Subnanomolar ion detection by stripping voltammetry with solid-supported thin polymeric membrane

Subnanomolar limits of detection (LODs) are obtained for stripping voltammetry based on ion transfer at the interface between the aqueous sample and the thin polymeric membrane supported with a solid electrode. It has been predicted theoretically that a lower LOD can be obtained for a more lipophilic analyte ion, which can be preconcentrated at a higher equilibrium concentration in the solid-supported thin polymeric membrane to enhance a stripping current response. This study is the first to experimentally confirm the general theoretical prediction for both cationic and anionic analytes. Proof-of-concept experiments demonstrate that a subnanomolar LOD of (8 ± 4) × 10^-11 M tetrapropylammonium is significantly lower than a LOD of less lipophilic tetraethylammonium. Importantly, stripping voltammetry of the cationic analytes is enabled by newly introducing an oxidatively doped poly(3,4-ethylenedioxythiophene) film as the intermediate layer between a plasticized poly(vinyl chloride) membrane and a Au electrode. On the other hand, an undoped poly(3-octylthiophene) film is used as an intermediate layer for voltammetric detection of a lipophilic inorganic anion, hexafluoroarsenate, an arsenical biocide found recently in wastewater. A LOD of (9 ± 2) × 10^-11 M hexafluoroarsenate thus obtained by ion-transfer stripping voltammetry is comparable to a LOD of 80 pM by inductively coupled plasma mass spectrometry with anion-exchange chromatography. Great sensitivity for a lipophilic ion is potentially useful for environmental analysis because high lipophilicity of an ion is relevant to its bioaccumulation and toxicity.