Quartz Crystal Microbalance (QCM) coated with polyetherimide (PEI) by spin coating method was applied for carbon dioxide (CO<sub>2</sub>) gas detection at room temperature in this study. The experiments were performed in dry and humid air atmospheres, and the results revealed that the prepared CO2 sensor in moisture circumstance exhibited a larger sensing response than that at dry condition. An enhanced sensing response took place for CO<sub>2</sub> detection with the existence of water molecules. The effect of different humidity on QCM sensor performances was investigated as well in this paper. A curve, which displayed a proportional relationship between sensing response and water vapor concentration, was obtained. Moreover, the relevant sensing mechanisms were investigated.
In this paper, Multi-walled carbon nanotubes (MWCNTs)-tin oxide (SnO<sub>2</sub>) composites were selected as sensing materials to detect NO<sub>2</sub> with concentrations ranging from 10 ppm to 50 ppm. Two sensors with different mass fractions of SnO<sub>2</sub> and MWCNTs, i.e., 6.7% and 10%, were fabricated by airbrushing the composites on interdigitated electrodes (IDEs). Response performances of both sensors at room temperature were investigated. Results showed that the sensor with mass fraction of 10% exhibited a larger sensing response and a bigger sensitivity than the other one, and a good linearity was observed for both ones. In addition, both sensors had a good selectivity to NO<sub>2</sub> in comparison with the other five interfering gases.