Capacitive Model for Coulometric Readout of Ion-Selective Electrodes

We present here a capacitive model for the coulometric signal transduction readout of solid-contact ion-selective membrane electrodes (SC-ISE) with a conducting polymer (CP) as an intermediate layer for the detection of anions. The capacitive model correlates well with experimental data obtained for chloride-selective SC-ISEs utilizing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with chloride as the ion-to-electron transducer. Additionally, Prussian blue is used as a simple sodium capacitor to further demonstrate the role of the transduction layer. The influence of different thicknesses of PEDOT as a conducting polymer transducer, different thicknesses of the overlaying ion-selective membranes deposited by drop casting and spin coating, and different compositions of the chloride-selective membrane are explored. The responses are evaluated in terms of current-time, charge-time, and charge-chloride activity relationships. The utility of the sensor with coulometric readout is illustrated by the monitoring of very small concentration changes in solution.