Ion selectivity obtained under voltammetric conditions when a TCNQ chemically modified electrode is presented with aqueous solutions containing tetraalkylammonium cations

The voltammetry of 7,7,8,8-tetracyanoquinodimethane (TCNQ) at an electrode-microparticle-aqueous(electrolye) interface has been proposed as a cation sensor on the basis that changes in electrolyte cation (analyte) concentrations result in reproducible shifts in the TCNQ(0/-) reversible potential. In order to probe the ion selective nature of the TCNQ sensor, the voltammetric response towards a series of tetraalkylammonium cations of variable size and hydrophobicity were studied. Both the thermodynamics (reversible potential) and kinetics (voltammetric peak separation) of the TCNQ(0/-) system were strongly dependant on the identity of the R4N+ cation. The reversible potential responded in a Nernstian manner to changes in cation concentration. When presented with mixed-analyte solutions, the TCNQ system exhibited Nicolsky type (or competitive) form of selectivity. However, the selectivity coefficients found in the present study were far greater than previously reported with group I cations. The order obtained for the tetraalkylammonium series indicates that ion selectivity is predominantly based on analyte solvation thermodyanics rather than a specific ionophore mechanism.