Unfortunately, some explosives are easier to detect than others and sniffing out the highly explosive PETN has, until now, been a time-consuming and expensive process. To reliably detect just a few small molecules of this material requires cutting-edge trace detector technology based on nanotubes. Working on his doctorate under Professor Wolfgang Ensinger, Head of the Materials Analytics Working Group at the Technical University in Darmstadt in Germany, Mario Boehme has developed an extremely sensitive sensor to do just that.

We can understand just why PETN has been so difficult to detect until now when we compare it with various everyday materials. We can smell a freshly mown lawn from quite some way away, and the baker‘s shop already invites us to stop by from some way down the street. Both the grass and the bread are giving off millions of molecules that our noses can easily detect. However, try to smell a porcelain cup or an aluminum door handle and, no matter how close you get, there‘s nothing much to smell. These materials don‘t give off molecules freely and therefore we can‘t smell them. The same goes for PETN - it is inherently ‚unsmelly‘ and therefore extremely difficult for electronic devices to detect. Further, just a few tiny grams of it are enough to cause a significant explosion.

Of all the various methods of explosives detection - mass spectrometry, chemiluminescence, x-ray diffraction, microcantilevers and electrochemical methods - the advent of nanotechnology over the past few years has brought the most impetus into the development of handheld trace detection devices.

It is clearly the way to go and promises astounding performance at sensitivity levels that, until recently, were only seen in science fiction films. In fact, the scale of measurement has taken on a new dimension; devices were previously rated at finding parts per million or billion (109), but now sensors are becoming effective - and reliable - in the area of parts per trillion (1012) or quadrillion (1015).

Who Nose Best
Whereas finding concealed explosives was once only commonplace at airports and embassies, it has now become an ever-higher priority at a number of other locations and situations; at vehicle entrances and major sporting events for example, at concerts and in popular clubs, at courts or correctional institutions or on public transport systems to name just a few.

The usual technique is to sample the air around vehicles, luggage or people, passing it over sensors that react to various specific triggers, although testing for PETN has until recently involved a time-consuming analysis of smears with a spectrometer.

An ‚electronic nose‘ is equipped with either a sensor specifically tuned to one type of explosive, or with many sensors that together provide a ‚fingerprint‘ of the substance. The sensors themselves use various methods to sound the alarm. There are biological sensors that rely on a chemical reaction and optical sensors that notice a change in the absorbence of specific wavelengths of light. There are also mechanical sensors - called cantilevers - that have one fixed end and bend when molecules of the target substance react with a chemical layer on one side. And now the nanosensors either change their natural fluorescence or their electrical conductivity in the presence of specific substances. The Massachusetts Institute of Technology in Cambridge, USA, did some pioneering work on improving the sensitivity of sensors using blue lasers some years ago and its continuing work has recently discovered a useful detection effect of bee venom applied to nanotubes.

Made to Measure
The production of nanotube-based sensors is now refined to a degree where they can be doped with various substances to provide both reliable and, importantly, very specific detection properties. The resultant sensors are cheap to produce commercially and are spawning a new generation of products that can be widely used to counter the terrorist threat. There is enormous potential in this technology that will also result in other new products appearing outside the security industry, and the development of nanosensors is already moving fast.

The challenge now, however, is to integrate these sensors into market-ready products that will alert to the presence of one or more of the more than one hundred different types of military and civilian explosives. There are a number of aspects to consider; for example, many different types of molecules go by as the sampled air is passed across the sensor. This can cause cross-contamination that obviously has to be removed before the sensor can work effectively again. Further, the humidity of the air currently has a significant effect on performance and must be controlled. Another potential issue is when molecules from another substance but nevertheless very similar to those being sought are detected. Certain synthetic products from the perfume industry can cause false positives.