Advances in sensing mechanisms and micro/nanostructured sensing layers for surface acoustic wave based gas sensors

Surface acoustic wave (SAW) technology has been extensively used in communications and highly sensitive sensing applications. For SAW based gas sensing applications, the sensitive material or sensing layer which is coated onto the SAW sensors is vital for its sensitivity, selectivity, limit of detection, and repeatability, as changes of sensing signals (including frequency, amplitude, and phase angle) are strongly linked to variations of mass loading, electrical conductivity and elastic modulus (or elastic loading) of this sensing layer. There are rapid development in this field recently but great challenges are still remained in choices of suitable sensing materials, structures and mechanisms of these sensing layers. This paper reviews recent advances of micro and nanostructured sensing materials and selections, their sensing mechanisms and designs towards enhancing the gas sensing performance of the SAW devices. We firstly discuss different sensing mechanisms based on SAW principles, along with key sensing influencing parameters. We then highlighted and categorized recently reported gas sensing materials into semiconductor metal oxides, carbon-based materials and polymers. We then focused the discussions of relationships among micro/nanostructures, compositions, and structure-sensing performance for the SAW based sensors. Finally, we highlighted the key challenges and potential solutions as well as future directions of sensing materials for SAW gas sensors.