20,197 research outputs found
Zeolite crystal layers coupled to piezoelectric sensors
Microporous zeolite crystals were successfully coupled onto the gold electrodes of
quartz crystal microbalances (QCM). A self-assembled monolayer of thiol-alkoxysilane
coupling agent on the gold surface was used as the interfacial layer to promote adhesion of
the zeolite crystals to the QCM. The resulting, densely packed single layers of zeolite
crystals were stable to at least 625 K. Transient sorption behavior of organic vapor
pulses, dynamic vapor sorption isotherms and nitrogen sorption isotherms at liquid
nitrogen temperature were examined to characterize the zeolite-coated QCMs. Depending on
the type of zeolite coating, the resonance frequency response to vapor pulses could be
increased up to 500-fold compared to the bare QCM. The regular micropores (0.3-0.8
nm) of the QCM-attached zeolite crystals were found to control molecular access into the
extensive intrazeolite volume. Selectivity of the frequency response in excess of 100:1
toward molecules of different size and/or shape could be demonstrated. An additional
recognition mechanism based upon intrazeolite diffusion rates was also established
Molecular recognition on acoustic wave devices
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 sensin
Gas Doping on the Topological Insulator Bi2Se3 Surface
Gas molecule doping on the topological insulator Bi2 Se3 surface with
existing Se vacancies is investigated using first-principles calculations.
Consistent with experiments, NO2 and O2 are found to occupy the Se vacancy
sites, remove vacancy-doped electrons and restore the band structure of a
perfect surface. In contrast, NO and H2 do not favour passivation of such
vacancies. Interestingly we have revealed a NO2 dissociation process that can
well explain the speculative introduced "photon-doping" effect reported by
recent experiments. Experimental strategies to validate this mechanism are
presented. The choice and the effect of different passivators are discussed.
This step paves the way for the usage of such materials in device applications
utilizing robust topological surface states
Wnt5a Signaling in Normal and Cancer Stem Cells.
Wnt5a is involved in activating several noncanonical Wnt signaling pathways, which can inhibit or activate canonical Wnt/β-catenin signaling pathway in a receptor context-dependent manner. Wnt5a signaling is critical for regulating normal developmental processes, including stem cell self-renewal, proliferation, differentiation, migration, adhesion, and polarity. Moreover, the aberrant activation or inhibition of Wnt5a signaling is emerging as an important event in cancer progression, exerting both oncogenic and tumor suppressive effects. Recent studies show the involvement of Wnt5a signaling in regulating normal and cancer stem cell self-renewal, cancer cell proliferation, migration, and invasion. In this article, we review recent findings regarding the molecular mechanisms and roles of Wnt5a signaling in stem cells in embryogenesis and in the normal or neoplastic breast or ovary, highlighting that Wnt5a may have different effects on target cells depending on the surface receptors expressed by the target cell
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