Enhanced resonator sensitivity with nanostructured porous silica coatings

We present a strategy to increase the sensitivity of resonators to the presence of specific molecules in the gas phase, measured by the change in resonant frequency as the partial pressure of the molecule changes. We used quartz crystals as the resonators and coated them with three different thin films (< 1 mu m thick) of porous silica: silica xerogel, silica templated by an ordered hexagonal phase of surfactant micelles, and silica templated by an isotropic L-3 phase surfactant micellar system. We compared the sensitivity of coated resonators to the presence of water vapor. The crystals coated with hexagonal phase-templated silica displayed a sensitivity enhancement up to 100-fold compared to an uncoated quartz crystal in the low-pressure regime where adsorption played a dominant role. L-3 phase-templated silica displayed the highest sensitivity (up to a 4000-fold increase) in the high partial pressure regimes where capillary condensation was the main accumulation mechanism. Three parameters differentiate the contributions of these coatings to the sensitivity of the underlying resonator: (i) specific surface area per unit mass of the coating, (ii) accessibility of the surfaces to a target molecule, and (iii) distribution in the characteristic radii of curvature of internal surfaces, as measured by capillary condensation.