Optimization of capacitance of conducting polymer solid contact in ion-selective electrodes

All-solid-state ion-selective electrodes with conducting polymer solid contact represent good analytical parameters, comparable with those of classical sensors with internal filling solution. One of parameters characterizing quality of the solid contact, related to ion-to-electron transduction, is the electrical capacitance. However, in many cases this capacitance is lower than for the solid contact only, in the absence of ion-selective membrane. This effect can be disadvantageous, particularly for sensors working under polarization conditions, in galvanostatic mode.The capacitance reduction effect was studied on example of model systems of anion- and cation-selective electrodes with poly(3,4-ethylenedioxythiophene) solid contact with anion- or cation-exchange properties. Basing on results obtained for these membranes and contacts as well as some model calculations, the reasons of reduced capacitance were ascribed to low amount of ions transferrable across the solid contact/membrane interface. This effect can result from low concentration of mobile ions in the conducting polymer contact or concentration polarization effects. Procedures or pretreatment methods were proposed to minimize the effect of capacitance decrease.

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