Doped Silica Sol Layer Coatings on Evanescent Field Fiber Bragg Gratings for Optical Detection of Nitroaromate Based Explosives

Evanescent field etched fiber Bragg gratings (eFBG) were prepared with doped and non-doped nanoporous silica sol coatings for optical detection of the nitroaromatic explosives 2,4-dinitrotoluene (DNT), 1,3-dinitrobenzene (DNB) and trinitrotoluene (TNT). We used 3-aminopropyltrimethoxysilane (APTMS) and tetrabutylammonium hydroxide based receptors (TBAH, C16H37NO) as dopants in order to increase the specific identification of the nitroaromates. In nitroaromatic trace gas atmosphere the induced wavelength shift of the Bragg reflection signal was highest for the sensor with TBAH-doped silica sol receptor coating. Within the first two minutes during exposure the change of the signal intensity, reflecting the adsorption dynamics of the nitroaromatic explosives, was most significant and characteristically depending on the type of receptor dopant used. The cross sensitivities of the sensor with TBAH-doped silica sol receptor coating with four common solvents like acetone, ethanol, 2propanol and tetrahydrofuran (THF) in comparison to the sensitivity of the tested explosives was more than six orders of magnitude (10(6)) lower than that of solvents, indicating a very high selectivity of the sensor. By applying a simplified diffusion model correlated to a series of measurements with different distances to the target we estimated the limit of detection for a sensor coated with TBAH doped silica sol with 35.8 +/- 0.9 ppb for DNB and 140.2 +/- 1.8 ppb for DNT in air at room temperature. Using a more etched FBG (but mechanically less robust) sensor we were able to detect TNT as well at concentrations about 5 ppb at room temperature in air. A major result of our investigations was to demonstrate, that the nanoporous silica sol host matrix is suited for fast adapting with low technological effort the affinity of the receptor coating for silica-based evanescent field sensors. For future applications a set of sensors of silica-sol coatings with different dopants in combination with pattern recognition of the sensor signals could be used for clear identification of explosives.