Making use of ion fluxes through potentiometric sensor membranes: ISEs with step responses at critical ion activities

Ion-selective membrane electrodes (ISEs) were designed for which the ideal response to primary ions is switched on at a given critical sample activity. Only above this value, the sensors exhibit the conventional selectivity behavior dictated by the membrane components (ionophore, solvent), whereas, below the critical activity, they turn out to be completely non-specific and yield apparent selectivity factors of about unity. ISEs of the novel type, so-called ‘switchtrodes’, were realized on the basis of Cu2+-selective ionophore membranes. The response curves of the sensors fully conformed to the theoretical expectations, which opens interesting applications of such devices for environmental monitoring.