Porous Organic Frameworks Constructed by the Synergetic Effect of Covalent Bonds and Hydrogen Bonds for the Selective Identification and Detection of Explosives

The syntheses of the flexible TAPDA-based covalent organic frameworks utilized for capturing iodine and fluorescence sensing 2,4-dinitrophenol

The hexa(4-formyl-phenoxy)cyclotriphosphazene (NOP-6--CHO) and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT-3-CHO) are common knots of the flexible covalent organic frameworks (COFs), which may undergo Schiff base reaction with diamide and ternary amine, but with no examples of quaternary amine. Herein, we chose quaternary amine -- N,N,N',N'-tetrakis(4-aminophenyl)-1,4-phenylenediamine (TAPDA) to react with NOP-6-CHO and TPT-3-CHO to synthesize flexible TAPDA-based COFs (HTAPDA and TTAPDA) for the first time. The flexible TAPDA-based COFs have flexible units and 3D triangular or 2D hexagonal pores which make the pores nimble and self-adaptable abilities. Delightly, the flexible TAPDA-based COFs have the high BET-specific surface areas of 1103 and 1048 m2/g and show some crystallization and high thermal stability. They can adsorp I2 in both the gaseous phases (4.08 and 3.80 g g-1 at 77 K) and the solution. The flexible TAPDA-based COFs possess excellent fluorescent properties whether at dispersion or solid states and may be used for fluorescent sensing 2,4-dinitrophenol (DNP) with high selectivity and sensitivity (KSV: 2.49 × 104 and 2.36 × 104 L mol-1). Fluorescence quenching of the flexible TAPDA-based COFs is observed upon coaction of photo-induced electron transfer process and fluorescence resonance energy transfer process.

Preparation of the conjugated hypercrosslinked polymers containing phenylenediamine and phenylenetriamine derivatives for fluorescent sensing of three nitrophenols

Hypercrosslinked polymers (HCPs) are the most promising porous organic polymers for large-scale production due to their easy preparation, extensive raw material source, good stability, and large specific surface area. However, due to the lack of extended conjugability, their application in fluorescence sensing is limited. Herein, three conjugated hypercrosslinked polymers (the conjugated HCPs: TPPDA-DMB, TDPAB-DMB, and MTDAB-DMB) were easily prepared by the Friedel-Craft arylation reactions with phenylenediamine or phenylenetriamine derivatives and p-dimethoxybenzene (DMB). The extended conjugated structures and the 3D networks give the conjugated HCPs excellent fluorescence properties and fluorescence sensing performance for nitrophenols. TPPDA-DMB can sense 2,4-dinitrophenol (DNP) and picric acid (PA) with KSV values of 1.31 × 104 and 1.43 × 104 L mol-1, TDPAB-DMB can sense PA and o-nitrophenol (o-NP) with KSV values of 9.97 × 103 and 7.54 × 103  L mol-1, and MTDAB-DMB can sense DNP and PA with KSV values of 2.42 × 103 and 1.01 × 104 L mol-1, respectively. Experiments and theoretical calculations show that the fluorescence quenching mechanisms of the nitrophenols to the conjugated HCPs include electron transfer and energy transfer processes.

One-pot synthesized multifunctional Zn-MOF/HOF heterostructure sensor array assisted by machine learning for efficient capture, target discrimination and optosmart sensing of doxycycline analogs

The ideal multifunctional platform that combines the capabilities of effective capture, sensitive detection, and accurate identification of doxycycline analogs (DCs) remains a serious challenge for ensuring the environment and food security. This work constructs heterostructure Zn-MOF/HOF asynchronous response fluorescence sensor using a multicomponent one-pot method for high-efficiency capturing and sensitive detecting DCs. Metal nodes and functional groups in Zn-MOF/HOF provide sites for specifically recognizing and sensitizing DCs that induce asynchronous response with blue/green fluorescence emission. Fluorescence spectra of Zn-MOF/HOF show characteristic differences due to different spatial conformations and substituents of DCs. Machine learning-assisted Zn-MOF/HOF fluorescent sensing array accurately discriminates DCs with a high precision of 100 %. An exceptional adsorption capacity of DCs up to 569.00 mg/g realizes the effective pre-enrichment of DCs, improving the sensitivity of the Zn-MOF/HOF sensor. The limits of detection of the Zn-MOF/HOF sensor are as ultra-low as 2.2 nmol/L. Satisfactory recoveries of 91.78 %-113.16 % are obtained for detecting DCs in real-world water and food samples. A portable optosmart sensing system integrating the Zn-MOF/HOF sensor and smartphone realizes visual quantitation and on-site monitoring DCs. This work innovatively reveals the great potential of Zn-MOF/HOF heterostructure as a multifunctional platform for simultaneous capture, identification, and sensing of emerging contaminants.

Rational design of uniform SiO2-based afterglow microparticles for photonic crystals

Despite recent advancements in organic phosphors, the synthesis of monodisperse afterglow microparticles (MPs) suitable for creating photonic crystals remains challenging. The SiO2 matrix is an attractive host material for activating the long-lived emissions of doped molecules due to several factors, including its cross-linked polymer-like structure, abundance of -OH groups, robustness, and presence of numerous emitter defects. However, the Stöber method struggles to produce monodisperse molecule-doped SiO2 MPs due to the complexity of the system. Our reported pseudomorphic transformation-assisted doping method shows promise in addressing this issue by using monodisperse SiO2 MPs as parent materials in the presence of dopants under hydrothermal conditions. This method offers flexibility in controlling the optical properties of the resulting monodisperse molecule-doped SiO2 MPs. The uniformity allows for the assembly of afterglow SiO2 MPs into photonic crystals, which demonstrate not only afterglow but also angle-dependent structural colors. Furthermore, adjusting the match between the stopband of the photonic crystals and the emission bands of the doped molecules presents additional opportunities to tune the optical properties of the assemblies. Our findings significantly expand the applications of afterglow materials in fields such as information storage and anticounterfeiting.

A practical fluorometric and colorimetric dual-mode sensing platform based on two-dimensional porous organic nanosheets for rapid determination of trifluralin

Trifluralin, a widely used dinitroaniline herbicide, poses significant toxic risks, necessitating the development of rapid detection methods for food safety. In this study, we prepared ultrathin two-dimensional triphenylamine porous organic nanosheets (TPA-PONs) through a facile liquid-phase exfoliation process. The TPA-PONs, characterized by their exceptional fluorescence properties and nanoscale thickness (1.65 ± 0.3 nm), demonstrated a remarkable fluorescence quenching response upon exposure to trifluralin. Spectroscopic analysis combined with DFT calculations revealed that the quenching mechanism is driven by electron and energy transfer. TPA-PONs-based fluorescence sensor exhibited a linear response to trifluralin concentrations ranging from 0.01 to 10.0 μmol L-1 with a limit of detection as low as 3.50 nmol L-1. Additionally, the sensor was applied to detect trifluralin residues in vegetables, achieving recoveries of 89.08-102.84%. To facilitate on-site detection, a novel TPA-PONs-based colorimetric film sensor has been developed, enabling visual analysis of trifluralin using a smartphone. This dual-mode sensing platform holds significant potential for enhancing food safety monitoring.

The N,N,N',N'-tetraphenylbenzidine-based conjugated hypercrosslinked polymers for adsorping iodine and fluorescence sensing 2,4-dinitrophenol, iodine

Although progress has been made in the development of hypercrosslinked polymers (HCPs) for iodine uptake, their fluorescence performance and fluorescence sensing performance are rare due to the lack of conjugated behavior in common HCPs. Herein, the N,N,N',N'-tetraphenylbenzidine-based conjugated hyper-crosslinked polymers (conjugated TPB-based HCPs) were efficiently fabricated via a one-step Friedel-Crafts arylation reaction by selecting N,N,N',N'-tetraphenylbenzidine (TPB) and N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (DPDB) as the basic building blocks and p-dimethoxybenzene (DMB) as an external crosslinker which will lead to the formation of the conjugated structures. The derived TPB-based HCPs (denoted as, TPB-DMB and DPDB-DMB) possessed excellent stability and high surface areas, Their iodine adsorption capacities are up to 4.25 and 3.56 g·g-1. The high iodine adsorption is caused by chemical adsorption. The fully conjugated structure and the 3D aryl network give the conjugated TPB-based HCPs excellent luminescence properties and fluorescence sensing properties for 2,4-dinitrophenol and iodine. The fluorescence sensing mechanisms of the conjugated  TPB-based HCPs for DNP and I2 includes photo-induced electron process and the resonance energy transfer process. This study provides a viable approach for the development of highly efficient iodine sorbents and fluorescence sensors of DNP and iodine for addressing environmental issues.

Preparation and properties of thermal responsive 2,3-diethyl-5-methylpyrazine fragrance microcapsules with β-CD/CS as wall materials

2,3-Diethyl-5-methylpyrazine (DEMP) is recognized for its unique nutty scent but faces limitations due to rapid evaporation. The primary objective of this study was to explore the effect of incorporating DEMP with β-cyclodextrin (β-CD) and chitosan (CS) as wall material on the microstructure and thermal release behavior, antibacterial, and antioxidant characteristics. Initially, the microcapsules preparation process underwent optimization with embedding rate of 78.03 % through response surface by ultrasonic technique. The characterization of microcapsules was confirmed through SEM, FT-IR and TEM, with the majority exhibiting smooth and shell core structures that overlapped. Through sustained release kinetics analysis, the release of microcapsules under 80 °C, 50 °C and room temperature was more in line with the first-order kinetic and Avrami kinetic equation. The heat release kinetics analysis yielded a well-matched linear fitting curve. Additionally, microcapsules effectively suppressed the growth of S. aureus and E. coli germs, and demonstrated strong antioxidant properties, compared with DEMP. Adding 10 mg microcapsules to the Heat Not Burning (HNB) cigarette, the sensory quality was significantly improved. This discovery has the potential to pave a new route for the encapsulation of fragrance molecules, and expanding their multifunctional usages for enhancing the flavor of cigarettes and food.

A hydroxyl-rich covalent organic framework for the precisely selective fluorescence sensing of explosives with high sensitivity

Covalent organic Frameworks (COFs) have become a new platform for functional research and material design. A novel covalent organic skeleton (DHB-TFP COF) was synthesized from 2-hydroxybenzene-1,3,5-tricarbaldehyde and 3,3'-dihydroxybenzidine using Schiff base reaction. DHB-TFP COF is a highly stable porous crystalline material and exhibits exceptional thermal and chemical resistance. DHB-TFP COF exhibited a selective and sensitive "turn-off" fluorescence response to 4-NP in ethanol, and TNP not only significantly quenched the fluorescence of DHB-TFP COF but also caused the obvious red-shift. The fluorescence intensity of DHB-TFP COF exhibited a linear correlation with the concentration of 4-NP with a detection limit of 0.40 μM. Furthermore, the maximum fluorescence peak observed for DHB-TFP COF demonstrated a linear relationship with TNP concentration with a detection limit of 11.15 μM. DHB-TFP COF exhibited satisfactory recovery in the detection of 4-NP and TNP in actual water sample indicating its practical application potential. The O atoms of rich hydroxyl and N atoms of C = N present on the surface of DHB-TFP COF scaffold can establish strong hydrogen bonds with 4-NP and TNP, facilitating their mutual interaction. The spectra studies indicated that the fluorescence quenching mechanism can be attributed to the absorption competitive quenching (ACQ) and fluorescence resonance energy transfer (FRET) mechanism. This study not only proposed the approach for synthesizing novel structured organic frameworks, but also developed a highly selective and sensitive fluorescence chemical sensor for identifying and detecting 4-NP and TNP.

Solvent Vapor/Gas-Induced Guest Transport and Exchange of a Nonporous Organic Crystal to Construct Smart Host-Guest Energetic Materials

Supramolecular materials with advanced properties constructed by intermolecular interactions have attracted extensive attention in many fields, such as sensing, catalysis, and biomedicine. However, in the field of energetic materials, limited by the tight-packed crystal structure of explosives and the strong intermolecular interaction forces, most supramolecular explosives can only be obtained in organic solution or under extreme external loading (high temperature/high pressure). Given the practical issues such as safety risks, operational difficulties, serious environmental pollution, and large-scale production of the existing technology, a new method of constructing host-guest explosives by solvent vapor/gas induction is proposed. This gas-solid reaction method takes advantage of the metastable properties from the explosives solvate (HNIW/ACN), and cleverly opens a fast channel for gas molecules to enter the explosives cell cavities, which results in the highly efficient preparation of the host-guest explosives (HNIW/CO2 and HNIW/N2O). The embedding of functional gas molecules greatly improves the structural stability and comprehensive performance of the explosive skeleton, and the detonation velocity of HNIW/N2O even reaches 9802 m·s-1, which is higher than that of ε-HNIW (9455 m·s-1). In addition, compared with ε-HNIW, HNIW/CO2 and HNIW/N2O exhibit high energy but low sensitivity, enhanced thermal stability, and combustion properties, which present a potential prospect in the field of energetic materials. The new method effectively overcomes the high-energy barrier of nonporous organic explosives, offering the advantages of simplicity, safety, efficiency, and environmental friendliness. This study provides a valuable pathway for constructing advanced supramolecular energetic materials, which contributes to the enrichment of supramolecular engineering systems.

Novel Covalent Bonds and Hydrogen Bonds Linked Porous Organic Frameworks as Chemosensor for Detecting 2,4,6-Trinitrophenol in Water and Soil Samples

A novel and unconventional structural porous organic framework combined through the synergistic effect of covalent bonds and hydrogen bonds was prepared with the combination of 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetraaniline (Py) and 5-hydroxyisophthalaldehyde (HP). It was the second example of CHOF until now and had been designated as Py-HP CHOF. The suspension of Py-HP CHOF in various solvents, such as ethanol, CH3CN, and methanol, exhibited a remarkably selective and sensitive "on-off" fluorescence response toward 2,4,6-trinitrophenol (TNP) compared with other explosives, with exceptionally low detection limits. The X-ray diffraction (XRD) spectra confirmed that the framework of Py-HP CHOF collapsed after interaction with TNP and acid, further indicating the existence of hydrogen bonds in the framework of Py-HP CHOF. The fluorescence quenching can be ascribed to the photoinduced electron transfer and the absorption competition quenching, as supported by XRD, X-ray photoelectron spectroscopy results, UV-vis absorption spectra, and density functional theory calculations. Fluorescence channels can be utilized by Py-HP CHOF to function as chemosensor, enabling the identification and detection of TNP in water and soil, and Py-HP CHOF is also the second CHOF example of sensing TNP reported to date. The application of this technique exhibits considerable potential in the analysis and detection of environmental pollutants, thereby presenting substantial practical implications.

A novel ethylene linkage-based covalent organic framework for turn-on fluorescence sensing for Al3+ with excellent selectivity and sensitivity

Covalent organic Framework (COFs) has become a new platform for functional research and material design. A novel covalent organic framework (CN-COF) was first synthesized with p-xylylene dicyanide and 2-hydroxy-1,3,5-benzenetrialdehyde through the Knoevenagel condensation reaction. CN-COF is a porous crystal material with strong thermal and chemical stability. CN-COF exhibits a selective "turn-on" fluorescence response to Al3+ in ethanol with blue-shifted emission spectra over the other tested metal ions. The color changes from pink to earth yellow, and the fluorescence effect is clearly visible. The fluorescence intensity of CN-COF was linearly related to the concentration of Al3+, and the detection limit was 1.815 μM. Importantly, CN-COF exhibits a satisfactory recovery for detecting Al3+ in drinking water and fish samples. CN-COF also showed the intuitive semi-quantitative detection ability for Al3+ via the color change with the naked eyes. The special pore structure is conducive to allow Al3+ enter to coordinate with O and N atoms on the wall of CN-COF scaffold. The revisable fluorescence change upon the selective addition of Al3+ and XRD, EDTA, XPS and DFT results demonstrated the complex process. The inhibition of the photoinduced electron transition from O atoms to Al3+ induced the fluorescence enhancement. This study not only presents a synthesis idea for a new structural organic framework, but also offers a highly selective and sensitive fluorescence chemical sensor for the identification and detection of Al3+.

Selective and Rapid Detection of 4-Nitrophenol in River and Treated Industrial Wastewater by a Luminescent Lanthanide Metal-Organic Framework Sensor

Phenolic organic compounds are widely used industrial chemicals that exist extensively in the environment and have a significant impact on human health. 4-Nitrophenol (4-NP) is a typical phenolic organic compound found in aqueous environments. Efficient detection of 4-NP in wastewater is highly challenging due to the complexity of testing environmental samples. Herein, a luminescent lanthanide metal-organic framework (MOF) sensor based on the Eu3+ ion {[Eu(HL)(L)(H2O)]·2H2O}n (EuMOF; H2L = 5-(4H-1,2,4-triazol-4-yl)benzene-1,3-dicarboxylic acid) was successfully synthesized for efficient 4-NP detection in wastewater. Fluorescence sensing experiments revealed that 4-NP could greatly quench the EuMOF fluorescence. Subsequently, EuMOF was applied to 4-NP detection in distilled water, tap water, river water, and treated industrial wastewater, exhibiting high sensitivity, a fast response within 30 s, high selectivity, excellent reusability, and a low detection limit. Finally, the fluorescence quenching mechanism was explored and attributed to competitive absorption of irradiated light between 4-NP and the ligand.

Mixed-Linkage Donor-Acceptor Covalent Organic Framework as a Turn-On Fluorescent Sensor for Aliphatic Amines

Amine determination is crucial to our daily life, including the prevention of pollution, the treatment of certain disorders, and the evaluation of food quality. Herein, a mixed-linkage donor-acceptor covalent organic framework (named DSE-COF) was first constructed by the polymerization between 2,4-dihydroxybenzene-1,3,5-tricarbaldehyde (DTA) and 4,4'-(benzo[c][1,2,5]selenadiazole-4,7-diyl)dianiline (SEZ). DSE-COF displayed superior turn-on fluorescent responses to primary, secondary, and tertiary aliphatic amines, such as cadaverine, isopropylamine, sec-butylamine, cyclohexylamine, hexamethylenediamine, di-n-butylamine, and triethylamine in absolute acetonitrile than other organic species. Further experiments and theoretical calculations demonstrated that the combination of intramolecular charge transfer (ICT) and photoinduced electron transfer (PET) effects between the DSE-COF and aliphatic amines resulted in enhanced fluorescence. Credibly, DSE-COF can quantitatively detect cadaverine content in actual pork samples with satisfactory results. In addition, DSE-COF-based test papers could rapidly monitor cadaverine from real pork samples, manifesting the potential application of COFs in food quality inspection.

Covalent organic frameworks: linkage types, synthetic methods and bio-related applications

Covalent organic frameworks (COFs) are composed of small organic molecules linked via covalent bonds, which have tunable mesoporous structure, good biocompatibility and functional diversities. These excellent properties make COFs a promising candidate for constructing biomedical nanoplatforms and provide ample opportunities for nanomedicine development. A systematic review of the linkage types and synthesis methods of COFs is of indispensable value for their biomedical applications. In this review, we first summarize the types of various linkages of COFs and their corresponding properties. Then, we highlight the reaction temperature, solvent and reaction time required by different synthesis methods and show the most suitable synthesis method by comparing the merits and demerits of various methods. To appreciate the cutting-edge research on COFs in bioscience technology, we also summarize the bio-related applications of COFs, including drug delivery, tumor therapy, bioimaging, biosensing and antimicrobial applications. We hope to provide insight into the interdisciplinary research on COFs and promote the development of COF nanomaterials for biomedical applications and their future clinical translations.