Two‐Dimensional Metal Organic Frameworks and Covalent Organic Frameworks

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Two-Dimensional Conjugated Metal-Organic Frameworks with a Ring-in-Ring Topology and High Electrical Conductance

Electrically conducting two-dimensional (2D) metal-organic frameworks (MOFs) have garnered significant interest due to their remarkable structural tunability and outstanding electrical properties. However, the design and synthesis of high-performance materials face challenges due to the limited availability of specific ligands and pore structures. In this study, we have employed a novel highly branched D3h symmetrical planar conjugated ligand, dodechydroxylhexabenzotrinaphthylene (DHHBTN) to fabricate a series of 2D conductive MOFs, named M-DHHBTN (M=Co, Ni, and Cu). This new family of MOFs offers two distinct types of pores, elevating the structural complexity of 2D conductive MOFs to a more advanced level. The intricate tessellation patterns of the M-DHHBTN are elucidated through comprehensive analyses involving powder X-ray diffraction, theoretical simulations, and high-resolution transmission electron microscope. Optical-pump terahertz-probe spectroscopic measurements unveiled carrier mobility in DHHBTN-based 2D MOFs spanning from 0.69 to 3.10 cm2 V-1 s-1. Among M-DHHBTN famility, Cu-DHHBTN displayed high electrical conductivity reaching 0.21 S cm-1 at 298 K with thermal activation behavior. This work leverages the "branched conjugation" of the ligand to encode heteroporosity into highly conductive 2D MOFs, underscoring the significant potential of heterogeneous double-pore structures for future applications.

Baicalin nanodelivery system based on functionalized metal-organic framework for targeted therapy of osteoarthritis by modulating macrophage polarization

Intra-articular drugs used to treat osteoarthritis (OA) often suffer from poor pharmacokinetics and stability. Nano-platforms as drug delivery systems for drug delivery are promising for OA therapy. In this study, we reported an M1 macrophage-targeted delivery system Bai@FA-UIO-66-NH2 based on folic acid (FA) -modified metal-organic framework (MOF) loaded with baicalin (Bai) as antioxidant agent for OA therapy. With outstanding biocompatibility and high drug loading efficiency, Bai@FA-UIO-66-NH2 could be specifically uptaken by LPS-induced macrophages to serve as a potent ROS scavenger, gradually releasing Bai at the subcellular level to reduce ROS production, modulate macrophage polarization to M2, leading to alleviation of synovial inflammation in OA joints. The synergistic effect of Bai@FA-UIO-66-NH2 on macrophage polarization and ROS scavenging significantly improved the therapeutic efficacy of OA, which may provide a new insight into the design of OA precision therapy.

Surface-supported metal-organic frameworks with geometric topological diversity via scanning tunneling microscopy

Surface-supported metal-organic frameworks (SMOFs) are long-range ordered periodic 2D lattice layers formed by inorganic metal nodes and organic ligands via coordination bonds on substrate surfaces. The atomic resolution STM lays a solid foundation for the conception and construction of SMOFs with large area, stable structure, and special function. In this review, the cutting-edge research of SMOFs from design strategy, preparation process, and how to accurately achieve structural and functional diversity are reviewed. Furthermore, we focus on the design and construction of novel and fascinating periodic and fractal structures, in which some typical honeycomb structures, Kagome lattice, hexagonal geometry, and Sierpiński triangles are summarized, and the related prospects for designing functional nanoscale systems and architectures are prospected. Finally, the challenges faced in the design and synthesis of SMOFs are denoted, and the application prospect and development trend of SMOFs are forecasted based on the current research status.

Covalent Organic Framework Enhanced Solid Polymer Electrolyte for Lithium Metal Batteries

High ionic conductivity, outstanding mechanical stability, and a wide electrochemical window are the keys to the application of solid-state lithium metal batteries (LMBs). Due to their regular channels for ion transport and tailored functional groups, covalent organic frameworks (COFs) have been applied to solid electrolytes to improve their performance. Herein, we report a flexible polyethylene oxide-COF-LZU1 (abbreviated as PEO-COF) electrolyte membrane with a high lithium ion transference number and satisfactory mechanical strength, allowing for dendrite-free and long-time cycling for LMBs. Benefiting from the interaction between bis(triflfluoromethanesulonyl)imide anions (TFSI-) and aldehyde groups in COF-LZU1, the Li+ transference number of the PEO-5% COF-LZU1 electrolyte reached up to 0.43, much higher than that of neat PEO electrolyte (0.18). Orderly channels are conducive to the homogenous Li-+ deposition, thereby inhibiting the lithium dendrites. The assembled LiFePO4|PEO-5% COF-LZU1/Li cells delivered a discharge specific capacity of 146 mAh g-1 and displayed a capacity retention of 80% after 200 cycles at 0.1 C (60 °C). The Li/Li symmetrical cells of the PEO-5% COF-LZU1 electrolyte presented a longer working stability at different current densities compared to that of the PEO electrolyte. Therefore, the enhanced comprehensive performance of the solid electrolyte shows potential application prospects for use in LMBs.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

Optimizing Porous Metal-Organic Layers for Stable Zinc Anodes

Aqueous zinc-ion batteries (ZIBs) have been considered as alternative stationary energy storage systems, but the dendrite and corrosion issues of Zn anodes hinder their practical applications. Here we report a series of two-dimensional (2D) metal-organic frameworks (MOFs) with Zr12 clusters, which act as artificial solid electrolyte interphase (SEI) layers to prevent dendrites and corrosion of Zn anodes. The Zr12-based 2D MOF layers were formed by incubating 3D layer-pillared Zr-MOFs in ZnSO4 aqueous electrolytes, which replaced the pillar ligands with terminal SO42-. Furthermore, the pore sizes of Zr12-based 2D MOF layers were systematically tuned, leading to optimized Zn2+ conduction properties and protective performance for Zn anodes. In contrast to the traditional 2D-MOFs with Zr6 clusters, Zr12-based 2D MOF layers as artificial SEI significantly reduced the polarization and increased the stability of Zn anodes in MOF@Zn||MOF@Zn symmetric cells and MOF@Zn||MnO2 full cells. In situ experiments and DFT computations reveal that the enhanced cell performance is attributed to the unique Zr12-based layered structure with intrinsic pores to allow fast Zn2+ diffusion, surface Zr-SO4 zincophilic sites to induce uniform Zn deposition, and inhibited hydrogen evolution by 2D MOF Zr12 layers.

Stepwise construction of multi-component metal-organic frameworks

Multi-component metal-organic frameworks (MC-MOFs) are crystalline porous materials containing multiple organic ligands or mixed metals, which manifest new properties beyond the linear combination of the single component. However, the traditional one-pot synthesis method for MOFs is not always applicable for synthesizing MC-MOFs due to the competitive coordination of multiple ligands and metals. Therefore, the stepwise construction of MC-MOFs has been explored, which enables more precise control of the heterogeneity within the ordered MC-MOFs. This review provides a summary of the synthesis strategies, namely, ligand exchange, coordinative modification, covalent modification, ligand metalation, cluster metalation, and use of mixed-metal precursors, for the stepwise construction of MC-MOFs. Furthermore, we discuss the applications of MC-MOFs with ordered arrangements of multiple functionalities, focusing on gas adsorption and separation, water remediation, heterogeneous catalysis, luminescence, and chemical sensing.

A series of polyoxometalate-based COF composites by one-pot mechanosynthesis of thioether to sulfone

An effective combination of polyoxometalates (POMs) and porous materials is a feasible method to solve the homogeneity of POMs and synthesize extremely stable POM-based catalysts. Herein, by using simple mechanochemical synthesis, we fabricated a series of composites constructed by Keggin-POMs, p-phenylenediamine (Pa-1), and 1,3,5-triformylphloroglucinol (Tp), which in situ form a stable covalent organic framework (Keggin-POMs@TpPa-1). Notably, the different Keggin-POMs@TpPa-1 composites showed different catalytic effects on thioether oxidation reaction under mild conditions. From the comparison, the catalytic effect of PW12@TpPa-1 with its added amount of 27% H3PW12O40 is superior to that of other composites, whose catalytic efficiency can reach 99%. This study provides some inspiration for designing diverse POM-modified catalysts with outstanding stability and efficiency using COFs as supports.

Aptamer-modified metal organic frameworks for measurement of food contaminants: a review

The measurement of food contaminants faces a great challenge owing to the increasing demand for safe food, increasing consumption of fast food, and rapidly changing patterns of human consumption. As different types of contaminants in food products can pose different levels of threat to human health, it is desirable to develop specific and rapid methods for their identification and quantification. During the past few years, metal-organic framework (MOF)-based materials have been extensively explored in the development of food safety sensors. MOFs are porous crystalline materials with tunable composition, dynamic porosity, and facile surface functionalization. The construction of high-performance biosensors for a range of applications (e.g., food safety, environmental monitoring, and biochemical diagnostics) can thus be promoted through the synergistic combination of MOFs with aptamers. Accordingly, this review article delineates recent innovations achieved for the aptamer-functionalized MOFs toward the detection of food contaminants. First, we describe the basic concepts involved in the detection of food contaminants in terms of the advantages and disadvantages of the commonly used analytical methods (e.g., DNA-based methods (PCR/real-time PCR/multiplex PCR/digital PCR) and protein-based methods (enzyme-linked immunosorbent assay/immunochromatography assay/immunosensor/mass spectrometry). Afterward, the progress in aptamer-functionalized MOF biosensors is discussed with respect to the sensing mechanisms (e.g., the role of MOFs as signal probes and carriers for loading signal probes) along with their performance evaluation (e.g., in terms of sensitivity). We finally discuss challenges and opportunities associated with the development of aptamer-functionalized MOFs for the measurement of food contaminants.

Interfacial chemistries in metal-organic framework (MOF)/covalent-organic framework (COF) hybrids

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been attracting tremendous attention in various applications due to their unique structural properties. Recent interest has been focused on their combination as hybrids to enable the engineering of new classes of frameworks with complementary properties. This review gives a comprehensive summary on the interfacial chemistries in MOF/COF hybrids, which play critical roles in their hybridization. The challenges and perspectives in the field of MOF/COF hybrids are also provided to inspire more efforts in diversifying this hybrid family and their cross-disciplinary applications.

Mn-doped Ti-based MOFs for magnetic resonance imaging-guided synergistic microwave thermal and microwave dynamic therapy of liver cancer

Currently, precise ablation of tumors without damaging the surrounding normal tissue is still an urgent problem for clinical microwave therapy of liver cancer. Herein, we synthesized Mn-doped Ti MOFs (Mn–Ti MOFs) nanosheets by in-situ doping method and applied them for microwave therapy. Infrared thermal imaging results indicate Mn–Ti MOFs can rapidly increase the temperature of normal saline, attributing to the porous structure improving microwave-induced ion collision frequency. Moreover, Mn–Ti MOFs show higher ¹O₂ output than Ti MOFs under 2 W of low-power microwave irradiation due to the narrower band-gap after Mn doping. At the same time, Mn endows the MOFs with a desirable T₁ contrast of magnetic resonance imaging (r₂/r₁ = 2.315). Further, results on HepG2 tumor-bearing mice prove that microwave-triggered Mn–Ti MOFs nearly eradicate the tumors after 14 days of treatment. Our study offers a promising sensitizer for synergistic microwave thermal and microwave dynamic therapy of liver cancer.

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Mn-doped Ti-MOFs nanosheets (Mn–Ti MOFs) were synthesized as novel microwave sensitizers.

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Mn–Ti MOFs can significantly generate heat and produce ROS under low-power microwave irradiation.

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The combination of microwave thermal therapy and microwave dynamic therapy can effectively inhibit the growth of tumor cells in vitro and in vivo.

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The microwave sensitizers have potential application in MRI-guided microwave therapy for liver cancer.

The Preparation of Covalent Bonding COF-TpBD Coating in Arrayed Nanopores of Stainless Steel Fiber for Solid-Phase Microextraction of Polycyclic Aromatic Hydrocarbons in Water

Covalent organic framework (COF)-TpBD was grafted on the arrayed nanopores of stainless steel fiber (SSF) with (3-aminopropyl) triethoxysilane as the cross-linking agent. The prepared SSF bonded with COF-TpBD showed high thermal and chemical stability and excellent repeatability. The prepared SSF bonded with COF-TpBD was also used for the solid-phase microextraction (SPME) of seven kinds of polycyclic aromatic hydrocarbons (PAHs) in actual water samples, followed by gas chromatography with flame ionization detection (GC-FID) determination, which exhibited low limits of detection (LODs), good relative standard deviation (RSD) and high recoveries.