Detection of ammonia in the ppt range based on a composite optical waveguide pH sensor

An optical waveguide (OWG) pH sensor with two thin guiding layers (composite OWG) was fabricated, and its application to sensing extremely low concentrations of ammonia was demonstrated. The highly sensitive element based on a titanium dioxide (TiO2) film was deposited onto the surface of a potassium ion (K+) exchanged glass OWG by RF sputtering. The surface of the TiO2 film was coated with a thin film of a pH indicator dye (bromothymol blue, BTB) by spin coating. With optimum thickness of BTB film at about 46 nm and of TiO2 films at 18–20 nm, this system proved to be an extremely sensitive ammonia sensor. The experimental results of the optimum conditions on BTB and TiO2 film thicknesses were close to theoretically calculated values. The sensor easily detected 1 parts per trillion (ppt) ammonia reversibly, and had a short response time. The present sensor is also characterized by low cost, simple structure and facile fabrication.