Current conductance/capacitance sensors

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. 

Microelectrodes exhibit faster response, reduced iR-drop as a consequence of reduced capacitance, and lower currents. However, owing to the hemispherical diffusion profile in front of the microelectrode the mass transfer is enhanced leading to a higher current density and an improved signal-to-noise ratio (see Chapter 2.5). They not only permit measurements in small volumes and very low concentrations but additionally in samples with low conductivity. Because of the reduced size of miniaturized sensors, smaller amounts of rare and expensive... 

Single wall carbon nanotube (SWNT) network sensors were prepared as previously described (7-9). The carbon nanotube network (CNN) sensors were spray coated with a 0.1% by weight solution of polymer to a thickness of approximately 100 nm. These CNN sensors can be monitored simultaneously in capacitive and resistive mode. The capacitance was measured by applying a 0.1 V, 30 kHz, AC voltage between a conducting Si substrate and a S T network deposited on a thermal SiOa layer. The induced AC current was measured using a Stanford Research Systems SR830 lock-in amplifier. 

Measurement Principles. The equipment used for measuring conductance usually consists of a conductivity sensor, a measuring attachment, and a temperature-compensation unit. Conductance values are obtained by establishing either the current flow between a set of electrodes subjected to a constant AC voltage, or the current induced in the secondary coil of a transformer connected to the primary coil by the electrolyte of interest. Capacitive coupled cells have been developed as well, but because of their expensive construction they are no longer of interest (98)-[101]. 

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