Wireless Surface Acoustic Wave Humidity Sensor With Chitosan/porous Cyclodextrin-TiO₂ Composites For Monitoring Air And Human Respiration

Humidity is the physical quantity that is most closely related to people in life.The measurement of humidity is particularly important both in daily life and in industrial manufacturing.Relative humidity（RH）is the ratio of absolute humidity to maximum humidity,and its value shows how high the saturation of water vapor is.Because of its low cost,small size,low energy consumption,sensitive response,large measuring range and strong anti-interference ability,saw sensor has gradually become the key research direction of the new generation of sensors.Wireless surface acoustic wave（SAW）technology can be used for real-time and non-contact humidity measurement,and can be used to monitor human respiration and environmental humidity.In this paper,a surface acoustic wave resonator with center frequency of 433 MHz based on chitosan（CS）/cyclodextrin（CD）-titanium dioxide（Ti O₂）composite as sensitive film is designed and fabricated,and a wireless humidity sensor is developed.The mesoporous distribution and hydrophilic composites of CD-Ti O₂in the sensitive membrane affect the sensitivity of CS/β-CD-Ti O₂/SAW.The response of SAW sensor to humidity can be controlled by CD type and Ti O₂content in the sensitive film.The frequency offset of the optimized CS/β-CD-Ti O₂/SAW humidity sensor varies linearly in 0%-76%RH and 76%-90%RH,and the sensitivity is-2.96 k Hz/%RH and-99.71 k Hz/%RH,respectively.The response and recovery time under 70%RH are 13.3 s and 28.8 s respectively,and the lag is 0.61%.There are two reasons for the excellent humidity response,one is that the heat-treatedβ-CD-Ti O₂soft template has a hydrophilic outer edge and a hydrophobic inner cavity to form a porous structure,and the other is that the surface of CS/β-CD-Ti O₂has a reversible hydrogen bond which can adsorb H₂O molecules.The response speed of wireless SAW humidity sensor system to normal air humidity and human respiratory humidity is 3.13%RH/s and 30%RH/s respectively,and the wireless transmission distance is 30 m.Wireless CS/β-CD-Ti O₂/SAW humidity sensors have great potential applications in scenarios requiring remote monitoring of air humidity and human respiration,such as intelligent agriculture,forest fire prevention,and respiratory monitoring of workers in special environments.