UC Riverside

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. An important attribute of xerogels in chemical sensing applications is the porosity of the sol-gel matrix, which results from the formation of the matrix, allows analytes to diffuse into the glass and come in contact with the sensing element. Our main goal, driven in part by the environmental applications, was directed toward incorporating sensing fluorescent chromophores into the sol-gel matrix and characterizing the effects of the matrix on the fluorophores' respective chemistry.

The mechanism by which ions diffuse into the sol-gel matrix, illustrated by measuring of the proton diffusion into the glass, was elucidated by the time-dependent absorbance change of the pH indicator dye, fluorescein. Small pore xerogel glasses were found to possess proton diffusion rates that depend on the direction of the pH change. The fluorescein-doped xerogels showed a slower time response for a decrease in pH and a longer time response for an increase in pH. The difference in these two rates could indicate that the sol-gel matrix provides a kinetic barrier to proton diffusion into the pores of the glass.

Sol-gel clad fiber-optic waveguides were investigated as intrinsic distributed fiber-optic chemical sensors. The porous sol-gel cladding allows diffusion of analytes into the evanescent field region close to the fiber-optic core. Pulsed optical excitation (0.5 ns) and time-resolved emission detection were used to simultaneously monitor several sensor regions along a fiber optic waveguide. Narrow band excitation and spectrally resolved emission provide additional means for discriminating between specific regions offering spatial sensitivity and kinetic-based sensing. A fluorescein-doped silica xerogel clad pH sensor and an undoped xerogel clad lucigenin sensor were demonstrated as intrinsic sol-gel clad fiber-optic sensors.