Sol-gel derived porous silica as a constituent material for designing optical fiber chemical sensors
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Sol-gel derived porous silica as a constituent material for designing optical fiber chemical sensors Shiquan Tao*, Joseph C. Fanguy and Lina Xu Diagnostic Instrumentation and Analysis Laboratory, Mississippi State University 205 Research Blvd, Starkville, MS 39759 (*e-mail: [email protected]) Abstract: Sol-gel processes were developed to prepare nano porous silica materials. The obtained porous sol-gel silica (PSGS) materials have been used as constituent materials in designing optical fiber chemical sensors. A PSGS membrane coated on the surface of an optical fiber was used as a transducer for sensing humidity level in air. A PSGS membrane doped with an ammonia indicator dye has been coated on an optical fiber to sense ammonia in air. Both of the coating based sensors are reversible and fast response. In the tested range, relative humidity (RH) in air down to 3% can be detected with the PSGS coated fiber optic sensor. The fiber optic ammonia sensor with ammonia indicator doped PSGS coating can be used to sense ammonia in air down to sub-ppm level. PSGS has also been used as a constituent material in preparing porous silica optical fibers. The obtained porous optical fibers have been used to design optical fiber chemical sensors for sensing humidity, ammonia and volatile organic compounds. A CuCl2 doped PSGS fiber has been tested for sensing ammonia in a high temperature gas sample. Ammonia in the high temperature air gas diffuses into the PSGS fiber, reversibly reacts with CuCl2 doped in the PSGS fiber to form a complex. The formed complex was detected with fiber optic spectrometric method. This sensor can detect ammonia in a high temperature (450 o C) air gas stream down 0.3 ppm. Techniques of preparing PSGS, coating PSGS on an optical fiber, making a porous optical fiber with PSGS as a constituent material will be presented. Examples of optical fiber sensors using PSGS coatings and a PSGS fiber as transducers for gas sensing are presented.
Introduction: A sol-gel process is a low temperature wet chemical process for the synthesis of an inorganic or a composite organic/inorganic polymer. The sol-gel process is based on the hydrolysis of a molecular precursor and subsequent polycondensation reactions. This is a versatile wet chemical process for preparing glasses, ceramics, thermal isolating materials, catalysts, sorbents for gas storage, separation and water purification, bioactive materials for catalyzing biochemical reaction and separation, special photoluminescence materials for laser design and optolectronic application, semiconductor materials for gas sensing, coating materials for fiber optic sensing, etc.1-3 The chemistry of a sol-gel process includes the hydrolysis of a precursor, condensation of the hydrolysis product to form a sol solution, gelation of the sol solution to form a gel, gel aging, drying the formed gel, and heat treatment of the dried sol-gel material. The precursor of a sol-gel material is usually a silicon alkoxide or metal alkoxide. In some reports, silicate salts, metal nitrate, and
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