Surface acoustic wave sensor based on Au/TiO₂/PEDOT with dual response to carbon dioxide and humidity

A surface acoustic wave (SAW) gas sensor with an Au/TiO₂/poly(3,4-ethylenedioxythiophene) (PEDOT, which is a conductive polymer with photoelectric conversion function) sensing film was constructed for the quantitative detection of water vapor and CO₂. The Au/TiO₂/PEDOT sensing film was assembled on the delayed region of the 204 MHz SAW delay line, which was used as the base device for the gas sensor. The center frequency of the sensor decreases with an increase in relative humidity (RH), and the center frequency increases with increasing CO₂ concentration, so that not only can the two gases be identified, but quantitative analysis can also be performed. The SAW sensor has a response range of 5%-90% for RH and a response range of 500-2000 ppm for CO₂ gas. The shifts in center frequency varied linearly with the concentrations, giving rise to the sensitivities of -0.0068 and -0.1880 kHz %^-1 for RH and ∼0.003 kHz ppm^-1 CO₂. The response/recovery time is 9 s/9.2 s for 700 ppm CO₂ and 15 s/14 s for 70% RH. The experimental results show that the SAW sensor offers excellent selectivity, wide response range, rapid response, and good stability and repeatability. The mechanism of humidity and CO₂ sensing is attributed to the hydrophilic porous structure of the Au/TiO₂/PEDOT sensing film, and also to the reversible variation of its viscoelasticity under illumination conditions. The sensor, combined with the communication function of its own SAW device, has several prospective applications in the monitoring of atmospheric conditions.