Bein, Thomas; Yan, Yongan
Molecular recognition on acoustic wave devices. Zeolite thin films coated with organosilane gate layers.
In: Microporous Materials, Vol. 1, Nr. 6: S. 401-411
Microporous thin films composed of a molecular coupling layer, zeolite crystals, and a porous silica overlayer, were formed
on the gold electrodes of quartz crystal microbalances (QCMs). The silica overlayer enhances the mechanical stability of the
zeolite films, and results in additional surface area and porosity as characterized by the sorption isotherms and transient
sorption of vapors with different molecular diameters and different polarities. The protecting silica glass layer is gas permeable
such that the regular zeolite micropores with molecular sieving capability are still accessible in the composite film. A novel
surface tailoring technique for the microporous thin films was developed, in which organosilane molecules were chemisorbed
on the silica overlayer via siloxane linkages, forming a molecular "gate" at the gas thin film interface. The adsorption of vapors
into the microporous zeolite films is therefore controlled by the permeability of the gate layer. Selective adsorption based on
kinetic or equilibrium exclusion from the microporous films could be achieved, as demonstrated by discrimination of molecules
with similar polarity but different molecular diameters (water vs. ethanol), and effective exclusion of larger molecules such as rt-hexane. As a result of the increase in the vapor sorption selectivity and reduction of the external surface area of the thin
films, the modified QCMs show high selectivity towards water over other molecules.
Keywords: acoustic wave device; sensor; zeolite film; organosilane coating; humidity sensing