Magnetoelastic resonators functionalized with metal–organic framework water harvesters as wireless humidity sensors

Relative humidity is a crucial parameter in several fields, such as air quality monitoring, food storage, or control of industrial processes. In this study, we propose a wireless humidity sensor based on magnetoelastic resonance sensors functionalized with Metal–Organic Frameworks (MOFs). Different MOF water harvesters were tested as sensor active coatings: MOF-801, MOF-808, UiO-66-NH2, Al-Fumarate, and CAU-23. Their water absorption capacity and overall performance, when integrated into the sensors, were evaluated. As expected, the selected MOFs showed promising water harvesting capacity, enabling a successful sensor response to humidity in a wide range of relative humidity: 3%–85%. These humidity sensors showed a great sensitivity, stability, and selectivity to water molecules. The response time of the devices was around 15 s, with stabilization and recovery times in adsorption and desorption processes of less than 1 min for certain ranges of operation. In addition to the satisfactory behavior as humidity sensors, magnetoelastic resonators have emerged as a promising tool for the characterization of the dynamic adsorption capacity of MOF materials, as they easily provide a quantitative measure of the water mass adsorbed by the material.