An Analogy of an Ion-Selective Electrode Sensor Based on the Voltammetry of Microcrystals of Tetracyanoquinodimethane or Tetrathiafulvalene Adhered to an Electrode Surface

The voltammetry of solid 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) at an electrode−microparticle−aqueous (electrolyte) interface generates characteristic current−potential profiles associated with solid−solid-phase transformations. During the reactions, electrolyte ions are included into the TCNQ (cations) and TTF (anions) lattice sites as part of the charge neutralization process. Consequently, electrolyte ion concentration is associated with the reversible potential of the TCNQ0/- and TTF0/+ reactions, making these processes candidates for the development of novel voltammetric cation and anion sensors, respectively. Electrode potential−analyte ion concentration dependence studies exhibited highly reproducible potential shifts of 45 (±1) mV/decade change in ion analyte concentration for both the TCNQ cation sensor and the TTF anion sensor. When presented with mixed-analyte solutions, both ion-sensing systems exhibited a degree of ion selectivity. Ion selectivity trends may be modeled using equations based on a Nicolsky-type selectivity relationship, in accordance with the concept that these are the voltammetric analogies of potentiometric ion-selective membrane electrodes.