Tunable Optical Sensing with PVC Membrane Based Ion-Selective Bipolar Electrodes.

We show here that the response of ion-selective membrane electrodes (ISEs) based on traditional PVC membranes can be directly translated into a colorimetric readout by a closed bipolar electrode (BPE) arrangement. Because the resulting optical response is based on the turnover of the redox probe, ferroin, dissolved in a thin layer compartment, it directly indicates the potential change at the ISE in combination with a reference electrode. This class of probes measures ion activity, analogous to their ISE counterparts. Unlike other ion optodes, the response if also fully tunable over a wide concentration range by the application of an external potential and occurs in a compartment that is physically separate from the sample. To allow for the electrical charge to pass across the ion-selective electrodes, the membranes are doped with inert lipophilic electrolyte, ETH 500, but are otherwise of established composition. The observed response behavior corre-lates well to theory. A wide range of ion-selective membranes are confirmed to work with this readout principle, demon-strating the detection of potassium, sodium, calcium and carbonate ions. The corresponding sigmodal calibration curve is used for the quantitative analysis in a range of samples including commercial beverages and river and lake samples. The data are successfully correlated with atomic emission spectroscopy and direct potentiometry.