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2.3.1 Direct detection
Nanostructured materials such as carbon nanotubes and metal oxide nanowires promise
superior performance over conventional materials due to selective uptake of gaseous
species (based on controlled pore size and chemical properties) and increased adsorptive
capacity (due to increased surface area).
Nanotubes
The binding of gas molecules to the surface of a carbon nanotube affects its electronic
properties which can be exploited in sensor technologies. Such sensors can be based on
single CNTs or CNTs assembled in arrays to provide simultaneous detection of different
molecular species. Single walled carbon nanotubes (SWCNT) can be combined with a
silicon-based microfabrication and micromachining process that enables the fabrication of
sensor arrays with the advantages of high sensitivity, low power consumption,
compactness, high yield and low cost (see Figure 2.10). SWCNT can be coated with
specific chemical groups providing a selective adsorption of different analytes. By
measuring changes in surface enhanced capacitance, real-time detection and
quantification of different target molecules such as explosives and neurotoxins can be
achieved (A.S Snow et al., 2005).
Figure 2.10 Illustration of a CNT-based sensor. Source:
http://people.nas.nasa.gov/~cwei/Publication/cnt_sensor.pdf
Various projects involving CNTs for sensor technologies have been awarded: Rensselaer
researchers were awarded a 1.3M$ NSF grant in 2003 to develop CNT sensors for
homeland security; Nanomix, a molecular electronics start-up which developed CNT
based electronic devices secured a 1M$ grant from the Department of Homeland Security
to develop sensing technology in 2007; finally NASA Ames research centre has already
developed a CNT based chemical sensor array. 4
Nanowires
Zinc oxide nanowires can be used to detect several substances as a result of changes in
electrical conductivity due to chemical adsorption (e.g. nitrogen dioxide gas reduces
current whereas carbon monoxide increases it). ZnO can also be used in the form of a
thin film, however the nanowire has a larger surface to volume ratio. An additional
benefit is the rapid dissociation of adsorbed chemical, allowing the nanowire to be
“reset”. Such sensors are under development at the University of South Carolina where
researchers are working on the integration of several sensing units in the form of an
array (to create a sort of electronic nose) and also on the feasibility of configuring an
array of vertical ZnO nanowires vertically in an array for use as a solar powered battery. 5
4 “carbon nanotube sensor for gas detection”, http://www.nasa.gov/centers/ames/research/technologyonepagers/gas_detection.html
5 “ZnO nanowires may lead to better chemical sensors, high-speed electronics”, physorg, 09/2006,
http://www.physorg.com/news77303473.html
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