Sol-Gel-Coated Mid-Infrared Fiber-Optic Sensors

Abstract

Application of organically modified sol-gels as novel recognition membranes for mid-infrared fiber-optic sensors is demonstrated for the first time by in situ detection of nitro-based aromatic compounds in aqueous media. Sol-gels were prepared by acid- and base-catalyzed copolymerization of alkyltrimethoxysiloxanes and applied onto the surface of silver halide (AgCl_0.3Br_0.7) fibers by drip coating. The coating process was monitored in situ using Fourier transform infrared (FT-IR) spectroscopy. Homogeneity of the layers was analyzed by scanning electron microscopy. Sol-gel-coated evanescent field sensors were investigated with respect to their capacity to suppress interfering water background absorptions, repeatability of dissolved analyte enrichment, and sensor response time. Nitrobenzene and parathion are the investigated analytes; figures of merit are derived from calibration curves determined to assess sensitivity and reproducibility of the developed sensor system. It can be concluded that sol-gel-coated infrared fiber-optic sensors enable reproducible detection of nitro-based aromatic compounds in the low ppm concentration range. Due to wide flexibility in tuning chemical properties of sol-gel films along with superior mechanical and chemical stability, organically modified sol-gels represent highly interesting coating materials for mid-infrared sensing applications.