Chapter 4 - New trends in ion-selective electrodes

Abstract

Ion-selective electrodes (ISEs) are the oldest class of chemical sensors and they are still superior to other sensor types used in various applications in biomedical, industrial, and environmental fields. Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application. In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications, as health disorders are usually correlated to ion activity. Pharmaceutical analysis is another important area for ISEs where a large number of drugs were reported to be detectable by ISEs in pharmaceutical formulations and during manufacturing processes. The concentration of drugs and their metabolites can be measured in real biological fluids. Most drug-selective electrodes are based on ion exchangers and exploit commonly high lipophilicities of drugs and metabolites. The possibility of detecting polyionic macromolecules added a new thrust to the area of ion-selective electrodes in the past decade. Professors Ma and Meyerhoff from the University of Michigan described the first polymeric membrane electrodes that respond to the polyanion heparin. In contrast to other methods, the heparin-selective electrode was able to detect heparin concentration directly in whole blood or plasma samples. In order to improve the biocompatibility of ISEs and reduce adsorption of cells and polypeptides, several approaches including immobilization of anticoagulants such as heparin and the continuous release of biologically active molecules such as nitric oxide are used. An interesting approach was used by the group of Bachas, who prepared a potassium-selective electrode coated with a copolymer containing phosphorylcholine that mimicked the polar groups on cell surfaces.