Yongan Yan

Molecular recognition on acoustic wave devices: Zeolite thin films coated with organosilane gate layers

Abstract 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 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 n-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.

Molecular recognition in zeolite thin film sensors. Growth of oriented zeolite films

Summary We describe chemical sensors based on selective sorption in zeolite thin films on piezoelectric devices. Zeolite crystals with different pore sizes were coupled onto the gold electrodes of quartz crystal microbalances (QCM), via molecular attachment on the surface. A self-assembled monolayer of thiol-alkoxysilane coupling agent on the gold surface was used to attach the zeolite crystals to the QCM, and the resulting films were coated with thin glass layers. The sorption behavior and selectivity of the devices was determined via dynamic vapor sorption isotherms, and nitrogen sorption isotherms at liquid nitrogen temperature. Uptake of molecules small enough to enter the zeolite pores can be one hundred times greater than that of molecules with kinetic diameters greater than the pores. The selective adsorption can be tailored by variation of the zeolite composition, pore sizes, and other parameters. The selectivity of these stable films was further controlled by ion exchange into the film. We will compare the above films with recently discovered, oriented zeolite films grown on organic multilayers on gold . 1

Molecular Recognition on Acoustic Wave Devices: Modified Zeolite-Silica Thin Films With Tailored Adsorption Properties

Microporous thin films composed either of zeolite crystals embedded in sol-gel derived glass or of a molecular coupling layer, zeolite crystals and a porous silica overlayer, were formed on the gold electrodes of Quartz Crystal Microbalances (QCM). The microporosity of the thin films was characterized by in situ nitrogen and vapor sorption isotherms. Both preparation methods result in thin films with substantial microporosity. Selective adsorption based on molecular size exclusion from the microporous films could be achieved.