α-Co(OH)${}_2$-based quartz crystal microbalance humidity sensor for real-time monitoring of respiration

Respiration is an important physiological process of the human body and can assess an individual's health status. Humidity detection is an effective method for real-time breath monitoring. Herein, the α-Co(OH)${}_2$-based quartz crystal microbalance humidity sensor (Co-QCM sensor) is prepared via the hydrothermal method. The Co-QCM sensor is characterized by scanning electron microscopy, Fourier-transform infrared, and X-ray diffraction. The α-Co(OH)${}_2$ with hierarchical nanostructure and hydroxy functional groups leads to good humidity-sensing performance. The Co-QCM sensor exhibits high sensitivity (23.1 Hz/% relative humidity), fast response and recovery times (within 3 and 4 s, respectively), high linearity (R${}^2$ = 0.994), and long-term stability (≈30 days) in the humidity range of 4-73.3%. In addition, different human breathing signals, including free breathing, shallow breathing, and deep breathing, can be detected and distinguished by this humidity sensor, showing the potential application in human respiratory detection. (© 2023 Wiley‐VCH GmbH)