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Copyright © 2008, Emerald Group Publishing Limited
Nanosensors: when can I buy one?
The field of nanotechnology is multidisciplinary, as with most sensor technologies. In general, nanotechnology refers to devices that are between 1 and 100nm in scale, drawing on the latest developments in the disciplines of physics, chemistry, mechanics, electronics, optics and biology. Advances in the areas of materials and fabrication techniques are particularly relevant to emerging nanosensors.
One of the most intensely studied types of nanomaterial over the past decade or so, are carbon nanotubes (CNTs), in particular single-walled carbon nanotubes (SWCNTs), which take the form of hollow cylinders of single layers of carbon atoms. The lengths of the tubes can be several microns and the diameters are several nm, leading to high-aspect ratio structures. High purity SWNTs without imperfections have been shown to be capable of carrying very high-current densities (up to 109 A/cm2). Furthermore, depending on their structural symmetry, SWNTs can behave as either metals or semiconductors. Coupled with their highly elastic behaviour (Young's modulus around 1TPa) and it becomes clear that they offer a wide range of mechanical sensor applications such as pressure, force, mass and displacement. Functioning as a strain gauge, for example, a SWCNT device has a gauge factor of up to 2,000 under controlled circumstances; compare that with a figure of around 2 for a traditional metal foil gauge. Such properties offer the possibility of applications in the area of measuring extremely small strain levels, perhaps even at the nano-strain level. At such high sensitivities, the question of associated electrical noise levels and the effects of fabrication processes on these is one that many researchers are currently investigating.
SWCNTs can also be used as gas sensors; the interaction of gas molecules with the nanotubes can lead to conductance changes, allowing detection limits of a few parts-per-million for gases such as NO2, NH3, N2 and O2. The US Naval Research Labs have used CNT sensors to detect nerve agents such as Sarin. There are also plenty of examples in the literature of using CNTs for biosensing applications to detect glucose, DNA, cholesterol and urea using multi-walled CNT structures.
Structures with very high aspect ratios are sometimes referred to as “nanowires” and can offer possibilities for new types of sensor structures. As an example, nanowire arrays of Bisthmuth alloys can lead to the formation of densely packed thermoelectric junctions, which could find application in the area of alternative battery technology by converting small temperature differences (<5ºC) into useful amounts of electrical power.
So, when will nanosensors be available on a commercial scale? Well, if the trends in nano systems follow those of their microscale counterparts, then probably within a decade. As we all know, however, forecasting is a risky business – the following statement is taken from the Met Office web site “Summer 2007 is likely to be the warmest ever year on record going back to 1850, beating the current record set in 1998”. Hmmm. Maybe if we had arrays of nanosensors available for measuring all possible environmental parameters ...
Neil WhiteElectronic Systems Design Group, School of Electronics and Computer Science, University of Southampton, Southampton, UK