Stripping voltammetry of nanomolar potassium and ammonium ions using a valinomycin-doped double-polymer electrode

Here, we report on the first application of an ionophore-doped double-polymer electrode for ion-transfer stripping voltammetry (ITSV) to explore the nanomolar limit of detection (LOD) and multiple-ion detectability. We developed a theoretical model for ITSV at a thin ionophore-doped membrane on the solid supporting electrode to demonstrate that its LOD is controlled by the equilibrium preconcentration of an aqueous analyte ion as an ionophore complex into the thin polymer membrane and is lowered by the formation of a more stable ion-ionophore complex. The theoretical predictions were confirmed using valinomycin as a K(+)-selective ionophore, which forms a ∼60 times more stable complex with K(+) than with NH(4)(+), as confirmed by cyclic voltammetry. A LOD of 0.6 nM K(+) was achieved by ITSV using commercial ultrapure water as a K(+)-free media, where NH(4)(+) contamination at a higher concentration was also detected by ITSV. The dependence of the ITSV response on the preconcentration time was monitored under the rotating-electrode configuration and analyzed theoretically to directly determine ∼100 nM NH(4)(+) and ∼5 nM K(+) contaminations in commercial ultrapure water and laboratory-purified water, respectively, without the background ITSV measurement of an analyte-free blank solution.