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.
Figure 1. Sensitive portable explosive sensors could be attached to bomb disposal robots like this one, to help soldiers identify IEDs remotely. Image Credits: 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.
Figure 2. Sensors made from functionalized carbon nanotubes can be used to selectively detect incredibly small concentrations of gas phase molecules. Image Credits: Pacific Northwest National Laboratory
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.
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.
Sources and Further Reading