An optical detection NH₃ gas by means of a-WO₃ thin films based on SPR technique*

Abstract
An optical sensor for ammonia gas, based on the surface plasmon resonance (SPR) method has been investigated. The surface plasmon resonance (SPR) is very sensitive, and so the optical technique used in chemical sensing. The angle of incident of light at which a resonant effect is observed, as well as the dip of a resonant are very sensitive to variations of the optical parameters of the medium on the surface active-plasmon metal layer. The sensing structure was made as follows. The gold layers were coated by means of vacuum evaporation on a substrate, 1 mm thick, made of BK7 glass slide. The thickness of Au was about 48 nm. An active layer of WO₃ was deposited by thermal evaporation on the gold film. The sensor structure was coupled on immersion oil with a prism coupler. A change of the intensity of light of the plasmon dip was observed when chemical active films (WO₃) were exposed to varying concentrations of NH₃. Optical ammonia gas sensors display a very fast response time and a fast regeneration time at room temperature.