"Smelltronics"-From Gas to Smell Sensing.

This review discusses recent trends in smell and odor-sensing studies, including materials, devices, and interfaces. Since the invention of chemical sensors, particularly over the past few decades, gas sensor materials and devices for small molecules have been extensively investigated. Recently, the social value of odor information has been strongly reaffirmed across diverse research fields, triggered by the rapid development of artificial intelligence (AI) and machine learning (ML). Compared with conventional gas sensing, the requirements for sensing materials and devices for extracting odor information are fundamentally different. In the fields of health diagnosis, food inspection, and environmental monitoring, target molecules that express odor information are frequently composed of molecules with relatively large molecular weights. Because the structural isomers of such relatively large molecules essentially exhibit variations in odor information, their discrimination of such structural isomers is an important issue for smell/odor sensing. Various sensors that operate based on different principles, including electrical, gravimetric, and optical sensors, were discussed in terms of their characteristics. In addition, we describe the sensing materials and interfaces for smell/odor sensing-smelltronics. Finally, we discuss the current challenges and perspectives of smelltronics.