Detection of organic vapours with Si cantilevers coated with inorganic (zeolites) or organic (polymer) layers

Microfabricated Si cantilevers with different sensitive layers have been used for the detection of volatile hydrocarbons (ethanol and toluene) in air at ppm concentration levels. A variety of coatings has been explored, including several polymers (PEUT, PECH, PIB) and zeolites both organophilic (silicalite) and hydrophilic (zeolite Y, zeolite A). Also, different sensor dimensions have been studied, in an attempt to understand the trade off between a larger sensor surface (and therefore a larger sensitive area) and better mechanical properties. The main sensor performance parameters have been calculated and compared for the different coatings and sensor designs. Also, some scarcely studied aspects such as the evolution of the Q factor with zeolite loading have been studied. Among the tested, detection limits below 24 ppmV for ethanol and 26 ppmV for toluene are obtained with zeolite type coatings; while the corresponding values for the polymer-coated are over 260 and 43 ppmV, respectively. The performance of zeolite-coated cantilevers could be further increased by degassing at a sufficiently high temperature to remove the adsorbed species, thus releasing their nanoporous structure for the next sensing cycle. To this end, cantilevers with integrated heaters have been developed which enabled lowering the limits of detection down to the single ppm level.