PRINCIPLES AND MECHANISMS OF ION-SELECTIVE OPTODES

A great variety of different types of optical sensing layers for ionic species have been described, but comparative studies on the physicochemical principles involved in the sensing process have not been made. The response of bulk optodes can be described straightforwardly as a function of activity ratios or products. In contrast, the description of optodes based on interfacial phenomena is much more complicated. This is mainly due to an additional and sophisticated dependence of the signal on the sample matrix. In many ion-selective optodes, the pH as well as the ionic strength of the sample play an integral part in the sensor response. With emphasis on this aspect, various ion-selective optode systems, including pH optodes, are discussed and compared.