Wireless surface acoustic wave humidity sensor with chitosan/porous cyclodextrin–TiO2 composites for monitoring air and human respiration

Abstract

Wireless surface acoustic wave (SAW) technology can be used for real-time and contactless humidity measurements, as well as for monitoring human respiration and environmental humidity. In this study, we developed a wireless humidity sensor based on a 433 MHz SAW resonator covered with chitosan (CS)/heat-treated cyclodextrin(α-, β-, γ-CD)–TiO2 sensitive films. The mesoporous distribution and hydrophilicity of the CD–TiO2 composite, which affect the sensitivity of the CS/CD–TiO2/SAW sensor response to humidity, can be controlled through the CD type and TiO2 content of the composite. The frequency shift of the optimized CS/β-CD–TiO2/SAW sensor varied linearly with the relative humidity (RH) in the RH ranges of 0%76% and 76%90% with the corresponding sensitivities of − 2.96 and − 99.71 kHz%−1, respectively. The response and recovery times at 70% RH were 13.3 and 28.8 s, respectively, with an insignificant hysteresis of 0.61%. The excellent humidity response was due to the hydrophobicity of the inner cavity in the β-CD template molecule and the hydrogen bond adsorption between the surface of the CS/β-CD–TiO2 and H2O molecules. The response rates of the wireless SAW sensor system to normal air humidity and human exhaled/inhaled humidity were 3.13% and 30% RH s−1, respectively, and the wireless transmission distance was 30 m. The wireless CS/β-CD–TiO2/SAW humidity sensor has considerable application potential in forest fire early warning and firefighter tracking systems that require the remote monitoring of air humidity and human respiration.