Exploring molecular behavior at nanometer dimensions in silica gel with single molecule spectroscopy and confocal microscopy

The study of molecular behavior at a solid/liquid interface in porous materials, such as silica gel, is of great interest for their applications in the fields of chemical sensing, heterogeneous catalysis, and in chemical separations. To probe the molecular behavior in these inherently microheterogeneous materials, single molecule spectroscopy removes the problem of ensemble averaging and reveals fluctuation phenomena that are usually obscured by bulk spectroscopic measurements. As a result, spatial variations in the properties of heterogeneous materials can be directly investigated. Single molecule spectroscopy, combined with the optical sectioning capability of confocal microscopy, thus provides a powerful technique for exploring the individual nanoscale behavior of molecules inside porous materials. A stage-scanning confocal microscope capable of quantitative imaging and single molecule detection was successfully built, characterized, and implemented in the lab.;Molecular diffusion and adsorption at a solid/liquid interface in silica gel are related to chromatographic retention processes. Single molecule spectroscopy was used to visualize diffusion and reversible strong adsorption of individual dye molecules at the interface of C18-modified silica gel and acetonitrile. Fluorescence correlation imaging revealed that the strong adsorption sites were randomly distributed throughout the silica particle, with a wide range of desorption times spanning from milliseconds to seconds. Ratiometric single-molecule study of individual strong adsorption events revealed that the strong adsorption sites are highly polar and heterogeneous in nature. The feasibility of ratiometric imaging of environmental polarities of freely diffusing individual molecules was also theoretically explored and experimentally demonstrated.;Micropolarity distribution of C18-modified silica gel was studied by confocal ratiometric fluorescence imaging of the C18 interface loaded with a polarity-sensitive dye. The results showed a fairly uniform micropolarity distribution within each particle, but a high degree of heterogeneity from particle to particle. This heterogeneity in polarity could be partially responsible for band broadening in chromatography. The wettability and environmental polarity of C18-modified silica mesopores were also studied under various wetting conditions. It was shown that the environmental polarity of C18-modified silica mesopores strongly depends on the composition of the wetting solution and the prior wetting history of the mesopores.