Fluorescence Quenching as an Indirect Detection Method for Nitrated Explosives

A water-soluble small molecular fluorescent sensor based on phosphazene platform for selective detection of nitroaromatic compounds

Nitro-aromatic compounds have a deleterious effect on the environment and they are extremely explosive. Therefore, societal concern about exposure to nitro-aromatic compounds encourages researchers to develop selective and sensitive detection platforms for nitro-aromatic compounds in recent years. In this paper, a new 100% water-soluble cyclotriphosphazene-based bridged naphthalene material (4) was prepared as a small molecule fluorescent sensor for ultra-selective detection of nitro-aromatic compounds. The chemical structure of 4 was extensively characterized by mass spectrometry and nuclear magnetic resonance spectroscopies (³¹P, ¹³C, ¹H). The photo-physical properties of the newly developed sensing system were investigated by steady-state fluorescence and UV-Vis absorption spectroscopies. The fluorescence sensor behaviors were extensively evaluated after treatment with the most commonly used metal cations, anions, competitive aromatic compounds, saccharides, and organic acids. The developed fluorescent sensing system (4) demonstrated ultra-selective fluorescence "turn-off" signal change toward nitro-aromatic compounds while other tested competitive species caused negligible changes. To evaluate selectivity, time-resolved, steady-state 3D-fluorescence and UV-Vis absorption spectroscopies were used in fully aqueous media. Moreover, theoretical calculations (density functional theory and time-dependent density functional theory) were applied and discussed to identify fluorescence sensing mechanisms toward nitroaromatic compounds for the presented sensing system.

Investigation of Chemosensing and Color Properties of Schiff Base Compounds Containing a 1,2,3-triazole Group

A series of Schiff base compounds (ER¹-ER⁵) containing a 1,2,3-triazole and carboxylic acid groups were synthesized and their chemosensory properties towards anions (I^-, CO₃^2-, SO₄^2-, NO₂^-, NO₃^-, CH₃COO^-, ClO₃^-, CNO^-, N₃^-) and cations (Al³⁺, Ag⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺). The compounds were also used as fluorescence probs for the detection of nitroaromatic compounds. The structural characterization of the synthesized compounds was elucidated using methods such as FT-IR, UV, FL, LC-MS, MALDI-TOF MS, ¹H(¹³C) NMR. The effect of substitute groups (-CH₃, -OCH₃, -OH, -Cl and -Br) on the synthesized Schiff bases (ER¹-ER⁵) on the chemosensory properties were compared. As the groups changed, the sensor and quenching effects of the molecule against anions and cations changed. Compound ER³ having methoxy (OCH₃) group exhibited selective sensor properties against Fe³⁺ ion while compound ER⁵ with a chloride substitute (Cl) group showed selectivity for Cr³⁺ ion under 254 nm UV-lamp. The substitute effect was also observed for the sensing of anions. Under 254 nm UV-lamp, ER² having the -OH group has a selective sensing property for CNO^- and ER⁴ with the bromide (Br) group exhibited selectivity for N₃^- ion. The synthesized Schiff base compounds were also tested as fluorescence probs for the sensing of some nitroaromatic explosives.

Cluster of Hexamolybdenum [Mo6Cl14]2- for Sensing Nitroaromatic Compounds

This contribution describes a novel method for the detection of trace amounts of trinitrotoluene (TNT) using a cluster of hexamolybdenum with general formula [Mo₆Cl₁₄]^2-. The molybdenum cluster was characterized by UV-visible, FT-IR, and fluorescence techniques, and the synthesis was efficient and reproducible. The evaluation of the molybdenum cluster by TNT detection was perfomed by fluoresecent measurements, and the results were interpreted by the Stern-Volmer equation, obtaining K _SV values of 2.9 × 10⁵ and 1.6 × 10⁴ M^-1 in different concentration ranges. Further, the results suggest that at TNT concentrations higher than 4 × 10^-5 mM (0.01 mg L^-1) it is possible to measure the quenching of the cluster fluorescence. The DFT calculations indicate that the contribution of the TNT in the active lowest unoccupied molecular orbitals that are involved in the higher intensity transitions in the complex cluster-TNT are significant. This situation differs from all the luminescent [M₆X₈L₆]^2- clusters (M = Mo; X = facial bridging ligand, and L = labile axial ligands), where most of the closely spaced excited states are located in the {M₆X₈} ^q+ core. Thus, the TNT switches off the cluster luminescence. The approach using a [Mo₆Cl₁₄]^2--based fluorescence sensor has the potential to be a sensing technology for the detection of nitroaromatic explosives.

CTV Based Sensor for the Rapid Detection of Nitro Toluene With Computational Studies and Molecular Modelling

A new tri-naphthoylated Cyclotriveratrylene molecule has been synthesized for the rapid and sensitive detection of 4-nitrotoluene (4-NT) among various nitro aromatic compounds (NACs) by using a spectrofluorimetric method. The newly derrivatized cyclotriveratrylene compound is successfully confirmed by using the available techniques of ¹H-NMR, ¹³CNMR, and ESI-MS. This synthesised molecule is 1NC-CTV (1-Naptholy Chloride-Cyclotriveratrylene). Strong quenching in the fluorescence intensity of 1NC-CTV was observed upon the addition of 4-NT. Further quantum yield studies were carried out and by using the stern volmer it was concluded that the fluorescence quenching mechanism is dynamic or static. The molecule 1NC-CTV was further studied with the help of computational methods such as molecular docking to study the binding interactions and properties of the molecule.

Quantification of 2-chlorohydroquinone based on interaction between N-doped carbon quantum dots probe and photolysis products in fluorescence system

As a member of chlorophenolic compounds, 2-chlorohydroquinone (H₂QCl) has been widely used as intermediates in various chemical industries and leaded to serious threat on the environment. It is urgent to develop simple and robust analytical method for sensitive and selective determination of H₂QCl. Carbon quantum dots (CQDs), a promising photoluminescence nanomaterial, have gained sufficient concern as optical sensors owing to their outstanding photochemical properties. In this work, nitrogen doped carbon quantum dots (N-CQDs) were successfully synthesized by a simple secondary hydrothermal method and applied as a fluorescent probe for the quantitation of H₂QCl. A new fluorescence region centered at excitation wavelength of 310 nm and emission wavelength of 390 nm appeared after nitrogen doping. It was found that the N-CQDs exhibited a high selectivity towards H₂QCl with sensitive fluorescence response and the fluorescence quenching of N-CQDs was linear with the concentration of H₂QCl in the range of 30-90 μM (Y = 0.0049X + 0.1255, R² = 0.996). This is the first time that the dual role of excitation light was observed in the fluorescence detection system. The ultraviolet light acted as not only the excitation energy source for N-CQDs photoluminescence, but also the light source for photolysis of H₂QCl. In the detection process, H₂QCl was degraded to p-benzoquinone by light, and then the CQDs combined with p-benzoquinone through Michael addition reaction under the action of doped nitrogen. The electron transfer from N-CQDs to the linked p-benzoquinone caused the quenching of fluorescence originated from the edge state of N-CQDs. Furthermore, this established method can be applied for the quantitative determination of H₂QCl in environmental water samples with satisfactory recoveries between 94.31 and 105.51%.

Covalent organic frameworks as multifunctional materials for chemical detection

Sensitive and selective detection of chemical and biological analytes is critical in various scientific and technological fields. As an emerging class of multifunctional materials, covalent organic frameworks (COFs) with their unique properties of chemical modularity, large surface area, high stability, low density, and tunable pore sizes and functionalities, which together define their programmable properties, show promise in advancing chemical detection. This review demonstrates the recent progress in chemical detection where COFs constitute an integral component of the achieved function. This review highlights how the unique properties of COFs can be harnessed to develop different types of chemical detection systems based on the principles of chromism, luminescence, electrical transduction, chromatography, spectrometry, and others to achieve highly sensitive and selective detection of various analytes, ranging from gases, volatiles, ions, to biomolecules. The key parameters of detection performance for target analytes are summarized, compared, and analyzed from the perspective of the detection mechanism and structure-property-performance correlations of COFs. Conclusions summarize the current accomplishments and analyze the challenges and limitations that exist for chemical detection under different mechanisms. Perspectives on how future directions of research can advance the COF-based chemical detection through innovation in novel COF design and synthesis, progress in device fabrication, and exploration of novel modes of detection are also discussed.

Synthesis and Tetraphenylethylene-Based Aggregation-Induced Emission Probe for Rapid Detection of Nitroaromatic Compounds in Aqueous Media

Tetraphenylethylene (TPE) can be used to construct fluorescent probes with typical aggregation-induced emission (AIE) behavior for next-generation sensing applications. McMurry coupling and Suzuki cross coupling strategies provided the desired sensor thiophene-substituted tetraphenylethylene (THTPE). The synthesized TPE analogues were characterized by NMR spectroscopy and mass spectrometry. Maximum AIE of THTPE was observed in 90% water (H2O/THF) content due to extensive formation of aggregates. The AIE properties of THTPE have been utilized for facile detection of nitroaromatic compounds (NACs) (1.0 nM) through a fluorescence quenching mechanism. A paper strip adsorbed with the AIE-based THTPE fluorophore is developed for rapid and convenient detection of NAC-based analytes. Further, interaction of THTPE with analytes is also studied via Gaussian software at the DFT/B3LYP/6-31G(d) level of theory. Interaction energy, frontier molecular orbitals (FMOs), and non-covalent interaction (NCI) analyses are studied by using the same method. Computational results revealed that nitrobenzene (NB) has the strongest interaction while 1,3-dinitrobenzene (DNB) exhibits the least interaction with the sensor molecule. These computational results clearly demonstrate good agreement with experimental data.

Field determination of hazardous chemicals in public security by using a hand-held Raman spectrometer and a deep architecture-search network

With the advanced development of miniaturization and integration of instruments, Raman spectroscopy (RS) has demonstrated its great significance because of its non-invasive property and fingerprint identification ability, and extended its applications in public security, especially for hazardous chemicals. However, the fast and accurate RS analysis of hazardous chemicals in field test by non-professionals is still challenging due to the lack of an effective and timely spectral-based chemical-discriminating solution. In this study, a platform was developed for the field determination of hazardous chemicals in public security by using a hand-held Raman spectrometer and a deep architecture-search network (DASN) incorporated into a cloud server. With the Raman spectra of 300 chemicals, DASN stands out with identification accuracy of 100% and outweighs other machine learning and deep learning methods. The network feature maps for the spectra of methamphetamine and ketamine focus on the main peaks of 1001 and 652 cm^-1, which indicates the powerful feature extraction capability of DASN. Its receiver operating characteristic (ROC) curve completely encloses the other models, and the area under the curve is up to 1, implying excellent robustness. With the well-built platform combining RS, DASN, and cloud server, one test process including Raman measurement and identification can be performed in tens of seconds. Hence, the developed platform is simple, fast, accurate, and could be considered as a promising tool for hazardous chemical identification in public security on the scene.

Fluorescent Calix[4]arene-Carbazole-Containing Polymers as Sensors for Nitroaromatic Explosives

Two highly fluorescent calix[4]arene-containing phenylene-alt-ethynylene-carbazolylene polymers (Calix-PPE-CBZs) were used in the detection of explosives from the nitroaromatic compounds (NACs) family, in solution and in vapour phases. Both fluorophores exhibit high sensitivity and selectivity towards NACs detection. The quenching efficiencies in solution, assessed through static Stern-Volmer constants (KSV), follow the order picric acid (PA) >> 2,4,6-trinitrotoluene (TNT) > 2,4-dinitrotoluene > (2,4-DNT) > nitrobenzene (NB). These correlate very well with the NACs electron affinities, as evaluated from their lowest unoccupied molecular orbitals (LUMOs) energies, indicating a photo-induced electron transfer as the dominant mechanism in fluorescence quenching. Moreover, and most interesting, detection of TNT, 2,4-DNT and NB vapours via thin-films of Calix-PPE-CBZs revealed a remarkably sensitive response to these analytes, comparable to state-of-the-art chemosensors. The study also analyses and compares the current results to previous disclosed data on the detection of NACs by several calix[4]arene-based conjugated polymers and non-polymeric calix[4]arenes-carbazole conjugates, overall highlighting the superior role of calixarene and carbazole structural motifs in NACs’ detection performance. Density functional theory (DFT) calculations performed on polymer models were used to support some of the experimental findings.

Highly fluorescent scandium-tetracarboxylate frameworks: selective detection of nitro-aromatic compounds, sensing mechanism, and their application

Recently, exploring new luminescent metal-organic frameworks (LMOFs) to selectively detect nitro-aromatic compounds (NACs) has been a hot topic of research. Simultaneously, it is still a challenging issue to understand the sensing mechanism of luminescent MOFs interacting with NACs at the molecular level. In this work, highly fluorescent Sc-tetracarboxylate frameworks (Sc-EBTC) have been successfully synthesized through a solvothermal method. The as-prepared Sc-EBTC crystals have good thermal stability, chemical stability as well as strong fluorescence (λ_ex = 320 nm and λ_em = 400 nm), and they can detect various NACs rapidly (as short as 30 s), selectively and efficiently by the "turn-off" fluorescence mechanism. The detection limits of Sc-EBTC toward 2,4-DNP and 4-NP are quantified to be 5.71 ppb and 6.26 ppb, respectively. Furthermore, to better understand the sensing mechanism, we attempt to use solid-state NMR and X-ray photoelectron spectroscopy to vividly characterize the charge transfer caused by the interaction between NAC molecules and the MOF at the molecular level. Additionally, test strips were made successfully for the practical detection of the NACs. This study demonstrates that the MOF constructed from the H₄EBTC ligands might be a promising candidate for the detection of trace NACs.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

Fabrication of amine functionalized CdSe@SiO2 nanoparticles as fluorescence nanosensor for highly selective and sensitive detection of picric acid

In this work, amino functionalized CdSe-silica core-shell nanoparticles (NH₂-CdSe@SiO₂ NPs) were constructed as probe to detect picric acid (PA). The CdSe QDs were embedded in SiO₂ nanoparticles and modified with amino groups on the surface. The nitro explosives are electron deficient in nature, which will have stronger affinity for amines and resulted in fluorescence quenching of quantum dots. It was proved that this strategy is selective, easy and sensitive enough for sensing PA with a detection limit of 0.5 × 10^-7 M.

Anthracene based AIEgen for picric acid detection in real water samples

In this work, a new anthracene Schiff base derivative (AS) was successfully synthesized and characterized by pivotal analytical techniques. Based on the contented results, the AS molecule was employed for photophysical investigation using UV-Visible absorption, steady state and time resolved fluorescence techniques. The photophysical studies reveal that the AS possesses modest molar absorption coefficient (10⁴) and weak fluorescence (ϕ = 0.006). The weak fluorescence of AS is due to intramolecular photoinduced electron transfer (PET). Intriguingly, the weak fluorescence intensity of AS is enhanced dramatically by the gradual addition of water up to 90% as well as appearance of long lived fluorescence decay. The enhancement in the fluorescence intensity and lifetime clearly indicates that this molecule has aggregation-induced emission (AIE) property. Further, the AIE property of AS is utilized for sensitive detection of picric acid (PA). The fluorescence of aggregated AS is quenched regularly with the sequential addition of PA concentration. The higher Stern-Volmer constant (2.61 × 10⁵ M^-1) and excellent detection limit of 93 nM designate the AS aggregates as potential candidate for explosive detection. The mechanism behind the quenching of fluorescence can be ascribed to inner filter effect, which is supported by spectral overlap analysis and fluorescence lifetime measurements. The suitability of AS aggregates for practical applications is realized by the detection of trace amounts of PA in real water samples.

A series of trinuclear zinc(ii) complexes with reduced Schiff base ligands: turn-off fluorescent chemosensors with high selectivity for nitroaromatics

A series of trinuclear zinc(ii) complexes have been synthesized and characterized by X-ray crystallography. The ability of the complexes to act as sensors for the detection of nitroaromatics in DMF has been assessed.

Constitutional isomers of dendrimer-like pyrene substituted cyclotriphosphazenes: synthesis, theoretical calculations, and use as fluorescence receptors for the detection of explosive nitroaromatics

Two constitutionally isomeric bis-pyrenyl phenol dendrons (4 and 6) and their dendrimer-like cyclotriphosphazene derivatives (5 and 7) are designed, synthesized and fluorescence detection behaviors are evaluated for nitro aromatic compounds (NACs).

Turn-on fluorescent sensor for the detection of lipopolysaccharides based on a novel bispyrenyl terephtalaldehyde-bis-guanylhydrazone

A “turn-on” bispyrenyl sensor for lipopolysaccharide detection with unique molecular conformations exhibits excimer emission with sensitivity down to nanomolar.

Conjugated Nanowire Sensors via High-Energy Single-Particle-Induced Linear Polymerization of 9,9'-Spirobi[9 H-fluorene] Derivatives

Nanostructures composed of conjugated polymers or π-conjugated molecules provide sensing platforms with large specific surface areas. One of the feasible approaches to accessing such nanostructured miniaturized sensors with ultrahigh sensitivity is to develop a network of organic nanowires with optical/electronic properties that can measure signals upon interacting with the analytes at their surfaces. In this work, organic nanowires with controlled number density and uniform length were fabricated by one-dimensional solid-state polymerization of 9,9'-spirobi[9 H-fluorene] (SBF) derivatives triggered by high-energy single particles. SBF was chosen as a conjugated molecular motif with the interplay of high density of π-electrons, high solubility, and uniform solid-state structures, allowing us to fabricate sensing platforms via solution processing. The as-deposited energy density in linear polymerization nanospace was theoretically analyzed by a collision model, interpreting nanowire sizes at subnanometer levels. The substitution of bromine atoms was confirmed to be effective not only for the higher collision probability of the incident particles but also for the remarkable increase in radiolytic neutral radical yield via C-Br cleavages or electron-dissociative attachments onto the bromine atoms. The fluorescence spectra of SBF-based nanowires were different from those of SBF derivatives due to extended bond formation as a result of polymerization reactions. Fluorescence was quenched by the addition of nitrobenzene, indicating the potential use of our nanowires for fluorometric sensing applications. Microwave-based conductivity measurements revealed that the SBF-based nanowires exhibited charge carrier transport property upon photoexcitation, and that the conductivity was changed upon treatment with nitrobenzene vapors. The presented strategy of bromination of aromatic rings for efficient fabrication of controlled nanowire networks with favorable fluorescent and charge transport properties of nanowires advances the development of nanostructured sensing systems.

A quinoline-based compound for explosive 2,4,6-trinitrophenol sensing: experimental and DFT-D3 studies

A quinoline-based compound, 2,5-dimethylbis(quinolin-2-ylmethylene)benzene-1,4-diamine (DQB), has been found to be a turn-off chemosensor for 2,4,6-trinitrophenol.

Design and application of a fluorogenic receptor for selective sensing of cations, small neutral molecules, and anions

An unprecedented single multi-analyte fluorogenic receptor, a sodium salt of N-(methyl-2-thiophenyl)-tyrosine (NaHTyrthio), is reported for the selective sensing of cations (Cu²⁺), small neutral molecules (nitrobenzene and aniline) and anions (F⁻) by variable spectral responses.

Toward wearable sensors: optical sensor for detection of ammonium nitrate-based explosives, ANFO and ANNM

An imine-functionalized polymer displays selective fluorimetric response to the component of ANFO and ANNM, ammonium nitrate and nitromethane!

Extended cavity pyrene-based iptycenes for the turn-off fluorescence detection of RDX and common nitroaromatic explosives

Extended cavity pyrene-based iptycenes were synthesized for the turn-off fluorescence detection of RDX and common nitro-aromatic explosives.

Bigger and Brighter Fluorenes: Facile π-Expansion, Brilliant Emission and Sensing of Nitroaromatics

π-Expanded butterfly-like 2D fluorenes and 3D spirobifluorenes 1-5 were synthesized via a DDQ-mediated oxidative cyclization strategy with a high regioselectivity. Through structural modification via π-expansion, it was possible to achieve near-ultraviolet absorption, bright-blue emission, very high near-unity fluorescence quantum yields in solution as well as in film states, and deep-lying HOMO energy levels with excellent thermal stabilities. Furthermore, these electron-rich compounds displayed a notable behavior towards sensing of nitroaromatic explosives, such as picric acid, up to a detection limit of 0.2 ppb.

Probing Energy and Electron Transfer Mechanisms in Fluorescence Quenching of Biomass Carbon Quantum Dots

The recent discovery of biomass-derived carbon quantum dots (CQDs) offers the potential to extend the sensing and imaging capabilities of quantum dots (QDs) to applications that require biocompatibility and environmental friendliness. Many studies have confirmed the exciting optical properties of CQDs and suggested a range of applications, but realizing the potential of CQDs will require a deeper fundamental understanding of their photophysical behavior. Here, biomass-derived CQDs were synthesized by hydrothermal processing methods from the aminopolysaccharide chitosan, and their fluorescence quenching behaviors were investigated. A family of nitroaromatics with different ring substituents was used to generate systematically varying CQD-quenching behaviors. Experimental evidence including a correlation between quenching constant and spectral overlap, fluorescence lifetime decay, and donor-acceptor distance all demonstrate that the primary mechanism for QCD-quenching is Förster resonance energy transfer (FRET) and not electron transfer. Spectroelectrochemical studies with redox-dependent quenching molecules and studies with complex dye molecules further support this conclusion. We envision this fundamental understanding of CQDs will facilitate the application of these emerging nanomaterials for sensing and imaging.

Photophysics of detection of explosive vapours via luminescence quenching of thin films: impact of inter-molecular interactions

Fluorescence-based detection of explosive analytes requires an understanding of the nature of the excited state responsible for the luminescence response of a sensing material.

Fluorescence based explosive detection: from mechanisms to sensory materials

The detection of explosives is one of the current pressing concerns in global security. In the past few decades, a large number of emissive sensing materials have been developed for the detection of explosives in vapor, solution, and solid states through fluorescence methods. In recent years, great efforts have been devoted to develop new fluorescent materials with various sensing mechanisms for detecting explosives in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time. This review article starts with a brief introduction on various sensing mechanisms for fluorescence based explosive detection, and then summarizes in an exhaustive and systematic way the state-of-the-art of fluorescent materials for explosive detection with a focus on the research in the recent 5 years. A wide range of fluorescent materials, such as conjugated polymers, small fluorophores, supramolecular systems, bio-inspired materials and aggregation induced emission-active materials, and their sensing performance and sensing mechanism are the centerpiece of this review. Finally, conclusions and future outlook are presented and discussed.

Fluorescent aggregates of AIEE active triphenylene derivatives for the sensitive detection of picric acid

Fluorescent aggregates of triphenylene derivatives 3 and 5 having aggregation induced emission enhancement (AIEE) characteristics have been developed which selectively detect picric acid.

SPECTROSCOPIC AND ELECTROCHEMICAL CHARACTERIZATION OF IRON(II) AND 2,4-DINITROTOLUENE

The objective of this work was the development of reliable methods to determine 2,4-dinitrotoluene, a precursor to explosives. A complex between Fe(II) ion and 2,4-dinitrotoluene was formed in solution and characterized by ultraviolet-visible absorption spectroscopy using Job's plots and attenuated total reflection-Fourier transform infrared spectroscopy. Surface modification of glassy carbon electrodes were performed with iron nanoparticles via electrochemical reduction of iron(II). The modified electrode was employed for the determination of 2,4-dinitrotoluene. Scanning electron micrographs showed that the iron nanoparticles were incorporated on the surface of glassy carbon electrode. The electrochemical determination of 2,4-dinitrotoluene was performed by cyclic voltammetry using the modified electrode. The iron modified electrode produced larger reduction currents than the unmodified electrode for the same concentration of 2,4-dinitrotoluene. Concentrations of 2,4-dinitrotoluene as low as 10 parts per billion were determined using the modified electrode.

π-Electron rich small molecule sensors for the recognition of nitroaromatics

In this review article we provide an overview of the recent developments made in small molecule-based turn-off fluorescent sensors for nitroaromatic explosives with special focus on organic and H-bonded supramolecular sensors.

Self-assembled monolayers of thiols adsorbed on Au/ZnO-functionalized silica nanosprings: photoelectron spectroscopy-analysis and detection of vaporized explosives

Self-assembled monolayers (SAMs) of thiols of L-cysteine, 6-mercaptohexanol, 4-mercaptobenzoic acid, DL-thioctic acid and 11-(1-pyrenyl)-1-undecathiol, which have been selected for their propensity to interact with vaporized explosives, have been attached from solution onto gold decorated ZnO-coated nanosprings. X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) have been used to investigate the surface electronic structure of the SAMs coated nanosprings. On the basis of XPS analysis, it has been determined that the packing densities of L-cysteine, 6-mercaptohexanol, 4-mercaptobenzoic acid, DL-thioctic acid and 11-(1-pyrenyl)-1-undecathiol on gold (zinc oxide) are 5.42 × 10(14) (2.83 × 10(14)), 3.26 × 10(14) (2.54 × 10(14)), 9.50 × 10(13), 2.55 × 10(14) (1.12 × 10(14)), and 5.23 × 10(13) molecules/cm(2), respectively. A single S 2p core level doublet is observed for 4-mercaptobenzoic acid and 11-(1-pyrenyl)-1-undecathiol, which is assigned to the S-Au bond. The S 2p core level for L-cysteine, 6-mercaptohexanol, and DL-thioctic acid consist of two doublets, where one is S-Au bond and the other is the S-Zn bond. Analysis of the C/S ratios agrees well with the stoichiometry of the respective thiols. UPS analysis shows that the hybridization of S 3p states and Au d-bands produces antibonding and bonding states, above and below the Au d-bands, which is characteristic of molecular chemisorption on Au nanoparticles. Gas sensors were constructed with thiolated nanosprings and their responsiveness to ammonium nitrate at 100-150 °C was tested. Nanosprings sensors functionalized with 4-mercaptobenzoic acid and 6-mercaptohexanol showed the strongest responses by a factor of 4 to 5 relative to the less responsive thiols. The response to ammonium nitrate can be correlated to the packing density and ordering of the SAMs.

Coupling paper-based microfluidics and lab on a chip technologies for confirmatory analysis of trinitro aromatic explosives

A new microfluidic paper-based analytical device (μPAD) in conjunction with confirmation by a lab on chip analysis was developed for detection of three trinitro aromatic explosives. Potassium hydroxide was deposited on the μPADs (0.5 μL, 1.5 M), creating a color change reaction when explosives are present, with detection limits of approximately 7.5 ± 1.0 ng for TNB, 12.5 ± 2.0 ng for TNT and 15.0 ± 2.0 ng for tetryl. For confirmatory analysis, positive μPADs were sampled using a 5 mm hole-punch, followed by extraction of explosives from the punched chad in 30 s using 20 μL borate/SDS buffer. The extractions had efficiencies of 96.5 ± 1.7%. The extracted explosives were then analyzed with the Agilent 2100 Bioanalyzer lab on a chip device with minimum detectable amounts of 3.8 ± 0.1 ng for TNB, 7.0 ± 0.9 ng for TNT, and 4.7 ± 0.2 ng for tetryl. A simulated in-field scenario demonstrated the feasibility of coupling the μPAD technique with the lab on a chip device to detect and identify 1 μg of explosives distributed on a surface of 100 cm(2).

Fabrication of magnetically separable fluorescent terbium-based MOF nanospheres for highly selective trace-level detection of TNT

In this work, we present novel kinds of Fe3O4@Tb-BTC magnetic metal-organic framework (MOF) nanospheres which possess both magnetic characteristics and fluorescent properties using a layer by layer assembly technique. The structure and morphology of the as-prepared Fe3O4@Tb-BTC were systematically characterized and it was applied in detection of nitroaromatic explosives, such as 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), nitrobenzene (NB) and picric acid (PA). The results indicate that the fluorescence intensity of Fe3O4@Tb-BTC can be quenched by all analytes studied in the present work. Remarkably, the as-synthesized nanospheres exhibit high sensitivity for 2,4,6-trinitrotoluene (TNT) detection with Ksv value of (94 800 M(-1)). Besides, the magnetic nanospheres can be easily recycled, which makes it more convenient for reutilization and friendly to the environment. The results show that it has broad application prospects in the detection of nitroaromatic explosives.