| Xerogels are porous glasses formed from the hydrolysis and polycondensation of metal alkoxides. Xerogels are used as insulators, catalysts, hosts in electro-optical devices, as well as solid-state matrices in chemical sensors. Advantages of using xerogels include high chemical purity and material homogeneity. An important attribute of xerogels in chemical sensing applications is the porosity that results from the formation of the matrix. The degree of porosity and the sizes of the pores of the xerogel can be controlled during the formation of the material. This is accomplished by controlling reaction parameters such as pH and water content.; In this research the structures of xerogels produced from four chemical protocols were characterized by small-angle neutron scattering, quasielastic neutron scattering, and gas adsorption isotherms. From the information gained by these techniques we have been able to develop an understanding of the role of chemistry in xerogel structure for the glasses used in our chemical sensors. The information gained includes the role of pH and water content on xerogel porosity, pore size, and surface area.; The role of xerogel structure on rates of analyte diffusion in pH sensors was also studied. In these experiments, pH indicator-doped xerogels were placed in buffered solutions and exposed to a pH change. The rates of proton diffusion were determined by measuring the time-dependent change in the absorbance of the indicator-doped xerogel. From these studies we have determined that the mean pore size as well as xerogel surface properties have a strong effect on proton diffusion rates.; The dye lucigenin was studied in porous xerogels as a candidate for kinetic-based halide sensing. Lucigenin possesses a ∼15 nsec fluorescence lifetime that is quenched by millimolar concentrations of chloride, bromide, and iodide. The sensitivity of lucigenin to halide ions was found to be affected by the xerogel matrix. |
