Zeolite-coated interdigital capacitors

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Urbiztondo MA, Pellejero I, Rodriguez A, Pina M.P, Santamarfa J. Zeolite-coated interdigital capacitors for humidity sensing. Sens Actuators B Chem 2011 157 450-459. 

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

Thus, zeolite-coated IDCs have been tested for sensing n-butane [317] and also, NH3, NO, and CO [318,319] on Na-Y and NaPtY zeolite-based sensors at temperatures high enough to where chemical reactions may also occur (above 200°C). The response time is of the order of seconds and the cross-sensitivity to water is small at high temperatures, at which no water condensation occurs in the zeolite-pore system. Under certain conditions, selectivity of these reactive chemical sensors is remarkable. Thus, the detection of 10 ppm of n-butane with a NaPtY interdigitated capacitor with no response to CO and H2 has been reported [318]. Similarly, Moos et al. [320] described a ZSM-5 based capacitor sensor with on-chip heating for temperatures up to 450°C capable of detecting NH3 with no cross-sensitivity to CO, hydrocarbons, and O2. 

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