COF Material Research Articles

The Feasibility of Using Electrostatic Interactions for Immobilizing Ru-bda Catalysts in Covalent Organic Framework: A Proof-of-concept.

Heterogenetization of molecular catalysts effectively resolves the separation issues of homogeneous catalysts and expands their application scenarios. In recent years, more and more studies have been using non-covalent interactions to achieve the heterogenization of molecular catalysts. Herein, electrostatic attraction was used to immobilize molecular catalysts, Ru-bda small molecular catalysts in COF materials, where the charged Ru-bda catalysts were immobilized in the oppositely charged COF with a high [Ru] loading content of ~0.2 mmol [Ru] g-1 COF. The leakage experiment verified that the immobilization of Ru-bda catalysts in COF by electrostatic interactions is stable in 0.1 M HClO4 and less than 5% of molecular Ru-bda catalysts were leached into the solution in 2 hours. The chemical water oxidation experiment was conducted as a model catalysis reaction to verify the feasibility of using electrostatic interactions for immobilizing Ru-bda catalysts in COFs. The prepared Ru(bda)@COFs demonstrate a high catalytic activity of 268 μmol L-1 s-1 O2 for chemical water oxidation, illustrating the electrostatic attractions between COF and small molecules that can be used to immobilize homogeneous catalysts in heterogeneous materials. However, the robustness of COF material itself under catalytic conditions should be considered in practical applications.

Selective lead capture using amide-containing COFs: A novel strategy for efficient soil remediation

A critical consideration in the application of phytoremediation to remediate sludge soil contaminated with heavy metals is the potential for leaching risks that prevail prior to the efficient uptake of these metals by plants. The most cost-effective method is to use heavy metal stabilizers with selective adsorption. A novel amide-based COF material (COF-TH) has been synthesized as a heavy metal stabilizer for Pb. COF-TH exhibits significant selectivity for Pb in five-metal-mixed solutions, with a distribution coefficient KD as high as 3279 mL·g–1, which was more than 7.3 times that of other heavy metals. The maximum adsorption capacity of COF-TH for Pb was 189 mg·g–1. The adsorption fitted Langmuir model and intra-particle diffusion model, and satisfied pseudo-second-order kinetic model. The excellent selectivity and adsorption performance originate from the complexation between abundant amide groups and Pb ions. Pot experiments and leaching assays confirm that COF-TH decreased Pb leachate concentrations by 77.8 % without significantly decreasing total phytoextracted amounts of other heavy metals, due to the high selectivity of COF-TH to Pb. Additionally, its positive impact on plant growth and microbial diversity makes it a promising soil remediation agent. This investigation offers a novel approach to mitigate the leaching risk of a specific heavy metal Pb during sludge land application by integrating soil phytoremediation with stabilization techniques.

The benzothiadiazole-based COF/CNT composite as host for efficient Li-S batteries with excellent cycling stability

The cycle stability and dynamic performance of lithium-sulfur (Li-S) batteries have been major challenges, which primarily arise from the dissolution and migration of the soluble reaction intermediates during cycling, as well as the low conductivity of sulfur. In this study, a covalent organic framework containing heteroatoms (B/N/S) is uniformly wrapped on the outer surface of carbon nanotubes to form a nanocomposite (COF@CNT). The composite combines the excellent electrical conductivity of CNTs and the COF material containing evenly distributed heteroatoms that can adsorb and catalyze polysulfides. B atom on COF can be chemically adsorbed by the negatively charged SX2−, S and N atoms can interact with Li+ to reduce the “shuttle effect”. At the same time, the heteroatoms promote the liquid-to-solid phase conversion of the soluble long-chain Li2Sx (4 ≤ x ≤ 8), and the subsequent conversion of solid Li2S2 to Li2S, thereby enhancing the dynamic kinetics. As a cathode material, COF@CNT/S exhibits an initial specific capacity of 1176 mAh/g at 0.2 C, maintaining a capacity of 688 mAh/g after 1000 cycles at 1 C with a Coulombic efficiency of 99.7 %. Notably, a quite low capacity decay rate of 0.029 % per cycle has been achieved, making the COF@CNT/S cathode one of the most stable electrode material for Li-S batteries. This work provides insights into improving cycle performance of Li-S batteries, and also enriches the current study on host materials.

A Type of Ferrocene-Based Derivative FE-1 COF Material for Glycopeptide and Phosphopeptide Selective Enrichment.

In this work, a new type of FE-1 COF material is prepared by a reversible imine condensation reaction with diaminoferrocene and diaminodiformaldehyde as materials. The material is connected by imine bonds to form a COF skeleton, and the presence of plenty of nitrogen-containing groups gives the material good hydrophilicity; the presence of metal Fe ions provides the material application potential in the enrichment of phosphopeptides. According to the different binding abilities of N-glycopeptide and phosphopeptide on FE-1 COF, it can simultaneously enrich N-glycopeptide and phosphopeptide through different elution conditions to realize its controllable and selective enrichment. Using the above characteristics, 18 phosphopeptides were detected from α-casein hydrolysate, 8 phosphopeptides were detected from β-casein hydrolysate and 21 glycopeptides were detected from IgG hydrolysate. Finally, the gradual elution strategy was used; 16 phosphopeptides and 19 glycopeptides were detected from the α-casein hydrolysate and IgG hydrolysate. The corresponding glycopeptides and phosphopeptides were identified from the human serum. It proves that the FE-1 COF material has a good enrichment effect on phosphopeptides and glycopeptides.

Covalent Organic Framework Interlayer Spacings as Perfectly Selective Artificial Proton Channels

AbstractBiological proton channels have perfect selectivity in aqueous environment against almost all ions and molecules, a property that differs itself from other biological channels and a feature that remains challenging to realize for bulk artificial materials. The biological perfect selectivity originates from the fact that the channel has almost no free space for ion or water transport but generates a hydrogen bonded wire in the presence of protons to allow the proton hopping. Inspired by this, we used the interlayer spacings of covalent organic framework materials consisting of hydrophilic functional groups as perfectly selective artificial proton channels. The interlayer spacings are so narrow that no atoms or molecules can diffuse through. However, protons exhibit a diffusivity in the same order of magnitude as that in bulk water. Density functional theory calculations show that water molecules and the COF material form hydrogen bonded wires, allowing the proton hopping. We further demonstrate that the proton transport rate can be tuned by adjusting the acidity of the functional groups.

Covalent Organic Framework Interlayer Spacings as Perfectly Selective Artificial Proton Channels.

Biological proton channels have perfect selectivity in aqueous environment against almost all ions and molecules, a property that differs itself from other biological channels and a feature that remains challenging to realize for bulk artificial materials. The biological perfect selectivity originates from the fact that the channel has almost no free space for ion or water transport but generates a hydrogen bonded wire in the presence of protons to allow the proton hopping. Inspired by this, we used the interlayer spacings of covalent organic framework materials consisting of hydrophilic functional groups as perfectly selective artificial proton channels. The interlayer spacings are so narrow that no atoms or molecules can diffuse through. However, protons exhibit a diffusivity in the same order of magnitude as that in bulk water. Density functional theory calculations show that water molecules and the COF material form hydrogen bonded wires, allowing the proton hopping. We further demonstrate that the proton transport rate can be tuned by adjusting the acidity of the functional groups.

Metal/covalent organic framework@silica double-layer core–shell composite: A high-performance liquid chromatographic stationary phase

Nowadays, metal/covalent organic frameworks (MOFs/COFs) have shown bright application prospects in the field of chromatographic separation due to their unique porous characteristics. However, chromatographic stationary phases prepared with only a single MOF or COF material have a few inherent defects, which keep them from satisfying the actual application fully. In contrast, MOF/COF composites have more superior properties that are more favorable for separation. Therefore, this study innovatively proposed a new strategy to apply MOF/COF modified silica composites for high performance liquid chromatography (HPLC). To be specific, using ethylenediamine as a linker, Zn-based MOF and imine-based COF were modified on the aminated silica gel layer by layer, and a MOF-5/COF@silica bilayer core–shell composite was successfully prepared. The composite of MOF and COF can complement each other in realizing the advantages of the two materials. The research results show that the MOF-5/COF composite can provide a large number of active sites with a variety of interaction forces for the analytes. The MOF-5/COF@silica stationary phase can separate a wide range of hydrophilic and hydrophobic substances, which can also be used for the effective detection of environmental hormones in lake water samples. This study verifies the feasibility of preparing MOF and COF co-modified silica gel composite to improve the separation performance of stationary phase, and demonstrates the superiority of the double-layer porous core–shell structure for efficient HPLC analysis. This provides a new idea for the further development and application of porous materials in the field of HPLC.

A pH-regulated fluorescence covalent organic framework for quantitative water content detection in methanol

In this paper, we designed and synthesized a two-dimensional fluorescent covalent organic framework (TAPB-DMTP-COF) for the precise determination of H2O content in methanol. The COF was synthesized using two typical monomers by grinding method, which significantly reduced the synthesis time. By adjusting the pH value of the COF suspension to 4.0, the portion of the COF material structure is disrupted, thereby mitigating π-π stacking and resolving the aggregation-caused quenching (ACQ) effect. Consequently, the non-fluorescent TAPB-DMTP-COF exhibited blue-purple fluorescence emission in methanol. At the same time, it is observed that in the presence of H2O, there is a red shift in the maximum fluorescence emission peak of TAPB-DMTP-COF, which correlates with the H2O content within a specific range. Notably, this redshift demonstrates a linear relationship with H2O content from 4% to 80% in methanol. Our work presents novel insights for efficient analysis and detection of H2O content in methanol and could be used for H2O detection in other organic solvents.

A Pt nanoenzyme- and BODIPY-loaded nanoscale covalent organic framework for relieving intratumoural hypoxia to enhance photodynamic therapy.

Herein, we report a composite COF material loaded with a Pt nanoenzyme and an organic photosensitizer BODIPY, synthesized via a stepwise post-synthetic modification. The obtained Pt@COF-BDP nanoparticles can efficiently and continuously convert H2O2 to O2, thereby increasing the efficiency of single-linear oxygen production and achieving efficient tumor inhibition.

Integrated High Voltage, Large Capacity, and Long‐Time Cyclic Stability of Lithium Organic Battery Based on a Bipolar‐Type Cathode of Triphenylamine‐Hydrazone COF Material

AbstractCovalent organic frameworks (COFs) are a promising class of electrode materials for lithium‐ion batteries. However, the reported cathodes of COFs typically exhibit low potential and voltage plateaus, which limits their further applications. In this work, a new hydrazone three‐dimensional (3D) COF material with bipolar characteristics, PTB‐DHZ‐COF, is constructed by Schiff base condensation of monomeric PTB with DHZ. The cell with PTB‐DHZ‐COF as the cathode exhibits a wide potential range of 1.2–4.2 V and a discharge voltage plateau of over 3.6 V. PTB‐DHZ‐COFX composites are successfully prepared by compounding PTB‐DHZ‐COF with carbon nanotubes. PTB‐DHZ‐COF40 cells show high specific capacity (114.24 mAh·g−1 at 1000 mA·g−1), excellent cycling capability (86.3% capacity retention after 5000 cycles), and ultra‐high energy density (489 Wh·kg−1 at 50 mA·g−1). This work provides a new strategy for the design of new COF materials and the development of high‐performance organic energy storage electrodes.

Thioether-functionalized covalent organic framework for mercury removal

Post-synthetic modification of COFs via pendant groups or secondary functional groups of the skeleton structure is an important strategy to introduce new properties or enhance existing properties of COFs. Herein, we take advantage of the inherent advantages of COFs and introduce thioether functional groups into a reasonably designed novel COF (TAPA-TFPD) through a post-synthetic modification strategy to create a promising thioether functionalized adsorbent for mercury removal. The thioether functionalized COF material, TAPA-TFPD-S-S, exhibited high mercury removal efficiency from aqueous solution, and the adsorption capacity of Hg2+ was up to 593 mg/g. This efficient mercury removal performance can be attributed its well-defined pore channel sufficient to facilitate Hg2+ diffusion together with high concentration of sulfide chelating sites distributed uniformly throughout the pore wall. The strong interaction between Hg2+ and the thioether groups of TAPA-TFPD-S-S was confirmed by XPS. In addition, TAPA-TFPD-S-S also has the ability to capture Hg0 from the air. The efficient capture of mercury species from aqueous solution and air and the excellent reusability make the prepared thioether-functionalized COFs have great development potential in practical applications. Our work not only demonstrates the utilization of thioether-functionalized COFs in the removal of heavy metal contaminants, but also highlights the exceptional potential of post-synthetic modification strategies in the development of environmental remediation materials.

Bimetallic atom synergistic covalent organic framework for efficient electrochemical nitrate reduction

Electrochemical reduction has emerged as an effective method to remove nitrate from industrial wastewater. Nevertheless, this method has been largely restricted by the lack of low-cost and efficient electrocatalysts. Here, we demonstrate a porous two-dimensional covalent organic framework (2D COF) material as a promising electrocatalyst, which is obtained via a Schiff base reaction by combining copper phthalocyanine with bipyridine sites for precise copper coordination. The bidentate coordinated COF material has a robust framework and stable chemical property, allowing the isolated Cu sites to be embedded into the regular pores with controlled and uniformly dispersed active centers. The well-defined design of the reaction monomers makes the COF material to trap nitrate ions more easily from aqueous solution. By rationally combining the synergistic effect of 2D COF and Cu active sites, the CuTAPc-CuBPy-COF electrocatalyst shows much higher nitrate reduction efficiency than CuTAPc-BPy-COF under low superpotential and different nitrate concentrations. The high NO3− conversion (90.3 %) and NH3 selectivity (69.6 %) are achieved. To our best acknowledge, this is the first demonstration of bi-copper-based COF material for NO3−RR electrocatalysis, which provides a new direction for the rational design of COFs as significant electrocatalysts for nitrate reduction.

Construction of two-dimensional fluorescent covalent organic framework nanospheres for the detection and removal of tetracycline

The abuse of tetracycline (TCH) leads to a large number of residues in water and agricultural products, which poses a serious threat to the ecological environment and human health. Therefore, it is necessary to develop materials that can simultaneously detect and effectively remove tetracycline. In this paper, we designed and synthesized a two-dimensional fluorescent covalent organic framework nanosphere (ETTA-Nap-COF) for the detection and efficient removal of tetracycline. Due to the efficient photoinduced electron transfer (PET) between ETTA-Nap-COF and TCH, a significant fluorescence enhancement of the COF material could be observed in the presence of TCH under UV light illumination, thereby realizing a selective and sensitive fluorescence sensing of tetracycline. At the same time, the combination of π-π stacking and the porous network structure of ETTA-Nap-COF is conducive to the efficient adsorption and removal of tetracycline, and the maximum adsorption capacity can reach 457.72 mg/g. The ETTA-Nap-COF material has good recyclability and high structural stability. Our work provides a new research idea for fluorescence detection and efficient removal of tetracycline.

Dual application of a cyano-containing covalent organic framework: Photocatalytic degradation of dyes with fluorescence detection studies

In this work, a donor-acceptor type COF material named HDU-105 was synthesized and applied as a metal-free catalyst for the photocatalytic degradation of organic dyes. The photocatalytic experiments demonstrated that the degradation rate of RhB and MB reached 99.9% under various concentrations (5–20 ppm) and over a wide pH range (3–10), but consuming only 4 mg of HDU-105. Furthermore, the degradation rate for both organic dyes still maintained approximately 98% after four cycles. ESR spectra experiments revealed that ⋅O2- and ‧OH in HDU-105 may dominated the photocatalytic degradation as the main reactive substances. HDU-105 also exhibited an extremely low limit for MB detection with merely 5.7 ng L−1. All of this proves the high stability, remarkable reusability and fluorescence stability of HDU-105. To conclude, the cyano-containing COF material has potential for dyes removal application with an ultra-low MB detection limit.

Application of fully conjugated covalent organic frameworks in photocatalytic carbon dioxide reduction performance.

Photocatalytic reduction of carbon dioxide into useful feedstocks has attracted increasing attention. In this study, a fully conjugated COF material COF-TMT-A with the main structure containing an alkyne group and triazine part was synthesized using sp2-carbon-carbon double bond (CC) linked COF as a research target. The prepared COF materials were characterized in detail by FT-IR, PXRD, and 13C solid-state NMR. The introduction of an alkyne group not only enhanced the conjugated π-electron leaving domain but also optimized the electronic band structure and significantly improved the photocatalytic activity. The selectivity for the product HCOO was as high as 99%. A 10 h photocatalytic CO2 reduction experiment was carried out, and COF-TMT-A showed a significantly higher HCOO- yield of about 43 μmol compared with COF-701 and the ligand.

A covalent organic framework-based nanoreactor for enhanced photothermal therapy via inhibiting intracellular heat defense systems.

Herein, we report an indocyanine green (ICG)-decorated and glucose oxidase (GOx)-loaded nanoscale composite COF material via a stepwise post-synthetic modification. The obtained GOx@COF-ICG can achieve synergistic inhibition of intracellular heat defense systems through starvation therapy to enhance photothermal therapy of tumors.

Dynamic Disulfide Bonds Contained Covalent Organic Framework Modified Separator as Efficient Inhibit Polysulfide Shuttling in Li–S Batteries

First, a new covalent organic framework (HUT9) with disulfide bonds have been synthesized, which can be grown in situ on carbon nanotubes (CNT) as separator modification material for Li–S batteries. Experiments and DFT showed that disulfide bond could effectively capture soluble polysulfides (LiPSs) during discharge process, while promoting the conversion of LiPSs to Li2S/Li2S2 and improve the utilization rate of sulfur. The polar groups of the covalent organic framework provide chemisorption, and an interwoven network of HUT9@CNT (physical constraints) cooperates to inhibit the transport of LiPSs. The modified battery has a capacity retention rate of 83% (decay rate of 0.032% per cycle) after 500 cycles at 1 C. Specifically, the battery with 2-HUT9@CNT modified separators (sulfur loading: 4 mg cm–2, sulfur content: 80%, electrolyte/sulfur ratio: 10 mL g–1) can reach 3.4 mAh cm–2 area capacity after 50 cycles. This work enriched the COF material system and provided guidance for the application of organic materials in an energy storage system.

Ionic modification on COF with rare earth ions for the selective optical sensing and removal of picronitric acid.

In this work, we reported a modified COF material for trinitrophenol (TPA) ratiometric sensing and removal. Here a cationic covalent organic framework (C-COF) was prepared as host, while two Tb(III)-based ions were doped into C-COF as probe by ionic exchange reaction with probe loading level of ∼15%. In the absence of TPA, weak Tb(III) emission (489nm, 545nm, 585nm) and bright red COF emission were observed (633nm). The addition of TPA increased Tb(III) emission and decreased COF emission, following linear response within TPA concentration region of 0-9μM. Their limit of detection values were determined as 0.9μM and 4.5μM, respectively. Corresponding working equations were fitted as I/I0=1.225+6.914×105M-1[TPA], R2=0.997 for TbCF3-COF and I/I0=1.063+9.222×104M-1 [TPA], R2=0.993 for TbDBM-COF. TbCF3-COF showed better sensing performance than TbDBM-COF, due to its suitable ligand triplet energy level. Their sensing mechanism was revealed as dopant "replacement", where dopant molecules loaded in COF micropore were replaced by TPA molecules, accompanied with energy competing on Tb(III) 5D4 level, showing ratiometric signals. Good selectivity and removal capacity (∼7.4wt%) for TPA were achieved.

Functionalized Graphene Quantum Dots Modified Dioxin-Linked Covalent Organic Frameworks for Superior Lithium Storage.

Covalent organic framework, as an emerging porous nano-frame structure with pre-designed structure and custom properties, has been demonstrated as a prospective electrode for rechargeable Li-ion batteries. For improving the reversible capacity and long-term cycle stability of COF materials, we propose a GQDs modified COF material (COF-GQDs) and apply it as the anode for LIBs for the first time. This COF-GQDs electrode delivers enhanced long-term cycling performance with a large capacity of ∼820 mAh g-1 after 300 cycles at 100 mA g-1 and an improved rate performance. The enhanced lithium-storage performance, in terms of obvious-shortened activation process and high reversible capacities, can be attributed to the modification of carboxyl GQDs, which would activate more active sites (activated C=C groups from benzene rings) for lithium-storage, and provide fast lithium-ion transportation kinetic. Besides, the decreased interphase resistance, enhanced electronic conductivity, and prevented aggregation of needle-flake COF structure, originated from the addition of GQDs, which lead to the enhanced improved cycling stability of the COF-GQDs electrode. This manuscript can promote the further exploration on the design of COF-related materials with modification of functionalized carbonaceous materials to achieve enhanced lithium-storage properties for next-generation energy storage.

Novel pyridine-based covalent organic framework containing N,N,N-chelating sites for selective detection and effective removal of nickel

A novel pyridine-based COF material with functional N,N,N chelating sites has been designed and synthesized, which has the dual functions of fluorescence detection and efficient removal of Ni2+ from aqueous solution.