Plastics to Detect Explosives

Chris -   With the advent of global terrorism, it's become apparent that people will go to extreme lengths to conceal bombs.  So quickly and accurately detecting trace amounts of explosives could not only save lives, but it could make air travel safer, quicker and a lot more convenient.  

Now, researchers at St. Andrews University have invented an explosive-detecting plastic that could be used at airports and even on mine fields.  Dr. Graham Turnbull is the inventor.  He is with us.  Hello, Graham.

Graham -   Good evening.

Chris -   So tell us, how did this work get started, plastics to pull out explosives?  I've heard of plastic explosives, but not the other way around.

Graham -   We're using plastics to find explosives.  So, in St. Andrews, we're developing a set of materials called organic semi-conductors for a range of different applications in optoelectronics.  These materials are a special class of plastic that unusually can conduct electricity, which is an unusual property for a plastic, but also can be stimulated either by illuminating it with light or by passing electricity through it to emit light.  So these materials are now being used for a range of applications as plastic light emitting diodes in Samsung mobile phones, plastic solar cells, and plastic lasers.  It turns out the same set of materials can also be used as a sensor.  And following on from some research by a group at MIT, we started to look and see how we could use our plastic lasers as very sensitive sensors for detecting explosives.

Chris -   So what's the technique then that the laser shines on the plastic and it changes the electrical activity of the plastic, but if the explosive is present, you get a different signal or something similar?

Graham -   It's something along those lines.  Essentially, what our sensors are doing is they're replacing a sniffer dog's nose.  So, we operate our plastics by illuminating them with ultraviolet light and then the plastic then re-emits that energy as visible light.  Now, if we take a thin film of these light emitting plastic materials and we bring a small number of TNT molecules or molecules similar to TNT into contact with the film, the TNT molecules can interrupt this light emission process.  Essentially, it switches off the light.  So if there's explosives present in the atmosphere surrounding the plastic film, the light emission is switched or made dimmer.  If you blow clean air across the film then the light emission returns.  So it's a reversible process that can detect the presence of very, very low concentrations of explosive molecules in the surrounding atmosphere around the film.

Chris -   How specific is it? Because you mentioned TNT - trinitrotoluene - the stuff that's in the majority of the 100 million or so land mines that we know are lurking out there somewhere around the world, but other explosives are used too and people are getting cleverer and cleverer in terms of the explosive cocktails they will use.  So, is it just TNT or could it work with other chemicals as well?

Graham -   Okay, so as you say, TNT is the major component of all military high explosives, but also, when you make TNT, you also fabricate other similar molecules like DNT - dinitrotoluene instead of trinitrotoluene - and various analogues.  This technique is able to detect a wide range of these nitro-aromatic molecules.  It also has potential to detect some of the other plastic explosive molecules.  The process works on a difference in energy states between the light emitting plastic and the explosive molecules, and essentially works by switching off the light emission by electrons hopping from the light emitting plastic onto the explosive molecules.  So, it can detect a wide range of different materials.  It doesn't intrinsically discriminate between those, but that's one of the next steps that we're trying to look at, to get selectivity.

Chris -   What about the problem of selectivity? Because one of the big holdups at airports is that people might have something - their shampoo or something - that triggers this agent and in fact, it's completely innocuous?  So, is there a false positivity with this and if so, is that a problem or is it at a level that is going to be tolerable?

Graham -   I think with all sensing approaches, there is some danger of false positives.  There are things that could be in your luggage that conceivably could trigger these things.  Angina sprays contain trinitroglycerine in them in very small quantities, but in general, we believe that it's a workable solution.  In a different context which we're interested in, in the area of land mine clearance, which has security implications as well because they're in former war zones, there's a lot of explosives lying around that terrorists in principle could get their hands on.  So clearing land mines has a security benefit for the western world.

Chris -   How would the technique work though and what would you do?  Would you have a box where some of the plastic being illuminated by the UV is in there and is being watched by a camera for example and a gas stream is pulled across your luggage or across the mine field, and that air then interacts with the plastic?

Graham -   That's right.  So that's the sort of the approach. We take one of our little plastic lasers which are postage stamp-sized lasers and you essentially can draw some of the air across the plastic film, the little polymer laser, and you then monitor the amount of light emission that is being emitted.

Chris -   And so, it is very portable.  So, the concept of mounting this on some kind of mobile device that could explore a mine field and find hot spots of the gas that could signify an explosive buried underground coming up, that's not beyond the realms of possibility.

Graham -   Not at all.  Our plastic lasers are very compact.  Within a collaborative project in the UK called Hipix, we've been developing very miniaturised compact versions of these plastic lasers where essentially, we take a high power light emitting diode, we remove the plastic dome lens, and replace it with one of our plastic lasers.  And so, the actual source can be the size of an LED.  In other work in the Hipix programme, we've also been developing fluorescence based explosive sensors that are based on two silicon chips.  So these are all laboratory prototypes, but they show a lot of promise for being able to make very compact sensors.

Chris -   And relative to a sniffer dog's nose, where would your plastic sit, more sensitive, less sensitive, or about the same?  And what's the absolute numbers in terms of parts per billion or so of explosives that they can pick up?

Graham -   Sniffer dog's noses are regarded as the gold standard in sensitivity.  I'm not sure I have a quantification for how low they can go in sensitivity.  This sort of approach, is the one that gives you the closest to the sniffer dog's nose in sensitivity.  In the laboratory, we routinely do tests on new materials with parts per billion quantities of TNT in the air, and the technique can go much lower than that in terms of sensitivity.

Chris -   But unlike a sniffer dog, I suppose your plastics won't get tired, don't need to play with a ball periodically to keep it interested!

Graham -   That's right.  A sniffer dog needs to be playing a game in order to detect explosives and in practice - depending on the environmental conditions the dogs are working in - they may typically work for half an hour at a time.  So for long, repetitive searches, or if you want to go into very hazardous areas, there are problems with using dogs in those sorts of conditions and so, a high tech alternative that doesn't get bored is attractive possibility.