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

Abstract

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.

Mass Sensors Based on Capacitive and Piezoelectric Micromachined Ultrasonic Transducers—CMUT and PMUT

“…As selectivity is a critical component for sensory performances, the sensing material cross-sensitivity can be addressed by using advanced microfabrication technology, wherein, individual CMUT cells can be configured in an array format [30]. In order to use CMUT configuration as a gas sensor, the top membrane can be coated by a polymer using different techniques including spin coating, inkjet dispensing, dip coating, inkjet printing, layer-by-layer deposition and electrodeposition [16,[31][32][33][34][35].…”

“…Analytes in the gas phase, such as carbon dioxide, sulfur dioxide, volatile organic compounds (VOCs) including acetone, methanol, ethanol, toluene, benzyl methyl ketone (BMK) and DMMP, and water, are viable for sensing based on mass changes of polymer films [4,27,38,39]. These gas-sensitive films comprising polymers such as poly(styrene-co-allyl alcohol), poly(vinylacetate), poly(capralactone), methylated poly(ethylene imine), poly (vinyl pyrrolidine) and poly {methyl [4-(2-hydroxy-4, 6-bistrifluoromethyl) phenyl] propylsiloxane} have been reported for detecting the analytes mentioned above [4,17,34,39,40]. Polymer properties such as specific chemical functional groups promote temporary weak interactions between analyte molecules and the polymer thus favoring physisorption.…”

“…Polymer properties such as specific chemical functional groups promote temporary weak interactions between analyte molecules and the polymer thus favoring physisorption. It has been shown that CO 2 physisorbs onto methylated poly(ethylene imine) via weak molecular coordination whereas SO 2 forms stronger irreversible hydrogen bonds with the amine group, which in turn degrades the polymer film [34].…”

Selective Ultrasonic Gravimetric Sensors Based on Capacitive Micromachined Ultrasound Transducer Structure—A Review

“…The main adsorption mechanism at the solid–gas interface, the main concepts, and thermodynamics are closely detailed in the reference [ 136 ], and Fourier transform infrared spectroscopy (FT-IR) can be used to investigate these gas absorption mechanisms [ 7 , 137 , 138 ].…”

“…Here, sensitivity describes the steepness of the main sensor function, usually characterized as a mass unit per the frequency unit. For this parameter, CMUT structure has little competition, since it was demonstrated by several groups to reach numbers down to 10 −18 g/Hz and below [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. As for the other two parameters, they are more related to the properties of the functional material of the sensor [ 1 , 7 , 9 , 10 , 11 , 12 ], and the topic of the functional materials suitable for SUGS remains highly underexplored.…”