Infrared and Raman spectroscopy techniques applied to identification of explosives

Novel Covalent Bonds and Hydrogen Bonds Linked Porous Organic Frameworks as Chemosensor for Detecting 2,4,6-Trinitrophenol in Water and Soil Samples

A novel and unconventional structural porous organic framework combined through the synergistic effect of covalent bonds and hydrogen bonds was prepared with the combination of 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetraaniline (Py) and 5-hydroxyisophthalaldehyde (HP). It was the second example of CHOF until now and had been designated as Py-HP CHOF. The suspension of Py-HP CHOF in various solvents, such as ethanol, CH3CN, and methanol, exhibited a remarkably selective and sensitive "on-off" fluorescence response toward 2,4,6-trinitrophenol (TNP) compared with other explosives, with exceptionally low detection limits. The X-ray diffraction (XRD) spectra confirmed that the framework of Py-HP CHOF collapsed after interaction with TNP and acid, further indicating the existence of hydrogen bonds in the framework of Py-HP CHOF. The fluorescence quenching can be ascribed to the photoinduced electron transfer and the absorption competition quenching, as supported by XRD, X-ray photoelectron spectroscopy results, UV-vis absorption spectra, and density functional theory calculations. Fluorescence channels can be utilized by Py-HP CHOF to function as chemosensor, enabling the identification and detection of TNP in water and soil, and Py-HP CHOF is also the second CHOF example of sensing TNP reported to date. The application of this technique exhibits considerable potential in the analysis and detection of environmental pollutants, thereby presenting substantial practical implications.

Rapid Quantification of Ammonium Nitrate and Urea Nitrate Using Liquid Chromatography-High-Resolution Orbitrap Mass Spectrometry

Resolution and sensitivity improvements in mass spectrometry technology have enabled renewed attempts at solving challenging analytical issues. One such issue involves the analysis of energetic ionic species. Energetic ionic species make up an important class of chemical materials, and a more robust and versatile analytical platform would provide tremendous value to the analytical community. Initial attempts at quantification of energetic ionic species employed high-resolution time-of-flight measurements with crown ether (CE) complexation and flow injection analysis (FIA). In this investigation, ammonium nitrate (AN) and urea nitrate (UN) in the presence of a crown ether complexation agent were explored by using high-resolution orbitrap mass spectrometry. Product ion scans of these signature complexes reveal positive identification of these energetic ionic species. Finally, quantification was demonstrated for both flow injection and liquid chromatography-mass spectrometry (LC-MS) analysis, suggesting the capability for routine and rapid analysis of these energetic ionic materials.

A novel electrochemical sensor based on phosphate-stabilized poly-caffeic acid film in combination with graphene nanosheets for sensitive determination of nitro-aromatic energetic materials

This work offers a novel approach and sensor electrode for electrocatalytic reduction of nitro-aromatic explosives (NAEs). This sensor was created by combining electrochemically reduced graphene nanosheets (GNSs) -through cyclic voltammetric reduction of a graphene oxide colloidal solution- with phosphate-stabilized poly-caffeic acid (pCAF) film-modified glassy carbon electrode (GCE). The poly-caffeic acid-modified nonconductive electrode was stabilized with a H2PO4-/HPO42- phosphate buffer at pH 7 and made conductive. The novel electrode, called phosphate stabilized-GC/GNSs/pCAF, was characterized by electrochemical methods and scanning electron microscopy (SEM). The sensor exhibited high performance for trinitrotoluene (TNT) detection with a linear response between 50 and 500 μg L- 1 and a detection limit of 6 μg L-1. In addition to TNT, precise determinations of NAEs such as 2,4-dinitrotoluene (2,4-DNT), tetryl (2,4,6-trinitrophenyl methyl nitramine), trinitro phenol (TNP or picric acid; PA), 2,4-dinitrophenol (2,4-DNP), and 4-amino dinitrotoluene (4A-DNT, an aerobic bacterial degradation product of TNT) were made using the developed sensor electrode and DPV technique. Simultaneous quantification of TNT and DNT was performed with the aid of a computational technique known as multiple linear regression (MLR). The optimized electrode was resistant to interference effects. Satisfactory results on real samples were obtained by applying the modified electrode to the determination of TNT, tetryl, and TNP in contaminated soil. The validation of the proposed method was made against a literature LC-MS/MS method. A statistical comparison of the obtained results was provided using F- and Student's t-tests.

Analytical Tools and Methods for Explosive Analysis in Forensics: A Critical Review

This review summarizes (i) compositions and types of improvised explosive devices; (ii) the process of collection, extraction and analysis of explosive evidence encountered in explosive and related cases; (iii) inter-comparison of analytical techniques; (iv) the challenges and prospects of explosive detection technology. The highlights of this study include extensive information regarding the National & International standards specified by USEPA, ASTM, and so on, for explosives detection. The holistic development of analytical tools for explosive analysis ranging from conventional methods to advanced analytical tools is also covered in this article. The most important aspect of this review is to make forensic scientists familiar with the challenges during explosive analysis and the steps to avoid them. The problems during analysis can be analyte-based, that is, interferences due to matrix or added molding/stabilizing agents, trace amount of parent explosives in post-blast samples and many more. Others are techniques-based challenges viz. specificity, selectivity, and sensitivity of the technique. Thus, it has become a primary concern to adopt rapid, field deployable, and highly sensitive techniques.

Applications of Raman spectroscopy in the analysis of biological evidence

During the past few decades, Raman spectroscopy has progressed and captivated added attention in the field of science. However, the application of Raman spectroscopy is not limited to the field of forensic science and analytical chemistry; it is one of the emerging spectroscopic techniques, utilized in the field of forensic science which in turn could be a supporting tool in the law and justice system. The advantage of Raman spectroscopy over the other conventional techniques is that it is rapid, reliable, and non-destructive in nature with minimal or no sample preparation. The quantitative and qualitative analysis of evidence from biological and non-biological origins could easily be performed by using Raman spectroscopy. The forensic domain is highly complex with multidisciplinary branches, and therefore a plethora of techniques are utilized for the detection, identification, and differentiation of innumerable pieces of evidence for the purpose of law and justice. Herein, a systematic review is carried out on the application of Raman spectroscopy in the realm of forensic biology and serology considering its usefulness in practical perspectives. This review paper highlights the significance of modern techniques, including micro-Raman spectroscopy, confocal Raman spectroscopy, surface-enhanced Raman spectroscopy, and paper-based surface-enhanced Raman spectroscopy, in the field of Raman spectroscopy. These techniques have demonstrated notable advancements in terms of their applications and capabilities. Furthermore, to comprehensively capture the progress in the development of Raman spectroscopy, all the published papers which could be retrieved from the available databases from the year 2007 to 2022 were incorporated.

Tunnel junction sensing of TATP explosive at the single-molecule level

Triacetone triperoxide (TATP) is a highly potent homemade explosive commonly used in terrorist attacks. Its detection poses a significant challenge due to its volatility, and the lack of portability of current sensing techniques. To address this issue, we propose a novel approach based on single-molecule TATP detection in the air using a device where tunneling current in N-terminated carbon-nanotubes nanogaps is measured. By employing the density functional theory combined with the non-equilibrium Green's function method, we show that current of tens of nanoamperes passes through TATP trapped in the nanogap, with a discrimination ratio of several orders of magnitude even against prevalent indoor volatile organic compounds (VOCs). This high tunneling current through TATP's highest occupied molecular orbital (HOMO) is facilitated by the strong electric field generated by N-C polar bonds at the electrode ends and by the hybridization between TATP and the electrodes, driven by oxygen atoms within the probed molecule. The application of the same principle is discussed for graphene nanogaps and break-junctions.

Amine-decorated Zirconium Based Metal Organic Framework for Ultrafast Detection of 2,4,6-Trinitrophenol in Aqueous Samples

An amine-decorated zirconium based metal organic framework (MOF) UiO-66-NH2 with rod shape morphology was synthesized by solvothermal process using 2-aminoterephthalic acid as an organic linker. Crystallinity of synthesized MOF material was confirmed with PXRD technique. MOF was employed as selective and sensitive sensor for ultra-trace detection of 2,4,6-trinitrophenol (TNP) in aqueous matrix, even in coexistence with other competitive nitroaromatic analytes. High value of Stern-Volmer quenching constant Ksv (1.106 × 105 M- 1), plausible photoluminescent quenching efficiency (97.8%) and lower detection limit (0.95 µM/217ng mL- 1) ascertained extraordinary sensitivity of developed MOF for TNP. Density functional theory calculations and electrostatic interactions (i.e. ionic interaction, H-bonding and π-π interaction) indicated that electron and energy transfer processes play a key role in turn-off quenching response of UiO-66-NH2 sensor. Spiked real samples were analysed to validate the developed method, which satisfactorily established the developed MOF sensor as an efficient tool for analysis.

Long wavelength mid-infrared multi-gases spectroscopy using tunable single-mode slot waveguide quantum cascade laser

Single-mode tunable quantum cascade lasers (QCLs) are promising for high-resolution and highly sensitive trace gases sensing across the mid-infrared (MIR) region. We report on the development of a tunable single-mode slot waveguide QCL array in the long wavelength part of the MIR regime (>12 µm). This laser array exhibits a tuning range of around 12 cm-1, from 735.3 to 747.3 cm-1. Using this developed single-mode tunable QCL, we demonstrate individual gas sensing, yielding the detection limit of 940 ppb and 470 ppb for acetylene and o-xylene, respectively. To verify the potential of the developed QCL array in multi-species gas detection, laser absorption measurements of two mixed gases of acetylene and o-xylene were conducted, showing the absorption features of the corresponding gases agree well with the theoretical predictions.

Machine Learning Identification of Organic Compounds Using Visible Light

Identifying chemical compounds is essential in several areas of science and engineering. Laser-based techniques are promising for autonomous compound detection because the optical response of materials encodes enough electronic and vibrational information for remote chemical identification. This has been exploited using the fingerprint region of infrared absorption spectra, which involves a dense set of absorption peaks that are unique to individual molecules, thus facilitating chemical identification. However, optical identification using visible light has not been realized. Using decades of experimental refractive index data in the scientific literature of pure organic compounds and polymers over a broad range of frequencies from the ultraviolet to the far-infrared, we develop a machine learning classifier that can accurately identify organic species based on a single-wavelength dispersive measurement in the visible spectral region, away from absorption resonances. The optical classifier proposed here could be applied to autonomous material identification protocols and applications.

Calibration of spectra in presence of non-stationary background using unsupervised physics-informed deep learning

Calibration is a key part of the development of a diagnostic. Standard approaches require the setting up of dedicated experiments under controlled conditions in order to find the calibration function that allows one to evaluate the desired information from the raw measurements. Sometimes, such controlled experiments are not possible to perform, and alternative approaches are required. Most of them aim at extracting information by looking at the theoretical expectations, requiring a lot of dedicated work and usually involving that the outputs are extremely dependent on some external factors, such as the scientist experience. This work presents a possible methodology to calibrate data or, more generally, to extract the information from the raw measurements by using a new unsupervised physics-informed deep learning methodology. The algorithm allows to automatically process the data and evaluate the searched information without the need for a supervised training by looking at the theoretical expectations. The method is examined in synthetic cases with increasing difficulties to test its potentialities, and it has been found that such an approach can also be used in very complex behaviours, where human-drive results may have huge uncertainties. Moreover, also an experimental test has been performed to validate its capabilities, but also highlight the limits of this method, which, of course, requires particular attention and a good knowledge of the analysed phenomena. The results are extremely interesting, and this methodology is believed to be applied to several cases where classic calibration and supervised approaches are not accessible.

Classification of local diesel fuels and simultaneous prediction of their physicochemical parameters using FTIR-ATR data and chemometrics

Class identification and prediction of physicochemical variables of eight diesel fuel brands collected from several stations within the Atlanta metropolitan area in the State of Georgia were investigated using principal component analysis (PCA), partial least squares discriminant analysis (PLS2-DA), and partial least squares regression (PLSR) as modeling techniques. The fuels were from a common pipeline, therefore, assumed to have very similar characteristics. Ten FTIR-ATR spectra per fuel brand were collected over the 650 - 4000 cm^-1 mid-infrared region, and the 80 x 3351 matrix was submitted to PCA to determine if there were any clusters. Following PCA, the 80 x 3351 matrix was split into a training matrix (56x3351) and a test matrix (24x3351). PLS2-DA models were built and evaluated for class identification using dummy variables (I,0) as input matrix. For physicochemical variable predictions, models were developed via PLSR using the FTIR-ATR spectra training matrix and physicochemical variables obtained from the Georgia Department of Agriculture Labs as input. Correlation coefficients of the eight fuels ranged from 0.9960 to 0.9998. PCA revealed all eight clusters of the diesel fuels, regardless of the tight correlation coefficients range. With a 1.0 ± 0.1 cut-off for fuel identification, the PLS2-DA models showed 100% correct predictions for four or five fuel brands, and 75% correct prediction for all eight fuel brands. PLSR predicted 100% correct physicochemical variables, with a RMSEP range of 0.019 to 1.132 for all 80 variables targeted.

Miniaturized Raman Instruments for SERS-Based Point-of-Care Testing on Respiratory Viruses

As surface-enhanced Raman scattering (SERS) has been used to diagnose several respiratory viruses (e.g., influenza A virus subtypes such as H1N1 and the new coronavirus SARS-CoV-2), SERS is gaining popularity as a method for diagnosing viruses at the point-of-care. Although the prior and quick diagnosis of respiratory viruses is critical in the outbreak of infectious disease, ELISA, PCR, and RT-PCR have been used to detect respiratory viruses for pandemic control that are limited for point-of-care testing. SERS provides quantitative data with high specificity and sensitivity in a real-time, label-free, and multiplex manner recognizing molecular fingerprints. Recently, the design of Raman spectroscopy system was simplified from a complicated design to a small and easily accessible form that enables point-of-care testing. We review the optical design (e.g., laser wavelength/power and detectors) of commercialized and customized handheld Raman instruments. As respiratory viruses have prominent risk on the pandemic, we review the applications of handheld Raman devices for detecting respiratory viruses. By instrumentation and commercialization advancements, the advent of the portable SERS device creates a fast, accurate, practical, and cost-effective analytical method for virus detection, and would continue to attract more attention in point-of-care testing.

Detection of Polynitro Compounds at Low Concentrations by SERS Using Ni@Au Nanotubes

The identification of high-energy compounds in trace concentrations not only in the laboratory, but also in field conditions is of particular interest. The process should be clear, easy, and well-recognizable. We formed SERS-active substrates by using elongated nickel nanotubes synthesized by electrochemical deposition in the pores of ion-track membranes and coated them with gold for further application in the detection of low concentrations of analytes. The substrates were characterized using various techniques to determine the morphology of the nanotubes and modifying gold layer. The possibility of obtaining two types of gold-layer morphology was shown: in the form of a smooth film up to 20–50 nm thick and a coating with nanoneedles up to 250 nm long. The electric fields around the nanotubes were simulated at a laser wavelength of 532 nm to demonstrate the influence of the gold-layer morphology on the field distribution. The “needle” morphology was chosen to form the most effective SERS-active substrates for detection of low concentrations of aromatic polynitro compounds. The spectral peaks were identified by comparing the model and experimental Raman spectra at concentrations down to 10−5 M. Within this limit, all peaks (“fingerprints” of the substance) were clearly distinguishable.

Fluorescence turn-off sensing of TNT by polyethylenimine capped carbon quantum dots

In recent years, the determination of 2,4,6-trinitrotoluene (TNT) explosive residues in various matrices has attracted great interest from the perspective of national security and public health. Here, a fluorescent polyethylenimine capped carbon quantum dots (PEI-C-dots) probe was synthesized by a microwave-assisted technique using polyethylenimine and citric acid precursors and used to detect TNT. The sensing mechanism of TNT is based on fluorescence quenching as a result of the donor-acceptor interaction between Meisenheimer anion form of TNT and PEI on the PEI-C-dots surface. The fluorescence quantum yield of the synthesized PEI-C-dots was 54% and the detection limit for TNT was 93 μg/L. It was observed that neither the nitramine group (HMX and RDX) explosives with similar structures nor common soil ions and camouflage agents interfered with the determination of TNT. The interference effect of picric acid was eliminated by removing it with a basic anion exchanger before the determination. This nanosensor allows rapid, simple, selective, and sensitive determination of TNT residues in complex matrices and has the potential to be converted into a kit format.

Curcumin Is an Iconic Ligand for Detecting Environmental Pollutants

The rapid increase in industrial revolution and the consequent environmental contamination demands continuous monitoring and sensitive detection of the pollutants. Nanomaterial-based sensing system has proved to be proficient in sensing environmental pollutants. The development of novel ligands for enhancing the sensing efficiency of nanomaterials has always been a challenge. However, the amendment of nanostructure with molecular ligand increases the sensitivity, selectivity, and analytical performance of the resulting novel sensing platform. Organic ligands are capable of increasing the adsorption efficacy, optical properties, and electrochemical properties of nanomaterials by reducing or splitting of band gap. Curcumin (diferuloylmethane) is a natural organic ligand that exhibits inherent fluorescence and electrocatalytic property. Due to keto-enol tautomerism, it is capable of giving sensitive signals such as fluorescence, luminescence, ultraviolet absorption shifts, and electrochemical data. Curcumin probes were also reported to give enhanced meterological performances, such as low detection limit, repeatability, reproducibility, high selectivity, and high storage stability when used with nanosystem. Therefore, research on curcumin-modified nanomaterials in the detection of environmental pollution needs a special focus for prototype and product development to enable practical use. Hence, this article reviews the role of curcumin as a natural fluorophore in optical and electrochemical sensing of environmentally significant pollutants. This review clearly shows that curcumin is an ideal candidate for developing and validating nanomaterials-based sensors for the detection of environmental pollutants such as arsenic, lead, mercury, boron, cyanide, fluoride, nitrophenol, trinitrotoluene, and picric acid and toxic gases such as ammonia and hydrogen chloride. This review will afford references for future studies and enable researchers to translate the lab concepts into industrial products.

Electrochemical determination of nitroaromatic explosives using glassy carbon/multi walled carbon nanotube/polyethyleneimine electrode coated with gold nanoparticles

The on site/in field detection of explosives has become a rising priority for homeland security and counter-terrorism measures. This work presents the sensitive detection of nitroaromatic explosives using glassy carbon/multi-walled carbon nanotubes/polyethyleneimine (GC/MWCNTs/PEI) electrode coated with gold nanoparticles (AuNPs). MWCNTs and PEI could be well dispersed in ethanol/water solution, giving rise to a thin and homogeneous film on GCE. The GC/MWCNTs/PEI electrode was electrochemically modified with AuNPs and used for the differential pulse voltammetric (DPV) detection of nitroaromatics. The enhanced detection sensitivities were achieved through π-π and charge-transfer (CT) interactions between the electron-deficient nitroaromatic explosives and donor amine groups in PEI to which gold nanoparticles were linked, providing increased analyte affinity toward the modified GCE. Calibration curves of current intensity versus concentration were linear in the range of 0.05-8 mg L^-1 for TNT, 0.2-4 mg L^-1 for 2,4-dinitrotoluene (DNT), 1-20 mg L^-1 for 2,4-dinitrophenol (2,4-DNP), 0.25-10 mg L^-1 for picric acid (PA), and 0.05-4 mg L^-1 for 2,4,6-trinitrophenyl-N-methylnitramine (tetryl) with detection limits (LOD) of 15 μg L^-1, 45 μg L^-1, 135 μg L^-1, 30 μg L^-1, and 12 μg L^-1, respectively. The proposed method was successfully applied to the analysis of nitroaromatics in synthetic explosive mixtures and military composite explosives (comp B and octol). The electrochemical method was not affected by possible interferents of electroactive camouflage materials and common soil ions. Method validation was performed against the reference LC-MS method on TNT and PA-contaminated clay soil samples separately.

Spectral Considerations for Standoff Infrared Detection of RDX on Reflective Aluminum

This paper examines infrared spectroscopic effects for the standoff detection of an explosive material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), inkjet printed on an aluminum surface. Results of a spectroscopic study are described, using multiple optical setups. These setups were selected to explore how variations in the angles of incidence and collection from the surface of the material result in corresponding variations in the spectral signatures. The goal of these studies is to provide an understanding of these spectral changes since it affects standoff detection of hazardous materials on a reflective substrate. We demonstrate that variations in spectral effects are dependent on the relative surface concentration of the deposited RDX. We also show that it is reasonable to use spectroscopic data collected in a standard laboratory infrared spectrometer outfitted with a variable angle reflectometer set at 0° as reference spectra for data collected in a standoff configuration. These results are important to provide a systematic approach to understanding infrared (IR) spectra collection using standoff systems in the field, and to allow for comparison between such data, and data collected in the laboratory. Although the precise results are constrained to a specific material system (thin layers on a reflective substrate), the approach and general discussion provided are applicable to a broad range of IR standoff sensing techniques and applications.

Reduced graphene oxide-supported smart plasmonic AgPtPd porous nanoparticles for high-performance electrochemical detection of 2,4,6-trinitrotoluene

Smart plasmonic AgPtPd NPs/rGO exhibited a wide linear range for TNT from 0.1 to 8 ppm with a sensing limit of 0.95 ppb. The remarkable features are probably attributed to the integrated advantages of the plasmonic properties and synergistic effect.

Rapid synthesis of self-propelled tubular micromotors for "ON-OFF" fluorescent detection of explosives

We report a rapid strategy to construct self-propelled functional tubular micromotors. Based on the established strategy, magnetic covalent-organic-framework-functionalized micromotors were fabricated to implement sensing of explosives in water. Such micromotors can complete fluorescent "On-Off" detection of trace explosive 2,4,6-trinitrophenol within 10 min.

Colorimetric optical nanosensors for trace explosive detection using metal nanoparticles: advances, pitfalls, and future perspective

Warfare threats and acts of terror are challenging situations encountered by defense agencies across the globe and are of growing concern to the general public, and security-minded policy makers. Detecting ultra-low quantities of explosive compounds in remote locations or under harsh conditions for anti-terror purposes as well as the environmental monitoring of residual or discarded explosives in soil, remains a major challenge. The use of metal nanoparticles (NPs) for trace explosive detection has drawn considerable interest in recent years. For nano-based explosive sensor devices to meet real-life operational demands, analytical parameters such as, long-shelf life, stability under harsh conditions, ease-of-use, high sensitivity, excellent selectivity, and rapid signal response must be met. Generally, the analytical performance of colorimetric-based nanosensor systems is strongly dependent on the surface properties of the nanomaterial used in the colorimetric assay. The size and shape properties of metal NPs, surface functionalisation efficiency, and assay fabrication methods, are factors that influence the efficacy of colorimetric explosive nanosensor systems. This review reports on the design and analytical performances of colorimetric explosive sensor systems using metal NPs as optical signal transducers. The challenges of trace explosive detection, advances in metal NP colorimetric explosive design, limitations of each methods, and possible strategies to mitigate the problems are discussed.

Fiber-dispersive Raman spectrometer with single-photon sensitivity

The two major challenges in Raman spectroscopy are the low intensity of spontaneous Raman scattering and often accompanying luminescence. We overcome these two issues with a novel fiber-dispersive Raman spectrometer utilizing pulsed excitation and a superconducting nanowire single-photon detector (SNSPD). By exploiting chromatic dispersion in the fiber material, we stretched propagation times of Raman photons and performed correlated measurements in the time domain, where the two emission processes, Raman scattering and luminescence, can be effectively separated. The spectrometer greatly benefits from SNSPD metrics, i.e. broad spectral sensitivity (from UV to near-IR wavelength range) on a single-photon level and high timing resolution (small timing jitter), which outperform those of competing avalanche single-photon detectors. The spectral resolution achievable with a fiber-dispersive spectrometer for the optimized components is estimated to be as good as 3 - 10 cm^-1 over the Stokes shifted range up to 4400 cm^-1 with an excitation wavelength of 785 nm and below 5 cm^-1 covering the same range with an excitation wavelength of 532 nm.

Ethylenediamine-bound magnetite nanoparticles as dual function colorimetric sensor having charge transfer and nanozyme activity for TNT and tetryl detection

A reusable, low-cost, and convenient ethylenediamine (EDA)-bound magnetite nanoparticles (MNPs)-based colorimetric sensor has been developed for dual function colorimetric determination of nitroaromatic explosives such as TNT and tetryl. Colorimetric detection of analytes may occur through two independent routes: (1) nano-Fe₃O₄- EDA- NH₂ as σ-donor may interact with the σ- and π-acceptor aromatic-poly(NO₂) groups to produce a colored charge-transfer (CT) complex; (2) nano-Fe₃O₄-EDA-NH₂ as a Fenton-type nanozyme may generate reactive species that comprise hydroxyl radicals (^•OH) with H₂O₂ to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to a blue-colored diimine (oxTMB-TMB) CT complex, where this color is bleached with TNT/tetryl because of donor-acceptor interactions between the explosive -NO₂ groups and the -NH₂ group of Fe₃O₄-EDA nanoparticles of restricted nanozyme activity. Both methods can quantify TNT well below the EPA recommended TNT residential screening level in soil, LOD being in the micromolar range. As EDA was covalently bound to MNPs, the same sensor can be separately reused six times for TNT and eight times for tetryl determination, using method (1). Common metal ions, anions, energetic materials, several camouflage materials, and soil components such as humates did not interfere with the nanosensor performance for TNT and tetryl. The combination of charge-transfer and nanozyme ability of Fe₃O₄- EDA-NH₂ nanoparticles may bring a new approach to dual function colorimetric sensor design. To the best of our knowledge, this is the first dual function colorimetric sensor for TNT and tetryl using the same nanoparticles as sensing elements in two different detection systems involving either formation or bleaching of colored species. The proposed colorimetric sensor can determine nitroaromatic explosives in two different ways: method-1 for TNT and tetryl sensing with EDA-MNPs relies on the donor-acceptor interaction between the electron-deficient nitroaromatics and electron-rich amine groups covalently functionalized on MNPs to produce an absorbance at 512 nm. In method-2, EDA-MNPs having nanozyme activity react with H₂O₂ to form reactive species that can oxidize TMB to its blue-colored charge-transfer (CT) complex, where TNT and tetryl addition may partially inhibit the nanozyme activity of EDA-MNPs and cause color bleaching (decrement of 650 nm absorbance) by disrupting the CT complex formed from TMB. This is the first dual function colorimetric sensor for nitro explosives uniquely combining charge-transfer and nanozyme ability of EDA-Fe₃O₄ nanoparticles in the same nano-sensor.

Non-Invasive Identification of Nutrient Components in Grain

Digital farming is a modern agricultural concept that aims to maximize the crop yield while simultaneously minimizing the environmental impact of farming. Successful implementation of digital farming requires development of sensors to detect and identify diseases and abiotic stresses in plants, as well as to probe the nutrient content of seeds and identify plant varieties. Experimental evidence of the suitability of Raman spectroscopy (RS) for confirmatory diagnostics of plant diseases was previously provided by our team and other research groups. In this study, we investigate the potential use of RS as a label-free, non-invasive and non-destructive analytical technique for the fast and accurate identification of nutrient components in the grains from 15 different rice genotypes. We demonstrate that spectroscopic analysis of intact rice seeds provides the accurate rice variety identification in ~86% of samples. These results suggest that RS can be used for fully automated, fast and accurate identification of seeds nutrient components.

Preliminary stable isotope analyses for propellant discrimination in shotshells

RATIONALE

This study aimed to develop methods to determine the identity and trace the origin of propellants used in shotshells. Specifically, the use of organic component and stable isotope analysis techniques, such as bulk stable isotope analysis (BSIA) and compound-specific isotope analysis (CSIA) techniques, for the study of shotshell propellants was investigated.

METHODS

Nine samples of shotshell propellants from different manufacturing countries and brands were analyzed for explosive and additive components by gas chromatography/mass spectrometry and thin-layer chromatography. BSIA of the propellants was achieved using elemental analysis/isotope ratio mass spectrometry without a pretreatment process. For the CSIA of nitroglycerin, double-base powder propellants were extracted with ether, and the isotope ratios of carbon and nitrogen were measured by gas chromatography/isotope ratio mass spectrometry.

RESULTS

Nine samples drawn from seven brands in four countries were classified into five groups by organic component analysis, while eight classification groups were identified by BSIA. Thus, two samples could not be distinguished from each other by either BSIA or organic component analysis. Subsequently, with the use of results obtained with CSIA for nitroglycerin, all the samples could be classified into different groups. These findings suggest that the nine propellant samples were all composed of different ingredients or raw materials from different sources.

CONCLUSIONS

Stable isotope ratio analyses were performed for propellant discrimination. The combined BSIA, CSIA and organic component analysis techniques were able to successfully distinguish the nine shotshell propellants from seven brands sourced from four different countries, and the results suggested that the samples contained different ingredients or raw materials from different sources. We therefore can conclude that reliable results can be obtained using combined isotope analysis methods such as CSIA and BSIA for origin tracing and identity determination.

A Simple Determination of Trinitrotoluene (TNT) Based on Fluorescence Quenching of Rhodamine 110 with FRET Mechanism

Sensitive and selective detection of nitroaromatic explosives is an important issue in regard to human health, environment, public security and military issues. In this study, a simple and sensitive fluorescence quenching - based assay utilizing Rhodamine 110 as fluorophore probe was developed for the determination of trinitrotoluene (TNT). This sensitive fluorometric method could measure the decrease in fluorescence of Rhodamine 110 (λ_ex = 490 nm, λ_em = 521 nm) owing to the primary amine groups of Rhodamine 110 (different from other rhodamines) capable of donor-acceptor interaction with TNT. The resulting TNT-amine complex can strongly quench the fluorescence emission of Rhodamine 110 by fluorescence resonance energy transfer (FRET) which occurs as the excited Rhodamine 110 fluorophore (donor) transfers its energy to TNT (acceptor) by non-radiative dipole-dipole interaction. Fluorescence quenching varied linearly with TNT concentration, with LOD and the LOQ of 0.71 and 2.38 mg L^- 1 TNT, respectively. Similar explosives, common soil ions, and possible camouflage materials were found not to interfere with the proposed method, offering significant advantages with its easy methodology, low-cost, sensitivity, and rapidity of analysis. FRET mechanism based on dye donor-TNT acceptor interaction.

Study of photo induced charge transfer mechanism of PEDOT with nitro groups of RDX, HMX and TNT explosives using anti-stokes and stokes Raman lines ratios

The paper reports the charge transfer mechanism between poly (3,4-ethylenedioxythiophene) (PEDOT) and high energy materials such as RDX, HMX and TNT, respectively in terms of ratios of anti-stokes (AS) and stokes(S) Raman lines of NO₂ bands. Generally it works as an effective sensing medium for the detection of explosives when mixed in an equal proportion and are subjected to 532 nm wavelength without any chemical treatment [1]. The pristine PEDOT is less sensitive to 532 nm wavelength (2.33 eV) but influences the Raman S and AS lines of explosives in the mixture. The study also reveals that a small quantity (one milligram) of PEDOT is sufficient to initiate the positive charge transfer mechanism between its oxidized state to the lone pairs of electrons on the oxygen atoms of the nitro group of the explosive molecules. Consequently, the intensity of the Raman spectra of RDX, HMX and TNT is dropped by an order of 22.5, 11.45 and 17.2 times, respectively along with the shift of the NO₂ vibrational modes. It is also attributed to Photon-electron-phonon interaction. Finally, we have estimated the reduced mass of the functional group to ascertain the force constant and the intensity ratios of AS /S lines to confirm the charge transfer mechanism. The effect of charge transfer mechanism is also reflected in drastic change in transmission /absorption characteristics of FTIR spectra of same PEDOT and explosive mixtures.

Inorganic oxidizer detection from propellants, pyrotechnics, and homemade explosive powders using gradient elution moving boundary electrophoresis

Advancement in rapid targeted chemical analysis of homemade and improvised explosive devices is critical for the identification of explosives-based hazards and threats. Gradient elution moving boundary electrophoresis (GEMBE), a robust electrokinetic separation technique, was employed for the separation and detection of common inorganic oxidizers from frequently encountered fuel-oxidizer mixtures. The GEMBE system incorporated sample and run buffer reservoirs, a short capillary (5 cm), an applied electric field, and a pressure-driven counterflow. GEMBE provided a separation format that allowed for continuous injection of sample, selectivity of analytes, and no sample cleanup or filtration prior to analysis. Nitrate, chlorate, and perchlorate oxidizers were successfully detected from low explosive propellants (e.g., black powders and black powder substitutes), pyrotechnics (e.g., flash powder), and tertiary explosive mixtures (e.g., ammonium nitrate- and potassium chlorate-based fuel-oxidizer mixtures). Separation of these mixtures exhibited detection without interference from a plethora of additional organic and inorganic fuels, enabled single particle analysis, and demonstrated semiquantitative capabilities. The bulk counterflow successfully excluded difficult components from fouling the capillary, yielding estimated limits of detection down to approximately 10 μmol/L. Finally, nitrate was separated and detected from postblast debris collected and directly analyzed from two nitrate-based charges.

Aggregation-directed High Fidelity Sensing of Picric Acid by a Perylenediimide-based Luminogen

Widespread use of picric acid (PA) in chemical industries and deadly explosives poses dreadful impact on all living creatures as well as the natural environment and has raised global concerns that necessitate the development of fast and efficient sensing platforms. To address this issue, herein, we report a perylenediimide-peptide conjugate, PDI-1, for detection of PA in methanol. The probe displays typical aggregation caused quenching (ACQ) behaviour and exhibits a fluorescence "turn-off" sensory response towards PA which is unaffected by the presence of other interfering nitroaromatic compounds. The sensing mechanism involves PA induced aggregation of the probe into higher order tape like structures which leads to quenching of emission. The probe possesses a low detection limit of 5.6 nM or 1.28 ppb and a significantly high Stern-Volmer constant of 6.87×10⁴  M^-1 . It also exhibits conducting properties in the presence of PA vapours and thus represents a prospective candidate for vapour phase detection of PA. This is, to the best of our knowledge, the first example of a perylenediimide based probe that demonstrates extremely specific, selective and sensitive detection of PA and thus grasps the potential for application in practical scenarios.

Trace Detection and Chemical Analysis of Homemade Fuel-Oxidizer Mixture Explosives: Emerging Challenges and Perspectives

The chemical analysis of homemade explosives (HMEs) and improvised explosive devices (IEDs) remains challenging for fieldable analytical instrumentation and sensors. Complex explosive fuel-oxidizer mixtures, black and smokeless powders, flash powders, and pyrotechnics often include an array of potential organic and inorganic components that present unique interference and matrix effect difficulties. The widely varying physicochemical properties of these components as well as external environmental interferents and background challenge many sampling and sensing modalities. This review provides perspective on these emerging challenges, critically discusses developments in sampling, sensors, and instrumentation, and showcases advancements for the trace detection of inorganic-based explosives.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

Multiple tier detection of TNT using curcumin functionalized silver nanoparticles

The rapid, selective and sensitive detection of trinitrotoluene (TNT), which is widely used in terrorist activities and also a major environmental contaminant is prime concern for the scientific community dealing with environmental problems and national security. This paper described unprecedented CAgP based multiple tier probe employing U.V.–Vis., DLS & SERS techniques for highly selective, rapid and ultrasensitive detection of TNT up to 0.1 nM level. The as synthesized CAgP made possible the naked eye detection of TNT in the form of flakes in real time. The developed method due to its multiple tier approach utilizing the same sample could easily be extended to a high-throughput format and can be utilized for rapid and reliable trace detection of TNT, for on-site screenings in airports, analysis of forensic samples, and environmental analysis.

∙

TNT can interact with curcumin functionalized Ag NPs through p-donor-acceptor interaction.

∙

Curcumin functionalized Ag NPs can be used as an ultrasensitive optical probe for TNT detection.

∙

The probe has an excellent selectivity for TNT against other nitro compounds.

∙

The probe can detect TNT at multiple levels using U.V–Vis., DLS and SERS.

Raman Spectroscopy Allows for the Determination of Elephant Ivory Age

Determination of the age of ivory is important for controlling illegal trafficking and the proper identification of ivory artifacts. Radiocarbon dating is the standard method of determining the age of ivories; however, it requires the destruction of a fragment of the sample. Raman spectroscopy is a nondestructive technique, and therefore can be used on artwork. Moreover, Raman measurements can be done using a portable system, and the data analysis can be performed on the spot once the groundwork is done. Ivories contain two primary components: collagen and bioapatite. Raman spectrum of ivory material is mainly a sum of the vibrational bands of these components. As collagen deteriorates with time, its Raman signal decreases; therefore, the ratio of collagen to bioapatite peaks is smaller in the older samples compared to the younger ones, providing a basis for sample dating. We have compared the results of Raman and radiocarbon measurements applied to a set of elephant ivory fragments and have successfully calibrated the Raman data set using radiocarbon measurements. We found that the Raman collagen to bioapatite peak ratios of the samples can be used as a metric to determine their age, providing a nondestructive technique to assess the age of ivory samples. We have also used singular value decomposition (SVD) to analyze the whole Raman spectra. We have observed clear separation between samples of different ages in the SVD component space. The samples also tended to align along the timeline diagonal in the correct order. The changes in multiple collagen and bioapatite peaks contribute to the differences in Raman spectra of ivory samples of different age.

Determination of Cadmium (II) in Aqueous Solutions by In Situ MID-FTIR-PLS Analysis Using a Polymer Inclusion Membrane-Based Sensor: First Considerations

Environmental monitoring is one of the most dynamically developing branches of chemical analysis. In this area, the use of multidimensional techniques and methods is encouraged to allow reliable determinations of metal ions with portable equipment for in-field applications. In this regard, this study presents, for the first time, the capabilities of a polymer inclusion membrane (PIM) sensor to perform cadmium (II) determination in aqueous solutions by in situ visible (VIS) and Mid- Fourier transform infrared spectroscopy (MID-FTIR) analyses of the polymeric films, using a partial least squares (PLS) chemometric approach. The influence of pH and metal content on cadmium (II) extraction, the characterization of its extraction in terms of the adsorption isotherm, enrichment factor and extraction equilibrium were studied. The PLS chemometric algorithm was applied to the spectral data to establish the relationship between cadmium (II) content in the membrane and the absorption spectra. Furthermore, the developed MID-FTIR method was validated through the determination of the figures of merit (accuracy, linearity, sensitivity, analytical sensitivity, minimum discernible concentration difference, mean selectivity, and limits of detection and quantitation). Results showed reliable calibration curves denoting systems' potentiality. Comparable results were obtained in the analysis of real samples (tap, bottle, and pier water) between the new MID-FTIR-PLS PIM based-sensor and F-AAS.

Benzene Derivatives Analysis Using Aluminum Nitride Waveguide Raman Sensors

Raman spectroscopy using aluminum nitride (AlN) optical waveguides was demonstrated for organic compound analysis. The AlN waveguide device was prepared by reactive sputtering deposition and complementary-metal-oxide semiconductor (CMOS) processes. A fundamental waveguide mode was observed over a broad visible spectrum and the waveguide evanescent wave was used to excite the Raman signals of the test analytes. The performance of the waveguide sensor was characterized by measuring the Raman spectra of the benzene derivative mixtures consisting of benzene, anisole, and toluene. The compositions and concentrations were resolved by correlating the obtained Raman spectrum with the characteristic Raman peaks associated with C-C, C-H, and C-O functional groups. With the advantages of real-time detection and enhanced Raman signal intensity, the AlN waveguides provided a sensor platform for nondestructive and online chemical compound monitoring.

A reliable and facile fluorescent sensor from carbon dots for sensing 2,4,6-trinitrophenol based on inner filter effect

2,4,6-Trinitrophenol (TNP) has absorbed much concerns because of its toxic effect and threat on the environment, which results from the fact that it is an important and universal reagent widely utilized for manufacturing many products. It is of great necessity to explore facile and efficient methods for monitoring TNP. In present study, carbon dots (CDs), a new carbonaceous nanomaterial with strong fluorescence, was applied to build a novel sensor for highly sensitive and selective detection of TNP. In the sensing procedure, the fluorescence intensity of as-prepared CDs was diminished with the presence of TNP due to inner filter effect (IFE) quenching mechanism. The sensitivity of the fluorescent sensor was very high with limit of detection down to 5.37 ng mL^-1. This fluorescent sensor was evaluated and excellent spiked recoveries were gained, which demonstrated that the developed sensor would be a robust tool for environmental applications.

Raman spectroscopy for detection of ammonium nitrate as an explosive precursor used in improvised explosive devices

Raman spectroscopy was evaluated as a sensor for detection of ammonium nitrate (NH₄NO₃, AN), fuel oil (FO), AN-water solutions, and AN- and FO-soil mixtures deposited on materials such as glass, synthetic fabric, cardboard and electrical tape to simulate field conditions of explosives detection. AN is an inorganic oxidizing salt that is commonly used in fertilizers and mining explosives, however, due to its widespread accessibility, AN-based explosives are also utilized for the manufacture of improvised explosive devices (IED). Pure AN crystals were ground to powder size and deposited on several substrates for Raman analysis, whereas FO was analysed in a quartz cuvette. To simulate field conditions samples of powdered AN, AN-water solutions (0.1% to 10.0% AN w/w), AN-soil (50% to 90% AN w/w) and FO-soil (50% to 75% FO w/w) were prepared and deposited on the clutter materials. Raman spectra were acquired at integration times between 0.1 and 30 s, and 3 replicate Raman measurements were carried out for each sample. The spectral window observed ranged from 300 to 3800 cm^-1. Several characteristic Raman bands were found, namely, at 710 cm^-1 (NO₃^-) and 1040 cm^-1 (NO₃^-) for AN; 1440-1470 cm^-1 (CH) and 2800-3000 cm^-1 (CH) for FO; 3000-3500 cm^-1 (OH) for water; and 615 cm^-1 (CCl), 1254 cm^-1 (CH), 1400 cm^-1 (CH₂) and 1600 cm^-1 (aromatic ring) for polyvinyl chloride (PVC, electrical tape). The effect of the AN concentration and integration time on the total and net Raman intensities, relative standard deviation, signal-to-noise ratio and relative limit of detection was evaluated. The relative limit of detection of AN in water was 0.1% (1 mg/g), and absolute limit of detection was 1.0 μg. The optimum integration time (≈10 s) for the Raman sensor to capture the analyte signals was estimated based on the Raman figures of merit as a function of the integration time.

Optimization of ultraviolet Raman spectroscopy for trace explosive checkpoint screening

Raman spectroscopy has long been considered a gold standard for optically based chemical identification, but has not been adopted in non-laboratory operational settings due to limited sensitivity and slow acquisition times. Ultraviolet (UV) Raman spectroscopy has the potential to address these challenges through the reduction of fluorescence from background materials and increased Raman scattering due to the shorter wavelength (relative to visible or near-infrared excitation) and resonant enhancement effects. However, the benefits of UV Raman must be evaluated against specific operational situations: the actual realized fluorescence reduction and Raman enhancement depend on the specific target materials, target morphology, and operational constraints. In this paper, the wavelength trade-space in UV Raman spectroscopy is evaluated for one specific application: checkpoint screening for trace explosive residues. The optimal UV wavelength is evaluated at 244, 266, and 355 nm for realistic trace explosive and explosive-related compound (ERC) residues on common checkpoint materials: we perform semi-empirical analysis that includes the UV penetration depth of common explosive and ERCs, realistic explosive and ERC residue particle sizes, and the fluorescence signal of common checkpoint materials. We find that while generally lower UV wavelength provides superior performance, the benefits may be significantly reduced depending on the specific explosive and substrate. Further, logistical requirements (size, weight, power, and cost) likely limit the adoption of optimal wavelengths. Graphical abstract.

Non-invasive identification of potato varieties and prediction of the origin of tuber cultivation using spatially offset Raman spectroscopy

High starch content, simplicity of cultivation, and high productivity make potatoes (Solanum tuberosum) a staple in the diet of people around the world. On average, potatoes are composed of 83% water and 12% carbohydrates, and the remaining 4% includes proteins, vitamins, and other trace elements. These proportions vary depending on the type of potato and location where they were cultivated. At the same time, the chemical composition determines the nutritional value of potato tubers and can be proved using various wet chemistry and spectroscopic methods. For instance, gravity measurements, as well as several different colorimetric assays, can be used to investigate the starch content. However, these approaches are indirect, often destructive, and time- and labor-consuming. This study reports on the use of Raman spectroscopy (RS) for completely non-invasive and non-destructive assessment of nutrient content of potato tubers. We also show that RS can be used to identify nine different potato varieties, as well as determine the origin of their cultivation. The portable nature of Raman-based identification of potato offers the possibility to perform such analysis directly upon potato harvesting to enable quick quality evaluation. Graphical abstract.

Chemical Classification of Explosives

This work comprehensively reviews some fundamental concepts about explosives and their two commonly used classifications based on either their velocity of detonation or their application. These classifications are highly useful in the military/legal field, but completely useless for the chemical determination of explosives. Because of this reason, a classification of explosives based on their chemical composition is comprehensively revised, discussed and updated. This classification seeks to merge those dispersed chemical classifications of explosives found in literature into a unique general classification, which might be useful for every researcher dealing with the analytical chemical identification of explosives. In the knowledge of the chemical composition of explosives, the most adequate analytical techniques to determine them are finally discussed.

Electrochemical determination of nitroaromatic explosives at boron-doped diamond/graphene nanowall electrodes: 2,4,6-trinitrotoluene and 2,4,6-trinitroanisole in liquid effluents

The study is devoted to the electrochemical detection of trace explosives on boron-doped diamond/graphene nanowall electrodes (B:DGNW). The electrodes were fabricated in a one-step growth process using chemical vapour deposition without any additional modifications. The electrochemical investigations were focused on the determination of the important nitroaromatic explosive compounds, 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitroanisole (TNA). The distinct reduction peaks of both studied compounds were observed regardless of the pH value of the solution. The reduction peak currents were linearly related to the concentration of TNT and TNA in the range from 0.05-15 ppm. Nevertheless, two various linear trends were observed, attributed respectively to the adsorption processes at low concentrations up to the diffusional character of detection for larger contamination levels. The limit of detection of TNT and TNA is equal to 73 ppb and 270 ppb, respectively. Moreover, the proposed detection strategy has been applied under real conditions with a significant concentration of interfering compounds - in landfill leachates. The proposed bare B:DGNW electrodes were revealed to have a high electroactive area towards the voltammetric determination of various nitroaromatic compounds with a high rate of repeatability, thus appearing to be an attractive nanocarbon surface for further applications.

Polyhydroquinoline nanoaggregates: A dual fluorescent probe for detection of 2,4,6-trinitrophenol and chromium (VI)

Fluorescent polyhydroquinoline (PHQ) derivative was fabricated utilizing one-pot engineered course. The PHQ derivative indicated aggregation induced emission enhancement (AIEE) with arrangement of nanoaggregates of size 11-13 nm in 95% watery DMF medium. The fluorescence emission of PHQ nanoaggregates was extinguished by including TNP and Cr (VI). They indicated prevalent fluorescence quenching towards both TNP and Cr (VI) over other meddling nitro-compounds and metal particles. In light of results got we presumed that both photo-induced fluorescence quenching of PHQ nanoaggregates by TNP, while Inner Filter Effect (IFE) was in charge of fluorescence quenching of PHQ nanoaggregates by Cr (VI). The PHQ nanoaggregates empowered identification of TNP and Cr (VI) down to 0.66 μM (TNP) and 0.28 μM (Cr (VI)). The use of PHQ nanoaggregates were reached out for location of TNP and Cr (VI) in genuine water tests.