A flexible room-temperature CO ₂ sensor based on ethylenediamine-functionalized graphene oxide and polypyrrole microparticles composite film

A novel flexible, room-temperature chemiresistive CO₂ sensor was developed by coating a composite film of polypyrrole microparticles (PPy MPs) and ethylenediamine-functionalized graphene oxide (EDA-GO/PPy MPs) on a polyethylene terephthalate (PET) substrate. The microstructural and chemical features of the composite were characterized by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). The effects of amino functionalization, PPy incorporation, and ambient humidity on electrical behavior and CO₂-sensing performance were systematically evaluated. The EDA-GO/PPy MPs composite exhibited nearly tenfold higher response than pristine GO at 5000 ppm CO₂ in wet air. Quartz crystal microbalance (QCM) measurements demonstrated that the enhanced performance arises from selective CO₂ adsorption <i>via</i> hard-soft acid-base interactions with amino groups and efficient electron transport through PPy MPs, which reduce material resistance. The CO₂-sensing mechanism involves adsorption of CO₂ onto amino-functionalized sites and formation of carbonates, resulting in resistance changes in the p-type composite. The sensor demonstrated high sensitivity, rapid response and recovery, mechanical flexibility, high selectivity, and long-term stability under ambient conditions. This work not only demonstrates a mechanistically informed approach for tuning gas-material interactions but also provides a practical design strategy for next-generation flexible CO₂ sensors with potential applications in wearable electronics, indoor air-quality monitoring, and low-power IoT devices.