Uncovering two kinetic factors in the controlled growth of topologically distinct core-shell metal-organic frameworks

Leveraging Ligand Steric Demand to Control Ligand Exchange and Domain Composition in Stratified Metal-Organic Frameworks

Incorporating diverse components into metal-organic frameworks (MOFs) can expand their scope of properties and applications. Stratified MOFs (sMOFs) consist of compositionally unique concentric domains (strata), offering unprecedented complexity through partitioning of structural and functional components. However, the labile nature of metal-ligand coordination handicaps achieving compositionally distinct domains due to ligand exchange reactions occurring concurrently with secondary strata growth. To achieve complex sMOF compositions, characterizing and controlling the competing processes of new strata growth and ligand exchange are vital. This work systematically examines controlling ligand exchange in UiO-67 sMOFs by tuning ligand sterics. We present quantitative methods for assessing and visualizing the outcomes of strata growth and ligand exchange that rely on high-angle annular dark-field images and elemental mapping via scanning transmission electron microscopy-energy dispersive X-ray spectroscopy. In addition, we leverage ligand sterics to create 'blocking layers' that minimize ligand exchange between strata which are particularly susceptible to ligand exchange and inter-strata ligand mixing. Finally, we evaluate strata compositional integrity in various solvents and find that sMOFs can maintain their compositions for >12 months in some cases. Collectively, these studies and methods enhance understanding and control over ligand placement in multi-domain MOFs, factors that underscore careful tunning of properties and function.

Recent Advances of Bimetallic-Metal Organic Frameworks: Preparation, Properties, and Fluorescence-Based Biochemical Sensing Applications

Bimetallic-metal organic frameworks (BiM-MOFs) or bimetallic organic frameworks represent an innovative and promising class of porous materials, distinguished from traditional monometallic MOFs by their incorporation of two metal ions alongside organic linkers. BiM-MOFs, with their unique crystal structure, physicochemical properties, and composition, demonstrate distinct advantages in the realm of biochemical sensing applications, displaying improvements in optical properties, stability, selectivity, and sensitivity. This comprehensive review explores into recent advancements in leveraging BiM-MOFs for fluorescence-based biochemical sensing, providing insights into their design, synthesis, and practical applications in both chemical and biological sensing. Emphasizing fluorescence emission as a transduction mechanism, the review aims to guide researchers in maximizing the potential of BiM-MOFs across a broader spectrum of investigations. Furthermore, it explores prospective research directions and addresses challenges, offering valuable perspectives on the evolving landscape of fluorescence-based probes rooted in BiM-MOFs.

Interfacial Engineering of Heterogeneous Reactions for MOF-on-MOF Heterostructures

Metal-organic frameworks (MOFs), as a subclass of porous crystalline materials with unique structures and multifunctional properties, play a pivotal role in various research domains. In recent years, significant attention has been directed toward composite materials based on MOFs, particularly MOF-on-MOF heterostructures. Compared to individual MOF materials, MOF-on-MOF structures harness the distinctive attributes of two or more different MOFs, enabling synergistic effects and allowing for the tailored design of diverse multilayered architectures to expand their application scope. However, the rational design and facile synthesis of MOF-on-MOF composite materials are in principle challenging due to the structural diversity and the intricate interfaces. Hence, this review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF hybrids can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, encompassing inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc. Meanwhile, these intriguing MOF-on-MOF materials offer a wide range of advantages in various application fields, such as adsorption, separation, catalysis, and energy-related applications. Finally, this review highlights current complexities and challenges while providing a forward-looking perspective on future research directions.

Hollow Ni3S4@Co3S4 with core-satellite nanostructure derived from metal-organic framework (MOF)-on-MOF hybrids as an electrode material for supercapacitors

Metal-organic frameworks (MOFs) have found wide applications in the field of supercapacitors due to their highly controllable porous structure, big specific surface area, and abundant chemical functional groups. MOF-on-MOF hybrids not only enhance the composition of MOFs (such as ligands and/or metal centers) but also provide greater structural diversity. By utilizing MOFs as precursors for preparing sulfides, the unique characteristics and inherent structure of MOFs are preserved but their conductivity and capacitance are enhanced. This study successfully synthesized hollow-structured Ni3S4@Co3S4 derived from an Ni-MOF@ZIF-67 hybrid structure, where the Ni-MOF serves as the core and ZIF-67 as the satellite. The Ni3S4@Co3S4 electrode demonstrated a specific capacity as high as 747.3 C g-1 at 1 A g-1, and it could still maintain 77% of its initial capacity at 10 A g-1. Furthermore, the assembled Ni3S4@Co3S4//AC hybrid supercapacitor (HSC) device achieved a maximum energy density of 30.8 W h kg-1 when the power density was 750 W kg-1. The device exhibited remarkable cycling durability, retaining 85.4% of its initial capacitance after 5000 cycles. Therefore, the derived functional materials based on MOF-on-MOF provide a more scalable and promising approach for the preparation of efficient electrode materials.

Implementation of a Core-Shell Design Approach for Constructing MOFs for CO2 Capture

Adsorption-based capture of CO₂ from flue gas and from air requires materials that have a high affinity for CO₂ and can resist water molecules that competitively bind to adsorption sites. Here, we present a core-shell metal-organic framework (MOF) design strategy where the core MOF is designed to selectively adsorb CO₂, and the shell MOF is designed to block H₂O diffusion into the core. To implement and test this strategy, we used the zirconium (Zr)-based UiO MOF platform because of its relative structural rigidity and chemical stability. Previously reported computational screening results were used to select optimal core and shell MOF compositions from a basis set of possible building blocks, and the target core-shell MOFs were prepared. Their compositions and structures were characterized using scanning electron microscopy, transmission electron microscopy, and powder X-ray diffraction. Multigas (CO₂, N₂, and H₂O) sorption data were collected both for the core-shell MOFs and for the core and shell MOFs individually. These data were compared to determine whether the core-shell MOF architecture improved the CO₂ capture performance under humid conditions. The combination of experimental and computational results demonstrated that adding a shell layer with high CO₂/H₂O diffusion selectivity can significantly reduce the effect of water on CO₂ uptake.

Synthesis and Peroxide Activation Mechanism of Bimetallic MOF for Water Contaminant Degradation: A Review

Metal-organic framework (MOF) materials possess a large specific surface area, high porosity, and atomically dispersed metal active sites, which confer excellent catalytic performance as peroxide (peroxodisulfate (PDS), peroxomonosulfate (PMS), and hydrogen peroxide (H₂O₂)) activation catalysts. However, the limited electron transfer characteristics and chemical stability of traditional monometallic MOFs restrict their catalytic performance and large-scale application in advanced oxidation reactions. Furthermore, the single-metal active site and uniform charge density distribution of monometallic MOFs result in a fixed activation reaction path of peroxide in the Fenton-like reaction process. To address these limitations, bimetallic MOFs have been developed to improve catalytic activity, stability, and reaction controllability in peroxide activation reactions. Compared with monometallic MOFs, bimetallic MOFs enhance the active site of the material, promote internal electron transfer, and even alter the activation path through the synergistic effect of bimetals. In this review, we systematically summarize the preparation methods of bimetallic MOFs and the mechanism of activating different peroxide systems. Moreover, we discuss the reaction factors that affect the process of peroxide activation. This report aims to expand the understanding of bimetallic MOF synthesis and their catalytic mechanisms in advanced oxidation processes.

Toward MOF@Polymer Core-Shell Particles: Design Principles and Potential Applications

ConspectusCompositing MOFs with polymers brings out the best properties of both worlds. The solubility and excellent mechanical properties of polymers endow the brittle, powdery MOFs with enhanced processability, thereby enriching their functions as solid sorbents, filters, membranes, catalysts, drug delivery vehicles, and so forth. While most MOF-polymer composites are random mixtures of two materials with little control over their fine structures, MOF@polymer core-shell particles have recently emerged as a new platform for precise composite design. The well-defined polymer coating can keep the rich pore characteristics of the MOF intact while furnishing the MOF with new properties such as improved dispersibility in various media, tunable surface energy, enhanced chemical stability, and regulated guest diffusion. Nevertheless, the structural and chemical complexity of MOFs poses a grand challenge to the development of a generalizable and feasible strategy for constructing MOF@polymer. Examples in the literature that showcase the presence of a well-defined polymer shell on the MOF with fully reserved porosity are rare. Moreover, methods for coating MOFs with condensation polymers (e.g., polyimide, polysulfone) are severely underexplored, despite their clear potential as membrane materials. In this Account, we present our group's effort over the past 4 years on the synthesis and applications of MOF@polymer composites. We first described a highly generalizable surface polymerization method that utilizes the rapid physisorption of a random copolymer (RCP) to carry initiating groups to the MOF surfaces. Subsequent controlled radical polymerization led to the formation of a uniform methacrylate or styrenic polymer on the MOF with tunable thickness and composition. To utilize the properties of condensation polymers, we pioneered the covalent grafting of polyimide (PI) brushes to UiO-66-NH₂ surfaces. In addition, to circumvent the need for a covalent anchoring group, we further developed an MOF surface grafting method based on mechanical linkage. Instead of connecting to the ligand, polyimide (PI) oligomer was linked to a functionalized linear polymer physically entangled within an MOF, thus realizing surface grafting with PI. Alternatively, PIs, polysulfone (PSF), and polycarbonate (PC) can also be grafted to various MOF surfaces through a metal-organic nanocapsule (MONC)-mediated method using a combination of electrostatic interaction and coordination bonds. To find a rapid and low-cost surface coating method suitable for commercialization, a new approach called non-solvent-induced surface-aimed deposition (NISAP) was developed. The action of the solvent phase separation drives dianhydrides and polyamines to the MOF surface, thus realizing accelerated polymerization and the rapid formation of a polymer coating on the MOF. Finally, we provided an overview of the unique properties and potential applications of MOF@polymer composites, including improved stability, MMMs, porous liquids (PLs), and immobilizing homogeneous catalysts. We hope that this Account can inspire more researchers to further develop and optimize the synthetic strategies for MOF@polymer and uncover its full application potential.

A robust hollow metal-organic framework with enhanced diffusion for size selective catalysis

Single crystalline (SC) hollow metal-organic frameworks (MOFs) are excellent host materials for molecular and nanoparticle catalysts. However, due to synthetic challenges, chemically robust SC hollow MOFs are rare. This work reports the construction of a defect-free and chemically stable SC hollow MOF, MOF-801(h), through templated growth from a unit cell mismatched core, UiO-66. Under the protection of excess MOF-801 ligand, fumaric acid, the MOF-801 shell was perfectly retained while the isoreticular UiO-66 core was selectively and completely etched away by formic acid. The combination of a large cavity, small aperture and short diffusion length allows the Pt nanoparticle encapsulated composite catalyst, Pt⊂MOF-801(h), to perform size selective hydrogenation of nitro compounds at an accelerated speed. Impressively, the catalyst can undergo concentrated HCl or boiling water treatment while maintaining its crystallinity, morphology, catalytic activity, and size selectivity. In addition, Au nanoparticles encapsulated catalyst, Au⊂MOF-801(h), was used for the size selective nucleophilic addition of HCl to terminal alkynes for the first time, which is a harsh reaction involving high concentrations of a strong acid.

Semi-hydrogenation of α,β-unsaturated aldehydes over sandwich-structured nanocatalysts prepared by phase transformation of thin-film Al2O3 to Al-TCPP

The semi-hydrogenation of α,β-unsaturated aldehydes to the desired unsaturated alcohols with both high conversion and high selectivity remains a big challenge. Herein, we designed a sandwich-structured nanocatalyst for the highly selective hydrogenation of various α,β-unsaturated aldehydes (e.g., cinnamaldehyde, furfural, crotonaldehyde, and 3-methyl-2-butenal) to the targeted unsaturated alcohols. Highly accessible platinum nanoparticles were sandwiched between a metal-organic framework (MOF) core (i.e., MIL-88B(Fe)) and a MOF shell (i.e., Al-TCPP). In particular, the growth of the Al-TCPP shell was achieved by atomic layer deposition (ALD) of thin-film Al₂O₃ followed by phase transformation with a tetrakis(4-carboxyphenyl)porphyrin (H₄TCPP) linker. The thickness of the Al-TCPP shell can be finely controlled by adjusting the cycle number of alumina ALD and the concentration of the H₄TCPP linker during the phase transformation of Al₂O₃ to Al-TCPP. It was proven that the permeable MOF shells could serve as selectivity regulators for the activation of the CO bonds in α,β-unsaturated aldehydes (in preference to the CC bonds), leading to higher selectivity towards unsaturated alcohols as compared to the conventional surface supported Pt catalysts. Mechanistic insights showed that the enhanced catalytic performance was attributed to (i) the modified electronic state of sandwiched Pt nanoparticles by the two MOF layers and (ii) the steric hindrance effect on substrate diffusion through the sandwich-structured catalysts.

Energetic Systematics of Metal-Organic Frameworks: A Case Study of Al(III)-Trimesate MOF Isomers

Thermal stability and thermodynamic properties of aluminum(III)-1,3,5-benzenetricarboxylate (Al-BTC) metal-organic frameworks (MOFs), including MIL-96, MIL-100, and MIL-110, have been investigated through a suite of calorimetric and X-ray techniques. In situ high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) revealed that these MOFs undergo thermal amorphization prior to ligand combustion. Thermal stabilities of Al-BTC MOFs follow the increasing order MIL-110 < MIL-96 < MIL-100, based on estimated amorphization temperatures. Their thermodynamic stabilities were directly measured by high-temperature drop combustion calorimetry. Normalized (per mole of Al) enthalpies of formation (ΔH*_f) of MIL-96, MIL-100, and MIL-110 from Al₂O_3, H₃BTC, and H₂O (only Al₂O₃ and H₃BTC for MIL-100) were determined to be -56.9 ± 13.7, -36.2 ± 17.9, and 62.8 ± 11.6 kJ/mol·Al, respectively. Our results demonstrate that MIL-96 and MIL-100 are thermodynamically favorable, while MIL-110 is metastable, in agreement with thermal and hydrothermal stability trends. The enthalpic preferences of MIL-96 and MIL-100 may be attributed to their shared trinuclear μ³-oxo-bridged (Al₃(μ₃-O)) secondary building units (SBUs) promoting stabilization of Al polyhedra by the ligands within these frameworks, in comparison to the sterically strained Al₈ octamer cluster cores formed in MIL-110. Furthermore, similar ΔH*_f of MIL-96 and MIL-100 explain their concurrent formation as physical mixtures often encountered during synthesis, implying the importance of kinetic factors that may facilitate the formation of Al-BTC framework isomers. More importantly, the normalized formation enthalpies of Al-BTC MOF isomers follow a negative correlation with the ratio of charged coordinated substituents to linkers (normalized per mole of Al within the MOF formula unit), with enthalpic preference given to systems with smaller (O^2- + OH^-)/ligand ratios. This trend has been successfully extended to the previously measured ΔH*_f of several Zn₄O-based frameworks (e.g., MOF-5, MOF-5(DEF), MOF-177, UMCM-1), all of which have been found to be metastable with respect to their dense phases (ZnO, H₂O, and ligands). The result suggests that carboxylate MOFs with higher metal coordination environments attain more enthalpic stabilization from the coordinated ligands. Thus, the formation of some lanthanide/actinide, transition metal, and main group carboxylate frameworks may be energetically more favored, which, however, requires further studies.

Mind the gap! tailoring sol–gel ceramic mesoporous coatings on labile metal–organic frameworks through kinetic control

Kinetic control allows for the synthesis of mesoporous silica shells on top of labile ZIF-8 cores without compromising MOF stability.

Connectivity Replication of Neutral Eu3+- and Tb3+-Based Metal-Organic Frameworks (MOFs) from Anionic Cd2+-Based MOF Crystallites

Neutral three-dimensional Eu³⁺- and Tb³⁺-based metal-organic frameworks (MOFs) with 4-fold interpenetration can be produced by seeding with anionic Cd²⁺-based MOF crystallites of identical connectivity. In the absence of these crystallites, two-dimensional networks are formed.

Lattice-Guided Construction and Harvest of a Naturally Nonpreferred Metal-Organic Framework

Constructing metal-organic frameworks (MOFs) to have a desired structure from the given components is critical to achieve ideal MOFs with optimal properties. However, thermodynamics and/or kinetics typically impose a restriction on MOF structures. Here, we report the MOF farming concept to produce a naturally nonpreferred structure from the given components. The HKUST-1 template offers ideal places for the efficient seeding and epitaxial growth of Ga-MIL-88B that is a naturally nonpreferred structure however intentionally produced instead of the preferred Ga-MIL-68. The MOF growth on the differently shaped HKUST-1 templates (octahedral, cuboctahedral, and cubic), containing different exposed lattices, proves that a hexagonal lattice with an exposed {111} plane of HKUST-1 selectively directs the perpendicular growth of Ga-MIL-88B, owing to the lattice matching with the {001} plane of Ga-MIL-88B. The grown Ga-MIL-88B is isolated in a pure form, and the refreshed template is reused to grow additional Ga-MIL-88B.

Rational Design and Growth of MOF-on-MOF Heterostructures

Multiporous metal-organic frameworks (MOFs) have emerged as a subclass of highly crystalline inorganic-organic materials, which are endowed with high surface areas, tunable pores, and fascinating nanostructures. Heterostructured MOF-on-MOF composites are recently becoming a research hotspot in the field of chemistry and materials science, which focus on the assembly of two or more different homogeneous or heterogeneous MOFs with various structures and morphologies. Compared with one single MOF, the dual MOF-on-MOF composites exhibit unprecedented tunability, hierarchical nanostructure, synergistic effect, and enhanced performance. Due to the difference of inorganic metals and organic ligands, the lattice parameters in a, b, and c directions in the single crystal cells could bring about subtle or large structural difference. It will result in the composite material with distinct growth methods to obtain secondary MOF grown from the initial MOF. In this review, the authors wish to mainly outline the latest synthetic strategies of heterostructured MOF-on-MOFs and their derivatives, including ordered epitaxial growth, random epitaxial growth, etc., which show the tutorial guidelines for the further development of various MOF-on-MOFs.

Solvent-Dependent Adsorption-Driven Mechanism for MOFs-Based Yolk-Shell Nanostructures

Metal-organic frameworks (MOFs)-based yolk-shell nanostructures have drawn enormous attention recently due to their multifunctionality. However, the regulations of the size and morphology of yolk-shell nanostructures are still limited by the unclear formation mechanism. Herein, we first demonstrated a solvent-dependent adsorption-driven mechanism for synthesizing yolk-shelled MOFs-based nanostructures coated with mesoporous SiO₂ shells (ZIF-8@mSiO₂ ) with tunable size and morphology. The selective and competitive adsorption of methanol (CH₃ OH) and water (H₂ O) on ZIF-8 core were found to have decisive effects on inducing the morphology evolution of yolk-shell nanostructures. The obtained yolk-shelled ZIF-8@mSiO₂ nanostructures show great promise in generating acoustic cavitation effect for sonodynamic cancer therapy in vitro. We believe that this work will not only help us to design novel MOFs-based yolk-shell nanostructures, but also promote the widespread application of MOFs materials.

Reverse synthesis of yolk-shell metal-organic frameworks

We report the first examples of yolk-shell metal-organic framework (MOF) heterostructures based on topologically distinct MOFs: ZIF-8/ZIF-67 and UiO-66. This was accomplished through an innovative reverse synthesis strategy: A hollow UiO-66 was first constructed; the precusors of the ZIFs were then loaded into the cavity of hollow UiO-66 through a mixed solvent impregnation method; subsequent crystallization under solvothermal condition led to the formation of yolk-shell MOFs containing one or multiple ZIF particles confined within a chemically robust single crystalline UiO-66 shell.

One-Pot Fabrication of Hollow Porphyrinic MOF Nanoparticles with Ultrahigh Drug Loading toward Controlled Delivery and Synergistic Cancer Therapy

Hollow nanostructures have attracted significant research interest in drug delivery systems due to their high capacities for drug loading and unique physicochemical properties, showing great potential in specific biomedical applications. Herein, hollow porphyrinic metal-organic framework (H-PMOF) nanoparticles with a mesoporous spherical shell have been fabricated via a facile self-sacrificial ZIF-8 nanoparticle template strategy. The H-PMOF nanoplatform not only demonstrates a greatly enhanced photodynamic therapy efficacy compared with nonhollow porphyrinic MOF nanoparticles but also can be used as a superior drug carrier to co-load doxorubicin (DOX) and indocyanine green (ICG) with an ultrahigh drug-loading capacity of 635%. Furthermore, cancer cell membrane camouflage of the (DOX and ICG)@H-PMOF composite nanoparticles affords a biomimetic nanoplatform, that is, (DOX and ICG)@H-PMOF@mem (DIHPm for short), with an outstanding homologous tumor-targeting and immune-escaping ability. Interestingly, DIHPm shows both pH-controlled and near-infrared laser-triggered DOX release. Both in vitro and in vivo studies of DIHPm demonstrate an excellent imaging-guided synergistic photodynamic/photothermal/chemotherapy anticancer activity with negligible systemic toxicity. The development of the high-performance H-PMOF nanoplatform provides new insights into the design of MOF-based multifunctional nanomedicines for combination cancer therapy and precise theranostics.

Novel Strategy for Engineering the Metal-Oxide@MOF Core@Shell Architecture and Its Applications in Cataluminescence Sensing

Cataluminescence is an attractive oxydic luminescence on the gas-solid interface, and metal-oxide@MOF core@shell architectures show great potential for cataluminescence sensing due to their integrated synergistic effect from core and shell components. However, restricting the direct nucleation and growth of metal-organic frameworks (MOFs) on the topologically distinct surface of metal oxides is a great challenge, owing to the high interface energy from the topology mismatch. Herein, for the first time, a novel liquid-phase concentration-controlled nucleation strategy is exploited to induce the direct assembly of a ZIF-8 layer on the surface of CeO₂ nanospheres without any sacrificial templates or further surface modifications. The results show that the construction of the CeO₂@ZIF-8 core@shell architecture can be accomplished within 1 min under the mediation of boosted nucleation kinetics. Furthermore, the universality of this developed strategy is demonstrated by the encapsulation of other metal-oxide cores such as magnetic Fe₃O₄ and ZnCo₂O₄ core particles with a ZIF-8 shell. Notably, compared to the pure CeO₂ and ZIF-8, the obtained CeO₂@ZIF-8 nanocomposite exhibits enhanced analytical performance for the cataluminescence sensing of propanal, in which the shell acts as the major catalytic reaction center, while the core contributes to further improving the catalytic efficiency. The proposed facile synthesis strategy with excellent simplicity, rapidity, and universality brings new insights into the engineering of core@shell advanced functional materials with mismatched topologies for catering to the diverse application demands.

Overcoming Symmetry Mismatch in Vaccine Nanoassembly through Spontaneous Amidation

Matching of symmetry at interfaces is a fundamental obstacle in molecular assembly. Virus-like particles (VLPs) are important vaccine platforms against pathogenic threats, including Covid-19. However, symmetry mismatch can prohibit vaccine nanoassembly. We established an approach for coupling VLPs to diverse antigen symmetries. SpyCatcher003 enabled efficient VLP conjugation and extreme thermal resilience. Many people had pre-existing antibodies to SpyTag:SpyCatcher but less to the 003 variants. We coupled the computer-designed VLP not only to monomers (SARS-CoV-2) but also to cyclic dimers (Newcastle disease, Lyme disease), trimers (influenza hemagglutinins), and tetramers (influenza neuraminidases). Even an antigen with dihedral symmetry could be displayed. For the global challenge of influenza, SpyTag-mediated display of trimer and tetramer antigens strongly induced neutralizing antibodies. SpyCatcher003 conjugation enables nanodisplay of diverse symmetries towards generation of potent vaccines.

Overcoming Symmetry Mismatch in Vaccine Nanoassembly through Spontaneous Amidation

Matching of symmetry at interfaces is a fundamental obstacle in molecular assembly. Virus-like particles (VLPs) are important vaccine platforms against pathogenic threats, including Covid-19. However, symmetry mismatch can prohibit vaccine nanoassembly. We established an approach for coupling VLPs to diverse antigen symmetries. SpyCatcher003 enabled efficient VLP conjugation and extreme thermal resilience. Many people had pre-existing antibodies to SpyTag:SpyCatcher but less to the 003 variants. We coupled the computer-designed VLP not only to monomers (SARS-CoV-2) but also to cyclic dimers (Newcastle disease, Lyme disease), trimers (influenza hemagglutinins), and tetramers (influenza neuraminidases). Even an antigen with dihedral symmetry could be displayed. For the global challenge of influenza, SpyTag-mediated display of trimer and tetramer antigens strongly induced neutralizing antibodies. SpyCatcher003 conjugation enables nanodisplay of diverse symmetries towards generation of potent vaccines.

Synthesis of MOF-on-MOF architectures in the context of interfacial lattice matching

This highlight summarises the previously reported MOF-on-MOF systems, with a focus on the presented crystallographic information and classification of the systems according to lattice parameter matching.

Core-Shell MOFs@MOFs: Diverse Designability and Enhanced Selectivity

Metal-Organic frameworks (MOFs), especially MOF-based composites, performed an irreplaceable role in the material fields. By virtue of the tailorability of MOFs, core-shell MOFs@MOFs composites with diverse designability and enhanced selectivity have inspired infinite scientific interest. This review will highlight an up-to-date overview of the designability and enhanced selectivity of core-shell MOFs@MOFs composites, covering the synthetic strategies of an epitaxial growth method, postsynthetic modification, and one-pot synthesis as well as the synergistic selective performance of the synthesized MOFs@MOFs in catalysis, adsorption and separation, and molecular recognition. Finally, the potential development trend and challenges toward core-shell MOFs@MOFs are addressed.

H2/CO2 separations in multicomponent metal-adeninate MOFs with multiple chemically distinct pore environments

Metal-organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn-Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H₂/CO₂ separation ability of bMOF-200. We examine the H₂/CO₂ separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H₂/CO₂ separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H₂ and CO₂ and a significantly improved H₂/CO₂ separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H₂/CO₂ separations.

Enhancing the Gas Separation Selectivity of Mixed-Matrix Membranes Using a Dual-Interfacial Engineering Approach

We report a dual-interfacial engineering approach that uses a sub-20 nm polycrystalline MOF-74 shell as a transition phase to engineer the MOF-polymer interface. The application of a shell MOF layer divides the original single interface problem into two interfaces: MOF-MOF and MOF-polymer, which can be individually addressed. The greater external surface area created by the uneven MOF-74 shell containing high-density open metal sites allows the MOF to interact with 300% polymer at the interface compared to traditional MOF, thereby ensuring good interfacial compatibility. When applied on UiO-66-NH₂, its respective mixed-matrix membranes exhibit a simultaneous increase of CO₂/CH₄ separation selectivity and CO₂ permeability with increasing MOF loading, implying a defect-free interface. When applied on MOF-801, the mixed-matrix membranes exhibit an ethylene/ethane separation selectivity up to 5.91, a drastic 76% increase compared to that of the neat polymer owing to a "gas focusing" mechanism promoted by the preferred pore orientation in the MOF-74 layer. This represents one of the most selective ethylene/ethane separation membranes reported to date.

Bimetallic metal-organic frameworks and their derivatives

Bimetallic metal-organic frameworks (MOFs) have two different metal ions in the inorganic nodes. According to the metal distribution, the architecture of bimetallic MOFs can be classified into two main categories namely solid solution and core-shell structures. Various strategies have been developed to prepare bimetallic MOFs with controlled compositions and structures. Bimetallic MOFs show a synergistic effect and enhanced properties compared to their monometallic counterparts and have found many applications in the fields of gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs can serve as excellent precursors/templates for the synthesis of functional nanomaterials with controlled sizes, compositions, and structures. Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications to the catalysis of more challenging reactions and electrochemical energy storage and conversion. This review provides an overview of the significant advances in the development of bimetallic MOFs and their derivatives with special emphases on their preparation and applications.

Hierarchy in Metal-Organic Frameworks

Sequence-defined nucleic acids and proteins with internal monomer sequences and arrangement are vital components in the living world, as a result of billions of years of molecular evolution. These natural hierarchical systems have inspired researchers to develop artificial hierarchical materials that can mimic similar functions such as replication, recognition, and information storage. In this Outlook, we describe the conceptual introduction of hierarchy into the design of metal-organic framework (MOF) materials. Starting with a history and background of hierarchical MOF synthesis and applications, we discuss further mesoscopic assembly strategies of MOF crystallites into hierarchical primary, secondary, tertiary, and quaternary architectures. This is followed by a highlight of the utilization of modular total synthesis for crafting MOFs with hierarchical compositions. The multiscale control over hierarchical MOF architecture formation can be rationally achieved by designing stepwise synthetic routes based on the knowledge from various fields including coordination chemistry, organic chemistry, reticular chemistry, and nanoscience. Altogether, this outlook is expected to shed light on these essential but embryonic materials and might offer inspiration for the development of the next generation of smart MOF materials with controllable heterogeneity and tailorable architectures.

Tracking and Visualization of Functional Domains in Stratified Metal-Organic Frameworks Using Gold Nanoparticles

We report here a new technique for the identification and visualization of functional domains in stratified metal-organic frameworks (MOFs). The technique, namely, gold diffusion enabled domain identification, utilizes the diffusion of Au nanoparticles within MOF cavities to track and selectively stain the more Au-philic domain in an MOF particle thereby allowing direct observation of domains, determination of domain sequences, and, in certain cases, domain boundaries under transmission electron microscopy. This method is an excellent tool for studying MOF materials with complex domain hierarchy.

Seed-mediated evolution of hierarchical metal-organic framework quaternary superstructures

Varying levels of hierarchy in metal–organic framework (MOF) superstructures are achieved through seed-mediated evolution of multiple MOF modules.

The idea of hierarchy, widely observed in natural and artificial worlds, has been extensively explored in chemistry and materials science. Similar to proteins which contain primary, secondary, tertiary and quaternary structures, varying levels of hierarchy in metal–organic framework (MOF) superstructures can also be achieved. In this work, we initiate a systematic study on the morphological evolution of hierarchical superstructures with the assistance of seeded growth and explore the assembly of multiple modular MOFs into superstructures with enhanced hierarchy and diversity. By utilizing MOF-74-III spherulite superstructures as seeds, multiple quaternary architectures were obtained depending on the lengths of organic linker precursors. The resulting superstructures with superior hierarchy represent a unique porous material which contains multiple modules with diverse morphologies. To the best of our knowledge, this is the first report that utilizes tertiary superstructures as seeds in MOF synthesis, which leads to unusual and diverse behaviors during secondary growth. This synthetic approach not only provides a facile method to establish hierarchy in porous materials, but also enables the fabrication of multiscale heterostructures through secondary growth on MOF seeds.

Recent highlights and future prospects on mixed-metal MOFs as emerging supercapacitor candidates

Mixed-metal metal-organic frameworks (M-MOFs) consist of at least two different metal ions as nodes in the same framework. The incorporation of a second or more metal ions provides structural/compositional diversity, multi-functionality and stability to the framework. Moreover, the periodical array of different metal ions in the framework may alter the physical/chemical properties of M-MOFs and result in fascinating applications. M-MOFs with exciting structural features offer superior supercapacitor performances compared to single metal MOFs due to the synergic effect of different metal ions. In this review, we summarize several synthetic methods to construct M-MOFs by employing various organic ligands or metalloligands. Further, we discuss the electrochemical performance of several M-MOFs and their derived composite materials for supercapacitor applications.