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.