Highly selective capacitive micromachined ultrasonic transducer–based miniature gravimetric CO2 sensor with in-situ calibration for relative humidity

The prototype gravimetric microelectromechanical imine functionalized CO2 sensor equipped with custom electronics and significantly increased stability under sub-percent concentrations is investigated. Instabilities caused by the relative air humidity were eliminated by two in-situ calibration methods: two-channel readings correction and single-channel correction by the reference disturbance. Capacitive micromachined ultrasound transducers (CMUT) were designed for 15 MHz nominal resonance frequency, while actual sensor readings were lower (over 12 MHz) due to the functionalization. The resonance frequency of polyethyleneimine (PEI) functionalized CMUT during sub-percent CO2 detection experienced instabilities of several hundred Hz while the relative humidity of the air was changed between 10% and 35%. Readings of graphene oxide (GOx) functionalized CMUT working in parallel with PEI functionalized CMUT were used for two-channel correction, resulting in highly stable sensor output, characterized by 3.2 Hz standard deviation during almost one hour experiment, while relative humidity varied between 10% and 25%. Single channel reading correction method resulted in larger residual instability, characterized by a standard deviation of up to 36 Hz over a two-hour experiment, however, it was found to be more effective for longer humidity-induced drifts, since reference disturbance of known relative humidity of 75% maintained by saturated NaCl solution technique was used.