ES (@NH+/@N) and then confer to the polymerits pH sensitivity. The protonation of those two imines isachieved simultaneously (the second-one being much easierthan the first-one) at a pKa ranging between 5.5 and 8depending on the polymer synthetic route, its chemicalstructure (ratio of imine/amine in the EB form), its polydis-persity, the nature of the substituent on the aniline ring, thepH and content of the electrolytic solution used during theelectropolymerisation, etc. [30–34]. Moreover, at moreacidic pH, the amine groups can also be protonated, inthe same ratio 1 over 4 amine group as in the LEB struc-ture, with a pKa around or lower than 2.5 [30,33]. Changesof the oxidation states of any form of PANI are accompa-nied by exchanges of protons or/and anions; the latter aregenerally those incorporated into the polymer structureduring the anodic electropolymerization (typically Cl�).The small size of Cl� and consequently its high mobilityallow quick exchanges that are favourable for the sensitiv-ity of PANIs towards pH changes [17]. Since redox changesare accompanied by anion exchanges, the nature of the

anions present in the medium play a major role in the redoxprocess. When using polyelectrolyte anions such as poly(2-acrylamido-2-methyl-1-propanesulfonic acid, PAMPS)[18,35], a fixed anionic environment for PANI is presentwhatever the redox state and ionic exchanges accompany-ing redox changes are of cationic nature [18].

Here, we have polymerized anodically anilines (aniline,ANI, o-methylaniline, MeANI, and o-methoxyaniline,OMeANI) on UMEs in the presence of either Cl� orPAMPS anions. We have studied the parameters that inter-vene in the electropolymerization, characterized the modi-fied electrodes and tested their potentiometric responseduring an acid/base titration [36]. Finally, we have testeda PANI modified UME to probe a pH variation over aNafion� film.

2. Experimental

2.1. Chemicals

Aniline (reagent grade) and [poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PAMPS, 10% wt% inwater] were purchased from Acros, o-methylaniline (99%)and o-methoxyaniline (99%) from Aldrich.

HCl, KCl, KOH, H2O2 and H2SO4 were purchasedfrom Prolabo. Ethanol (analytical grade) was purchasedfrom SDS and milliQ grade water was obtained from thelaboratory.

The buffer solutions (2 < pH < 9) were obtained fromthe following solutions using the phoebus software forthe proportions: 1 M citric acid, 0.1 M KH2PO4, 0.1 MK2HPO4, 0.1 M (trihydroxymethyl)aminomethane (tris),

C. Slim et al. / Journal of Electroanalytical Chemistry 612 (2008) 53–62 55

0.1 M NaB4O7, 0.1 M KCl, 1 M or 0.1 M hydrochloric acidor 1 M or 0.1 M sodium hydroxide.

Platinum wires, used in the ultramicroelectrodes, were of99.9% purity (Goodfellow, United Kingdom). Ultramicro-electrode were fabricated according to literature [37].

2.2. Electrochemical measurements

The experiments were performed in a glass electrochem-ical cell containing 10 mL of aqueous solution and 50 mMof monomer. A platinum wire was used as counter-elec-trode (radius = 0.5 mm) and an SCE as reference electrode.The solution was degassed by nitrogen for 15–20 minbefore operation.

To obtain a polymer film with Cl� as counter-anion, theelectropolymerization reactions were performed in 1 Mchlorohydric acid solution. To obtain a polymer film withPAMPS as counter-anion, the electropolymerization reac-tions were performed in a polyanion solution whose pHwas adjusted to 2 by addition of 1 or 2 droplets of HCl.

The electropolymerization reactions were performed bycyclic voltammetry at scan rates ranging from 0.01 to0.1 V s�1 according to two procedures. In the first-one,the potential was swept between 0 and 1 or 0.8 V/SCEfor a certain number of cycles. The second one consistedof increasing the anodic limit of the scanning of 50 mVevery 10 scans, starting from 0.7 V and up to 1 V (70scans).

Electropolymerization reactions, characterization of thedeposited films and potentiometric measurements were per-formed with a CH Instruments potentiostat/galvanostat(CH660A, CH Instruments, USA).

For titration curves, pH were measured independentlyby a glass electrode; titration was performed by addingsmall volumes of a KOH solution to an HCl solution. Toestimate precisely potentials and pH around neutralization,addition of volumes as low as �10�1–10�2 lL was per-formed by simple dipping of a syringe needle (our labora-tory is not equipped with a titrimeter offering suchaccuracy). The added volume was derived from the solu-tion pH, measured independently by a glass electrode.

To probe pH over polymer films, Nafion� films of 50 to200 nm thickness were obtained by spin coating over glasssubstrates. PH measurements over such thin films wereachieved either (i) with a two-electrode assembly, madeof a 50 lm diameter PANI/PAMPS UME and a 250 lmdiameter Ag/AgCl wire or (ii) with a 700 lm diameter com-mercial glass electrode (Tacussel). The electrode was heldvertically above the film surface by a motorized verticalstage and positioned a few millimeters above the film. Withthe PANI/PAMPS UME, it was possible to deposit a waterdroplet of a known pH on the modified UME disc. Then,the electrodes + droplet set was moved downwards untilthe droplet contacted the film. Conversely, with the com-mercial electrode, that was impossible, due to the electrodeshape (drop, the surface for water deposition was not flat);the water droplet was then deposited between the Nafion�

surface and the pH electrode. The contact surface area wasmeasured with a stereomicroscope (Zeiss).

All potentials are given versus SCE. Solutions contained0.1 M KCl.

2.3. SEM

An Hitachi S3600 scanning electron microscope wasused to examine the film morphology. For the elementalanalysis, an Oxford Instrument INCA 4.06 was used. Thesamples were set in epoxy resin and in some cases, theywere coated with gold using a Polaron 550 coating system.

2.4. Characterization of the modified UMEs

The modified UMEs were characterized by cyclic vol-tammetry. The amount of polymer deposited on the elec-trode was estimated from the charge exchanged, Q, onthe latter voltammogram during the oxidation or thereduction step and also by such charge normalized by theelectrode surface area, q.

In some cases, the film thickness, d, was estimatedaccording to:

d ¼ QM=nF pr2q

with Q the charge corresponding to the complete polymerfilm oxidation (from LEB to P state), M, the monomer mo-lar mass (M = 93.1 g mol�1 for aniline), n, the number ofelectrons exchanged in the peak for one monomer unit(n = 1), F, the Faraday constant (F = 96,500 C mol�1), r,the UME radius and q the density of the polymer (takenas 1.2 g cm�3 [38]).

3. Results and discussion

In the following, all our results will be discussed but notnecessarily shown to avoid a boring reading. The mainresults will be illustrated for some representative examples.

3.1. Electropolymerization by cyclic voltammetry

Electropolymerizations of the different anilines mono-mers were carried out on platinum disk UMEs whosediameters varied between 25 and 100 lm. They were per-formed by cyclic voltammetry in an aqueous solution con-taining either HCl or PAMPS and 50 mM of the monomer.

For the lowest UME diameter (25 lm), electropolymer-ization between 0 and 0.8–1 V did not allow to obtain afilm modified UME, whatever the nature of the monomer.This is likely due to propriety of UMEs to sustain very highcurrent density at the UME edge. The polymerization isthen expected to occur at a much faster rate at the edgeof the UME than on a millimetric electrode. In order tostart the polymerization at a reasonable and constructiverate, the current density should be lowered, this is achievedby lowering the anodic potential for the polymerization.


Here, the limit anodic potential was progressively shiftedanodically of 50 mV every 10 cycles, starting at 0.7 V.

3.1.1. Polyaniline (PANI)

We show in Fig. 1a the first four voltammetric cycles ofANI polymerization between 0.0 and 1.0 V/SCE in thepresence of PAMPS ([ANI]/[PAMPS], 1/2), with a 50 lmdiameter UME. On the first scan, an oxidation peak isdetected at �0.90 V/SCE. When the potential is reversed,the reduction curve crosses the oxidation peak, and areduction peak is observed at �0.55 V/SCE. During thenext scans, the charge involved in each redox peakincreases and the redox system moves slightly towards neg-ative potentials. On the 4th cycle, the peak characteristicsare �0.63 V/SCE for the oxidation and �0.54 V/SCE forthe reduction. On the first scan, the oxidation peak corre-sponds to aniline oxidation into its radical cation and thereduction peak to the reduction of an aniline oligomeror/and polyaniline itself. Oligomers and polyaniline are

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Fig. 1. Aniline (ANI) electropolymerization. 50 lm diameter Pt UME.Sweep rate = 0.01 V s�1. [ANI] = 50 mM. [PAMPS] = (a), 100 mM (b)200 mM. Potential swept between 0.00 and (a) 0.95 V/SCE, (b) 0.80 V/SCE. (a) pH 1.7, ( ) 1st, ( ) 2nd, ( ) 3rd, and (—) 4th cycle. (b) pH 1.4,( ) first, ( ) 10th, ( ) 20th, ( ) 30th, and ( ) 36th cycle.

obtained at the aniline oxidation peak potential, they aremore difficult to reduce than the aniline radical cation.The next scans involve successive and alternate oxidationreactions of oligomers and polymer, which increase thechain length. As the scans number increases, the oligomersoxidation becomes easier, which explains the negativepotential shift.

Electropolymerization was also performed with anupper potential of 0.8 V instead of 1.0 V/SCE in order toavoid over-oxidation as mentioned by Ivaska [27]. Thesame redox system at �0.60/0.55 V/SCE may then beobserved (see Fig. 1b). As the number of cycles increases,two small redox systems at �0.15/0.10 V/SCE and �0.40/0.37 V/SCE may also be detected. The first system corre-sponds to the LEB/ES transition, the second-one to degra-dation products and the third-one to the ES/P transition[27].

3.1.2. Poly-o-methylaniline (PMeANI)

With o-methylaniline, it was very difficult to obtain athick film by electropolymerization on a 25 lm diameterUME, even by progressive electropolymerization; the cur-rent intensity recorded on the first voltammetric scans istoo low to give a nice voltammogram. The voltammogramfor the 70th scan exhibits two badly resolved reversible sys-tems at �0.5 V/SCE that correspond to the LEB/ES andES/P transitions.

3.1.3. Poly-o-methoxyaniline (POMeANI)

With o-methoxyaniline three redox systems are observedon the successive voltammograms of the electropolymeriza-tion, as depicted in Fig. 2, which represents a scan every 10scans of a progressive electropolymerization in the presence

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0 0.2 0.4 0.6 0.8 1

E (V) / SCE

I (1

0-8A

)

Fig. 2. Methoxyaniline (OMeANI) progressive electropolymerization.25 lm diameter Pt UME. Sweep rate = 0.1 V s�1. [OMeANI] = 50 mM.[HCl] = 1 M. (—) 10th cycle (Emax = 0.7 V), ( ) 20th cycle (Emax =0.75 V), ( ) 30th cycle (Emax = 0.8 V), ( ) 40th cycle (Emax = 0.85 V),(—) 50th cycle (Emax = 0.9 V), ( ) 60th cycle (Emax = 0.95 V), and ( )70th cycle (Emax = 1 V).

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0 0.2 0.4 0.6 0.8 1

E(V) / SCE

I (1

0-8A

)


of Cl� as counter-anion, with a 25 lm diameter UME. Theintensities corresponding to the 10th cycle are so low thatthe voltammogram, which should appear in blue,1 cannotbe seen in Fig. 2. For the 20th cycle, only the most anodicpart of the voltammogram (in pink) can be slightly seen.

The different redox systems potentials are similar tothose observed for PANI UMEs. The first system at�0.15/0.10 V/SCE corresponds to the LEB/ES transition,the second-one at �0.40/0.37 V/SCE to different possiblereactions (irregular couplings, dimers and degradationproducts) [26] and the third-one at �0.60/0.55 V/SCE tothe ES/P transition. Similar features are observed withPAMPS as counter-anion instead of Cl�. When using alower upper potential no significant difference is observedon the electropolymerization voltammograms.

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E(V) / SCE

I (1

0-8A

)I

(10-8

A)

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2

4

6

-0.2 0 0.2 0.4 0.6 0.8

E(V) / SCE

Fig. 3. Cyclic voltammetry of a modified PANI/PAMPS Pt UME. 50 lmdiameter Pt UME. Sweep rate = 0.05 V s�1. (a) and (b) Electropolymer-ization for four cycles between 0.00 and 0.95 V/SCE with [ANI] = 50 mM,[PAMPS] = 200 mM; characterization in: (a) HCl, pH 1.4 + 0.1 M KCl;(b) HCl, pH 3 + 0.1 M KCl; (c) electropolymerization for 36 cyclesbetween 0.00 and 0.80 V/SCE with [ANI] = 50 mM, [PAMPS] = 200 mM;characterization in HCl, pH 0.0. Charge exchanged (a) 7.1 · 10�7 C(q = 3.6 · 10�2 C cm�2) between 0.35 and 0.85 V; (c) Qox = 4.0 · 10�7 Cand Qred = 4.4 · 0�7 C (qox = 2.0. · 10�2 C cm�2, qred = 2.2. · 10�2

C cm�2) between �0.15 and 0.70 V.

3.2. Characterization of the modified UMEs by cyclic

voltammetry

3.2.1. Polyaniline

We have characterized different 50 lm diameter UMEsmodified by PANI/PAMPS.

First, the effect of the pH used to characterize the elec-trodes was tested. For that, characterizations of an UMEobtained under the experimental conditions of Fig. 1awas performed in different solutions by cyclic voltammetrybetween 0 and 0.85 V/SCE. At pH 1.4, similar voltammo-grams are observed in HCl and HCl + 0.1 M KCl (seeFig. 3a), which means that the K+ cation does not playany role at pH 1.4 in HCl and confirms that at such pHonly H+ and Cl� may be exchanged. Moreover, no changein the voltammogram is observed when characterization isperformed in 100 mM PAMPS at pH 1.4; the anionicexchange in the presence of Cl� has then been convertedinto a proton exchange.

Conversely a pH increase from 1.4 to 3 results in a dra-matic change of the shape, potentials and charges in thevoltammogram (see Fig. 3b). The oxidation and reductionpeaks become wider, which could be due to the inference ofK+ exchange instead of H+.

PANI modified UMEs may be properly characterizedwhen electropolymerization has been carried out underpotential conditions where over-oxidation has beenavoided (Fig. 3c). The characterization voltammogramgiven is reminiscent of that obtained with a millimetric elec-trode by Ivaska [27]. It presents three systems: (i) a welldefined redox system at Eox �0.15 V/SCE and Ered�0.00 V/SCE, that may be attributed to the transitionbetween the non conductive LEB form into the conductiveES form, (ii) a small system around 0.5 V/SCE that corre-sponds to degradation products of PANI and (iii) thebeginning of a third system that corresponds to the ES/Ptransition, which is concomitant with the degradation of

1 For interpretation of color in Figs. 1 and 2, the reader is referred to theweb version of this article.

PANI. Most authors also observe two characteristic redoxsystems for PANI whose relative intensities depend on theexperimental conditions [27,39–42].

We have shown by impedance spectroscopy that whenpH varied from 2 to 4, the PANI film impedance logarithm(at zero current potential on a 50 lm diameter UME)increased from 3.8 to 5.2, then remained constant untilpH 6. Actually, at this potential, the impedance resistancepart, Z 0, is largely higher than the imaginary part, Z00, and

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Fig. 4. Cyclic voltammetry of a modified POMeANI Pt UME (25 lmdiameter). Characterization in 0.1 M HCl. Sweep rate = (a) and (b)0.1 V s�1, (c) 0.05 V s�1. (a) and (b) Progressive electropolymerizationbetween 0.00 and 1.00 V/SCE with [OMeANI] = 50 mM and (a)[HCl] = 1 M, (b) [PAMPS] = 200 mM. (c) Electropolymerization between0.00 and 0.70 V/SCE for 45 cycles with [OMeANI] = 50 mM,[HCl] = 1 M. Charge exchanged: (a) 2.2 · 10�7 C (q = 4.5 · 10�2 C cm�2);(b) 3.5 · 10�7 C (q = 7.1 · 10�2 C cm�2); (c), 1.5 · 10�6 C (q = 1.1C cm�2).


the observed effect is mainly due to Z 0 (Z00 increases in thewhole pH range); for higher pH (pH > 8), both impedancecomponents increase dramatically owing likely to the lossof conductivity of the polymer. This increase of the PANIfilm impedance with pH should be responsible for slowerionic exchanges inside the film that result in the lowerslopes mentioned above. The inference of cations exchangeat higher pH might be taken into account [43].

The reproducibility of the deposition was tested by theevaluation of the amount of polymer deposited onto fiveUMEs synthesized under identical conditions (UME diam-eter: 50 lm, electrosynthesis for 4 voltammetric cyclesbetween 0 V and 1 V at 0.01 V s�1). The five voltammo-grams are identical, and lead to a mean charge, Qox, of3.7 · 10�7 C (q = 1.9 · 10�2 C cm�2, standard deviationratio: 3.6%).

It has been reported that millimetric PANI modifiedelectrodes had variable lifetimes depending on the potentialto which the film had been submitted [32]. We can confirmthese trends for PANI UMEs. We have shown, for exam-ple, that successive and long uses of the modified UMEat a constant imposed potential of 0.45 V resulted in a shiftof the UME characteristic cyclic voltammogram towardsnegative potentials and a decrease of the current intensities;this corresponds to a loss of the amount of conductingmaterial in contact with the electrode. For pH measure-ments with PANI modified UMEs, it is then more reason-able that no current flows through the electrode andactually, measurements made by potentiometry at zero cur-rent had lesser consequences on the PANI modified UMEcharacteristics. Besides, the UME characteristics do notchange much with time if it is not on operation (no contactwith a solution).

3.2.2. Poly-o-methylaniline

In the case of PMeANI, the film obtained with a 25 lmdiameter UME was not stable enough to resist to charac-terization. Such lack of stability is related to the smalldiameter of the electrode. The voltammogram obtainedfor the 70th scan of the electropolymerization exhibits aredox system at �0.5 V, that corresponds to the ES/P tran-sition. The charge exchanged on the oxidation system islower than with the other PANI modified UMEs(Qox = 3.7 · 10�8 C, q = 7.5 · 10�3 C cm�2).

3.2.3. Poly-o-methoxyaniline

We have characterized a 25 lm diameter POMeANI/ClUME modified under the experimental conditions ofFig. 2. Characterization was performed in HCl, pH 1, bycyclic voltammetry between 0 and 0.85 V/SCE (seeFig. 4a). It confirms the results obtained during the elec-tropolymerization (cf. Fig. 2). The first peak (Epox�0.15 V, Epred �0.10 V) corresponds to the LEB/ES tran-sition, the broad second peak to irregular couplings, dimersand degradation products [26] and the third-one to the ES/P transition. The negative shift of the latter, compared tothat on the electropolymerization voltammogram, results

from the lower HCl concentration for the measurement(0.1 M) than for the electropolymerization (1 M).

We have also characterized a 25 lm diameter UMEmodified by POMeANI in the presence of PAMPS as thecounter anion. The characterization was achieved in0.1 M HCl (see Fig. 4b). The first peak observed withCl� is present at the same potential (Epox �0.15 V, Epred�0.10 V), but the broad second peak is resolved in a firstsystem with Epox �0.40 V, Epred �0.37 V and a second-one with Epox �0.50 V, Epred �0.45 V.


The characterization voltammogram of a film obtainedunder potential conditions where over-oxidation is avoidedis given in Fig. 4c. It presents the three systems mentionedabove; as already observed with PANI, only the beginningof the third system corresponding to the ES/P transitionwas plotted to avoid over oxidation [27].

SEM secondary electrons images of a 25 lm diameter PtUME modified by POMeANI/Cl� are given in Fig. 5 forrelatively thick films (see Fig. 5a–c) and a thinner film(see Fig. 5d). The estimated mean thickness for the firstthickest film is �2 lm, which is in agreement with a roughestimation by AFM.

Fig. 5a shows that the polymer is present mainly in a�25 lm diameter disk with some scratches. This disk is sur-rounded by a nearly unmodified glass zone; however, somepolymer can also be seen on glass.

Higher resolution images of the same film or of the othertwo (see Fig. 5b–d) evidence a compact microspheroid sur-face morphology. Such morphology has already beenobserved for an emeraldine hydrofluoroborate film whilstthe same authors observed fibrils in the case of an emeral-dine hydroperchlorate film [44] and other authors observedsponge-like structures with PANI/Cl� [10].

Analysis of platinum by X fluorescence of the same sam-ples (not shown) shows some heterogeneity of the modifica-tion since at the level of the metallic wire, the Pt imageexhibits several zones that are less covered by the polymer.Analysis of carbon by X fluorescence confirms the presenceof the polymer on the platinum UME but also on somepart of the glass.

Fig. 5. SEM images for POMeANI/Cl� Pt UMEs (25 lm diameter). P[OMeANI] = 50 mM. [HCl] = 1 M. Charge: (a, b), 8.2 · 10�7 C; q = 0.11.3 · 10�2 C cm�2. (c) and (d) Gold coated.

3.3. Potentiometric response to pH

The potentiometric response of millimetric electrodesmodified by polyaniline and substituted polyanilines isdescribed in the literature. It may be linear with pH inbuffered solutions in a broad pH range (2 < pH < 10)[10,11,15,17]. In unbuffered solutions, some linearity maybe observed, but generally not in the whole pH range [13].

First, we have studied the potentiometric response ofmodified UMEs with time during an acid-base titration.A typical chronopotentiogram (E–t curve) obtained duringthe acid-base titration (addition of base to an acidic solu-tion) is given in Fig. 6 for a 25 lm diameter POMeANIUME. It shows that the UME response time to a pHchange is shorter in acidic media (�20 s) than in neutralones (�100 s); it is even higher in alkaline media (200 saround pH 8, not shown). This is likely due to the insulat-ing character of PANIs in alkaline media, which slowsdown charge and therefore mass transfers, such as iontransfers into the film.

UMEs modified by PMeANI were not tested due to thepoor stability of the film.

3.3.1. Polyaniline

We have studied the response of a 100 lm diameterPANI UME in the presence of Cl� as a function of pH.First, we have checked that in buffered solutions, theresponse was linear in the whole range studied (slope�60 mV per pH unit, see Fig. 7a). Then, the response ofa 50 lm diameter UME modified by polyaniline in the

rogressive electropolymerization between 0.00 and 1.00 V/SCE with7 C cm�2, (c), 5.0 · 10�7 C, q = 0.10 C cm�2; (d), 0.65 · 10�7 C, q =

0

0.04

0.08

0.12

0.16

0.2

0 200 400 600 800 1000Time (s)

E(V

) / S

CE

2.72.9

3.1 3.3 3.43.7

4.0

4.5

5.35.5

6.26.5

Fig. 6. Potentiometric response with time during the titration of2 · 10�3 M HCl by 2 · 10�2 M KOH. POMeANI/PAMPS modified PtUME (25 lm diameter). Progressive electropolymerization between 0.00and 1.00 V/SCE with [OMeANI] = 50 mM, [HCl] = 1 M. Successive pHare indicated above the curve.


presence of PAMPS (PANI/PAMPS, 1/4) under conditionswhere over-oxidation is avoided (same as for Fig. 1b) wasstudied during an acid/base titration. For that, a 10�3 MHCl solution was titrated by a 5 · 10�3 M KOH solutionfrom pH 3 to pH 9. As can be seen in Fig. 7b, the responseis linear on the whole pH range with a slope of �60 mV perpH unit (correlation coefficient: 0.998).

0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10

pH

E/V

/SC

E

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10pH

E/V

/SC

E

Fig. 7. PANI potentiometric response to pH. (a) PANI/Cl� modified PtUME (100 lm diameter). Electropolymerization between 0.00 and 1.00 V/SCE for 50 cycles; test in buffered solutions. (b) PANI/PAMPS modifiedPt UME (50 lm diameter). Electropolymerization between 0.00 and0.80 V/SCE for 75 cycles with [ANI] = 50 mM. [PAMPS] = 200 mM.Titration of 10�3 M HCl by 5 · 10�3 M KOH.

3.3.2. Poly-o-methylaniline

With PMeANI, it was only possible to test millimetricelectrodes since modified UMEs were not stable enoughto resist to any measurement. During the direct titration,the response is linear for 2 < pH < 5 with a slope of�40 mV/unit pH and also for 5 < pH < 10 with a slopeof �30 mV/unit pH. Since the slopes of both segmentsare not so different, the whole curve may be approximatedby a straight line with a slope of �35 mV per pH unit. Thesame behaviour is observed on the reverse titration and thewhole curve can be approximated by a straight line with aslope of �40 mV per pH unit.

3.3.3. Poly-o-methoxyaniline

A 25 lm diameter POMeANI UME prepared by pro-gressive electropolymerization and conditioned for 24 hin 0.1 M HCl presents a nearly linear behaviour in the 3–10 pH range with a slope of �50 mV per pH unit (correla-tion coefficient: 0.991). It may be noticed that the slope isslightly lower between pH 5 and 7. Moreover, some hyster-esis between the direct and reverse titrations is present (seeFig. 8a, respectively (¤) and (n)). When plotted as a func-tion of the added volume, pH variations are represented bythe classical curve for the titration of a strong acid by astrong base (see Fig. 8b). The potentiometric response ofa 25 lm diameter POMeANI UME electropolymerized in

-0.1

0

0.1

0.2

0.3

0 1 2 3 4

V (mL)

E (

V)

/ SC

E

-0.2

-0.1

0

0.1

0.2

0.3

0 2 4 8 10 12

pH

E (

V)

/ SC

E

6

Fig. 8. Titration of a 12.5 mL solution of 10�3 M HCl by 5 · 10�3 MKOH with a POMeANI/Cl� modified Pt UME (25 lm diameter).Progressive electropolymerization between 0.00 and 1.00 V/SCE with[OMeANI] = 50 mM, [HCl] = 1 M. (a) potentiometric response to pH and(b) titration curve. (r) direct, (j) reverse titrations.


the presence of PAMPS ([OMeANI]/[PAMPS], 1/4) is sim-ilar to that observed in the presence of Cl�.

3.4. Estimation of a pH variation in a nafion film

The objective of this work is to probe ion exchangesbetween a polyelectrolyte thin film and a solution with apH sensitive UME. From the preceding sections, it is quitedifficult to make reproducible UMEs usable several times.However, we have tested a PANI/PAMPS 50 lm diameterUME to probe the acidity of a 200 nm thin Nafion filmspin-coated onto a glass surface.

For that purpose, a two-electrode assembly, made of a50 lm diameter PANI/PAMPS UME and a 250 lm diam-eter Ag/AgCl wire is held by a motorized vertical stage. A10 lL droplet of an unbuffered pH 8 solution is depositedat the extremity of the electrodes assembly. The assemblyis positioned a few millimeters above the Nafion film. Then,the electrodes + droplet set is moved stiffly until the dropletcontacts the Nafion film. During the whole procedure, theopen circuit potential difference between the two electrodesis recorded as a function of time as presented by the D sym-bols in Fig. 9. At time t < 0 the droplet is only in contactwith the electrodes and the whole assembly moves towardthe Nafion film. At t = 0, the droplet contacts the Nafionfilm surface and the PANI UME potential increases toreach a stationary value, �100 mV higher than the valuebefore contact, after a transitory period of �100–150 s.

This potential variation cannot be ascribed to the pHdecrease due to CO2 dissolution into the droplet becausewhen a droplet of the same solution is contacted with aglass surface, the PANI UME potential does not changemuch within 100 s (Fig. 9, +). The potential variationobserved is significant, even though it is situated at theupper limit of pH use for a PANI electrode. It is thereforedifficult to precisely estimate the change of the droplet pHafter it has been put in contact with the acidic Nafion poly-

0.1

0.2

0.3

0.4

-50 50 150 250 350 450

t (s)

EP

AN

I - E

Ag/

AgC

l (V

)

-0.1

-0.05

0

0.05

ΔEgl

assµ

E (

V)

Fig. 9. Local pH variation resulting from H+ exchange in a Nafion� film.Starting pH 8. (D), (+), PANI/Cl� UME (50 lm diameter) obtained byelectropolymerization between 0.00 and 1.00 V/SCE for 50 cycles. (s),700 lm diameter pH glass microelectrode. (D), (s), over a Nafion� film.(+) Over a glass surface. At t = 0, the droplet of solution is contacted withthe polymer surface.

mer. However from the titration curve of the PANI UMEused (see Fig. 7b), the final pH value should range between6 and 6.5. This corresponds to the delivery of about3 · 10�12–10�11 M of H+ from the Nafion� film into thedroplet. Since the investigated Nafion film has a thicknessk = 200 nm (estimated by profilometry) and a concentra-tion of �SO�3 sites of C�SO3

¼ 1:7 10�3 mol cm�3 (estimatedfrom the Nafion density q = 1.9 and its monomer molarmass, M = 1100 g mol�1), the maximum amount of H+

deliverable by the Nafion� film along the surface exposedto the droplet (S = 9 mm2) is then the amount of �SO�3sites of the film potentially exposed with the droplet:n�SO3¼ C�SO3

Sk ¼ 3� 10�9 mol. Even though slightly alka-line, the droplet of solution put in contact with the acidicNafion� thin film allows the delivery of only 0.1–0.3% ofits H+ content. This low value is in agreement with thelow value of the ionic strength of the solution used (about10�5–10�6 M due to the addition of NaOH to impose theinitial pH value of 8), which does not drive efficiently, froma thermodynamic point of view, the process of ionexchange between the film and the solution.

This is corroborated by a comparative experiment per-formed with a commercial 700 lm diameter pH glassmicroelectrode (Fig. 9, �). In this experiment, the glassmicroelectrode is mounted vertically and held so that itssensing extremity is positioned approx. 300–400 lm abovea 90 nm thick Nafion� film. A 20 lL droplet of a pH 8solution is then deposited, at time t = 0, between the Naf-ion� film and the microelectrode; the geometry of the glassmicroelectrode would not allow the use of the former pro-cedure with such small solution volumes. The potential val-ues for t < 0 are meaningless as they correspond tomeasurements in the air. For t > 0, a behaviour similar tothat of the PANI UME is observed: a 100–200 s transientregime and the attainment of a steady value that corre-sponds to a final pH 7. In this comparative experiment,the amount of H+ delivered from the Nafion film areaexposed to the electrode (12 mm2) also corresponds to0.1% of the film content.

4. Conclusion

We have tried to develop conducting polymer based sys-tems on a micrometric scale in order to subsequently probelocal pH variations. For that, we have tested different poly-anilines, whose sensitivity to pH is well known. We haveperformed electropolymerizations in the presence of eithera molecular anion (Cl�) or a polyelectrolyte (poly(2-acry-lamido-2-methyl-1-propanesulphonic acid), PAMPS).

We have evidenced that for UME diameters P50 lm,poly-aniline and o-methoxyaniline films can be easilyobtained directly on an UME through the anodic electrop-olymerization of the monomer during successive potentialcycles. Progressive electropolymerization allowed to obtainstable films on smaller UMEs (25 lm diameter). It consistsof (i) starting the electropolymerization cycles with an ano-dic limit at the foot of the monomer oxidation peak and (ii)


increasing gently the anodic limit with the number ofcycles.

For o-methylaniline, we were not able to obtain stablefilms on 25 lm diameter UMEs even by progressiveelectropolymerization.

The potentiometric response depends on the nature ofthe anion incorporated during the electropolymerization.At best, a linear dependence on pH is observed for3 < pH < 9–10, with a slope of �60 or 50 mV per pH unitfor polyaniline and poly(o-methoxyaniline) UMEs, respec-tively. Such UMEs can be used in acid-base titrations.

Finally, we have tested a 50 lm diameter polyanilinemodified UME to probe a pH variation resulting fromion exchange in a thin Nafion� film. The result is in agree-ment with that obtained with a commercial 700 lm diame-ter glass microelectrode. The advantage of our system is itssmall size, which allows measurements in smaller volumes.Such preliminary experiments need further investigation.

Acknowledgement

Sandra Nunige is gratefully acknowledged for SEMmeasurements.

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Polyaniline films based ultramicroelectrodes sensitive to pH

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