Electrochemical preparation of semipermeable polymer membranes on carbon fiber microelectrodes for selective amperometric detection of cations

An electrochemical procedure is presented for in situ preparation and subsequent deposition of semipermeable membranes on ultramicroelectrodes. The membranes are based on a matrix of poly(oxyphenylene) bearing carboxyl and sulfonic groups, i.e. poly(1,2-oxyphenylene-4-sulfonic acid) or poly(1,2-oxyphenylene-3-carboxylic acid). These membranes exhibit a cation exchange behavior whereas the transport of anions is inhibited. The diffusion coefficients of Ru(NH3)6·3+ within the semipermeable membranes could be estimated by means of chronoamperometric and steady-state measurements. The values obtained are in the range of 1.0–6.9 · 10−7 cm2/s. The permeability of the cation through the membranes is high. Therefore, no distortion of the voltammetric response due to the attached membrane is observed. It is shown that the transport rate can be modulated by copolymerization of the functionalized phenolic monomer with varying amounts of a cross-linking agent. The ultramicroelectrodes modified with the above membranes can be used as amperometric sensors displaying a linear current/concentration relationship.