Composite films of poly-(ester-sulphonated) and poly-(3-methylthiophene) for ion-exchange voltammetry in acetonitrile solutions
Introduction
The possibility to introduce electrochemical control in the uptake and release of electroactive cations in polymer-coated electrodes is a very attractive property which has been examined in numerous studies [1]. To this aim, two main approaches were followed: one based on the use of electroactive films with electrochemically controllable ion-exchange properties such as redox polymers [2], [3], [4], [5] or conducting polymers [6], [7], the other taking advantage of the special properties of composite films in which the (switchable) anion-exchange properties of oxidized conducting polymers were compensated and combined with the cation-exchange properties of polyelectrolytes [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Very recently, these studies brought to the proposal of using electrochemically controlled films for performing and controlling electrochemically the solid phase micro extraction (SPME) of anions or cations [9], [15], [18], [19].
However, all these studies were devoted in examining the electrochemical switching of the uptake-release of trace analytes in water solutions, but few, if none, studies in this direction were devoted to applications in non-aqueous media. This appears quite a relevant lack in view of the fact that many trace environmental pollutants are characterized by poor water solubility and water is not the only relevant matrix where these pollutants can be present. Moreover, the preparation of ion-exchange coatings whose properties can be controlled electrochemically in organic solvents appears relevant not only for SPME aims but also for their possible application as electrochemically modulated stationary phases in liquid chromatography [20], [21], [22], [23], where organic solvents, in general, and acetonitrile, in particular, are very often used as mobile phases.
Until now, developments in this direction were limited by the impossibility to use the most applied ion-exchange film coating, namely Nafion®, in non-aqueous solutions. A valid alternative to Nafion is offered by the poly-(ester-sulphonated) Eastman AQ55® (AQ55), which is stable and presents good cation-exchange properties in acetonitrile solutions [24], [25], [26], [27].
In order to develop a layer suitable as electrode coating for electrochemically controlled ion-exchange in non-aqueous solvent, in the present work we report on the preparation and characterization of an electrode coating obtained by electropolymerisation of 3-methylthiophene (MeT) on AQ55, with the aim of combining the good cation exchange properties of the latter with the redox switchability of the former [9], [28], [29], [30], [31], [32].
Quantitative data on the ion-exchange properties and electrochemical switching of the film in acetonitrile solutions are obtained studying the uptake and release of the cationic complex tris(2,2′bipyridil) osmium(II) (Os(bpy)32+), used as reversible redox probe of well known behaviour [26]. By this way, it was possible to gain fundamental information on the properties of the mixed film in acetonitrile solutions as a necessary basis for future possible application for the uptake-release of redox analytes of environmental interest.
Section snippets
Chemicals
All chemicals used were of analytical grade quality. Reagent grade acetonitrile was purified further by distillation from phosphorus pentoxide and stored under nitrogen atmosphere. The supporting electrolytes tetrabutylammonium tetrafluoroborate (TBATFB) and tetrabutylammonium hexafluorophosphate (TBAHFP) (both from Fluka, puriss.) were dried overnight in a vacuum oven at 50 °C before use. The Eastman AQ55® polymer (supplied as irregular shaped pellets) was kindly provided as a gift by Eastman
Electropolymerization of 3-methylthiophene
The electrochemical polymerization of MeT has been studied in detail [28], [29], [30], [31], [32], however, some points of interest for the preparation of the mixed film are revisited here. Fig. 1 shows the growth of the PMeT film on the Pt–quartz electrode, studied by cyclic voltammetry and concomitant EQCM measurements. In agreement with the literature [17], the oxidation branch of the first scan is flat until an uprise of the current is observed at E > 1.5 V. The monomer is oxidized indeed at
Conclusions
The AQ55/PMeT composite is stable in acetonitrile solutions, displaying ion-exchange properties which can be modulated electrochemically, but only in the portion of the AQ55/PMeT mixture where an intimate contact between ionomer and conductive polymer is possible. This fraction increases when operating in the absence of supporting electrolyte added to the acetonitrile solution.
Really, uptake and entrapment of redox cations are ruled mainly by the oxidation state of the PMeT layer: when neutral,
Acknowledgements
This work was supported by MIUR(Rome). We thank Eastman Italia for the AQ55 sample. The authors wish to thank an anonymous reviewer whose suggestions were helpful for gaining insight in the present work.
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Present address: CIVEN Coordinamento Interuniversitario Veneto Nanotecnologie, Via delle Industrie 17/a, Marghera, Venice, Italy.