Detection of Marginally Discriminated Ions with Differential Solid Contact Ion-Selective Electrodes

A differential sensor is reported for decreasing the interference from multiple ionic and non-ionic species in droplet-sized samples, hence lowering the detection limit for ions that are particularly hard to measure by conventional ion-selective potentiometry. We believe that the proposed approach represents significant progress beyond state-of-the-art by making the application of ion-selective membranes possible for ions that do not have selective ionophores. Two solid contact ion-selective electrodes (SCISEs) are utilized, which are prepared similarly, except for adding ionophore to one of the membranes. We show experimentally and theoretically that a significant signal is observed only when the analyte ion is present in the sample solution. The dynamic response of the sensor is investigated theoretically by Nernst-Planck-Poisson finite element simulations. Symbolic regression is used to generate human-readable expressions. Differential SCISEs are tested under potentiometric and amperometric conditions. While a large potential response is seen in the conventional SCISE vs reference electrode setup at high concentrations of interfering ions, the signal of interfering species is eliminated in the differential SCISE setup. Consequently, it is possible to measure a few μ M levels of ammonium in droplets without the interference of potassium using an ionophore that is not especially selective for ammonium over potassium.