Rational Design of Semiconductor‐Based Chemiresistors and their Libraries for Next‐Generation Artificial Olfaction

Abstract

With rapid progress in device integration and computing power, the realization of highly miniaturized one-chip artificial olfaction with superior performance is close and should be further supported by the rational design of chemical sensors and chemical sensing materials. The number of gas sensors tends to increase in order to sniff more complex odors with the progress of civilization. This explains the evolution from a single chemical sensor to complicated artificial olfaction using sensor arrays. At the advent of oxide chemiresistive gas sensors in the 1960s and 1970s, a single gas sensor was widely used to detect toxic, explosive, dangerous, and harmful gases, such as CO, C 3 H 8 , CH 4 , H 2 S, and NO 2 , in a selective manner. [26,27] However, a single sensor is often insufficient for recognizing most of the natural odors encountered in daily life, which consist of hundreds to thousands of different chemicals. [28] In the mammalian olfactory system, odorants are initially detected by olfactory receptors (ORs) covering the surface of the cilia projected from olfactory sensory neurons (OSNs), which are located in the olfactory epithelium lining the nasal cavity. These signals are transmitted to the glomeruli in the olfactory bulb, where a high degree of signal integration happens (Figure 1a). [29] Finally, the signals are sent through mitral cells to a higher brain region (olfactory cortex), and the signals are subsequently combined or modified in a variety of ways by parallel processing for analysis and interpretation (Figure 1b). [30] Each OR can detect various kinds of odorants, and similarly, each odorant can also be recognized by a number of ORs. That is, the olfactory system determines multiple odorants from various combinations of ORs (Figure 1c). [31] For better olfaction, both OSNs and ORs should be abundant. A larger number of OSNs is advantageous for detecting trace concentrations of analytes and ligands. [32] For instance, dogs with more OSNs are better than humans at detecting explosives, searching and rescuing, diagnosing disease, and assessing agricultural products. [33] If a mammal can detect larger numbers of faint gas components within odors, he can perceive and identify the odors more accurately. From this perspective, gas sensors with lower detection limits would be assembled to the stronger artificial olfaction. Kinds of ORs are also important. Mammals are known to have ≈1000 different kinds of ORs, and combinations of dissimilar ORs enable the discrimination of a myriad of odorants. [29] To mimic this, sensor arrays consisting of small number (5-20) of sensors that show Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machinelearning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable onc...

How Chemoresistive Sensors Can Learn from Heterogeneous Catalysis. Hints, Issues, and Perspectives

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