Selective NH3 gas sensor based on Langmuir-Blodgett polypyrrole film

Polypyrrole thin films have been deposited onto a glass substrate by the Langmuir-Blodgett technique to fabricate a selective ammonia (NH3) gas sensor. The d.c. electrical resistance of the sensing elements is found to exhibit a specific increase upon exposure to different gases such as NH3, CO, CH4, H-2 in N-2 and pure O-2. The polypyrrole thin-film detector shows a considerable increase of resistance when exposed to NH3 in N-2, and negligible response when exposed to comparable concentrations of interfering gases such as CO, CH4, H-2 in N-2 and pure O-2. The calibration curve for NH3 in N-2 at room temperature is measured in the concentration range from 0.01 to 1%. The relative change of the electrical resistance is about 10% for the lower detectable limit of 100 ppm of NH3 in N-2. The sensitivity of the Langmuir-Blodgett polypyrrole towards ammonia is considerably higher than that of the electrochemical polypyrrole. The fast rise time and the high sensitivity of the detector are reported as a function of number of the polypyrrole layers. Long-term aging tests of the selective NH3 gas sensor are performed.