Fluorescent Metal-organic Framework Research Articles

Self‐Assembly of UiO‐66 on Conjugated Polyelectrolytes: Toward Enhanced Photophysical Properties

AbstractIn this study, the synthesis and characterization of the fluorescent metal–organic framework (MOF)‐polymer composites is reported based on zirconium‐based MOFs (UiO‐66 and UiO‐66(OH)₂) and a conjugated polyelectrolyte, poly(phenylene ethynylene) carboxylate (PPE‐CO₂‐108). The defect‐rich nature of these Zr‐MOFs facilitates strong interactions between the polyelectrolyte's carboxylic acid moieties and the open metal sites on the Zr clusters within the MOF nanoparticles. The composites are prepared by a simple impregnation approach, where MOF nanocrystals suspended in HEPES buffer are added to the polymeric solution under sonication for 10 min. The obtained MOF composites (i.e., UiO‐66‐CPE and UiO‐66 (OH)2‐CPE) are characterized using powder X‐ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Brunauer‐Emmett‐Teller (BET) surface area, zeta potential measurements, and UV–vis spectroscopy. The characterization confirmed the homogeneous distribution of the MOF nanocrystals on the polymer as seen in the SEM images, and the interaction with PPE‐CO₂‐108 is confirmed by the decrease in the surface areas from 1050 to 590 m2 g−1 for UiO‐66‐CPE and from 500 to 137 m2 g−1, for UiO‐66(OH)2‐CPE. Fluorescence microscopy revealed strong fluorescence in the microparticles, confirming robust polymer‐MOF interaction. The composites also exhibit improved photostability of the conjugated polyelectrolyte, suggesting their potential for applications in biomedical imaging and other areas requiring stable and bright fluorescent systems.

Metal-organic framework-based off-on fluorescent probe for highly selective and sensitive phosphate detection

The escalating demand for versatile and precise diagnostic tools across various biomedical applications has led to the development of an advanced assay utilizing a fluorescent metal-organic framework (MOF). Our novel approach centers on the synthesis of a controlled-explosive NH2-MIL-53(Fe) MOF, composed of trivalent iron ions and 2-aminoterephthalic acid. This framework possesses the unique capability to produce explosively enhanced fluorescence upon environmental triggers such as phosphate conditions, where it disintegrates to release intensely fluorescent molecules. The surface of NH2-MIL-53(Fe) MOF was further functionalized with poly(acrylic acid-co-maleic acid) polymers (PAAcoMA) to form PAAcoMA@MIL-53(Fe) MOF, thereby enhancing its stability. The presence of phosphate ions is detected by the degree of fluorescence resulting from the unquenching of PAAcoMA@MIL-53(Fe) MOF. This system demonstrates not only high sensitivity but also a broad dynamic range, making it suitable for phosphate ion detection. This innovative technology holds promise for significant advancements in the field of phosphate indicator-based biosensing, with potential applications in the development of PCR kits, thereby supporting a wide range of diagnostic and therapeutic applications.

Regulation on the ligand fluorescence in UiO-66-Y metal-organic frameworks for effective detection of nitro-aromatic explosives

Rapid, sensitive and highly selective detection of nitro-aromatic explosives (NAEs) has become one of the most pressing environmental and safety issues. Metal-organic frameworks (MOFs) offer new possibilities for the development of new photoactive materials with excellent sensing properties. In this paper, three robust UiO-66-type fluorescent MOFs were prepared from the self-assembly of yttrium cations (Y3+) and linear dicarboxylic acid ligands, named terephthalic acid (BDC), 2-hydroxyterephthalic acid (OHBDC) and 2,5-dihydroxyterephthalic acid (DHBDC). The solid and solution fluorescence properties of three MOFs were successfully modulated by regulating the number of –OH functional groups from zero to two. At 370 nm excitation, Y-BDC MOF without –OH group was almost non-fluorescent, Y-OHBDC MOF with one –OH group emitted blue fluorescence, and Y-DHBDC MOF with two –OH groups emitted green fluorescence. Thanks to the suitable porous structures and stable frameworks, Y-OHBDC and Y-DHBDC MOFs both can quantificationally detect at least seven nitro-aromatic explosives (NAEs) by a fluorescence quenching process. Remarkably, the detection performance for 2,4,6-trinitrophenol (TNP) is the most outstanding with the KSV values of 8.4 × 104 M−1 and 2.1 × 105 M−1. Fast response, high sensitivity, strong anti-interference ability and good reusability make UiO-66-Y MOFs potential fluorescent sensors for TNP.

Exploring the Photophysical and Mechanical Behavior of Fluorescent Metal-Organic Framework Monoliths.

Luminescent metal-organic frameworks exhibit great potential as materials for nanophotonic applications because of their programmable properties and tunable structures. In particular, luminescent guests (LG) can be hosted by metal-organic frameworks due to their porosity and guest confinement capacity, forming LG@MOF composite systems. However, such guest-host systems are mainly produced as loose powders, preventing their widespread use in practical devices and technological applications that require implementation of a stable continuum solid. In this regard, using monolithic MOF hosts might be a workable option to solve this challenge. Herein, we reported the facile synthesis and fabrication of novel prototypical sol-gel monolithic systems, designated as LG@monoMOF. Red (rhodamine B), blue (7-methoxycoumarin), and yellow (fluorescein) emitting dyes were encapsulated in a robust UiO-66 monolithic host, resulting in the red, blue, and yellow light-emitting luminescent monoliths. The mechanical and photophysical characterization of the three LG@monoMOF systems was systematically carried out in order to unravel the role of guest-host interactions in the mechanical and optical response of the bespoke LG@monoMOF composites.

Assembly of unique interpenetrated metal–organic frameworks based on flexible tetra-carboxylate ligands

Entangled metal–organic frameworks (MOFs) are attracting more and more attentions attributed to their special performances, such as adsorption, fluorescence and catalysis. It is still of significant to synthesize new MOF with unique interpenetrating motif. In this work, two new MOFs, namely [(Me2)NH2]·[In(TCM)]·4DMF (JOU-32), and [Cd2(TCM)(DMF)2]·5DMF (JOU-33), have been successfully synthesized using flexible tetra-carboxylate ligand tetrakis(4-carboxyphenoxymethyl)methane (H4TCM). The structures of JOU-32 and JOU-33 were well characterized by single-crystal X-ray diffraction. It is shown that JOU-32 reveals a rare example of interpenetrating MOF with two different networks disorderedly distributed within the same crystal. JOU-33 is a (4,8)-connected flu topological structure based on [Cd4(COO)8(DMF)2] cluster. The fluorescent properties for both JOU-32 and JOU-33 have also been tested. The average life times for JOU-32 and JOU-33 were detected to be 2.39 and 2.42 ns, respectively. In addition, both JOU-32 and JOU-33 can be utilized as fluorescent probes for detection of NB. This work provides valuable clues for preparing new fluorescent MOFs with attractive structures.

Flexible Metal-Organic Frameworks: From Local Structural Design to Functional Realization.

ConspectusFlexible metal-organic frameworks (MOFs), also known as soft porous crystals, exhibit dynamic behaviors in response to external physical and chemical stimuli such as light, heat, electric or magnetic field, or the presence of particular matters, on the premise of maintaining their crystalline state. The reversible structural transformation of flexible MOFs, a unique characteristic seldomly found in other types of known solid-state materials, affords them distinct properties in the realms of molecule separation, optoelectronic devices, chemical sensing, information storage, biomedicine applications, and so on. The mechanisms underlying their dynamic behaviors can be comprehensively investigated at the molecular level by means of in situ single-crystal or powder X-ray diffraction as well as other in situ spectroscopic techniques due to the high regularity of these crystalline materials during stimuli-responsive phase transitions. Through the introduction of specific stimuli-responsive groups/moieties into the well-defined and ordered molecular arrays, targeted applications can be achieved, and the performance of flexible MOFs can also be further improved via rational structural design.In this Account, we summarize our progress on the design, synthesis, and applications of flexible MOFs over the past few years. First, we highlight the construction principle of flexible MOFs, emphasizing the pivotal role of local structural design. Using an F-modified ligand, a flexible MOF with remarkable structural transformations can be obtained; the regulation of the metal coordination environment and interpenetrating frameworks is also crucial for achieving flexible MOFs. We also propose a strong correlation strategy based on the supramolecular interactions between the guest molecules and the framework, which realizes the temperature-responsive dynamic spatial "open-closed" regulation. Mechanisms of the dynamic behaviors investigated by the in situ techniques were also presented for the obtained materials. Second, some representative specific applications of the newly developed dynamic coordination systems were reviewed. The gas molecule responsive flexible MOFs show efficient short-chain alkane separation properties with discriminatory sorption behavior toward similar gaseous substrates. Smart sensing of temperature, pressure, and volatile organic compounds was achieved by several novel flexible fluorescent MOFs, with optimization potential through state-of-the-art chemical design. Furthermore, multiferroic materials with multiple bistable states and high working temperatures were also obtained based on flexible MOFs.Finally, we provide a discussion of the challenges of flexible MOFs in future research, including precise and efficient synthesis, in-depth structure-property relationship investigation, performance optimization, and industrialization. We hope that this Account will stimulate further research interest in developing next-generation smart materials based on flexible MOFs for applications in challenging chemical separation, extreme environmental sensing, massive information storage, and beyond.

Novel fluorescent sensor with 2,5-furandicarboxylic acid as the ligand for histamine detection

Using effective and sustainable technologies for food quality evaluation has become imperative due to the growing concerns regarding food safety and security. Fluorescent-based sensing is a rapid monitoring technique for food spoilage detection; however, the technique employs fossil-based compounds for sensor development. Here, a fluorescent sensor for histamine (biogenic amine) as an indicator for the detection of spoilage in the fish matrix was fabricated using 2,5-furandicarboxylic acid (FDCA) as the organic ligand. FDCA is a biobased molecule and serves as a replacement for petroleum-derived terephthalic acid, which is widely used in the synthesis of fluorescent metal-organic frameworks (MOF). Herein we report a MOF that was synthesized using two organic ligands: FDCA and adenine with Fe as the metal center, denoted as Fe-AD-FDCA. The fluorescent sensor was created by covalently bonding methyl red (MR) with an NH2-rich Fe-AD-FDCA framework by post-synthetic modification. When exposed to histamine, the sensor showed a fluorescent emissive response that increased MR emission and produced a distinctly visible color shift from dull pink to blue. By spreading and solidifying MR@Fe-AD-FDCA in the water-phase sodium salt of carboxy methyl cellulose (CMC-Na), portable sensory hydrogels were produced. This fluorescence sensor's ability could be utilized for real-time monitoring of the freshness of raw fish samples.

Portable on–off visual-mode detection using intrinsic fluorescent zinc-based metal–organic framework for detection of diclofenac in pharmaceutical tablets

On-site, robust, and quantitative detection of diclofenac (DCF) is highly significant in bioanalysis and quality control. Fluorescence-based metal–organic frameworks (MOFs) play a pivotal role in biochemical sensing, offering a versatile platform for detecting various biomolecules. However, conventional fluorescent MOF sensors often rely on lanthanide metals, which can pose challenges in terms of cost, accessibility, and environmental impact. Herein, an intrinsic blue fluorescent zinc-based metal–organic framework (FMOF-5) was prepared free from lanthanide metals. Coordination-induced emission as an effective strategy was followed wherein a non-fluorescent ligand is converted to a fluorescent one after insertion in a framework. Conventional fluorometry and smartphone-assisted visual methods were employed for the detection of DCF. The fluorescence emission of the FMOF-5 was effectively quenched upon the addition of the DCF, endowing it an “off” condition, which permits the construction of a calibration curve with a wide linear range of 30–670 µM and a detection limit of about 4.1 µM. Other analytical figures of merit, such as linearity, sensitivity, selectivity, accuracy, and precision were studied and calculated. Furthermore, the proposed sensor was successfully applied to quantify DCF in pharmaceutical tablets with reliable recovery and precision. Importantly, the elimination of lanthanide metals from the fluorescence detection system enhances its practicality and sustainability, making it a promising alternative for DCF detection in pharmaceutical analysis applications.

Chiral Liquid Crystalline Metal-Organic Framework Thin Films for Highly Circularly Polarized Luminescence.

Combining metal-organic frameworks (MOFs) with liquid crystals to construct liquid crystalline MOFs (LCMOF) offers the advantage of endowing and enhancing their functionality, yet it remains a challenging task. Herein, we report chiral liquid crystalline MOF (CLCMOF) thin films by cross-linking the chiral liquid crystals (CLC) with MOF thin films to realize highly circular polarization luminescence (CPL) performance with photo and thermal switching. By layer by layer cross-linking stilbene-containing CLC with stilbene-based MOF (CLC/MOF) thin film, the CLCMOF thin films were successfully obtained after UV irradiation due to the abundant [2 + 2] photocycloaddition. The resulted CLCMOF thin films have strong chirality, obvious photochromic fluorescent, and strong CPL performance (the asymmetry factor reaches to 0.4). Furthermore, due to the photochromic fluorescent MOF and thermotropic CLC, the CPL can be reversed and red-shifted after heating and UV irradiation treatment, showing photo- and thermal CPL switching. Such MOF-based CPL thin films with photo/thermal CPL switching were prepared to patterns and codes for the demonstration of potential application in advanced information anticounterfeit and encryption. This study not only opens a strategy for developing chiral thin films combining MOFs and liquid crystals but also offers a new route to achieve CPL switching in optical applications.

Rational Designed Metal–Organic Framework with Nanocavity Traps for Selectively Recognizing and Separating of Radioactive Thorium in Rare Earth Wastewater

AbstractFacing the complex coexistence of various metal ions in wastewater and mineral resources, metal–organic frameworks (MOFs) with diverse functionalities and tunable pores are poised to serve as distinctive solutions for efficiently recognizing and selectively separating radioactive thorium (Th(IV)). Herein, a fluorescent MOF (DBT‐DHTA‐Cd) with unique thorium nanocavity traps is rationally designed by using two rigid ligands, including azole and hydroxyl groups, respectively. Notably, the synergistic effect of the appropriate pore size and dense hydroxyl···N nanocavity traps in DBT‐DHTA‐Cd results in selective capture of Th(IV), arising from the high charge density of nano‐trap and extremely low binding energy (Eads = −602.7 KJ mol−1). The fast adsorption kinetics (30 min) and rapid fluorescence response (1 min) of DBT‐DHTA‐Cd toward Th(IV) are attributed to the stronger electron holding capacity of the d and f orbitals belonged to Th(IV), making it more prone to accepting electrons transferred from the N/O active sites in the nano‐trap. As a verification test, tailing breakthrough experiments confirm that DBT‐DHTA‐Cd can efficiently purify Th(IV) from monazite mining solid waste in one step. This work provides profound insights into relationship between structure and property of MOFs and has valuable guidance for designing materials in the radioactive purification field.

Aromatimide tetracarboxylate metal-framework materials: Fluorescence sensing application towards NACs and small drug molecules

Three multifunctional fluorescent Metal-Organic Framework (MOF) sensors are synthesized based on N, N’-bis(3, 5-dicarboxyphenyl)isophthalic diimide (H4BDCPPI) with Zn, Cu, and Cd ions. Structural analysis shows that although using the same organic conjugated aromatic imide tetracarboxylic acids ligand, their corresponding structures are different. Based on luminescence characteristics of the organic components, they exhibit good luminescent properties. They can sensitively detect nitro aromatic compounds (p-nitrophenol (PNP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP)) and small drug molecules (colchicine and balsalazide disodium) with low limit of detection. The complex 1 is more sensitive for detection of TNP, compared to the other complexes. The mechanism of fluorescence sensing is proposed in detail based on analyses of PXRD, fluorescence lifetimes, and UV–vis spectra. This study explores the potential of MOFs based on H4BDCPPI in the field of fluorescence sensing for the first time, laying the theoretical foundation for application of complexes with conjugated aromatic imide tetracarboxylic acids.

Miniaturized Implantable Fluorescence Probes Integrated with Metal-Organic Frameworks for Deep Brain Dopamine Sensing.

Continuously monitoring neurotransmitter dynamics can offer profound insights into neural mechanisms and the etiology of neurological diseases. Here, we present a miniaturized implantable fluorescence probe integrated with metal-organic frameworks (MOFs) for deep brain dopamine sensing. The probe is assembled from physically thinned light-emitting diodes (LEDs) and phototransistors, along with functional surface coatings, resulting in a total thickness of 120 μm. A fluorescent MOF that specifically binds dopamine is introduced, enabling a highly sensitive dopamine measurement with a detection limit of 79.9 nM. A compact wireless circuit weighing only 0.85 g is also developed and interfaced with the probe, which was later applied to continuously monitor real-time dopamine levels during deep brain stimulation in rats, providing critical information on neurotransmitter dynamics. Cytotoxicity tests and immunofluorescence analysis further suggest a favorable biocompatibility of the probe for implantable applications. This work presents fundamental principles and techniques for integrating fluorescent MOFs and flexible electronics for brain-computer interfaces and may provide more customized platforms for applications in neuroscience, disease tracing, and smart diagnostics.

A o-phenanthroline-based metal-organic framework for fluorescence sensing toward CrVI and antibiotics in aqueous phase

Herein, a novel o-phenanthroline-based ligand involving two naphthalenecarboxylate groups was first synthesized, and successfully employed to fabricate a fluorescent metal–organic framework 1 under solvothermal conditions. According to the single-crystal X-ray analysis of compound 1, the Cd(II) ions connected by organic ligands form a 2D layer structure with a unique metallacycle building unit, and adjacent 2D layers are further stacked into a 3D porous framework via numerous supramolecular interactions. Powder X-ray diffraction revealed that 1 exhibits exceptional stability in both water and common organic solvents. The fluorescence sensing experiments conducted with compound 1 toward anions and antibiotics, demonstrated its selective detection capabilities for CrVI anions (CrO42−/Cr2O72−) and nitrofuran antibiotics in aqueous solutions. Additionally, fluorescent 1 serves as an effective fluorescent probe for sensing nitro-aromatic compounds in dimethyl sulfoxide. Its robust anti-interference properties, recyclability, and resistance to water make compound 1 a promising candidate for pollutant detection in wastewater.

Second near-infrared fluorescent Metal–Organic framework sensors for in vivo extracellular adenosine triphosphate monitoring

Plant nanobionic sensors enable real-time monitoring of signaling molecules in plants by interfacing them with specifically designed nanoprobes, which have been acknowledged as species-independent analytical tools. In this study, we developed a plant nanobionic sensor for in vivo detection of extracellular adenosine triphosphate (eATP) in living plants by designing a novel second near-infrared (NIR-II) fluorescent metal–organic framework (MOF) nanoprobe. The NIR-II fluorescent nanoprobe (IR-1061 micelle@ZIF-90) with a sandwich structure was synthesized by successive encapsulation of the hydrophobic NIR-II dye IR-1061 with the amphipathic polymer DSPE-mPEG 2000 and MOF ZIF-90. Interestingly, coating ZIF-90 around IR-1061 micelles increased the NIR-II fluorescence 16.6-fold. Utilizing the ultrahigh NIR-II fluorescent emission of the designed nanoprobes and specific recognition of ZIF-90 to ATP, the nanoprobes were applied to spatial and temporal monitoring eATP in model and non-model plants under environmental stress.

Multifunctional Luminescent 3D Ln-MOFs with High Sensitivity for Trace Detection of Micronutrients.

The synthesis of two versatile fluorescent metal-organic frameworks (MOFs), [Eu(4-NCP)(1,4-bdc)]n·0.5H2O (1) and [Eu(4-NCP)(4,4'-bpdc)]n·0.75H2O (2) (HNCP = 2-(4-carboxyphenyl)imidazo(4,5-f)-(1,10)phenanthroline, 1,4-H2bdc = benzene-1,4-dicarboxylic acid, 4,4'-H2bpdc = 4,4'-biphenyldicarboxylic acid), was carried out using a hydrothermal method. These MOFs were characterized through various advanced technologies to determine their structural information. The results indicate that both MOFs exhibited 3D network structures with specific topologies. Furthermore, these MOFs demonstrated exceptional thermal stabilities and adsorption capabilities. Additionally, complex 2 was utilized for studying the fluorescence sensing properties of various micronutrients including metal ions, nitro aromatic compounds, and biological small molecules. Notably, complex 2 showed promising potential as a multifunctional sensor for selectively detecting Fe3+, nitrobenzene, and ascorbic acid in aqueous solutions through fluorescence quenching with low limits of detection (LODs ∼ 10-7 M) and high quenching constants (Ksv ∼ 103 M-1). Moreover, the detection mechanism of complex 2 was further investigated by using experimental methods and DFT calculations.

Two Indium(III)-based Fluorescent Metal-Organic Frameworks for Highly Sensing Fe3+, 2,4-DNP, and TNP

Two Indium(III)-based Fluorescent Metal-Organic Frameworks for Highly Sensing Fe3+, 2,4-DNP, and TNP

Unveiling the unique mechanism of the UV-sensitive phosphonate-based metal-organic frameworks

Fluorescent metal-organic frameworks (MOFs) exhibit distinguished applications in sensing, especially in UV dosimeters. Although the designing strategy focuses on choosing metal ions and ligands, utilizing the host-guest interactions of phosphonate-based...

The {Cu2I2} cluster bearing metal organic frameworks: crystal structures and fluorescence detecting performances towards cysteine and explosive molecules.

Two {Cu2I2} cluster-bearing metal organic frameworks (MOFs) of {[Eu(CuI)2(INA)3DMF]·0.95DMF}n (Eu-CuI-INA) and {K[(CH3)2NH2]Sr4(INA)2(DMF)2{(Cu2I2)2(INA)8}·2H2O}n (Sr-K-CuI-INA, HINA = isonicotinic acid, DMF = N,N-dimethyl formamide) were prepared and characterized in this work. Both materials feature a three-dimensional (3-D) structure, in which the {Cu2I2} clusters and Eu3+ (or Sr2+) metal ions are coordinated by INA- ligands with pyridine and carboxylic groups, respectively. Impressively, Sr-K-CuI-INA exhibits sensitive fluorescence sensing behaviors towards cysteine and nitro-bearing molecules, demonstrating potential FL sensing applications for bio and explosive molecules. This work would provide a good reference for designing fluorescent MOF probes containing CuI molecules.

Assembly of three new triphenylamine functionalized MOFs for the fluorescent sensing of nitrobenzene

The detection of nitrobenzene (NB) is very important because it has a series of irreversible effects on human health. Metal-organic frameworks (MOFs) show potential applications in this field due to their tunable and porous structures. In this work, three new MOFs, namely [Mn3(TCA)2(DMF)4]·DMF (JOU-28), [Cd21(TCA)14(H2O)9]·21DMF·21H2O (JOU-29), and [Cd3Cl3(TCA)(DMF)2]·H2O (JOU-30), were successfully prepared by solvothermal method (H3TCA = 4,4′,4″-tricarboxytriphenylamine; DMF = N,N′-Dimethylformamide). Single crystal X-ray diffractions showed that JOU-28 was a 3D porous supramolecular structure based on 2D coordination networks, while both JOU-29 and JOU-30 were 3D porous structures based on 1D chains. Due to its strong fluorescence emission, JOU-29 was selected as a representative to study the NB sensing performance. A linear relationship between the NB concentration and the fluorescence emission intensity of JOU-29 could be obtained as I0/I = 0.952 + 8089[NB]. This result showed that JOU-29 can be used for quantitative detection of NB. Further studies revealed that the fluorescence quenching of JOU-29 may be due to the fluorescence resonance transfer effect and the competitive absorption. This work provides clues for the design and synthesis of fluorescent MOFs for NB detection.

Metal-Organic Framework Based on Pyrazinoquinoxaline Tetracarboxylic Acid for Fluorescence Sensing for Nitro Explosives.

The rapid and selective detection of nitro explosives has become one of the current urgent environmental and safety issues. Fluorescent metal-organic frameworks (MOFs) provide strong support for the development of photoactive materials with excellent sensing performances. In this work, Zn2+ and pyrazinoquinoxaline tetracarboxylic acid with high nitrogen content were selected to construct a MOF structure termed Zn-MOF, which had excellent optical properties. The fluorescence sensing performance of Zn-MOF for nitro explosives was also investigated. The structural advantages of Zn-MOF, such as its porous structure, abundant host-guest interaction sites, and stable framework, ensure the prerequisites for various applications. Zn-MOF is not only capable of responding to a wide range of substrates, such as Fe3+, Cr2O72-, and MnO4-, to achieve fluorescence quenching detection but also able to achieve sensitive fluorescence sensing behavior for nitro explosives. In particular, for trinitrotoluene, the Ksv value can reach 8.72 × 103 M-1. The results show that the introduction of pyrazinoquinoxaline groups into MOFs can be an effective strategy for the preparation of highly efficient fluorescent sensing materials for nitro explosives.