Selectivity comparison of neutral carrier-based ion-selective optical and potentiometric sensing schemes

The selectivities of analogous carrier-based potentiometric and optical sensors are directly compared. This is done on the basis of recent developments in both fields that allow for a thermodynamically founded description of selectivity. Extended equations are shown that allow for the interconversion of selectivity values obtained with different definitions. The response of optodes toward different ions should generally be reported with the overall ion-exchange constants, Kexch, together with the assumed stoichiometry of the formed complexes in the optical film. In addition, the selectivities for both sensing schemes can principally be reported with two sets of selectivity coefficients, KIJpot and kIJPsel for ion-selective electrodes and KIJopt and kIJOsel for optodes. While KIJpot and KIJopt values refer to the Nicolskii and Nicolskii-like selectivity coefficients, kIJPsel and kIJOsel values are obtained graphically as the horizontal distance of the separately measured calibration curves on a logarithmic activity scale. With ion-selective electrodes, the reporting of Nicolskii coefficient is generally preferred since such data are sample independent for Nernstian response slopes to all measured ions. In contrast, ion optode selectivities are preferably reported as kIJOsel values since the continuous concentration changes within the optical film during measurement make Nicolskii-like selectivity coefficients generally sample-dependent. Nonetheless, the reporting of Nicolskii-like selectivities is recommended to compare optode and ISE selectivities. Care must be taken to choose equivalent equilibrium concentration ratios of complex and free ionophore in the sensing film for such comparative experiments. The clear distinction in symbols and formalisms allows one to avoid confusion in future reports on carrier-based ion sensors. Possible reasons why experimental selectivities may still deviate significantly from each other are discussed, as well as why one type of sensor may in some cases be a superior choice over the other.