No abstract

A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO2) and sulfur dioxide (SO2) mixtures in nitrogen. The created measurement setup of continuous data collection, integrated with an in-situ Fourier Transform Infrared (FT-IR) spectroscopy, allows for better understanding of the mechanisms and molecular interactions with the sensing layer (methylated poly(ethylene)imine) and its need of surface functionalization for multiple gas detection. During experimentation with CO2 gases, weak molecular interactions were observed in spectroscopy data. Linear sensor response to frequency shift was observed with CO2 concentrations ranging from 0.16 vol % to 1 vol %. Moreover, the Raman and FT-IR spectroscopy data showed much stronger SO2 and the polymer interactions, molecules were bound by stronger forces and irreversibly changed the polymer film properties. However, the sensor change in resonance frequency in the tested region of 1 vol % to 5 vol % SO2 showed a linear response. This effect changed not only the device resonance frequency but also affected the magnitude of electroacoustic impedance which was used for differentiating the gas mixture of CO2, SO2, in dry N2.

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.