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

Light and matter co-confined multi-photon lithography

Mask-free multi-photon lithography enables the fabrication of arbitrary nanostructures low cost and more accessible than conventional lithography. A major challenge for multi-photon lithography is to achieve ultra-high precision and desirable lateral resolution due to the inevitable optical diffraction barrier and proximity effect. Here, we show a strategy, light and matter co-confined multi-photon lithography, to overcome the issues via combining photo-inhibition and chemical quenchers. We deeply explore the quenching mechanism and photoinhibition mechanism for light and matter co-confined multiphoton lithography. Besides, mathematical modeling helps us better understand that the synergy of quencher and photo-inhibition can gain a narrowest distribution of free radicals. By using light and matter co-confined multiphoton lithography, we gain a 30 nm critical dimension and 100 nm lateral resolution, which further decrease the gap with conventional lithography.

Polymers and Polymer-Based Materials for the Detection of (Nitro-)explosives

Methods for the remote detection of warfare agents and explosives have been in high demand in recent times. Among the several detection methods, fluorescence methods appear to be more convenient due to their low cost, simple operation, fast response time, and naked-eye-visible sensory response. For fluorescence methods, a large variety of fluorescent materials, such as small-molecule-based fluorophores, aggregation-induced emission fluorophores/materials, and supramolecular systems, have been reported in the literature. Among them, fluorescent (bio)polymers/(bio)polymer-based materials have gained wide attention due to their excellent mechanical properties and sensory performance, their ability to recognize explosives via different sensing mechanisms and their combinations, and, finally, the so-called amplification of the sensory response. This review provides the most up-to-date data on the utilization of polymers and polymer-based materials for the detection of nitroaromatic compounds (NACs)/nitro-explosives (NEs) in the last decade. The literature data have been arranged depending on the polymer type and/or sensory mechanism.

Optical sensors based on electrospun membranes – principles, applications, and prospects for chemistry and biology

Electrospun membranes are promising substrates for receptor layer immobilization in optical sensors. Either colorimetric, luminescence, or Raman scattering signal can be used to detect the analyte.

Self-Assembled Photonic Microsensors with Strong Aggregation-Induced Emission for Ultra-Trace Quantitative Detection

Ultratrace quantitative detection based on fluorescence is highly desirable for many important applications such as environmental monitoring or disease diagnosis, which however has remained a great challenge because of limited and irregular fluorescence responses to analytes at ultralow concentrations. Herein the problem is circumvented via local enrichment and detection of analytes within a microsensor, that is, photonic porous microspheres grafted with aggregation-induced emission gens (AIEgens). The obtained microspheres exhibit dual structural and molecular functions, namely, bright structural colors and strong fluorescence. Large fluorescence quenching induced by nitrophenol compounds in an aqueous environment is observed at ultralow concentrations (10^-12-10^-8 mol/L), enabling quantitative detection at a ppb level (ng/L). This is achieved within a porous structure with good connectivity between the nanopores to improve analyte diffusion, an internal layer of poly(ethylene oxide) (PEO) for analyte enrichment via hydrogen bonding, and homogeneous distribution of AIEgens within the PEO layer for enhanced fluorescence quenching. The fluorescent porous microspheres can be readily obtained in a single step templated by well-ordered water-in-oil-in-water double emulsion droplets with AIE amphiphilic bottlebrush block copolymers as the effective stabilizer.

Detection of Trace-Level Nitroaromatic Explosives by 1-Pyreneiodide-Ligated Luminescent Gold Nanostructures and Their Forensic Applications

By attaching the1-pyreneiodide ancillary ligand to the surface of polyvinylpyrrolidone-stabilized gold (Au:PVP) cluster or the cetyltrimethyl ammonium bromide-stabilized gold (Au:CTAB) nanorod, a new class of luminescent mixed ligand-stabilized gold nanostructures is synthesized. This postsynthetic surface modification method followed by us is a comparatively easier and hassle-free technique to acquire surface-active luminescent "functional nanomaterials". Careful analyses of transmission electron microscopy images revealed that the sizes of these Au-clusters or Au-nanorods remain unchanged without any noticeable aggregation in the medium. Owing to the formation of an excimer within the neighboring pyrenes mounted on the surface of core nanostructures (i.e., Au:PVP nanocluster and Au:CTAB nanorod), the resulting pyrene-grafted nanocomposites exhibit strong emission characteristics. The strong excimer emission is significantly quenched in the presence of electron-deficient chemical inputs, and this phenomenon can be used for analytical purposes. Using these luminescent Au-nanomaterials, we demonstrate a selective detection and sensing of trace-level nitroaromatic explosives (e.g., trinitrotoluene, trinitrophenol (TNP), dinitrotoluene, 4-nitrotoluene, etc.). It was observed that the Py-Au:PVP nanocluster is equally effective for explosive detection in both solution and solid phases with the limit of detection up to 10 nanomolar. A high Stern-Volmer constant of up to 3.88 × 10⁶ M^-1 was seen in the case of TNP in anhydrous methanol at 298 K. The deactivation pathway operating within the Py-Au:PVP nanocluster and the analytes is thought to be a result of a predominating static quenching process, where a nonfluorescent D-A supramolecular adduct is formed in the medium. Py-Au:PVP has also been successfully used to develop latent fingerprints from nonporous surfaces under an exposure of 365 nm UV light. The results suggest that these new composite materials could behave as potential "functional nanomaterials", which might be a promising alternative for on-the-spot detection of explosive traces as well as for easy visualization of latent fingerprints.

Two highly sensitive and efficient salamo-like copper(II) complex probes for recognition of CN. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2020;228:117775. [PMID: 31718968 DOI: 10.1016/j.saa.2019.117775]

Two salamo-like copper(II) complex probes, L¹-Cu²⁺ and L²-Cu²⁺, were designed and synthesized for sensitive and efficient identification of CN^-. UV spectroscopy, high resolution mass spectrometry, RGB analysis and naked eye recognition were performed to explore their recognition mechanisms. High resolution mass spectra indicated that the probes L¹-Cu²⁺ and L²-Cu²⁺ formed complexes with CN^-. The two probes could recognize CN^- by the naked eye and the color of the solution changed from light yellow to red. In terms of application, the contents of CN^- in the environmental water samples were tested. In addition, the optimal pH ranges for probe detection of CN^- were investigated by pH value measurement.

Time-resolved photoluminescence of 6-thienyl-lumazine fluorophores in cellulose acetate nanofibers for detection of mercury ions

Time-resolved photoluminescence measurements were used to characterize the photophysical properties of 6-thienyllumazine (TLm) fluorophores in cellulose acetate nanofibers (NFs) in the presence and absence of mercuric acetate salts. In solution, excited-state proton transfer (ESPT) from TLm to water molecules was investigated at pH from 2 to 12. The insertion of thienyl group into lumazine introduces cis and trans conformers while keeping the same tautomerization structures. Global and target analyses were employed to resolve the true emission spectra of all prototropic, tautomeric, and rotameric species for TLm in water. The results support the premise that only the cis conformers are related to the ESPT process. However, no ESPT from TLm to a nearby water molecule was observed in NFs. The addition of NFs increases the excited-state lifetime of TLm in the solid state because of combined polarity/confinement effects. The solid-state fluorescence of TLm (in NFs) was quenched by mercuric acetate through different mechanisms-dynamic and static-depending on the applied pressure-atmospheric and vacuum, respectively. The new solid-state sensor is simple, ecofriendly, and instantly fabricated. TLM-loaded NFs can detect mercuric ions at a concentration of 50 picomolar. The formation of non-fluorescent ground-state complex between TLm molecules and mercuric ions inside the pores of NFs was achieved under vacuum condition.

Detection of Nitroaromatics by Pyrene-Labeled Starch Nanoparticles

Starch nanoparticles (SNPs) were hydrophobically modified by using 1-pyrenebutyric acid (PyBA) with degrees of substitution (DS) between 0.0006 and 0.11. Fluorescence quenching studies were conducted on the pyrene-labeled starch nanoparticles (Py-SNPs) in dimethyl sulfoxide (DMSO) and water with nitromethane (NM), 4-mononitrotoluene (MNT), 2,6-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) to assess the mode of quenching of the pyrene labels in the two solvents. In DMSO where pyrene, starch, and the quenchers were soluble, a decrease in fluorescence signal was the result of dynamic encounters between the excited pyrene labels and the nitrated quenchers. In water where starch could be dispersed but pyrene and the nitroaromatic compounds (NACs) were sparingly soluble, quenching took place through the binding of NACs to pyrene aggregates. Py(11)-SNPs (Py-SNPs with a DS of 0.11)-coated filter papers (Py-CFPs) were prepared as fluorescence sensors. The fluorescence emitted by Py-CFPs was quenched to 25% of its original value within 10 ± 2, 72 ± 20, and 23 ± 4 s upon exposure to vapors of MNT, DNT, and TNT, respectively. When known amounts of NACs were deposited onto Py-CFPs, their limit of detection (LOD) when the fluorescence decreased by more than 3 standard deviations (3σ) from its original value equaled 9.2 ± 0.8, 3.3 ± 0.5, and 0.20 ± 0.02 ng/mm² for MNT, DNT, and TNT, respectively. These response times and LODs were among the best values reported to date in the scientific literature for fluorescence sensors. The selectivity of the Py-CFPs toward NACs was also investigated by comparing their response to the presence of non-nitrated aromatics, amines, and aromatic ketones. Quenching was only observed with the latter family of chemicals tested, but with much lower efficiency compared to TNT, thus reflecting some level of selectivity toward this specific NAC.

A convenient and universal platform for sensing environmental nitro-aromatic explosives based on amphiphilic carbon dots

2,4,6-trinitrophenol (TNP) is environmentally deleterious substance that has been of pressing societal concern. Therefore, developing a convenient and reliable platforms for its fast and efficient detection is of paramount importance from security point of view. Herein, amphiphilic fluorescent carbon dots (CDs) were prepared by a simple solvothermal method. CDs exhibit high selectivity and sensitivity on TNP in the polar and apolar solvent and even natural water samples. Moreover, the simple and portable indicator paper can be prepared conveniently and used for sensing TNP visually with high sensitivity and fast response. Research findings obtained from this study would assist in the development of portable devices for the on-site and real-time detection of environmental hazards.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

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.

Detection of Sudan Dyes Based on Inner-Filter Effect with Reusable Conjugated Polymer Fibrous Membranes

Developing effective methods for detecting illegal additives in food or seasoning is of great significance. In this study, a sensing strategy for selective detection of Sudan dyes was designed based on the fluorescence inner-filter effect (IFE) by using poly(phenylenevinylene) (PPV) solid materials in combination with an optimized experimental protocol. Two types of fluorescent solid materials, electrospun fibrous membranes and drop-cast films, were fabricated with PPV as the fluorophore and poly(vinyl alcohol) as the matrix, respectively. Sudan dyes greatly quenched the fluorescence of the membrane and film, whereas other food colorings or possible food ingredients displayed a much smaller or negligible quenching effect. The sensing mechanism was studied, and the selectivity was ascribed to IFE, which requires the overlap between the absorption of the analyte and absorption/emission of the sensing material. The form of materials (membrane or film), the content of PPV, and the cross-linking process did not have much influence on the selectivity and sensitivity, which is consistent with the IFE mechanism and demonstrates the advantage of not requiring strict control of the preparative process. All the cross-linked materials were found to be stable against water/humidity and displayed good reversibility in sensing and can be reused at least for 10 cycles with negligible influence on the sensing performance. A cross-linked membrane was selected for detecting Sudan dyes in chili powder because folding did not affect the mechanical stability of the membrane. Two different protocols were used to pretreat the chili samples, which allowed the detection of Sudan dyes in chili powder as well as the discrimination of Sudan dyes from synthetic food coloring such as allura red. This study provides a facile and cost-effective method for preparing reusable sensing materials for detecting some dyes in commercial foods or food seasonings.

Amplified polarization properties of electrospun nanofibers containing fluorescent dyes and helical polymer

Well-aligned nanofibers containing cationic fluorescent dyes and anionic chiral polymers prepared via electrospinning exhibit an enhanced circular dichroism, which is mainly caused by linear dichroism and linear birefringence.

Ormosil-coated conjugated polymers for the detection of explosives in aqueous environments

A fluorescence-based sensor for detecting explosives, based on a conjugated polymer coated with an ormosil layer, has been developed for use in aqueous environments. The conjugated polymer Super Yellow was spin-coated onto glass substrates prior to a further spin-coating of an MTEOS/TFP-TMOS-based ormosil film, giving an inexpensive, solution-based barrier material for ruggedization of the polymer to an aqueous environment. The sensors showed good sensitivity to 2,4-DNT in the aqueous phase at micromolar and millimolar concentrations, and also showed good recovery of fluorescence when the explosive was removed.

Multichannel Discriminative Detection of Explosive Vapors with an Array of Nanofibrous Membranes Loaded with Quantum Dots

The multichannel fluorescent sensor array based on nanofibrous membranes loaded with ZnS quantum dots (QDs) was created and demonstrated for the discriminative detection of explosives. The synergistic effect of the high surface-to-volume ratio of QDs, the good permeability of nanofibrous membranes and the differential response introduced by surface ligands was played by constructing the sensing array using nanofibrous membranes loaded with ZnS QDs featuring several surface ligands. Interestingly, although the fluorescence quenching of the nanofibrous membranes is not linearly related to the exposure time, the fingerprint of each explosive at different times is very similar in shape, and the fingerprints of the three explosives show different shapes. Three saturated vapors of nitroaromatic explosives could be reliably detected and discriminated by the array at room temperature. This work is the first step toward devising a monitoring system for explosives in the field of public security and defense. It could, for example, be coupled with the technology of image recognition and large data analysis for a rapid diagnostic test of explosives. This work further highlights the power of differential, multichannel arrays for the rapid and discriminative detection of a wide range of chemicals.

Bulky Isopropyl Group Loaded Tetraaryl Pyrene Based Azo-Linked Covalent Organic Polymer for Nitroaromatics Sensing and CO2 Adsorption

An azo-linked covalent organic polymer, Py-azo-COP, was synthesized by employing a highly blue-fluorescent pyrene derivative that is multiply substituted with bulky isopropyl groups. Py-azo-COP was investigated for its sensing and gas adsorption properties. Py-azo-COP shows selective sensing toward the electron-deficient polynitroaromatic compound picric acid among the many other competing analogs that were investigated. Apart from its chemosensing ability, Py-azo-COP (surface area 700 m2 g-1) exhibits moderate selectivity toward adsorption of CO2 and stores up to 8.5 wt % of CO2 at 1 bar and 18.2 wt % at 15.5 bar at 273 K, although this is limited due to the electron-rich -N=N- linkages being flanked by isopropyl groups. Furthermore, the presence of a large number of isopropyl groups imparts hydrophobicity to Py-azo-COP, as confirmed by the increased adsorption of toluene compared to that of water in the pores of the COP.

Dual-Emitting Dye@MOF Composite as a Self-Calibrating Sensor for 2,4,6-Trinitrophenol

An anionic metal-organic framework (MOF) {(NH₂Me₂)[Zn₃(μ₃-OH)(tpt)(TZB)₃](DMF)₁₂}_n (1, tpt = 2,4,6-tri(4-pyridyl)-1,3,5-triazine, H₂TZB = 4-(1H-tetrazol-5-yl)benzoic acid and DMF = N,N-dimethylformamide), with both nanosized cages and partitions, has been solvothermally synthesized, which can serve as a crystalline vessel to encapsulate the fluorescent dye rhodamine 6G (Rh6G) via a "bottle around ship" approach. As a result, the obtained dye@MOF composite system features a blue emission of the ligand at 373 nm and a red emission of Rh6G at 570 nm when dispersed in solution, which could be used for decoding the trace amount of 2,4,6-trinitrophenol (TNP) by referring the peak-height ratio of each emission, even in coexistence with other potentially competitive nitroaromatic analytes. Furthermore, the observed fluorescence responses of the composite toward TNP are highly stable and reversible after recycling experiments. To the best of our knowledge, this is the first example of an MOF-implicated self-calibrated sensor for TNP detection.

Dimensionally controlled water-dispersible amplifying fluorescent polymer nanoparticles for selective detection of charge-neutral analytes

Fluorescent nanoparticles composed of poly(p-phenylenevinylene) block copolymers were prepared by the facile one-step process and exhibited discriminative detection of neutral explosives against charged molecules.

Dynamic fluorescence quenching by 2,4,6-trinitrophenol in the voids of an aggregation induced emission based fluorescent probe

PY-DNSfollows dynamic fluorescence quenching with TNP to elicit a linear change in fluorescence quenching over eight orders of concentration of TNP (10⁻¹³–10⁻⁵M).

pH-Responsive and pyrene based electrospun nanofibers for DNA adsorption and detection

A labeling-free DNA adsorption and detection method based on pH-responsive volume changes poly(DMAEMA-co-SA-co-Py) nanofibers shows its potential applications in DNA adsorption and separation.

Highly Photoluminescent Molybdenum Oxide Quantum Dots: One-Pot Synthesis and Application in 2,4,6-Trinitrotoluene Determination

As a well-studied transition-metal semiconductor material, MoOx has a wider band gap than molybdenum disulfide (MoS2), and its property varies dramatically for the existence of several different allotropes and suboxide phases of molybdenum oxides (MoOx, x < 3). In this manuscript, a one-pot method possessing the advantages of one pot, easily prepared, rapid, and environmentally friendly, has been developed for facile synthesis of highly photoluminescent MoOx quantum dots (MoOx QDs), in which commercial molybdenum disulfide (MoS2) powder and hydrogen peroxide (H2O2) are employed as the precursor and oxidant, respectively. The obtained MoOx QDs can be further utilized as an efficient photoluminescent probe, and a new turn-off sensor is developed for 2,4,6-trinitrotoluene (TNT) determination based on the fact that the photoluminescence of MoOx QDs can be quenched by the Meisenheimer complexes formed in the strong alkali solution through the inner filter effect (IFE). Under the optimal conditions, the decreased photoluminescence of MoOx QDs shows a good linear relationship to the concentration of TNT ranging from 0.5 to 240.0 μM, and the limit of detection was 0.12 μM (3σ/k). With the present turn-off sensor, TNT in river water samples can be rapidly and selectively detected without tedious sample pretreatment processes.

An inner filter effect based sensor of tetracycline hydrochloride as developed by loading photoluminescent carbon nanodots in the electrospun nanofibers

The inner filter effect (IFE), which results from the absorption of the excitation or emission light by absorbers, has been employed as an alternative approach in sensing systems due to its flexibility and simplicity. In this work, highly photoluminescent carbon nanodots (CDs), which were simply prepared through a new one-step microwave synthesis route, were loaded in electrospun nanofibers, and the obtained nanofibers were then successfully applied to develop a fluorescent IFE-based visual sensor for tetracycline hydrochloride (Tc) sensing in milk. This developed visual sensor has high selectivity owing to the requirements of the spectral overlap between the CDs and Tc, showing high promise in sensing chemistry with an efficient response and economic effect.

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.