Novel Materials for High Explosive Detection
A review of the literature summarising a range of explosive detection methods that are currently available is documented. The review takes a glimpse of primitive methods such as canines and metal detectors, through a progression of technology to analytical based methods and more recently, advanced science by the use of chemical methods, in particular fluorescence detection.
A range of fluorescent materials and their properties is discussed and the strengths that the new detection methods provide, in a time where terrorism concerns are prevalent, and the need for increased national defence and security measures crucial.
Novel Materials for High Explosive Detection
Explosives have been used for many applications within the military, employed by industry and are more recently involved in acts of terrorism. With the increasing global concerns regarding terrorist action, enhancements in national security and defence are required to rapidly monitor and detect the presence of high explosives, in a fast, efficient and economic fashion.
Explosive materials are presented in a plethora of complex environments in the field, and within a range of matrices and containments. 1 2 3 4 5 In addition to high explosives, there are concerns over nerve agents. These are based on organophosphates that block the neurochemical acetylcholine from transmitting nervous responses. 6 3
As nerve agents are a large area of research, they are out of the scope of this review.
Nitrated compounds are a popular choice of explosives and come in many forms, such as nitroaromatics, nitramines, nitrate esters and nitroaliphatics. The detection of explosives is a key area of interest within research and development. Improving current techniques or generating new methods which have the desirable properties needed for a diverse analysis is vital. A simple, easily operable, quick, selective, economical, easily interpreted, and portable detection method is sought to detect at trace levels. As the range of explosives that are in use is large, this is a complicated task as one method is not diverse enough to cover all the detection criteria. 1
Methods of explosive detection to date have covered a number of areas including the use of canines, analytical techniques and chemical approaches. An introductory review of canines and analytical techniques will be covered, with a more comprehensive discussion of chemical methods, in particular fluorescence detection, that are currently in use or in development stages for the detection of nitroaromatic explosives.
2.0 Properties of Explosives
Explosions fall into three main categories; physical, atomic and chemical. The focus of this review is on chemical explosions. Chemical explosions are generated when a large amount of heat energy and gas is produced in a short space of time due to a change in the state of a compound or a chemical reaction occurring. In a confined container, a rapid exothermic reaction is generated causing the release of gas and an increase in pressure since the gas cannot expand instantaneously. As the pressure becomes strong enough to burst the surrounding container, a blast wave is generated and causes damage to the container and nearby objects. 7
Chemical explosives are classified into two categories; matter that is explosive, and matter that contains explosive mixtures. Explosive matter compounds are known to have explosive properties, for example nitroaromatics, nitramines, azides, nitrate esters and peroxides.7 (Table 1) High explosives are categorised into two groups; primary and secondary. ( 1)
Table 1 - Examples of explosive classes
(Tri acetone triperoxide)
2.1 Primary High Explosives
Primary high explosives are detonated easily in the presence of shock or by heat generation and are also known as initiating explosives, because of their ability to initiate a secondary explosive. Lead azide (Pb(N3)3 and lead styphnate are classified as primary explosives. ( 2) 7, 8
2.2 Secondary High Explosives
Secondary explosives do not detonate from heat and are more powerful than primary explosives. These are usually used in military applications as they are more easily controlled and can only be detonated by the shock produced when a primary explosive explodes. 7 Examples include nitroaromatics such as TNT (2,4,6- trinitrotoluene), picric acid, and tetryl (2,4,6-Trinitrophenyl-N-methylnitramine), (Table 2) and nitramines such as RDX. (Table 1) Low explosives can be classified as propellants or pyrotechnics.
2.3 Nitroaromatic explosives
Nitroaromatic compounds are electron deficient aromatic substances that are commonly used as explosives. They are found in improvised explosive devices and landmines utilized by terrorist organisations. 9 As they are relatively cheap and easily available, the importance in detecting them is great. (Table 2) 4 10
Table 2 - Examples of some common nitroaromatic explosives
2, 4, 6-trinitrotoluene
2.4 Explosive vapours
Explosives such as ethylene glycol dinitrate have a high vapour pressure that can be detected by sensing techniques.11 Nitroaromatic compounds are insufficiently volatile thus have low vapour pressures making them difficult to detect without the presence of a taggant. 112. For example, at room temperature TNT has a vapour pressure of 5 p.p.b but this is much lower, by up to 6 times, when it is confined in casing, or mixed with other explosives. 10
Taggants are used within explosives to allow easier detection before they are detonated and are legally required. These compounds are either solids or liquids that emit a vapour and are added to explosives that have low self producing vapours. Compounds such as 2-nitrotoluene (2NT), 4-nitrotoluene (4NT), 2,3-dimethyl-2,3-dinitrobutane (DNMB) and ethylene glycol dinitrate (EGDN) (Table 3) have properties that are desirable for this kind of detection. They pose no environmental damage, are not found naturally in the environment, consistently release vapours for up to ten years, do not thwart the explosive properties of the compound that has been tagged, and are not susceptible to binding to other media the taggant may be in contact with. 13 These advantages greatly improve the ability of explosives to be detected.
Table 3- Structures of chemical taggants
2, 3-dimethyl-2, 3-dinitrobutane
Ethylene glycol dinitrate
Sensors for explosive detection appear in a variety of forms, the three main categories are; canines, analytical or electronic noses and chemical detection. In the past, metal detectors were vastly used for signalling the locations of landmines due to the metal cases they were encapsulated in. This now however, is almost an invalid approach, as many casings of explosives are principally plastics. As a result more technical methods of explosive detection are required.
The most commonly used method of explosive detection is that of sniffer dogs. Sniffer dogs have been used for many years in the search for humans, drugs and explosives. They are more able to perform this task than a human due to having an olefactory system four times larger and thus having a significant number of receptors that, once trained, can distinguish between many compounds. They can also follow a scent due the separation of the nostrils by the septum.3 2
This method of detection is very sensitive, and their use is advantageous, but dogs are expensive to keep and train, and cannot be left to work alone, so costs rise due to the salary of the handler. In addition to this, they tire easily and can become distracted by other scents in the environment they are searching. Their findings are also difficult to quantify.
3.2 Electronic noses- analytical methods
In light of this, electronic noses as they have become known are an improvement on canines. Analytical techniques are used to detect the vapours released by the explosive compounds and some solid residues. These include but are not exhaustive, instruments such as mass spectrometry (MS), ion mobility spectrometry (IMS) and chromatographic methods including gas chromatography, (GC), high performance liquid chromatography (HPLC) and hyphenated techniques (GC-MS etc.)
3.2.1 Ion Mobility Spectrometry
Ion mobility spectrometry is a method of explosive detection commonly used in airports and security check points. 14 The principle of the method is a filtering system based on the size and charge of the ion which travels through a drift tube against a drift gas and an electric current. ( 3) Larger ions are heavier, have a larger cross section and consequently progress through the tube at a slower speed than the smaller ions that travel faster. The ion mobility chromatogram that is produced displays the ion current as a function of the drift time. ( 5)11 The advantages of the technique include its high sensitivity, reasonable selectivity and rapidity (1 to 7 seconds).13 It is of a portable size, is easy to use, economic, requires little power to run and the data are easy to interpret. ( 4) IMS excludes the need for any sample preparation prior to the analysis, is able to detect trace levels of explosives at atmospheric pressure in little time and its detection limits are low.5 This technique has now also been engineered to be a hand held device that shows the same and, in some cases, better detection limits than the table-top versions of the instrument. However, this method does show an overlap in the drift times of some explosives detected, which makes the detection of multiple samples difficult when ran simultaneously. ( 5) 5 It is also dependant on calibrating the instrument carefully in order to reference the results to the chemical database and confirm the identity. 2
3.2.2 Mass spectrometry
Mass spectrometry has a broad range of approaches to detect and quantify explosive vapours and can also be coupled to other techniques to enhance the information that is generated prior to the mass spectrum.
3.2.3 Chromatography techniques
There are a range of chromatographic techniques employed in the detection of explosives. GC and LC separate the explosive mixture according to the retention time of the explosive on a column. The time it takes for each explosive compound to be eluted is referenced to a library database and the compound can be identified. These methods are generally coupled to another separation method such as MS, which confirms the presence of the explosive and quantifies the initial qualitative result. All these methods of detection and others are great in the detection of vapour samples and in some explosive residues and traces, but they are rarely low powered, hand-held, portable devices that can be made cost effectively. In addition, the vapour phase methods are deficient when looking for low vapour compounds. 10
4.0 Chemical Methods of detection
Chemical methods of explosive detection are quickly becoming popular. These methods are cheap, easy to interpret by non-specialists and portable. Optical sensors are a major use of chemical explosive detection and show a visual change when a chemical reaction takes place.
Chemical methods of detection include colorimetric analysis, electrochemistry and luminescence. Other methods are also used, and vary in size, cost, sensitivity and ease of use. Sample preparation is required in many analyses which are usually at a different site to the place of detection. Transport to the site is therefore required and results are consequently not immediate. This is a disadvantage to some chemical methods. However, methods that give an immediate response have been developed.
4.1 Colorimetric analysis
Colorimetric methods of detection are based on a colour change that occurs due to altering the absorption of visible light when an explosive chemically reacts with a sensor. A visual examination allows ease of interpretation. The signal that is produced is a ratio of light absorption and interferences, and the signal: noise ratio can be high in some cases. The explosive is usually a particulate residue rather than a vapour for this method to work, and is employed to detect nitramines and nitroaromatics. These compounds produce high selectivity due the chemical reactions generating identifying colour changes. Colorimetric analysis also demonstrates low limits of detection. 2
4.2 Electrochemical analysis
Electrochemical sensors pass an electrical current through the electrodes that interacts with the chemicals. Three methods of electrochemical detection are used, potentiometric (measuring voltage), conductometric (measuring the conductivity) and amperometric (measuring the current). As a result of the electrical current, the explosive chemicals are modified. As nitroaromatic chemicals are redox active, their detection by electrochemical means is idyllic.
Electrochemical methods can be used to identify a particular explosive due to the degradation scheme that they follow. For example, the scheme shown below ( 6) illustrates the reduction of nitroaromatics to hydroxylamines, and further conversion to amine groups. The reduction potentials at each step determine the nitroaromatic being reduced and the current that is needed per unit of time is relative to the concentration of the explosive in a liquid media.
However, the electrochemical technique does carry a disadvantage; there is a limited sensitivity of electrochemical sensors. For this method to be effective an electrolyte must be mobile within the system that can keep the charge balances once an electron has been taken into the chemical that is being detected. In addition, the carbon electrodes are easily fouled; coating to prolong their life makes the sensor more complex than it should be for its application.8
Some chemical compounds have the ability to produce luminescence; the emission of light when atoms ormolecules undergo a radiative transition from an electronically excited state to the ground state. Luminescence can be divided into two main categories: fluorescence and phosphorescence. Fluorescence emission occurs via the decay of atoms or molecules in the excited singlet state. The kinetic lifetime (t) of a fluorophore is on average in the order of nanoseconds, is the time taken for 69% or 1/e of the excited state to decay to the ground state. Phosphorescence is an analogous process that occurs via emission from triplet excited states. Such electronic transitions are (spin) forbidden and therefore lifetimes are much longer in comparison to fluorophores, typically in the order of milliseconds to seconds. These processes can be summarised in a Jablonski diagram shown in ( 7).15 16 17
5.0 Fluorescence as a detection method
Fluorescence as an explosive detection method is almost the opposite of colorimetric analysis as the light intensity is reduced by the analyte instead of amplified. For fluorescence to occur, a conjugated or aromatic system is required that prevents immediate vibrational relaxation. Fluorescence is a beneficial technique for explosive detection as many naturally occurring chemicals do not inherently fluoresce and therefore, fluorescence is measured against a zero or low background, making the identification of a signal more evident. 1 It has minimal requirements on the equipment, as only a source of excitation and a detector is needed, and this is easily combined into a portable device. 1
The main disadvantages with the fluorescence sensing method are problems with photo degradation, photobleaching, slow response times in some cases and can sometimes give non-specific responses, depending on the set up. Turn-on and turn-off techniques have been employed and fall under the categories of direct and indirect detection.
5.1 Direct detection by fluorescence
Excitation of the explosive by UV or visible light that generates fluorescence inherently is known as direct detection. Fluorescence can be instigated in some explosives by a high energy excitation source such as gamma rays or x-rays, or by chemical reactions that generate or cause degradation of fluorescent products. 1
Turn-on techniques use the chemical or redox reaction between the explosive and another compound to generate a product that fluoresces. 1 RDX and PETN (Table 1) are both non-aromatic nitro explosives. A report by Andrew and Swagger 18 describes a fluorescence detection turn-on technique that can distinguish between the two. A zinc-coordinated acridine dye ( 8) is photooxidised to acridinium a fluorescent molecule, in the presence of PETN and RDX. The detection limits were low, at concentrations of 130 mM and 70 mM respectively. The method does not work for the nitroaromatic, TNT, but demonstrates a rare example of direct detection of nitrated species. ( 9) 4
5.2 Nitroaromatic explosives in fluorescence
Nitroaromatics are exceptionally electron deficient because the nitro groups bonded to the aromatic ring pull electron density away from the delocalised system. They are strongly electron withdrawing and form strong transfer complexes by p stacking with electron rich conjugated or aromatic fluorophores. 2 Nitroaromatic explosives have low energy unoccupied p* orbitals that can accept an electron from the excited state of fluorescent molecules, accounting for high quenching constants, (see section 5.3).10 19 20 This is a property exploited in fluorescence sensor detection as nitroaromatics cannot inherently fluoresce. 1 8 9
5.3 Indirect detection by fluorescence
Indirect detection methods are used in nitroaromatic explosive detection and identification, where a secondary fluorescent molecule is quenched by the nitroaromatic. This is also known as a turn off technique.
Quenching is when the excited molecule is deactivated by an external stimulus such as a quencher. It occurs either by collision of the fluorophore and quencher or by the production of a ground state complex. ( 10)
There are two main mechanisms of fluorescence quenching; static and dynamic. In static quenching, a complex is formed between a ground state fluorophore and a ground state quencher. The complex that is formed is stable, and the properties of the spectrum produced are different to the spectrum of the free fluorophore. In dynamic quenching, the interaction of an excited state fluorophore and a quencher in its ground state collide whilst the fluorophore is in its excited lifetime. The complex that forms is initially in the excited state, but this dissipates via deactivation radiatively and or non-radiatively and the quencher and fluorophore are left in the ground state. 20
When fluorescence quenching is used for explosive detection, the fluorophore used must be chosen carefully to ensure that the selectivity and sensitivity are adequate. The intensity of the signal produced by a fluorophore in the absence of a quencher should be strong, and on addition of the quencher the signal should be reduced significantly in a relation proportional to the concentration. The fluorescent molecules can either be in solid or liquid phases.
Quenching constants of a range of nitrated explosives were studied by Goodpaster et al. They found that aliphatic nitrated compounds have the lowest quenching constants in comparison to nitramines and nitroaromatics, with the latter confirming an observation found previously, that aromatic systems are more efficient at quenching fluorescence than aliphatic compounds. This is largely thought to be due to the ability of aromatic groups to accept electrons in a charge transfer complex with the fluorophore. 20 Goodpaster also discovered that the more abundant the nitro groups were on the aromatic explosives, the higher the quenching constant became. Electron density being withdrawn from the delocalised system, results in the affinity of the aromatic ring for electrons to increase. This was seen in 2NT, 2,4, DNT and 2,4,6-TNT. 20 However, when there are many nitro groups attached to the aromatic ring, the quenching efficiency is decreased with respect to fewer nitro groups bonded. Quenching efficiency is therefore not just controlled by the number of nitro groups attached to the aromatic, but is also affected by the diffusion coefficient of the quencher, and the electronic properties it possesses.
6.0 Liquid fluorescent sensors
A study conducted by Hughes et. al, 4 demonstrated the ability to sequester pyrene ( 11) within a micelle to encourage the interaction between pyrene itself and nitrated explosives. As the hydrophobic environment within a micelle would favour the sequestration of hydrophobic explosives, an increase in the concentration of the analyte would form in these areas of the solution.
Three advantages were bestowed to sensing explosives; first, a sensitivity increase of the assay; as the area for collisions to occur within the micelle is smaller and more concentrated than in the bulk solution, quenching of pyrene can more readily occur. Secondly, pyrene is almost exclusively hydrophobic in nature and is found largely within the micelle environment, whereas the explosives that are small and nitrated are distributed a little more between the two surroundings. The range of nitrated explosives had varying hydrophobicities, and therefore had a degree of differentiation amid their relative distributions of the micelle and bulk solution. As such, their abilities to quench the pyrene differed and a selective discrimination method could be developed. Finally, within the micelle, pyrene is protected from molecular oxygen that has damaging effects to its fluorescence. The ability to sequester pyrene within the micelle eliminated the need for guarding against O2 contamination.
Within the same class of explosives quenching efficiencies are more difficult to differentiate than between classes. The use of fluorescence in micelle systems was developed into a sensor array, providing different fluorophores in the micelluar solution that show varying responses to the interactions with the quencher molecules.
Pyrene, pyrene excimer, pyrene-perylene fluorescence resonance energy transfer (FRET) pair and diphenylanthracene (DPA) were present in a commercial surfactant, Tween 80. When different nitrated explosives were added to the solution, distinct patterns of fluorescence quenching that are indicative of the explosive present were noticed. These were plotted as a 2D pattern using linear discriminant analysis (LDA), where the location of each point on the plot indicates the chemical identity.
The method designed for the detection of nitrated organic explosives has shown to be an inexpensive and eloquent sensor. With good sensitivity and differentiation powers between similar structures such as RDX and HMX, the sensor has the potential to be used for determination of other explosives, as alternative surfactants and fluorophores can be used in place of the current system and consequently expand its use. 4
Work by Goodpaster and McGuffin explored separating explosives by the use of high efficiency capillary liquid chromatography. To the eluted mixture, pyrene fluorophore was added and its presence was detected by laser induced fluorescence as a selective indirect fluorescence detection technique. It was found that the interactions of nitroaromatics stabilise the excited state of the pyrene and cause a shift in the emission to longer wavelengths. Therefore the pyrene reacts selectivity towards different nitroaromatics and could be a measure of selective identification of nitroaromatic quenchers, when other nitro based compounds are present. 20
7.0 Solid fluorescent sensors
Solid based sensors have also been investigated as a detection method and have proven to be a keen area of interest.
In a study by Anandakathir et al, 12 the direct fluorescence quenching by explosives of a thiophene based conjugated polymer was explored. The polymer, poly [2-3(3-thienyl)ethanol n-butoxy carbonyl methyl-urethane] (PURET), ( 13) was synthesised in two steps. Initially the monomer, from 2-(3-thienyl) ethanol and butyl isocyanato acetate, in the presence of dibutyltin dilaurate was synthesised. Polymerisation of the monomer then by dehydrogenation in the presence of anhydrous ferric chloride yielded the PURET polymer.
In solution, the fluorescence quenching of the PURET was poor and ineffective, and was thought to be due to the side groups that were bonded to the polymer backbone, rotating to form a hindrance sphere and preventing interaction of the quencher and the backbone. For effective electron transfer to occur, and consequently quenching of the polymers' fluorescence, the analyte and polymer need to be within 10 Å of each other.
However, when a spin-coated thin film of the polymer was made and exposed to the vapours of a range of nitroaromatics, strong fluorescent quenching was observed within 3 minutes of exposure to DNT. This was repeated with TNT and 2NT and the quenching occurred at different rates depending on the nitroaromatic present. This could be an attribute of the analyte properties or the interaction of the polymer with the analyte. The study concluded that the thin film polymer could be used as a method of explosive detection. 12
Germain and Knapp 21 conducted a study using Zn(salicyladimine) (ZnL) sensors ( 14) to differentiate between nitroaromatics within the same class. A sensor array was developed using ZnL, which are powerful fluorophores, and transfer electrons to nitroaromatics and nitroalkanes. With varying the ligands (L) coordinated, different intensities of fluorescencing molecules were accumulated into the array and various degrees of fluorescence quenching were observed depending on the nitroaromatic that was introduced. ( 15)
Static and dynamic quenching are both involved in the quenching process and the steric bulk of the ZnL complex and redox potential balance the quenching. This balance means the quenching mechanism is dependent on the structure of the ZnL and the nitroaromatic. An array of ZnL sensors has been formed and each has an individual response to the nitroaromatics. The type of quenching mechanism that reduces the fluorescence is dependant on the nitroaromatic present. 21
Toal et al 10 synthesised metallole-containing polymers (PSi, PGe and PSF (Table 4) that were luminescent in a thin film for the detection of nitroaromatic explosives. The method was low-cost, straightforward and rapid. The metallole containing polymers gave a visual detection of the quenching and identified nitroaromatics at nanogram levels.
Table 4 - Structures of Toal's metallole- containing polymers
A sensing film prepared from the alkoxycarbonyl-substituted, carbazole-cornered, arylene-ethynylenetetracycle (ACTC) ( 16) was developed by Naddo et al, 22 as a fluorescent sensor for the detection of oxidative explosives.
The structure is large and planar, and p-p stacking of molecules is therefore easily achieved. The material is porous and as such gaseous molecules can be detected. Naddo et al principally focused on the detection of 2, 4-DNT and 2, 4, 6-TNT. A saturated vapour of the DNT and TNT were passed over the sensor and the fluorescence of ACTC was quickly quenched. TNT quenched ACTC slower than DNT and this is thought to be due to the higher vapour pressure that DNT yields. ( 17) However, after equilibrium the quenching efficiencies of the two nitroaromatics were comparable and this is thought to be because the TNT integrates well into the film.
Naddo's study has found that in comparison to other studies, such as Toal's polymers, 10 where the thickness of the film controls the quenching efficiency, the porosity and one-dimensional structure of ACTC allows p-p stacking to occur, and for the quencher to gain easier access to excited states. As such the thickness of the film is no relation to the ability of fluorescence quenching from occurring.
The film once dosed in the nitroaromatic vapours could be reversed to fluoresce again by exposing the film to air over a couple of days. This process could be accelerated by leaving it in the presence of hydrazine vapour. The oxidised defects that were present in the film were reduced by the strong reducing properties of hydrazine. 22
Ponnu and Anslyn 23 demonstrated the use of a non-fluorescent cyclodextrin, CD, a cyclic ogliosaccharide, combined with the fluorophore 9, 10-bis(phenylethynyl)antharacene (BPEA) to produce a fluorescent complex ( 18). TNT and other nitrated explosives were exposed to the complex, and the ability for them to bind or be included was studied. The aromatic nitrated compounds that were analysed (TNT and Tetryl) had different quenching capacities to the sensor than the non-aromatic explosives such as RDX, PETN and HMX. TNT and tetryl both quenched the complexes fluorescence, yet the non-aromatic compounds, RDX, PETN and HMX had no effect on the sensor, suggesting they did not bind to the CD or quench the fluorescence of the BPEA. The quenching of the TNT and tetryl were compared and found that the TNT quenched the fluorescence more strongly than tetryl. ( 19) The cavity size of the CD was thought to be the reason for this and may suggest that TNT fits better into the cavity than tetryl, which gave a higher quenching constant. Ponnu et al concluded that this method could potentially be used as a fluorescence sensor for the detection of nitroaromatic explosives. 23
1,4-diarylpentiptycenes were synthesised by Zyryanov et al 9, in a two step preparation to yield a relatively cheap sensor for the detection of the nitroaromatics, 2,4 DNT and TNT. The 1e and 1d (Table 5, 20) 1,4-diarylpentiptycenes gave the highest fluorescence quenching for the two nitroaromatic explosives. When solution cast polyurethane films were doped with the successful 1,4-diarylpentiptycenes, the quenching abilities were still good, even in the solid state. As a result of this study, the preparation of the sensors for explosive detection is in progress. 9
Table 5 - Structure of Zyryanov's nitroaromatic sensor
Where Ar = 1d in 20
Ar= 1e in 20
Many methods of explosive detection have been employed historically, ranging from canines, to analytical methods and more recently on chemical techniques. The advantages and disadvantages vary and as such, one technique for the exclusive detection of all explosives is elusive.
Fluorescence sensors are proving to be a valuable technique in the detection of nitrated explosives and are a sensitive and convenient method in their detection .23 The sensitivity is due to the low background the fluorescence is against, and there being few inherently fluorescent compounds in the environment.
With the addition of taggants to aid uncovering explosives and the low limits of detection fluorescence sensing ascertains, significant advances in security and defence can be accomplished, along with forensic applications. A range of methods have been studied from direct excitation of the explosive via high energy techniques to indirect fluorescent quenching, in liquids and on solids. Arrays of fluorophores have been synthesised in order to test for an assortment of nitrated explosives simultaneously. Not only have these techniques been able to detect the presence of nitrated explosive vapours, they have differentiated between classes of nitrated explosives, (nitroaromatics, nitramines and non-aromatised nitro compounds) and shown differences in their spectra between closely related structures, such as TNT and DNT.
In addition, the equipment for the detections is relatively cheap and accessible, the measurements need minimal kit that is small enough to make into a portable device and the results are simple to interpret. The quenching affects of the nitrated compounds is reversible and therefore one sensor can be used repeatedly, before a replacement is required, making the method cost effective.
Overall the fluorescence detection is a strong candidate for the detection of explosives, but other methods of detection are valuable and provide advantages that fluorescence cannot give. IMS is a technique currently used at airports and despite the advances in fluorescence sensing, a range of techniques are required to cover the diverse and complex task of screening for high explosives.
The future of explosive detection requires a technique that will encompass all varieties of explosive to be detected in a single screen, and one that is not dependant on the type of structure the explosive yields. This is an elusive task, and in the meantime an accumulation of the aforementioned techniques can be employed to increase national security and defence against terrorist organisations.