New and improved explosives detectors are bringing hope to the war against terror and helping the search for unexploded landmines, as Gaetano Mancino reports
New and improved explosives detectors are bringing hope to the war against terror and helping the search for unexploded landmines, as Gaetano Mancino reports
The misuse and proliferation of conventional and improvised explosives threatens our security more now than ever before. This year saw a large number of terrorist attacks around the world. What is more, the problem of landmines and other unexploded ordnance is not abating. All of this adds urgency to the search for improved explosives detectors.
According to the International Campaign to Ban Landmines, between 15 000 and 20 000 people in at least 60 countries are either killed or maimed by landmines each year. This equates to roughly 40 people per day or two fatalities every hour. Landmines also deprive the poorest countries of land needed for infrastructure development, hamper the delivery of aid, kill livestock and cause environmental havoc.
In 1997, the Ottawa Treaty was set up to ensure that countries do not use, produce, stockpile or sell landmines. It has been ratified or signed by 154 states, but 40 countries have yet to sign. According to Steve Wilson, director of international relations at the Mines Advisory Group, ’there are still about 15 countries that produce landmines. Unfortunately the problem is still huge and landmines are still being laid’.
Sniffing out explosives
Detecting and neutralising landmines is extremely difficult and newer generations are often harder to locate than previous models. One approach is to use metal detectors, but this can be slow and painstaking and often picks up false alarms from shell casings and scrap metal. An alternative strategy is to detect the explosive materials themselves, which are generally based on 2,4,6-trinitrotoluene (TNT) and similar nitroaromatic compounds.
Dogs can be trained to smell TNT vapour, but this is not considered to be the most reliable method. The most commonly used artificial TNT detection system is based on ion mobility spectroscopy and involves sampling explosive particles from contaminated surfaces. However, this technique requires easy access to the particles, which is impractical when devices are buried underground.
The explosive threat is not limited to conventional weapons; terrorist groups frequently use improvised devices. These makeshift explosives can be made in large quantities from everyday household chemicals. The most common of these explosives are cyclic peroxides such as triacetonetriperoxide (TATP) and hexamethylene triperoxide (HMTD). TATP can be manufactured using hydrogen peroxide, acetone and acid. The first reported criminal use of TATP was in Israel in 1980 and since then a range of cyclic peroxides have been implicated in several terrorist attacks, including the bombings in London, UK, in July 2005. Richard Reid, the so-called shoe bomber, used TATP as a detonator in his failed attempt to blow up a plane using explosives in the soles of his shoes .
Molecules such as TATP boast similar explosive strength to TNT. Furthermore, TATP is extremely sensitive to heat and vibrational shock and can be ignited with an open flame or small electrical discharge. Unlike conventional explosives it does not need a primer or accelerator.
The growing threat from TATP has spurred on the search for an effective detection tool. These efforts have focused mainly on a combination of liquid- and gas-phase chromatography, mass spectrometry, and infrared spectroscopy. However, these methods are not well suited to rapid testing in the field. Andrea Sella, a chemist at University College London, UK, summarises the situation: ’What we need are incredibly cheap, simple and robust methods that can screen an enormous number of people, and that’s really tough to do’.
Despite the considerable challenges faced when developing TNT and TATP detection devices, researchers are making important advances.
Artificial chemical noses
In 1996 the US Defense Advanced Research Projects Agency launched its dog’s nose programme, which sponsors scientists and technologists to develop artificial gas-phase sensors that can imitate the ability of canines to detect TNT. Timothy Swager, head of chemistry at the Massachusetts Institute of Technology (MIT), US, is a key researcher in this area (see Chem. Brit., May 2000, p36).
Since 1996, Swager and his group have developed a range of sensory materials that can be used to detect TNT. Their screening method is based on the principle that an analyte such as TNT can be detected when it binds to a receptor and produces an easily measurable signal change (called a transduction event).
Swager’s team uses a conjugated organic polymer, a polyarylethynylene, as a receptor. These polymers are model receptors for TNT because they form strong co-facial
-complexes with electron deficient nitroaromatic compounds.
Swager’s polyarylethynylenes are highly fluorescent in both solution and the solid state. However, TNT and other nitroaromatics dramatically quench this fluorescence by taking electrons from the excited state polymer. Crucially, a single TNT molecule quenches the fluorescence from several chromophores on the conjugated polymer. The quenching effect is thus amplified by several orders of magnitude relative to a single chromophore, leading to high levels of sensitivity.
But as Swager recognises, sensitivity is not the only factor in detection, and ’any analytical chemist will tell you that sensitivity without specificity is nothing’. Specificity is necessary to filter out false alarms and Swager’s technique is versatile in this regard. His polymers are designed to detect a wide range of nitroaromatic compounds, rather than relying on identifying unique compounds. ’We have very broad specificity in these systems,’ he says.
In collaboration with Nomadics, a US-based technology firm, Swager has incorporated his sensory polymers into the Fido, a portable, hand-held detection device. Nomadics claims that Fido is the most sensitive device of its kind, capable of detecting TNT vapours at the femtogram level, which is similar to the sensitivity of a dog’s nose.
The Fido system consists of a glass capillary in which the sensory polymer is embedded. Light activation makes the polymer fluoresce and at the same time air is pulled into the capillary. Any trace of explosive vapour will bind to the polymer and quench its fluorescence. This process is monitored by a photodetector positioned at the far end of the capillary tube, and the information is relayed to an LCD screen on the user interface.
Fido sensors have already been used in a variety of field applications, ranging from demining operations to screening air and sea cargo. Nomadics has also commercialised an underwater version of Fido that uses the same organic polymer technology developed at MIT.
Swager’s group has recently developed polymers that, under the appropriate conditions, undergo stimulated emission - lasing (see box below). TNT and other nitroaromatics cause a remarkable drop in lasing intensity that is at least 30-times more sensitive than that observed for spontaneous emission - conventional fluorescence.
Swager is keen to integrate his lasing polymer into a live device, and believes that sensitivity enhancement is possible. ’Thirty-fold [increase in sensitivity] for the lasing system is really a conservative estimate. My bet is that if you put these polymers into a much better photonic structure we will see thousand-fold improvements.’
According to Swager, his group’s current priority is to create a single device capable of detecting multiple analytes. ’We’d like to make something that looks like a tricorder from Star trek; something that detects lots of different things,’ he says. ’We have in my group developed materials that are able to address completely different odours of space with high sensitivity and stability. It’s now a case of deciding how you implement them into a sensor.’
Swager believes there are two ways this can be done. One approach is to use multiple transduction materials integrated into a single unit. An alternative method is to make use of a temporal response, whereby variable rates of diffusion allow differentiation between a mixture of analytes. He says that several research groups are now working on these different strategies, and their contributions range from designing new sensory materials to developing mathematical algorithms that remove false signals.
If future advances are to be realised it is vital that all these strings are pulled together. Swager is optimistic: ’Do I think it will be possible for us to have the full range of detection in the future?...Let me put it this way, it will definitely not happen next year, but it’s not inconceivable to me that it can be accomplished.’
Polymers in explosive detectors
The tendency for fluorescent polymers to aggregate in thin films has limited their applicability as chemosensory materials. This aggregation phenomenon causes self-quenching of the fluorescent chromophores and a subsequent drop in quantum yield. In addition, dense polymer networks impede the diffusion of analytes, such as TNT, through the polymer. Both effects cause a reduction in the transduction signal intensity and reproducibility.
Timothy Swager and his team from the Massachusetts Institute of Technology (MIT), US, have solved this problem by attaching bulky pentiptycene moieties to the polymer flanks, while leaving the intrinsic polyarylethynylene backbone intact. The pentiptycene groups prevent the polymers from stacking in a co-facial arrangement. This minimises fluorescence self-quenching and improves thin film stability.
The pentiptycene groups also create cavities inside the polymer network, which allows the diffusion and intercalation of flat molecules such as TNT. Swager has found that different types of pentiptycenes induce subtle changes in cavity size and electron density inside the polymer. These difference act as filters, allowing discrimination between different nitroaromatic analytes.
Polymer stability is also a major concern in the lasing sensors developed by Swager. In the past, these polymers have not been robust enough to withstand the high pump powers needed to achieve lasing, limiting their use as sensors.
To overcome this, Swager’s group has functionalised the polymer with pendant aromatic side chainsThese chains act as molecular sheaths, encapsulating the polymer backbone within a protective shell. When protected in this way the polymer does not undergo photochemical bleaching, maintaining the lasing quantum yield and extending its operational lifetime.
The protected lasing polymers can be processed into a range of different forms, such as conventional thin films, and coated onto the surface of silica fibres.
The search for a TATP detection system has gone through a more tortuous route than TNT detectors. Ehud Keinan, professor of chemistry at the Technion Institute, Israel, has spent the past 18 years working on a recognition tool for TATP. This endeavour has proved difficult because, according to Keinan, the cyclic peroxides are fairly boring molecules. ’TATP is essentially a great big chunk of grease. People have talked about developing synthetic receptors [for TATP] but it’s hydrophobic and there’s nothing to recognise it,’ he says.
To compound the problem, TATP and similar compounds have no interesting or useful spectroscopic properties. Apart from some infrared and Raman-active bands they possess no UV chromophores and do not fluoresce. Despite this intrinsic difficulty, Keinan and his team have recently created a TATP detection tool. The key to Keinan’s approach lies in detecting trace quantities of hydrogen peroxide, which is generated when TATP is exposed to acid.
A considerable body of work concerning H2O2 detection already exists and involves experiments with peroxidase enzymes. These catalyse the oxidation of certain substrates by H2O2 and are often used in molecular biology binding studies. If the oxidised substrates differ from the reduced form by a change in either state or appearance this can be used as evidence for the presence of H2O2 and, in this case, TATP.
From this simple tenet, Keinan and his group designed and built a TATP-detection kit that was unveiled this summer (see Chemistry World, September 2005, p7). The small device is called a peroxide explosives tester (PET) and resembles a multi-coloured ballpoint pen.
The main components include a silicone rubber test pad, a central transparent section and three syringes containing a strong acid, a solution of enzyme and a colourless pigment.
The tester applies a sample to the rubber test pad and washes the residue with acid. The pigment is added next, followed by the enzyme. Samples containing TATP-type substances produce H2O2 which oxidises the pigment to produce a blue-green solution. This distinct colour change is used to indicate the presence of TATP.
The PET has a fast response time and is reported to be simple to operate. It is also highly sensitive and selective, detecting sub-milligram quantities of material and helping to differentiate between genuine explosive residues and miscellaneous oxidants.
Keinan says that law enforcement agencies are impressed with the PET invention. ’There has been a lot of interest, even from British companies. Hopefully this will be commercialised very soon.’ He envisages a situation where security personnel will carry these kits on their belts and test suspected material in real time.
The PET is still only a prototype and needs further refinement. Keinan is currently trying to make the device smaller and reusable. It would also be advantageous to couple the PET methodology with existing detection techniques, particularly for aviation security.
Ultimately, law enforcement agencies need a gas-phase detection system, minimising the risk to civilians and security personnel. The volatile nature of TATP makes this requirement a distinct possibility. ’In terms of getting a gas-phase detector, basically it’s a matter of engineering. It’s more a case of development than research,’ said Keinan. Sella agrees, but cautions that effective gas-phase screening will require detection limits in the nano range, something that Keinan and his group are working on.
Keinan hopes that the success of the PET will inspire others to design and build advanced detectors for TATP and similar improvised explosives. ’If 100 people start working on this tomorrow morning then the result will be different than if one person works on this part-time,’ he says.
Keinan also urges governments not to underestimate the threat posed by TATP. ’For many years they [western governments] probably thought it was just an Israeli problem, but really it is a global problem and has to be treated as such. The British got a wake-up call on 7 July 2005 and that is just the tip of the iceberg.’
TATP’s decomposition theory
Triacetonetriperoxide (TATP) is commonly used by terrorist groups. To the untrained eye, the explosion of TATP resembles that of any other conventional explosive, namely the rapid release of energy in a violent exothermic process. However, investigate a little deeper and you find that TATP does not obey these assumptions.
The decomposition of TATP is, perhaps surprisingly, not a highly favoured thermodynamic process. Rather, the breakdown of TATP involves ’entropy burst’, whereby its disintegration produces four gas-phase molecules for every one of TATP.
For any reaction, the overall Gibbs free energy change,
The key to understanding the decomposition of TATP lies in analysing its molecular structure. TATP is composed of three acetone molecules linked by O-O peroxide bridges. The crystal structure of TATP shows the molecule exists in a boat-chair conformation, with each oxygen atom buried within the central cavity surrounded by the hydrophobic isopropylidene groups.
The isopropylidene units are described as molecular scaffolds from which the peroxide units are suspended in close proximity to one another. The decomposition of TATP begins when one of the O-O bridges cleaves homolytically. A complex cascade reaction then develops, involving the fragmentation of the molecule and neighbouring molecules in the solid state.
Computer modelling of the decomposition pathway of TATP predicts the formation of several minor byproducts, including highly reactive diradical oxygen species, methyl acetate, ethanol, CO2, O2, and ethane, all of which have been identified experimentally at blast sites.
The major decomposition products of TATP are ozone and acetone, which produce considerable pressure increases during an explosion.
The ensuing shockwaves can accelerate as fast as 5000 ms-2. It is for this reason that TATP and other members of the cyclic peroxide family are so explosive.
A step ahead
Terrorist groups will always strive to develop new explosives that evade detection techniques. So long as terrorists continue to use improvised weapons, and more sophisticated landmines continue to flourish, thousands of people’s lives will continue to be at risk.
Swager and Keinan are not alone in their fierce determination to combat the threat of explosives. Their individual successes act as focal points for others who are similarly inspired to put science to good use.
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