Attogram sensing of trinitrotoluene with a self-assembled molecular gelator

Self-assembly of a tetrapodal adamantane with carbazole branches into hollow spherical aggregates in organic media

A hydrophobic tetrapodal molecule is composed of carbazole units at the periphery linked by a phenyl spacer on an adamantane core. Tetrapodal adamantane self-assembles into hollow spherical aggregates with a multilayer membrane in organic media. The spherical assembly size is dependent on the organic solvent used. Hollow spheres can entrap guest molecules within their internal spaces. By increasing the concentrations of tetrapodal molecules, hollow spheres fused into necklace-shaped nanostructures and two-dimensional networks were obtained.

Multifunctional white-light-emitting metal-organic gels with a sensing ability of nitrobenzene

In this study, three novel luminescent nanofibrous metal-organic gels (MOGs) have been synthesized by the reaction of 1,3,5-tris(3-pyridylmethoxyl)benzene (L) with chloride salts of Cd(II), Hg(II), and Cu(II). The metal-ligand coordination, intermolecular π-π stacking and several other weak interactions found to play an important role in the formation of nanofibrous materials. The gel materials are characterized by rheology, diffuse reflectance spectra and various microscopic techniques such as TEM, FESEM, and AFM. The gels MOG-1 and MOG-2 were found to exhibit significant white photoluminescence, whereas the MOG-3 exhbits green emission upon excitation at 325 nm. Furthermore, the MOG-1 has shown its application as a chemosensor for the remarkable detection of nitroaromatics such as nitrobenzene (NB), 2,4-dinitrophenol (DNP). The significant quenching response for NB and DNP is attributed to the strong charge-transfer interactions between the electron-deficient aromatic ring of NB and the electron rich aromatic group of L in MOG-1. The crystal structure of Cd(II) complex of L reveals the formation one-dimensional network which contains strong π-π interactions within and between the networks and these strong π-π interactions generate the free charge carrier in all these nanofibrous gels.

Micelle-induced versatile sensing behavior of bispyrene-based fluorescent molecular sensor for picric acid and PYX explosives

The effect of surfactant micelles on the photophysical properties of a cationic bispyrene fluorophore, Py-diIM-Py, was systemically examined. The results from series of measurements including UV-vis absorption, steady-state fluorescence emission, quantum yield, fluorescence lifetime, and time-resolved emission spectra reveal that the cationic fluorophore is only encapsulated by the anionic sodium dodecyl sulfate (SDS) surfactant micelles and not incorporated in the cationic dodecyltrimethylammonium bromide (DTAB) and neutral Triton X-100 (TX100) surfactant micelles. This different fluorophore location in the micellar solutions significantly influences its sensing behavior to various explosives. Fluorescence quenching studies reveal that the simple variation of micellar systems leads to significant changes in the sensitivity and selectivity of the fluorescent sensor to explosives. The sensor exhibits an on-off response to multiple explosives with the highest sensitivity to picric acid (PA) in the anionic SDS micelles. In the cationic DTAB micelles, it displays the highest on-off responses to PYX. Both the sensitivity and selectivity to PYX in the cationic micelles are enhanced compared with that to PA in the anionic micelles. However, the poor encapsulation in the neutral surfactant TX100 micelles leads to fluorescence instability of the fluorophore and fails to function as a sensor system. Time-resolved fluorescence decays in the presence of explosives reveal that the quenching mechanism of two micellar sensor systems to explosives is static in nature. The present work demonstrates that the electrostatic interaction between the cationic fluorophore and differently charged micelles plays a determinative role in adjusting its distribution in micellar solutions, which further influences the sensing behavior of the obtained micellar sensor systems.

Multistimuli-responsive supramolecular gels: design rationale, recent advances, and perspectives

This manuscript presents a brief overview of recent advances in multistimuli-responsive supramolecular gels (MRSGs). The synthesis of MRSGs with faster and smarter responsive abilities to a variety of external stimuli, such as redox reagents, pH changes, ligands, and coupling reagents, is one key issue for the upgrade of current molecular motors, signal sensors, shape memory devices, drug delivery systems, display devices, and other devices. However, the design rules of MRSGs are still not well understood. The lack of information about the relationship between the spatial structure and gelation behavior of existing gelators means that the knowledge required to design new gelators by the addition of functional moieties to well-known gelators is lacking. Insights into the gelation pathway of known gelators may bring inspiration to researchers who want to exploit elegant designs and specific building blocks to obtain their own MRSGs with predictable stimuli-responsive abilities.

Femtogram level detection of nitrate ester explosives via an 8-pyrenyl-substituted fluorene dimer bridged by a 1,6-hexanyl unit

Compared with nitroaromatic explosives detection, nitrate esters are far from wide attention possibly because of their shortage of aromatic ring and difficulty in being detected. Three fluorescent chemical probes for trace nitrate ester detection: an intramolecular dimer (P3) of 8-pyrenyl-substituted fluorenes bridged by a 1,6-hexanyl unit as well as its counterparts 2PR-F (P1) and 2PR-Cz (P2) has been synthesized and characterized. Their chemical structures and photophysical and electrochemical properties show that the dimer P3 film has a higher molar extinction coefficient, larger steric hindrance, higher area-to-volume ratio, and matching energy level with nitrate ester explosives, which contributes to higher sensitivity and moderate selectivity for sensing of nitrate ester explosives such as nitroglycerin (NG). The fluorescence of the P3 film is rapidly about 90% quenched upon exposure to a saturated vapor of NG for 50 s and almost 100% quenched for 300 s at room temperature due to photoinduced electron transfer between the probe and analyte. In addition, a very sensitive, rapid, simple, and low-cost surface-sensing method by disposable filter-paper-based test strips is demonstrated. The contact-mode approach exhibits a detection limit as low as 0.5 fg/cm(2) for NG. These results reveal that the multiple-pyrenyl-unit-substituted fluorene dimer P3 is suitable for preparing a highly sensitive and efficient thin-film device for detecting nitrate esters.

Fluorescence ratiometric sensor for trace vapor detection of hydrogen peroxide

Trace vapor detection of hydrogen peroxide (H2O2) represents a practical approach to nondestructive detection of peroxide-based explosives, including liquid mixtures of H2O2 and fuels and energetic peroxide derivatives, such as triacetone triperoxide (TATP), diacetone diperoxide (DADP), and hexamethylene triperoxide diamine (HMTD). Development of a simple chemical sensor system that responds to H2O2 vapor with high reliability and sufficient sensitivity (reactivity) remains a challenge. We report a fluorescence ratiometric sensor molecule, diethyl 2,5-bis((((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)carbonyl)amino)terephthalate (DAT-B), for H2O2 that can be fabricated into an expedient, reliable, and sensitive sensor system suitable for trace vapor detection of H2O2. DAT-B is fluorescent in the blue region, with an emission maximum at 500 nm in the solid state. Upon reaction with H2O2, DAT-B is converted to an electronic "push-pull" structure, diethyl 2,5-diaminoterephthalate (DAT-N), which has an emission peak at a longer wavelength centered at 574 nm. Such H2O2-mediated oxidation of aryl boronates can be accelerated through the addition of an organic base such as tetrabutylammonium hydroxide (TBAH), resulting in a response time of less than 0.5 s under 1 ppm of H2O2 vapor. The strong overlap between the absorption band of DAT-N and the emission band of DAT-B enables efficient Förster resonance energy transfer (FRET), thus allowing further enhancement of the sensing efficiency of H2O2 vapor. The detection limit of a drop-cast DAT-B/TBAH film was projected to be 7.7 ppb. By combining high sensitivity and selectivity, the reported sensor system may find broad application in vapor detection of peroxide-based explosives and relevant chemical reagents through its fabrication into easy-to-use, cost-effective kits.

Redox-switchable copper(I) metallogel: a metal-organic material for selective and naked-eye sensing of picric acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper(II) chloride in aqueous medium. The chemistry involves the reduction of Cu(II) to Cu(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal-ligand complexation, i.e., Cu(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet-Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu(I) and chloride ions as the key ingredients in the metal-organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel-sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations.

Hexaphenylbenzene-based fluorescent aggregates for ratiometric detection of cyanide ions at nanomolar level: set-reset memorized sequential logic device

A hexaphenylbenzene-based receptor 3 has been synthesized that forms a fluorescent spherical aggregate in mixed aqueous media due to its aggregation-induced emission enhancement attributes. These fluorescent spherical aggregates show ratiometric response toward cyanide ions via nucleophilic addition and undergo deaggregation to form smaller nanoaggregates. In addition, the solution-coated paper strips of 3 can detect cyanide ions in the range of ∼2.6 ng/cm(2), thus, providing a simple, portable, and low-cost method for detection of cyanide ions in aqueous media. Receptor 3 also behaves as a set-reset memorized sequential logic circuit with chemical inputs of CN(-) ions and trifluoroacetic acid or H(+) (pH ≤ 3).

Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications

The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials which are respectively important for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the rapid development of nanotechnology in the past few years, nanomaterials-enhanced surface plasmon resonance sensors have been developed and used as effective tools to sense hard-to-detect molecules within the concentration range between pmol and amol. In this review article, we reviewed and discussed the latest trend and challenges in engineering and applications of nanomaterials-enhanced surface plasmon resonance sensors (e.g., metallic nanoparticles, magnetic nanoparticles, carbon-based nanomaterials, latex nanoparticles and liposome nanoparticles) for detecting "hard-to-identify" biological and chemical analytes. Such information will be viable in terms of providing a useful platform for designing future ultrasensitive plasmonic nanosensors.

Terthiophene derivatives of cholesterol-based molecular gels and their sensing applications

Three novel terthiophene derivatives of cholesterol (TtGC, TtLPC, TtDPC), of which the two building blocks are linked by a structure of glycine, l-phenylalanine, or d-phenylalanine, respectively, were designed and prepared, and their gelation behaviors in 26 liquids were tested. It was demonstrated that the compounds show different gelation abilities with the variation of the linker structures even though the variation is small. FTIR, (1)H NMR, and UV-vis measurements revealed that intermolecular hydrogen bonding and van der Waals interaction are the main driving forces for the gel formation. As for TtDPC, CD and AFM measurements revealed that it aggregated into chiral structures of left-helical feature in benzene. Importantly, the morphologies of the gel networks could be subtly adjusted via alteration of the gelator concentration. Considering the brightness in fluorescence and the unique micro/nanostructures of the gel networks, a fluorescent film (film 1) was fabricated by simple dip-coating of TtDPC/benzene solution (before gelation) onto a glass plate surface. Fluorescent studies demonstrated that the film is photochemically unstable. Two hours UV irradiation of the film results in film 2, which is almost fluorescent silence. However, the presence of HAc vapor or the vapors of some other volatile organic liquids induces new fluorescence emission, laying the foundation for creating a turn-on type fluorescent sensor of the organic vapors. Furthermore, as a new type of low-molecular-mass gelators (LMMGs), of which oligothiophene was employed as a building block, the present study has provided a possibility to explore the photo-/electronic applications of oligothiophenes via a molecular gel strategy.

Surface plasmon resonance enhanced upconversion luminescence in aqueous media for TNT selective detection

We present a novel report on a surface plasmon resonance enhanced upconversion luminescence strategy in aqueous media for highly sensitive and selective detection of 2,4,6-trinitrotoluene (TNT).

A metal–organic framework with a 9-phenylcarbazole moiety as a fluorescent tag for picric acid explosive detection: collaboration of electron transfer, hydrogen bonding and size matching

An electron-rich luminescent metal–organic framework, Zn–PDA, displaying high selectivity in detection of picric acid explosive as a fluorescent sensor is reported.

Response of strongly fluorescent carbazole-based benzoxazole derivatives to external force and acidic vapors

The nanofibers have enhanced emission and exhibited an isothermal reversible mechanochromism. Furthermore, acetic acid vapor may selectively act as stabilizer and developer to retain the information imparted by mechanical force.

ZnO–Ag hybrids for ultrasensitive detection of trinitrotoluene by surface-enhanced Raman spectroscopy

Low-cost SERS sensors were fabricated by 4-ATP-functionalized ZnO–Ag hybrid substrates for TNT detection with high sensitivity, selectivity and reproducibility.

A carbazole–fluorene molecular hybrid for quantitative detection of TNT using a combined fluorescence and quartz crystal microbalance method

Self-assembled fluorescent rods and nanoparticles prepared from a carbazole–fluorene molecular hybrid have been used for the sensing of TNT.

Synthesis of novel benzimidazole-carbazole-N-glycosylamines and their self-assembly into nanofibers

A new class of benzimidazole-carbazole-N-glycosylamines were synthesized and self-assembled through non-covalent interaction into mechanically and thermally stable organogels with nanofibrous morphology.

Fast detection of nitroaromatics using phosphonate pyrene motifs as dual chemosensors

A new class of dual fluorescent chemosensors for nitroaromatic compounds (NACs) based on phosphonated pyrene derivatives is reported, showing high selectivity towards trinitrotoluene (TNT).

Selective fluorescence sensing of polynitroaromatic explosives using triaminophenylbenzene scaffolds

C₃-Symmetric 1,3,5-tris(4′-aminophenyl)benzene has been employed as a selective fluorescence chemosensor for polynitroaromatic compounds.

Luminescent hybrid perovskite nanoparticles as a new platform for selective detection of 2,4,6-trinitrophenol

CH₃NH₃PbBr₃based luminescent perovskite nanoparticles have been used for the selective detection of an explosive, 2,4,6-trinitrophenol (picric acid) with high sensitivity in solution and vapour state.

Synthesis of gold nanoparticles and nanoclusters in a supramolecular gel and their applications in catalytic reduction of p-nitrophenol to p-aminophenol and Hg(ii) sensing

Seven gelator molecules giving supramolecular gels produced Au-nanoparticles and fluorescent, small Au-nanoclusters. Such Au-nanoparticle containing gels catalyzed the reduction of p-nitrophenol to p-aminophenol without NaBH₄. The fluorescent Au-nanoclusters acted as a Hg(ii) sensor.