CO₂ and SO₂ Interactions with Methylated Poly(ethylenimine)-Functionalized Capacitive Micromachined Ultrasonic Transducers (CMUTs): Gas Sensing and Degradation Mechanism

A capacitive micromachined ultrasonic transducer (CMUT)-based sensor modified with methylated poly(ethylenimine) (mPEI) was designed and tested for the detection of two acidic gases: carbon dioxide (CO2) and sulfur dioxide (SO2). Combined gas sensing and Fourier transform infrared spectroscopy of the adsorbed products allowed to simultaneously and in situ determine the types and strength of the molecular interactions responsible for sensing. For CO2, the limit of detection was calculated to be 0.011 CO2 vol % and the limit of quantification was calculated to be 0.033%. For SO2, the limit of detection was calculated as 0.232 SO2 vol % and the limit of quantification was 0.704%. The sensing system exhibited a linear response at lower concentrations for CO2 and linear response for all the tested concentrations of SO2. In situ IR and ex situ Raman showed that CO2 was observed to undergo weak molecular coordination with mPEI while SO2 bound strongly and irreversibly degraded the thin mPEI films.