A novel wireless, passive CO₂sensor incorporating a surface acoustic wave reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed for CO 2 detection. The developed SAW gas sensor is composed of single phase unidirectional transducers (SPUDTs), three shorted grating reflectors, and CO 2 -sensitive polymer film on 41° YX LiNbO 3 substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted design parameters, the SAW device was fabricated. Teflon AF 2400 was used as the sensitive film because it provides high CO 2 solubility, permeability and selectivity. In wireless device testing using a network analyzer, four sharp reflection peaks with high signal-to-noise (S/N) ratio, small signal attenuation, and few spurious peaks were observed in the time domain. The time positions of the reflection peaks were well matched with the predicted values from the simulation. Infusion of CO 2 into the chamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observed for a CO 2 concentration of 0–450 ppm. The obtained sensitivity was 1.98° ppm −1 . Temperature and humidity effects were also investigated during the sensitivity evaluation process.