A Highly Linear and Sensitive QCM Humidity Sensor Based on High-Frequency Quartz Crystal Transducer

Sensitivity and linearity are two crucial parameters in the context of quartz crystal microbalance (QCM) humidity sensors. Achieving a highly sensitive response in a QCM-based humidity sensor involves employing a significant mass of humidity-sensitive material; however, this approach may introduce a viscosity effect, leading to a nonlinear response from the sensor. The Sauerbrey equation suggests that a QCM device exhibits a linear mass-frequency response when a micro-mass is deposited on its surface. In this study, we present a design strategy for a QCM humidity sensor based on a small-size, high-frequency QCM transducer. A minimal mass ( $1 ~\mu \text{g}$ ) of graphene oxide (GO) serves as the moisture-sensitive material and is deposited on a 16-MHz QCM device enclosed in a 5032 package, resulting in the fabrication of a QCM humidity sensor. The sensor demonstrates both high sensitivity (82.40 Hz/%RH) and high linearity (adjusted R-square: 0.9892) across a relative humidity (RH) range of 11%–97%; furthermore, the sensor exhibits low humidity hysteresis (2.5% RH), excellent stability, and repeatability. The methodology proposed in this article is anticipated to contribute to the development of high-performance QCM humidity sensors.