Study of silica sol-gel materials for sensor development

Silica sol-gel is a transparent, highly porous silicon oxide glass made at room temperature by sol-gel process. The name of silica sol-gel comes from the observable physical phase transition from liquid sol to solid gel during its preparation. Silica sol-gel is chemically inert, thermally stable, and photostable, it can be fabricated into different desired shapes during or after gelation, and its porous structure allows encapsulation of guest molecules either before or after gelation while still retaining their functions and sensitivities to surrounding environments. All those distinctive features make silica sol-gel ideal for sensor development. Study of guest-host interactions in silica sol-gel is important for silica-based sensor development, because it helps to tailor local environments inside sol-gel matrix so that higher guest loading, longer shelf-life, higher sensitivity and faster response of silica gel based sensors could be achieved.;We focused on pore surface modification of two different types of silica sol-gel by post-grafting method, and construction of stable silica hydrogel-like thin films for sensor development. By monitoring the mobility and photostability of rhodamine 6G (R6G) molecules in silica alcogel thin films through single molecule spectroscopy (SMS), the guest-host interactions altered by post-synthesis grafting were examined. While physical confinement remains the major factor that controls mobility in modified alcogels, both R6G mobility and photostability register discernable changes after surface charges are respectively reversed and neutralized by aminopropyltriethoxysilane (APTS) and methyltriethoxysilane (MTES) grafting. The change in R6G photostability was found to be more sensitive to surface grafting than that of mobility. In addition, silica film modification by 0.4% APTS is as efficient as that by pure MTES in lowering R6G photostability, which suggests that surface charge reversal is more effective than charge neutralization in disrupting R6G/silica attraction.;Similar post-grafting method was applied to highly hydrated silica hydrogel monoliths. Rhodamine 6G (R6G) and fluorescein (Fl) molecules were used as probes to monitor the surface modification inside silica hydrogel by measuring anisotropy values of doped dyes. Due to the larger pore sizes, pore surface modification inside hydrogel was more effective than in alcogel. Surface modification by chemical reactions of 3-Aminopropyltrimethoxysilane (APTS) and methyltriethoxysilane (MTES) showed dramatic effect on guest molecule mobility, whereas surface modification by physical method, that is to increase ionic strength by using 1.0 M sodium chloride or to neutralize pore surfaces by adding pH 2.0 hydrochloric acid, barely showed any effect. Charge-reversal by APTS is a more effective way to modify pore surfaces in hydrogel than hydrophobic capping from MTES. The ease of tracking surface modification inside hydrogel by simply locating R6G dye band, and the negligible pore fluid effect on R6G in modified hydrogel makes R6G a better probe than Fl to monitor the pore surface modification process in silica hydrogel monoliths.;During the study of post-grafting on silica alcogel thin film, a new approach to produce stable silica hydrogel-like thin films was discovered. Homogeneous thin film hydrogel-like samples with thickness between 100 nm and 300 nm were produced, and they showed a very hydrophilic surface, high dye loading capacity, and the support of molecular diffusion. The reactive stage of starting silica gel matrix was elongated by increasing environmental humidity, the reproducibility of sample preparation was greatly improved by controlling environmental humidity, and the dye loading capacity of samples was improved more than ten times by using phosphate buffer solutions (PBS). The concentration of R6G trapped inside hydrogel-like thin film could reach as high as 900 times of its saturated aqueous solution. Dye encapsulation can simply be accomplished by dipping a chemically reactive alcogel thin film into a dye-doped buffer solution. Since alcohol exposure can be kept to a minimum during dye encapsulation, this new silica film makes a promising candidate for biomolecule encapsulation and thus biosensor development. A prototype silica hydrogel-like thin film pH sensor was also constructed and tested, and it showed faster response than the corresponding alcogel thin film sensor.