Microwave humidity sensor based on carbon dots-decorated MOF-derived porous Co₃O₄ for breath monitoring and finger moisture detection

This research investigates a microwave transduction-based humidity sensor that is a promising candidate for real-time clinical healthcare applications and green miniaturized wearable electronic devices. Optimization of sensing material, sensing platform, and device fabrication techniques produces a carbon dots (CDs)-decorated metal organic framework (MOF)-derived porous Co₃O₄ (CDs-Co₃O₄) microwave resonator-based sensor with excellent real-time humidity detection. Inspired by the water absorption component polyacrylamide in baby diapers, the acrylamide is adopted to synthesize CDs for microwave humidity sensor. Combining CDs with MOF-derived porous Co₃O₄ enhances humidity sensitivity under microwave excitation, with a frequency shift of 3.40 MHz/% RH and a loss variation of 0.15 dB/% RH between 5% and 99% RH. These values are 49.7% (for frequency shift) and 20.5% (for return loss) higher than Co₃O₄ sensor. Moreover, CDs-Co₃O₄ exhibits high selectivity towards water vapor against other volatile organic compounds, and the response or recovery time are both less than 5 s. Fabricated by an integrated passive device technology, the sensing platform is miniaturized at 0.98 × 0.80 × 0.22 mm³ with superb device stability and reliability. The CDs-Co₃O₄ sensor remarkably monitors respiratory patterns of breathing or apnea, as well as subtle changes in the humidity levels of an approaching finger. A charge transfer process and microwave interactions are the mechanisms for improved humidity sensitivity.