Multilayer fiber-optic chemical sensors employing organically modified SiO2 and mixed TiO2/SiO2 sol-gel membranes

Fiber-optic sensors have been developed that incorporate multi-layer organically modified silica sol-gel membranes. pH sensors use a single layer hydrophilic organo-silica sol-gel membrane with a covalently attached pH sensitive dye, hydroxypyrene trisulfonic acid. The hydrophilic coating is made by copolymerizing silanol-terminated polydimethylsiloxane and tetraethylorthosilicate with 3-aminopropyltriethoxysilane. Unlike previous methods, which use acid as a catalyst, we have found that a base catalyst produces optically transparent gels. The sol-gel coated sensors are simple to make and require drying and aging times of as little as one day. Sensors made using these gels exhibit very good long-term stability, fast response times and no dye leaching. pCO(2) sensors were fabricated using the same pH sensitive sol-gel layer overcoated with a hydrophobic high organic content sol-gel membrane. The response of the pH and pCO(2) sensors is very fast due to the high porosity of the sol-gel membranes. Although in-situ sensors have been described for a number of organic and inorganic species, many volatile organochloride compounds (VOC's), such as perchloroethylene (PCE) and trichloroethylene (TCE), have been difficult to measure using current fiber-optic sensor transduction schemes. One of the optical sensors described here is a multilayer (3-4) sol-gel system that incorporates a TiO2/SiO2 membrane to degrade VOC's into smaller, detectable products. Upon exposure to UV light, TiO2, a semiconductor with a bandgap of 3.2 eV, produces highly reactive electron-hole pairs that are capable of photodegrading most organic compounds. The VOC's mentioned above are sensitive to degradative oxidation on TiO2 surfaces. During photodegradation of VOC's a number of products are formed including H+, HCl, CO2 and a number of smaller hydrocarbons. These products are produced in the TiO2 membrane and on TiO2 surfaces and the products diffuse into the nearby indicator membrane where they are detected. Carbon dioxide and protons produced are detected by the pH sensitive indicator layer described above. pH and CO2 sensors, and preliminary data for the measurement of VOCs will be presented.