reportExplosives Detection
9.5.3 Short description

There are a number of methods used to detect trace vapour such as ion-mobility spectrometry, mass spectrometry and electronic noses. Probing radiation techniques include X-ray techniques, millimetre wave imaging, terahertz technology, neutron gamma ray techniques, and nuclear quadrapole resonance. The third method uses two or more technology solutions in a complimentary way [91]. Techniques such as X- rays, gamma rays, millimetre imaging have been used for detecting explosives and weapons [92]. Based on the type of measurement obtained,
explosive sensors are largely categorised into – electrochemical, mass, optical sensors and
biosensors [93].

Electrochemical sensors - Electrochemical sensors convey changes in the environment through changes in current, when chemicals interact with the electrodes. Three main types of electrochemical sensors are in use namely, potentiometric, amperometric, and conductometric. Such sensors can be used for the detection of TNT in marine environments [94]. Detection of explosives has also been demonstrated with amperometric bioelectrochemical sensors [95].
These sensors have limited sensitivity, and need mobile electrolytes [93]. Nanocomposites of metal nanoparticles with carbon nanotubes solubilised in Nafion have been demonstrated for the
detection of TNT and other nitroaromatic explosives [96]. Glassy carbon electrodes containing copper nanoparticles and single walled carbon nanotubes have shown a reproducible detection limit of 1 ppb. Glass carbon electrode modified by single walled carbon nanotubes has been demonstrated to detect TNT [97].

Mass based sensors - Mass based sensors generally adsorb, chemicals on to the surface and the change in mass is detected by the device. The detection of the explosive is done by a travelling acoustic wave or by bending of the surface. Polymer films are used in fabrication SAW sensors that are used for detecting explosives and explosive devices [98]. Sorbent coatings for SAW sensors have demonstrated a DNT detection limit of less than 100 parts per trillion [99].

Fibre optic based sensors - Fibre optical sensors have been used for detecting explosives. They rely on changes in frequency, or intensity of electromagnetic radiation for detection of explosives [93]. Explosives such as DNT and DNB have been detected at low ppb level within seconds using optical sensors that rely on changes in fluorescence properties [100]. Optic fibre based explosive detection is based on defect free zeolite film, utilises a change in sensor reflectivity on exposure. Such sensors have not demonstrated selectivity and sensing time is about 200 seconds [101].

A number of spectrophotometric methods are used for detecting explosives such as absorption based detection, photoluminescence based detection, fluorescence based detection, laser
induced breakdown spectroscopy and terahertz based detection [93]. Absorption based detection based on the change in colour, has been demonstrated to detect nitrous explosives and explosive related compounds. This method has demonstrated a detection limit of 0.2 ng for DNT [102 ]. Nanosized molybdenum hydrogen bronze react with TATP to change colour from dark blue to
yellow. The colour change property can be used both for titration neutralisation and for detection of explosives [103].

Photoluminescence based detection - Photoluminescence based detection, is based on monitoring the photoluminescence of a nanocrystalline porous silicon film that is exposed to an analyte in flowing air stream. Nitro aromatic compound explosives have been detected using this method [104]. Fluorescence based detection of explosives relies on quenching of fluorescence
when a target molecule is acquired. The advantage of this technique is the ability to detect from a distance. Fluorescent sensory material spread over the suspected area is detected, which is illuminated with fluorescent light identifying the explosive in question. Nitro aromatic explosives have been detected with electron rich polymer semiconductors [105]. Fluorescence quenching method using pyrene as fluorophore is applied for the detection of RDX, HMX, TNT, nitromethane
and ammonium nitrate [106]. Quantum dots of cadmium selnide with zinc sulphide shell have been used to detect TNT [107]. Fluorescent nanofibrous membranes prepared by electrospinning have demonstrated very high sensitivity to trace vapours of TNT. Highly porous structures of these nanofibres have been reported to provide it high sensitivity to analytes with detection limit of 10 parts per billion [108]. Nanofibrous membranes have been reported to act as both chemiresistor sensors and fluorescence quenched sensors. These sensors based on conductive polymer nanotubes have uses in detection of explosive, biological and chemical
agents [109].

Spectroscopic methods - Explosives have been detected using techniques such as laser
induced breakdown spectroscopy. The explosive is detected by means of laser that is used to create plasma over the explosive surface. High pulsed lasers have been demonstrated to create plasma that is detected using an optical probe to determine the explosive material composition. Detection of TNT on brass and molybdenum substrates and RDX on molybdenum substrates has
been demonstrated [110]. Nuclear Resonance fluorescence has been demonstrated in detection of explosives using the signatures of carbon, hydrogen and oxygen of the elements. The experimentally demonstrated technique offers advantages such as short detection times and high probability of detection [111].

Terahertz detection –Terahertz explosive sensors are based on differential absorption. A
sample region is illuminated with two frequencies, chosen for a specific explosive, and to maximise the contrast between presence and absence of an explosive. This technique has been demonstrated as a detection tool for explosive and also identifying specifically the unique terahertz spectral fingerprints of TNT, TDX, HMX, and Semtax [93]. Carbon nanotubes based antennas for THz detection have been mentioned in the literature [112]. Potassium ions interaction with carbon nanotubes have shown to induce a strong dielectric response. The binding
event causes the carbon nanotubes to vibrate at 0.4Tz. This effect has potentially applications as a THz detector operating at room temperature [113].

Terahertz sensing for explosives detection at airports is being developed. A product demonstrator in a portable unit with data processing has been produced [114]. Improvements in growth, design,
and characterisation of low temperature grown gallium arsenide photomixers has enabled its use for sensing applications. An important component of these photomixers is nanoparticulates that reduce the charge carrier lifetime to sub-picosecond therefore allowing optical mixing to terahertz range. This approach is limited in the power output and device reliability [115]. This technique has
been demonstrated in detecting bombs and explosives in envelopes, clothes, luggage and soil [116]. Terahertz method has been demonstrated to detect RDX and RDX related explosive when are covered by opaque material [117]. One of the main challenges and limitations in Terahertz in stand off detection has been reported to be environmental conditions and barrier materials as
picosecond pulsed measurement cannot be accurately measured. Quantum dot based detectors have been claimed to be the detector of choice for terahertz detection. Multiband tunnelling quantum dots infrared photo detectors, with nanometre dimensions, have been produced using molecular beam epitaxy for security applications [118]. The design and characteristic of a indium aluminium arsenide and gallium arsenide quantum dot which respond to terahertz radiation has
been mentioned in the literature [119].

Surface enhanced Raman scattering - Surface enhanced Raman scattering has been used to
detect vapours of explosives. Raman systems for detecting explosives such as RDX, TNT, and
PETN have been demonstrated at a distance of 50 metres and over [120]. Large enhancements of Raman signals, using noble metal nanoparticles and nanostructures have been observed for adsorbed molecules. Electrochemically roughened gold and silver substrates have been demonstrated as a field SERS sensor to detect vapour signatures of TNT [121]. Enhancement of
TNT Raman signal on non-noble metal materials is being researched [122]. SERS enhancement has been demonstrated for the detection of TNT using colloidal silver suspension. High sensitivity
was demonstrated in the experimental research with detection of limit at 10-15g for DNT and 10-19g for TNT [123].

Cataluminescence - Cataluminescence is the emission of light during the catalytic oxidation of a molecule on the surface of a solid state catalyst [124]. A sensor array based on
cataluminescence, using strontium, barium and aluminium carbonates as catalysts have been
demonstrated for explosive gas mixtures [125].

Biosensors - Biosensors are devices that integrate a biological element on a solid state surface, enabling interaction with an analyte and signal transduction. Biological elements such as peptides, enzymes, receptors, single strand DNA [93]. Detection of TNT and DNT has been demonstrated using proteins immobilised on electrodes [126]. Immunoassays use antibodies as
recognition elements in biosensors. Electrochemiluminescent immunoassays have been developed for TNT detection in which enzyme labelled antibodies are bound to paramagnetic beads on the electrode surface are used. TNT has been detected with this method in 80s to a sensitivity of 31 ppb [127]. Development and comparison of two immunoassays for detection of TNT has been based on competitive inhibition [128].

Nanosensors - Nanosensors have been mentioned to be one of the most effective platforms for detection of explosives. The most important characteristics of a trace explosive sensor are sensitivity, selectivity, reversibility and real time operation. A number of sensing elements and platforms have been mentioned for nanosensors platform such as micro-nano structures, quantum dots, nanowires, nanotubes and nanobelts [129].

Selectivity to sensors is provided by coatings such as self assembled monolayers, polymers,
metal oxides and single stranded DNA. Important factors for consideration are response time and recovery. Self assembled monolayers of 4 mercaptobenzoic acid are good for explosive vapour
detection [130]. 6-Mercaptonicotinic acid monolayer has been reported to produce good results with TNT detection [131]. Molecular imprinted polymers for the detection of TNT have also been developed and demonstrated [132].

Nanomechanical sensors have been considered an ideal platform for detection of explosives.
Molecular adsorption of vapour on the surface of the cantilever, results in bending of cantilever structure, which is used to detect the presence of explosives. Piezoresistive cantilever deflection leads to a change in the resistance which is measured. Beams coated with 4-MBA have been demonstrated to detect TNT explosive vapours [133]. Nanoporus coatings tend to produce micrometer responses in the presence of vapour phase TNT and DNT. A detection limit of 520
ppt was demonstrated [134].

A number of different approaches have been considered such as thermally induced
decomposition, have demonstrated a sensitivity of 40 picogram [135]. Another approach used is photo thermal deflection spectroscopy, where a bimaterial cantilever demonstrates high sensitivity to temperature changes. The bending of the cantilever results from absorbing the IR energy on the surface of the explosive molecule [136]. Amplifying fluorescence polymers have been successfully used in detection explosives. The adsorption of the explosive molecule on the surface of the polymer leads to a change in the fluorescence characteristic. AFP’s with TNT adsorbed on the surface continuously fluorescence under the ultraviolet light source. Sensors
based on thin films of AFP have been commercialised by Nomadics [137,138].

Cantilever based sensors have been used in detecting explosives by identifying compounds
trinitrotoluene (TNT), 2,4 dinitrotoluene (DNT); pentaerythritol tetranitrate (PETN), and
hexahydro-1,3,5-triazine (RDX). TNT is a commonly used explosive and DNT remains a by product of TNT. PETN and RDX are high end explosives used for sabotaging aircrafts. The
detection of the explosive vapours was demonstrated. The sensitivity of microcantilevers in detecting such explosives is approximately 14 part per trillion within 20 seconds. Active detection of explosive vapours was allowed to deposit on a piezoresistive microcantilever which has been detected using optical signals such as laser scattering [139,140,141].

Nanowires - Nanowires also provide an effective platform for sensing explosives. Interdigitated electrode capacitors modified with single walled carbon nanotube has been demonstrated to detect chemical vapours with high sensitivity. The change in capacitance is used to detect the molecules and its class [142].

Table EW.2- Comparative assessment of the sensor performance used for detection of explosives, as cited in review paper [93]

Sensor Type	Field of application	Explosive detected	Detection limit
Electrochemical	Soil samples	RDX	0.12 ppm
Electrochemical	Marine water	TNT	25 ppb
Electrochemical	Forensic laboratory	DNB and TNT,	60 ppb for both
Electrochemical	Soil extract and ground water	RDX, TNT, 2,4-DNT, 2,3-DNT, 2,4-DNT	RDX 0.2 ppm, TNT 0.11 ppm, 2,4-DNT 0.15 ppm, 2,6-DNT 0.16 ppm, 2,3-DNT-0.15 ppm
SAW	Laboratory samples	2,4-DNT
SAW	Laboratory samples	DNT	92 ppt
SAW	Laboratory samples	2,4-DNT, TNT
Micro cantilever	For detection of explosive vapors	PETN and RDX	A low femtogram (10-15 g)
Micro cantilever		TNT	520 ppt
Optical	Field test (soil samples)	DNT	120 ppb
Optical (fiber optic based)	Ground water and soil extracts	TNT and RDX	0.1 ppm
Optical (photoluminescence based)	Air and sea water	TNT, Picric acid	4 ppb for TNT vapor in air, 1.5 ppt for TNT in sea water, 6 ppb for picric acid in sea water
Optical (fluorescence based)	Laboratory samples	TNB, TNT, DNB, tetryl, and 2,4-DNT	1 ppm for all these explosives
Optical (fluorescence based)		TATP and HMTD	2×10-6 mol L-1 for both TATP and HMTD
Optical (fluorescence based)	Water samples	DNP	1.0×10-6 mol L-1
Optical (LIBS)		DNT	NB 40 ppb for DNT and 17-24 ppb for nitrobenzene
Optical fiber (biosensor)	Ground water	TNT and RDX	0.05 ppb for both RDX and TNT
Electrochemical (biosensor)	Laboratory samples	TNT	31 ppb
Optical (immunosensor)	Artificial sea water	TNT	0.05 ppb
Electrochemical (immunosensor)	Environmental samples and clinical assay	TNT	1 ppt
Optical (fluoroimmunoassay)	Soil and water samples	TNT
Optical (immunosensor)	Laboratory samples	TNT and RDX	450 fmol for TNT, 1 ppb for RDX
Optical (immunosensor)	Seawater	TNT	250 ppt
Electrochemical (immunosensor)	Seawater	TNT	2.5 ppb in saline buffer and 25 ppb in seawater
Optical (SPR based immunosensor)	On-site detection of landmines	TNP	10 ppt
Optical (SPR based Immunosensor)	Laboratory samples	TNT	6 ppt

A combination of different sensor platforms and different optical modes has been suggested as an optimal method for successfully detecting explosive trace vapours. Mass, stress and thermal signals of TNT vapours measured by cantilevers are independent of one another and could
provide pattern recognition. Increased selectivity and sensitivity could also be achieved by combining platforms in a single sensing unit that could measure different physical and chemical properties. An example of such a combination would be SWCNT and cantilever. A SWCNT could measure the electrical polarisation of explosive molecule adsorbed on its surface by change in
capacitance. The cantilever would determine the mass and stress measurements of the same explosive molecule [129].

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Tags: electrochemical sensing, mass based sensing, fibre optic sensor, photoluminescence, spectroscopic methods, terahertz detection, SERS, Cataluminiscence, biosensors, Nanomechanical sensors, nanowires

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reportExplosives Detection 9.5.3 Short description

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reportExplosives Detection
9.5.3 Short description