Nanomaterials for luminescence detection of nitroaromatic explosives

Strategic Design and Functionalization of an Amine-Decorated Luminescent Metal Organic Framework for Selective Gas/Vapor Sorption and Nanomolar Sensing of 2,4,6-Trinitrophenol in Water

On the basis of the strategic design of a triazine-based dicarboxylate ligand with two primary amino groups and one secondary amino group, an amine-functionalized autofluorescent and polar three-dimensional metal organic framework (MOF) {[Cd(ATAIA)]·4H₂O} _n (1), where H₂ATAIA = 5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isophthalic acid, has been synthesized under two different solvothermal conditions and structurally characterized. Single-crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic polar space group Fdd2, where each ATAIA ligand acts as a linear linker to connect four Cd(II) centers, resulting in the formation of a three-dimensional framework with a repeat of a double helical metal chain. It has been further characterized by elemental analysis, UV-vis and Fourier transform infrared spectroscopy, and thermogravimetric analysis. Its bulk phase purity and stability in aqueous acid and base solutions are confirmed by powder X-ray diffraction. Both field emission scanning electron microscopy and high resolution transmission electron microscopy images of 1 reflect the formation of microflowers by self-assembly of nanopetals. With the dehydrated framework of 1, sorption studies of different gases (N₂, H₂, and CO₂) as well as polar and nonpolar solvents, such as water, benzene (Bz), and cyclohexane (Cy), have been performed. The CO₂ sorption isotherm depicts type I isotherm at 298 and 273 K and type IV isotherm at 195 K. Furthermore, with an uptake of 129.2 cm³ g^-1 (25.62 wt %) at 195 K, sorption of CO₂ is selective over N₂ (77 K) and H₂ (77 K) because of the strong adsorbate-adsorbent interaction as clearly evident from an isosteric heat of adsorption ( Q_st) at zero coverage of 37.5 kJ mol^-1, which is exceptionally higher than that of other functionalized MOFs. Using the ideal adsorption solution theory calculation for a CO₂/N₂ (15:85) mixture, selectivity values are found to be 54.08 (298 K) and 46.96 (273 K) at 100 kPa. For a major application, activated 1 has been utilized for selective and ultrafast detection of 2,4,6-trinitrophenol (TNP) in water with a limit of 0.94 nM (0.2 ppb), which supersedes any previous reported value. Excellent recyclability and stability of 1 for sensing experiments have been established. Time-resolved fluorescence studies and density functional theory calculations have been used to establish its mechanism of action. Furthermore, a prototype experiment for the real-time sensing of TNP in the vapor phase by fluorescence microscopy provides an easy colorimetric monitoring.

A dual spectroscopic fluorescence probe based on carbon dots for detection of 2,4,6-trinitrophenol/Fe (III) ion by fluorescence and frequency doubling scattering spectra and its analytical applications

A convenient, highly sensitive and reliable assay for 2,4,6‑trinitrophenol (TNP) and Fe (III) ion (Fe³⁺) in the dual spectroscopic manner is developed based on novel carbon dots (CDs). The CDs with highly blue emitting fluorescent were easily prepared via the one-step potassium hydroxide-assisted reflux method from dextrin. The as-synthesized CDs exhibited the high crystalline quality, the excellent fluorescence characteristics with a high quantum yield of ~13.1%, and the narrow size distribution with an average diameter of 6.3±0.5nm. Fluorescence and frequency doubling scattering (FDS) spectra of CDs show the unique changes in the presence of TNP/Fe³⁺ by different mechanism. The fluorescence of CDs decreased apparently in the presence of TNP via electron-transfer. Thus, after the experimental conditions were optimized, the linear range for detection TNP is 0-50μM, the detection limit was 19.1nM. With the addition of Fe³⁺, the FDS of CDs appeared to be highly sensitive with a quick response to Fe³⁺ as a result of the change concentration of the scattering particle. The emission peak for FDS at 450nm was enhanced under the excitation wavelength at 900nm. The fluorescence response changes linearly with Fe³⁺ concentration in the range of 8-40μM, the detection limits were determined to be 44.1nM. The applications of CDs were extended for the detection of TNP, Fe³⁺ in real water samples with a high recovery. The results reported here may become the potential tools for the fast response of TNP and Fe³⁺ in the analysis of environmental pollutants.

Luminescent metal–organic frameworks as chemical sensors: common pitfalls and proposed best practices

In this review we approach the emerging field of sensors based on luminescent metal–organic frameworks from the perspective of the most commonly encountered pitfalls and we suggest best practices so that they can be avoided.

Pt(II)C∧N∧N-Based Luminophore-Micelle Adducts for Sensing Nitroaromatic Explosives

Two luminescent cyclometalated Pt(II)-complexes, 1•Pt and 2•Pt, respectively, were synthesized by using unsymmetrical C^∧N^∧N ligands having different alkyl substituents. These π-electron-rich complexes are used for sensing various electron deficient nitroaromatic explosives, e.g., 4-nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol (TNP), in aqueous, nonaqueous, as well as in the solid state as a paper strip with maximum detection limit of ca. 10^-9 M. It was demonstrated that the sparingly soluble 2•Pt complex becomes water-soluble in the presence of all kinds of surfactants, viz., cationic (e.g., cetyltrimethylammonium bromide, CTAB), anionic (e.g., sodium dodecyl sulfate, SDS), and neutral (e.g., Triton X-100). This may be due to the incorporation of its long lyophilic tail group (-C₁₂H₂₅) inside the micellar core, exposing planar Pt(II)C^∧N^∧N headgroup to the aqueous bulk phase. It was also observed that the extent of solubility of these Pt(II)-complexes in micellar media strongly depends on the length of the existing alkyl chain. For instance, the presence of longer dodecyl chain makes 2•Pt complex ca. 1000-fold more soluble than the complex 1•Pt, which contains a shorter propyl chain. Their sensing behavior essentially arises by the quenching of Pt(II)-based intense luminescence due to the supramolecular charge transfer (CT) process originating from Pt(II)C^∧N^∧N-antenna to the electron deficient nitroaromatic explosives. Our present work shows that the micellar adducts formed by highly luminophoric material and surfactant molecules could effectively detect such explosives in aqueous medium with better sensitivity compared to what were observed in other media.

Highly Fluorescent Polyimide Covalent Organic Nanosheets as Sensing Probes for the Detection of 2,4,6-Trinitrophenol

A new fluorescent polyimide covalent organic framework (PI-COF) has been successfully synthesized through solvothermal route using tetra(4-aminophenyl) porphyrin and perylenetracarboxylic dianhydride, which possesses porous crystalline and excellent thermal stability (>500 °C). Furthermore, few-layered PI covalent organic nanosheets (PI-CONs) can be easily obtained from the fluorescent PI-COF through a facile liquid phase exfoliation approach, which were confirmed by atomic force microscopy and transmission electron microscopy analysis. It is interesting that the fluorescent intensity of PI-CONs is obviously enhanced relative to that of PI-COF. The PI-CONs have been successfully utilized as an efficient fluorescent probe for the highly sensitive and selective detection of 2,4,6-trinitrophenol (TNP). The mechanism might be attributed to the combination of electron transfer and inner filter effect based on DFT calculations and spectral overlap data. The system exhibits a good linear response toward TNP over the range from 0.5 to 10 μM with a detection limit of 0.25 μM.

Approach Based on Polyelectrolyte-Induced Nanoassemblies for Enhancing Sensitivity of Pyrenyl Probes

We have developed a unique approach for enhancing the sensitivity of pyrenyl probes based on polyelectrolyte-induced nanoassemblies and explored its sensing application toward 2,4,6-trinitrophenol (TNP). The key issue of the method is the formation of the nanoassemblies which possess high-density charges, specific surface area, and inner hydrophobic regions. These properties would help increase the loading of analytes and promote probe-analyte interactions, thereby leading to the prominent enhancement of the sensitivity. In the course of TNP detection, pyrene nanoassemblies can bind TNP efficiently through cooperative noncovalent interactions including electrostatic, π-π stacking, and charge-transfer interactions, resulting in the distinct fluorescent responses of pyrene moieties. This system has excellent selectivity and sensitivity for TNP detection. The detection limit is as low as 5 nM. It may be used for monitoring the TNP concentrations in real-world samples.

DNA assembled metal nanoclusters: synthesis to novel applications

In this review, we have discussed the emergence of promising environmental-benign DNA assembled fluorescent metal nanoclusters and their unique electronic structures, unusual physical and chemical properties.

Signal enhancement of sensing nitroaromatics based on highly sensitive polymer dots

A new, rapid, sensitive, selective and portable fluorescence detection method for nitroaromatics based on polymer dots (Pdots) had been successfully developed not only in aqueous media but also in the solid state with test strips. The fluorescence quenching rates were proportional to the concentrations of 2,4,6-trinitrophenol (TNP) in the range of 0.2-20.0 μg mL(-1) and p-nitrophenol (PNP) in the range of 0.05-6.0 μg mL(-1), when Pdots were used as ratiometric fluorescent sensors in aqueous solution. The 3σ limit of detection of PNP reached 18.8 ng mL(-1). Compared with polymer-based detection for nitroaromatics in the organic phase, the signal enhancement effect was initially found when Pdots were used to detect nitroaromatics in the aqueous phase. The mechanism of the interaction between Pdots and nitroaromatics was revealed as an electron transfer phenomenon from the electron-rich chromophoric probe to the electron deficient nitroaromatics. The results indicated that Pdots-based detection was particularly suitable for on-site qualitative detection and quantitative analysis of nitroaromatics.

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.

Peptide Functionalized Nanoplasmonic Sensor for Explosive Detection

In this study, a nanobiosensor for detecting explosives was developed, in which the peptide was synthesized with trinitrotoluene (TNT)-specific sequence and immobilized on nanodevice by Au-S covalent linkage, and the nanocup arrays were fabricated by nanoimprint and deposited with Au nanoparticles to generate localized surface plasmon resonance (LSPR). The device was used to monitor slight change from specific binding of 2,4,6-TNT to the peptide. With high refractive index sensing of ~104 nm/RIU, the nanocup device can detect the binding of TNT at concentration as low as 3.12 × 10-7 mg mL-1 by optical transmission spectrum modulated by LSPR. The nanosensor is also able to distinguish TNT from analogs of 2,4-dinitrotoluene and 3-nitrotoluene in the mixture with great selectivity. The peptide-based nanosensor provides novel approaches to design versatile biosensor assays by LSPR for chemical molecules.

A turn-on near-infrared fluorescent chemosensor for selective detection of lead ions based on a fluorophore-gold nanoparticle assembly

A turn-on fluorescent chemosensor of Pb(2+) in the near-infrared (NIR) region, which is based on the Pb(2+)-tuned restored fluorescence of a weakly fluorescent fluorophore-gold nanoparticle (AuNPs) assembly, has been reported. In this fluorophore-AuNP assembly, NIR fluorescent dye brilliant cresyl blue (BCB) molecules act as fluorophores and are used for signal transduction of fluorescence, while AuNPs act as quenchers to quench the nearby fluorescent BCB molecules via electron transfer. In the presence of Pb(2+), fluorescent BCB molecules detached from AuNPs and restored their fluorescence due to the formation of a chelating complex between Pb(2+) and glutathione confined on AuNPs. Under the optimal conditions, the present BCB-AuNP assembly is capable of detecting Pb(2+) with a concentration ranging from 7.5 × 10(-10) to 1 × 10(-8) mol L(-1) (0.16-2.1 ng mL(-1)) and a detection limit of 0.51 nM (0.11 ng mL(-1)). The present BCB-AuNP assembly can be used in aqueous media for the determination of Pb(2+) unlike common organic fluorescent reagents, and also shows advantages of NIR fluorescence spectrophotometry such as less interference, lower detection limit, and higher sensitivity. Moreover, the present method was successfully applied for the detection of Pb(2+) in water samples with satisfactory results.

Oligomer-coated carbon nanotube chemiresistive sensors for selective detection of nitroaromatic explosives

High-performance chemiresistive sensors were made using a porous thin film of single-walled carbon nanotubes (CNTs) coated with a carbazolylethynylene (Tg-Car) oligomer for trace vapor detection of nitroaromatic explosives. The sensors detect low concentrations of 4-nitrotoluene (NT), 2,4,6-trinitrotoluene (TNT), and 2,4-dinitrotoluene (DNT) vapors at ppb to ppt levels. The sensors also show high selectivity to NT from other common organic reagents at significantly higher vapor concentrations. Furthermore, by using Tg-Car/CNT sensors and uncoated CNT sensors in parallel, differential sensing of NT, TNT, and DNT vapors was achieved. This work provides a methodology to create selective CNT-based sensors and sensor arrays.

Electron-rich π-extended phthalocyanine–thiophene–phthalocyanine triad for the sensitive and selective detection of picric acid

An electron-rich phthalocyanine–thiophene–phthalocyanine triad 3 (Pc-triad 3), newly synthesized, can function as a highly selective chemosensor against picric acid (PA).