Potentiometry ISFETs

The ion-selective field-effect transistor (ISFET) represents a remarkable new construction principle [7, 63], Inverse potentiometry with ion-selective electrodes is the electrolysis at the interface between two immiscible electrolyte solutions (ITIES) [28, 55],... 

In contrast to potentiometry with ISEs, the drain current is measured with the ISFET and not the voltage. As the drain current depends only approximately linearly on A 0 and as the aK (>ri) value depends on the properties of the membrane surface (for example, on the adsorption of surfactants), measurement of activities using an ISFET requires careful calibration. The response time depends on the membrane properties and is not affected by the components of the solid-phase sensor [162]. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

Progress in analytical potentiometry in recent years has brought a miniaturization of detection systems via the hyphenation of the sensor with a large extent of integration electronics. In this context, chemically sensitive layers are placed directly onto field effect transistors (FETs) to produce ion-selective FETs (ISFETs). The use of two identical ISFETs in a flow-through cell offers the possibility of differential measurements without the influence of the liquid junction potential. 

The ISFET, developed from the fabrication techniques of semiconductor devices, is an important sensor device used in potentiometry. The main advantages are the extremely small size, solid-state structure and the ability to fabricate multi-ion sensors. More than 30 years ago, methods have been proposed to work with a differential arrangement, i.e. the integration of an ion-sensitive and an ion-insensitive structure, the later one working as the reference element (R(E)FET). The main problem is that semiconductor-modified surfaces required for R(E)FET are also not always in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or interfering dissolved species or not well characterised aging phenomena. 

Potentiometric measurements by ion-selective electrode potentiometry (ISE) or ion-selective field effect transistors (ISFET), stripping voltammetry (SV) and stripping chronopotentiometry (SCP) are analytical techniques capable of giving not only the total concentration of analytes in water samples, but also metal speciation information, since they are sensitive to the free metal fractiOTi. This capability is especially useful for the study of metal distribution and transfer among environmental compartments in water, as previously commented in this subchapter (Fig. 3.1). It s important to stress that SV and SCP are dynamic techniques, whereas ISE and ISFET are equilibrium techniques. For more details on potentiometric measurements, see Chap. 9 vol 2. 

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