8 · CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 1: GENERAL APPROACHES 3. CAPACITANCE SENSORS A capacitor is an electrical circuit component which can store electrical energy in response to an ap- plied electric field. It consists of a dielectric material which is sandwiched between two parallel metal electrodes. When a voltage is applied to these plates, equal and opposite charges accumulate on the electrodes. Capacitance can be expressed as C = 0 r A d (1.6) where 0 represents the permittivity in vacuum, r is the relative permittivity of the dielectric, A is the area of the electrode, and d is the distance between the two electrodes. The main operating principle of capacitance sensors is that the analyte of interest can change r , A, or d, and therefore, by measuring the change in the capacitance, the presence of the measurand can be detected (Ishihara et al. 1998). With reference to polymer coating, the mechanisms which can be ac- companied by the change of these parameters are shown in Figure 1.5. Upon analyte absorption, the physical properties of the polymer layer change as a result of the incorporation of the analyte molecules into the polymer matrix. Changes in two physical properties influence the sensor capacitance: (1) vol- ume (swelling) and (2) dielectric constant. The resulting capacitance change is detected by the read-out electronics (Kummer et al. 2004).