Electrochemical sensors

Electrochemical sensors are of two types: ion-selective electrodes and ion-sensitive field-effect transistors. Both are designed to measure the electrochemical potential of a specific solute. They differ only in their method of analysis.

Ion-selective electrodes

Ion-selective electrodes (Fig, 1) are found most commonly in large batch analyzers in the clinical chemistry laboratory. The sample is diluted to minimize interference from other solutes. The ion-selective electrode allows a particular solute or metabolite through a semipermeable membrane, while excluding others, so that a potential gradient is generated at the electrode surface. This potential difference is then used to calculate the specific ion or solute concentration ( Eberhart and Weigelt 1989).

Internal reference electrode

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Fiq. 1 Ion-selective electrode. The ion-selective membrane allows only the chemical of interest to contact the electrode. (Reproduced with permission from Koryta

Internal reference electrode

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Ion-sensitive field-effect transistor

The ion-sensitive field-effect transistor ( Fig 2) differs from the ion-selective electrode in the method by which it measures the electrochemical potential produced by the ionic species in question. An electrochemical potential is generated at the substrate-gate insulator interface. This potential acts as an effective gate voltage which modulates electrocurrent drain. The circuit is completed by a reference electrode in contact with the solution. Thus a major difference between the ion-selective electrode and the ion-sensitive field-effect transistor is that the latter requires a reference electrode to be in contact with the solution of interest to complete the circuit (Eberhart_and_WejfleJU9.ยง.9). Both types of sensor have shown reliability in vitro; however, in vivc clinical application is still minimal.

Fig. 2 Diagram of the pH-sensing ion-sensitive field-effect transistor (p-type is the solution or plasma; n-type is the electrochemical current source). Hydrogen ions accumulate at the liquid-gate insulator interface, or are sequestered in the ion-selective membrane, if used. An electric field is produced across the silicon oxide-nitride gate insulator, modulating the current flow between source and drain. (Reproduced with permission from Eberhart eta,L (19.82.) )

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