There's never been a greater need for explosives detectors. From detecting IEDs on the battlefields of Afghanistan to screening cargoes at airports and sea terminals for bombs or illegal arms shipments, the need to seek out and identify explosives means deploying detectors to hundreds of thousands of locations around the world. Unfortunately, there has always been an unpleasant trade off when it comes to explosives detection sensors - they are either cheap, but not very sensitive or they are very sensitive, but also expensive. They are also often difficult to manufacture, use a good deal of power, are relatively delicate and require a trained operator.

Along with traditional sniffer dogs, many ideas are under development to improve explosives detection technology such as lasers, UV-sensitive sprays, terrahertz radiation scanners and even sensors using bee-venom. Most of the very sensitive sensors that can detect very minute quantities of explosives at a distance are based on ion mobility spectrometers (IMS). Put simply, these work by ionizing the molecules in an air sample and then measuring how fast they pass through a "drift tube". It's a very sensitive and accurate way of detecting tiny traces of explosives, but it still suffers from requiring very expensive, hard to construct equipment. If only there was an alternative that was potentially as sensitive, but as cheap as printing a document. Then it would be possible to deploy detectors far and wide at much lower cost.


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It turns out, there is now such an alternative. At the Georgia Tech Research Institute (GTRI), a team lead by principal research scientist Dr. Krishna Naishadham have created an ink-jet printable ammonia sensor capable of cheap, practical explosives detection. Ammonia is a key ingredient in the manufacture of many explosives.

The process of creating the sensor involves printing carbon nanotubes on paper or "paper-like" materials, such as the plastic polyethylene terephthalate. The ink consists of silver nanoparticles held in an emulsion that can be passed through an ink-jet printer at a temperature of only 212 F (100 C). This ink is treated with ultrasonic waves in a process known as sonification, which alters the viscosity and makes the ink more homogeneous for greater effectiveness. As it sets, the ink forms into nanoscale cylinders called nanotubes. These are only one-billionth of a meter in diameter-about 1/50,000th the width of a human hair. When these nanotubes are coated with a conductive polymer that attracts ammonia it becomes an effective explosives sensor capable of detecting trace amounts of ammonia as low as five parts per million. With different coatings, the nanotubes can detect other gases.

Xiaojuan (Judy) Song and Krishna Naishadham and display two prototype wireless explosives detector devices (Photo: Greg Meek, Georgia Tech)

Explosives detector and communication device

Not only is this process cheap and effective, but the nanotubes can also be formed into RF circuits, components and antennae. This means that the sensor can be printed with a built-in communications device already installed to transmit data. These components can be printed on a suitable plastic and be formed out of flexible organic materials, such as liquid crystal polymer to make them more robust and water resistant. The device also uses very little power, which makes it suitable for running off of thin-film batteries or solar cells. And, being printed, the device can be stuck on any surface where it might be required. The GTRI team is also working to make the device capable of operating passively without an internal power source-something like RFID tags used in shops that get their power from the shop's scanning devices..

This makes for a very flexible little package. The GTRI team designed the detector to act as an integrated detection/transmission system that provides stand-off explosives detection that allows personnel to remain at a safe distance while the detector transmits the results back to the operators.

"This prototype represents a significant step toward producing an integrated wireless system for explosives detection, says Dr. Naishadham. "It incorporates a sensor and a communications device in a small, low-cost package that could operate almost anywhere."

If it lives up to its promise, the printable explosives detector could deliver vitally needed, life-saving detectors that can be mass produced cheaply in any corner of the world.

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