Advances in nanostructured chemiresistive sensors for formaldehyde detection

Formaldehyde (HCHO) is deleterious both as an indoor volatile organic compound, contributing to respiratory and carcinogenic risks, and as an adulterant in food products such as fish, where ingestion poses severe systemic toxicity. Sub-ppm monitoring is essential, given that exposure limits are ≤0.1 ppm and adverse health effects emerge at 50-100 ppb. Chemiresistive sensors address this demand through an adsorption-driven modulation of surface depletion layers and charge-transfer dynamics, thereby enabling real-time, ultrasensitive detection. Owing to their architectural simplicity, miniaturisation potential, and cost-effectiveness, these sensors have emerged as a dominant platform for HCHO detection with detection limits of a few ppb, response times of 5-60 seconds and selectivity factors typically 5 times over interfering VOCs under optimised conditions. This review highlights recent progress in nanostructured materials as sensing layers of chemiresistive sensors including single and binary metal oxides, two-dimensional nanomaterials, conjugated polymers, and their heterostructural hybrids, emphasizing how surface chemistry, defect modulation, and heterointerface engineering enhance sensitivity (10⁴-10^6% ppm^-1), selectivity, operating temperature (from 250-350 °C to 80-150 °C), humidity resistance and detection limits. In addition, critical assessment of morphology modification (0D-3D architecture) and the role of noble-metal decoration in modulating surface charge transfer and accelerating adsorption-desorption kinetics typically shows a reduction of response/recovery cycles by 20-60%. Emerging strategies such as light activation, plasma treatment, vacancy engineering, and gas treatments to enhance sensor metrics have been studied for their role in stabilizing sensor performance in humid and variable environments. The integration of machine learning (ML) is also discussed as a complementary approach to improve selectivity (>90% class accuracy), optimizing feature extraction, and enabling predictive modelling of sensor behaviour across varying temperatures, humidities, and interfering gases. Finally, the review outlines current challenges and envisages future opportunities toward scalable, humidity-resilient, and intelligent chemiresistive platforms for next-generation HCHO monitoring. By uniting nanoscale material design with intelligent data analytics, next-generation chemiresistive sensors are poised to transition from laboratory prototypes to robust, scalable technologies that address urgent societal needs in environmental monitoring and public health.