Advances in the Conceptual Understanding of Membrane Electrodes

Our conceptual understanding of polymeric membrane electrodes has evolved significantly over the years and this new knowledge has helped to open exciting novel applications. At the beginning stood ground-breaking work that demonstrated the usefulness of organic membranes doped with electrically neutral receptors (ionophores) for selectively detecting ionic species in complex samples such as whole blood. But it turned out to be very difficult to understand how these systems function, and a kinetic steady-state model based on electrical migration principles was required to rationalize the findings. Years later, new experimental evidence helped to establish a simplified equilibrium theory to relate the characteristics of such polymeric membrane electrodes to extraction principles. Still, diffusional mass transport at zero current was later shown to be important to understand deviations from ideality, especially for the determination of selectivity and the understanding and improvement of the lower detection limit. Further advances and a plethora of new applications were later made possible by using dynamic electrochemistry principles to impose and control transmembrane ion fluxes, departing from a zero current protocol. Additionally, if one may tolerate transient currents of limited amplitude, self-powered sensing systems with optical readout may be realized that otherwise still work like potentiometric sensing probes. The use of solid-contact electrodes with membranes of very limited thickness may be probed with dynamic electrochemistry, achieving an electrochemical equilibration step at each applied potential during a linear sweep. Such systems provide a rich tool to assess ion extraction and interaction processes in polymeric membranes.