In this paper, a fiber optic sensing system, designed, and developed for the detection of ammonia in aqueous ambient at room temperature, is presented. The sensor is constituted by a standard silica optical fiber (SOF) coated by a tin dioxide sensitive layer. The SnO<sub>2</sub> films have been transferred onto the distal end of the SOF by means of the simple and low-cost electrostatic-spray-pyrolysis deposition technique. The spectral characterization of the fabricated samples has been carried out in the wavelength range 400–1750 nm in order to estimate the thickness of the SnO<sub>2</sub> fiber coatings. The morphology and the elemental composition of the deposited layers have also been investigated by means of scanning-electron-microscopy observation and energy-dispersive-spectrometer analysis, respectively. Single-wavelength reflectance measurements have been carried out to test the sensing performances of the realized sensors toward ammonia traces in water. A fiber-Bragg-grating temperature sensor has also been used for monitoring the temperature changes occurring inside the test ambient during the experimental measurements, in order to identify the effects of thermal drifts on the sensor response. The results here presented demonstrate that the developed refractometric chemical sensor is able to provide measurements of ammonia concentration in water and at room temperature with a high sensitivity, response times of few minutes, and a resolution as low as 2 ppm.
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