Properties of xerogel layers for the detection of toluene in water

This paper presents results of experimental investigation of optical properties of xerogel layers applied on silica optical fibers and their sensitivity to toluene dissolved in water. Several types of layers, namely the layers prepared from sols based on tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and of a mixture of methyltriethoxysilane and phenyltriethoxysilane (MTES/PTES) were applied on the bare core of silica fibers by the dip-coating method. For comparison, thin layers of a polydimethylsiloxane (PDMS) polymer were also applied on the fibers from toluene solutions of the prepolymer.Properties of the layers were characterized by measuring angular distributions of optical power from the fiber and by time and spectral responses of the layers to toluene dissolved in water. The angular and time response characteristics were measured at 670 nm, the spectral changes were measured in a wavelength range of 1600–1800 nm. Inclined beams were used for fiber excitation in order to increase measurement accuracy.The experimental results obtained show that the MTES-based xerogel layer exhibits higher sensitivity to toluene dissolved in water than the PTES-based or TEOS-based one. With the MTES-based layers a detection limit of about 1 ppm (5 mg/l) was achieved. The spectral measurements provided experimental evidence of the presence of toluene in the layers.