Room temperature ethanol sensor with sub-ppm detection limit: Improving the optical response by using mesoporous silica foam

Abstract

In this paper, the improvement in room temperature ethanol sensing characteristics of zinc peroxide (ZnO2) based hybrid thin films is presented by the combination of the beneficial sensing properties of mesoporous materials and reflectometric interference spectroscopy (RIfS). The hybrid thin films were prepared by Layer-by-Layer (LbL) self-assembly method from ZnO2 nanoparticles, polyelectrolyte [poly(acrylic acid), PAA] and/or mesoporous silica (MPS). The expected improved sensing properties were attributed to the fractal properties and high specific surface area (as) of the mesoporous coating/interlayer material, which was evidenced by small angle X-ray scattering (SAXS) and N2 sorption measurements (as>650m2/g). The sensor tests showed that the detection limit of the thin films is in the sub-ppm range (<500 ppb). Applying silica foam (SF) as surface coating or interlayer material in the sandwich-structured thin film (ZnO2/SF) improved the optical response (Δl: wavelength shift) compared to the ZnO2/PAA thin layer, but the sensitivity showed non-linear characteristic and signal drift. The thin film with mixed structure (ZnO2/PAA/ZnO2/SF) showed linear sensitivity (Δλ/Δc=0.6nm/ppm) in the 0.5–12ppm range with an acceptable selectivity and stable baseline. Testing the sensor in extended (up to 40ppm) concentration range showed only a slight quadratic deviation from linear behavior with R2=0.9987.