Detection of electroinactive ions using conducting polymer microelectrodes

Abstract

The detection of electroinactive ions using conducting polymers has been described previously. In this work. we shon-that the analyrical performance of this method is greatly improved using microelectrodes. The use of microelectrodes overcomes the problems associated with the implementation of this detection scheme in lorn-conductivig-eluents. KEY WORDS: Conducting polymers, Electroinactive ions, Microelectrodes LWRODUCTIONThe use of polymer sensors has attracted considerable attention in recent years [l-31. Of the conducting polymers considered, polypyrrole ( P e ) has attracted the most attention because of the stability and flexibili? available with this system. This latter feature enables the production of electrode surfaces capable of diverse molecular interactions and, hence, a range of sensing operations. The chemical properties of P4; are determined bl-the counterion (A-) incorporated during synthesis according to the following:For example, A-may be a simple anion such as chloride in which case the polymer may be used as a sensor to incorporate appropriate anal>ies via ion exchange [4,5]. However, at the other end of the molecular spectrum, A-may be a complicated biomolecule such as an enzyme [6,7] o r an antibody [8,9] capable of extremely specific molecular recognition.These materials can then be used for sensing with electrical signals based on current flow [ l ] as a result of analge oxidation/reduction, changes in resistance [ 10,11], or capacitance [9] utilized. An additional unique signal generation mechanism exists with conducting polymers such as PPy [12,13]. This involves the generation of a signal owing to oxidation/reduction of the polymer in the presence of the analyte of interest. This mechanism is most appropriately used with flow-injection analysis (FIA) and an eluent whose ions are not readily incorporated during polymer oxidation/reduction. The oxidation/reduction of the polymer is then dependent on the presence of more easily incorporated ions (the analytes) and the degree of osidation/reduction is dependent on the concentration of these species. When used with FIA and pulsed amperometric detection, this gives rise to a unique and sensitive sensing technology. The use of this scheme is appreciated when one realizes that the ion affinity of the conducting polymer for particular anions or cations is readily modified by adjusting the polymer counterion incorporated during synthesis [ 141.This is a usehl mechanism for electrochemical detection of electroinactive ions. However, a quandary exists. The use of carrier eluents with anions that are not readily incorporated inevitably leads to the use of eluenrs containing larger ionic species and/or electrolytes of lower concentration. Such solutions have low conductivity. In a conventional electrochemical cell, this leads to increased iR drop and a loss of control over the applied potential. Consequently, the analytical signal is distorted.The use of microelectrodes, electrodes <20 pm in at least one dimension, in eluents with l...

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