Investigation of common fluorophores for the detection of nitrated explosives by fluorescence quenching

Design and Synthesis of an Efficient Fluorescent Probe Based on Oxacalix[4]arene for the Selective Detection of Trinitrophenol (TNP) Explosives in Aqueous System

We present the synthesis of a new oxacalix[4]arene system, DMANSOC, wherein two 5-(dimethylamino)-1-naphthalene sulfonamide subunits are attached to the lower rims of the basic oxacalix[4]arene platform. Extensive spectrophotometric studies were conducted to investigate the selectivity and sensitivity of DMANSOC towards nitroaromatic explosives. Detailed analysis of spectrophotometric data, utilizing techniques such as Stern-Volmer, Benesi-Hildebrand, Job's plot, and interference study, unequivocally demonstrated the effectiveness of DMANSOC as a highly efficient fluorescent sensor for 2,4,6-trinitrophenol explosive (TNP) detection in an aqueous medium. The sensor exhibited a linear concentration range of 7.5 μM to 50 μM, with a low detection limit of 4.64 μM and a high binding affinity of 2.45 × 104 M towards TNP. Furthermore, the efficiency of the sensor in environmental samples contaminated with TNP was evaluated, yielding excellent recovery rates. Complementary DFT calculations and molecular dynamics simulations were performed to elucidate the mechanism behind the selective fluorescence quenching of DMANSOC in the presence of TNP.

Interactions of ferulic acid and ferulic acid methyl ester with endogenous proteins: Determination using the multi-methods

Ferulic acid (FA) and ferulic acid methyl ester (FAM) are important phenolic compounds in Baijiu. In this study, the interaction of FA and FAM with human serum albumin (HSA) and lysozyme (LZM) was investigated using multispectral methods and molecular dynamics simulation. FA and FAM could interact with HSA and LZM, changing the conformation and hydrophilicity of the protein. The quenching mechanisms of FA-HSA, FA-LZM, FAM-HSA, and FAM-LZM were all static-quenching. In the FA-HSA, FAM-HSA, and FA-LZM systems, the interaction forces were mainly hydrophobic interactions and hydrogen bonding. In the FAM-LZM system, the interaction forces were mainly hydrophobic interactions, hydrogen bonding, and van der Waals force. Common metal ions such as K+, Ca2+, Cu2+, Mg2+, and Mn2+ could affect the binding ability of FA and FAM to HSA and LZM. Moreover, FA and FAM could increase the stability of HSA and LZM, and the protein bound to FA/FAM was more stable than the free protein. FA and FAM had varying degrees of impact on the physiological activities of HSA and LZM. This study provides relevant information on the interactions and metabolic mechanisms of FA and its derivatives with endogenous proteins.

Further Develop 1,3,4-Thiadiazole Based Probe to Effectively Detect 2,4,6-Trinitrophenol with the Help of DFT Calculations

Four fluorimetric probes had been developed to rapidly detect 2,4,6-trinitrophenol (TNP). They were designed and synthesized on the basis of 1,3,4-thiadiazole framework combining calculation with experiment. Among them, SK-1 displayed strong blue emission with fluorescence quantum yield as high as 63.6% in solution. Further evaluation demonstrated that SK-1 displays good selectivity and high sensitivity for rapid and visual detection of TNP. It brought significant changes in both colour and fluorescence emission spectrum. The detection limit was as low as 38 nM. Quenching mechanism was confirmed as photo-induced electron transfer (PET) by nuclear magnetic titration and DFT calculations. What's more, application in real water samples and solid phase paper tests illustrated the practical significance of detection of TNP in both vapor and solution.

Accurately selected 1,3,4-thiadiazole and coumarin unit to construct fluorescent probes that effectively detect 2,4,6-trinitrophenol

Two remarkablely fluorimetric probes were developed to rapidly detect 2,4,6-trinitrophenol (TNP). With the help of density functional theory (DFT) calculations, we confirmed that using 1,3,4-thiadiazole skeleton as recognition group and coumarin unit as fluorophore would show excellent application prospects in terms of TNP detection. The probes LK-1 and LK-2 displayed green and orange emission with fluorescence quenching yield as high as 83.7% and 75.1% in solution. Further evaluation demonstrated that they display outstanding selectivity and sensitivity for rapid and visual detection of TNP. Both fluorescent color and fluorescence emission spectrum had significant changes and these phenomena could easily observe via naked-eye and analytical instrument. The detection limits of them were 97 nM and 71 nM. What's more, application in real water samples and solid phase paper tests illustrated the practical significance of detection of TNP.

Spherical silver oxide nanoparticles for fabrication of electrochemical sensor for efficient 4-Nitrotoluene detection and assessment of their antimicrobial activity

This research article reports an economic and affordable microwave-assisted synthesis of spherical silver oxide nanoparticles (Ag₂O NPs) (80-90 nm) for an efficient electrochemical sensing of a hazardous organic pollutant 4-nitrotoluene (4-NT). Such well-characterized Ag₂O NPs were utilized to modify gold (Au) electrode in order to fabricate Ag₂O-NPs/Au sensor to detect 4-NT using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques. Ag₂O-NPs/Au sensor exhibited a distinguished electrical response as a function of varying 4-NT concentration in neutral medium samples. Ag₂O-NPs/Au sensor demonstrated an ultrahigh sensitivity as 15.33 μA (μM)^-1 cm^-2, a low detection limit of 62.3 nM, and linear response in detection ranges of 0.6-5.9 μM and 37-175 μM. The sensing performance of fabricated Ag₂O-NPs/Au sensor is reproducible, reusable, selective in presence of other chemical species, and validated using real samples. The Ag₂O/Au sensor had much rapid and easy fabrication process and offered much lower LOD for 4-NT detection than many previously reported sensors. Along with efficient electrochemical activity, the spherical Ag₂O NPs also exhibit remarkable antimicrobial activity against harmful gram negative Escherichia coli (E. coli) and gram positive Staphylococcus aureus (S. aureus) bacteria. Herein projected efficient Ag₂O-NPs/Au electrochemical sensor for 4-NT is affordable with the capability of scaling up to perform point-of-care 4-NT testing needed for successful environmental monitoring and water quality assurance.

Sensitive vapor detection with hollow thin film arrays

In this manuscript, we explored the performance of a hollow thin film array (HTFA) for the detection of HCl vapor based on fluorescence quenching. The HTFA structure was fabricated by manually stacking layers of an active thin film and a supporting film, alternately, with a hollow structure in each supporting film. The total penetration depth of vapor molecules in the HTFA sample is 2n times increased, where n is the layer number of the active thin film. We tested the sensing performance of the HTFA sample using fluorescence emission and laser emission in a Fabry-Pérot (FP) microcavity. In the fluorescence sensing, the sensing efficiency increases with the vapor concentration, and can be as high as 80% with a vapor concentration of 400 ppm. While in the laser sensing, the efficiency can achieve 100% with an external pump intensity three times of the lasing threshold at a vapor concentration of 85 ppm. The HTFA sample is not only suitable for vapor detection but also suitable for molecule detection in liquid.

Hydrazone connected stable luminescent covalent-organic polymer for ultrafast detection of nitro-explosives

Developing promising luminescent probes for the selective sensing of nitro-explosives remains a challenging issue. Porous luminescent covalent–organic polymers are one of the excellent sensing probes for trace hazardous materials. Herein, fluorescent monomers 1,1,2,2-tetrakis(4-formyl-(1,1′-biphenyl))ethane (TFBE) and 1,3,5-benzenetricarboxylic acid trihydrazide (BTCH) were selected to build a novel hydrazone connected stable luminescent covalent–organic polymer (H-COP) of high stability by typical Schiff-base reaction. The N₂ sorption study, BET surface area analysis, and TGA profile indicate the porosity and stability of this H-COP material. Such properties of the H-COP material enable a unique sensing platform for nitro-explosives with great sensitivity (Ksv ∼ 10⁶ M) and selectivity up to μM. This polymer material shows attractive selectivity and sensitivity towards phenolic nitro-explosives and other common explosives among earlier reported COP-based sensors.

A novel H-COP was synthesized through Schiff-base condensation reaction, which shows high sensitivity (K_sv ∼ 10⁶ M⁻¹) and selectivity (μM level) towards nitro-explosives.

Recent Advancements for the Recognization of Nitroaromatic Explosives Using Calixarene Based Fluorescent Probes

In this era, explosives are easily available compared to the early days. Thus, more effective detection of explosives has become the main concern of homeland security. In the past decades, a large number of sensing materials have been developed for the detection of explosives in solid, vapor, and solution states through fluorescence methods. In recent years, great efforts have been devoted to developing new fluorescent materials with various sensing mechanisms for detecting explosives in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time. Modified calixarenes have high potentials to detect nitroaromatic compounds (NACs) due to their favorable structural properties. It summarizes the detection of NACs by the modified calixarene system formed by the complex. Various methodologies responsible for complex formation and binding mechanisms (PET, FRET, EE, etc.) are the centerpiece of this review. Finally, conclusions and future outlook are presented and 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.

Side-Chain Type Polysulfates: Their Synthesis, AIE Properties and Applications for p-Nitrophenol Detection in Water

Two small molecular monomers, ph-TPE and ph-TPE-CN, and their homopolymers Poly (ph-TPE) and Poly (ph-TPE-CN) containing tetra phenylethylene and sulfate structures, were synthesized by a sulfur (VI) fluorine exchange click reaction (SuFEx) and radical polymerization. All the monomers and polymers exhibit a typical aggregation-induced emission (AIE) effect both in the solid state and aggregated state. Moreover, based on the intermolecular charge transfer (ICT) effect between the tetra phenylethylene chromophore and p-nitrophenol, both polymers could be used for the selective detection of p-nitrophenol. The detection limit and reactivity coefficient of Poly (ph-TPE) are 0.081 μM and 5.15×10⁴  M^-1 , respectively, whereas the detection limit and reactivity coefficient of Poly (ph-TPE-CN) are 0.077 μM and 1.81×10⁴  M^-1 , respectively. This can be attributed to the greater sensitivity of Poly (ph-TPE-CN) to p-nitrophenol than that of Poly (ph-TPE). This work provides a new methodology for the preparation and broadening application of side-chain type AIE-active polysulfate fluorescent probes.

Ultrastable cucurbit[6]uril-based multifunctional supramolecular assembly for efficient detection of nitroaromatic compounds and antibiotics

A cucurbit[6]uril-based supramolecular assembly was used for rapid and sensitive detection of nitroaromatic compounds and antibiotics through luminescence quenching.

Four imidazole derivative AIEE luminophores: sensitive detection of NAC explosives

Four imidazole sensors with aggregation-induced emission enhancement (AIEE) properties were used for the sensitive detection of NAC explosives.

A perylene monoimide probe based fluorescent micelle sensor for the selective and sensitive detection of picric acid

A hydroxyl functionalized perylene monoimide probe (PMI-OH) was prepared and self-assembled with the nonionic surfactant Triton X-100 (TX100) to fabricate a fluorescent micelle sensor for the selective and sensitive detection of picric acid (PA), a common explosive and environmental pollutant. The synthesized PMI-OH probe exhibited excimer fluorescence emission, and the intensity of the excimer fluorescence emission was significantly enhanced after the PMI-OH probe formed micelles with TX100. The obtained PMI-OH@TX100 micelles presented excellent photoluminescence properties and had a maximum fluorescence emission at 630 nm. The red fluorescence of the PMI-OH@TX100 micelles was quenched upon introduction of the nitro explosive PA due to electron transfer from the donor (PMI-OH) to the acceptor (PA). The fluorescence quenching of the fluorescent micelle sensor was proportional to the concentration of PA in the range of 2 to 10 μM. The limit of detection was 500 nM using 3σ/k. Thus, the developed PMI-OH@TX100 micelle sensor has great potential to detect PA in ordinary samples.

p-Pyridine BODIPY-based fluorescence probe for highly sensitive and selective detection of picric acid

A novel fluorescence probe p-PBP for PA was synthesized based on a basic N atom as the electronic donor. The probe could detect PA over TNT, CE, PETN, RDX, HMX, NB, NT, DNT, NP, DNP, and common inorganic explosive ions (K⁺, Ba²⁺, NH₄⁺, NO₃^-, ClO₃^-, and ClO₄^-), and common ions (Na⁺, Ca²⁺, and Mg²⁺) with high selectivity. The fluorescence quenching was attributed to the photo-induced electron transfer (PET) processes from the excited state of p-PBP to the ground state PA. The detection limit of probe p-PBP for PA was as low as 13.06 nmol/L, which is far lower than the concentration stipulated by the Environmental quality standards for surface water. The response time was less than 30 s. Hence, the fluorescence probe p-PBP was successfully developed to detect the concentration level of PA in real samples, which would provide a novel quantitative analysis method of PA in forensic science.

Electron-Rich π-Extended Diindolotriazatruxene-Based Chemosensors with Highly Selective and Rapid Responses to Nitroaromatic Explosives

A series of electron-rich π-extended diindolotriazatruxene-based compounds DIT, 4Py-DIT (bearing pyrene units) and 4PyF-DIT (bearing fluorene units) have been explored and investigated as fluorescence chemosensors. Quantitative analysis through fluorescence titrations showed that the resulting DIT molecules exhibited highly selective response to electron-deficient nitroaromatic explosives. The calculated Stern-Volmer quenching constants (>4.0×10³  M^-1 ) revealed that these sensors were much more sensitive in solution compared to most of the existing small-molecule fluorescence chemosensors based on pyrene, triphenylene, triphenylamine, and triazatruxene skeletons. Fluorescence quenching showed that the sensors adsorbed on paper were sensitive to explosives in the solid, solution, and vapor phases, with fast response times of about 10 s. Moreover, these chemosensors are reusable for the detection of nitroaromatic compounds as they recover their fluorescence intensity after quenching.

Three AIE-ligand-based Cu(i) coordination polymers: synthesis, structures and luminescence sensing of TNP

Cu(i) CPs were constructed for the first time with the AIE ligand DPMF; they exhibited distinct architectures and efficient detection of TNP.

A down converting serine-functionalised NaYF4:Ce3+/Gd3+/Eu3+@NaGdF4:Tb3+ photoluminescent probe for chemical sensing of explosive nitroaromatic compounds

In this contribution, we explored a novel serine-functionalised NaYF₄:Ce³⁺/Gd³⁺/Eu³⁺@NaGdF₄:Tb³⁺ core–shell nanophosphor as a down-converting photoluminescent probe for efficient sensing of nitroaromatic explosives.

Stepwise elucidation of fluorescence based sensing mechanisms considering picric acid as a model analyte

The precise study of fluorescence-based sensing mechanisms and a step-by-step design experiment for the elucidation of the mechanism of sensing for newly designed sensing systems can be ascertained using the presented tutorial review.

Kinetic Understanding of the Ultrahigh Ionization Efficiencies (up to 28%) of Excited-State CH2Cl2-Induced Associative Ionization: A Case Study with Nitro Compounds

Excited-state CH₂Cl₂-induced associative ionization (AI) is a newly developed ionization method that is very effective for oxygenated organics. However, this method is not widely known. In this study, an unprecedented ionization efficiency and ultrafast reaction rate of AI toward nitro compounds were observed. The ionization efficiencies of o-nitrotoluene (o-NT), m-nitrotoluene (m-NT), and nitrobenzene (NB) were as high as (28 ± 3)%, (27 ± 2)%, and (13 ± 1)%, respectively (∼1-3 ions for every 10 molecules). The measured reaction rate coefficients of these nitroaromatics were (0.5-1.3) × 10^-7 molecule^-1 cm³ s^-1 (∼300 K). These unusual rate coefficients indicated strong long-range interactions between the two neutral reactants, which was regarded as a key factor leading to the ultrahigh ionization efficiency. The detection sensitivities of the nitroaromatics, (1.01-2.16) × 10⁴ counts pptv^-1 in 10 s acquisition time, were obtained by an AI time-of-flight mass spectrometer (AI-TOFMS). These experimental results not only provide new insight into the AI reaction but also reveal an excellent ionization method that can improve the detection sensitivity of nitroaromatics to an unprecedented degree.

β-Octaalkoxyporphyrins: Versatile fluorometric sensors towards nitrated explosives

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

Aggregation-induced emission (AIE)-active polymers for explosive detection

This review is to summarize the latest progress on aggregation-induced-emission (AIE)-active polymers for explosive detection.

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.

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.

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.

Reliability of Aerosol Jet Printed Fluorescence Quenching Sensor Arrays for the Identification and Quantification of Explosive Vapors

One of the primary challenges in explosive detection using fluorescence quenching is the identification and quantification of detected targets. In this work, we explore the reliability of aerosol jet printed sensor arrays for the discrimination of nitroaromatic traces using linear discriminant analysis (LDA). We varied the amount of the deposited material by controlling the printer's shutter to investigate the impact on the detection reliability. For a twofold variation of the amount of the deposited material, we report excellent classification rates between 81 and 96% for the discrimination of nitrobenzene, 1,3-dinitrobenzene, and 2,4-dinitrotoluene at 1, 3, and 10 parts per billion in air, respectively. Our results close to the detection limits indicate a remarkable identification and quantification of explosive trace vapors because of high control of the printing process. This work demonstrates the high potential of digitally printed fluorescence quenching sensor arrays and the excellent capabilities of LDA as a simple supervised statistical learning technique.

Direct Detection of Dilute Solid Chemicals with Responsive Lateral Organic Diodes

Organic field-effect transistors (OFETs) have emerged as promising sensors targeting chemical analytes in vapors and liquids. However, the direct detection of solid chemicals by OFETs has not been achieved. Here for the first time, we describe the direct detection of solid chemical analytes by organic electronics. An organic diode structure based on a horizontal side-by-side p-n junction was adopted and shown to be superior to OFETs for this purpose. The diodes showed more than 40% current decrease upon exposure to 1 ppm melamine powders. The estimated detection limit to melamine can potentially reach the ppb range. This is the first demonstration of an electronic signal from an interaction between a solid and an organic p-n junction directly, which suggests that our lateral organic diodes are excellent platforms for the development of future sensors when direct detection of solid chemicals is needed. The approach developed here is general and can be extended to chemical sensors targeting various analytes, opening unprecedented opportunities for the development of low-cost and high-performance solid chemical sensors.

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!

Highly sensitive amperometric biosensor based on electrochemically-reduced graphene oxide-chitosan/hemoglobin nanocomposite for nitromethane determination

Nitromethane (CH3NO2) is an important organic chemical raw material with a wide variety of applications as well as one of the most common pollutants. Therefore it is pretty important to establish a simple and sensitive detection method for CH3NO2. In our study, a novel amperometric biosensor for nitromethane (CH3NO2) based on immobilization of electrochemically-reduced graphene oxide (rGO), chitosan (CS) and hemoglobin (Hb) on a glassy carbon electrode (GCE) was constructed. Scanning electron microscopy, infrared spectroscopy and electrochemical methods were used to characterize the Hb-CS/rGO-CS composite film. The effects of scan rate and pH of phosphate buffer on the biosensor have been studied in detail and optimized. Due to the graphene and chitosan nanocomposite, the developed biosensor demonstrating direct electrochemistry with faster electron-transfer rate (6.48s(-1)) and excellent catalytic activity towards CH3NO2. Under optimal conditions, the proposed biosensor exhibited fast amperometric response (<5s) to CH3NO2 with a wide linear range of 5 μM~1.46 mM (R=0.999) and a low detection limit of 1.5 μM (S/N=3). In addition, the biosensor had high selectivity, reproducibility and stability, providing the possibility for monitoring CH3NO2 in complex real samples.