Imine-based Covalent Organic Framework Research Articles

Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate.

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

Needle trap devices packed with an imine-based 2D COF: An innovative tool for the sampling of polycyclic aromatic hydrocarbons in air

This study introduces a new method for sampling and microextraction of polycyclic aromatic hydrocarbons (PAHs) in the air using needle trap devices (NTDs) packed with an imine-based covalent-organic framework (COF) as an adsorbent. The NTD was coupled with gas chromatography-flame ionization detection (GC-FID) for analytes’ separation and quantification. The COF adsorbent was synthesized using a solvothermal method at room temperature and packed inside a stainless-steel needle. The adsorbent was characterized using X-ray diffraction analysis (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), field-emission scanning electron microscopy (FE-SEM-EDS) and Brunauer-Emmett-Teller (BET) analysis. The method was developed in the laboratory under various conditions and subsequently applied in the field to compare the results with those obtained using sampling and analytical method-NIOSH 5528. The desorption conditions and breakthrough volume (BtV) were optimized at recommended exposure limits concentration using response surface methodology (RSM) with a central composite design (CCD). Accordingly, the optimal values for desorption temperature and BtV were obtained 310 °C and 2840 mL, respectively. The limits of detection (LODs) and the limits of quantification (LOQs) were found to be 0.021–0.027 and 0.065–0.091 μg L−1, respectively. The repeatability and reproducibility of the method (R%) were estimated to be 4.02–6.83 % and 8.51–10.34 %, respectively. Also comparing the mean concentrations of the samples collected by the NTD and the NIOSH 5528 method, revealed no statistically significant differences between the methods (p > 0.05). Overall, the COF-packed NTD was identified as an appropriate and reliable method for sampling and extracting occupational and ambient contaminants in the workplace air.

Regenerable and Highly Stable Two-Dimensional Imine-Based Covalent Organic Framework for Simultaneous Rapid Detection and Adsorption of Cu2+ Ions.

The development of efficient fluorescent probes and adsorbents for detecting and removing Cu2+, which pose potential environmental and health risks, is a highly active area of research. However, achieving simultaneously improved fluorescence detection efficiency and enhanced adsorption capacity in a single porous probe remains a significant challenge. In this study, we successfully synthesized a two-dimensional imine-based TAP-COF using 2,4,6-triformylphloroglucinol and tri(4-aminophenyl)amine as raw materials. TAP-COF exhibited excellent properties, including a large specific surface area of 685.65 m2·g-1, exceptional thermal stability (>440 °C), chemical stability, temporal stability, and recyclability. Fluorescence testing revealed that TAP-COF exhibited remarkable specificity and high sensitivity for detecting Cu2+. The fluorescence mechanism, in which the excited state intramolecular proton transfer was impeded by the interaction of Cu2+ with C═O and C-N bonds on TAP-COF upon the addition of Cu2+, was further elucidated through experimental and theoretical methods. Furthermore, the adsorption capacity of TAP-COF toward Cu2+ was investigated, confirming the excellence of TAP-COF as a fluorescent probe and adsorbent for the specific detection and removal of Cu2+. This work holds significant implications for improving environmental and human health concerns associated with Cu2+ contamination.

Aptasensor based on high surface area covalent organic framework for simple and ultrasensitive detection of sarcosine in the diagnosis of prostate cancer

In this study, an electrochemical aptasensor was developed for the specific detection of sarcosine using a covalent organic framework (COF). The imine-based two-dimensional COF was synthesized through a solvothermal process using terephthaldehyde and melamine. This resulted in the formation of a structure that is highly porous and has a unique surface area of 908 m2/g. The produced biosensor demonstrated a significant linear relationship between charge transfer resistance (Rct) and various concentrations of sarcosine in blood serum samples. The aptasensor had two linear ranges, spanning from 0.5 fM to 700 fM and 10 pM to 0.12 nM, respectively, with a detection limit of 0.15 fM. The incorporation of high surface area COFs in the aptasensor design offers a promising combination of sensitivity, stability, and specificity. This combination creates a valuable device for diagnosing and monitoring of prostate cancer and potentially other diseases.

Missing-linker defects in a covalent organic framework photocatalyst for highly efficient synthesis of tetrahydroquinoline

A unique imine-based covalent organic framework heterogeneous catalyst with missing-linker defects was designed and synthesized, and the catalyst exhibited excellent photocatalytic activity for efficient synthesis of tetrahydroquinolines.

Push–pull–pull interactions of 2D imide–imine-based covalent organic framework to promote charge separation in photocatalytic hydrogen production

This study demonstrated the synthesis of imide–imine based COFs, boosting the electrical conductivity in the network due to the presence of dual electron-acceptor centers. This makes them efficient for light-induced hydrogen evolution reactions.

Covalent organic framework (COF)-decorated NH2-functionalized MXene sheets for thermomechanical and UV-shielding performance of epoxy nanocomposite coatings

Designing a multi-purpose epoxy nanocomposite with promising mechanical and weathering properties is yet a challenge to be explored. Meeting all above mentioned properties in one nanocomposite requires a rational design of hybrid nanomaterials benefiting from their synergistic effect. To this end, a robust imine-based COF was synthesized on an MXene substrate, forming an inorganic-organic assembly as filler for a multi-functional epoxy coating. As a proof of concept, the tensile strength of the loaded coating was almost twice the unloaded one, confirming the efficient stress transfer due to mechanically robust MXene and compatibility improvement of the hybrid assembly in the presence of the COF. Furthermore, the physical barrier of MXene, along with the nitrogen-rich structure of COF, resulted in efficient thermal insulating properties of the fabricated nanocomposite coatings. Moreover, the weathering test illustrated that the conjugated structure of the COF enhanced the epoxy film weathering stability after 7 days of UV exposure, which is promising among competitors. This high yield, facile processing, and rational design provide new insight into the application of MXene-conjugated COF in epoxy nanocomposite coatings.

A novel needle trap device containing an imine-based covalent organic framework for sampling of PAHs in soil

A new adsorbent based on covalent organic frameworks (COFs) was synthesized through imine condensation and utilized for the needle trap device (NTD) microextraction. The structure and surface properties of the synthesized adsorbent were investigated using Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Brunauer-Emmett-Teller (BET) surface area analysis, and X-ray diffraction analysis (XRD), techniques. The designed microextraction system was coupled with a gas chromatography flame ionization detector (GC-FID) and used for the measurement of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil samples. The statistical response surface methodology (RSM) with a Box-Behnken design (BBD) was used to investigate and optimize the effective variables in the extraction process, including extraction temperature, extraction time, and sample flow rate. Under optimal conditions, the calibration curves were linear over the range of 0.2–10000 ng g−1. The detection limit of the method for the PAHs was obtained in the range of 0.01–0.05 ng g−1. The RSD values were obtained 7.6–12.9%. The NTD-GC-FID procedure based on COF adsorbent was successfully utilized for the analysis of PAHs in contaminated soil samples.

Palladium-Nanoparticle-Decorated Covalent Organic Framework Nanosheets for Effective Hydrogen Gas Sensors

Covalent organic frameworks (COFs) are attractive for chemical sensing applications because their structures contain delocalized π electrons and active sites that are evenly distributed. COF-based chemical sensors are largely restricted to luminescence and absorption-based mechanisms of sensing. The present study demonstrates the potentiality of exfoliated COF nanosheets hybridized with palladium nanoparticles (Pd NPs) for effective chemiresistive H2 sensing, for the first time. The hybridization of Pd NPs with the nanosheets of an imine-based covalent organic framework (ePd@TpPa-SO3H COF) has turned an innocent bulk COF into an effective chemiresistive H2 sensor.

Orally delivered 2D covalent organic frameworks releasing kynurenine generate anti-inflammatory T cell responses in collagen induced arthritis mouse model

Covalent organic framework (COF) crystalline biomaterials have great potential for drug delivery since they can load large amounts of small molecules (e.g. metabolites) and release them in a controlled manner, as compared to their amorphous counterparts. Herein, we screened different metabolites for their ability to modulate T cell responses in vitro and identified Kynurenine (KyH) as a key metabolite that not only decreases frequency of pro-inflammatory RORgt + T cells but also supports frequency of anti-inflammatory GATA3+ T cells. Moreover, we developed a methodology to generate imine-based TAPB-PDA COF at room temperature and loaded these COFs with KyH. KyH loaded COFs (COF-KyH) were able to then release KyH in a controlled manner for 5 days in vitro. Notably, COF-KyH when delivered orally in mice induced with collagen-induced rheumatoid arthritis (CIA) were able to increase frequency of anti-inflammatory GATA3+CD8+ T cells in the lymph nodes and decrease antibody titers in the serum as compared to the controls. Overall, these data demonstrate that COFs can be an excellent drug delivery vehicle for delivering immune modulating small molecule metabolites.

Development of the new pH-driven carrier from alginate/carboxymethyl starch bio-coated co-drugs@COF-OH for controlled and concomitant colon cancer treatment

To develop a new more efficient colon cancer treatment bio-vehicle, in frontier research, for the first time, an attempt has been made to design a unique colon-targeted bio-carrier containing polysaccharides along with nanoporous materials. So, at first, an imine-based covalent organic framework (COF-OH) with respectively an average pore diameter and surface area at 8.5058 nm and 208.29 m2·g−1 was fabricated. In the next step, about 41.68 % and 95.8 % of 5-fluorouracil (5-Fu) and curcumin (CUR) respectively were loaded on COF-OH, and 5-Fu + CUR@COF-OH was achieved. Due to the higher rate of drug releases in simulated stomach media, 5-Fu + CUR@COF-OH was coated with a mixture of alginate (Alg) and carboxymethyl starch (CMS) via the ionic crosslinking (Alg/CMS@(5-Fu + CUR@COF-OH)). Findings displayed that the use of polysaccharide coat reduce the drug releases in simulated gastric and improved it in simulated intestinal and colonic fluids. The beads swelled about 93.33 % under simulated gastrointestinal conditions, but this value was found higher in the simulated colonic environment and reached 326.67 %. The hemolysis rate lower than 5 %, as well as the cell viability higher than 80 %, were the main showing signs of system biocompatibility. Altogether, the results of the preliminary investigations can highlight the potential of the Alg/CMS@(5-Fu + CUR@COF-OH) for colon-specific drug delivery.

In situ growth of imine-based covalent organic framework as stationary phase for high-efficiency electrochromatographic separation

Design of the smart stationary phases, which can improve the separation efficiency is an essential work in the capillary electrochromatography (CEC). Owing to good excellent properties, covalent organic frameworks (COFs) have showed promising potential in the area of separation science. Here, a micro- and mesoporous COF TAPB-BTCA with adequate available interaction sites and outstanding mass transfer performance was first exploited as a stationary phase for high-efficiency in capillary electrochromatography. Through in situ growth approach, the COF TAPB-BTCA coated capillary column was facilely prepared at room temperature. The separation ability of the COF TAPB-BTCA coated capillary column was studied. The fabricated column showed high efficiency for the separation of six types of small molecular compounds, including alkylbenzenes, chlorobenzenes, phenols, parabens, vanillin and related phenolic compounds, and non-steroidal anti-inflammatory drugs (NSAIDs). The maximum theoretical plates reached up to 293,363 N/m for phloroglucinol, showing significantly improved column efficiency in comparison to previous reported COFs-based columns. In addition, the mass loadability for methylbenzene was achieved to 1.44 mg/mL. Also, excellent reproducibility and stability were obtained on the COF TAPB-BTCA coated columns. The relative standard deviations of intra-day (n = 3), inter-day (n = 3) and three batch tubes were all less than 2%, and no obvious change was presented in separation performance after the column was used 120 runs. This COF TAPB-BTCA-based stationary phase would be a promising candidate for high-efficiency chromatographic separation.

Templated synthesis of imine-based covalent organic framework hollow nanospheres for stable potassium-ion batteries

Covalent organic frameworks (COFs), as highly tunable porous crystalline materials, have promising applications in potassium-ion batteries (PIBs) due to their abundant charge carrier transport channels and excellent structural stability. However, the excessive stacking of interlayer electron clouds makes it difficult to expose internal active sites. Strategies to design functional COFs with controllable morphology and copious active sites are promising but still challenging. Herein, by utilizing the condensation between 1,3,5-triformylbenzene (TFB) and p-phenylenediamine (PPD) and using amino-modified SiO2 nanospheres as templates, we synthesize core-shell NH2-SiO2@TP-COF. Through NaOH etching of NH2-SiO2@TP-COF, we obtain imine-based TP-COF hollow nanospheres, which shows excellent potassium storage performance when applied to the anode for PIBs. Ex-situ analysis and density functional theory calculations reveal that CN groups and benzenes are active sites for K+ storage.

Pd(II)-immobilized on a novel covalent imine framework (COF-BASU1) as an efficient catalyst for asymmetric Suzuki coupling

This study reports the synthesis of a new imine-based covalent organic framework (COF) stabilizer through the reaction between 1,4-phenylenediamine and 4,4′,4″-(benzene-1,3,5-triyltris(oxy))tribenzaldehyde (COF-BASU1). Next, palladium (Pd) species were decorated on the synthesized COF, and the obtained Pd/COF-BASU1 composite was used as a heterogeneous catalyst in the asymmetric Suzuki-Miyaura coupling (SMC) reaction. The easily prepared imine-linked COF-BASU1 possesses a two-dimensional eclipsed layered-sheet structure, making its incorporation with metal ions feasible. Via a simple post-treatment, a Pd(II)-containing COF, Pd/COF-BASU1, was accordingly synthesized, which showed excellent catalytic activity in catalyzing the asymmetric Suzuki-Miyaura coupling reaction with high stereoselectivity. The eclipsed imine structure provide chiral environment to achieve the asymmetric Suzuki-Miyaura coupling reactionplay. Afterward, the chiral high-performance liquid chromatography (HPLC) was conducted to examine the axially chiral products generated through these reactions. These products were separated via the HPLC method. This technique is based on the function of some columns whose packing groups can segregate chiral products. The fabricated nanocatalyst was structurally authenticated using various analytical techniques such as FT-IR, XRD, FE-SEM, TGA, EDS, ICP-AES, and BET. The outcomes indicated that Pd/COF-BASU1 nanocomposite is a suitable catalyst to synthesis axially chiral biaryl derivatives with high stereoselectivity (up to 85.37% ee).

Catalyst regulated interfacial synthesis of self-standing covalent organic framework membranes at room temperature for molecular separation

Covalent organic framework (COF) membranes have shown enormous potential for molecular separation due to their large surface areas and pre-designable structures. However, the mild and convenient preparation of COF membranes with high crystallinity has remained a significant challenge. In this work, we reported on a facile liquid–liquid interfacial polymerization method to fabricate self-standing imine-based COF membranes with excellent crystallinity and a tunable thickness at room temperature. Polymerization was confined at the immiscible organic solvent–water interface when the monomers in the dichloromethane met the catalyst aqueous solution. This unique design concept exploited the rapid formation of COF monolayers at the liquid–liquid interface to control catalyst diffusion and structural rearrangement, achieving high crystallinity of the COF membrane. Moreover, the thickness of the self-standing COF membranes could be regulated from 50 nm to 1 μm through the flexible regulation of the growth process. Benefiting from the large surface area of the COF membranes (378 m2/g) and the intensive π–π conjugate effect between the COFs and organic dyes, the obtained COF membranes exhibited high adsorption capacities toward Chrome Black T and Rose Bengal. This work may open a viable avenue to easily and mildly prepare COF membranes for water treatment.

Influence of the Building Unit on Covalent Organic Frameworks in Mediating Photo-induced Energy-Transfer Reversible Complexation-Mediated Radical Polymerization (PET-RCMP).

Two imine-based covalent organic framework photocatalysts with different building units, TPB-DMTA-COF and TAT-DMTA-COF, for photo-induced energy transfer reversible complexation-mediated radical polymerization (PET-RCMP) were developed and investigated, producing ideal polymers with accurate molecular weight and moderate dispersity under visible light irradiation. The chain extension and spatiotemporal control experiments revealed the high chain-end fidelity of polymers and the compatibility of RCMP processes in both bulk and aqueous system. Moreover, density functional theory (DFT) calculations verified that heteroatom-doped TAT-DMTA-COF exhibits higher activities for weakening C-I bond energy barrier, which promotes PET-RCMP polymerization performance. This work demonstrates that rational adjustment of building block for constructing COF heterogeneous photocatalyst can enhance the catalytic performance of PET-RCMP, providing a design methodology for the development of polymeric organic photoelectric semiconductor catalysts to mediate RCMP.

Exploration of an imine-based covalent organic framework for the solid phase extraction of nitroimidazoles in milk and meat samples

TAPT-AN-COF, an imine-based covalent organic framework, was synthesized by a solvothermal method of 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (TP) and 4,4′,4″-(1,3,5-Triazine-2,4,6-triyl)trianiline (TAPT). The structure was characterized and tested by several techniques, revealing that the material had good stability and high specific surface area. The adsorption experiment demonstrates that the adsorption isotherm of TAPT-AN-COF followed the Freundlich isothermal equation, while its adsorption kinetics conformed to the pseudo second-order kinetic model. After characterization, the prepared TAPT-AN-COF was used to separate and enrich nitroimidazoles (NDZs) in milk and meat as the solid-phase extraction (SPE) adsorbent. The effects of adsorbent dosage, pH value, washing solvent, elution solvent type and volume on recoveries were studied. Under the optimal conditions, a method for the determination of NDZs in milk and meat samples was established based on high performance liquid chromatography-ultraviolet detection(HPLC-UVD).The result showed that NDZs had good linearity in the concentration range of 25-500 µg·kg−1, and the determination coefficients (r2) were all above 0.99. When spiked at 25, 50 and 125 µg·kg−1, the recoveries of three kinds of food samples ranged from 64.5% to 85.3%, the limits of detection (LODs) were between 2 and 10 µg·kg−1, and the relative standard deviations were all below 15.9%. In addition, the recoveries of NDZs didn't decrease significantly after being reused for 6 times, showing that TAPT-AN-COF has excellent reusability. Compared with HLB and MCX sorbents, TAPT-AN-COF had better extraction efficiency and qualified purification efficiency. The established method had a satisfying performance on the determination of NDZs in food samples.

In Situ Synthesis of Gold Nanoclusters in Covalent Organic Frameworks with Enhanced Photodynamic Properties and Antibacterial Performance

In this work, ultrasmall gold nanoclusters (AuNCs) have been in situ synthesized in nanopores of covalent organic framework (COF) nanoparticles, which exhibited enhanced fluorescence, improved photosensitizing capabilities, and promising antibacterial performance. A small organic molecule, 1-vinylimidazole (Vim), was diffused into the nanopores of imine-based COFs and served as a reducing agent and capping ligand for the in situ synthesis of ultrasmall AuNCs. The as-obtained AuNCs were homogeneously distributed throughout the COF nanoparticles whose fluorescence intensity was enhanced remarkably. Due to the efficient electron transfer between AuNCs and COFs and increased separation of photogenerated electron-hole pairs, the light-triggered reactive oxygen species (ROS) production of COFs was prominently enhanced by AuNCs. Moreover, the obtained nanocomposites exhibited an efficient photodynamic killing behavior on Escherichia coli under visible light exposure. Thus, we provide a facile strategy to prepare COF/AuNC nanocomposites for ROS-related applications.

Synthesis and Characterization of a Crystalline Imine-Based Covalent Organic Framework with Triazine Node and Biphenyl Linker and Its Fluorinated Derivate for CO2/CH4 Separation.

A catalyst-free Schiff base reaction was applied to synthesize two imine-linked covalent organic frameworks (COFs). The condensation reaction of 1,3,5-tris-(4-aminophenyl)triazine (TAPT) with 4,4′-biphenyldicarboxaldehyde led to the structure of HHU-COF-1 (HHU = Heinrich-Heine University). The fluorinated analog HHU-COF-2 was obtained with 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxaldehyde. Solid-state NMR, infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis confirmed the successful formation of the two network structures. The crystalline materials are characterized by high Brunauer–Emmett–Teller surface areas of 2352 m2/g for HHU-COF-1 and 1356 m2/g for HHU-COF-2. The products of a larger-scale synthesis were applied to prepare mixed-matrix membranes (MMMs) with the polymer Matrimid. CO2/CH4 permeation tests revealed a moderate increase in CO2 permeability at constant selectivity for HHU-COF-1 as a dispersed phase, whereas application of the fluorinated COF led to a CO2/CH4 selectivity increase from 42 for the pure Matrimid membrane to 51 for 8 wt% of HHU-COF-2 and a permeability increase from 6.8 to 13.0 Barrer for the 24 wt% MMM.

Ultralarge Free-Standing Imine-Based Covalent Organic Framework Membranes Fabricated via Compression.

Demand continues for processing methods to shape covalent organic frameworks (COFs) into macroscopic objects that are needed for their practical applications. Herein, a simple compression method to prepare large‐scale, free‐standing homogeneous and porous imine‐based COF‐membranes with dimensions in the centimeter range and excellent mechanical properties is reported. This method entails the compression of imine‐based COF‐aerogels, which undergo a morphological change from an elastic to plastic material. The COF‐membranes fabricated upon compression show good performances for the separation of gas mixtures of industrial interest, N2/CO2 and CH4/CO2. It is believed that the new procedure paves the way to a broader range of COF‐membranes.