Potentiometric Ion-, Gas-, and Bio-Selective Membrane Electrodes

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Abstract
Membrane electrodes are relatively simple electrochemical devices that can be used for the direct measurement of ions, gases, and biomolecules in complex samples. Selectivity for one species over another is determined by the nature and chemical composition of the membranes and associated reaction layers used to fabricate the devices. All membrane electrode probes employ at least one ion-selective membrane as the ultimate transduction element. This indicator membrane serves as an additional component of a classic two-electrode galvanic cell. The potential developed at the membrane/sample interface is directly or indirectly related to the activity or concentration of analyte in the sample. Because cell potentials are detected under essentially zero-current conditions, analytical measurements with these probes are generally not subject to the mass transfer and electron transfer kinetic limitations that often plague voltammetric or amperometric techniques. In this report, we review the current state of the art with respect to development and application of potentiometric membrane electrode probes. As a starting point, we provide a brief introduction to the fundamentals of membrane-based galvanic cell potentials and their measurement. Subsequent sections summarize the various membranes now in use for direct ion sensing, the approaches taken to miniaturize these same ion-selective systems, and the use of such membranes in conjunction with secondary membranes to devise a variety of potentiometric gas sensing systems. Finally, we conclude with an overview of the approaches being employed presently to devise potentiometric bioprobes suitable for the selective, in situ detection of biologically important molecules.