By Will Soutter
Topics Covered
Introduction
Nanosensors for Explosives
"Electronic Nose" Sensors
Nanomaterials in
Explosive Detection Systems
Nanomechanical Sensors
Conclusion
References and Further Reading
Introduction
Detection of explosives has become a very important issue in the
last few years. The number of terrorist attacks, and suicide bombings
in particular, has risen dramatically in the last few years. A
significant number of the casualties in Iraq and Afghanistan have been
caused by IEDs (Improvised Explosive Devices). Early detection of
hidden explosive devices therefore has the potential to save many lives.
Nanosensors for Explosives
The explosives typically used by terrorists and insurgents only give
off incredibly small amounts of detectable gas. Existing systems which
are capable of detecting compounds in the gas phase at such low levels
are large, unwieldy and very expensive. They also have limited
sensitivity and selectivity - there is no way to be certain which
explosive substance has been detected.
As nanomaterials become more readily available for commercial
devices, great interest has been shown in using them to develop
trace-level detection systems for explosives which overcome all of
these issues. Because of the unique nature and tuneable
properties of nanomaterials like carbon nanotubes, nanowires, and other
nanostructures, handheld or portable systems which are sensitive down
to the molecular level could well be possible.
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Figure 1. Sensitive
portable explosive sensors could be attached to bomb disposal robots
like this one, to help soldiers identify IEDs remotely. Image credit: US Department of Defense.
"Electronic Nose" Sensors
The concept of an "electronic nose" has been in development since
the 1980's - the aim is to use electronic sensors and pattern
recognition technology to imitate the sensing capabilities of the human
nose.
With the addition of nano-enhanced sensors and advances in
artificial intelligence technologies like neural networks, electronic
noses have been developed which can detect and identify incredibly
small quantities of airborne chemicals.
Nanomaterials in
Explosive Detection Systems
Carbon nanotubes, nanowires, and other nanostructured materials have
very high surface areas and a unique set of optical, mechanical and
electrical properties, which make them ideal for exploitation in
high-sensitivity detection of molecules.
A typical setup would involve an array of nano-sized sensors
connected in a circuit. Each sensor unit reacts to the adsorption of
analyte molecules, such as trace explosives, changing the electrical
signal in a unique way. The combination of responses from the whole
array produces a complicated, fingerprint-like measurement.
Analysis of these signals by pattern-learning neural networks
creates
a database of signatures for known substances - these can then be
applied in the field to detect tiny traces of explosives, and determine
which chemicals are present. This will be of great use to security
personnel working on the front line, to determine the nature and
magnitude of the potential risk when explosives are detected.
.jpg)
Figure 2. Sensors made from
functionalized carbon nanotubes can be used to selectively detect
incredibly small concentrations of gas phase molecules. Image credit: Pacific Northwest National
Laboratory
Nanomechanical Sensors
An alternative approach to chemical adsorption sensors is to use the
nanomechnical response of cantilevers. When molecules adsorb to a
nanoscale cantilever, they cause a mechanical stress which could be
used to detect masses down to a single molecule. The sensor system can
be made selective to a particular substance with a chemical coating.
Whilst the potential of nanomechanical sensors has not yet been
fully realized, they are a more mature technology than sensors based on
nanowires and nanotubes, as these materials have not been available to
researchers for as long, and have high fabrication costs associated
with them.
Conclusion
Terrorist attacks and warfare in the Middle East are providing huge
incentives for development of more portable and more sensitive systems
for detecting explosive devices. Nanotechnology is providing the means
for researchers and companies to create sensor devices which will make
a huge difference to the way these battles are fought.
Whilst the technology is some way off commercial use, the military
applications mean that more funding is available for novel
technologies. This drive to bring nanotechnology into play in this
particular market may have repercussions across other, less
military-focused applications, by allowing the intitial cost of making
nanomaterials market-ready to be overcome more easily.
References and Further
Reading