Trace Detection Of Explosives Research Articles

Very Sensitive Optical System with the Concentration and Decomposition Unit for Explosive Trace Detection

AbstractThe vapour pressure of most explosives is very low. Therefore, the explosive trace detection is very difficult. To overcome the problem, concentration units can be applied. At the Institute of Optoelectronics MUT, an explosive vapour concentration and decomposition unit to operate with an optoelectronic sensor of nitrogen dioxide has been developed. This unit provides an adsorption of explosive vapours from the analysed air and then their thermal decomposition. The thermal decomposition is mainly a chemical reaction, which consists in breaking up compounds into two or more simple compounds or elements. During the heating process most explosive particles, based on nitro aromatics and alkyl nitrate, release NO2 molecules and other products of pyrolysis. In this paper, the most common methods for the NO2 detection were presented. Also, an application of the concentration and decomposition unit in the NO2 optoelectronic sensor has been discussed.

Contributed review: quantum cascade laser based photoacoustic detection of explosives.

Detecting trace explosives and explosive-related compounds has recently become a topic of utmost importance for increasing public security around the world. A wide variety of detection methods and an even wider range of physical chemistry issues are involved in this very challenging area. Optical sensing methods, in particular mid-infrared spectrometry techniques, have a great potential to become a more desirable tools for the detection of explosives. The small size, simplicity, high output power, long-term reliability make external cavity quantum cascade lasers (EC-QCLs) the promising spectroscopic sources for developing analytical instrumentation. This work reviews the current technical progress in EC-QCL-based photoacoustic spectroscopy for explosives detection. The potential for both close-contact and standoff configurations using this technique is completely presented over the course of approximately the last one decade.

Trace detection of nitro aromatic explosives by highly fluorescent g-C3N4 nanosheets.

Highly fluorescent g-C3N4 nanosheets were facilely fabricated by exfoliating bulk g-C3N4 under ultrasonic irradiation for 1 h. The atomic force microscopy (AFM) image shows that the resultant g-C3N4 nanosheets are ∼6-14 nm thick, and the suspension is stable in air for several weeks. Remarkably, the obtained nanosheets exhibited strong fluorescence with an extremely high quantum yield (QY) up to 32%, and high sensitivity, selectivity, as well as a fast response to nitro aromatic explosives were observed. Typically, the quenching efficiency coefficient Ksv for PNP was 30,460 M(-1), which proved that the resultant nanosheets possessed an extremely high sensitivity for nitro-phenol PNP detection.

Fluorescent porous film modified polymer optical fiber via "click" chemistry: stable dye dispersion and trace explosive detection.

In this paper, we report a facile strategy to fabricate fluorescent porous thin film on the surface of U-bent poly(methyl methacrylate) optical fiber (U-bent POF) in situ via "click" polymerization for vapor phase sensing of explosives. Upon irradiation of evanescent UV light transmitting within the fiber under ambient condition, a porous film (POSS-thiol cross-linking film, PTCF) is synthesized on the side surface of the fiber by a thiol-ene "click" reaction of vinyl-functionalized polyhedral oligomeric silsesquioxanes (POSS-V8) and alkane dithiols. When vinyl-functionalized porphyrin, containing four allyl substituents at the periphery, is added into precursors for the polymerization, fluorescence porphyrin can be covalently bonded into the cross-linked network of PTCF. This "fastened" way reduces the aggregation-induced fluorescence self-quenching of porphyrin and enhances the physicochemical stability of the porous film on the surface of U-bent POF. Fluorescent signals of the PTCF/U-bent POF probe made by this method exhibit high fluorescence quenching toward trace TNT and DNT vapor and the highest fluorescence quenching efficiency is observed for 1, 6-hexanedimercaptan-based film. In addition, because of the presence of POSS-V8 with multi cross-linkable groups, PTCF exhibits well-organized pore network and stable dye dispersion, which not only causes fast and sensitive fluorescence quenching against vapors of nitroaromatic compounds, but also provides a repeatability of the probing performance.

Fabrication of SERS swab for direct detection of trace explosives in fingerprints.

Swab sampling is of great importance in surface contamination analysis. A cotton swab (cotton Q-tip) was successfully transformed into surface-enhanced Raman scattering (SERS) substrate (SERS Q-tip) through a bottom-up strategy, where Ag NPs were first self-assembled onto the Q-tip followed by in situ growing. The capability for direct swab detection of Raman probe Nile Blue A (NBA) and a primary explosive marker 2,4-dinitrotoluene (2,4-DNT) using the SERS Q-tip was explored. It was found that at optimum conditions, a femotogram of NBA on glass surface could be swab-detected. The lowest detectable amount for 2,4-DNT is only ∼1.2 ng/cm(2) (total amount of 5 ng) on glass surface, 2 orders of magnitude more sensitive than similar surface analysis achieved with infrared technique, and comparable even with that obtained by ion mobility spectrometry-mass spectrometry. Finally, 2,4-DNT left on fingerprints was also analyzed. It was found that SERS signal of 2,4-DNT from 27th fingerprint after touching 2,4-DNT powder can still be clearly identified by swabbing with the SERS Q-tip. We believe this is the first direct SERS swabbing test of explosives on fingerprint on glass. Considering its relative long shelf life (>30 d), the SERS Q-tip may find great potential in future homeland security applications when combined with portable Raman spectrometers.

Direct detection and speciation of trace explosives using a nanoporous multifunctional microcantilever.

We have developed a highly selective and sensitive nanomechanical infrared (IR) calorimetric spectrometer for use in the direct detection of ultralow concentrations of explosive vapors using a nanoporous TiO2 cantilever. These cantilevers were fabricated using a two-step anodization and photolithography process. By patterning nanoscale wells onto a cantilever, its surface area is increased by 2 orders of magnitude and the surface is converted into a preconcentrator. Resonant excitation of adsorbed molecules using IR radiation causes the cantilever to bend due to temperature changes originating from the nonradiative decay process. The porous structure of the cantilever increases its thermomechanical sensitivity as well as the number of adsorbed molecules. The system performance was demonstrated by detecting binary explosive mixtures under ambient conditions. The TiO2 sensor surface also allows regeneration through the photocatalytic decomposition of adsorbates under UV irradiation.

Fast Switching of CO3–(H2O)n and O2–(H2O)n Reactant Ions in Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Explosives Detection

Ion mobility spectrometry (IMS) has become the most deployed technique for on-site detection of trace explosives, and the reactant ions generated in the ionization source are tightly related to the performances of IMS. Combination of multiform reactant ions would provide more information and is in favor of correct identification of explosives. Fast switchable CO3(-)(H2O)n and O2(-)(H2O)n reactant ions were realized in a dopant-assisted negative photoionization ion mobility spectrometer (DANP-IMS). The switching could be achieved in less than 2 s by simply changing the gas flow direction. Up to 88% of the total reactant ions were CO3(-)(H2O)n in the bidirectional mode, and 89% of that were O2(-)(H2O)n in the unidirectional mode. The characteristics of combination of CO3(-)(H2O)n and O2(-)(H2O)n were demonstrated by the detection of explosives, including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For TNT, RDX, and BP, product ions with different reduced mobility values (K0) were observed with CO3(-)(H2O)n and O2(-)(H2O)n, respectively, which is a benefit for the accurate identification. For ANFO, the same product ions with K0 of 2.07 cm(2) V(-1) s(-1) were generated, but improved peak-to-peak resolution as well as sensitivity were achieved with CO3(-)(H2O)n. Moreover, an improved peak-to-peak resolution was also obtained for BP with CO3(-)(H2O)n, while the better sensitivity was obtained with O2(-)(H2O)n.

Theory and simulation of diffusion–adsorption into a molecularly imprinted mesoporous film and its nanostructured counterparts. Experimental application for trace explosive detection

To understand the diffusion–adsorption of small gas molecules in molecularly imprinted porous (MIP) systems, two general and suitable physicomathematical models have been developed for the molecularly imprinted mesoporous film and its nanostructured counterparts.

Risley prism scan-based approach to standoff trace explosive detection

A number of optical techniques are available to perform active standoff trace explosive detection. Integrating a laser scanner provides the ability to detect explosives over a wide area as well as to assess the full extent of a threat. Risley prism laser-beam steering systems provide a robust alternative to conventional scanner solutions and are ideal for portable and mobile systems due to their compact size, low power, large field-of-view, and fast scan speed. The design of a long-wave infrared Risley prism-scanned diffuse reflectance spectroscopy system along with data obtained from a prototype system is presented for both simulant and live explosive materials.

Adsorption of 2,4,6-trinitrotoluene on the ZnO (21¯1¯) surface: A density functional theory study of the detection mechanism of ZnO nanowire chemiresistors

We report first-principles calculations of the adsorption of 2,4,6-trinitrotoluene (TNT), a prototypical nitroaromatic explosive, on the ZnO (21¯1¯0) surface. This surface is common among ZnO chemiresistors being considered for trace explosive detection. Recent work has achieved 60 ppb detection of TNT using a ZnO nanowire array, but the physical mechanism of sensing is unclear. Our results indicate that TNT strongly chemisorbs via interactions between the oxygen on the nitro groups and surface zinc, creating surface states within the gap. We present a theoretical estimate showing the strong effect of these surface states on the depletion layer of ZnO nanowires.

Evaluation of false positive responses by mass spectrometry and ion mobility spectrometry for the detection of trace explosives in complex samples

Secondary electrospray ionization-ion mobility-time of flight mass spectrometry (SESI-IM-TOFMS) was used to evaluate common household products and food ingredients for any mass or mobility responses that produced false positives for explosives. These products contained ingredients which shared the same mass and mobility drift time ranges as the analyte ions for common explosives. The results of this study showed that the vast array of compounds in these products can cause either mass or mobility false positive responses. This work also found that two ingredients caused either enhanced or reduced ionization of the target analytes. Another result showed that an IMS can provide real-time separation of ion species that impede accurate mass identifications due to overlapping isotope peak patterns. The final result of this study showed that, when mass and mobility values were used to identify an ion, no false responses were found for the target explosives. The wider implication of these results is that the possibility exists for even greater occurrences of false responses from complex mixtures found in common products. Neither IMS nor MS alone can provide 100% assurance from false responses. IMS, due to its low cost, ease of operation, rugged reliability, high sensitivity and tunable selectivity, will remain the field method of choice for the near future but, when combined with MS, can also reduce the false positive rate for explosive analyses.

Standoff explosives trace detection and imaging by selective stimulated Raman scattering

We introduce a sensitive method for laser based standoff detection of chemicals based on stimulated Raman scattering. Selective excitation of a particular Raman transition is detected by measuring the diffusely reflected laser light from a distant surface. The method simultaneously measures stimulated Raman loss and gain within a single laser shot and is insensitive to the optical properties (reflectivity/absorptivity) of the substrate. We demonstrate the specificity and sensitivity by detecting and imaging nanogram analyte micro-crystals on paper, fabric, and plastic substrates at 1 to 10 m standoff distance using only 10 mW of laser power from a single femtosecond laser.

Recognition of explosives fingerprints on objects for courier services using machine learning methods and laser-induced breakdown spectroscopy

During recent years laser-induced breakdown spectroscopy (LIBS) has been considered one of the techniques with larger ability for trace detection of explosives. However, despite of the high sensitivity exhibited for this application, LIBS suffers from a limited selectivity due to difficulties in assigning the molecular origin of the spectral emissions observed. This circumstance makes the recognition of fingerprints a latent challenging problem. In the present manuscript the sorting of six explosives (chloratite, ammonal, DNT, TNT, RDX and PETN) against a broad list of potential harmless interferents (butter, fuel oil, hand cream, olive oil, …), all of them in the form of fingerprints deposited on the surfaces of objects for courier services, has been carried out. When LIBS information is processed through a multi-stage architecture algorithm built from a suitable combination of 3 learning classifiers, an unknown fingerprint may be labeled into a particular class. Neural network classifiers trained by the Levenberg–Marquardt rule were decided within 3D scatter plots projected onto the subspace of the most useful features extracted from the LIBS spectra. Experimental results demonstrate that the presented algorithm sorts fingerprints according to their hazardous character, although its spectral information is virtually identical in appearance, with rates of false negatives and false positives not beyond of 10%. These reported achievements mean a step forward in the technology readiness level of LIBS for this complex application related to defense, homeland security and force protection.

A Microcalorimeter Integrated With Carbon Nanotube Interface Layers for Fast Detection of Trace Energetic Chemicals

Detection of trace explosives is still a challenging task because of the extremely low vapor concentrations. This paper reports a new microcalorimeter for detection of trace explosives with low detection limit and fast detection rate. The microcalorimeter consists of a suspended micromembrane with integrated heaters and thermistors and a carbon nanotube (CNT) interface layer in-situ synthesized on the membrane surface. Due to the large surface areas, the CNT interface layer improves adsorption to target chemical molecules. By operating the microcalorimeter in differential scanning calorimetry mode and differential thermal analysis mode, trace chemical detection is achieved through heating the adsorbed chemicals to deflagration and measuring the induced thermal response features and the total heat. The microcalorimeter is verified by explosive detection, and an equivalent limit of detection of 2.6 pg has been achieved by extrapolating the measurement results. Theoretically, detection is distinguishable upon 10-s fast exposure to saturated explosive vapors. These preliminary results demonstrate the ability of the microcalorimeter in detection of trace energetic chemicals.

A novel fluorescent polymer brushes film as a device for ultrasensitive detection of TNT

In this work, we first report the application of fluorescent polymer brushes film as a device for ultrasensitive and selective detection of TNT. As positively charged fluorescent molecules of quaternary ammonium tetraphenylethylene derivatives (d-TPE) were self-assembled on the brushes of poly(acrylic acid) (PAA) by coulomb forces, we obtained a novel fluorescent polymer brushes film with excellent photoluminescent properties, and the emission could be quantitatively and sensitively responsive to 2,4,6-trinitrotoluene (TNT) due to the electron transfer between d-TPE and TNT. The limit of detection of TNT was established to be 0.1 ppb in water. The fluorescent device could also be used repeatedly for TNT detection, which indicated a promising application in trace explosive detection.

Reducing cross-sensitivity of TiO2-(B) nanowires to humidity using ultraviolet illumination for trace explosive detection

Environmental humidity is an important factor that can influence the sensing performance of a metal oxide. TiO2-(B) in the form of nanowires has been demonstrated to be a promising material for the detection of explosive gases such as 2,4,6-trinitrotoluene (TNT). However, the elimination of cross-sensitivity of the explosive detectors based on TiO2-(B) toward environmental humidity is still a major challenge. It was found that the cross-sensitivity could be effectively modulated when the thin film of TiO2-(B) nanowires was exposed to ultraviolet (UV) light during the detection of explosives under operating conditions. Such a modulation of sensing responses of TiO2-(B) nanowires to explosives by UV light was attributed to a photocatalytic effect, with which the water adsorbed on the TiO2-(B) nanowire surface was split and therefore the sensor response performance was less affected. It was revealed that the cross-sensitivity could be suppressed up to 51% when exposed to UV light of 365 nm wavelength with an intensity of 40 mW cm(-2). This finding proves that the reduction of cross-sensitivity to humidity through UV irradiation is an effective approach that can improve the performance of a sensor based on TiO2-(B) nanowires for the detection of explosive gas.

SWNT Based Nanosensors for Wireless Detection of Explosives and Chemical Warfare Agents

The detection of concealed explosives and chemical warfare agents is an urgent need to protect the safety of people and cities around the world. This requires the development of highly sensitive, portable, and stand-off sensors. Here, we present a wireless sensing unit based on single-walled carbon nanotubes (SWNTs) integrated with complementary metal oxide semiconductor (CMOS) circuitry, which can effectively detect trace explosives and chemical agents. The response of SWNT sensors to Dinitrotoluene (DNT) (a byproduct of TNT) and Dimethyl methylphosphonate (DMMP) (an analog of nerve agent sarin) vapors is improved dramatically after decoration with single stranded-DNA (ss-DNA). The response of carbon nanotube sensors to DNT and DMMP vapors is reversible and repeatable. The nanosensors integrated on a CMOS chip are tested with DMMP (DNT) vapors with concentrations varying from 1.57 to 130.49 ppm (9.41-45.73 ppm), and the corresponding change in resistance of the SWNT sensor varied from 7.5% to 27.5% (6.53-22.76%). The detected signal is processed by off-chip components on a circuitry board and is transmitted wirelessly to a computer. This versatile sensing system provides a promising platform to detect explosives and chemical agents at a trace level in a wireless manner and stand-off distance.

Surface-sampling and analysis of TATP by swabbing and gas chromatography/mass spectrometry

The method of sample recovery for trace detection and identification of explosives plays a critical role in several criminal investigations. After bombing, there can be difficulties in sending big objects to a laboratory for analysis. Traces can also be searched for on large surfaces, on hands of suspects or on surfaces where the explosive was placed during preparatory phases (e.g. places where an IED was assembled, vehicles used for transportation, etc.).In this work, triacetone triperoxide (TATP) was synthesized from commercial precursors following reported methods. Several portions of about 6mg of TATP were then spread on different surfaces (e.g. floors, tables, etc.) or used in handling tests. Three different swabbing systems were used: a commercial swab, pre-wetted with propan-2-ol (isopropanol) and water (7:3), dry paper swabs, and cotton swabs wetted with propan-2-ol. Paper and commercial swabs were also used to sample a metal plate, where a small charge of about 4g of TATP was detonated. Swabs were sealed in small glass jars with screw caps and Parafilm® M and sent to the laboratory for analysis. Swabs were extracted and analysed several weeks later by gas chromatography/mass spectrometry. All the three systems gave positive results, but wetted swabs collected higher amounts of TATP. The developed procedure showed its suitability for use in real cases, allowing TATP detection in several simulations, including a situation in which people wash their hands after handling the explosive.

Methodologies for Removing/Desorbing and Transporting Particles from Surfaces to Instrumentation

Explosive trace detection (ETD) continues to be a key technology supporting the fight against terrorist bombing threats. Very selective and sensitive ETD instruments have been developed to detect explosive threats concealed on personnel, in vehicles, in luggage, and in cargo containers, as well as for forensic analysis (e.g. post blast inspection, bomb-maker identification, etc.) in a broad range of homeland security, law enforcement, and military applications. A number of recent studies have highlighted the fact that significant improvements in ETD systems’ capabilities will be achieved, not by increasing the selectivity/sensitivity of the sensors, but by improved techniques for particle/vapor sampling, pre-concentration, and transport to the sensors. This review article represents a compilation of studies focused on characterizing the adhesive properties of explosive particles, the methodologies for removing/desorbing these particles from a range of surfaces, and approaches for transporting them to the instrument. The objectives of this review are to summarize fundamental work in explosive particle characterization, to describe experimental work performed in harvesting and transport of these particles, and to highlight those approaches that indicate high potential for improving ETD capabilities.

Magnetic Resonance Detection of Explosives and Illicit Materials

This special issue of Applied Magnetic Resonance is devoted to magnetic resonance detection of explosives and illicit materials. This special issue presents papers on a very specific and important area of nuclear quadrupole and magnetic resonance (NQR and NMR) applications for detection of various solid-state and liquid explosive materials, illicit substances and medicine counterfeiting. The detection of explosives is an issue of extremely importance for modern civilization. There are remains of explosives after the wars and local conflicts as well as the explosives which are used by terrorists around the world. In the aviation and public security, there is a problem of non-invasive detection of the explosives in the baggage, suits, cars, and others. In spite of the availability of various detectors, there are a number of problems to be resolved, such as increasing the sensitivity, shortening the detection time, decreasing the scanner cost, etc. In addition, an enormous number of various explosive materials are known that imposes a requirement for an effective technique to detect a vast range of possible threats. A bomb or a mine has plastic or metal enclose (usually hermetically sealing the content) and may also contain extra materials in addition to the explosive itself that makes a number of detection techniques not possible to use. Available detection techniques can be divided on the two large group of bulk and trace explosive detection methods. Although some trace detection techniques have proven their high effectiveness (such as dogs, ion mobility/mass/Raman/terahertz spectrometry, etc.), their application is unlikely to resolve an issue of the reliable explosive