Attogram sensing of trinitrotoluene with a self-assembled molecular gelator

Promoting Spontaneous Second Harmonic Generation through Organogelation

An organogelator based on the Disperse Red nonlinear optical chromophore was synthesized according to a simple and efficient three-step procedure. The supramolecular gel organization leads to xerogels which display a spontaneous second harmonic generation (SHG) response without any need for preprocessing, and this SHG activity appears to be stable over several months. These findings, based on an intrinsic structural approach, are supported by favorable intermolecular supramolecular interactions, which promote a locally non-centrosymmetric NLO-active organization. This is in sharp contrast with most materials designed for SHG purposes, which generally require the use of expensive or heavy-to-handle external techniques for managing the dipoles' alignment.

Functionality-Oriented Derivatization of Naphthalene Diimide: A Molecular Gel Strategy-Based Fluorescent Film for Aniline Vapor Detection

Modification of naphthalene diimide (NDI) resulted in a photochemically stable, fluorescent 3,4,5-tris(dodecyloxy)benzamide derivative of NDI (TDBNDI), and introduction of the long alkyl chains endowed the compound with good compatibility with commonly found organic solvents and in particular superior self-assembly in the solution state. Further studies revealed that TDBNDI forms gels with nine of the 18 solvents tested at a concentration of 2.0% (w/v), and the critical gelation concentrations of five of the eight gels are lower than 1.0% (w/v), indicating the high efficiency of the compound as a low-molecular mass gelator (LMMG). Transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy studies revealed the networked fibrillar structure of the TDBNDI/methylcyclohexane (MCH) gel. On the basis of these findings, a fluorescent film was developed via simple spin-coating of the TDBNDI/MCH gel on a glass substrate surface. Fluorescence behavior and sensing performance studies demonstrated that this film is photochemically stable, and sensitive and selective to the presence of aniline vapor. Notably, the response is instantaneous, and the sensing process is fully and quickly reversible. This case study demonstrates that derivatization of photochemically stable fluorophores into LMMGs is a good strategy for developing high-performance fluorescent sensing films.

Supercoiled fibres of self-sorted donor-acceptor stacks: a turn-off/turn-on platform for sensing volatile aromatic compounds

To ensure the comfortable survival of living organisms, detection of different life threatening volatile organic compounds (VOCs) such as biological metabolites and carcinogenic molecules is of prime importance. Herein, we report the use of supercoiled supramolecular polymeric fibres of self-sorted donor-acceptor molecules as "turn-off/turn-on" fluorescent sensors for the detection of carcinogenic VOCs. For this purpose, a C3-symmetrical donor molecule based on oligo(p-phenylenevinylene), C3OPV, and a perylene bisimide based acceptor molecule, C3PBI, have been synthesized. When these two molecules were mixed together in toluene, in contrast to the usual charge transfer (CT) stacking, supramolecular fibres of self-sorted stacks were formed at the molecular level, primarily driven by their distinct self-assembly pathways. However, CT interaction at the macroscopic level allows these fibres to bundle together to form supercoiled ropes. An interfacial photoinduced electron transfer (PET) process from the donor to the acceptor fibres leads to an initial fluorescence quenching, which could be modulated by exposure to strong donor or acceptor type VOCs to regenerate the respective fluorescence of the individual molecular stacks. Thus, strong donors could regenerate the green fluorescence of C3OPV stacks and strong acceptors could reactivate the red fluorescence of C3PBI stacks. These supercoiled supramolecular ropes of self-sorted donor-acceptor stacks provide a simple tool for the detection of donor- or acceptor-type VOCs of biological relevance, using a "turn-off/turn-on" fluorescence mechanism as demonstrated with o-toluidine, which has been reported as a lung cancer marker.

Picogram sensing of trinitrophenol in aqueous medium through a water stable nanoscale coordination polymer

A water stable nanoscale coordination polymer (CP) can detect trinitrophenol (TNP) in an aqueous medium at a record-picogram level (∼1.66 pg cm(-2)) with a detection limit of 1.66 ppb. This is a simple and low-cost method for the detection of TNP in aqueous media in contact mode, taking advantage of the unique structural arrangement of the as-synthesized CP and the associated photophysical properties.

A Comparative Account of the Kinetics of Light-Induced E-Z Isomerization of an Anthracene-Based Organogelator in Sol, Gel, Xerogel, and Powder States: Fiber to Crystal Transformation

The organogel of (E)-N'-(anthracene-10-ylmethylene)-3,4,5-tris(dodecyloxy)benzohydrazide (I) in methyl cyclohexane having a fibrillar network structure exhibits excellent fluorescence, which decreases sharply with time upon photoirradiation at λ = 365 nm. It has been attributed to the transformation of the E isomer of I to the Z isomer, and the kinetics of E-Z isomerization are compared for the sol, gel, xerogel, and powder states. The rate constants at different temperatures are measured from Avrami plots and its increase with an increase in temperature, indicating temperature acts as a promoter for photoirradiated E-Z isomeization along the imine (C═N) bond. In the powder form, the rate constant values are the lowest compared to those of other states for all temperatures and the xerogels exhibit the highest rate of E-Z isomerization. The rate constants of sol and gel states mostly lie between the two. The wide-angle X-ray scattering pattern changes after ultraviolet (UV) irradiation with the generation of new sharp peaks whose intensities increase with an increase in irradiation time. A polarized optical microscopic study indicates formation of small crystalline dots on the fibers in the gels, dendritic morphology on the xerogel fibers, and large needlelike morphology at the surface boundary of the solid. The dried I gel exhibits a melting peak at 96.7 °C, but upon irradiation, two peaks are observed at 98.5 and 152.7 °C; the latter has been attributed to the melting of crystals of Z isomers. Similar higher melting peaks are observed both for the xerogel and for powders after UV irradiation; the powders exhibit the highest meting peak at 159.4 °C. Possible reasons for the variation of rate constant values in the four different states and the difference in morphology and melting points of crystals of Z isomers of I are discussed.

Fluorescent Polystyrene Microbeads as Invisible Security Ink and Optical Vapor Sensor for 4-Nitrotoluene

Color-tunable solid-state emitting polystyrene (PS) microbeads were developed by dispersion polymerization, which showed excellent fluorescent security ink characteristics along with sensitive detection of vapors of nitro aromatics like 4-nitro toluene (4-NT). The fluorophores pyrene and perylenebisimide were incorporated into the PS backbone as acrylate monomer and acrylate cross-linker, respectively. Solid state quantum yields of 94 and 20% were observed for the pyrene and perylenebisimide, respectively, in the PS/Py and PS/PBI polymers. The morphology and solid state fluorescence was measured by SEM, fluorescence microscopy, and absorbance and fluorescence spectroscopy techniques. The ethanol dispersion of the polymer could be used directly as a fluorescent security "invisible" ink, which became visible only under ultraviolet light. The color of the ink could be tuned depending on the amounts of the pyrene and perylenebisimide incorporated with blue and orange-green for pyrene alone or perylenebisimide alone beads respectively and various shades in between including pure white for beads incorporating both the fluorophores. More than 80% quenching of pyrene emission was observed upon exposure of the polymer in the form of powder or as spin-coated films to the vapors of 4-NT while the emission of perylenebisimide was unaffected. The limit of detection was estimated at 10(-5) moles (2.7 ppm) of 4-NT vapors. The ease of synthesis of the material along with its invisible ink characteristics and nitro aromatic vapor detection opens up new opportunities for exploring the application of these PS-based materials as optical sensors and fluorescent ink for security purposes.

Our Expedition in Linear Neutral Platinum-Acetylide Complexes: The Preparation of Micro/nanostructure Materials, Complicated Topologies, and Dye-Sensitized Solar Cells

During the past few decades, the construction of various kinds of platinum-acetylide complexes has attracted considerable attention, because of their wide applications in photovoltaic cells, non-linear optics, and bio-imaging materials. Among these platinum-acetylide complexes, the linear neutral platinum-acetylide complexes, due to their attractive properties, such as well-defined linear geometry, synthetic accessibility, and intriguing photoproperties, have emerged as a rising star in this field. In this personal account, we will discuss how we entered the field of linear neutral platinum-acetylide chemistry and what we found in this field. The preparation of various types of linear neutral platinum-acetylide complexes and their applications in the areas of micro/nanostructure materials, complicated topologies, and dye-sensitized solar cells will be summarized in this account.

Anion triggered metallogels: demetalation and crystal growth inside the gel matrix and improvement in viscoelastic properties using Au-NPs

Progelator complex Zn-TRPA-2 undergoes Cl(-) triggered gelation to afford ZTP2G, while Zn-TRPA-2 capped Au-NPs under similar conditions gave another gel GNZTP2G which also represents a rare nano-composite metallogel. When Zn-TRPA-2 was triggered by Cl(-) and NO3(-) simultaneously, crystals of demetalated species NA-TRPA-2 grew inside the ZTP2G matrix. Interestingly, GNZTP2G exhibits superior viscoelastic properties over ZTP2G.

Photochemistry of the α-Al₂O₃-PETN Interface

Optical absorption measurements are combined with electronic structure calculations to explore photochemistry of an α-Al₂O₃-PETN interface formed by a nitroester (pentaerythritol tetranitrate, PETN, C₅H₈N₄O₁₂) and a wide band gap aluminum oxide (α-Al₂O₃) substrate. The first principles modeling is used to deconstruct and interpret the α-Al₂O₃-PETN absorption spectrum that has distinct peaks attributed to surface F⁰-centers and surface—PETN transitions. We predict the low energy α-Al₂O₃ F⁰-center—PETN transition, producing the excited triplet state, and α-Al₂O₃ F⁰-center—PETN charge transfer, generating the PETN anion radical. This implies that irradiation by commonly used lasers can easily initiate photodecomposition of both excited and charged PETN at the interface. The feasible mechanism of the photodecomposition is proposed.

MPTTF-containing tripeptide-based organogels: receptor for 2,4,6-trinitrophenol and multiple stimuli-responsive properties

A series of monopyrrolotetrathiafulvalene-tripeptide conjugates have been synthesized and investigated as new low-molecular mass organogelators. It was found that most of these compounds could immobilize low-polarity solvents readily and the gelation behaviors of these gelators showed a dependence on the amino acid residues. These organogels were thoroughly studied using various techniques including atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), circular dichroism (CD) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, (1)H NMR spectroscopy, UV-Vis absorption spectroscopy and X-ray diffraction (XRD). The results showed that the cooperative interplay of hydrogen bonding, π-π stacking and SS interactions were the main driving force for the formation of the gels. Of all the organogels, the aromatic solvent gels, such as toluene gel, exhibited multiple-stimulus responsiveness towards heating, shaking, chemical redox activity and the presence of anions, thus leading to reversible sol-gel phase transitions. Most interestingly, gelation in the presence of 2,4,6-trinitrophenol (TNP) in organic solvents could be observed visually with a concomitant color change through donor-acceptor interactions. The strength of the charge-transfer interaction between gelators and TNP was proportional to the incubation time and increasing critical gelation concentration (CGC). The gels could function as efficient absorbents for potential application in removal of crystal violet and rhodamine B dyes from water.

Fiber-optic detection of nitroaromatic explosives with solution-processable triazatruxene-based hyperbranched conjugated polymer nanoparticles

Solution-processable triazatruxene-based hyperbranched conjugated polymer nanoparticles have been developed as homogeneous, transparent fluorescent films coated on glass substrates and on optic-fiber tips for the detection of TNT vapor.

Detection of nitroaromatic explosives using π-electron rich luminescent polymeric nanocomposites

Novel electron rich luminescent nanocomposite polymers for sensing nitroaromatic explosives by PET and FRET phenomenon.

“Not quenched” aggregates of a triphenylene derivative for the sensitive detection of trinitrotoluene in aqueous medium

“Not quenched” porous aggregates of triphenylene derivative 4 have been utilized for the detection of TNT in solution, solid and vapour phases with detection limits of 22.7 attograms cm⁻².

Fluorescence tuning of Zn(ii)-based metallo-supramolecular coordination polymers and their application for picric acid detection

A series of metallo-supramolecular coordination polymers displayed strong and tunable visible luminescent emission and possessed the capability for detection of picric acid (PA) with high sensitivity and selectivity.

How paramagnetic and diamagnetic LMOCs detect picric acid from surface water and the intracellular environment: a combined experimental and DFT-D3 study

Diamagnetic and paramagnetic luminescent metal organic complexes (LMOCs) have been reported for explosive and pollutant nitro aromatic (epNAC) recognition.

Halogen-bonding for visual chloride ion sensing: a case study using supramolecular poly(aryl ether) dendritic organogel systems

A convenient and straightforward method for the visual recognition of chloride ion has been established through a chloride-responsive dendritic organogel.

Two bent-shaped π-organogelators: synthesis, fluorescence, self-assembly and detection of volatile acid vapours in gel films and in gel-gel states

Two novel bent-shaped π-organogelators 6a and 6b having different terminal pyridine rings as responsive sites were designed, synthesized and fully characterized. A subtle difference in the position of the N atom at the pyridine ring greatly affected their fluorescence and gelation properties. 6b showed remarkably stronger fluorescence both in solution and in the solid state as compared to 6a. Theoretical calculation revealed a clear discrepancy in the electron distribution between them. Furthermore, driven by π–π stacking interaction and hydrophobic interaction, both 6a and 6b can gelate several organic solvents with different polarities. Rheological studies, spectroscopic tests and powder X-ray diffraction showed that 6a displayed a closer stacking mode leading to stronger gel robustness. The xerogel films of 6a and 6b were prepared and utilized to detect acid vapours. Both of them can fulfil the detection of acid vapours through a distinct fluorescence change which could be seen by the naked eye under a UV lamp, but with different sensing modes. A rare gel to gel transformation was also observed upon exposure to acid vapours accompanied by a morphological change.

Conjugated Polymer Nanoparticles for the Amplified Detection of Nitro-explosive Picric Acid on Multiple Platforms

Spontaneously formed conjugated polymer nanoparticles (CPNs) or polymer dots displayed remarkable fluorescence response toward nitroexplosive-picric acid (PA) in multiple environments including 100% aqueous media, solid support using portable paper strips and vapor phase detection via two terminal device. This new cationic conjugated polyelectrolyte (CPE) poly(3,3'-((2-phenyl-9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(1-methyl-1H-imidazol-3-ium)bromide) (PFMI) was synthesized by Suzuki coupling polymerization followed by post functionalization method without employing any hectic purification technique. Highest quenching constant value (K(sv)) of 1.12 × 10(8) M(-1) and a very low detection limit of 30.9 pM/7.07 ppt were obtained exclusively for PA in 100% aqueous environment which is rare and unique for any CPE/CPNs. Contact mode detection of PA was also performed using simple, cost-effective and portable fluorescent paper strips for achieving on-site detection. Furthermore, the two terminal sensor device fabricated with nanoparticles of PFMI (PFMI-NPs) provides an exceptional and unprecedented platform for the vapor mode detection of PA under ambient conditions. The mechanism for the ultrasensitivity of PFMI-NPs probe to detect PA is attributed to the "molecular-wire effect", electrostatic interaction, photoinduced electron transfer (PET), and possible resonance energy transfer (RET).

A Suite of Tetraphenylethylene-Based Discrete Organoplatinum(II) Metallacycles: Controllable Structure and Stoichiometry, Aggregation-Induced Emission, and Nitroaromatics Sensing

Materials that organize multiple functionally active sites, especially those with aggregation-induced emission (AIE) properties, are of growing interest due to their widespread applications. Despite promising early architectures, the fabrication and preparation of multiple AIEgens, such as multiple tetraphenylethylene (multi-TPE) units, in a single entity remain a big challenge due to the tedious covalent synthetic procedures often accompanying such preparations. Coordination-driven self-assembly is an alternative synthetic methodology with the potential to deliver multi-TPE architectures with light-emitting characteristics. Herein, we report the preparation of a new family of discrete multi-TPE metallacycles in which two pendant phenyl rings of the TPE units remain unused as a structural element, representing novel AIE-active metal-organic materials based on supramolecular coordination complex platforms. These metallacycles possess relatively high molar absorption coefficients but weak fluorescent emission under dilute conditions because of the ability of the untethered phenyl rings to undergo torsional motion as a non-radiative decay pathway. Upon molecular aggregation, the multi-TPE metallacycles show AIE-activity with markedly enhanced quantum yields. Moreover, on account of their AIE characteristics in the condensed state and ability to interact with electron-deficient substrates, the photophysics of these metallacycles is sensitive to the presence of nitroaromatics, motivating their use as sensors. This work represents a unification of themes including molecular self-assembly, AIE, and fluorescence sensing and establishes structure-property-application relationships of multi-TPE scaffolds. The fundamental knowledge obtained from the current research facilitates progress in the field of metal-organic materials, metal-coordination-induced emission, and fluorescent sensing.

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.

Modifying a known gelator scaffold for nitrite detection

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

Contrasting Response of Two Dipolar Fluorescence Probes in a Leucine-Based Organogel and Its Implications

The microenvironments of a leucine-based organogel are probed by monitoring the fluorescence behavior of coumarin 153 (C153) and 4-aminophthalimide (AP). The steady-state data reveals distinctly different locations of the two molecules in the gel. Whereas AP resides close to the hydroxyl moieties of the gelator and engages in hydrogen-bonding interactions, C153 is found in bulk-toluene-like regions. In contrast to C153, AP exhibits excitation-wavelength-dependent emission, indicating that the environments of the hydrogen-bonded AP molecules are not all identical. A two-component fluorescence decay of AP in gel, unlike C153, supports this model. A time-resolved fluorescence anisotropy study of the rotational motion of the molecules also reveals the strong association of only AP with the gelator. That AP influences the critical gelation concentration implies its direct involvement in the gel-formation process. The results highlight the importance of guest-gelator interactions in gels containing guest molecules.

Design of Nanohybrid Systems from a Partially Fluorinated Organogelator and Syndiotactic Polystyrene Thermoreversible Gel

Nanohybrid systems are prepared from organogels of a partially fluorinated molecule and from thermoreversible gels of syndiotactic polystyrene. The thermodynamic behavior, morphology, and structure are investigated by using differential scanning calorimetry, atomic force microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The outcomes of these investigations suggest that the fibrils of the organogel coil around the sPS fibrils, probably through a heterogeneous nucleation process. These systems therefore differ from previously investigated sPS/OPV systems (oligo vinylene phenylene) where OPV fibrils pervade the sPS network.

Chemical sensing in two dimensional porous covalent organic nanosheets

Two new imide-based crystalline, porous, and chemically stable covalent organic frameworks (COFs) (TpBDH and TfpBDH) have been successfully synthesized employing solvothermal crystallization route. Furthermore, thin layered covalent organic nanosheets (CONs) were derived from these bulk COFs by the simple liquid phase exfoliation method. These 2D CONs showcase increased luminescence intensity compared to their bulk counterparts (COFs). Notably, TfpBDH-CONs showcase good selectivity and prominent, direct visual detection towards different nitroaromatic analytes over TpBDH-CONs. Quite interestingly, TfpBDH-CONs exhibit a superior "turn-on" detection capability for 2,4,6-trinitrophenol (TNP) in the solid state, but conversely, they also show a "turn-off" detection in the dispersion state. These findings describe a new approach towards developing an efficient, promising fluorescence chemosensor material for both visual and spectroscopic detection of nitroaromatic compounds with very low [10-5 (M)] analyte concentrations.

Synthesis of photoresponsive cholesterol-based azobenzene organogels: dependence on different spacer lengths

A series of azobenzene–cholesterol organogel compounds (M₀–M₁₂) with different spacers were designed and synthesized. The molecular structures were confirmed by ¹H NMR and ¹³C NMR spectroscopy. The rapid and reversible photoresponsive properties of the compounds were investigated by UV–vis spectroscopy. Their thermal phase behaviors were studied by DSC. The length of the spacer plays a crucial role in the gelation. Compound M₆ is the only one that can gelate in ethanol, isopropanol and 1-butanol and the reversible gel–sol transitions are also investigated. To obtain visual insight into the microstructure of the gels, the typical structures of the xerogels were studied by SEM. Morphologies of the aggregates change from flower-like, network and rod with different sizes. By using IR and XRD characterization, it is found that intermolecular H-bonding, the solvents and van der Waals interaction are the main contributions to the specific superstructure.

Fundamental Study of Electrospun Pyrene-Polyethersulfone Nanofibers Using Mixed Solvents for Sensitive and Selective Explosives Detection in Aqueous Solution

Fluorescent pyrene-polyethersulfone (Py-PES) nanofibers were prepared through electrospinning technique using mixed solvents. The effects of mixed solvent ratio and polymer/fluorophore concentrations on electrospun nanofiber's morphology and its sensing performance were systematically investigated and optimized. The Py-PES nanofibers prepared under optimized conditions were further applied for highly sensitive detection of explosives, such as picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) in aqueous phase with limits of detection (S/N = 3) of 23, 160, 400, and 980 nM, respectively. The Stern-Volmer (S-V) plot for Py excimer fluorescence quenching by PA shows two linear regions at low (0-1 μM) and high concentration range (>1 μM) with a quenching constant of 1.263 × 10(6) M(-1) and 5.08 × 10(4) M(-1), respectively. On the contrary, S-V plots for Py excimer fluorescence quenching by TNT, DNT, and RDX display an overall linearity in the entire tested concentration range. The fluorescence quenching by PA can be attributed to the fact that both photoinduced electron transfer and energy transfer are involved in the quenching process. In addition, pyrene monomer fluorescence is also quenched and exhibits different trends for different explosives. Fluorescence lifetime studies have revealed a dominant static quenching mechanism of the current fluorescent sensors for explosives in aqueous solution. Selectivity study demonstrates that common interferents have an insignificant effect on the emission intensity of the fluorescent nanofibers in aqueous phase, while reusability study indicates that the fluorescent nanofibers can be regenerated. Spiked real river water sample was also tested, and negligible matrix effect on explosives detection was observed. This research provides new insights into the development of fluorescent explosive sensor with high performance.