An artificial neural network model based on CDs@MOF/COF composites for the ultra-sensitive fluorescence detection of glyphosate.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Advances in Emerging Crystalline Porous Materials.

Mapping out the Degree of Freedom of Hosted Enzymes in Confined Spatial Environments

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

Integrating Superwettability within Covalent Organic Frameworks for Functional Coating

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Carbon dots (CDs) are environmentally benign, strongly photoluminescent, metal free nanoparticles. Interfacing them with tailor-made organic semiconductors possesing an ordered channel structure such as covalent organic frameworks (COFs) promises to yield multifunctional materials. In this study, microwave-derived CDs are successfully incorporated into the porous structure of COF in a one-pot synthesis in which the condensation reaction between benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxaldehyde (BDT) and 1,1,2,2-tetra(p-aminophenyl)ethylene (ETTA) is conducted in the presence of CDs. A detailed structural and optoelectronic characterization of the COF/CDs composite reveals that upon tuning the CDs loadings encapsulated in COF the interaction between both components can be controlled allowing the switch between energy and charge transfer. At CDs loadings ⩽20 wt%, strong binding of CDs to the COF enables charge transfer evinced from the quenched photoluminescence (PL) of both components and accelerated exciton decay kinetics of the COF. At CDs loadings ⩾30 wt% Förster resonance energy transfer from CDs to COF prevails, leading to enhanced COF PL. Our study underlines the interaction mechanism in organic composites and provides the knowledge required for the design of novel functional materials with applications in photocatalysis, optoelectronics and sensing.

Metal clusters exhibit diverse structures, emerging functions, and applications; thus, incorporating clusters into metal–organic frameworks (MOFs) brings tremendous merits. Although the constructio...

Electroactive covalent organic frameworks: a new choice for organic electronics

This paper reviews the application of deep eutectic solvents (DESs) in the synthesis of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as well as their prospects in the field of solid-phase extraction (SPE). Porous organic frameworks (POFs) have unique properties such as a large specific surface area, high porosity, and easy modification. Thus, these materials are widely applied in the fields of catalysis, adsorption, drug delivery, gas storage, and separation. POFs include MOFs, COFs, conjugated microporous polymers (CMPs), porous aromatic frameworks (PAFs), and covalent triazine frameworks (CTFs). MOFs are constructed from metal ions/clusters and organic ligands through coordination bonds and can be extended in two or three dimensions by repeated coordination with potential voids. COFs are formed from two monomers containing light elements (such as carbon, hydrogen, oxygen, nitrogen, boron, and other elements) via coordination bonds and have large two- or three-dimensional structures. However, conventional POF synthesis methods generally suffer from disadvantages such as long synthesis times, high temperature and pressure requirements, and the use of toxic and hazardous reaction solvents. DES consists of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD) bound by hydrogen-bonding interactions. It is a promising green solvent for material synthesis owing to its low vapor pressure, high stability, and ease of preparation. DES can be used to prepare MOFs and COFs and, in specific cases, acts as a structure-directing agent, which has an important impact on the structure and properties of the resulting frameworks. Using appropriate DES formulations, researchers can modulate the crystal structures, pore sizes, and surface properties of MOFs and COFs, resulting in materials with excellent characteristics. SPE is an analytical technique in which a sample solution is added to an SPE column; the sample solution is forced through the stationary phase, and the target compounds are collected for analysis by elution with an organic solvent. Therefore, suitable stationary-phase materials are critical for SPE. Owing to their large specific surface areas and abundant active sites, MOFs and COFs exhibit outstanding adsorption capacity and selectivity in SPE and can effectively enrich target analytes from complex samples. DES-based MOFs and COFs have shown potential use in a wide range of applications, such as in environmental analysis, food testing, and biological sample analysis. Although DES-based MOFs and COFs for SPE are still in the early stages of development, their properties such as efficient enrichment and high selectivity offer good prospects for practical applications. Future research should continue to explore DES-based synthesis methods in depth to prepare other MOFs and COFs with the desired properties and investigate their potential applications in various fields. These efforts are expected to apply these novel materials in commercialized solid-phase extraction methods, bringing new development opportunities in the field of analytical chemistry.

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr4+ ion and 2-aminoterephthalic acid (BDC-NH2). High affinity of Zr4+ ion to phosphate species improves sensing selectivity. The surface -NH2 reacts 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. Hydrogen bond formed between hydroxyl group in Tp and imine nitrogen realizes excited state intramolecular proton transfer, so multi-emission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470 and 613 nm for UiO@COF1 and 370, 470, and 572 nm for UiO@COF2. When added phosphate ion in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multi-emission, the affinity of Zr4+ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multi-emission, affinity, and structural selectivity to improve sensing performance for differentiation and ratiometric detection.

ConspectusMembrane technology plays an increasingly important role for sustainable development of our society owing to its huge capability to tackle the energy crisis, water scarcity, environmental pollution, and carbon neutrality. To fully unlock the potential of membranes, it is in high demand to develop advanced membrane materials that significantly outperform conventional polymer membrane materials in separation performance and long-term stability. The emergent covalent organic frameworks (COFs) have been deemed as potent membrane materials because of their unique structure and properties in comparison with polymers, zeolites, and metal organic frameworks (MOFs). (i) First, the highly tunable and ordered crystalline pore structure, high porosity, and excellent stability render COFs an ideal membrane material. COFs are more stable than MOFs and, in some cases, are even more stable than zeolite. Moreover, it is easier to introduce functional groups into the COF nanochannels compared with zeolite and MOFs. Further, COFs are ideally suitable for constructing ordered nanochannels with size in the range of 0.6–3 nm which is difficult to be realized by other materials. (ii) Second, along with the unremitting discovery of diverse platform chemistries such as reticular chemistry, the in-depth understanding of nucleation/growth mechanisms of COFs as well as the rapid progress of manufacturing technologies and various routes to fabricating COF membranes with favorable physical and chemical structures inside the nanochannels are being actively exploited. COFs generally show better membrane-formation ability owing to their abundant 2D structures, which make it easier to fabricate ultrathin membranes compared with zeolite and MOFs. (iii) Last, a great number of COF membranes exhibit exceptionally high separation performance and stability, establishing their position as the next-generation membranes.In this Account, we discuss three types of engineering toward COF membranes based on Schiff base reaction for high-efficiency molecules/ion separations, i.e., reticular engineering, crystal engineering, and nanochannel engineering. First, we discuss the reticular engineering of COF membranes with a focus on the bond types, chemical structure, and architecture design. The membrane-formation ability and methods of COFs are also analyzed. Second, we discuss the crystal engineering of COF membranes with a focus on the key thermodynamical and kinetic factors to drive the disorder-to-order transition where we attempt to dig deeper into the crystallization habit of COF membranes. Third, we discuss nanochannel engineering of COF membranes with a focus on the construction and modulation of the physical and chemical microenvironments of nanochannels for efficient and selective transport of molecules/ions. Last, we conclude with a perspective on the opportunities and major challenges in the R&D of COF membranes, targeting at identifying the future directions.

Dual-functional lanthanide metal organic frameworks for visual and ultrasensitive ratiometric fluorescent detection of phosphate based on aggregation-induced energy transfer

Covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) are two emerging classes of extended porous structures, which seek to develop the reticular chemistry beyond molecules and open up new horizons for compositions, structures, properties, and applications (Yaghi, 2019; Lyu et al., 2020). Like MOFs that extend inorganic metal complexes into 2D and 3D frameworks, COFs extend organic chemistry from molecules and polymers into 2D and 3D organic structures (Diercks and Yaghi, 2017). MOFs/COFs are built with the aim of extending porous frameworks through strong bonds (coordinate/covalent interactions) between molecular building blocks (metal-containing unit-organic linker/organic-organic monomer) based on topological guides. The advantages of these approaches include controllable synthesis, pre-designable structures, and manageable functionality (Geng et al., 2020). In addition to possessing high surface area and tunable pores, both MOFs and COFs display a lot of intriguing properties including layered crystalline structures through π-π stacking and high stability which is only exhibited in graphene (Fritz and Coskun, 2020) owing to the presence of strong covalent bonds. However, metal-free COFs far from meet the growing demands of numerous fields where the role of metal in the framework structure is emphasized. This includes applications such as gas adsorption and separation, heterogeneous catalysis, electronics, electrocatalysis, and electrochemical energy storage. An effective way to address these challenges is to introduce targeted metal ions into COFs frameworks to form metal-covalent organic frameworks (MCOFs) (Dong et al., 2020). Compared with metal-free COFs, MCOFs not only have superior electrocatalytic activity but also display higher intrinsic conduction due to the involvement of the metal component. Developing distinctive synthesis methods/strategies to achieve novel MOFs and COFs holds much promise toward promoting their application. For instance, flexible and free-standing pure COFs membranes were prepared by liquid-liquid interfacial polymerization at room temperature and atmospheric pressure, which solves a major problem since COFs are generally insoluble and unprocessable powders (Liu et al., 2020). A vast number of organic monomers have been reported to date with infinite possibilities of functionalization in their resulting structures. This leads to “digital reticular chemistry” based on laboratory robotics and artificial intelligence (AI), which could achieve high-throughput experiments involving synthesis and characterization. This approach is poised to make the discoveries in MOFs and COFs more significant and easily achievable (Lyu et al., 2020).

Capillary electrochromatography (CEC) has received increased attention from the academic community because it combines the excellent selectivity of high performance liquid chromatography (HPLC) and the high efficiency of capillary electrophoresis (CE). Selecting the most appropriate stationary phase material is crucial to achieve better separation effects in CEC. In recent years, a considerable number of materials, such as graphene oxide, proteins, metal organic frameworks, and covalent organic frameworks (COFs), have been widely used as stationary phases in CEC to further improve its separation performance and extend its scope of potential applications. Among these materials, COFs have shown great application prospects in CEC owing to their unique properties, which include high porosity, large surface area, excellent stability, tunable pore size, and high designability of the framework structure. This review systematically summarizes published papers on the development and application of COFs in CEC from 2016 to 2023. First, two COF-based capillary columns (i. e., open-tube CEC columns and monolithic CEC columns) and their preparation methods are introduced. Second, the applications of CEC based on COF stationary phases in the separation of environmental endocrine disruptors, pesticides, aromatic compounds, amino acids, and drugs, particularly chiral drugs, are systematically summarized. The separation mechanism of CEC based on COF stationary phases is also introduced. At present, the good separation ability of COF-based CEC is mainly attributed to two factors: 1) The size exclusion effect of the pores of the COF stationary phase. Because of differences in the sizes of their organic molecular building units and side chains, COFs have varying pore sizes and topological structures. Thus, target analytes smaller than the pores of the COFs can enter the frameworks and interact with them during separation. On the other hand, target analytes larger than the pores of the COFs cannot enter the frameworks and interact with them during separation; thus, they can be separated. 2) The interactions between the target analytes and side chains (e. g., hydrophobic interactions, hydrogen bonding, π-π interactions, etc.) of the COFs. Since COFs usually contain alkyl side chains, aromatic structures, and oxygen and/or nitrogen atoms with high electronegativity, various interactions could occur between the COFs and target analytes. Finally, directions for the future development and strategic application of CEC based on COF stationary phases are proposed. We believe that future research in CEC based on COF stationary phases should focus on the following aspects: 1) The use of cheminformatics to design and construct COFs to improve the efficiency of COF capillary column preparation; 2) the development of milder methods to synthesize COFs that can meet the requirements of high performance COF capillary columns; and 3) in-depth research to explore the separation mechanism of CEC based on COF stationary phases to provide theoretical guidance for developing CEC methods suitable for the separation and analysis of complex samples.