Lowering the Detection Limit of Ion-Selective Plastic Membrane Electrodes with Conducting Polymer Solid Contact and Conducting Polymer Potentiometric Sensors

The detection limit of conducting polymer (CP) poly(pyrrole)-based potentiometric chloride-sensitive electrodes was lowered over 3 orders of magnitude, applying anodic current of density in the range microamperes per centimeter squared. This effect was attributed to compensation of doping chloride ion leakage from the membrane into adjacent solution layer, caused by self-discharge of CP. This method was successfully applied to lower the detection limit of all-solid-state chloride-selective potentiometric sensors containing plastic, poly(vinyl chloride) chloride ion-selective membrane, and conducting poly(pyrrole) film applied as ion-to-electron transducer. The control of the leakage of chloride ions from the CP transducer phase to the plastic membrane resulted in linear response of ion-selective electrode shifted down to lower activities. A 2 orders of magnitude lowered detection limit, equal to 4 × 10-7 M, was achieved for current densities corresponding to extended linear range of CP film electrode, i.e., in the range of microamperes per centimeter squared. Experiments with all-solid-state potentiometric sensors were carried out using open-sandwich arrangement (two poly(pyrrole) electrodes coated by the same ionexchanging membrane): one part of the sensor was working under open circuit conditions, while the second electrode was polarized using anodic current. Due to arrangement used, the lowered detection limit observed for nonpolarized electrode is attributed to modification of ion fluxes in the membrane; it is interfered by neither ohmic drop nor anion depletion layer formation due to anodic current flow.