Chapter 17
Electrical Impedance-Based Sensors
The flow of direct current (DC) through an electric circuit is impeded only by the electrical resistance presented by the circuit. By contrast, in the case of alternating currents (AC) other mechanisms impede the flow of current, in addition to the resistance. For example, a capacitor cannot be crossed by direct current but it allows alternating current to flow across it by means of alternating charging–discharging processes. Nevertheless, a capacitor presents certain opposition to the AC flow by a mechanism that is fundamentally different from that of the electrical resistance. The physical quantity that quantifies the opposition of a capacitor to the flow of alternating current is called electrical impedance. Among other factors, the impedance of a capacitor depends on the dielectric constant of its insulator. This property can be used to develop chemical sensors based on the interaction of the analyte with a suitable insulator material. A sensor of this type is a capacitive sensor and is based on the use of impedance measurements for transduction purposes.
The electrical resistance itself represents a particular case of electrical impedance. Sensors based on resistance monitoring have been developed using materials that can change the electrical resistivity in response to their interaction with an analyte.
Of particular relevance to chemical-sensor science is the electrical impedance of electrochemical cells. As shown in Chapter ...
