Resistive and Capacitive Sensors for Liquids

Chemoresistors for hquid phase (impedimetric sensors) have a design similar to that of gas sensors (Fig. 5.7). In contact with electrolytic solution, a specific electrochemical cell is estabhshed. With this cell, the measming set-up cannot be arranged to respond to effects of a single electrode alone, as was possible with classical electrochemical impedance experiments (Sect. 2.2.6). Hence, with chemoresistors the equivalent circuit must consider both electrodes. For a sensitive layer with some intrinsic conductivity, for the low frequency range the conditions can be symbolized approximately by Fig. 5.9. Cf and R symbolize the film s capacity and resistance, respectively. Q and R are the corresponding quantities of the sensor interface. 

Impedance measiuements are useful for resistive as well as for capacitive sensors. Analytes can affect the different components of the equivalence circuit in various ways. By impedance spectroscopy, i.e. by phase selective determination of the complex quantities, maximiun sensitivity and selectivity can be achieved. Impedance in this case is not the preferred method in fundamental research its greatest usefulness is in analytical applications. It is aimed at a cahbration curve as hnear as possible. 

Chemoresistors have proved useful for biosensors in particular. Many enzymes catalyse reactions which generate ions. Such enzymes can be immobilized easily in thin hydrogel films covering interdigitated structures. Increases in the ionic concentrations as a result of enzymatic reactions will strongly enhance the conductivity. The latter can be measured easily with little effort. Very useful is an electronic circuit measuring the difference in conductances of one sensor with an enzyme and another one without it (Fig. 5.10). 

Nowadays, chemoresistive biosensors are very common. Most widespread are urea sensors, which are a good way to explain the function principle. The enzyme urease catalyses the following reaction  

Urea is a non-ionic substrate. As a result of the reaction, the ionic substances carbonate and ammonia are formed. The conductance increases, and this effect is measured by means of a differential measuring circuit (Hendji et al. 1994). Alternatively, a potentiometric urea sensor can be constructed on the basis of a pH-sensitive electrode. Such electrodes are much more complex and fragile than resistive sensors. Furthermore, in potentiometry a reference electrode is necessary. 

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