Chemical sensing in spatial/temporal domains.

Abstract

Sensors are devices to receive physical or chemical signals and to convert them into electrical signals. Physical signals are carried by waves such as electromagnetic, optical, and acoustic ones. Sensing technology for physical signals has been well-understood and has already been established. On the other hand, sensing technology for chemical signals, which are carried by chemical substances, is not matured. It is thought that chemical substances are moved by molecular diffusion. However, diffusion velocities of gas molecules are too slow to transport chemical signals under many conditions. The transport of chemical substances onto chemical sensors is actually governed by fluid dynamics. Fluid dynamics offers two aspects such as a signal in spatial domain and one in time domain. A signal in spatial domain is tightly coupled with a plume, a flowing trail of a chemical substance. Although the electromagnetic and acoustic waves mainly propagate straight and their behaviors are easily predicted, it is difficult to predict the behavior of the plume. Thus, it is helpful to see the plume dynamics so that people can understand the plume behavior in spatial domain. Then, the gas distribution can be measured using a homogeneous sensor array. The two types of sensor arrays such as sparse and packed sensor arrays are available. The sparse sensor array can show the global behavior of the plume, whereas the packed one reveals the local detailed behavior of the plume. Although the measurement of gas distribution is the typical method to reveal the chemical-signal behavior in spatial domain, one of the recent topics is the plume generated in a virtual environment, where people perceive sensory stimuli even if they do not stay in the actual environment. In virtual reality, people can perceive an object with smell. The direction to an odor source, the feeling of approaching or going away from it, might be realized even if the actual smell source is not in front of people; it is difficult for a chemical sensor to follow the true dynamic concentration change of the chemical substance. Generally, temporal behavior of a chemical sensor has not been well-studied in comparison with steady-state response. However, the temporal signal sometimes has information of chemical substance. Thus, the technique to know the sensor dynamics such as time constant is required. In some cases, time constant must be obtained even if the concentration profile is irregular and is not known. In some cases, the peaks of the chemical signal over time provide information of chemical substance. The temporal data from preconcentrator is also useful to obtain information on the chemical substance. In addition to raising the sensitivity, the preconcentrator with variable temperature can be used to enhance the pattern separation among odor samples. The sensing in both spatial and time domains is complicated. Although there have been many works in a single domain, a limited number of works have been addressed to combining both domains. The straightforward method to understand the combination of both domains is to observe change in spatial distribution with time. Another approach * Corresponding author. Tel.: +81-3-5734-2579. Fax: +81-3-5734-2828. E-mail: nakamoto@mn.ee.titech.ac.jp. † Tokyo Institute of Technology. ‡ Tokyo University of Agriculture and Technology. 680 Chem. Rev. 2008, 108, 680−704

DOI: 10.1021/cr068117e

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Cite this paper

@article{Nakamoto2008ChemicalSI, title={Chemical sensing in spatial/temporal domains.}, author={Takamichi Nakamoto and Hiroshi Ishida}, journal={Chemical reviews}, year={2008}, volume={108 2}, pages={680-704} }