PEDOT films: multifunctional membranes for electrochemical ion sensing

Properties of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied from the point of view of direct use as ion-sensing membranes in potentiometric or amperometric sensors. Stable and reproducible potentiometric characteristics were obtained for PEDOT doped by poly(4-styrenesulfonate) ions, PEDOT(PSS) (cationic characteristics), and PEDOT doped by hexacyanoferrate(II) anions, PEDOT(HCF) (anionic characteristics). As shown by voltammetric and EDAX results, the anion exchange properties of the latter polymer result from gradual replacement of HCF ions by Cl- anions from solution. The zero-current potentiometric detection limit of PEDOT(PSS), equal to 3x10(-6) M, can be shifted to 7x10(-7) M by polarization using a cathodic current density of 3x10(-7) A cm(-2). PEDOT films doped by Cl- or PSS- ions can be used as membranes for sensing anions or cations, respectively, under pulse amperometric conditions, within the range from 10(-4) to 1 M, comparable with that accessible by zero-current potentiometry. Dissolved oxygen (redox interferent of low charge transfer rate) exerts a minor influence on the slope of the potentiometric and amperometric characteristics of PEDOT films. Although the presence of redox reactants characterized by a high rate of charge transfer [Fe(CN)(6)(3-/4-)] results in the disappearance of the potential dependence on KCl concentration, this disadvantageous effect is much less significant under pulse amperometric conditions.