Hypothesis: A novel slab optical waveguide sensor exploiting an ionic liquid (trihexyl(tetradecyl)phospho nium chloride)-based film can be used to detect gaseous ammonia. Bromothymol blue (BTB) incorporated into the ethyl cellulose-supported ionic liquid film can absorb the evanescent wave generated in the sensor and act as an indicator. The diffusion of gaseous ammonia across the sensing film is thus expected to change the absorbance of bromothymol blue, providing the basis for sensitive ammonia detection. Experiments: The above sensor was exposed to different concentrations of ammonia gas, showing fast and reproducible responses and being superior to previously reported waveguiding-based sensors. Moreover, the fabricated system was used to detect ammonia in the breath of human volunteers. Findings: Under optimal conditions, gaseous ammonia was accurately detected in the range of 100-1800 ppb, with a detection limit of 69 ppb (3 sigma/s). The developed sensor was re-usable due to exhibiting reversible gas sorption/desorption dynamics and was successfully applied to the determination of ammonia content in human breath. (C) 2017 Elsevier Inc. All rights reserved.