Analytical determination of inorganic and organic ions is a matter of great interest to various areas such as biomedical sciences, environment monitoring and industry. Ion sensors respond to the demand of continuous ion monitoring, field analysis and in vivo ion determination.
Ion sensors rely on the general property of ions of bearing an electric charge. It is therefore not surprising that ion sensors were first developed based on electrochemical principles in the framework of potentiometry, that is, electrochemistry under equilibrium conditions. However, practical potentiometric ion sensors are not generally based on electrochemical oxidation or reduction processes because such processes display poor selectivity. Rather, potentiometric ion sensors are based on ion-transfer processes occurring at the interface of two immiscible phases. One phase is the test solution and the second phase is constructed so as to interact specifically with the analyte ion. The second phase, usually called an ion-sensitive membrane, can be in the solid state form or a liquid supported on an inert material. Due to the analyte interaction with the membrane, a nonuniform ion distribution appears at the membrane/solution interface. This results in a nonuniform electric charge distribution that gives rise to an electric potential difference. It is this potential difference that produces the response signal of potentiometric sensors. Such ion sensors ...