Polymer substrates for fiber-optic chemical sensors

The primary goal of this research was to examine chemical and mechanical properties of polymers designed for a fiber optic chemical sensor based on polymer swelling. A second goal of this research was to develop a temperature sensor that could be used with other fiber optic chemical sensors. Fiber optic chemical sensors have the capability for remote use, accuracy, sensitivity, selectivity, and ruggedness. Fiber optic sensors based on polymer swelling exploit the rugged features of polymers and fiber optic communications technology for use in remote sensing. A crosslinked polymer changes size in the presence of an analyte. As the polymer changes size, it changes the position of a reflective surface. Light is transmitted to and collected from this surface using fiber optics. As the reflective surface moves, the light intensity collected by the fiber optic changes. The effect of derivatization on swelling and mechanical properties of a series of crosslinked polymers was investigated. Polymers were chosen for their selectivity for either ionic or neutral molecules, and included poly(ethylenimine), poly(phenylboronic acid), cyclodextrin, polystyrene, tertiary-butyl polystyrene, and polyisoprene. It was determined that small, short chain polymers do not have the mechanical integrity needed for use in this sensor. Crosslinked polystyrene beads were tested in the fiber optic sensor in pure organic solvents and water samples that contained organic solvents. In pure toluene and methylene chloride signal changes of up to 30% were observed, but in solutions of water saturated with these solvents, small signal changes of 3-4% were observed. The mechanical properties of the polymers need to be matched more closely to the force exerted by the sensor diaphragm in order to see a larger signal response. The temperature sensor is based on a change in the ratio of absorbances of phenol red at two different wavelengths. Because it involves a ratio measurement, it is not affected by drift at the source and detector. The phenol red was incorporated in a polyacrylamide gel along with TRIS buffer, which has a highly temperature-dependent pKa. The experimental slope of the response of the sensor was $-$1.80 $\times$ 10$\sp{-2}\sp\circ$C$\sp{-1}$