Effects of the porous texture and surface chemistry of silica xerogels on the sensitivity of fiber-optic sensors toward VOCs
Résumé
Fiber-optic sensors based on porous silica films, and operating in reflectance mode are very suitable for detecting volatile organic compounds (VOCs) because they combine the advantages of the optical fiber with the flexibility of the sol-gel process to obtain porous silica and organically modified silica films. The aim of this study was to assess the effect of the porous texture, refractive index, and surface chemistry of silica xerogels on their sensitivity toward dichloromethane, acetone, and cyclohexane. One xerogel was synthesized at pH 4.5 using tetraethylorthosilane (TEOS) as a silicon precursor (XG-4.5-00); two were synthesized at pH 10, one using TEOS as precursor (XG-10-00) and the other using a mixture of methyltriethoxisilane (MTEOS) and TEOS in a 20:80 molar percentage (XG-10-20). FTIR spectroscopy confirmed the presence of Si-C bonds in XG-10-20, and that the methyl group is incorporated into the xerogel. XG-4.5-00 had the largest area, 727 m(2) g(-1), followed by XG-10-00, 606 cm(2) g(-1), and XG-10-20, 342 m(2) g(-1). The sensing elements were tested with a measuring cell operating under static volumetric conditions. For each VOC, the highest sensitivity was obtained using the xerogel synthesized from TEOS at pH 4.5, followed by the xerogel synthesized from TEOS at pH 10, and then by the xerogel synthesized from MTEOS-TEOS in a 20:80 molar percentage. Neither acetone nor cyclohexane could be quantified using the hybrid xerogel. For each sensing element, the sensitivity of the response was dichloromethane > acetone > cyclohexane. The specific surface area and the surface chemistry of the xerogels explain, at least partially, the changes in the sensitivity of the response to each analyte among the various xerogel films.