Capacitance measurements, signal transduction

Capacitance-based chemical sensors are in the class of devices that transduce analytes into electrical currents. Such sensors are typically comprised of a dielectric, chemically-sensitive film coated onto a substrate electrode these films pass low conduction current, making amperometric or conductimetric measurements less sensitive or attractive for signal transduction. To detect an analyte, changes in the chemically-sensitive film s capacitive properties (associated with its dielectric constant, charge uptake, or formation of interface dipole layers) are measured when an active species is present or generated. 

In physical sensors, such as pressure sensors and accelerometers, capacitance measurements are a common means of signal transduction. Such methods can also be used with chemical sensors34, however, they have not received extensive attention to date. For sensors applied to the liquid phase, this may be due to the complexities associated with the impedance of the double layer that forms in solution at a charged interface. This tends to make the sensor sensitive to changes in ionic strength, unless the time constant of the selective coating that is relevant to the analytical measurement is considerably different from that of the double layer. Further, many chemical reactions associated with a coating... 

Impedance is another possible transduced signal, and it can be measured by a spectroscopic analyzer or by LCR (L = inductance, C = capacitance, R = resistance) bridges. It may be useful to identify the different contributions to the sensor response (grain, bulk and contact) but, due to higher costs, there are no commercial devices based on this transduction. 

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