Ion-Selective Electrodes R De Marco and G Clarke, Curtin University of Technology, Perth, Western Australia, Australia & 2009 Elsevier B.V. All rights reserved. Introduction An ion-selective electrode (ISE) is an example of an electrochemical sensor utilizing the principle of po- tentiometry, or measurement of the cell potential (i.e., ISE against a standard reference electrode) at near-zero current. Under these conditions, the boundary potential at the ISE–solution interface is governed by the laws of electrochemical thermodynamics or is compliant with the famous Nernst equation. To ensure a compliance with the Nernst equation, the very best commercially available potentiometers utilize a high input impedance >10 13 O, and an appropriate operational amplifier in the electrometer to handle the associated minute currents. The heart of the ISE is the permselective membrane that is capable of species recognition, and Figure 1 presents schematic representations of conventional solid- contact (left-hand side) and liquid-contact (right-hand side) ISEs. Ion-selective electrode sensors have been classified according to the membrane type and include the following electrode classifications: (1) homogeneous crystalline membrane electrodes (e.g., LaF 3 for F À ,Ag 2 S for Ag þ or S 2À , AgCl/Ag 2 S for Cl À ); (2) heterogeneous crystalline membrane electrodes (e.g., silicone rubbers comprising crystalline powders such as LaF 3 ,Ag 2 S, AgCl/Ag 2 S); (3) rigid noncrystalline membrane elec- trodes embodying silicate and chalcogenide glasses; (4) nonrigid noncrystalline electrodes employing plasticized polymer membranes incorporating neutral ionophores as receptor molecules; (5) sensitized electrodes such as carbon dioxide or amonia ISEs separating a filling solu- tion housing an immersed pH indicator electrode from the sample solution by a gas diffusion membrane, or enzyme-sensitized electrodes incorporating a pH indi- cator electrode encapsulated by agel film of enzyme (e.g., glucose oxidase) for the selective detection of the analyte (e.g., glucose). Ion-selective electrodes have a history of over one century,noting that the best and most widely utilized ISE based on the glass pH electrode was invented in 1906 by Cremer, and that the pH electrode was quickly adapted as a routine analytical tool by the 1930s. Despite many attempts to make glass electrodes that were sensitive to alternate ions to H þ , this imperative was achieved only in the late 1950s. With Frant’s and Ross’ pioneering de- velopment of an excellent LaF 3 single-crystal ISE for F À in 1966, along with Stefanac’s and Simon’s innovation of an outstanding K þ ISE using valinomycin as a neutral ionophore in a plasticized membrane, also in 1966, these ISE types soon became the gold standard in analytical chemistry, and the glass ISEs for analytes other than H þ were quickly superseded. More recently (1970s), the Vlasov Group at St Petersburg University in Russia introduced a class of chalcogenide crystalline glassy materials that have been utilized as ISEs for a wide range of heavy metal ions. Furthermore, if the ISE membrane is coated onto the surface of the missing metallic gate in a field-effect transistor (FET), then the current passed between the source and the drain of the FET device is governed by the ISE membrane potential, and these so- called ion-selective field-effect transistor devices were first described by Bergveld in 1970 and further developed by Janata and coworkers as well as Japanese researchers. Most significantly, the seminal paper in the Journal of the Solid contact Liquid contact Electrode body Solid gold contact PVC tubing Ion-selective membrane Ag/Agcl wire Inner filling solution Figure 1 Schematic diagrams showing typical solid-contact (left-hand side) and liquid-contact ion-selective electrodes. PVC, poly(vinyl chloride). 103 . Ion-Selective Electrodes R De Marco and G Clarke, Curtin University of Technology, Perth, Western Australia, Australia & 2009 Elsevier B.V. All rights reserved. Introduction An ion-selective. or enzyme-sensitized electrodes incorporating a pH indi- cator electrode encapsulated by agel film of enzyme (e.g., glucose oxidase) for the selective detection of the analyte (e.g., glucose). Ion-selective electrodes. gold contact PVC tubing Ion-selective membrane Ag/Agcl wire Inner filling solution Figure 1 Schematic diagrams showing typical solid-contact (left-hand side) and liquid-contact ion-selective electrodes.