Polyfunctional Lewis Acids: Intriguing Solid‐State Structure and Selective Detection and Discrimination of Nitroaromatic Explosives

Synthesis and crystal structures of three porphyrin‐based polyfunctional Lewis acids 1 – 3 are reported. Intermolecular HgCl ??? HgCl (linear and μ‐type) interactions in the solid state of the peripherally ArHgCl‐decorated compound 3 lead to a fascinating 3D supramolecular architecture. Compound 3 shows a selective fluorescence quenching response to picric acid and discriminates other nitroaromatic‐based explosives. For the first time, an electron‐deficient polyfunctional Lewis acid is shown to be useful for the selective detection and discrimination of nitroaromatic explosives. The Stern–Volmer quenching constant and detection limits of compound 3 for picric acid are the best among the reported small‐molecular receptors for nitroaromatic explosives. The electronic structure, Lewis acidity, and selective sensing characteristics of 3 are well corroborated by DFT calculations.     

Iptycene‐Based Fluorescent Sensors for Nitroaromatics and TNT

Small‐molecule fluorescent sensors ( 1 – 5 ) for the recognition of nitroaromatic compounds, such as 2,4‐dinitrotoluene and the explosive TNT, were obtained by using a three‐step dehydrohalogenation cycloaddition protocol. The interaction of the receptors and nitroaromatics was studied both in solution and in the solid state by using fluorescence spectroscopy and X‐ray crystallography, respectively. It is shown that the iptycene receptors 1 – 5 provide a cavity suitable for binding nitroaromatic compounds in an edge‐to‐face mode, rather than simple ring‐stacking interactions. The results obtained inspired us to develop an inexpensive, reliable and robust sensor for vapour detection of explosives. Polymer nanofibres are particularly suitable for the production of such TNT sensors as they accelerate the mass exchange between the polymer and the vapours of TNT. Quenching of the sensors took place within 1 min compared to 10 min for a glass‐slide assay. Hence, the sensor performance can be improved by optimising the matrix material and morphology without resynthesising the sensor moieties.     

Supramolecular polymer for explosives sensing: role of H-bonding in enhancement of sensitivity in the solid state

A π-electron rich supramolecular polymer as an efficient fluorescent sensor for electron deficient nitroaromatic explosives has been synthesized, and the role of H-bonding in dramatic amplification of sensitivity/fluorescence quenching efficiency in the solid state has been established.     

Prediction of toxicity of nitroaromatic compounds through their molecular structures

In this paper, a new simple method is presented for the estimation of the toxicity of nitroaromatic compounds including some well-known explosives. This method can predict the 50% lethal dose concentration for rats (LD ₅₀) as the estimation of toxicity in vivo. The prediction of LD ₅₀ of nitroaromatics through a new general correlation is based on the number of alkyl and nitro groups per molecular weight of the nitroaromatic compound as a core function. The existence of some specific structural parameters can decrease or increase the predicted results on the basis of the core function. The predicted results of various nitroaromatic compounds afford reliable prediction of LD ₅₀ with respect to experimental data. Prediction of toxicity for 28 nitroaromatic compounds, where the experimental data were available, and new nitroaromatic derivatives produce comparable results to those of several models of Quantitative Structure Activity Relation (QSAR).     

Diffusion-controlled detection of trinitrotoluene: interior nanoporous structure and low highest occupied molecular orbital level of building blocks enhance selectivity and sensitivity

Development of simple, cost-effective, and sensitive fluorescence-based sensors for explosives implies broad applications in homeland security, military operations, and environmental and industrial safety control. However, the reported fluorescence sensory materials (e.g., polymers) usually respond to a class of analytes (e.g., nitroaromatics), rather than a single specific target. Hence, the selective detection of trace amounts of trinitrotoluene (TNT) still remains a big challenge for fluorescence-based sensors. Here we report the selective detection of TNT vapor using the nanoporous fibers fabricated by self-assembly of carbazole-based macrocyclic molecules. The nanoporosity allows for time-dependent diffusion of TNT molecules inside the material, resulting in further fluorescence quenching of the material after removal from the TNT vapor source. Under the same testing conditions, other common nitroaromatic explosives and oxidizing reagents did not demonstrate this postexposure fluorescence quenching; rather, a recovery of fluorescence was observed. The postexposure fluorescence quenching as well as the sensitivity is further enhanced by lowering the highest occupied molecular orbital (HOMO) level of the nanofiber building blocks. This in turn reduces the affinity for oxygen, thus allocating more interaction sites for TNT. Our results present a simple and novel way to achieve detection selectivity for TNT by creating nanoporosity and tuning molecular electronic structure, which when combined may be applied to other fluorescence sensor materials for selective detection of vapor analytes.     

Relation between Electric Spark Sensitivity and Impact Sensitivity of Nitroaromatic Energetic Compounds

Impact and electric spark sensitivities of energetic compounds are two important sensitivity parameters, which are closely related to many accidents in working places. In contrast to electric spark sensitivity, impact sensitivity can be easily measured. A new simple method is introduced to correlate electric spark and impact sensitivities of nitroaromatic compounds. Two correcting functions are used to consider several molecular moieties for reliable prediction of electric spark sensitivity through the measured or estimated impact sensitivity of nitroaromatics. The model is optimized using a set of 28 CHNO polynitroaromatic explosives and then it is tested for some nitroaromatics containing the other atoms such as sulfur. The predicted electric sensitivities of the new method are also compared with the reported results of a new quantum mechanical approach. For 22 CHNO nitroaromatics, quantum mechanical calculations are within ±3.0 J of 18 measured values and more than ±3.0 J for remaining 4 experimental data. Meanwhile, the predicted results of the method are less than ±3.0 J for 28 CHNO nitroaromatics. The root‐mean‐square (rms) deviations of the new model and quantum mechanical are also 1.55 and 2.51 J, respectively.     

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.     

Discrimination of nitroaromatics and explosives mimics by a fluorescent Zn(salicylaldimine) sensor array

Detecting and identifying components of plastic explosive devices is a challenge to current optical sensing methods. We report a fluorescent Zn(salicylaldimine) sensor array that accurately discriminated nitroaromatics, which are mimics of plastic explosives. Nitroaromatics quench the fluorescence of Zn(salicylaldimine), with differential quenching for the various fluorophore/quencher partners. The response pattern of the Zn(salicylaldimine) array created unique fingerprints for each of the nine nitroaromatic analytes tested, including unique responses for dinitrotoluene and dinitrobenzene. Linear discriminant analysis showed that the discrimination is largely due to a combination of two physical properties of the nitroaromatics: redox potential and log P. The array was able to discriminate unknown nitroaromatic samples in solution.     

Textile‐based Electrochemical Sensing: Effect of Fabric Substrate and Detection of Nitroaromatic Explosives

This study examines the influence of textile substrates upon the behavior of wearable screen‐printed electrodes and demonstrates the attractive sensing properties of these sensors towards the detection of nitroaromatic explosives. Compared to electrodes printed on common cotton or polyester substrates, GORE‐TEX‐based electrochemical sensors display reproducible background cyclic voltammograms, reflecting the excellent water‐repellant properties of the GORE‐TEX fabric. The wetting properties of different printed textile electrodes are elucidated using contact angle measurements. The influence of laundry washing and mechanical stress is explored. The GORE‐TEX‐based printed electrodes exhibit favorable detection of 2,4‐dinitrotoluene (DNT) and 2,4,6‐trinitrotoluene (TNT) explosives, including rapid detection of DNT vapor.     

Multicomponent Assembly of Fluorescent‐Tag Functionalized Ligands in Metal–Organic Frameworks for Sensing Explosives

Detection of trace amounts of explosive materials is significantly important for security concerns and pollution control. Four multicomponent metal–organic frameworks ( MOFs‐12 , 13 , 23 , and 123 ) have been synthesized by employing ligands embedded with fluorescent tags. The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density in the MOFs. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by single‐crystal XRD. Single‐crystal structures of the MOFs‐12 and 13 showed the formation of three‐dimensional porous networks with the aromatic tags projecting inwardly into the pores. These electron‐rich MOFs were utilized for detection of explosive nitroaromatic compounds (NACs) through fluorescence quenching with high selectivity and sensitivity. The rate of fluorescence quenching for all the MOFs follows the order of electron deficiency of the NACs. We also showed the detection of picric acid (PA) by luminescent MOFs is not always reliable and can be misleading. This attracts our attention to explore these MOFs for sensing picryl chloride (PC), which is as explosive as picric acid and used widely to prepare more stable explosives like 2,4,6‐trinitroaniline from PA. Moreover, the recyclability and sensitivity studies indicated that these MOFs can be reused several times with parts per billion (ppb) levels of sensitivity towards PC and 2,4,6‐trinitrotoluene (TNT).     

Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron‐Rich and ‐Deficient Pore Surface

A porous covalent triazine framework (CTF) consisting of both an electron‐deficient central triazine core and electron‐rich aromatic building blocks is reported. Taking advantage of the dual nature of the pore surface, bimodal functionality has been achieved. The electron deficiency in the central core has been utilized to address one of the pertinent problems in chemical industries, namely separation of benzene from its cyclic saturated congener, that is, cyclohexane. Also, by virtue of the electron‐rich aromatic rings with Lewis basic sites, aqueous‐phase chemical sensing of a nitroaromatic compound of highly explosive nature (2,4,6‐trinitrophenol; TNP) has been achieved. The present compound supersedes the performance of previously reported COFs in both the aspects. Notably, this reports the first example of pore‐surface engineering leading to bimodal functionality in CTFs.     

基于有机光电材料的痕量爆炸物识别

随着国际恐怖袭击事件的增多,痕量爆炸物的识别技术研究越来越为重要.本文以作者在该领域的研究为例,综述了基于一维有机纳米材料、荧光金属配位聚合物(金属-有机框架化合物)和聚噻咯的荧光猝灭技术,在痕量爆炸物识别上的应用.同时介绍了酞菁薄膜和苝亚酰胺纳米线制备的电子传感器分别对过氧化物和硝基类爆炸物有灵敏的响应.     

A Rapid and Efficient Way to Dynamic Creation of Cross‐Reactive Sensor Arrays Based on Ionic Liquids

Based on the simple counterion exchange of ionic liquids, a rapid, facile, and efficient strategy to create a cross‐reactive sensor array with a dynamic tunable feature was developed, and exemplified by the construction of a sensor array for the identification and classification of nitroaromatics and explosives mimics. To achieve a good sensing system with fast response, good sensitivity, and low detection limit, the synthesized ionic liquid receptors were tethered onto a silica matrix with a macro‐mesoporous hierarchical structure. Through the facile anion exchange approach, abundant ionic‐liquid‐based individual receptors with diversiform properties, such as different micro‐environments, diverse molecular interactions, and distinctive physico‐chemical properties, were easily and quickly synthesized to generate a distinct fingerprint of explosives for pattern recognition. The reversible anion exchange ability further endowed the sensor array with a dynamic tunable feature as well as good controllability and practicality for real‐world application. With the assistance of statistical analysis, such as principal component analysis (PCA) and linear discrimination analysis (LDA), an optimized‐size array with a good resolution was rationally established from a large number of IL‐based receptors. The performed experiments suggested that the ionic‐liquid‐based sensing protocol is a general and powerful strategy for creating a cross‐reactive sensor array that could find a wide range of applications for sensing various analytes or complex mixtures.     

Electrochemical Sensing of Explosives

This article reviews recent advances in electrochemical sensing and detection of explosive substances. Escalating threats of terrorist activities and growing environmental concerns have generated major demands for innovative field‐deployable tools for detecting explosives in a fast, sensitive, reliable and simple manner. Field detection of explosive substances requires that a powerful analytical performance be coupled to miniaturized low‐cost instrumentation. Electrochemical devices offer attractive opportunities for addressing the growing explosive sensing needs. The advantages of electrochemical systems include high sensitivity and selectivity, speed, a wide linear range, compatibility with modern microfabrication techniques, minimal space and power requirements, and low‐cost instrumentation. The inherent electroactivity of nitroaromatic, nitramine and nitroester compounds makes them ideal candidates for electrochemical detection. Recent activity in various laboratories has led to the development of disposable sensor strips, novel electrode materials, submersible remote sensors, and electrochemical detectors for microchip (‘Lab‐on‐Chip’) devices for on‐site electrochemical detection of explosive substances. The attractive behavior of these electrochemical monitoring systems makes them very promising for addressing major security and environmental problems.     

A Fluorescent Sensor for Highly Selective Detection of Nitroaromatic Explosives Based on a 2D,Extremely Stable,Metal–Organic Framework

A 2D, extremely stable, metal–organic framework (MOF), NENU‐503 , was successfully constructed. It displays highly selective and recyclable properties in detection of nitroaromatic explosives as a fluorescent sensor. This is the first MOF that can distinguish between nitroaromatic molecules with different numbers of ?NO₂ groups.     

Influence of Methyl Substituent Position on Redox Properties of Nitroaromatics Related to 2,4,6‐Trinitrotoluene

The environmental remediation of military installation sites is very important due to frequently large presence of carcinogenic derivatives of explosives in the ground and in ground waters. These nitroaromatic explosives and their derivatives are assessed by sensing devices. It is highly important to have insight on the reasons affecting the reduction potentials of these compounds. The redox properties of mono‐, di‐ and tri‐nitroaromatic compounds are studied with cyclic voltammetry at a glassy carbon electrode for comparison. We show that the presence of a methyl group in the aromatic system leads into more negative reduction potentials. The ease of nitro group reduction vary from meta>para>ortho positions relative to a methyl group. The redox properties were also studied at various pH ranging from 2 to 10. Acidic environments facilitated the reduction processes at lower potentials. These findings will have a profound influence upon understanding the processes during reductive decontaminations of the polluted sites as well as for construction of highly sensitive sensors for their determination.     

Nanoporous Carbon Materials for Electrochemical Sensing

Nanoporous carbon materials are highly important materials for a wide array of applications. Here we show that nanoporous carbon can act as highly active materials for electrochemical sensing. We observed that nanoporous carbon material exhibits a faster heterogeneous electron transfer than graphite and pure carbon nanotubes. Nanoporous carbon exhibits a superior electrochemical performance for sensing of important biomarkers such as dopamine, ascorbic acid, uric acid, NADH, DNA bases, and forensic‐related compounds such as nitroaromatic explosives.     

Atmospheric pressure ion/molecule reactions for the selective detection of nitroaromatic explosives using acetonitrile and air as reagents

Acetonitrile vapor and air are useful reagents for the selective detection of nitroaromatic compounds using atmospheric pressure ion/molecule reactions. Reagent ions CH2CN- and CN- generated from acetonitrile, and O-*, OH- and OOH- produced from the oxygen in air, react with vapor-phase and condensed-phase nitroaromatics in the course of atmospheric pressure chemical ionization (APCI) and desorption atmospheric pressure chemical ionization (DAPCI), respectively. The homogeneous and the heterogeneous phase reactions both lead to the formation of the same anionic adducts. These adducts have characteristic fragmentation patterns upon collisional activation, which makes these two reagents valuable for the selective detection of particular nitroaromatics, including explosives present as components of complex mixtures. Complementary information is available from the two reagents because their different chemistry facilitates analyte identification. DAPCI is demonstrated to be a useful ambient detection method for nitroaromatic explosives absorbed on surfaces.     

Design Rules for Nanogap‐Based Hydrogen Gas Sensors

Nanoscale gaps, which enable many research applications in fields such as chemical sensors, single‐electron transistors, and molecular switching devices, have been extensively investigated over the past decade and have witnessed the evolution of related technologies. Importantly, nanoscale gaps employed in hydrogen‐gas (H₂) sensors have been used to reversibly detect H₂ in an On–Off manner, and function as platforms for enhancing sensing performance. Herein, we review recent advances in nanogap design for H₂ sensors and deal with various strategies to create these gaps, including fracture generation by H₂ exposure, deposition onto prestructured patterns, island formation on a surface, artificial manipulation methods, methods using hybrid materials, and recent approaches using elastomeric substrates. Furthermore, this review discusses a new nanogap design that advances sensing capabilities in order to meet the diverse needs of academia and industry.     

Sensing of 2,4,6‐Trinitrotoluene (TNT) and 2,4‐Dinitrotoluene (2,4‐DNT) in the Solid State with Photoluminescent RuII and IrIII Complexes

A series of metal–organic chromophores containing Ru^II or Ir^III were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long‐lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution‐based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal‐to‐ligand charge‐transfer (MLCT) excited state quenching upon addition of TNT and 2,4‐dinitrotoluene (2,4‐DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink‐jet printing and solid‐state titrations was used for the solid‐state detection of TNT; achieving a limit‐of‐quantitation (LOQ) as low as 5.6 ng cm^?2. The combined effect of a long‐lived excited state and an energetically favorable driving force for the PET process makes the Ru^II and Ir^III MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high‐energy compounds.