Surface acoustic wave humidity sensors based on Al-doped ZnO ultrathin nanosheets architecture and their applications for skin moisture detection

Surface acoustic wave (SAW) humidity sensor is considered one of the most potential sensors for the monitoring of the skin moisture. Herein, we proposed a SAW humidity sensor based on Al-doped ZnO ultrathin nanosheets (Al-ZnOUNSs) architecture as sensing film for the monitoring of skin humidity. A SAW resonator with quartz piezoelectric substrate and aluminum electrodes was chosen as a basic device and AlN film as protection layer was deposited on the SAW resonator by magnetron sputtering. The Al-ZnOUNSs with high specific surface area, more channels, and numerous defects as active sites for gas adsorption and diffusion were grown on the AlN film by hydrothermal synthesis. High sensitivity is reached at high humidity for the Al-ZnOUNSs/AlN/SAW humidity sensor. The fast response and recovery times are found to be of order ∼11 s and 3 s within 10 ̶ 70 % RH for the sensor. The sensor also displays superior repeatability, selectivity, and long-time stability after 30 days. The involved gas sensing mechanism has been also discussed by theoretical calculation, Grotthuss chain reaction mechanism, and the mass loading effect of SAW. Moreover, the proposed sensor can quickly monitor the movement of fingertip, detect the skin moisture of various parts of human hand under different physical conditions, and capture changes of fingertip moisture after drinking water and applying emollient cosmetics. The proposed SAW humidity sensor for the monitoring of skin moisture emerges promising potentials for the assessment of metabolic and physiological conditions, the wound healing process, and the effectiveness of dermatocosmetic products.