Strategic Defect Engineering of Metal–Organic Frameworks for Optimizing the Fabrication of Single-Atom Catalysts

Single-atom catalysts (SACs) have garnered enormous interest due to their remarkable catalysis activity. However, the exploitation of universal synthesis strategy and regulation of coordination environment of SACs remain a great challenge. Herein, a versatile synthetic strategy is demonstrated to generate a series of transition metal SACs (M SAs/NC, M = Co, Cu, Mn; NC represents the nitrogen-doped carbon) through defect engineering of metal-organic frameworks (MOFs). The interatomic distance between metal sites can be increased by deliberately introducing structural defects within the MOF framework, which inhibits metal aggregation and consequently results in an approximately 70% increase in single metal atom yield. Additionally, the coordination structures of metal sites can also be facilely tuned. The optimized Co SAs/NC-800 exhibits superior activity and excellent reusability for the selective hydrogenation of nitroarenes, surpassing several state-of-art non-noble-metal catalysts. This study provides a new avenue for the universal fabrication of transition metal SACs.     

Linker Defects in Metal–Organic Frameworks for the Construction of Interfacial Dual Metal Sites with High Oxygen Evolution Activity

Designing efficient electrocatalysts based on metal–organic framework (MOF) nanosheet arrays (MOFNAs) with controlled active heterointerface for the oxygen evolution reaction (OER) is greatly desired yet challenging. Herein, a facile strategy for the synthesis of MOF-based nanosheet arrays (γ-FeOOH/Ni-MOFNA) is developed with abundant heterointerfaces between Ni-MOF and γ-FeOOH nanosheets by introducing linker defects to the former. The experimental and theoretical results show the key role of linker defects in inducing the growth of secondary γ-FeOOH nanosheets onto the surface of Ni-MOFNAs, which further leads to the formation of interfacial Ni/Fe dual sites with high oxygen evolution activity. Notably, the resulting γ-FeOOH/Ni-MOFNA exhibits excellent OER performance with low overpotentials of 193 and 222 mV at 10 and 100 mA cm⁻², respectively. Furthermore, the study of the structure–performance relationship of MOF-based heterostructures reveals that Ni sites at the interface of the γ-FeOOH/Ni-MOFNA have higher activity than those at the interface of NiFe layered double hydroxide and Ni-MOFNA. This study provides a new prospect on heterostructured electrocatalysts with highly active sites for enhanced OER.     

Laser-Assisted Printing of Electrodes Using Metal–Organic Frameworks for Micro-Supercapacitors

Direct laser scribing, an advanced printing technique, has been recently developed to enable the carbonization of carbonaceous precursors in a rapid, precise, and cost-effective manner. Herein, it is reported that metal−organic frameworks (MOFs) can be converted into patterned derived carbon with desired structural features using a CO₂ infrared laser system. Metal species in MOFs play a key role in the morphology, porous structure, and crystallinity of the resulting laser-induced products by studying six representative MOFs. Diverse features such as ordered porous structure and continuous network microstructure can be obtained in the laser-induced MOF-derived carbon, which is influenced by the melting and boiling points of metals and their magnetic and catalytic behaviors. Furthermore, a core–shell structured composite (MOF-199@ZIF-67) has been designed and prepared for the fabrication of 12-interdigital electrodes derived from the composite by laser-assisted printing. The as-obtained electrodes with highly porous and hierarchical structure show an enhanced specific capacitance for micro-supercapacitors (MSCs). This work provides a complementary heat treatment method to produce MOF-derived carbon nanomaterials with desired structural features and patterns for MSCs and micro-device-related applications.     

Multimode Emission from Lanthanide-Based Metal–Organic Frameworks for Advanced Information Encryption

Although remarkable progress on luminescent materials is made in advanced optical information storage and anti-counterfeiting applications, many challenges still remain in these fields. Currently, most luminescent materials are based on a single photoluminescent model that can be easily imitated by substitutes. In this work, a series of multimodal emission lanthanide-based metal–organic frameworks (MOFs) are developed, where they emit red and green light originating from Eu³⁺ and Tb³⁺ under ultraviolet light irradiation. Meanwhile, under 980 nm near-infrared laser irradiation, these MOFs show cyan upconversion cooperative luminescence derived from Yb³⁺ and characteristic upconversion luminescence from lanthanide activators (Eu³⁺, Tb³⁺, or Ho³⁺), respectively. Based on the integrated optical functionality, the functional information storage applications are successfully designed, which indicates that multimodal emission features can be easily detected under ultraviolet lamps (254 or 393 nm) or 980 nm near-infrared laser. And, the unique optical features show a high level of security in the advanced information storage application, which would be sufficiently complex to be forged.     

Novel Metal–Organic Framework Cocrystal Strategy for Significantly Enhancing Photocatalytic Performance

Owing to their high carrier recombination speed and low spectral utilization, it is difficult to further improve the performance of photocatalysts. In this study, a novel metal–organic framework (MOF) self-assembled cocrystal material is developed. The guest molecule is inserted and self-assembled with the existing MOF ligand to form an organic cocrystal. The highly ordered molecular arrangement and tight intermolecular distances between donor and acceptor molecules promote a strong π–π charge transfer interaction, facilitating the migration and separation of photogenerated charge carriers. In addition, efficient redshifts in the absorption wavelength enhance the response to visible light. Further, the unique porous structure of MOFs is beneficial for increasing the interfacial area of photocatalytic reactions, and metal ions can become the center of photogenerated carrier capture, effectively inhibiting carrier recombination. Consequently, the MOF cocrystal demonstrates remarkable efficiency in the degradation of pollutants in water, achieving a noteworthy removal efficiency of 95.31% within 15 min. Moreover, the photocatalyic reaction kinetics constant of the MOF cocrystal is 46.5 times higher, indicating the success of this new strategy in developing highly efficient photocatalytic systems.     

Pulmonary Targeting Crosslinked Cyclodextrin Metal–Organic Frameworks for Lung Cancer Therapy

Lung cancer is a serious threat to human health with the highest morbidity and mortality; metastatic lung cancer accounts for a majority of cancer-related deaths. Hence, there is considerable interest in developing efficient lung-targeted drug delivery systems to improve overall survival and quality of life of lung cancer patients. Based on the lung-targeting characteristics of cubic crosslinked cyclodextrin metal–organic framework (CDF) nanoparticles, this study shows the synthesis of a nanoplatform using RGD-functionalized CDF to co-deliver low-molecular-weight heparin (LMWH) and doxorubicin (DOX) for treatment of lung cancer. Rational design of the DOX-loaded RGD-CDF-LMWH nanoplatform (RCLD) is carried out. RCLD nanoparticles are efficiently targeted to lung tumors following intravenous administration; RCLD accumulation in the lung is 5.8 times greater than that in the liver. Moreover, RCLD inhibits migration and invasion of cancer cells in vitro and significantly diminishes lung tumor nodule count and area of spread in human A549 and murine B16F10 lung cancer models in vivo. Furthermore, RCLD does not show serum enzyme or histopathologic indicators of tissue damage or adverse hematologic effects. Therefore, the multiple antitumor activities of this novel RCLD nanoplatform, alongside its safety profile for normal tissues, strongly support its use for targeted treatment of lung cancer.     

Hierarchical Oriented Metal–Organic Frameworks Assemblies for Water-Evaporation Induced Electricity Generation

Advances in metal–organic frameworks (MOFs) are stimulating interest in water-evaporation induced electricity generation. Establishing design principles for desirable MOFs and revealing the structure-activity relationship is essential to development of this field. Around this key issue, herein the concept of “hierarchical oriented MOFs” is proposed and the Cu(BDC-OH) MOFs are organized into hierarchical oriented assemblies by successively using methods of hydrolysis, anion exchange reaction, and heteroepitaxy growth. It is discovered that this hierarchical oriented Cu(BDC-OH) assemblies-based generator containing long-ranged ordered microporous channels exhibits a voltage of 0.6 V and electricity output of 1.56 nW cm^–2 in the natural state of water evaporation. Finite element analysis and density functional calculation elucidate that hierarchical oriented structure of Cu(BDC-OH) MOFs with certain surface charge density have an “aggregation effect,” dominating the final output voltages. This work not only offers valuable theoretical guidance for exploring self-assembly of MOFs superstructures and water-evaporation induced electricity generation, but also may open interesting perspectives for understanding some fundamental questions about structure-activity relationship in MOFs materials science.     

The Role of Defects in Metal–Organic Frameworks for Nitrogen Reduction Reaction: When Defects Switch to Features

The electrochemical nitrogen reduction reaction (NRR), a contributor for producing ammonia under mild conditions sustainably, has recently attracted global research attention. Thus far, the design of highly efficient electrocatalysts to enhance NRR efficiency is a specific focus of the research. Among them, defect engineering of electrocatalysts is considered a significant way to improve electrocatalytic efficiency by regulating the electronic state and providing more active sites that can give electrocatalysts better physicochemical properties. Recently, metal–organic frameworks (MOFs), along with their derivatives, have captured immense interest in electrocatalytic reactions owing to not only their large surface area and high porosity but also the ability to create rich defects in their structures. Hence, they can provide plenty of exposed active sites for electron transfer, NN cleavage, and N₂ adsorption to enhance NRR performance. Herein, the concept, the in situ characterizations techniques for defects, and the most common ways to create defects into MOFs have been summarized. Furthermore, the recent advances of MOF-based electrocatalysts towards NRR have been recapitulated. Ultimately, the major challenges and outlook of defects in MOFs for NRR are proposed. This paper is anticipated to provide critical guidelines for optimizing NRR electrocatalysts.     

Pristine Metal–Organic Frameworks and their Composites for Renewable Hydrogen Energy Applications

Metal–organic frameworks (MOFs) have emerged as ideal multifunctional platforms for renewable hydrogen (H₂) energy applications owing to their tunable chemical compositions and structures and high porosity. Their advanced component species and porous structure contribute greatly to the enhanced activity, electrical conductivity, photo response, charge-hole separation efficiency, and structural stability of MOF materials, which are promising for practical H₂ economy. In this review, we mainly introduce design strategies for the enhancement of electro-/photochemical behaviors or adsorption performance of porous MOF materials for H₂ production, storage, and utilization from compositional perspective. Following these engineering strategies, the correlation between composition and property-structure-performance of pristine MOFs and their composite with advanced components is illustrated. Finally, challenges and directions of future development of related MOFs and MOF composites for H₂ economy are provided.     

Post-Synthetic Modification of Metal–Organic Frameworks Toward Applications

Post-synthetic modification (PSM) of metal–organic framework (MOF) compounds is a useful technique for preparing new MOFs that can exhibit or enhance many of the properties of the parent MOFs. PSM can be carried out by a number of approaches such as modifying the linker (ligand) and/or metal node, and adsorption/exchange of guest species. The surface environment of the MOF can be modified to increase structural stability as well as introducing desired properties. There is considerable scope in widening the applications of the MOF with compatible metal or ligand employing the PSM. This review focuses on the recent developments of modified materials through PSM, which augers well for the chemical modification and functionalization of MOFs. In this review, different types of PSM methods are presented in an orderly manner, and the diverse applications of resultant frameworks are described and discussed.     

Laser-Induced Annealing of Metal–Organic Frameworks on Conductive Substrates for Electrochemical Water Splitting

The conventional thermal transformation of metal–organic frameworks (MOFs) for electrocatalysis requires high temperature, an inert atmosphere, and long duration that result in severe aggregation of metal particles and non-uniform porous structures. Herein, a precise and inexpensive laser-induced annealing (LIA) strategy, which eliminates particle aggregation and rapidly generates uniform structures with a high exposure of active sites, is introduced to carbonize MOFs on conductive substrates under ambient conditions within a few minutes. By systematically considering 8 substrates and 12 MOFs, a series of LIA-MOF/substrate devices with controllable sizes and good flexibility are successfully obtained. These LIA-MOF/substrate devices can directly serve as working electrodes. Remarkably, LIA-MIL-101(Fe) on nickel foam exhibits an ultralow overpotential of 225 mV at a current density of 50 mA cm⁻² and excellent stability over 50 h for facilitating the oxygen evolution reaction, outperforming most recently reported transition-metal-based electrocatalysts and commercial RuO₂. Physical characterizations and theoretical calculations evidence that the high activity of LIA-MIL-101(Fe) arises from the favorable adsorption of intermediates at its Ni-doped Fe₃O₄ overlayer that is formed during the laser treatment. Moreover, the LIA-MOF/substrate devices are assembled for overall water splitting. The proposed LIA strategy demonstrates a cost-effective route for manufacturing scalable energy storage and conversion devices.     

Assembling Amorphous Metal–Organic Frameworks onto Heteroatom-Doped Carbon Spheres for Remarkable Bifunctional Oxygen Electrocatalysis

Multifunctional electrocatalysts play an increasingly crucial role in various practical electrochemical energy conversion devices. Especially, on the air cathode of rechargeable zinc–air batteries (ZABs), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), requiring efficient bifunctional electrocatalysts, are switched during discharging and charging process. Here, supported by the theoretical computations, a facile strategy for the in situ assembly of NiFe-MOFs nanosheets on heteroatoms-doped porous activated carbon spheres is developed. The newly designed electrocatalyst (NP-ACSs@NiFe-MOFs) shows excellent performance toward bifunctional oxygen electrocatalysis. Specifically, a remarkable low value of potential gap (ΔE = 0.61 V), which is the difference between the potential to reach an OER current density of 10 mA cm⁻² and ORR half-wave potential, is achieved in 0.1 m KOH. Notably, the aqueous ZAB based on NP-ACSs@NiFe-MOFs shows super cycle stability with small voltage gap of only 0.79 V when cycled for 450 h at 10 mA cm⁻². Also, the quasi-solid-state ZAB indicates excellent flexibility and cycling stability. This study presents a facile strategy for the rational integration of different catalytically active components, and can be extended to prepare other strongly competitive multifunctional electrocatalysts.     

Electronic Modulation of Metal–Organic Frameworks by Interfacial Bridging for Efficient pH-Universal Hydrogen Evolution

Designing well-defined interfacial chemical bond bridges is an effective strategy to optimize the catalytic activity of metal–organic frameworks (MOFs), but it remains challenging. Herein, a facile in situ growth strategy is reported for the synthesis of tightly connected 2D/2D heterostructures by coupling MXene with CoBDC nanosheets. The multifunctional MXene nanosheets with high conductivity and ideal hydrophilicity as bridging carriers can ensure structural stability and sufficient exposure to active sites. Moreover, the Co–O–Ti bond bridging formed at the interface effectively triggers the charge transfer and modulates the electronic structure of the Co-active site, which enhances the reaction kinetics. As a result, the optimized CoBDC/MXene exhibits superior hydrogen evolution reaction (HER) activity with low overpotentials of 29, 41, and 76 mV at 10 mA cm⁻² in alkaline, acidic, and neutral electrolytes, respectively, which is comparable to commercial Pt/C. Theoretical calculation demonstrates that the interfacial bridging-induced electron redistribution optimizes the free energy of water dissociation and hydrogen adsorption, resulting in improved hydrogen evolution. This study not only provides a novel electrocatalyst for efficient HER at all pH conditions but also opens up a new avenue for designing highly active catalytic systems.     

New-Generation Anion-Pillared Metal–Organic Frameworks with Customized Cages for Highly Efficient CO2 Capture

The rational design of porous materials for CO₂ capture under realistic process conditions is highly desirable. However, trade-offs exist among a nanopore's capacity, selectivity, adsorption heat, and stability. In this study, a new generation of anion-pillared metal-organic frameworks (MOFs) are reported with customizable cages for benchmark CO₂ capture from flue gas. The optimally designed TIFSIX-Cu-TPA exhibits a high CO₂ capacity, excellent CO₂/N₂ selectivity, high thermal stability, and chemical stability in acid solution and acidic atmosphere, as well as modest adsorption heat for facile regeneration. Additionally, the practical separation performance of the synthesized MOFs is demonstrated by breakthrough experiments under various process conditions. A highly selective separation is achieved at 298–348 K with the impressive CO₂ capacity of 2.1–1.4 mmol g⁻¹. Importantly, the outstanding performance is sustained under high humidity and over ten repeat process cycles. The molecular mechanism of MOF's CO₂ adsorption is further investigated in situ by CO₂ dosed single crystal structure and theoretical calculations, highlighting two separate binding sites for CO₂ in small and large cages featured with high CO₂ selectivity and loading, respectively. The simultaneous adsorption of CO₂ inside these two types of interconnected cages accounts for the high performance of these newly designed anionic pillar-caged MOFs.     

Functional Metal–Organic Frameworks for Maximizing Transconductance of Organic Photoelectrochemical Transistor at Zero Gate Bias and Biological Interfacing Application

Organic electrochemical transistors showing maximum transconductance (g_m) at zero gate bias (V_G) is desired but has long been a challenge. To date, few solutions to this issue are available. Light-matter interplay is shown as rich sources for optogenetics, photodynamic therapy, and advanced electronics, but its potential in g_m modulation are largely untapped. Herein, the challenge is addressed by unique light-matter interplay in the newly emerged technique of organic photoelectrochemical transistor (OPECT), which is exemplified by dual-ligand photosensitive metal–organic frameworks (DL-PS-MOFs)/TiO₂ nanorods (NRs) gated poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) OPECT under 425 nm light irradiation. Interestingly, the light stimulation on the DL-PS-MOFs can de-dope PEDOT:PSS with altered transistor physics, achieving device showing maximum g_m at zero V_G and the simultaneous superior output of channel current. In connection to a cascade catalytic hairpin assembly-rolling circle amplification strategy, such a device is then biologically interfaced with a miRNA-triggered growth of DNA spheres for the sensitive detection of miRNA-21 down to 0.12 fm . This work features a proof-of-concept study using light-matter interplay to enable organic transistors showing maximum g_m at zero V_G and its sensitive biological interfacing application.     

A Dual−Functional Cationic Covalent Organic Frameworks Modified Separator for High Energy Lithium Metal Batteries

Separator modification is an efficient strategy to handle with the challenges of lithium metal batteries but its success is primarily subject to the modification of the materials. Herein, a cationic covalent organic framework (COF) composed of positively charged organic units and weakly bonded fluoride ions (F⁻) is introduced to modify the commercial polypropylene separator (COF−F@PP). It is found that the organic unit has abundant nanopores to homogenize the lithium ions (Li⁺) flux and can interact with electrolyte solvent molecules to form a desolvation structure of Li⁺. Meanwhile, the F⁻ within the nanopores is proved to assist in building a robust LiF−riched solid electrolyte interphase to avoid the side reactions between lithium anode and electrolyte. Hence, the COF−F@PP delivers feasible practicality for the outstanding cycling stability, high Coulombic efficiency, and superior rate capability of Li//LFP coin cell at 5 C, low N/P ratio (2.19) full cell, and pouch cell at 1 C.     

Smart Metal–Organic Frameworks with Reversible Luminescence/Magnetic Switch Behavior for HCl Vapor Detection

Smart luminescent metal–organic frameworks (MOFs) demonstrate promising performance in the detection of toxic gases. The incorporation of twisted or rotary organic ligands with aggregation-induced emission (AIE) characteristics can provide further opportunities in designing such smart MOFs with new topologies and stimuli-responsive behaviors. Herein, novel AIE MOFs are reported with reversible luminescence or a magnetic switch for HCl vapor detection. The twisted conformation of tetrakis(4-carboxyphenyl)ethylene (TCPE) ligand leads to the unique [M⁺–L–M⁻–L–M]_∞ (M = metal clusters, L = ligand) configuration for ZnMOF and CoMOF. Different from conventional MOFs with [M–L]_∞ topology, ZnMOF and CoMOF exhibit a blue-to-yellow greenish fluorescence transition and a ferrimagnetic-to-antiferromagnetic switch behavior, respectively, upon recognition of HCl vapor. The adsorbed HCl molecules rather than coordinated ones are determined to be the main reason, and such luminescence and magnetic switch can be induced in a reversible manner via HCl vapor adsorption/desorption processes with high reliability. This work of AIE MOFs with twisted and rotary ligands shall pave new avenue in design of smart MOFs with new topologies and stimuli-responsive behavior for real-time sensing and detection applications.     

Palladium Nanocrystals-Engineered Metal–Organic Frameworks for Enhanced Tumor Inhibition by Synergistic Hydrogen/Photodynamic Therapy

Hydrogen therapy, as a star therapeutic modality, has recently acquired much attention in the field of anticancer medicine. Evidence suggests that hydrogen can selectively reduce intratumoral overexpressed hydroxyl radicals (•OH) to break the redox homoeostasis and thereby result in redox stress and cell damage. As a reactive oxygen species-related noninvasive modality, photodynamic therapy (PDT) has been approved for varied tumor treatments clinically. For implementing tumor therapy with enhanced anticancer efficacy and attenuated side effects, here a biocompatible palladium nanocrystals-integrated nanoscale porphyrinic metal–organic framework (NPMOF) is designed to develop a novel combined therapy modality, that is, synergistic hydrogen/photodynamic therapy. The NPMOF is employed simultaneously as the photosensitizer for PDT and as the nanocarrier to support palladium nanocrystals, which is further used as the hydrogen vehicle. The final hydrogen-containing nanosystems exhibit a persistent reductive hydrogen release behavior and considerable light-activated singlet oxygen (¹O₂) generation without mutual interference, contributing to the adequate disturbance of tumor microenvironment redox steady-state for synergistically inducing tumor cell death. Ultimately, by coupling of tumor-selective hydrogen therapy and PDT, the designed nanosystems realize the augmented therapeutic outcome with minimal side effects, providing a safe and efficient tumor treatment for future clinical translation.     

Localized Surface Plasmon Resonance Promotes Metal–Organic Framework-Based Photocatalytic Hydrogen Evolution

Combining metal nanoparticles (NPs) featured with localized surface plasmon resonance (LSPR) with metal–organic framework (MOF)-based photocatalysts is a novel means for achieving efficient separation of electron–hole pairs. Herein, the Au@NH₂-UiO-66/CdS composites are successfully synthesized by encapsulating Au NPs with LSPR into the NH₂-UiO-66 nanocage, further growing CdS NPs on the surface of the NH₂-UiO-66, which exhibits higher photocatalytic activity in hydrogen evolution reaction under visible-light irradiation than that of NH₂-UiO-66/CdS and CdS, respectively. Transient absorption measurements reveal that MOF is not only a transit station for electrons generated from CdS to Au, but also a receiver for hot electrons generated from plasmonic Au in Au@MOF/CdS composites. Thus, the LSPR-induced hot electron transfer from Au NPs is an important manifestation to prolong the carrier lifetime and enhance the photocatalytic performance. This work provides insights into investigating the photoinduced carrier dynamics of nanomaterials with LSPR effects for enhancing the MOF-based photocatalytic performance.     

Implanting of Single Zinc Sites into 2D Metal–Organic Framework Nanozymes for Boosted Antibiofilm Therapy

Design of nanozymes with catalytic active sites at atomic-scale not only improves the atomic utilization, but also provides a well-defined coordination structure for nanozymes’ catalytic mechanism research. Herein, a surfactant-assisted method is reported for preparing 2D metal–organic frameworks based single zinc sites nanozyme (SZN-MOFs) through assembling preformed Zn single-atom coordinated porphyrin precursors into ultrathin MOF nanosheets. The Zn atom loading weight ratio in SZN-MOFs is up to 4.6 wt.%. The SZN-MOFs exhibit extraordinary peroxidase-like activity, which can effectively catalyze H₂O₂ into hydroxyl radicals. The catalytical mechanism is elucidated and the origin of the high peroxidase-like activity of SZN-MOFs is rationalized using density functional theory calculations. Finally, it is demonstrated that the SZN-MOFs present great promises for in vitro and in vivo antibiofilm activity under a low concentration of H₂O₂. This study not only develops a surfactant-assisted method for fabricating MOF-based single sites nanozyme, but also manifests the applications in the field of antibiofilm therapy.