Applications of voltammetric ion selective electrodes to complex matrices

The practical application of two different voltammetric ion selective electrodes (VISE) to measure ion activity in complex solutions has been explored. 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) microcrystals adhered to an electrode surface act as a low selectivity voltammetric ion sensor. Resistance drop effects and pH artifacts were minimised by the addition of an innocent supporting electrolyte (buffer) to the analyte solution. In this format, addition of an ionophore to improve selectivity resulted in a reduction in current magnitude, due to competition for the ion. In contrast, voltammetry of a thin film containing a redox active species, electrolyte, ionophore and membrane solvent provides a highly selective ion sensor. Choice of ionophore was shown to affect the upper concentration detection limit. Use of ionic liquids as a combined membrane solvent and electrolyte was demonstrated. Methods to attach both VISE types to low-cost screen-printed electrodes have been explored. Various potential referencing techniques were also investigated. Both the microcrystal and thin film VISEs could be used to determine ion activity in complex solutions, as demonstrated in seawater, beverages, plasma and whole blood. Dissolved oxygen does not need to be removed, as it does not affect the response. However calibration methods are important for sensor accuracy and issues relating to electrode fouling must be addressed.