Near-Room-Temperature CO₂ Capacitive Sensors Based on Hybrid Polymer Sensitive Materials

Driven by growing concerns over the health hazards linked to carbon dioxide emissions, the detection of this gas is garnering increasing attention. Continued advancements in air quality monitoring (AQM) technologies underscore the necessity for low-power gas sensors without sacrificing stability and sensor performance. Polymer-based sensing materials seem to be a very promising prospect for gas sensing at room temperature, offering key improvements over their counterparts, thanks to their lower operating temperature. Polyethylenimine (PEI), a hydrophilic cationic polymer has been studied extensively for its excellent carbon dioxide adsorption properties due to it being rich in amine groups. The use of silicate materials in conjunction with this polymer has been proven to further enhance its sensing capabilities. This study reports on the fabrication of the interdigitated electrode (IDE)-based capacitive sensor, as well as the sensing capabilities facilitated by the PEI-SiO2 nanoparticles (NPs) sensing layer. Specialized gas characterizations were carried out in a controlled environment simulating real-life operating conditions (RH =25%, 50%), and lower sensing temperatures (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T} = 55~^{\circ }$ </tex-math></inline-formula> C, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30~^{\circ }$ </tex-math></inline-formula> C). With CO2 concentrations ranging from 250 to 2000 ppm. Under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$55~^{\circ }$ </tex-math></inline-formula> C, the response demonstrated an overall capacitance change of about 52% for 2000 ppm of CO2 with a short response and recovery times of about 55 and 45 s, respectively. The sensor also demonstrated good reversibility and stability.