Gas sensing properties of Langmuir-Blodgett polypyrrole film investigated by surface acoustic waves

The gas sensing properties of organic polypyrrole (PPy) film, deposited onto LiNbO3 substrate by Langmuir-Blodgett (LB) technique, have been monitored by surface acoustic wave (SAW) delay lines and studied with respect to sensitivity, selectivity, response time, stability, repeatability, and aging. The SAW PPy elements demonstrate high sensitivity toward NH3 gas with high selectivity against CH4, CO, H-2, and O-2. The detectable threshold concentration has been estimated as 20 ppm NH3 in air; the response time is in the 10s seconds range, and the recovery time is about 15 min; the repeatability of the SAW response toward eight sequential NH3 gas exposures is within 6%; the aging of the PPy film is within 4% over a month; and the effect of humidity on SAW NH3 gas response is negligible for the typical conditions at room ambient air. Partially reversible SAW response recognizing NH3 gas as one component of an interfering gases-mixture has been observed. Simultaneous chemoresponses of SAW phase and insertion loss have been performed in order to investigate the sensing mechanisms. By merging with electrical conductivity gas response, the dominant SAW sensing effects for NH3 gas detection are defined as elastic loading.