Humidity-resilient, room-temperature NO2 sensing using activated carbon functionalized with gold nanoparticles

Growing concern over air pollution, particularly nitrogen dioxide (NO₂) and its adverse effects on human health, has intensified the demand for sensitive, low-power gas sensors capable of trace-level detection. This study systematically investigates the NO₂ sensing performance of bio-based activated carbon (L3S) and gold nanoparticle-functionalized hybrids (L3S-Au NPs) at 25°C. Successful Au NP decoration on L3S was confirmed by TEM, XRD and XPS analyses. Pristine and Au-modified sensors were exposed to NO₂ concentrations ranging from 1 to 10 ppm in 1 ppm increments. Au functionalization significantly enhanced sensor response, sensitivity, and response-recovery dynamics. Sensing performance increased with Au loading, reaching an optimum at 2 wt.% Au, which exhibited a response twice that of pristine L3S at 10 ppm NO₂. This enhancement is attributed to the synergistic catalytic and electronic sensitization effects of Au NPs, which promote NO₂ adsorption, accelerate charge transfer, and stabilize the p-type conduction of L3S. Importantly, the optimized L3S-Au2 sensor demonstrated excellent humidity tolerance, maintaining relatively stable responses (6.6–6.8%) over a broad relative humidity range of 0–87%. These findings identify L3S-Au hybrids as sustainable, cost-effective, and energy-efficient materials for robust room temperature NO₂ detection, with strong potential for environmental and industrial monitoring applications.