Designing polypyrrole-based sensors: selective electrochemical cation in aza crown ethers

SYQTMTII[: I IITnLS ELSEVIER

Synthetic Metals 67 (1994) 251-254

Designing polypyrrole-based sensors: selective electrochemical cation in aza crown ethers H. Korri Youssoufi, A. Yassar, S. Baiteche, M. Hmyene, F. Garnier Laboratoire des Mat~riaux Mol~culaires, CNRS, 2 rue Henri Dunant, 94320 Thiais, Franc(,

Abstract

The synthesis of new functionalized conducting polypyrroles bearing aza crown ethers of variable cage size at the 3-carbon atom of pyrrole is described. The electrochemical selectivity of these electropolymerized polymers towards cations has been analyzed in organic media using the alkaline series Li +, Na ÷ and K +. A selectivity towards Na + and K + is clearly evidenced and discussed in terms of cage size. This result brings an interesting example of transduction of a chemical information into an electrical signal. Keywords: Polypyrrole; Sensors; Crown ethers

1. Introduction

The electrochemical recognition of charged guest species by redox ionophores is an area of intense current interest with relevance to the development of chemical sensor technology [1]. The conjugated polymers are ideally suited for the design of such sensory materials since, on the one hand, the monomers can be modified by substitution with different groups and, on the other hand, the polymers exhibit high conductivity and electroactivity [2]. Functionalized conducting polymers constitute materials, which specifically interact with external physical or chemical quantities, depending on the nature of the functionalized group [3]. Furthermore, ion-selective crown ethers are a well-established example of molecular sensory components [4]. For this purpose aza crown ether of variable cage size have been grafted at the 3-carbon atom of pyrrole. We report here on the electrosynthesis and electrochemical properties of these polymers. The characteristics of sensitivity and selectivity towards alkaline cations are discussed.

2. Experimental

3-Aza crown ether pyrrole can be synthesized by a condensation of 3-pyrryl acetic acid with aza crown ether, which has been already described [5]. These monomers are electropolymerized in a one-compartment cell, onto a platinum electrode in acetonitrile,

0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 2 5 5 - W

0.5 M Bu4NC104 and 0.1 M monomer, deoxygenated by argon bubbling. The electropolymerization was carried out at a constant potential of 0.9 V(SCE), with a polymerization charge of 0.5 C/cm 2. The black shiny film grown on the electrode was rinsed and dried, and characterized by cyclic voltammetry in solution free of monomer.

3. Results and discussion

We have previously reported on the synthesis of pyrrole substituted by aza crown ether, and discussed the need to introduce an acetyl group between the pyrrole ring and the aza crown ether for preserving the conducting and electroactivc properties of the polypyrrole chains [5]. The aza crown ether-substituted pyrroles have been electropolymerized under the same conditions as for unsubstituted polypyrrole. The polymers, electrochemically characterized in an organic solution of 0.1 M Bu4NC104 in acetonitrile, show reversible cyclic voltammograms (Fig. 1) with symmetrical redox peaks at an anodic potential of 0.03 and 0.1 V(SCE) for poly(1) and poly(2), respectively. The electrochemical properties of these polymers are similar to those of unsubstituted polypyrrole, and show high electroactivity and reversibility.

~ Korri Youssoufi et aL / Synthetic Metals 67 (1994) 251-254

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for polypyrrole substituted with aza 18-crown-6 (poly(2)) significant differences in the cyclic voltammograms are observed depending on the cations. In a first step, the polymers were synthesized and analysed with the same electrolyte. When the electropolymerization is carried out with Li + instead of Bu4N +, no modification of the electrochemical properties are observed. When a film of poly(2) is realised in the presence of Na ÷ or K + ions, and analysed with the same ions, a distinct change of the original voltammetric response is observed (Fig. 2(a)). The curves show that the current peak is shifted towards positive potentials and that the charge reversibly exchanged is lowered, which indicates that the polymers become more difficult to oxidize. On the other hand, when a film of poly(2) is grown in the presence of Bu4N + and analysed in a solution containing Na + or K + instead of Li + or NBu4 +, the anodic wave gradually shifts to higher potentials and stabilizes at 0.5 and 0.54 V(SCE) for Na + and K +, respectively. The effect of concentration of the electrolyte has also been examined. Thus, films of poly(2) have been analysed in acetonitrile

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Aza crown ethers are known to bind different alkaline cations as a function of the ionic diameter in organic media [6]. To analyse the sensitivity of these polymers toward cations, the voltammetric responses in solutions containing Bu4N +, Li +, Na + and K + were investigated. In the case of polypyrrole substituted with aza 12crown-4, no modification of the electrochemical response of the polymers is observed with regard to the nature of the cations present in the solution. However,

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Fig. 2. (a) Voltammograms of films of poly(2) synthesized and analysed in CH3CN with the same electrolyte: (1) 0.1 M LiC104, (2) 0.1 M NaCIO4. Potential sweep rate, 20 mV/s. (b) Voltammograms of films of poly(2) synthesized in acetonitrile+0.5 M LiCIO4 and analysed in acetonitrile+0.1 M NaCIO4 (successive scans 1 to 5).

11. Korri Youssoufi et al. / Synthetic Metals 67 (1994) 251-254

containing 0.1 M Bu4NC104and various concentrations of NaC104. The cyclic voltammograms of poly(2) change with the concentration of Na ÷ in the solution (Fig. 3). The anodic peak potential is gradually shifted to higher positive values. Very low Na + concentrations of about 10 5 M affect the electrochemical response of poly(2), leading to an increase of the oxidation potential of 70 inV. The variation of peak potential with [Na+] concentration increases from 10 -5 to 10 -2 M (Fig. 4), and at higher concentrations becomes constant. The same behaviour has also been obtained with variable concentrations of [K+].

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Fig. 3. Voltammograms of films of poly(2) synthesized in CH3CN + 0.5 M Bu4NCIO4 and analysed in C H 3 C N + 0 . I M Bu4NCIO4 in the presence of increasing NaCIO4 concentration: - - , 0 M; - - - , 10 -5 M; - - - , 10 -3 M; . . . . . , 10 -2 M. Sweep rate, 20 mV/s.

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253

These results show that the electrochemical properties of poly(2) are highly sensitive to Na ÷ or K ÷, in contrast to Li÷, thus bringing clear evidence that functionalized polypyrrole shows selective voltammetric response towards alkaline cations. The effectiveness of this complexation process is dependent on the size of both the ions and the macrocyclic cavities. Aza 12-crown-4 forms a cavity of 1 ~ diameter, which is much smaller than that of Na ÷ and K ÷ ions, 1.94 and 2.66 ~, respectively. The sensitivity towards Na+ and K ÷ ions observed in poly(2) agrees with the ca~e size of aza 18-crown-6, which has a diameter of 3.2 A. The absence of sensitivity for Li ÷ ions both by poly(l) and poly(2) can be explained by the large difference between the cations and the cage size, and also by the fact that lithium is a small hard cation, more tightly solvated in acetonitrile than Na ÷ and K +. The binding of cations by aza crown ether, inducing a greater difficulty to oxidize the conjugated polymers, can be explained by the change of the conformational properties of the polymers. The metal complexation will induce a stiffening of the polymer chain, leading to a deviation from coplanarity of the conjugated chain and, thus, to an increase of the redox potential. Such electrochemical molecular recognition, associated with specifically functionalized conducting polymers, has already been described in the literature. Chiral conducting polymers have shown a differentiation toward the enantiomeric forms of a chiral electrolyte during the electrochemical doping process of these films [7,8]. Crown ether-functionalized polythiophenes have also shown a cationic dependence of their voltammetric waves [9].

4. Conclusions

The characterization of new conducting polypyrrole functionalized with aza crown ether has been described for the first time. The high electroactivity of these materials shows that the electrochemical properties of the substituted polypyrrole are not perturbed by the presence of a bulky substituent. Their analysis in the presence of alkaline metal ions shows that the electrochemical response of poly(2) is selective and sensitive to low concentrations of Na + and K ÷. The cation recognition is controlled by the relative sizes of both the aza crown substituent and of the ion. The combination of the complexing properties of aza crown ether and of the electroactive properties of polypyrrole chains leads to an observed transfer of the cationbinding information into the conducting polymer chain. This result can be considered as the transduction of a chemical information into an electrical signal. These

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H. Korri Youssoufi et al. / Synthetic Metals 67 (1994) 251-254

results appear very promising for realizing new materials for electrochemical metal cation sensors.

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[3] F. Gamier, Angew. Chem. Adv. Mater., 101 (1989) 117. [4] E. Weber (ed.), Supramolecular Chemistry I - Directed Synthesis and Molecular Recognition, Springer, New York, 1993. [5] H. Korri Youssoufi, M. Hmyene, F. Garnier and D. Delabouglise, J. Chem. Soc., Chem. Commun., (1993) 1550. [6] H.K. Frensdorff, J. Am. Chem. Soc., 93 (1971) 600. [7] M. Lemaire, D. Delabouglise, R. Garreau, A. Guy and J. Roncali, J. Chem. Soc., Chem. Commun., (1988) 658. [8] J.C. Moutet, E. Saint-Aman, F. Tran-Van, P. Angibeaud and J.P. UtiUe, Adv. Mater., 4 (1992) 511. [9] P. B/iuerle and S. Scheib, Adv. Mater., 5 (1993) 848.