Recent Progress on Nanomaterials for NO2 Surface Acoustic Wave Sensors

NO2 gas surface acoustic wave (SAW)sensors are under continuous development due to their high sensitivity, reliability, low cost and room temperature operation. Their integration ability with different receptor nanomaterials assures a boost in the performance of the sensors. Among the most exploited nano-materials for sensitive detection of NO2 gas molecules are carbon-based nanomaterials, metal oxide semiconductors, quantum dots, and conducting polymers. All these nanomaterials aim to create pores for NO2 gas adsorption or to enlarge the specific surface area with ultra-small nanoparticles that increase the active sites where NO2 gas molecules can diffuse. This review provides a general overview of NO2 gas SAW sensors, with a focus on the different sensors' configurations and their fabrication technology, on the nanomaterials used as sensitive NO2 layers and on the test methods for gas detection. The synthesis methods of sensing nanomaterials, their functionalization techniques, the mechanism of interaction between NO2 molecules and the sensing nanomaterials are presented and discussed.