Covalent organic frameworks for separation applications

Structure determination of a low-crystallinity covalent organic framework by three-dimensional electron diffraction

Covalent organic frameworks (COFs) have been attracting intense research due to their permanent porosity, designable architecture, and high stability. However, COFs are challenging to crystallize and their synthesis often results in tiny crystal sizes and low crystallinities, which hinders an unambiguous structure determination. Herein, we demonstrate that the structure of low-crystallinity COF Py-1P nanocrystals can be solved by coupling three-dimensional electron diffraction (3DED) with simulated annealing (SA). The resulting model is comparable to that obtained from high-crystallinity samples by dual-space method. Moreover, for low-resolution 3DED data, the model obtained by SA shows a better framework than those provided by classic direct method, dual-space method, and charge flipping. We further simulate data with different resolutions to understand the reliability of SA under different crystal quality conditions. The successful determination of Py-1P structure by SA compared to other methods provides new knowledge for using 3DED to analyze low-crystallinity and nanosized materials.

Computation-based regulation of excitonic effects in donor-acceptor covalent organic frameworks for enhanced photocatalysis

The strong excitonic effects widely exist in polymer-semiconductors and the large exciton binding energy (E_b) seriously limits their photocatalysis. Herein, density functional theory (DFT) calculations are conducted to assess band alignment and charge transfer feature of potential donor-acceptor (D-A) covalent organic frameworks (COFs), using 1,3,5-tris(4-aminophenyl)triazine (TAPT) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) as acceptors and tereph-thaldehydes functionalized diverse groups as donors. Given the discernable D-A interaction strengths in the D-A pairs, their E_b can be systematically regulated with minimum E_b in TAPT-OMe. Guided by these results, the corresponding D-A COFs are synthesized, where TAPT-OMe-COF possesses the best activity in photocatalytic H₂ production and the activity trend of other COFs is associated with that of calculated E_b for the D-A pairs. In addition, further alkyne cycloaddition for the imine linkage in the COFs greatly improves the stability and the resulting TAPT-OMe-alkyne-COF with a substantially smaller E_b exhibits ~20 times higher activity than the parent COF.

Single-Crystal 2D Covalent Organic Frameworks for Plant Biotechnology

Molecules chemically synthesized as periodic two-dimensional (2D) frameworks via covalent bonds can form some of the highest-surface area and -charge density particles possible. There is significant potential for applications such as nanocarriers in life sciences if biocompatibility can be achieved; however, significant synthetic challenges remain in avoiding kinetic traps from disordered linking during 2D polymerization of compatible monomers, resulting in isotropic polycrystals without a long-range order. Here, we establish thermodynamic control over dynamic control on the 2D polymerization process of biocompatible imine monomers by minimizing the surface energy of nuclei. As a result, polycrystal, mesocrystal, and single-crystal 2D covalent organic frameworks (COFs) are obtained. We achieve COF single crystals by exfoliation and minification methods, forming high-surface area nanoflakes that can be dispersed in aqueous medium with biocompatible cationic polymers. We find that these 2D COF nanoflakes with high surface area are excellent plant cell nanocarriers that can load bioactive cargos, such as the plant hormone abscisic acid (ABA) via electrostatic attraction, and deliver them into the cytoplasm of intact living plants, traversing through the cell wall and cell membrane due to their 2D geometry. This synthetic route to high-surface area COF nanoflakes has promise for life science applications including plant biotechnology.

Enantioseparation Membranes: Research Status, Challenges, and Trends

The purity of enantiomers plays a critical role in human health and safety. Enantioseparation is an effective way and necessary process to obtain pure chiral compounds. Enantiomer membrane separation is a new chiral resolution technique, which has the potential for industrialization. This paper mainly summarizes the research status of enantioseparation membranes including membrane materials, preparation methods, factors affecting membrane properties, and separation mechanisms. In addition, the key problems and challenges to be solved in the research of enantioseparation membranes are analyzed. Last but not least, the future development trend of the chiral membrane is expected.

Covalent organic framework: A state-of-the-art review of electrochemical sensing applications

Covalent organic framework (COF), a kind of porous polymer with crystalline properties, is a periodic porous framework material with precise regulation at atomic level, which can be formed by the orderly connection of pre-designed organic construction units through covalent bonds. Compared with metal-organic frameworks, COFs exhibit unique performance, including tailor-made functions, stronger load ability, structural diversity, ordered porosity, intrinsic stability and excellent adsorption features, are more conducive to the expansion of electrochemical sensing applications and the universality of applications. In addition, COFs can accurately integrate organic structural units with atomic precision into ordered structures, so that the structural diversity and application of COFs can be greatly enriched by designing new construction units and adopting reasonable functional strategies. In this review, we mainly summarized state-of-the-art recent advances of the classification and synthesis strategy of COFs, the design of functionalized COF for electrochemical sensors and COFs-based electrochemical sensing. Then, an overview of the considerable recent advances made in applying outstanding COFs to establish electrochemical sensing platform, including electrochemical sensor based on voltammetry, amperometry, electrochemical impedance spectroscopy, electrochemiluminescence, photoelectrochemical sensor and others. Finally, we discussed the positive outlooks, critical challenges and bright directions of COFs-based electrochemical sensing in the field of disease diagnosis, environmental monitoring, food safety, drug analysis, etc.

Chromatographic separation performance of silica microspheres surface-modified with triazine-containing imine-linked covalent organic frameworks

In this work, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 1,3,5-tris(4-formylphenyl)benzene (TFPB) were used as monomers to construct a triazine-containing imine-linked covalent organic framework (COF), which was then bonded onto the surface of aldehydized silica (SiO₂-CHO), and finally a COF@silica composite material (TAPT-TFPB COF@SiO₂) was successfully prepared. The chromatographic separation performance of SiO₂-CHO, TAPT-TFPB COF@SiO₂ and TAPT-TFPB COF@SiO₂/SiO₂-CHO (80/20, mass ratio) was evaluated and compared. It was found that separation efficiency was obviously enhanced by adding an appropriate amount of SiO₂-CHO into TAPT-TFPB COF@SiO₂. The obtained TAPT-TFPB COF@SiO₂/SiO₂-CHO showed more favorable separation ability than SiO₂-CHO and TAPT-TFPB COF@SiO₂. Various aromatic compounds including alkylbenzenes, polycyclic aromatic hydrocarbons, environmental endocrine disruptors, foodborne stimulants and phenyl ketones were effectively separated on the TAPT-TFPB COF@SiO₂/SiO₂-CHO column in reversed phase chromatography mode. The silica microspheres surface-modified with triazine-containing imine-linked COFs proved to be a new type of promising chromatographic packing materials.

Separation of C18 Fatty Acid Esters and Fatty Acids Derived from Vegetable Oils Using Nanometer-Sized Covalent Organic Frameworks Incorporated in Polyepoxy Membranes

Fatty acids (FAs) and FA methyl esters (FAMEs) are easily isolated from vegetable oil and are important starting materials for the chemical industry to produce commercial products that are green, biorenewable, and nontoxic. A challenge in these applications is that mixtures of five or more FAs and FAMEs are isolated from a vegetable oil source, and methods to separate these mixtures are decades old and have increasingly high costs associated with the production of high-purity single-component FAs or FAMEs. We developed a method to separate these mixtures using mixed matrix membranes containing nanometer-sized covalent organic frameworks. The 2D, crystalline COFs possessed narrow distributions of pore sizes of 1.3, 1.8, 2.3, and 3.4 nm that separated FAs and FAMEs based on their degrees of unsaturation. The COFs were synthesized, characterized, and then encapsulated at 10 or 20% by weight into a prepolymer of epoxy that was then fully cured. For all mixed matrix membranes, as the degree of unsaturation increased, the FAs or FAMEs had a slower flux. The largest difference in flux was obtained for a COF/epoxy membrane with a pore size of 1.8 nm, and methyl stearate had a 5.9× faster flux than methyl linolenate. These are the first membranes that can separate the important C18 FAs and FAMEs found in vegetable oil.

Wood-Inspired Ultrafast High-Performance Adsorbents for CO2 Capture

Under favorable regeneration conditions (120 °C, 100% CO₂), ultrafast adsorption kinetics and excellent long-term cycle stability are still the biggest obstacles for amine-based solid CO₂ adsorbents. Inspired by natural wood, a biochar with a highly ordered pore structure and excellent thermal conductivity was prepared and used as a carrier of organic amines to prepare ideal CO₂ adsorbents. The results showed that the prepared adsorbent has a very high adsorption working capacity (4.23 mmol CO₂·g^-1), and its performance remains stable even after 30 adsorption-desorption cycles in the harsh desorption environment (120 °C, 100% CO₂). Due to the existence of the hierarchical structure, the adsorbent exhibited ultra-fast adsorption kinetics, and the reaction rate constant is 37 times higher than that of traditional silica. This adsorbent also showed a very low regeneration heat of 1.64 MJ·kg^-1 (CO₂), which is especially important for the practical application. Therefore, these biochar-based adsorbents derived from natural wood make the CO₂ capture process promising.

Pore-in-Pore Engineering in a Covalent Organic Framework Membrane for Gas Separation

Covalent organic framework (COF) membranes have emerged as a promising candidate for energy-efficient separations, but the angstrom-precision control of the channel size in the subnanometer region remains a challenge that has so far restricted their potential for gas separation. Herein, we report an ultramicropore-in-nanopore concept of engineering matreshka-like pore-channels inside a COF membrane. In this concept, α-cyclodextrin (α-CD) is in situ encapsulated during the interfacial polymerization which presumably results in a linear assembly (LA) of α-CDs in the 1D nanochannels of COF. The LA-α-CD-in-TpPa-1 membrane shows a high H₂ permeance (∼3000 GPU) together with an enhanced selectivity (>30) of H₂ over CO₂ and CH₄ due to the formation of fast and selective H₂-transport pathways. The overall performance for H₂/CO₂ and H₂/CH₄ separation transcends the Robeson upper bounds and ranks among the most powerful H₂-selective membranes. The versatility of this strategy is demonstrated by synthesizing different types of LA-α-CD-in-COF membranes.

Topological Isomerism in Three-Dimensional Covalent Organic Frameworks

Although isomerism is a typical and significant phenomenon in organic chemistry, it is rarely found in covalent organic framework (COF) materials. Herein, for the first time, we report a controllable synthesis of topological isomers in three-dimensional COFs via a distinctive tetrahedral building unit under different solvents. Based on this strategy, both isomers with a dia or qtz net (termed JUC-620 and JUC-621) have been obtained, and their structures are determined by combining powder X-ray diffraction and transmission electron microscopy. Remarkably, these architectures show a distinct difference in their porous features; for example, JUC-621 with a qtz net exhibits permanent mesopores (up to ∼23 Å) and high surface area (∼2060 m² g^-1), which far surpasses those of JUC-620 with a dia net (pore size of ∼12 Å and surface area of 980 m² g^-1). Furthermore, mesoporous JUC-621 can remove dye molecules efficiently and achieves excellent iodine adsorption (up to 6.7 g g^-1), which is 2.3 times that of microporous JUC-620 (∼2.9 g g^-1). This work thus provides a new way for constructing COF isomers and promotes structural diversity and promising applications of COF materials.

Lignin-based covalent organic polymers with improved crystallinity for non-targeted analysis of chemical hazards in food samples

Lignin, the most abundant source of renewable aromatic compounds derived from natural lignocellulosic biomass, has great potential for various applications as green materials due to its abundant active groups. However, it is still challenging to quickly construct green polymers with a certain crystallinity by utilizing lignin as a building block. Herein, new green lignin-based covalent organic polymers (LIGOPD-COPs) were one-pot fabricated with water as the reaction solvent and natural lignin as the raw material. Furthermore, by using paraformaldehyde as a protector and modulator, the LIGOPD-COPs prepared under optimized conditions displayed better crystallinity than reported lignin-based polymers, demonstrating the feasibility of preparing lignin-based polymers with improved crystallinity. The improved crystallinity confers LIGOPD-COPs with enhanced application performance, which was demonstrated by their excellent performances in sample treatment of non-targeted food safety analysis. Under optimized conditions, phytochromes, the main interfering matrices, were almost completely removed from different phytochromes-rich vegetables by LIGOPD-COPs, accompanied by "full recovery" of 90 chemical hazards. Green, low-cost, and reusable properties, together with improved crystallinity, will accelerate the industrialization and marketization of lignin-based COPs, and promote their applications in many fields.

A Bipyridyl Covalent Organic Framework with Coordinated Cu(I) for Membrane C3 H6 /C3 H8 Separation

Covalent organic frameworks (COFs) mixed matrix membranes (MMMs) combining individual attributes of COFs and polymers are promising for gas separation. However, applying COF MMMs for propylene/propane (C₃ H₆ /C₃ H₈ ) separation remains a big challenge due to COF inert pores and C₃ H₆ /C₃ H₈ similar molecular sizes. Herein, the designed synthesis of a Cu(I) coordinated COF for membrane C₃ H₆ /C₃ H₈ separation is reported. A platform COF is synthesized from 5,5'-diamino-2,2'-bipyridine and 2-hydroxybenzene-1,3,5-tricarbaldehyde. This COF possesses a porous 2D structure with high crystallinity. Cu(I) is coordinated to bipyridyl moieties in the COF framework, acting as recognizable sites for C₃ H₆ gas, as shown by the adsorption measurements. Cu(I) COF is blended with 6FDA-DAM polymer to yield MMMs. This COF MMM exhibits selective and permeable separation of C₃ H₆ from C₃ H₈ (C₃ H₆ permeability of 44.7 barrer, C₃ H₆ /C₃ H₈ selectivity of 28.1). The high porosity and Cu(I) species contribute to the great improvement of separation performance by virtue of 2.3-fold increase in permeability and 2.2-fold increase in selectivity compared to pure 6FDA-DAM. The superior performance to those of most relevant reported MMMs demonstrates that the Cu(I) coordinated COF is an excellent candidate material for C₃ H₆ separation membranes.

Monomer Symmetry-Regulated Defect Engineering: In Situ Preparation of Functionalized Covalent Organic Frameworks for Highly Efficient Capture and Separation of Carbon Dioxide

Developing crystalline porous materials with highly efficient CO₂ selective adsorption capacity is one of the key challenges to carbon capture and storage (CCS). In current studies, much more attention has been paid to the crystalline and porous properties of crystalline porous materials for CCS, while the defects, which are unavoidable and ubiquitous, are relatively neglected. Herein, for the first time, we propose a monomer-symmetry regulation strategy for directional defect release to achieve in situ functionalization of COFs while exposing uniformly distributed defect-aldehyde groups as functionalization sites for selective CO₂ capture. The regulated defective COFs possess high crystallinity, good structural stability, and a large number of organized and functionalized aldehyde sites, which exhibit one of the highest selective separation values of all COF sorbing materials in CO₂/N₂ selective adsorption (128.9 cm³/g at 273 K and 1 bar, selectivity: 45.8 from IAST). This work not only provides a new strategy for defect regulation and in situ functionalization of COFs but also provides a valuable approach in the design and preparation of new adsorbents for CO₂ adsorption and CO₂/N₂ selective separation.

Quinoid-Thiophene-Based Covalent Organic Polymers for High Iodine Uptake: When Rational Chemical Design Counterbalances the Low Surface Area and Pore Volume

A novel 2D covalent organic polymer (COP), based on conjugated quinoid-oligothiophene (QOT) and tris(aminophenyl) benzene (TAPB) moieties, is designed and synthesized (TAPB-QOT COP). Some DFT calculations are made to clarify the equilibrium between different QOT isomers and how they could affect the COP formation. Once synthetized, the polymer has been thoroughly characterized by spectroscopic (i.e., Raman, UV-vis), SSNMR and surface (e.g., SEM, BET) techniques, showing a modest surface area (113 m² g^-1) and micropore volume (0.014 cm³ g^-1 with an averaged pore size of 5.6-8 Å). Notwithstanding this, TAPB-QOT COP shows a remarkably high iodine (I₂) uptake capacity (464 %wt) comparable to or even higher than state-of-the-art porous organic polymers (POPs). These auspicious values are due to the thoughtful design of the polymer with embedded sulfur sites and a conjugated scaffold with the ability to counterbalance the relatively low pore volumes. Indeed, both morphological and Raman data, supported by computational analyses, prove the very high affinity between the S atom in our COP and the I₂. As a result, TAPB-QOT COP shows the highest volumetric I₂ uptake (i.e., the amount of I₂ uptaken per volume unit) up to 331 g cm^-3 coupled with a remarkably high reversibility (>80% after five cycles).

One-Dimensional Covalent Organic Frameworks for the 2e- Oxygen Reduction Reaction

Two-dimensional covalent organic frameworks (2D COFs) are often employed for electrocatalytic systems because of their structural diversity. However, the efficiency of atom utilization is still in need of improvement, because the catalytic centers are located in the basal layers and it is difficult for the electrolytes to access them. Herein, we demonstrate the use of 1D COFs for the 2e^- oxygen reduction reaction (ORR). The use of different four-connectivity blocks resulted in the prepared 1D COFs displaying good crystallinity, high surface areas, and excellent chemical stability. The more exposed catalytic sites resulted in the 1D COFs showing large electrochemically active surface areas, 4.8-fold of that of a control 2D COF, and thus enabled catalysis of the ORR with a higher H₂ O₂ selectivity of 85.8 % and activity, with a TOF value of 0.051 s^-1 at 0.2 V, than a 2D COF (72.9 % and 0.032 s^-1 ). This work paves the way for the development of COFs with low dimensions for electrocatalysis.

Bottom-Up Synthesis of Covalent Organic Frameworks with Quasi-Three-Dimensional Integrated Architecture via Interlayer Cross-Linking

Developing strategies to enhance the structural robustness of covalent organic frameworks (COFs) is of great importance. Here, we rationally design and synthesize a class of cross-linked COFs (^CCOFs), in which the two-dimensional (2D) COF layers are anchored and connected by polyethylene glycol (PEG) or alkyl chains through covalent bonds. The bottom-up fabrication of these ^CCOFs is achieved by the condensation of cross-linked aldehyde monomers and tritopic amino monomers. All the synthesized ^CCOFs possess high crystallinity and porosity, and enhanced structural robustness surpassing the typical 2D COFs, which means that they cannot be exfoliated under ultrasonication and grinding due to the cross-linking effect. Furthermore, the cross-linked patterns of PEG units are uncovered by experimental results and Monte Carlo molecular dynamics simulations. It is found that all ^CCOFs are dominated by vertical cross-layer (interlayer) connections (clearly observed in high-resolution transmission electron microscopy images), allowing them to form quasi-three-dimensional (quasi-3D) structures. This work bridges the gap between 2D COFs and 3D COFs and provides an efficient way to improve the interlayered stability of COFs.

Construction of Covalent Organic Framework Nanofiber Membranes for Efficient Adsorption of Antibiotics

Techniques beyond crystal engineering are critical for manufacturing covalent organic frameworks (COFs) and to explore them for advanced applications. However, COFs are normally obtained as insoluble, unmeltable, and thus nonprocessible microcrystalline powders. Therefore, it is a significant challenge to implement COFs into larger architectures and structural control on different length scales. Herein, a facile strategy is presented to prepare flexible COF nanofiber membranes by in-situ growth of COFs on polyacrylonitrile (PAN) nanofiber substrates via a reversible polycondensation-termination approach. The obtained PAN@COF nanofiber membranes with vertically aligned COF nanoplates combine a large functional surface with efficient mass transport, thus making it a promising adsorbent, for example, for water purification. The antibiotic pollutant ofloxacin (OFX) is removed from water with a superior absorption capacity of ≈236 mg g^-1 and removal efficiency as high as 98%. The here presented in-situ growth of COFs on nanofiber membranes can be extended to various Schiff base-derived COF materials with different compositions, providing a highly efficient way to construct flexible COF-based membranes for several applications.

Record Ultralarge-Pores, Low Density Three-Dimensional Covalent Organic Framework for Controlled Drug Delivery

The unique structural characteristics of three-dimensional (3D) covalent organic frameworks (COFs) like high surface areas, interconnected pore system and readily accessible active sites render them promising platforms for a wide set of functional applications. Albeit promising, the reticular construction of 3D COFs with large pores is a very demanding task owing to the formation of interpenetrated frameworks. Herein we report the designed synthesis of a 3D non-interpenetrated stp net COF, namely TUS-64, with the largest pore size of all 3D COFs (47 Å) and record-low density (0.106 g cm^-3 ) by reticulating a 6-connected triptycene-based linker with a 4-connected porphyrin-based linker. Characterized with a highly interconnected mesoporous scaffold and good stability, TUS-64 shows efficient drug loading and controlled release for five different drugs in simulated body fluid environment, demonstrating the competency of TUS-64 as drug nanocarriers.

Electrochemical Interfacial Polymerization toward Ultrathin COF Membranes for Brine Desalination

Ultrathin covalent organic framework (COF) membranes are urgently demanded in molecular/ionic separations. Herein, we reported an electrochemical interfacial polymerization strategy to fabricate ultrathin COF membranes with thickness of 85 nm, by actively manipulate self-healing effect and self-inhibiting effect. The resulting COF membrane exhibited superior performance in brine desalination with the permeation flux of 92 kg m^-2  h^-1 and the rejection of 99.96 %. Our electrochemical interfacial polymerization strategy enriches the fabrication approach of COF membranes and facilitates the rational design of ultrathin membranes.

Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH3, CO2/CH4 and CO2/N2, C2H2/C2He and CeH6, C2H2/CO2, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H2 and CO2 reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

Application and Research Progress of Covalent Organic Frameworks for Solid-State Electrolytes in Lithium Metal Batteries

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with periodic networks that are constructed from small molecular units via covalent bonds, which have low densities, high porosity, large specific surface area, and ease of functionalization. The one-dimension nanochannels in COFs offer an effective means of transporting lithium ions while maintaining a stable structure over a wide range of temperatures. As a new category of ionic conductors, COFs exhibit unparalleled application potential in solid-state electrolytes. Here, we provide a comprehensive summary of recent applications and research progress for COFs in solid-state electrolytes of lithium metal batteries and discuss the possible development directions in the future. This review is expected to provide theoretical guidance for the design of high-performance solid-state electrolytes.

Tuning excited state electronic structure and charge transport in covalent organic frameworks for enhanced photocatalytic performance

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Tuning excited state electronic structure and charge transport in covalent organic frameworks for enhanced photocatalytic performance

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Converting an amorphous covalent organic polymer to a crystalline covalent organic framework mediated by a repairing agent

We herein report a new approach to converting an amorphous covalent organic polymer to a crystalline heteropore covalent organic framework (COF), which is promoted by using an additive for structure repair. This provides a new method for the construction of COFs from cross-linked polymers.

Building N-hydroxyphthalimide organocatalytic sites into a covalent organic framework for metal-free and selective oxidation of silanes

A novel crystalline covalent organic framework COF-NHPI was built by a bottom-up strategy to guarantee highly ordered embedment of the N-hydroxyphthalimide (NHPI) units as nitroxyl radical organocatalytic sites. The COF-NHPI was demonstrated to be a metal-free, highly selective and heterogeneous catalyst for the efficient oxidation of various silanes to the corresponding silanols. Mechanistic studies revealed that the critical phthalimido N-oxyl radical was generated in situ to govern the catalysis.

Covalent Organic Frameworks: Recent Progress in Biomedical Applications

Covalent organic frameworks (COFs) are a type of crystalline organic porous material with specific features and interesting structures, including porosity, large surface area, and biocompatibility. These features enable COFs to be considered as excellent candidates for applications in various fields. Recently, COFs have been widely demonstrated as promising materials for biomedical applications because of their excellent physicochemical properties and ultrathin structures. In this review, we cover the recent progress of COF materials for applications in photodynamic therapy, gene delivery, photothermal therapy, drug delivery, bioimaging, biosensing, and combined therapies. Moreover, the critical challenges and further perspectives with regards to COFs for future biology-facing applications are also discussed.

Designed Synthesis of Three-Dimensional Covalent Organic Frameworks: A Mini Review

Covalent organic frameworks are porous crystals of polymers with two categories based on their covalent linkages: layered structures with two dimensions and networks with three-dimensional structures. Three-dimensional covalent organic frameworks are porous, have large surface areas, and have highly ordered structures. Since covalent bonds are responsible for the formation of three-dimensional covalent organic frameworks, their synthesis has been a challenge and different structures are generated during the synthesis. Moreover, initially, their topologies have been limited to dia, ctn, and bor which are formed by the condensation of triangular or linear units with tetrahedral units. There are very few building units available for their synthesis. Finally, the future perspective of 3D COFs has been designated for the future development of three-dimensional covalent organic frameworks.

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Boosting Electroreduction of CO2 over Cationic Covalent Organic Frameworks: Hydrogen Bonding Effects of Halogen Ions

We present the first example of charged imidazolium functionalized porphyrin-based covalent organic framework (Co-iBFBim-COF-X) for electrocatalytic CO₂ reduction reaction, where the free anions (e.g., F^- , Cl^- , Br^- , and I^- ) of imidazolium ions nearby the active Co sites can stabilize the key intermediate *COOH and inhibit hydrogen evolution reaction. Thus, Co-iBFBim-COF-X exhibits higher activity than the neutral Co-BFBim-COF, following the trend of F^- <Cl^- <Br^- <I^- . Particularly, the Co-iBFBim-COF-I^- showed nearly 100 % CO₂ selectivity at a low full-cell voltage of 2.3 V, and achieved a high CO₂ partial current density of 52 mA cm^-2 with a turnover frequency of 3018 h^-1 at 2.4 V in the anion membrane electrode assembly, which is 3.57 times larger than that of neutral Co-BFBim-COF. This work provides new insight into the importance of free anions in the stabilization of intermediates and decreasing the local binding energy of H₂ O with active moiety to enhance CO₂ reduction reaction.

A sweat-responsive covalent organic framework film for material-based liveness detection and sweat pore analysis

Covalent organic frameworks have shown considerable application potential and exceptional properties in the construction of stimulus-responsive materials. Here, we designed a sweat-responsive covalent organic framework film for material-based fingerprint liveness detection. When exposed to human sweat, the COF_TPDA-TFPy film can transform from yellow to red. The COF_TPDA-TFPy film, when touched by living fingers, can produce the naked-eye-identified fingerprint pattern through the sweat-induced color change, while artificial fake fingerprints cannot. This technique, which we named material-based liveness detection, can thus intuitively discern living fingers from fake fingerprints with a 100% accuracy rate. Additionally, the distribution of sweat pores on human skin can also be collected and analyzed by shortening the contact time. By merely washing them with ethanol, all the samples can be utilized again. This work inventively accomplished material-based liveness detection and naked-eye-identified sweat pore analysis and highlighted their potential for use in clinical research and personal identification.

Recent advances in development of functional magnetic adsorbents for selective separation of proteins/peptides

Nowadays, proteins separation has attracted great attention in proteomics research. Because the proteins separation is helpful for making an early diagnosis of many diseases. Magnetic nanoparticles are an interesting and useful functional material, and have attracted extensive research interest during the past decades. Because of the excellent properties such as easy surface functionalization, tunable biocompatibility, high saturation magnetization etc, magnetic microspheres have been widely used in isolation of proteins/peptides. Notably, with the rapid development of surface decoration strategies, more and more functional magnetic adsorbents have been designed and fabricated to meet the growing demands of biological separation. In this review, we have collected recent information about magnetic adsorbents applications in selective separation of proteins/peptides. Furthermore, we present a comprehensive prospects and challenges in the field of protein separation relying on magnetic nanoparticles.

Ordered Macro-Microporous Single Crystals of Covalent Organic Frameworks with Efficient Sorption of Iodine

Fashioning microporous covalent organic frameworks (COFs) into single crystals with ordered macropores allows for an effective reduction of the mass transfer resistance and the maximum preservation of their intrinsic properties but remains unexplored. Here, we report the first synthesis of three-dimensional (3D) ordered macroporous single crystals of the imine-linked 3D microporous COFs (COF-300 and COF-303) via a template-assisted modulated strategy. In this strategy, COFs crystallized within the sacrificial colloidal crystal template, assembled from monodisperse polystyrene microspheres, and underwent an aniline-modulated amorphous-to-crystalline transformation to form large single crystals with 3D interconnected macropores. The effects of the introduced macroporous structure on the sorption performances of COF-300 single crystals were further probed by iodine. Our results indicate that iodine adsorption occurred in micropores of COF-300 but not in the introduced macropores. Accordingly, the iodine adsorption capacity of COF single crystals was governed by their micropore accessibility. The relatively long diffusion path in the non-macroporous COF-300 single crystals resulted in a limited micropore accessibility (48.4%) and thus a low capacity in iodine adsorption (1.48 g·g^-1). The introduction of 3D ordered macropores can greatly shorten the microporous diffusion path in COF-300 single crystals and thus render all their micropores fully accessible in iodine adsorption with a capacity (3.15 g·g^-1) that coincides well with the theoretical one.

Room temperature synthesis of flower-like hollow covalent organic framework for efficient enrichment of microcystins

The morphology of nanomaterials is one of essential factors for their unique properties. Herein, a hollow covalent organic framework with a flower-like structure (HFH-COF) was synthesized at room temperature. The synthesized HFH-COF has a very large specific surface area, mesoporous structure, excellent chemical stability, and good crystallinity. The special morphology endowed HFH-COF with high specific surface area utilization and rapid mass transfer rate, resulting in faster equilibration time and better extraction performance than spherical COF (S-COF). Subsequently, combined with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), an efficient and sensitive method was established for microcystins (MCs) detection. The developed method has low detection limits (0.6-0.8 pg mL^-1), wide linear ranges (1.5-1000.0 pg mL^-1, R ≥ 0.9993), and acceptable reproducibility (RSD ≤ 7.6%, n = 6). Real biological samples were analyzed by the developed method, and trace levels of MC-YR, MC-RR and MC-LR were detected. The results indicate that the synthesized HFH-COF is an ideal sorbent for efficient extraction of MCs from complex biological samples.

Dandelion-like covalent organic frameworks with high-efficiency fluorescence for ratiometric sensing and visual tracking-by-detection of Fe3

Iron ions, one of the most common heavy metal pollutants in industrial waste materials, are continuously actively or passively delivered to the environment. Meanwhile, the importance of Fe³⁺ in biological processes in vivo can not be neglected due to its crucial role in maintaining normal physiological function. Therefore, a ratiometric fluorescence covalent organic framework (TD-COF) was constructed for tracking-by-detection of Fe³⁺. Alkynes-extended 1,3,6,8-tetrakis(4-ethynyl benzaldehyde)-pyrene (TEBPY) with complete planar structure and 2,5-dihydroxyterephthalohydrazide (DHTH) with functional group -OH were selected as the building blocks. The ratiometric fluorescence TD-COF with a dandelion-like structure exhibited its dual emission peaked at 510 nm and 630 nm. It displayed an obvious fluorescence color variation of yellow-red-black in the presence of Fe³⁺. Benefiting from the high luminescent efficiency (QY of 36.4%) and multiple identical binding sites, TD-COF exhibited a wide linear range to Fe³⁺ (0.005-50 μM) with a detection limit of 10.9 nM. Additionally, a smartphone visual sensing platform integrated with TD-COF was developed based on the color transformation and successfully applied to visual smart real-time monitoring Fe³⁺. More surprisingly, the maximum adsorption capacity of TD-COF towards Fe³⁺ was 833.3 mg/g due to the coordination interaction and cationic π-effect. The practicability of the smartphone-integrated ratiometric sensing platform for visual tracking-by-detection of Fe³⁺ was verified by choosing tap water as the actual sample, and the recoveries were calculated to be 98.71-100.88%. This work thus developed COF-based ratiometric sensing of Fe³⁺, which is an attractive candidate for further application in fluorescent sensing and visual monitoring.

Modulating Uranium Extraction Performance of Multivariate Covalent Organic Frameworks through Donor-Acceptor Linkers and Amidoxime Nanotraps

Covalent organic frameworks (COFs) can be designed to allow uranium extraction from seawater by incorporating photocatalytic linkers. However, often sacrificial reagents are required for separating photogenerated charges which limits their practical applications. Herein, we present a COF-based adsorption-photocatalysis strategy for selective removal of uranyl from seawater in the absence of sacrificial reagents. A series of ternary and quaternary COFs were synthesized containing the electron-rich linker 2,4,6-triformylphloroglucinol as the electron donor, the electron-deficient linker 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde as the acceptor, and amidoxime nanotraps for selective uranyl capture (with the quaternary COFs incorporating [2,2'-bipyridine-5,5'-diamine-Ru(Bp)₂]Cl₂ as a secondary photosensitizer). The ordered porous structure of the quaternary COFs ensured efficient mass transfer during the adsorption-photocatalysis capture of uranium from seawater samples, with photocatalytically generated electrons resulting in the reduction of adsorbed U(VI) to U(IV) in the form of UO₂. A quaternary COF, denoted as COF 2-Ru-AO, possessed a high uranium uptake capacity of 2.45 mg/g/day in natural seawater and good anti-biofouling abilities, surpassing most adsorbents thus far. This work shows that multivariate COF adsorption-photocatalysts can be rationally engineered to work efficiently and stably without sacrificial electron donors, thus opening the pathway for the economic and efficient extraction of uranium from the earth's oceans.

Integrated interfacial design of covalent organic framework photocatalysts to promote hydrogen evolution from water

Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300-1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g^-1 h^-1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h^-1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems.

Janus Dione-Based Conjugated Covalent Organic Frameworks with High Conductivity as Superior Cathode Materials

The development of conductive covalent organic frameworks (COFs) with high stability is desirable for the practical applications in optoelectronics and energy storage. Herein, we developed a new kind of Janus dione-based COF, which is fully sp² carbon-conjugated through the connection by olefin units. The electrical conductivity and carrier mobility reached up to 10^-3 S cm^-1 and 7.8 cm² V^-1 s^-1, respectively. In addition, these COFs are strongly robust against various harsh conditions. The well-ordered two-dimensional crystalline structures, excellent porosity, high conductivity, and abundant redox-active carbonyl units render these COFs serviceable as high-performance cathode materials in lithium-ion batteries. It is worth noting that TFPPy-ICTO-COF exhibits a capacity of up to 338 mAh g^-1 at a discharge rate of 0.1 C, which sets a new capacity record among COF-based lithium-ion batteries. Its capacity retention was as high as 100% even after 1000 cycles, demonstrating the remarkable stability of these Janus dione-based COF materials. This work not only expands the diversity of olefin-linked COFs but also makes a new breakthrough in energy storage.

A self-standing three-dimensional covalent organic framework film

Covalent crystals such as diamonds are a class of fascinating materials that are challenging to fabricate in the form of thin films. This is because spatial kinetic control of bond formation is required to create covalently bonded crystal films. Directional crystal growth is commonly achieved by chemical vapor deposition, an approach that is hampered by technical complexity and associated high cost. Here we report on a liquid-liquid interfacial approach based on physical-organic considerations to synthesize an ultrathin covalent crystal film. By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimeter size scale with a uniform thickness of 13 nm. The film exhibits considerable mechanical robustness enabling a free-standing length of 37 µm, as well as a clearly anisotropic chemical structure and crystal lattice alignment.

Post-synthetic Fully π-Conjugated Three-Dimensional Covalent Organic Frameworks for High-Performance Lithium Storage

A fully π-conjugated nitrogen-rich three-dimensional covalent organic framework (PYTRI-COF-2) containing both pyrazine and triazine units was prepared through a post-synthetic strategy. The imine linkages in the pre-prepared PYTRI-COF-1 were converted into heterocyclic quinoline by the Povarov reaction. The obtained PYTRI-COF-2 displayed high Li-ion storage capacity and excellent cycling stability when it was used as the lithium (Li)-ion battery electrode.

Functional Covalent Organic Framework Films Based on Surface and Interfacial Chemistry for Molecular Separations

Covalent organic frameworks (COFs) are promising crystalline porous materials with highly tunable structures and functionalities. In the last decade, COF films have been synthesized and used as multifunctional materials for a diverse range of separation applications. However, there are still challenges in the scaling-up preparation of COF films with benchmarked performance for precise molecular separations. Recently, research has turned its attention to preparing functional COF films with an appropriate aperture size/functionality, facile preparation process, and superior stability. In this Perspective, we outline the recent advances in designing and preparing functional COF films based on surface and interfacial chemistry. On top of that, current obstacles and opportunities in the scaling-up preparation of functional COF films and their industrial applications are proposed and discussed. This Perspective strives to inspire the development of functional COF films with tailored structures and functionalities and promote their practical applications in diverse molecular separation processes.

Ordered Macro/Microporous Ionic Organic Framework for Efficient Separation of Toxic Pollutants from Water

In case of pollutant segregation, fast mass diffusion is a fundamental criterion in order to achieve improved performance. The rapid mass transport through porous materials can be achieved by availing large open pores followed by easy and complete accessibility of functional sites. Inducing macroporosity into such materials could serve as ideal solution providing access to large macropores that offer unhindered transport of analyte and full exposure to interactive sites. Moreover, the challenge to configure the ionic-functionality with macroporosity could emerge as an unparalleled avenue toward pollutants separation. Herein, we strategized a synthetic protocol for construction of a positively charged hierarchically-porous ordered interconnected macro-structure of organic framework where the size and number of macropores can easily be tuned. The ordered macropores with strong electrostatic interaction synergistically exhibited ultrafast removal efficiency towards various toxic pollutants.

Synthesis of Vinylene-Linked Thiopyrylium-, Pyrylium-, and Pyridinium-Based Covalent Organic Frameworks by Acid-Catalyzed Aldol Condensation

The development of new vinylene-linked covalent organic frameworks (COFs) with special ionic structure and high stability is challenging. Herein, we report a facile, general method for constructing ionic vinylene-linked thiopyrylium-based COFs from 2,4,6-trimethylpyrylium tetrafluoroborate and other common reagents by means of acid-catalyzed Aldol condensation. Besides, pyrylium-, and pyridinium-based COFs also can be prepared from the same monomer under slightly different reaction conditions. The COFs exhibited uniform nanofibrous morphologies with excellent crystallinities, special ionic structures, well-defined nanochannels, and high specific surface areas.

Reticular Synthesis of One-Dimensional Covalent Organic Frameworks with 4-c sql Topology for Enhanced Fluorescence Emission

Covalent organic frameworks (COFs) have been extensively investigated due to their unique structure, porosity, and functionality. However, at the topological level, COFs remain as two-dimensional (2D) or three-dimensional (3D) structures, while COFs with one-dimensional (1D) topology have not been systematically explored. In this work, we proposed a synthetic strategy for the construction of 1D-COFs based on non-linear edges and suitable high-symmetry vertices. Compared with their 2D-COFs counterparts, the 1D-COFs with AIEgens located at the vertex of the frame exhibited enhanced fluorescence. The density functional theory (DFT) calculations revealed that the dimensional-induced rotation restriction (DIRR) effect could spontaneously introduce additional non-covalent interactions between the strip frames, which could substantially diminish non-radiative transitions. This work also provides protocols for the design of 1D-COFs and a guidance scheme for the synthesis of emitting COFs.