PEI-SnO₂-MWCNT Functionalized CMUT Resonant Gas Sensor for Formaldehyde Sensing with Sub-ppm Detection Limit

Formaldehyde, a pervasive indoor and outdoor pollutant extensively used in household and industrial applications, poses substantial health risks involving respiratory issues and DNA damage. Most countries regulate their legal levels in public spaces and workplaces, with the World Health Organization recommending an indoor guideline value of 0.1 mg·m^-3. Although various formaldehyde detection methods exist, most require a laboratory setup and skilled personnel, highlighting the need for lightweight, portable sensors. Microelectromechanical systems (MEMS) and nano/microscale resonant sensors show promise for efficient gas detection. However, achieving a high level of selectivity and sensitivity in resonant gas sensing technologies remains challenging, necessitating advanced tuning for specific gas detection while minimizing interference. Addressing the demand for miniature gas sensors in environmental protection and chemical production, this paper investigates gas sensors based on capacitive micromachined ultrasound transducers (CMUTs). We focus on sensitive materials and CMUT functionalization technology for formaldehyde gas detection. Sensing materials are synthesized using polyetherimide (PEI), SnO₂ nanoparticles, and multiwalled carbon nanotubes. A spin-coating method is proposed to functionalize CMUTs-based chips, and the surface structure characteristics of these chips are studied. This study validates the feasibility of the synthesized sensing materials and the proposed functionalization strategy. The sensor exhibits significant performance in detecting HCHO gas, with a sensitivity of 95.5 Hz·ppm^-1 for concentrations from 1 to 20 ppm, achieving a limit of detection of 250 ppb.