Using a Multi‐Shelled Hollow Metal–Organic Framework as a Host to Switch the Guest‐to‐Host and Guest‐to‐Guest Interactions

Segregation of Constitutional Isomers of Some Aromatic Amines Exploiting the Changes of Photo-Luminescence Behaviour of Isoreticular Metal-Organic Frameworks

Three different two dimensional Cd(II)-based metal-organic frameworks (MOFs) have been synthesized by utilizing same N,N'-donor ligand and three different functionalized dicarboxylate linkers namely isophthalate, 5-nitroisophthalate and 5-hydroxyisophthalate for compound 1, 2 and 3 respectively. The compounds that are isoreticular bi-walled 2D frameworks, show dual fluorescence emission spectra for their π-π* and n-π* excitation. Compound 1 is consists of unsubstituted bridging isophthalate whereas 2 and 3 are made with bridging isophthalate that are substituted by electron withdrawing -NO2 group and electron donating -OH group respectively. These different electronic environments in three MOFs are found effective to respond differently towards the constitutional isomers of aromatic amines having suitable electronic environment. Compound 1 responses towards non-polar -CH2 group containing picolylamine moieties, showing maximum fluorescence emission enhancement for 2-picolylamine in comparison to 3- and 4- isomer of picolylamine derivates. Compound 2, consisting electron withdrawing -NO2 group, responses more significantly towards the electron rich phenylenediamines showing maximum quenching in case of m-phenylenediamine compared to their ortho and para isomers. The change of functionalisation from -NO2 to -OH in 3, helps to detect the electron deficient aminopyridines; where the maximum fluorescence quenching has been observed in presence of 3-aminopyridine in contrast to its other isomers.

A ternary heterostructure aptasensor based on metal-organic framework and polydopamine nanoparticles for fluorescent detection of sulfamethazine

Residue of sulfamethazine (SMZ), a typical short-acting drug to prevent bacterial infections, in food is a threat to human health. A ternary heterogeneous metal-organic framework hybrid (Zn/Fe-MOF@PDANSs) of Zn-TCPP-MOF, MIL-101 (Fe) and polydopamine nanoparticles (PDANSs) was proposed to establish an aptasensor for the sensitive and selective detection of SMZ. In this sensor, Zn-TCPP-MOF and FAM emitted fluorescence at 609 nm and 523 nm, respectively, and the fluorescence of FAM-ssDNA could be quenched when it was adsorbed on the surface of MOF hybrid. In the presence of SMZ, the fluorescence of FAM-ssDNA recovered due to the dropping from MOF hybrid, while the fluorescence of MOF hybrid remained. With this strategy, a wide concentration range and high sensitivity of SMZ were detection. And the ternary Zn/Fe-MOF@PDANSs sensor exhibited more excellent performance than binary Zn/Fe-MOF aptasensor. In addition, the sensor showed pleasurable selectivity, and was utilized for SMZ determination in authentic chicken and pork samples, implying the fascinating potential in practical application.

Major-auxiliary cooperative metal pairs in MOFs enable cascade oxidation of KA oil to ε-caprolactone

Direct oxidation of KA oil (the mixture of cyclohexanone and cyclohexanol) toward ε-caprolactone is in high demand yet hard to implement in need of juggling the activation of both methyne C-H bond of cyclohexanol and α-C-C bond of cyclohexanone. Here we demonstrate that in situ formed Cu1+δ-Oδ-• active site, which originates from relay reaction at Ni(II) and Cu(I) pairs in a metal-organic framework (known as NiCu-MOF-74) with O2 and benzaldehyde (PhCHO), efficiently oxidizes KA oil toward ɛ-caprolactone along with good stability. Mechanism investigation discloses that the auxiliary Ni(II) site first adsorbs O2 for abstracting formyl hydrogen in PhCHO followed by transfer of PhCO· to react with another O2 over the major Cu(I) site, leading to formation of Cu1+δ-Oδ-• and PhCOOH. This major-auxiliary cooperative strategy will be particularly suitable for multivariate MOFs as next generation catalysts towards complex reactions.

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.

Increasing Multienzyme Cascade Efficiency and Stability of MOF via Partitioning Immobilization

Enhancing the stability of multienzyme cascade reactions in metal-organic frameworks (MOFs) is a challenging task in the fields of biotechnology and chemistry. However, addressing this challenge could yield far-reaching benefits across the application range in the biomedical, food, and environmental sectors. In this study, multienzyme partitioning immobilization that sequentially immobilizes cascade enzymes with hierarchical MOFs is proposed to reduce substrate diffusion resistance. Conversion results of ginsenosides indicate that this strategy improves the cascade efficiency up to 1.26 times. The substrate diffusion model is used to investigate the dual-interenzyme mass transfer behavior of substrates in the restricted domain space and evaluate the substrate channeling effect under partitioning immobilization. Molecular docking and kinetic simulations reveal that the MOFs effectively limit the conformational changes of cascade enzymes at high temperatures and in organic solvents while maintaining a large pocket of active centers. This phenomenon increased efficient substrate docking to the enzyme molecules, further optimizing cascade efficiency. The results of the immobilization of GOX and horseradish peroxidase as model enzymes indicate that the partitioned MOF immobilization strategy could be used for universal adaptation of cascade enzymes.

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.

From nano- to macroarchitectures: designing and constructing MOF-derived porous materials for persulfate-based advanced oxidation processes

Persulfate-based advanced oxidation processes (PS-AOPs) have gained significant attention as an effective approach for the elimination of emerging organic contaminants (EOCs) in water treatment. Metal-organic frameworks (MOFs) and their derivatives are regarded as promising catalysts for activating peroxydisulfate (PDS) and peroxymonosulfate (PMS) due to their tunable and diverse structure and composition. By the rational nanoarchitectured design of MOF-derived nanomaterials, the excellent performance and customized functions can be achieved. However, the intrinsic fine powder form and agglomeration ability of MOF-derived nanomaterials have limited their practical engineering application. Recently, a great deal of effort has been put into shaping MOFs into macroscopic objects without sacrificing the performance. This review presents recent advances in the design and synthetic strategies of MOF-derived nano- and macroarchitectures for PS-AOPs to degrade EOCs. Firstly, the strategies of preparing MOF-derived diverse nanoarchitectures including hierarchically porous, hollow, yolk-shell, and multi-shell structures are comprehensively summarized. Subsequently, the approaches of manufacturing MOF-based macroarchitectures are introduced in detail. Moreover, the PS-AOP application and mechanisms of MOF-derived nano- and macromaterials as catalysts to eliminate EOCs are discussed. Finally, the prospects and challenges of MOF-derived materials in PS-AOPs are discussed. This work will hopefully guide the design and development of MOF-derived porous materials in SR-AOPs.

Fluorescence-quenching mechanisms of novel isomorphic Zn/Cd coordination polymers for selective nitrobenzene detection

Nitroaromatic compounds in aqueous undermine environmental sustainability and affect human health. The development of a fluorescent sensor capable of efficiently and selectively detecting trace amounts of nitroaromatic compounds presents a considerable challenge. This study introduced Zn/Cd isomeric coordination polymers (Zn-H2CIA-1/Cd-H2CIA-2), which are synthesized using 5-((4-carboxybenzyl)oxy)isophthalic acid (5-H3CIA) and 1,10-phenanthroline (Phen). The polymers have zero-dimensional discrete crystal structure with a six-coordinated scissor-like shape. The two coordination polymers can be used as fluorescent sensors for detecting nitrobenzene (NB) and demonstrated favorable sensitivity, with detection limits of 1.95 × 10-8 and 4.66 × 10-7 mol/L, respectively. Zn-H2CIA-1 exhibited stronger fluorescence and a more sensitive response to NB compared with Cd-H2CIA-2. To elucidate their fluorescence-quenching mechanisms, we analyzed Zn-H2CIA-1 by performing DFT and TD-DFT calculations. The pore structure, density of states, excitation energy, hole-electron distribution, and orbital composition were analyzed. The suitable size of pores in Zn-H2CIA-1 is the main reason for its high NB selectivity. Moreover, intermolecular π-π stacking interactions result in an orbital overlap between Zn-H2CIA-1 and NB, enabling the transfer of electrons from Zn-H2CIA-1 to NB. This electron transfer is identified as the fundamental cause of fluorescence quenching in Zn-H2CIA-1.

Yolk-shell composite optical sensors with chiral L-histidine/Rhodamine 6G for high-sensitivity "turn-on" detection of L-proline

Chirality is a ubiquitous phenomenon in nature and has attracted wide attention in the biomedicine, pharmaceutics and biosensing research fields. Enantiomeric recognition of chiral compounds, especially chiral drugs and chiral amino acids, is important for human health and nutrition. In this work, through the encapsulation of L-His&R6G (L-His = L-Histidine; R6G = Rhodamine 6G) into MOF@MOF framework ZIF-67@ZIF-8, composited material L-His&R6G@ZIF-67@ZIF-8 can be obtained. Additionally, through the etching process, a unique yolk-shell ZIF-8 chiral composite optical sensors L-His&R6G@ZIF-8 (1) can be successfully prepared. Photo-luminescent (PL) experiment also reveals that 1 can highly sensitively detect L-Proline (L-Pro) through the "turn-on" detection strategy (KBH = 1.22 × 104 M-1 and detection limit 1.9 μM). Further yolk-shell L-His&R6G@ZIF-8-based fabricate flexible mixed-matrix membranes has been prepared using doctor-blading technique, which show significant fluorescence enhancement effect under ultraviolet lamp. This work also provides the unique example of preparing chiral yolk-shell framework composite sensors, which have broad application in chiral sensing area.

One-Pot Synthesis of MOF@MOF: Structural Incompatibility Leads to Core-Shell Structure and Adaptability Control Makes the Sequence

Core-shell metal-organic frameworks (MOF@MOF) are promising materials with sophisticated structures that cannot only enhance the properties of MOFs but also endow them with new functions. The growth of isotopic lcore-shell MOFs is mostly limited to inconvenient stepwise seeding strategies with strict requirements, and by far one-pot synthesis is still of great challenge due to the interference of different components. Through two pairs of isoreticular MOFs, it reveals that the structural incompatibility is a prerequisite for the formation of MOFs@MOFs by one-pot synthesis, as illustrated by PMOF-3@HHU-9. It further unveils that the adaptability of the shell-MOF is a more key factor for nucleation kinetic control. MOFs with flexible linkers has comparably slower nucleation than MOFs with rigid linkers (forming PMOF-3@NJU-Bai21), and structural-flexible MOFs built by flexible linkers show the lowest nucleation and the most adaptability (affording NJU-Bai21@HHU-9). This degree of adaptability variation controls the sequence and further facilitates the synthesis of a first triple-layered core-shell MOF (PMOF-3@NJU-Bai21@HHU-9) by one-pot synthesis. The insight gained from this study will aid in the rational design and synthesis of other multi-shelled structures by one-pot synthesis and the further expansion of their applications.

A facile dual-template-directed successive assembly approach to hollow multi-shell mesoporous metal-organic framework particles

Hollow multi-shell mesoporous metal-organic framework (MOF) particles with accessible compartmentalization environments, plentiful heterogeneous interfaces, and abundant framework diversity are expected to hold great potential for catalysis, energy conversion, and biotechnology. However, their synthetic methodology has not yet been established. In this work, a facile dual-template-directed successive assembly approach has been developed for the preparation of monodisperse hollow multi-shell mesoporous MOF (UiO-66-NH2) particles through one-step selective etching of successively grown multi-layer MOFs with alternating two types of mesostructured layers. This strategy enables the preparation of hollow multi-shell mesoporous UiO-66-NH2 nanostructures with controllable shell numbers, accessible mesochannels, large pore volume, tunable shell thickness and chamber sizes. The methodology relies on creating multiple alternating layers of two different mesostructured MOFs via dual-template-directed successive assembly and their difference in framework stability upon chemical etching. Benefiting from the highly accessible Lewis acidic sites and the accumulation of reactants within the multi-compartment architecture, the resultant hollow multi-shell mesoporous UiO-66-NH2 particles exhibit enhanced catalytic activity for CO2 cycloaddition reaction. The dual-template-directed successive assembly strategy paves the way toward the rational construction of elaborate hierarchical MOF nanoarchitectures with specific physical and chemical features for different applications.

Reactive oxygen nanobiocatalysts: activity-mechanism disclosures, catalytic center evolutions, and changing states

Benefiting from low costs, structural diversities, tunable catalytic activities, feasible modifications, and high stability compared to the natural enzymes, reactive oxygen nanobiocatalysts (RONBCs) have become dominant materials in catalyzing and mediating reactive oxygen species (ROS) for diverse biomedical and biological applications. Decoding the catalytic mechanism and structure-reactivity relationship of RONBCs is critical to guide their future developments. Here, this timely review comprehensively summarizes the recent breakthroughs and future trends in creating and decoding RONBCs. First, the fundamental classification, activity, detection method, and reaction mechanism for biocatalytic ROS generation and elimination have been systematically disclosed. Then, the merits, modulation strategies, structure evolutions, and state-of-art characterisation techniques for designing RONBCs have been briefly outlined. Thereafter, we thoroughly discuss different RONBCs based on the reported major material species, including metal compounds, carbon nanostructures, and organic networks. In particular, we offer particular insights into the coordination microenvironments, bond interactions, reaction pathways, and performance comparisons to disclose the structure-reactivity relationships and mechanisms. In the end, the future challenge and perspectives for RONBCs are also carefully summarised. We envision that this review will provide a comprehensive understanding and guidance for designing ROS-catalytic materials and stimulate the wide utilisation of RONBCs in diverse biomedical and biological applications.

Emerging Pristine MOF-Based Heterostructured Nanoarchitectures: Advances in Structure Evolution, Controlled Synthesis, and Future Perspectives

Metal-organic frameworks (MOFs) can be customized through modular assembly to achieve a wide range of potential applications, based on their desired functionality. However, most of the initially reported MOFs are limited to microporous systems and are not sufficiently stable, which restricts their popularization. Heterogeneity is introduced into a simple MOF framework to create MOF-based heterostructures with fascinating properties and interesting functions. Heterogeneity can be introduced into the MOFs via postsynthetic/ligand exchange. Although the ligand exchange has shown potential, it is difficult to precisely control the degree of exchange or position. Among the various synthesis strategies, hierarchical assembly is particularly attractive for constructing MOF-based heterostructures, as it can achieve precise regulation of MOF-based heterostructured nanostructures. The hierarchical assembly significantly expands the compositional diversity of MOF-based heterostructures, which has high elasticity for lattice matching during the epitaxial growth of MOFs. This review focuses on the synthetic evolution mechanism of hierarchical assemblies of MOF-based nanoarchitectures. Subsequently, the precise control of pore structure, pore size, and morphology of MOF-based nanoarchitectures by hierarchical assembly is emphasized. Finally, possible solutions to address the challenges associated with heterogeneous interfaces are presented, and potential opportunities for innovative applications are proposed.

Yolk-Shell and Hollow Zr/Ce-UiO-66 for Manipulating Selectivity in Tandem Reactions and Photoreactions

One of the hallmarks of multicomponent metal-organic frameworks (MOFs) is to finely tune their active centers to achieve product selectivity. In particular, obtaining bimetallic MOF hollow structures with precisely tailored redox centers under the same topology is still challenging despite a recent surge of such efforts. Herein, we present an engineering strategy named "cluster labilization" to generate hierarchically porous MOF composites with hollow structures and tunable active centers. By partially replacing zirconium with cerium in the hexanuclear clusters of UiO-66, unevenly distributed yolk-shell structures (YSS) were formed. Through acid treatment or annealing of the YSS precursor, single-shell hollow structures (SSHS) or double-shell hollow structures (DSHS) can be obtained, respectively. The active centers in SSHS and DSHS differ in their species, valence, and spatial locations. More importantly, YSS, SSHS, and DSHS with distinct active centers and microenvironments exhibit tunable catalytic activity, reversed selectivity, and high stability in the tandem reaction and the photoreaction.

Lactate-Responsive Gene Editing to Synergistically Enhance Macrophage-Mediated Cancer Immunotherapy

Combination therapies involving metabolic regulation and immune checkpoint blockade are considered an encouraging new strategy for cancer therapy. However, the effective utilization of combination therapies for activating tumor-associated macrophages (TAMs) remains challenging. Herein, a lactate-catalyzed chemodynamic approach to activate the therapeutic genome editing of signal-regulatory protein α (SIRPα) to reprogram TAMs and improve cancer immunotherapy is proposed. This system is constructed by encapsulating lactate oxidase (LOx) and clustered regularly interspaced short palindromic repeat-mediated SIRPα genome-editing plasmids in a metal-organic framework (MOF). The genome-editing system is released and activated by acidic pyruvate, which is produced by the LOx-catalyzed oxidation of lactate. The synergy between lactate exhaustion and SIRPα signal blockade can enhance the phagocytic ability of TAMs and promote the repolarization of TAMs to the antitumorigenic M1 phenotype. Lactate exhaustion-induced CD47-SIRPα blockade efficiently improves macrophage antitumor immune responses and effectively reverses the immunosuppressive tumor microenvironment to inhibit tumor growth, as demonstrated by in vitro and in vivo studies. This study provides a facile strategy for engineering TAMs in situ by combining CRISPR-mediated SIRPα knockout with lactate exhaustion for effective immunotherapy.

Recent strategies for constructing hierarchical multicomponent nanoparticles/metal-organic framework hybrids and their applications

Hybrid nanoparticles with unique tailored morphologies and compositions can be utilized for numerous applications owing to their combination of inherent properties as well as the structural and supportive functions of each component. Controlled encapsulation of nanoparticles within nanospaces (NPNSs) of metal-organic frameworks (MOFs) (denoted as NPNS@MOF) can generate a large number of hybrid nanomaterials, facilitating superior activity in targeted applications. In this review, recent strategies for the fabrication of NPNS@MOFs with a hierarchical architecture, tailorability, unique intrinsic properties, and superior catalytic performance are summarized. In addition, the latest and most important examples in this sector are emphasized since they are more conducive to the practical applicability of NPNS@MOF nanohybrids.

Modulating Charges of Dual Sites in Multivariate Metal-Organic Frameworks for Boosting Selective Aerobic Epoxidation of Alkenes

Selective aerobic epoxidation of alkenes without any additives is of great industrial importance but still challenging because the competitive side reactions including C═C bond cleavage and isomerization are difficult to avoid. Here, we show fabricating Cu(I) single sites in pristine multivariate metal-organic frameworks (known as CuCo-MOF-74) via partial reduction of Cu(II) to Cu(I) ions during solvothermal reaction. Impressively, CuCo-MOF-74 is characteristic with single Cu(I), Cu(II), and Co(II) sites, and they exhibit the substantially enhanced selectivity of styrene oxide up to 87.6% using air as an oxidant at almost complete conversion of styrene, ∼25.8% selectivity increased over Co-MOF-74, as well as good catalytic stability. Contrast experiments and theoretical calculation indicate that Cu(I) sites contribute to the substantially enhanced selectivity of epoxides catalyzed by Co(II) sites. The adsorption of two O₂ molecules on dual Co(II) and Cu(I) sites is favorable, and the projected density of state of the Co-3d orbital is closer to the Fermi level by modulating with Cu(I) sites for promoting the activation of O₂ compared with dual-site Cu(II) and Co(II) and Co(II) and Co(II), thus contributing to the epoxidation of the C═C bond. When other kinds of alkenes are used as substrates, the excellent selectivity of various epoxides is also achieved over CuCo-MOF-74. We also prove the universality of fabricating Cu(I) sites in other MOF-74 with various divalent metal nodes.

Hierarchical Assembly of a Micro- and Macroporous Hydrogen-Bonded Organic Framework with Tailored Single-Crystal Size

Porous organic molecular materials represent an emergent field of research in Chemistry and Materials Science due to their unique combination of properties. To enhance their performance and expand the number of applications, the incorporation of hierarchical porosity is required, as exclusive microporosity entails several limitations. However, the integration of macropores in porous organic molecular materials is still an outstanding challenge. Herein, we report the first example of a hydrogen-bonded organic framework (MM-TPY) with hierarchical skeletal morphology, containing stable micro- and macroporosity. The crystal size, from micro to centimetre scale, can be controlled in a single step without using additives or templates. The mechanism of assembly during the crystal formation is compatible with a skeletal crystal growth. As proof of concept, we employed the hierarchical porosity as a platform for the dual, sequential and selective co-recognition of molecular species and microparticles.

Metal-organic framework nanocrystal-derived hollow porous materials: Synthetic strategies and emerging applications

Metal-organic frameworks (MOFs) have garnered multidisciplinary attention due to their structural tailorability, controlled pore size, and physicochemical functions, and their inherent properties can be exploited by applying them as precursors and/or templates for fabricating derived hollow porous nanomaterials. The fascinating, functional properties and applications of MOF-derived hollow porous materials primarily lie in their chemical composition, hollow character, and unique porous structure. Herein, a comprehensive overview of the synthetic strategies and emerging applications of hollow porous materials derived from MOF-based templates and/or precursors is given. Based on the role of MOFs in the preparation of hollow porous materials, the synthetic strategies are described in detail, including (1) MOFs as removable templates, (2) MOF nanocrystals as both self-sacrificing templates and precursors, (3) MOF@secondary-component core-shell composites as precursors, and (4) hollow MOF nanocrystals and their composites as precursors. Subsequently, the applications of these hollow porous materials for chemical catalysis, electrocatalysis, energy storage and conversion, and environmental management are presented. Finally, a perspective on the research challenges and future opportunities and prospects for MOF-derived hollow materials is provided.

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MOFs have garnered multi-disciplinary attention due to their unique inherent properties

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Various synthetic strategies of MOFs-derived hollow porous materials are summarized

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Emerging applications of MOFs-derived hollow porous materials are reviewed

Palladium decoration directed synthesis of ZIF-8 nanocubes with efficient catalytic activity for nitrobenzene hydrogenation

A palladium precursor (H₂PdCl₄) has been utilized as a novel structure-directing agent for controlling the morphology of ZIF-8. Using reverse micelles as nanoreactors, the Pd/ZIF-8 nanocomposite with a uniform size distribution is obtained. It is revealed that Pd(II) can selectively coordinate with the (100) plane of ZIF-8. As a result, the morphology of ZIF-8 is transformed from rhombic dodecahedral to cubic. After hydrogen treatment, the as-obtained Pd NPs/ZIF-8 nanocubes show efficient catalytic activity for nitrobenzene hydrogenation, which is higher than that of the commercially available Pd/C catalyst.

Tunable fluorescence emission based on multi-layered MOF-on-MOF

Luminescent metal-organic frameworks (MOFs) have garnered considerable attention in various fields. Herein, we proposed a hierarchical confinement strategy based on MOF-on-MOF to tune luminescence emission ranging from blue to red including white light in a flexible way. The easily available ZIF-8 MOF was used as a host for the confinement of two kinds of size-matching dyes (perylene and rhodamine B) to obtain a layered ZIF-8@dye@ZIF-8@dye via in situ encapsulation and seed-mediated synthesis. ZIF-8@dye@ZIF-8@dye materials with different fluorescence emission in dispersed and solid states were both obtained by tuning the initial encapsulation concentration of dye and changing the structure of the inner and outer ZIF-8@dye layers. To our delight, ZIF-8@0.125perylene@ZIF-8@25RhB with white light emission in the dispersed state was obtained; meanwhile, ZIF-8@0.125perylene + 25RhB and mechanically mixed ZIF-8@0.125perylene + ZIF-8@25RhB could not realize white light emission under the same conditions, indicating that the proposed hierarchical confinement strategy facilitated white light regulation. Similarly, the emission of ZIF-8@dye@ZIF-8@dye in the solid state has also been investigated; ZIF-8@perylene@ZIF-8@3RhB with white light emission was obtained, while white light emission could not be achieved in ZIF-8@perylene + 3RhB and ZIF-8@perylene + ZIF-8@3RhB, which further indicated the importance of the hierarchical confinement strategy based on MOF-on-MOF. The proposed hierarchical confinement strategy may also inspire the development of other functional optical MOF materials.

Ultra-stable two-dimensional metal-organic frameworks for photocatalytic H2 production

Two-dimensional (2D) metal-organic frameworks (MOFs) are some of the most promising photocatalysts owing to their high numbers of exposed active sites and excellent charge mobility. However, the synthesis of highly stable 2D MOF photocatalysts involves challenges, and examples have been rarely reported. Herein, a new kind of material, 2D indium-based porphyrin MOF cubic nanosheets (2D In-TCPP NS) with an average thickness of ∼3.97 nm, is synthesized via a surfactant-assisted approach, and it shows good chemical stability in the pH range of 2-11 in aqueous solution. In photocatalytic H₂-generation experiments, 2D In-TCPP NS exhibits activity that is enhanced by over one order of magnitude compared with the 3D bulk In-TCPP MOF, arising from its highly enhanced electron-hole separation abilities. Moreover, after 40 h of continuous photocatalysis testing, 2D In-TCPP NS shows nearly no activity decrease, which suggests its great potential for practical commercial use.

Recent Advances in the Application and Mechanism of Carbon Dots/Metal-Organic Frameworks Hybrids in Photocatalysis and the Detection of Environmental Pollutants

Metal-organic frameworks (MOFs) are a class of crystalline porous materials with simple synthesis conditions, large specific surface area, structural diversity, and a wide range of interesting properties. The integration of MOFs with other materials can provide new multifunctional composites that exhibit both component properties and new characteristics. In recent years, the integration of carbon dots (CDs) into MOFs to form composites has shown improved optical properties and fascinating new characteristics. This review focuses on the design and synthesis strategies of CDs@MOFs composites (including pore-confined synthesis, in situ encapsulation, post-synthesis modification and impregnation method) and their recent research progress in photocatalysis and detection of environmental pollutants. Both the achievements and problems are evaluated and proposed, and the opportunities and challenges of CDs@MOF composite are discussed.

Hierarchically encapsulating enzymes with multi-shelled metal-organic frameworks for tandem biocatalytic reactions

Biocatalytic transformations in living organisms, such as multi-enzyme catalytic cascades, proceed in different cellular membrane-compartmentalized organelles with high efficiency. Nevertheless, it remains challenging to mimicking biocatalytic cascade processes in natural systems. Herein, we demonstrate that multi-shelled metal-organic frameworks (MOFs) can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency. Encapsulating multi-enzymes with multi-shelled MOFs by epitaxial shell-by-shell overgrowth leads to 5.8~13.5-fold enhancements in catalytic efficiencies compared with free enzymes in solution. Importantly, multi-shelled MOFs can act as a multi-spatial-compartmental nanoreactor that allows physically compartmentalize multiple enzymes in a single MOF nanoparticle for operating incompatible tandem biocatalytic reaction in one pot. Additionally, we use nanoscale Fourier transform infrared (nano-FTIR) spectroscopy to resolve nanoscale heterogeneity of vibrational activity associated to enzymes encapsulated in multi-shelled MOFs. Furthermore, multi-shelled MOFs enable facile control of multi-enzyme positions according to specific tandem reaction routes, in which close positioning of enzyme-1-loaded and enzyme-2-loaded shells along the inner-to-outer shells could effectively facilitate mass transportation to promote efficient tandem biocatalytic reaction. This work is anticipated to shed new light on designing efficient multi-enzyme catalytic cascades to encourage applications in many chemical and pharmaceutical industrial processes.

Mimicking multi-enzyme catalytic cascades in natural systems with spatial organization in confined structures is gaining increasing attention in the emerging field of systems chemistry. Here, the authors demonstrate that multi-shelled metal-organic frameworks can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency.

Anisotropic MOF-on-MOF Growth of Isostructural Multilayer Metal-Organic Framework Heterostructures

Isostructural MOFs with similar crystallographic parameter are easily available for MOF-on-MOF growth and possible to form core–shell structure by isotropic growth. However, due to well-matched cell lattice, selective growth in isostructural MOF heterostructures remains a great challenge for engineering atypical MOF heterostructures. Herein, an anisotropic MOF-on-MOF growth strategy was developed to structure a range of multilayer sandwich-like ZIF-L heterostructures via stacking isostructural ZIF-L-Zn and ZIF-L-Co alternately with three-, five-, seven-, and more layer structures. Moreover, these heterostructures with highly designable feature were fantastic precursors for fabricating derivatives with tunable magnetic and catalytic properties. Such strategy explores a novel way of achieving anisotropic MOF-on-MOF growth between isostructural MOFs and opens up new horizons for regulating the properties by MOF modular assembly in versatile functional nanocomposites.

Tailoring Heterogeneous Catalysts at the Atomic Level: In Memoriam, Prof. Chia-Kuang (Frank) Tsung

Professor Chia-Kuang (Frank) Tsung made his scientific impact primarily through the atomic-level design of nanoscale materials for application in heterogeneous catalysis. He approached this challenge from two directions: above and below the material surface. Below the surface, Prof. Tsung synthesized finely controlled nanoparticles, primarily of noble metals and metal oxides, tailoring their composition and surface structure for efficient catalysis. Above the surface, he was among the first to leverage the tunability and stability of metal-organic frameworks (MOFs) to improve heterogeneous, molecular, and biocatalysts. This article, written by his former students, seeks first to commemorate Prof. Tsung's scientific accomplishments in three parts: (1) rationally designing nanocrystal surfaces to promote catalytic activity; (2) encapsulating nanocrystals in MOFs to improve catalyst selectivity; and (3) tuning the host-guest interaction between MOFs and guest molecules to inhibit catalyst degradation. The subsequent discussion focuses on building on the foundation laid by Prof. Tsung and on his considerable influence on his former group members and collaborators, both inside and outside of the lab.

Ultrasound-assisted continuous-flow synthesis of PEGylated MIL-101(Cr) nanoparticles for hematopoietic radioprotection

Metal-organic frameworks (MOFs) are useful as drug delivery carriers with high loading capacity and excellent biocompatibility. We fabricated a new drug carrier based on MIL-101(Cr) environmentally and loaded it with 47.2 wt% WR-1065 (active metabolite of amifostine). Moreover, the permeability and stability of these nanoparticles increased after PEGylation by the N-hydroxysuccinimide active ester protocol. Then, a "green" continuous-flow system equipped with an ultrasound applicator was newly designed to prepare the nanoparticles under the effect of acoustic cavitation. Response surface methodology (RSM) was used to optimize the large-scale process conditions with Box-Behnken design to obtain high space-time yield (5785 kg m^-3 day^-1). These less toxic MOFs nanoparticles increased cell viability by scavenging the accumulated reactive oxygen species and resisting DNA damage after irradiation. They are capable of mitigating radiation injury, achieving a 30-d survival rate of 90% in mice after lethal total body irradiation (8.0 Gy). This countermeasure significantly improved the peripheral blood cell count, hematopoietic stem and progenitor cells frequency, and clonogenic function of hematopoietic progenitor cells. It probably prevents irradiation-induced hematopoietic damage through the p53-dependent apoptotic pathway. Therefore, ultrasound-assisted continuous-flow synthesis is a sustainable method to produce MOFs on a large scale for radioprotection.

Nanomaterials for Targeted Delivery of Agrochemicals by an All-in-One Combination Strategy and Deep Learning

The development of modern agriculture has prompted the greater input of herbicides, insecticides, and fertilizers. However, precision release and targeted delivery of these agrochemicals still remain a challenge. Here, a pesticide-fertilizer all-in-one combination (PFAC) strategy and deep learning are employed to form a system for controlled and targeted delivery of agrochemicals. This system mainly consists of three components: (1) hollow mesoporous silica (HMS), to encapsulate herbicides and phase-change material; (2) polydopamine (PDA) coating, to provide a photothermal effect; and (3) a zeolitic imidazolate framework (ZIF8), to provide micronutrient Zn2+ and encapsulate insecticides. Results show that the PFAC at concentration of 5 mg mL-1 reaches the phase transition temperature of 1-tetradecanol (37.5 °C) after 5 min of near-infrared (NIR) irradiation (800 nm, 0.5 W cm-2). The data of corn and weed are collected and relayed to deep learning algorithms for model building to realize object detection and further targeted weeding. In-field treatment results indicated that the growth of chicory herb was significantly inhibited when treated with the PFAC compared with the blank group after 24 h under NIR irradiation for 2 h. This system combines agrochemical innovation and artificial intelligence technology, achieves synergistic effects of weeding and insecticide and nutrient supply, and will potentially achieve precision and sustainable agriculture.

Space-confined construction of two-dimensional nitrogen-doped carbon with encapsulated bimetallic nanoparticles as oxygen electrocatalysts

A space-confined strategy has been used to control the pyrolysis of two-dimensional (2D) NiCo-MOF@ZIF-L(Zn). A thin SiO₂ layer as a confined space could avoid the destruction of the 2D morphology during pyrolysis and expose more active sites. The obtained NiCo-NC material exhibits high ORR and Zn-air battery performance.

A Porous Metal-Organic Framework as an Electrochemical Sensing Platform for Highly Selective Adsorption and Detection of Bisphenols

The design of artificial receptors with a specific recognition function and enhanced selectivity is highly desirable in the electrochemical sensing field, which can be used for detection of environmental pollutants. In this facet, metal-organic frameworks (MOFs) featured adjustable porosities and specific host-guest recognition properties. Especially, the large hydrophobic cavity formed in the porous MOFs may become a potential artificial receptor. We herein designed a new porous MOF [Zn₂(L)(IPA)(H₂O)]·2DMF·2MeOH·3H₂O (Zn-L-IPA) by using a functionalized sulfonylcalix[4]arene (L¹) and isophthalic acid (H₂IPA) (DMF = N,N'-dimethylformamide). The specific pore size and pore shape of Zn-L-IPA made it efficiently selective for absorption of bisphenol A (BPA), bisphenol F (BPF), and bisphenol S (BPS). Therefore, a rapid, highly selective, and ultrasensitive electrochemical sensing platform Zn-L-IPA@GP/GCE was fabricated by using Zn-L-IPA as a host to recognize and absorb bisphenol guests (GP = graphite powder, GCE = glassy carbon electrode). Most strikingly, the extremely low detection limits were up to 3.46 and 0.17 nM for BPA and BPF, respectively, using the Zn-L-IPA@GP/GCE electrode. Furthermore, the "recognition and adsorption" mechanism was uncovered by density functional theory with the B3LYP function. This work offered a prospective strategy for selective absorption and detection of harmful bisphenols with the MOF-based porous material.

Metal-Organic Framework-Based Hierarchically Porous Materials: Synthesis and Applications

Metal-organic frameworks (MOFs) have been widely recognized as one of the most fascinating classes of materials from science and engineering perspectives, benefiting from their high porosity and well-defined and tailored structures and components at the atomic level. Although their intrinsic micropores endow size-selective capability and high surface area, etc., the narrow pores limit their applications toward diffusion-control and large-size species involved processes. In recent years, the construction of hierarchically porous MOFs (HP-MOFs), MOF-based hierarchically porous composites, and MOF-based hierarchically porous derivatives has captured widespread interest to extend the applications of conventional MOF-based materials. In this Review, the recent advances in the design, synthesis, and functional applications of MOF-based hierarchically porous materials are summarized. Their structural characters toward various applications, including catalysis, gas storage and separation, air filtration, sewage treatment, sensing and energy storage, have been demonstrated with typical reports. The comparison of HP-MOFs with traditional porous materials (e.g., zeolite, porous silica, carbons, metal oxides, and polymers), subsisting challenges, as well as future directions in this research field, are also indicated.

One-Step Construction of a Hollow Au@Bimetal-Organic Framework Core-Shell Catalytic Nanoreactor for Selective Alcohol Oxidation Reaction

Hollow core-shell catalytic nanoreactors have received tremendous attention due to their high mass transfer in catalysis applications. Herein, we present a novel type of well-arranged, hollow core-shell nanoreactors featured with a bimetallic porous Zn/Ni-MOF-2 shell and a tiny Au nanoparticle core. The well-designed hollow Au@Zn/Ni-MOF-2 nanoreactors were constructed through the strategy of a facile one step from a rare crystal-structure transformation without any additional template. These nanoreactors exhibit outstanding multifunctional catalysis for a broad range of alcohol oxidation under the green oxidant environment. Moreover, such hollow nanoreactors show excellent recyclability toward the selective alcohol oxidation. These findings might provide a promising platform for a general construct of various metal-organic framework-based hollow core-shell nanostructures and further highly augmented catalytic applications.

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.

Metal-Organic Framework (MOF)-Based Drug Delivery

Background:

Developing a controllable drug delivery system is imperative and important to reduce side effects and enhance the therapeutic efficacy of drugs. Metal-organic frameworks (MOFs) an emerging class of hybrid porous materials built from metal ions or clusters bridged by organic linkers have attracted increasing attention in the recent years owing to the unique physical structures possessed, and the potential for vast applications. The superior properties of MOFs, such as well-defined pore aperture, tailorable composition and structure, tunable size, versatile functionality, high agent loading, and improved biocompatibility, have made them promising candidates as drug delivery hosts. MOFs for drug delivery is of great interest and many very promising results have been found, indicating that these porous solids exhibit several advantages over existing systems.

Objective:

This review highlights the latest advances in the synthesis, functionalization, and applications of MOFs in drug delivery, and has classified them using drug loading strategies. Finally, challenges and future perspectives in this research area are also outlined.

Sequential drug release via chemical diffusion and physical barriers enabled by hollow multishelled structures

Hollow multishelled structures (HoMSs), with relatively isolated cavities and hierarchal pores in the shells, are structurally similar to cells. Functionally inspired by the different transmission forms in living cells, we studied the mass transport process in HoMSs in detail. In the present work, after introducing the antibacterial agent methylisothiazolinone (MIT) as model molecules into HoMSs, we discover three sequential release stages, i.e., burst release, sustained release and stimulus-responsive release, in one system. The triple-shelled structure can provide a long sterility period in a bacteria-rich environment that is nearly 8 times longer than that of the pure antimicrobial agent under the same conditions. More importantly, the HoMS system provides a smart responsive release mechanism that can be triggered by environmental changes. All these advantages could be attributed to chemical diffusion- and physical barrier-driven temporally-spatially ordered drug release, providing a route for the design of intelligent nanomaterials.

Controllable broadband multicolour single-mode polarized laser in a dye-assembled homoepitaxial MOF microcrystal

Multicolour single-mode polarized microlasers with visible to near-infrared output have very important applications in photonic integration and multimodal biochemical sensing/imaging but are very difficult to realize. Here, we demonstrate a single crystal with multiple segments based on the host-guest metal-organic framework ZJU-68 hierarchically hybridized with different dye molecules generating controllable single-mode green, red, and near-infrared lasing, with the lasing mode mechanism revealed by computational simulation. The segmented and oriented assembly of different dye molecules within the ZJU-68 microcrystal causes it to act as a shortened resonator, enabling us to achieve dynamically controllable multicolour single-mode lasing with a low three-colour-lasing threshold of ~1.72 mJ/cm2 (approximately seven times lower than that of state-of-the-art designed heterostructure alloys, as reported by Fan F et al. (Nat. Nanotechnol. 10:796-803, 2015) considering the single pulse energy density) and degree of polarization >99.9%. Furthermore, the resulting three-colour single-mode lasing possesses the largest wavelength coverage of ~186 nm (ranging from ~534 to ~720 nm) ever reported. These findings may open a new route to the exploitation of multicolour single-mode micro/nanolasers constructed by MOF engineering for photonic and biochemical applications.

Chiral Cux OS@ZIF-8 Nanostructures for Ultrasensitive Quantification of Hydrogen Sulfide In Vivo

In this study, a Cu_x OS@ZIF-8 nanostructure is fabricated to quantify the levels of hydrogen sulfide (H₂ S) in living cells and in vivo. Zeolitic lmidazolate framework-8 (ZIF-8) is chosen as an encapsulation shell to improve the selectivity of this probe. Using this unique nanostructure, ultrasensitive quantification and bioimaging of H₂ S in living cells are successfully achieved. The lower limit of detection is 0.8 and 5.3 nmol per 10⁶ cells for circular dichroism and fluorescence modes, respectively. It is found that the chiral Cu_x OS NPs transformed into achiral Cu_x S NPs contribute to the ultrasensitive detection. Notably, this probe can also be carried out to detect and track H₂ S levels in tumor-bearing animals. The discoveries put forward for the creation of a detection platform for quantitative tracking and analysis in clinic.

When hollow multishelled structures (HoMSs) meet metal-organic frameworks (MOFs)

Hollow multishelled structures (HoMSs) have distinguished advantages, such as a large effective surface area, an optimized mass transport route, and a high loading capacity, but the fabrication of HoMSs has been a big challenge. In 2009, we developed a universal and facile method for HoMS fabrication, i.e., the sequential templating approach (STA). Progress in the synthetic methodology has enabled the study of HoMSs to develop and has made it a research hotspot in materials science. To date, HoMSs have shown their advantages in a wide range of applications, including catalysis, energy conversion and storage, drug delivery, etc. Based on the understanding in this field, we recently revealed the unique temporal-spatial ordering properties of HoMSs. Furthermore, we have been wondering if the structure of a HoMS can be modulated at the molecular level. Encouragingly, metal-organic frameworks (MOFs) are star materials with clearly defined molecular structures. The compositions, geometries, functionalities and topologies of MOFs have been well tuned by rational design. Integrating the unique properties of MOFs and HoMS could realize the systemic design of materials from the molecular to the micro-level, which would provide a series of advantages for various applications, such as developing high performance catalysts for cascade and/or selective catalysis, combining the reaction and separation process for multiple reactions, releasing drugs in a certain environment for smart medicine, and so on. We believe it is time to summarize the recent progress in the integration of MOFs and HoMSs, including HoMSs coated with MOFs, MOF-derived HoMSs, and MOFs with a hollow multishelled structure, and we also put forward our personal outlook in relation to the future opportunities and challenges in this emerging yet promising research field.

Tuning Metal-Organic Framework Nanocrystal Shape through Facet-Dependent Coordination

We studied coordination-dependent surfactant binding on shaped MOF nanocrystals. Cetyltrimethylammonium bromide (CTAB) on the surface of ZIF-8 was used as a model system. Infrared spectroscopic analysis and molecular dynamics simulations reveal different coordination environments for Zn nodes on {100} and {110} facets, resulting in different CTAB adsorption. We found that we are able to fine-tune the ratio of {100} and {110} facets in the nanocrystals. We also observed that once the MOF nanocrystals are enclosed by pure {110} facets growth along the {100} facets is terminated because the MOF nanocrystal has no surface area for CTAB adsorption. Growth can then be reinitiated through the etching of these rhombic dodecahedral nanocrystals to form a small amount of undercoordinated sites. This work represents the first systematic study of the design principles underpinning the synthesis of shaped MOF nanocrystals.