The use of organic materials to detect the presence of explosives has been a hot area of chemical research for some time. One of the classes of plastics that I’ve worked a lot on in my career – compounds called “PPEs” – have been extensively developed by researchers at MIT and elsewhere to assist in the detection of trace TNT (trinitrotoluene) vapor. The fumes given off by landmines that are buried under the ground are rich in compounds containing aromatic rings and nitro groups. These molecules can drift up into a handheld sensor utilizing these PPE compounds and quench the otherwise high fluorescence of the polymers. So for TNT, we have a case where the presence of explosives diminishes the natural glow that is given off by the plastic when it’s irradiated with a UV light source.
This method works well for TNT, but it is incapable of detecting other types of high explosive, in particular the peroxide-based explosives such as triacetone triperoxide. This newer class of explosives don’t contain any aromatic rings or nitro substituents and so there is no mechanism for fluorescence quenching. Luckily, two chemists at the University of San Diego have developed a new plastic which takes the completely opposite approach, and is successful for sensing peroxides. Their results were published in the Journal of Materials Chemistry.
Imagine you’re on a hill overlooking a large city. During the night, you can look down at the city and see all of the combined light glare from the thousands of streetlights, automobile headlights, and house lights. If enough of the lights go out all at once – a blackout of a city block, or something like that – you’ll probably notice it. The overall light coming from the city will be reduced. You probably wouldn’t notice one individual car losing a headlight however, as it’s just lost in the background glare. Now, imagine looking out over a city that’s completely blacked out – no lights anywhere. If a house suddenly gains power and lights up its lawn, there’s a good chance that you’ll notice it. The human eye is very good at detecting a bright spot against a dark background. That’s the approach that this new polymer takes. Instead of quenching fluorescence, it produces fluorescence. The plastic doesn’t glow at all to start with, but upon exposure to even a tiny amount of peroxide, it rapidly begins to emit light.
This new polymer is synthesized by a combination of a protected boronic acid called 3,6-bis (pinacolatoboron) fluoran and the alcohol named pentaerythritol. The pinacol gets replaced by the other alcohol and the two molecules stitch into a long, robust polymer. The reaction is driven to completion because the starting material (a five-membered ring) becomes a six-membered ring in the polymer, which is energetically favorable as a six-membered ring has less strain. The reaction is complete after 2 days reaction time in a warm flask. Upon exposure of the finished product to peroxide vapor, the boron functionalites are oxidized and they release their grip on the fluorescein molecule. Once the fluorescein falls free from the polymer chain, it begins to glow. This method is so sensitive that it can detect peroxide at levels as low as 3 parts per billion – 3 molecules of peroxide in every billion molecules of air constituents.
This new polymer can be incorporated into a hand-held sensor quite easily, as it uses the same base instrumentation (fluorescence spectrometer) as the sensors developed for TNT. It’s an interesting take on how the appearance of fluorescence, as well as the disappearance of fluorescence, can be used to indicate the presence of explosive vapor.
The source of this article can be found at: http://www.rsc.org/Publishing/Journals/JM/article.asp?doi=b809674k