Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes.

We present here a general strategy to translate potential change at a potentiometric probe into a tunable color readout. It is achieved with a closed bipolar electrode where the ion-selective component is confined to one end of the electrode while color is generated at the opposite pole, allowing one to physically separate the detection compartment from the sample. An electrical potential is imposed across the bipolar electrode by solution contact such that the potentiometric signal change at the sample side modulates the potential at the detection side. This triggers the turnover of a redox indicator in the thin detection layer until a new equilibrium state is established. The approach is demonstrated in separate experiments with a chloride responsive Ag/AgCl element and a liquid membrane based calcium-selective membrane electrode, using the redox indicator ferroin in the detection compartment. The principle can be readily extended to other ion detection materials and optical readout principles.