Functionalization layers for CO2 sensing using capacitive micromachined ultrasonic transducers

Abstract Sensing of carbon dioxide (CO 2 ) using inexpensive, miniaturized, and highly sensitive sensors is of great interest for environmental and consumer applications. In this paper, we present four functionalization layers that are suitable for resonant sensors based on mass-loading for CO 2 detection. We compare the volume sensitivities of these layers to CO 2 and relative humidity (RH) by using a highly sensitive 50-MHz capacitive micromachined ultrasonic transducer (CMUT) as a resonant sensor. Among the four functionalization layers, the layer based on a guanidine polymer exhibits the highest volume sensitivity to CO 2 of 1.0 ppm/Hz in N 2 and 3.8 ppm/Hz in air (∼45%RH). Furthermore, we report on other important characteristics of the guanidine polymer for sensing applications, including polymer saturation, regeneration, and repeatability.

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