Explosive Taggant Research Articles

Stationary Explosive Trace Detection System Using Differential Ion Mobility Spectrometry (DMS).

Detecting trace amounts of explosives is important for maintaining national security due to the growing threat of terror attacks. Particularly challenging is the increasing use of homemade explosives. Therefore, there is a constant need to improve existing technologies for detecting trace amounts of explosives. This paper describes the design of a stationary device (a gate) for detecting trace amounts of explosives and explosive taggants and the design of differential ion mobility spectrometers with a focus on the gas system. Nitromethane (NM), trimeric acetone peroxide (TATP), hexamine peroxide (HMTD), and explosive taggants 2,3-dimethyl-2,3-dinitrobutane (DMDNB) and 4-nitrotoluene (4NT) were used in this study. Gate measurements were carried out by taking air from the hands, pocket area, and shoes of the tested person. Two differential ion mobility spectrometers operating in two different modes were used as explosive detectors: a mode with a semi-permeable membrane to detect explosives with high vapor pressures (such as TATP) and a mode without a semi-permeable membrane (using direct introduction of the sample into the measuring chamber) to detect explosives with low vapor pressures (such as HMTD). The device was able to detect trace amounts of selected explosives/explosive taggants in 5 s.

Microwave spectra of dinitrotoluene isomers: a new step towards the detection of explosive vapors.

The spectroscopic characterization of explosive taggants used for TNT detection is a research topic of growing interest. We present a gas-phase rotational spectroscopic study of weakly volatile dinitrotoluene (DNT) isomers. The pure rotational spectra of 2,4-DNT and 2,6-DNT were recorded in the microwave range (2-20 GHz) using a Fabry-Perot Fourier-transform microwave (FP-FTMW) spectrometer coupled to a pulsed supersonic jet. Rotational transitions are split by hyperfine quadrupole coupling at the two 14N nuclei leading to up to 9 hyperfine components. The spectral analysis was supported by quantum chemical calculations carried out at the B98/cc-pVTZ and MP2/cc-pVTZ levels of theory. Based on 2D potential energy surfaces at the B98/cc-pVTZ level of theory, the methyl group internal rotation barriers were calculated to be V3 = 515 cm-1 and 698 cm-1 for 2,4- and 2,6-DNT, respectively. Although no splitting due to internal rotation was observed for 2,6-DNT, several splittings were observed for 2,4-DNT. The microwave spectra of both species were fitted using a semi-rigid Hamiltonian accounting for the quadrupole coupling hyperfine structure. Based on the internal axis method (IAM), an additional analysis was performed to retrieve an accurate value of the rotationless A-E tunneling splitting which could be extracted from the rotational dependence of the tunneling splitting. This yielded in the case of 2,4-DNT to an experimental value of 525 cm-1 for the barrier height V3 which agrees well with the DFT value. The coupled internal rotations of -CH3 and -NO2 are investigated in terms of 2-D surfaces, as already done in the case of 2-nitrotoluene [A. Roucou et al., Chem. Phys. Chem., 2020, 21, 2523-2538].

Luminescent CdSe Quantum Dot Arrays for Rapid Sensing of Explosive Taggants

Chemical sensors based on fluorescent quantum dots have attracted intense interest because of their excellent optical and electronic properties compared to the routinely employed fluorescent organic dyes. This study reports a CdSe QD-polymer-based luminescent chemosensor, which is based on an array containing either green-emitting or red-emitting CdSe QDs embedded in polycaprolactone as a polymer host matrix. We evaluate the sensing capability of the nanocomposites by exposing both sensors to vapors of explosive taggants, explosive-like molecules, and some common solvents. Both nanocomposites exhibit a very fast response time of <30 s. The limit of detection of the sensors for 3-nitrotoluene, 4-nitrotoluene 2,3-dimethyl-2,3-dinitrobutane, and picric acid was found to be 0.055, 2.7, 0.7 and 916.4 ng, respectively. The sensor array constitutes a powerful tool to discriminate between explosive taggants (3-nitrotoluene, 4-nitrotoluene, and 2,3-dimethyl-2,3-dinitrobutane) and shows specific molecular recognition toward picric acid. This type of miniaturized luminescent QD-based nanocomposites might form the basis of a sensing platform technology to perform effective chemical detection and identification of explosive taggants preblast and postblast.

Molecularly imprinted nanocomposites of CsPbBr3 nanocrystals: an approach towards fast and selective gas sensing of explosive taggants

Solid-state luminescent gas sensor based on CsPbBr3 NCs embedded in a molecularly imprinted polymer (MIP) with fast response and high selectivity to nitro compounds.

Vapor pressure and thermophysical properties of explosive taggants

Knowledge of the vapor pressure is one of the key parameters that determine the application of detection taggants for a given explosive and useful lifetime. The objective of this work was to determine the vapor pressures and thermodynamically linked properties of selected explosive taggants, namely 2,3-dimethyl-2,3-dinitrobutane, 2-nitrotoluene, and 4-nitrotoluene. Experimental studies included vapor pressure measurements using a static method, investigation of phase behavior by differential scanning calorimetry, and measurement of condensed-phase heat capacities. On the basis of the above-mentioned data and in combination with theoretically calculated ideal-gas thermodynamic properties, the recommended vapor pressure equations for the explosive taggants in a wide temperature range were developed by the method of simultaneous correlation of vapor pressures and thermodynamically linked properties.

Molecularly Imprinted Silver Nanocomposites for Explosive Taggant Sensing

Sensing explosive taggants such as 3-nitrotoluene (3-NT) and 2,3-dimethyl-2,3-dinitrobutane has become a strategic priority in homeland security. This work reports the synthesis of a solid-state plasmonic sensor based on a nanocomposite of Ag nanoparticles (NPs) embedded in a molecularly imprinted polymer (MIP) for selective detection of 3-NT, an explosive taggant for 2,4,6-trinitrotoluene. In our approach, the in situ synthesis of Ag NPs and the molecular imprinting with 3-NT as a template take place simultaneously inside the polyethyleneimine (PEI) thin film during the baking step after spin coating. The MIP sensor fabrication is done by a low-cost, fast, and scalable one-step procedure. We demonstrate the chemosensing capabilities of Ag-PEI MIP nanocomposites to 3-NT using the localized surface plasmon resonance band intensity decay as a sensing parameter. The molecular imprinting approach results in an enhancement of specific sensor response to 3-NT, with a limit of detection of 54.8 ng for 3-NT and a sensitivity of 24.0% ± 3.0%. We tested the MIP sensor specificity by comparing the sensor response to several NO2-containing molecules. The Ag-PEI MIP sensor demonstrated a robust, specific molecular recognition toward 3-NT. Because the MIP nanocomposite sensor is easy to prepare, easy to use, and inexpensive, these plasmonic sensors can be easily implemented with portable reading platforms into remote explosive detection and bomb disposal robots.

Effects of delaying measurements of concentration using neutron activation analysis on explosive taggants

A nuclear barcode has been proposed to identify taggants added to explosives, which encodes identifying information by adding a unique combination of taggant elements at different concentration levels. Testing was conducted on two single element solutions at three concentrations to determine minimum detection concentration levels. The effect of delaying the time to obtain measurements and its effect on uncertainty over approximately four half-lives (5 days) was tested on five 1 part per million holmium solutions.

β-Octaalkoxyporphyrins: Versatile fluorometric sensors towards nitrated explosives

Real time detection of explosive residues is important to mitigate increasing security threats. Therefore, systematic studies are essential to optimize the performance of sensors. In this work, we have explored β-octamethoxyporphyrin and β-octabutoxyporphyrin to evaluate the effect of alkoxy groups in solution and in vapor phase sensing of nitrated explosives. Our systematic studies revealed a marked difference in sensitivity of these free-base porphyrins in solution state and vapor phase sensing of nitrated explosives simply by modulation of alkyl chain lengths. Alkoxyporphyrins exhibit very good sensitivity towards not only nitro aromatics but also alkyl nitro explosive taggants compared to β-octaethylporphyrin. Therefore, alkoxyporphyrins may act as versatile fluorescence turn-off based chemical sensors for nitrated explosives.

Cavitand-Decorated Silicon Columnar Nanostructures for the Surface Recognition of Volatile Nitroaromatic Compounds.

Nanocolumnar Si substrates (porous silicon (PSi)) have been functionalized with a quinoxaline-bridged (EtQxBox) cavitand in which the quinoxaline moieties are bonded to each other through four ethylendioxy bridges at the upper rim of the cavity. The receptor, which is known to selectively complex aromatic volatile organic compounds (VOCs) even in the presence of aliphatic compounds, has been covalently anchored to PSi. The larger surface area of PSi, compared to that of flat substrates, allowed one to study the recognition process of the surface-grafted receptors through different techniques: Fourier-transform infrared spectroscopy, thermal desorption, and X-ray photoelectron spectroscopy. The experiments proved that surface-grafted cavitands retain the recognition capability toward aromatic VOCs. In addition, the affinities of EtQxBox for various aromatic compounds (i.e., benzene, toluene, nitrobenzene, and p-nitrotoluene) have been studied combining density functional theory computations and thermal desorption experiments. Computational data based on the crystal structures of the complexes indicate that this cavitand possesses a higher affinity toward aromatic nitro-compounds compared to benzene and toluene, making this receptor of particular interest for the detection of explosive taggants. The results of computational studies have been validated also for the surface-grafted receptor through competitive recognition experiments. These experiments showed that EtQxBox-functionalized PSi can recognize nitrobenzene in the presence of a significant excess of aromatic vapors such as benzene (1:300) or toluene (1:100).

Dissociation Enthalpies of Chloride Adducts of Nitrate and Nitrite Explosives Determined by Ion Mobility Spectrometry.

The kinetics for thermal dissociations of the chloride adducts of the nitrate explosives 1,3-dinitroglycerin (1,3-NG), 1,2-dinitroglycerin (1,2-NG), the nitrite explosive 3,4-dinitrotoluene (3,4-DNT), and the explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) have been studied by atmospheric pressure ion mobility spectrometry. Both 1,3-NG·Cl(-) and1,2-NG·Cl(-) decompose in a gas-phase SN2 reaction in which Cl(-) displaces NO3(-) while 3,4-DNT·Cl(-) and DMNB·Cl(-) decompose by loss of Cl(-). The determined activation energy (kJ mol(-1)) and pre-exponential factor (s(-1)) values for the dissociations respectively are 1,3-NG·Cl(-), 86 ± 2 and 2.2 × 10(12); 1,2-NG·Cl(-), 97 ± 2 and 3.5 × 10(12); 3,4-DNT·Cl(-), 81 ± 2 and 4.8 × 10(13); and DMNB·Cl(-), 68 ± 2 and 9.7 × 10(11). Calculations by density functional theory show the structures of the nitrate ester adducts involve three hydrogen bonds: one from the hydroxyl group and the other two from the two nitrated carbons. The relative Cl(-) dissociation energies of the nitrates together with the previously reported smaller value for glycerol trinitrate and the calculated highest value for glycerol 1-mononitrate are explicable in terms of the number of hydroxyl hydrogen bond participants. The theoretical enthalpy changes for the nitrate ester displacement reactions are in agreement with those derived from the experimental activation energies but considerably higher for the nitro compounds.

Propylene Glycol Dinitrate (PGDN) as an Explosive Taggant

Propylene Glycol Dinitrate (PGDN) is a liquid nitrate ester explosive which has been used as a gelatinating agent in some energetic formulations. The aim of the present work was to assess whether PGDN could be used as a detection taggant. The PGDN was synthesized in the laboratory using laboratory grade propylene glycol (PG). The purity of the synthesized PGDN was assessed using gas chromatography-mass spectrometry (GC/MS) and Fourier transform infrared spectroscopy (FTIR). A study of the thermal decomposition of PGDN was carried out using both DSC and thermogravimetry-mass spectrometry analysis (TG/MS) methods. The gases produced during thermal decomposition were identified by mass spectrometry and the influence of the heating rate was investigated. The duality of DSC-TGA was highlighted by studying the complementarity between these two methods. Vapour pressure and enthalpy of vaporisation of PGDN were considered as the foremost taggant characteristics, and were estimated using TGA and taking benzoic acid as the reference. The vapour pressure of PGDN at ambient temperature is 2.54 Pa, therefore the PGDN could be a good candidate as a detection taggant compared to other explosive taggants (Nitroglycerin, EGDN, DMNB and PDCB).

Silicon quantum dot sensors for an explosive taggant, 2,3-dimethyl-2,3-dinitrobutane (DMNB).

Silicon quantum dots obtained by the reaction of magnesium silicide with ethylenediamine dihydrochloride were utilized to investigate the sensing mechanism and sensitivity for DMNB detection applications. Sensing DMNB provided us with evidence that Si QDs with a higher lying conduction band have better sensitivity compared to CdSe QDs.

Quantitative real time sensing reveals enhanced sensitivity of polar dendrimer thin films for plastic explosive taggants

A method of introducing pulses of analyte vapours has been developed to study the interactions of nitro-containing analytes with fluorescent sensing films.

Cavitand-Based Solid-Phase Microextraction Coating for the Selective Detection of Nitroaromatic Explosives in Air and Soil

A selective cavitand-based solid-phase microextraction coating was synthesized for the determination of nitroaromatic explosives and explosive taggants at trace levels in air and soil. A quinoxaline cavitand functionalized with a carboxylic group at the upper rim was used to enhance selectivity toward analytes containing nitro groups. The fibers were characterized in terms of film thickness, morphology, thermal stability, and pH resistance. An average coating thickness of 50 (±4) μm, a thermal stability until 400 °C, and an excellent fiber-to-fiber and batch to batch repeatability with RSD lower than 4% were obtained. The capabilities of the developed coating for the selective sampling of nitroaromatic explosives were proved achieving LOD values in the low ppbv and ng kg(-1) range, respectively, for air and soil samples.

Synchrotron FT-FIR spectroscopy of nitro-derivatives vapors: New spectroscopic signatures of explosive taggants and degradation products

We report on the first successful rovibrational study of gas phase mononitrotoluene and dinitrotoluene in the TeraHertz/Far-Infrared (THz/FIR) spectral domain. Using the AILES beamline of the synchrotron SOLEIL and a Fourier Transform spectrometer connected to multipass cells, the low-energy vibrational cross-sections of the different isomers of mononitrotoluene have been measured and compared to calculated spectra with the density functional theory including the anharmonic contribution. The active FIR modes of 2,4 and 2,6 dinitrotoluene have been assigned to the vibrational bands measured by Fourier Transform FIR spectroscopy of the gas-phase molecular cloud produced in an evaporating/recondensating system. This study highlights the selectivity of gas phase THz/FIR spectroscopy allowing an unambiguous recognition and discrimination of nitro-aromatic compounds used as explosive taggants.

Cooperative Effects in the Detection of a Nitroaliphatic Liquid Explosive and an Explosive Taggant in the Vapor Phase by Calix[4]arene-Based Carbazole-Containing Conjugated Polymers.

Two fluorescent molecular receptor based conjugated polymers were used in the detection of a nitroaliphatic liquid explosive (nitromethane) and an explosive taggant (2,3-dimethyl-2,3-dinitrobutane) in the vapor phase. Results have shown that thin films of both polymers display remarkably high sensitivity and selectivity toward these analytes. Very fast, reproducible, and reversible responses were found. The unique behavior of these supramolecular host systems is ascribed to cooperativity effects developed between the calix[4]arene hosts and the phenylene ethynylene-carbazolylene main chains. The calix[4]arene hosts create a plethora of host-guest binding sites along the polymer backbone, either in their bowl-shaped cavities or between the outer walls of the cavity, to direct guests to the area of the transduction centers (main chain) at which favorable photoinduced electron transfer to the guest molecules occurs and leads to the observed fluorescence quenching. The high tridimensional porous nature of the polymers imparted by the bis-calixarene moieties concomitantly allows fast diffusion of guest molecules into the polymer thin films.

New entities for sensory chemistry based on calix[4]arene-carbazole conjugates: from synthesis to applications

Two new calix[4]arene-carbazole conjugates (CALIX-CBZs) comprising 2- and 3-ethynyl-substituted carbazole derivatives attached to a central bis-calix[4]arene-containing phenylene ring have been designed for fluorescence-based detection of high explosive materials and explosive markers in vapour phase. The title compounds were prepared in good isolated yields and structurally fully characterised. CALIX-CBZs are highly fluorescent compounds that largely preserve their deep blue luminescence in solid state with no notorious emissions from electronic aggregated states. The excellent optical properties exhibited by casted films of both materials, including their photochemical stability, suggested their potential use as solid-state sensors. Remarkable high and fast responses were in fact retrieved upon contact with saturated vapours of 2,4,6-trinitrotoluene (TNT, a high explosive) and 2,4-dinitrotoluene (a common impurity in TNT batches, often used as its chemical marker), reaching near 80% of fluorescence quenching for the later on 30 s of exposure. Experiments performed with nitroaliphatic compounds (nitromethane (NM), a liquid explosive and 2,3-dimethyl-2,3-dinitrobutane, an explosive taggant) also revealed a high level of sensitivity (up to near 40% fluorescence quenching in only 10 s of exposure to NM). The quenching efficiencies were overall correlated with the extent and strength of CALIX-CBZs–analyte interactions, the vapour pressure of the analytes and the film thicknesses.

Electrochemical Detection of the Explosive Taggant 2,3-Dimethyldinitrobutane using a Single-Walled Carbon Nanotube Employed TiO2 Composite Film

A highly sensitive and fast method for electrochemical detection of 2,3-dimethyldinitrobutane (DMNB) on a graphite working electrode modified with a TiO2 thin film including single-walled carbon nanotubes (SWCNTs) is proposed. Incorporation of the SWCNTs into the inorganic matrix allowed the preparation of a highly stable film with an integrated electrochemical chip. The sensing mechanism is based on the current change generated at the specific voltage due to the redox reaction of the DMNB NO2 functional group. The detection limit and sensitivity were significantly improved due to the interaction between the SWCNTs and the analyte, which considerably enhanced the electron transferability. Under the optimized conditions, the linear range of the DMNB detection was 10 ppb to 10 ppm with a detection limit of 12 ppb, showing a sensitivity of 61 nA ppb-1. Keywords: Single-walled carbon nanotubes (SWCNTs), 2,3-dimethyldinitrobutane, Explosive taggant, Electrochemical detection, TiO2 matrix, chromatography-mass spectroscopy, differential pulse voltammetry (DPV), limit of detection (LOD), TiO2-carbon nanotube, corundum ceramic (CC) electrodes, DMNB compound, Amperometric Detection, TiO2+SWCNT film

Dual-transduction-mode sensing approach for chemical detection

Abstract Smart devices such as electronic nose have been developed for application in many fields like national security, defense, environmental regulation, health care, pipeline monitoring and food analysis. Despite a large array of individual sensors, the resolving power of these devices can still be improved to identify a target at a very low concentration out of a mixture of odors, if different types of transductions are employed in concert as a set of sensing responses to distinguish one odor from another. Here, we propose a new sensor architecture enabling different types of transduction signals in parallel on the same individual sensor. We demonstrate this architecture using a light emitting organic field-effect transistor (LEOFET) operated at a dual-transduction mode, as a proof-of-concept. Sensing response has been observed on both electrical and optical output signals from a green LEOFET upon exposure to an explosive taggant, with optical signal exhibiting much higher sensitivity. This new sensor architecture opens a field of devices of synergic capabilities to distinguish chemical and biological targets.

Terahertz Spectroscopy of the Explosive Taggant 2,3-Dimethyl-2,3-Dinitrobutane

The terahertz spectrum of the crystalline explosive taggant 2,3-dimethyl-2,3-dinitrobutane (C(6)H(12)N(2)O(4)) has been investigated as an alternative means of detecting solid-state explosives. The room-temperature spectrum exhibits two broad absorption features centered at 38.3 and 49.2 cm(-1). Once the sample is cooled to liquid-nitrogen temperatures, the resolution of three additional peaks occurs, with absorption maxima now appearing at 40.1, 47.5, 56.6, 63.9, and 73.6 cm(-1). Solid-state density functional theory simulations, both with and without London force dispersion corrections, have been used for the assignment of the experimental cryogenic THz spectrum to specific molecular motions in the crystalline solid. The B3LYP hybrid density functional paired with the 6-311G(2d,2p) basis set provides an excellent reproduction of the experimental data revealing that the THz spectrum arises from a mixture of intramolecular torsional vibrations localized primarily in the nitro groups and intermolecular lattice vibrations composed of rigid molecular rotations.