Effect of the Sol-Gel Matrix on the Performance of Ammonia Fluorosensors Based on Energy Transfer

We report on the effect of organomodified sol-gel materials on fluorosensors for ammonia. The fluorosensors are based on ion pairs composed of an inert fluorophore and a pH-sensitive absorber dye and are embedded in sol-gel glass. Upon exposure to ammonia, deprotonation of the pH-sensitive dye bromophenolblue occurrs, and consequently, energy is transferred from the fluorophore rhodamine B or tetramethylrhodamine ethyl ester to the absorber. The response of the fluorosensors using different ratios of precursors, such as tetramethoxysilane and phenyltrimethoxysilane, is investigated. Detection limits for sol-gel layers composed of 50% tetramethoxysilane and 50% phenyltrimethoxysilane are as low as 0.1 mg/L of aqueous ammonia. Response times are of the order of 3 to 6 min for forward response. The reversibility of the sensor is related to the composition of the organically modified sol-gel glasses and is fastest for sensor layers composed of pure phenyltrimethoxysilane. Conditioning, regeneration, and storage of the layers are shown to be of vital importance for the performance of the sensor layers.