Ion sensing based on receptor-mediated control of phase boundary potentials at liquid/liquid interfaces

The potentiometric responses of ion-selective electrodes (ISEs) based on electrically charged receptors and hydrogen-bonding neutral receptors were investigated experimentally and theoretically on the basis of the phase boundary potentials at liquid/liquid interfaces. Neutral derivatives of nucleobases were synthesized and applied to ISEs for nucleotides. Nernstian slopes obtained with these electrodes made it possible to interpret the potentiometric selectivities on the basis of the phase boundary equilibria, giving design strategies for hydrogen-bonding receptors that are suitable fbr ISE application. Two types of non-Nernstian responses, induced by charged receptors, were explained by taking account of the distribution of two different ions at the phase boundary. Apparently "twice-Nernstian" responses to dications, whose slopes are two-times larger than Nernstian slopes for dications, were shown to be the only examples of super-Nernstian responses that can be explained by the phase boundary equilibria. The anionic interference for ISEs based on charged receptors and neutral receptors was shown to occur at similar anion concentrations if the charged and neutral receptors form complexes of equal stabilities and stoichiometries. While the use of neutral receptors has been more common than that of charged ones, these results show the high potentiality of charged receptor-based ISEs.