Passive wireless surface acoustic wave CO2 sensor with carbon nanotube nanocomposite as an interface layer

A passive wireless CO2 sensing system based on surface acoustic wave (SAW) device and carbon nanotube nanocomposite thin film was studied. The electrical resistance change of the carbon nanotube nanocomposite thin film was examined first, and about 10% resistance increase was observed when the thin film is exposed to pure CO2. The frequency change for SAW sensor coated with nanocomposites film was around 0.03% when the device is exposed to pure CO2. However, the performance of the CO2 SAW sensor is severely influenced by environmental humidity. With additional ultrathin parylene film packaging, the SAW sensor frequency change from relative humidity of 0-100% at room temperature decreased from over 0.1% to less than 0.01%. It was found that the lowest detection limit of the CO2 SAW sensor is 1% CO2 concentration, with 0.0036% frequency change. Wireless module was tested and showed promising transmission distance at preferred parallel orientation.