Simultaneous humidity and temperature measurement with micropillar enhanced QCM sensors

In this study, we propose using a polymethylmethacrylate (PMMA) micropillar-based quartz crystal microbalance (micropillar-QCM) sensor for simultaneous measurement of exhaled breath temperature (EBT) and relative humidity (RH). The micropillar-QCM device, fabricated by nanoimprinting lithography, exhibits enhanced sensitivity due to the unique resonance phenomena formed between the vibrating micropillars and quartz substrate. The resonance frequency of the micropillar-QCM shifts in response to changes in the mechanical property - Young's modulus of PMMA due to the temperature and humidity variations. Experimental results demonstrate that the frequency shift is linearly dependent on the temperature change within the range of 25-35 degrees C and follows a power law relationship with relative humidity (RH) within the range of 50%- 90%. The sensor exhibits excellent sensitivity in temperature and humidity measurements with limit of detections (LODs) of 0.09 degrees C and 0.35% respectively. In addition, the temperature change has little effect on the bandwidth of the signal of micropillar-QCM while its bandwidth decreases linearly with the increasing relative humidity. As a result, the combination of the resonance frequency shift and bandwidth of the micropillar-QCM sensor can be utilized to predict the temperature and relative humidity simultaneously. The finding highlights the potential of the micropillar-QCM sensor for applications requiring accurate and real-time temperature and humidity measurements, such as breath analysis, environmental monitoring, and healthcare.