Toward Nanometer-Scale Sensing Systems: Natural and Artificial Noses as Models for Ultra-Small, Ultra-Dense Sensing Systems

The development of highly sensitive, selective, reliable, and compact sensing systems to detect toxic chemical and biological agents is of great importance to national security. This paper examines the best such naturally occurring sensing system, the sense of smell or "olfaction," as well as artificial sensing systems built to emulate the nose. The goal is to combine lessons learned from natural and artificial olfaction with opportunities presented by advances in nanotechnology, in order to further the development of nose-like sensing systems integrated on the nanometer scale. The olfactory processes are reviewed here in some detail. Dense arrays of olfactory neurons, acting as ultra-small, non-specific sensors, use molecular recognition to perform highly parallel molecular sensing. The sensory signals so generated are identified by the brain using a spatio-temporal coding scheme. In this way the olfactory system recognizes, with great accuracy and sensitivity, a broad range of chemical stimuli. The principles of olfaction have been applied to developing artificial noses that are composed of arrays of cross-reactive gas sensors of various types. Artificial noses based upon conductivity-change devices, mass-change devices, and fluorescent optical fibers are reviewed here. The smallest artificial noses at this time are devices that incorporate micron-scale sensing elements and are comparable in size to a credit card. To more closely approximate the capabilities and compact size of the natural nose, it will be necessary to shrink the individual sensor size even farther, integrating nanometer-scale sensors into systems. Individual nanometer-scale devices, such as carbon nanotubes and nanowires, already have been demonstrated to function as gas sensors, and their applicability to nose-like sensing is discussed. At this point in time, however, no complete, nose-like nanometer-scale sensing system has been developed. This paper concludes by presenting for consideration a proposal for an electronic nose composed of nanowire sensors.