Explosives have been used for many applications within the military, industrial applications and are more recently involved in acts of terrorism.
With the increasing concerns globally over terrorism, 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. 63 As nerve agents are a large area of research, nerve agents 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, in addition to peroxides.
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, straightforward to interpret, and portable detection method is sort 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
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Methods of explosive detection to date have covered a number of areas including the use of canines, analytical techniques or electronic noses 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.
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 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, because 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 matters have compounds that are known to have explosive properties, for example nitroaromatics, nitramines, azides, nitric esters and peroxides.7
High explosives are categorised into two groups; Primary and secondary.
Primary high explosives are detonated easily in the presence of a shock or by heat generation, and are also known as initiating explosives, because of their ability to initiate a secondary explosive. Lead azide and lead styphnate are classified as primary explosives. 7, 8
Secondary explosives do not detonate from heat, and are more powerful than the primary explosives. These are usually used in military applications as they are more easily controlled. They 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), and nitramines such as RDX (hexahydr-1,3,5,, trinitroazine). 7 8
Low explosives can be classified as propellants or pyrotechnics.
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 high. 4 10
Nitroaromatic compounds are insufficiently volatile so have low vapour pressures making them difficult to detect without the presence of a taggant. 1 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 high vapour pressure compounds added to the explosives to aid detection.
Nitroaromatics are exceptionally electron withdrawing, and form strong transfer complexes with fluorophores that have polyaromatic hydrocarbons. The strong electron withdrawing properties account for the high quenching constants that nitroaromatic compounds possess when interacted with a fluorophore. 11 This property is exploited in fluorescence sensor detection.
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Nitroaromatic explosives have low energy unoccupied ï°* orbitals that can accept an electron from the excited state of fluorescent molecules, readily quenching the fluorescence. 10 12 The nitro groups attached to the aromatic ring pull electron density away from the delocalised system, and this makes them highly electron deficient.
Table 1 shows the structures of some common nitroaromatics explosives.
2, 4, 6-trinitrotoluene
Some explosives have a high vapour pressures. These are vapours that are released by the explosive chemical, and can be detected by sensing techniques. Unfortunately, not all explosives yield this property due to low volatility as already discussed. 13. As a result taggants are added to low vapour pressure compounds.
Taggants are used within explosives which allow easier detection before they are detonated. These compounds are either solids or liquids that emit a vapour and are added to explosives that have a low self producing vapours. Compounds such as 2-nitrotoluene, 4-nitrotoluene, 2, 3-dimethyl-2, 3-dinitrobutane and ethylene glycol dinitro 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. 14
These advantages greatly improve the ability of explosives to be detected, and at a distance.
Table 2 shows the structures of the taggants described.
2, 3-dimethyl-2, 3-dinitrobutane
ethylene glycol dinitro
Sensors for explosive detection come 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 searches 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 than humans and therefore 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 of a high advantage, 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 and therefore concentration is lost. Their findings are also difficult to quantify.
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 exhausted, 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, (GM-MS etc.)
Ion mobility spectrometry is a method of explosive detection commonly used in airports and security check points. 15 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. Larger ions are heavier and 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. 16
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The advantages of the technique include its high sensitivity, reasonable selectivity, and rapidity, (1 to 7 seconds.) 14 The size is also moderately reliable, it is easy to use, is economic, requires little power to run and the data are easy to interpret.
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 high. 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 It is also dependant on calibrating the instrument carefully in order to reference the results to the chemical database, and confirm the identity. 2
Mass spectrometry has a broad range of methods 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. Mass spectrometry is a quantitative technique that allows relatively fast analysis, but its disadvantage is the need to employ trained analysts to interpret the results. 16
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 referred to a library, 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 poor when looking for low vapour compounds. 10
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, electrochemical, 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.
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 noise can disrupt the results. The explosive is usually a particulate residue rather than a vapour for this method to work, and is employed to detect nitramines and nitroaromatics. They produce high selectivity due the chemical reactions generating identifying colour changes. Colorimetric analysis also demonstrates low limits of detection. 2
Electrochemical sensors are derived from an electrical current passing through the electrodes that interact 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, there detection by electrochemical means is idyllic.
Electrochemical methods can be used to identify a particular explosive due to the degradation scheme that they follow. The scheme reduces nitroaromatics to hydroxylamines, and then these convert 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 technique does carry disadvantages; 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 emitance of light when an electron falls from an excited state, back to the ground state. Luminescence can be divided into two main categories: fluorescence and phosphorescence.
In fluorescence; an electron in the singlet excited state that is paired to an electron in the ground state orbital by opposite spin, is spin allowed, and falls from the excited state to the ground state with a photon being emitted. This is fluorescent light. The lifetime (ï´) of a fluorophore is on average 10 ns, and is the average time between excitation of the electron to a higher excited state, and falling back to the ground state.
Phosphorescence is similar to fluorescence but instead of the electron in a singlet excited state, the electron resides a triplet excited state that is not paired to a ground state electron. The electron in the ground state is of equal spin and therefore the transmission is forbidden, consequently meaning phosphorescence emission is much slower (103 to 100 s-1) than fluorescence emission, and its lifetimes longer, in the range of milliseconds to seconds. 17
Fluorescence as a detection method
Using fluorescence as an explosive detection method has been researched extensively. Turn-on and turn- off techniques have been employed, but the most successful has been the turn-off mechanisms.
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 are both non-aromatic nitro explosives. A report by Andrew and Swagger describes a fluorescence detection turn-on technique for these two explosives that can distinguish between the two. A zinc-coordinated acridine dye is photooxidised to acridinium, a fluorescent molecule, in the presence of PETN and RDX. The detection limits were low, at concentrations of 130 ïM and 70 ïM respectively. The method does not work for the nitroaromatic, TNT, but demonstrates a rare example of a direct detection method of nitrated species. 4
The use of fluorescence in detecting nitroaromatics has been extensively studied recently. Nitroaromatics are able to quench fluorescence as they are electron deficient, and give an optical reading of their presence. An electron transfer mechanism from the flourophore to the nitroaromatic (the quencher) occurs and there is no longer an excited singlet electron in a higher energy state to fall back to the ground state and emit a photon of fluorescent light. 8
Quenching constants of a range of nitrated explosives were studied by Goodplaster et al. Aliphatic nitrated compounds have the lowest quenching constants in comparison to nitramines and nitroaromatics, with the latter confirming an observation found previously, that the aromatic systems are more efficient at quenching fluorescence than aliphatic compounds. This is largely thought to be due to the ability of aromatics to accept electrons in a charge transfer complex with the fluorophore and quench the fluorescence. 11
The study 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 2-nitrotoluene, 2,4, DNT and 2,4,6-TNT. 11
However, when there are many nitro groups attached to the aromatic ring, the quenching efficiency is decreased with respect to the systems with fewer nitro groups attached.
Quenching efficiency is therefore not just controlled by the number of nitro groups attached to the aromatic, but is also found to be affected by the diffusion coefficient of the quencher, and the electronic properties it possesses.11
Fluorescence is a beneficial technique for explosive detection as many naturally occurring chemicals do not inherently fluoresce. As a result, fluorescence is measured against a zero or low background and makes the identification of a signal more evident. 1
The power of fluorescence is dependent on the concentration of the fluorophore, which is dependent on the power source. If a stronger power source is used to initiate fluorescence, such as a high powered laser, the sensitivity of the fluorescence technique can be increased, with respect to an absorbance method, which relates the concentration ratio of the power source before and after the interaction of the sample. Consequently, "fluorescence, with respect to absorbance is one to three times more sensitive and have expanded linear ranges compared to absorbance based methods." 1 Fluorescence 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
Fluorescence detection can be used either directly or indirectly.
In order to detect nitroaromatic compounds a fluorescent compound is used that forms a complex with the nitroaromatic that causes the fluorescence to be quenched, and the intensity to be reduced. 9
Direct use of fluorescence is when the explosive is inherently fluorescent when excited by visible or ultraviolet light. For fluorescence to occur a conjugated or aromatic system is required that prevents immediate vibrational relaxation.
Nitroaromatics are electron deficient due to the strong electron withdrawing properties of the nitro groups attached. As such they cannot inherently fluoresce.1
In other types of explosives, such as nitramines, nitrate esters and peroxide explosives, there is no element of conjugation and therefore vibrationally relax effectively.
Fluorescence can be instigated in some explosives by a high energy excitation source such as gamma rays or x-rays, as can chemical reactions that generate or cause degradation of fluorescent products. 1
The more common method of fluorescence in explosive detection however is indirect.
As nitroaromatic compounds are not inherently fluorescent, indirect detection methods are used in their detection and identification. A secondary fluorescent molecule is quenched by the nitroaromatic.
Quenching is one example of indirect fluoresce detection and occurs either by a collision of the fluorophore and quencher or by the production of a ground state complex, resulting in no electrons in the singlet excited state to fall and produce fluorescence. Quenching is when the fluorescence that is produced by a fluorophore is reduced.
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 "radiative and/or non-radiative deactivation" and the quencher and fluorophore are left in the ground state. This type of quenching is described by the Stern-Volmer equation. 11
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. Many studies of solid detectors have been explored, whereby the fluorescent molecule is adjoined to a polymer backbone and incorporated into thin films of materials. Micelles have been utilised to hold fluorescent molecules in liquids and antibody- antigen complexes as biosensors have also been developed. 1 8
Arrays of fluorescing molecules have also been synthesised, where a number of fluorescent molecules are arranged onto a solid surface by chemical reaction, and this enhances the signal by accumulating all of the individual quenching mechanisms.
Liquid fluorescent sensor
A study conducted by Hughes et. al, demonstrated the ability to sequester pyrene 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 concentrate 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 it fluorescence. The ability to sequester pyrene within the micelle eliminated the need for guarding against O2 contamination.
As already mentioned, the ability for nitrated explosives to quench fluorescence is decreased from nitroaromatics, to nitramines to nitroaliphatics, and the differences large. However, within the same class of explosives, this differentiation is more difficult to determine.
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) and diphenylantracene (DPA) were dissolved 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 that is present were noticed. These were plotted as a 2D pattern using linear discriminant analysis (LDA). Where each point is on the plot indicates of the chemicals 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 therefore expand its use. 4
Work by Goodplaster and McGuffin explored separating explosives by high efficiency capillary liquid chromatography, adding a pyrene fluorophore to the eluted mixture and detecting its presence 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 pyrene reacts selectivity towards different nitroaromatics, and could be a measure of selective identification of nitroaromatic quenchers, when other nitro based compounds are present. 11
Solid fluorescent sensors
Solid based sensors have also been investigated as a detection method and have proven to be a keen area of interest. Nitroaromatics have been detected by metallole containing polymers,10 nanofibril thin films,18 fluorescence based cyclodextrin sensors, 19 fluorescent polymers, 13, conjugated polymers containing triphenylamine groups, 20 and supramolecular complexes. 9
In a study by Anandakathir et al, 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), 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 attached to the polymer backbone, rotating to form a "sphere of hindrance" 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 angstroms 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. 13
Germain and Knapp conducted a study using Zn(salicyladimine) (ZnL) sensors 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 different degrees of fluorescence quenching were observed, depending on the nitroaromatic that was introduced.
The phenolate ring of the ZnL transfers electrons to the nitro compounds, and fluorescence quenching occurs. Static and dynamic quenching are both involved in the quenching process and are "balanced by the redox potential and the steric bulk of ZnL." 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 of the nitroaromatic present. 21
A sensing film prepared from the alkoxycarbonyl-substituted, carbazole-cornered, arylene-ethynylene
tetracycle (ACTC) was developed by Naddo et al, as a fluorescent sensor for the detection of oxidative explosives.
The structure is large and planar, and ï°ï€ï° 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. However, after equilibrium, the quenching efficiencies of the two nitroaromatics were comparable, and this is thought to be because the TNT has a "high partition into the film".
This study has found that in comparison to other studies where the thickness of the film controls the quenching efficiency, the porosity and one-dimensional structure of ACTC allows ï°ï€ï° 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 speeded up by leaving it in the presence of hydrazine vapour. The oxidised defects that were present in the film were reduced by the hydrazines strong reducing properties. 18
Toal et al synthesised metallole-containing polymers (PSi, PGe and PSF), that were luminescent in a thin film for the detection of nitroaromatic explosives. The method was low-cost, straightforward, and quick. It gave a visual detection of the quenching and identified nitroaromatics at nanogram levels. 10
Ponnu and Anslyn demonstrated the use of a non-fluorescent cyclodexrin, CD, a cyclic ogliosaccharide, combined with the fluorophore 9, 10-bis(phenlyethynyl)antharacene (BPEA) to produce a fluorescent complex. 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. 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. It was concluded that this method could potentially be used as a fluorescence sensor for the detection of nitroaromatic explosives. 19
1,4-diarylpentiptycenes were synthesised by Zyryanov et al, 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 3 and figure2) 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 high, even in the solid state. As a result of this study, the preparation of the sensors for explosive detection is in progress. 9
Where Ar = 1d
Many methods of explosive detection have been employed over time, ranging from canines, to analytical methods and more recently on chemical techniques.
The advantages and disadvantages of them all 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.19 The sensitivity is due to the black background the fluorescence is against, due to 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.
The main disadvantages with the fluorescence sensing method are problems with photodegradation, photobleaching, slow response times in some cases, and can sometimes give non-specific responses, depending on the set up.
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.