Interrogating Kinetic versus Thermodynamic Topologies of Metal–Organic Frameworks via Combined Transmission Electron Microscopy and X-ray Diffraction Analysis

Applications of metal organic frameworks in dispersive micro solid phase extraction (D-μ-SPE)

Metal-organic frameworks (MOFs) are a fascinating family of crystalline porous materials made up of metal clusters and organic linkers. In comparison with other porous materials, MOFs have unique characteristics including high surface area, homogeneous open cavities, and permanent high porosity with variable shapes and sizes. For these reasons, MOFs have recently been explored as sorbents in sample preparation by solid-phase extraction (SPE). However, SPE requires large amounts of sorbents and suffers from limited contact surfaces with analytes, which compromises extraction recovery and efficiency. Dispersive SPE (D-SPE) overcomes these limitations by dispersing the sorbents into the sample, which in turn increases contact with the analytes. Miniaturization of the microextraction procedure, particularly the amount of sorbent reduces the amount consumed of the organic solvent and shorten the time required to attain the equilibrium state. This may explain the reported high efficiency and applicability of MOFs in dispersive micro SPE (D-µ-SPE). This method retains all the advantages of solid phase extraction while also being simpler, faster, cheaper, and, in some cases, more effective in comparison with D-SPE. Besides, D-µ-SPE requires smaller amounts of the sorbents which reduces the overall cost, and the amount of waste generated from the analytical process. In this review, we discuss the applications of MOFs in D-µ-SPE of various analytes including pharmaceuticals, pesticides, organic dyes from miscellaneous matrices including water samples, biological samples and food samples.

Zr-based multivariate metal-organic framework for rapid extraction of sulfonamide antibiotics from water and food samples

Based on multiple ligands strategy, a series of multivariate metal organic frameworks (MTV-MOFs) named as PCN-224-DCDPSx were prepared using one-pot solvothermal method to extract and remove sulfonamide antibiotics (SAs). The pore structure and adsorption performance can be further regulated by modulating the doping ratios of medium-tetra(4-carboxylphenyl) porphyrin and 4,4'-dicarboxydiphenyl sulfones. The MTV-MOFs of PCN-224-DCDPS1.0 possesses very large specific surface area (1625 m2/g). Using PCN-224-DCDPS1.0 as sorbent, a dispersive solid-phase extraction method was developed to extract and preconcentrate SAs from water, eggs, and milk prior to high performance liquid chromatography analysis. The limits of detection of method were determined between 0.17 and 0.27 ng/mL with enrichment factors ranging 214-327. The adsorption can be finished within 30 s, and the recovery rate remains above 80 % after 10 repeated uses. The adsorption capacities of sorbent were determined from 300 to 621 mg/g for sulfadiazine, sulphapyridine, sulfamethoxydiazine, sulfachlorpyridazine, sulfabenzamide, and sulfadimethoxine. The adsorption mechanisms were investigated and can be attributed to π-π interactions, hydrogen bonds, and electrostatic interactions. This work represents a method for preparation of MTV-MOFs and uses as sorbent for extraction and enrichment of trace pollutants from complex samples.

Interface-Engineered 2D Heterojunction with Photoelectric Dual Gain: Mxene@MOF-Enhanced SPR Spectroscopy for Direct Sensing of Exosomes

MXene is widely used in the construction of optoelectronic interfaces due to its excellent properties. However, the hydrophilicity and metastable surface of MXene lead to its oxidation behavior, resulting in the degradation of its various properties, which seriously limits its practical application. In this work, a 2D metal-organic framework (2D MOF) with matching 2D morphology, excellent stability performance, and outstanding optoelectronic performance is grown in situ on the MXene surface through heterojunction engineering to suppress the direct contact between reactive molecules and the inner layer material without affecting the original advantages of MXene. The photoelectric dual gain MXene@MOF heterojunction is confirmed. As a photoelectric material, its properties are highly suitable for the demand of interface sensitization layer materials of surface plasmon resonance (SPR). Therefore, using SPR as a platform for the application of this interface material, the performance of MXene@MOF and its potential mechanism to enhance SPR are analyzed in depth using experiments combined with simulation calculations (FDTD/DFT). Finally, the MXene@MOF/peptides-SPR sensor is constructed for rapid and sensitive detection of the cancer marker exosomes to explore its potential in practical applications. This work offers a forward-looking strategy for the design of interface materials with excellent photoelectric performance.

Network topology diversification of porous organic salts

Hydrogen-bonded organic frameworks (HOFs) are porous organic materials constructed via hydrogen bonds. HOFs have solubility in specific high-polar organic solvents. Therefore, HOFs can be returned to their components and can be reconstructed, which indicates their high recyclability. Network topologies, which are the frameworks of porous structures, control the pore sizes and shapes of HOFs. Therefore, they strongly affect the functions of porous materials. However, hydrogen bonds are usually weak interactions, and the design of the intended network topology in HOFs from their components has been challenging. Porous organic salts (POSs) are an important class of HOFs, are hierarchically constructed via strong charge-assisted hydrogen bonds between sulfonic acids and amines, and therefore are expected to have high designability of the porous structure. However, the network topology of POSs has been limited to only dia-topology. Here, we combined tetrasulfonic acid with the adamantane core (4,4',4'',4'''-(adamantane-1,3,5,7-tetrayl)tetrabenzenesulfonic acid; AdPS) and triphenylmethylamines with modified substituents in para-positions of benzene rings (TPMA-X, X = F, methyl (Me), Cl, Br, I). We changed the steric hindrance between the adamantane and substituents (X) in TPMA-X and obtained not only the common dia-topology for POSs but also rare sod-topology, and lon- and uni-topologies that are formed for the first time in HOFs. Changing template molecules under preparation helped in successfully isolating the porous structures of AdPS/TPMA-Me with dia-, lon-, and sod-topologies which exhibited different gas adsorption properties. Therefore, for the first time, we demonstrated that the steric design of HOF components facilitated the formation, diversification, and control of the network topologies and functions of HOFs.

Atomic-level imaging of beam-sensitive COFs and MOFs by low-dose electron microscopy

Electron microscopy, an important technique that allows for the precise determination of structural information with high spatiotemporal resolution, has become indispensable in unravelling the complex relationships between material structure and properties ranging from mesoscale morphology to atomic arrangement. However, beam-sensitive materials, particularly those comprising organic components such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), would suffer catastrophic damage from the high energy electrons, hindering the determination of atomic structures. A low-dose approach has arisen as a possible solution to this problem based on the integration of advancements in several aspects: electron optical system, detector, image processing, and specimen preservation. This article summarizes the transmission electron microscopy characterization of MOFs and COFs, including local structures, host-guest interactions, and interfaces at the atomic level. Revolutions in advanced direct electron detectors, algorithms in image acquisition and processing, and emerging methodology for high quality low-dose imaging are also reviewed. Finally, perspectives on the future development of electron microscopy methodology with the support of computer science are presented.

Radical-Driven Crystal-Amorphous-Crystal Transition of a Metal-Organic Framework

Self-assembly-based structural transition has been explored for various applications, including molecular machines, sensors, and drug delivery. In this study, we developed new redox-active metal-organic frameworks (MOFs) called DGIST-10 series that comprise π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligands and Ni2+ ions, aiming to boost ligand-self-assembly-driven structural transition and study the involved mechanism. Notably, during the synthesis of the MOFs, a single-crystal-amorphous-single-crystal structural transition occurred within the MOFs upon radical formation, which was ascribed to the fact that radicals prefer spin-pairing or through-space electron delocalization by π-orbital overlap. The radical-formation-induced structural transitions were further confirmed by the postsynthetic solvothermal treatment of isolated nonradical MOF crystals. Notably, the transient amorphous phase without morphological disintegration was clearly observed, contributing to the seminal structural change of the MOF. We believe that this unprecedented structural transition triggered by the ligand self-assembly magnifies the structural flexibility and diversity of MOFs, which is one of the pivotal aspects of MOFs.

Surfactant-Mediated Crystalline Structure Evolution Enabling the Ultrafast Green Synthesis of Bismuth-MOF in Aqueous Condition

Green synthesis of stable metal-organic frameworks (MOFs) with permanent and highly ordered porosity at room temperature without needing toxic and harmful solvents and long-term high-temperature reactions is crucial for sustainable production. Herein, a rapid and environmentally friendly synthesis strategy is reported to synthesize the complex topological bismuth-based-MOFs (Bi-MOFs), [Bi9(C9H3O6)9(H2O)9] (denoted CAU-17), in water under ambient conditions by surfactant-mediated sonochemical approach, which could also be applicable to other MOFs. This strategy explores using cetyltrimethylammonium bromide (CTAB) amphiphilic molecules as structure-inducing agents to control the removal of non-coordinated water (dehydration) and enhance the degree of deprotonation of the ligands, thereby regulating the coordination and crystallization in aqueous solutions. In addition, another two new strategies for synthesizing CAU-17 by crystal reconstruction and one-step synthesis in binary solvents are provided, and the solvent-induced synthesis mechanism of CAU-17 is studied. The as-prepared CAU-17 presents a competitive iodine capture capability and effective delivery of the antiarrhythmic drug procainamide (PA) for enteropatia due to the broad pH tolerance and the unique phosphate-responsive destruction in the intestine. The findings will provide valuable ideas for the follow-up study of surfactant-assisted aqueous synthesis of MOFs and their potential applications.

How Reproducible is the Synthesis of Zr-Porphyrin Metal-Organic Frameworks? An Interlaboratory Study

Metal-organic frameworks (MOFs) are a rapidly growing class of materials that offer great promise in various applications. However, the synthesis remains challenging: for example, a range of crystal structures can often be accessed from the same building blocks, which complicates the phase selectivity. Likewise, the high sensitivity to slight changes in synthesis conditions may cause reproducibility issues. This is crucial, as it hampers the research and commercialization of affected MOFs. Here, it presents the first-ever interlaboratory study of the synthetic reproducibility of two Zr-porphyrin MOFs, PCN-222 and PCN-224, to investigate the scope of this problem. For PCN-222, only one sample out of ten was phase pure and of the correct symmetry, while for PCN-224, three are phase pure, although none of these show the spatial linker order characteristic of PCN-224. Instead, these samples resemble dPCN-224 (disordered PCN-224), which has recently been reported. The variability in thermal behavior, defect content, and surface area of the synthesised samples are also studied. The results have important ramifications for field of metal-organic frameworks and their crystallization, by highlighting the synthetic challenges associated with a multi-variable synthesis space and flat energy landscapes characteristic of MOFs.

Influence of Water Content on Speciation and Phase Formation in Zr-Porphyrin-Based MOFs

Controlled synthesis of phase-pure metal-organic frameworks (MOFs) is essential for their application in technological areas such as catalysis or gas sorption. Yet, knowledge of their phase formation and growth remain rather limited, particularly with respect to species such as water whose vital role in MOF synthesis is often neglected. As a consequence, synthetic protocols often lack reproducibility when multiple MOFs can form from the same metal source and linker, and phase mixtures are obtained with little or no control over their composition. In this work, the role of water in the formation of the Zr-porphyrin MOF disordered PCN-224 (dPCN-224) is investigated. Through X-ray total scattering and scanning electron microscopy, it is observed that dPCN-224 forms via a metal-organic intermediate that consists of Zr6O4(OH)4 clusters linked by tetrakis(4-carboxy-phenyl)porphyrin molecules. Importantly, water is not only essential to the formation of Zr6O4(OH)4 clusters, but it also plays a primary role in dictating the formation kinetics of dPCN-224. This multidisciplinary approach to studying the speciation of dPCN-224 provides a blueprint for how Zr-MOF synthesis protocols can be assessed and their reproducibility increased, and highlights the importance of understanding the role of water as a decisive component in Zr-MOF formation.

Dual-Mechanism-Driven Ratiometric Electrochemiluminescent Biosensor for Methicillin-Resistant Staphylococcus aureus

Design of a ratiometric method is a promising pathway to improve the sensitivity and reliability of electrochemiluminescent (ECL) assay, for which the signals produced at two distinct potentials change reversely as it is applied to the target analyte. Herein, a biosensor for ECL assay of methicillin-resistant Staphylococcus aureus (MRSA) was constructed by immobilizing porcine IgG for capturing MRSA onto an electrode that was precoated with β-cyclodextrin-conjugated luminol nanoparticles (β-CD-Lu NPs) as an anodic luminophore. MOF PCN 224 loaded with an atomically distributed Zn element (PCN 224/Zn) was conjugated with phage recombinant cellular-binding domain (CBD) to act as a cathodic luminophore for tracing MRSA. After the formation of the sandwich complex of β-CD-Lu NPs-porcine IgG/MRSA/PCN 224/Zn-CBD on the biosensor, two ECL reactions were triggered with cyclic voltammetry. The anodic process of the β-CD-Lu NPs-H2O2 system and the cathodic process of the PCN 224/Zn-S2O82- system competed to react with reactive oxygen species (ROS) for producing ECL emission, which led to a reverse change of the two signals. Meanwhile, the overlap of the β-CD-Lu NPs emission spectrum and PCN 224/Zn absorption spectrum effectively triggered ECL resonance energy transfer between the donor (β-CD-Lu NPs) and the acceptor (PCN 224/Zn). Thus, a ratiometric ECL method was proposed for assaying MRSA with a dual-mechanism-driven mode. The detection limit for assaying MRSA is as low as 12 CFU/mL. The biosensor was applied to assay MRSA in various biological samples with recoveries ranging from 84.9 to 111.3%.

Understanding and Controlling Molecular Compositions and Properties in Mixed-Linker Porphyrin Metal-Organic Frameworks

Porphyrin-based metal-organic frameworks (MOFs) are attractive materials for photo- and thermally activated catalysis due to their unique structural features related to the porphyrin moiety, guest-accessible porosity, and high chemical tunability. In this study, we report the synthetic incorporation of nonplanar β-ethyl-functionalized porphyrin linkers into the framework structure of PCN-222, obtaining a solid-solution series of materials with different modified linker contents. Comprehensive analysis by a combination of characterization techniques, such as NMR, UV-vis and IR spectroscopy, powder X-ray diffraction, and N2 sorption analysis, allows for the confirmation of linker incorporation. A detailed structural analysis of intrinsic material properties, such as the thermal response of the different materials, underlines the complexity of synthesizing and understanding such materials. This study presents a blueprint for synthesizing and analyzing porphyrin-based mixed-linker MOF systems and highlights the hurdles of characterizing such materials.

Kinetic Trapping of Photoluminescent Frameworks During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with potential utility in gas separations, drug delivery, sensing, and catalysis. Small variations in MOF synthesis conditions can lead to a range of accessible frameworks with divergent chemical or photophysical properties. New methods to controllably access phases with tailored properties would broaden the scope of MOFs that can be reliably prepared for specific applications. Herein, we demonstrate that simply increasing the reaction concentration during the solvothermal synthesis of M2(dobdc) (M = Mg, Mn, Ni; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) MOFs unexpectedly leads to trapping of a new framework termed CORN-MOF-1 (CORN = Cornell University) instead. In-depth spectroscopic, crystallographic, and computational studies support that CORN-MOF-1 has a similar structure to M2(dobdc) but with partially protonated linkers and charge-balancing or coordinated formate groups in the pores. The resultant variation in linker spacings causes CORN-MOF-1 (Mg) to be strongly photoluminescent in the solid state, whereas H4dobdc and Mg2(dobdc) are weakly emissive due to excimer formation. In-depth photophysical studies suggest that CORN-MOF-1 (Mg) is the first MOF based on the H2dobdc2- linker that likely does not emit via an excited state intramolecular proton transfer (ESIPT) pathway. In addition, CORN-MOF-1 variants can be converted into high-quality samples of the thermodynamic M2(dobdc) phases by heating in N,N-dimethylformamide (DMF). Overall, our findings support that high-concentration synthesis provides a straightforward method to identify new MOFs with properties distinct from known materials and to produce highly porous samples of MOFs, paving the way for the discovery and gram-scale synthesis of framework materials.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Induced Production of Atypical Naturally Nonpreferred Metal-Organic Frameworks and Their Detachment via Provoking Post-Mismatching

The structures of metal-organic frameworks (MOFs) are typically determined by the building blocks that compose them and the conditions under which they are formed. MOFs tend to adopt a thermodynamically and/or kinetically stable structure (naturally preferred form). Thus, constructing MOFs with naturally nonpreferred structures is a challenging task, as it requires avoiding the easier pathway toward a naturally preferred MOF. Herein, an approach to construct naturally nonpreferred dicarboxylate-linked MOFs employing reaction templates is reported. This strategy relies on the registry between the surface of the template and the cell lattice of a target MOF, which reduces the effort required to form naturally nonpreferred MOFs. Reactions of p-block trivalent metal ions (Ga3+ and In3+ ) with dicarboxylic acids typically produce preferred MIL-53 or MIL-68. However, the surface of UiO-67 (and UiO-66) template exhibits the well-defined hexagonal lattice, which induce the selective formation of a naturally nonpreferred MIL-88 structure. Inductively grown MIL-88s are purely isolated from the template via provoking a post-mismatch in their lattices and weakening the interfacial interaction between product and template. It is also discovered that an appropriate template for effective induced production of naturally nonpreferred MOFs shall be properly selected based on the cell lattice of a target MOF.

Removal of diclofenac by adsorption process studied in free-base porphyrin Zr-metal organic frameworks (Zr-MOFs)

As the world population continues to grow, there is also a rising concern regarding water pollution since this condition could negatively impact the supply of clean water. One of the most recent concerns is related to the pollution that comes from various pharmaceuticals, in particular non-steroidal anti-inflammatory drugs (NSAIDs) since they have been industrially produced at large scale and can be easily purchased as an over-the-counter medicine. Diclofenac is one of the most popular NSAIDs because of its high-effectiveness, which leads to its excessive consumption. Consequently, its presence in water bodies is also continuously increasing. An adsorption process could then be employed as a highly effective method to address this issue. In comparison to other conventional adsorbents such as activated carbon, the use of metal-organic frameworks (MOFs) as an alternative adsorbent is very attractive since it can offer various advantages such as tailorability and high adsorption capacity. In this study, the performance of three water-stable, free-base porphyrin MOFs assembled using zirconia-based nodes, namely MOF-525, MOF-545, and NU-902, for diclofenac adsorption was thoroughly investigated. Interestingly, although all three free-base porphyrin MOFs are assembled using the same building block and have a similar specific surface area (based on the experimental argon physisorption and calculation based on non-localized density functional theory), their diclofenac adsorption capacity is substantially different from one another. It is found that the highest diclofenac adsorption capacity is shown by MOF-525, which has maximum capacity around 792 mg g^-1. This is then followed by MOF-545 and NU-902 that have adsorption capacities around 591 and 486 mg g^-1, respectively. Some possible adsorption mechanisms are then thoroughly discussed that might contribute to this phenomenon. Lastly, their performance is also compared with other MOFs that are also studied for this purpose to show their performance superiority not only in terms of adsorption capacity but also their affinity towards the diclofenac molecule, which might be useful as an adsorption performance indicator in the real condition where the contaminant concentration is considerably low.

Fabrication of ZnO/Gypsum/Gelatine nanocomposites films and their antibacterial mechanism against Staphylococcus aureus

Staphylococcus aureus (S. aureus) has long been acknowledged as being one of the most harmful bacteria for human civilization. It is the main contributor to skin and soft tissue infections. The gram positive pathogen also contributes to bloodstream infections, pneumonia, or bone and joint infections. Hence, developing an efficient and targeted treatment for these illnesses is greatly desired. Recently, studies on nanocomposites (NCs) have significantly increased due to their potent antibacterial and antibiofilm properties. These NCs provide an intriguing way to control the growth of bacteria without causing the development of resistance strains that come from improper or excessive use of the conventional antibiotics. In this context, we have demonstrated the synthesis of a NC system by precipitation of ZnO nanoparticles (NPs) on Gypsum followed by encapsulation with Gelatine, in the present study. Fourier transform infrared (FTIR) spectroscopy was used to validate the presence of ZnO NPs and Gypsum. The film was characterized by X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM). The system exhibited promising antibiofilm action and was effective in combating S. aureus and MRSA in concentrations between 10 and 50 ug/ml. The bactericidal mechanism by release of reactive oxygen species (ROS) was anticipated to be induced by the NC system. Studies on cell survival and in-vitro infection support the film's notable biocompatibility and its potential for treating Staphylococcus infections in the future.

Unlocking New Topologies in Zr-Based Metal-Organic Frameworks by Combining Linker Flexibility and Building Block Disorder

The outstanding diversity of Zr-based frameworks is inherently linked to the variable coordination geometry of Zr-oxo clusters and the conformational flexibility of the linker, both of which allow for different framework topologies based on the same linker-cluster combination. In addition, intrinsic structural disorder provides a largely unexplored handle to further expand the accessibility of novel metal-organic framework (MOF) structures that can be formed. In this work, we report the concomitant synthesis of three topologically different MOFs based on the same M₆O₄(OH)₄ clusters (M = Zr or Hf) and methane-tetrakis(p-biphenyl-carboxylate) (MTBC) linkers. Two novel structural models are presented based on single-crystal diffraction analysis, namely, cubic c-(4,12)MTBC-M₆ and trigonal tr-(4,12)MTBC-M₆, which comprise 12-coordinated clusters and 4-coordinated tetrahedral linkers. Notably, the cubic phase features a new architecture based on orientational cluster disorder, which is essential for its formation and has been analyzed by a combination of average structure refinements and diffuse scattering analysis from both powder and single-crystal X-ray diffraction data. The trigonal phase also features structure disorder, although involving both linkers and secondary building units. In both phases, remarkable geometrical distortion of the MTBC linkers illustrates how linker flexibility is also essential for their formation and expands the range of achievable topologies in Zr-based MOFs and its analogues.

Metalloporphyrin-Based Metal–Organic Frameworks for Photocatalytic Carbon Dioxide Reduction: The Influence of Metal Centers

Photocatalysis is one of the most promising technologies to achieve efficient carbon dioxide reduction reaction (CO2RR) under mild conditions. Herein, metalloporphyrin-based metal–organic frameworks (MOFs) with different metal centers, denoted as PCN-222, were utilized as visible-light photocatalysts for CO2 reduction. Due to the combination of the conjugated planar macrocyclic structures of metalloporphyrins and the stable porous structures of MOFs, all PCN-222 materials exhibited excellent light-harvesting and CO2-adsorbing abilities. Among the studied MOFs of varied metal centers (M = Pt, Fe, Cu, Zn, Mn), PCN-222(2H&Zn) exhibited the highest photocatalytic CO2RR performance, with an average CO yield of 3.92 μmol g−1 h−1 without any organic solvent or sacrificial agent. Furthermore, this was three and seven times higher than that of PCN-222(Zn) (1.36 μmol g−1 h−1) and PCN-222(2H) (0.557 μmol g−1 h−1). The superior photocatalytic activity of PCN-222(2H&Zn) was attributed to its effective photoexcited electron–hole separation and transportation compared with other PCN-222(2H&M) materials. The obtained results indicate that Zn ions in the porphyrin’s center played an important role in the reaction of active sites for the adsorption–activation of CO2. In addition, PCN-222(2H&Zn) showed the highest CO2 selectivity (almost 100%) and stability. This work provides a clear guide for the design of efficient photocatalysts.

π-Stacks of radical-anionic naphthalenediimides in a metal-organic framework

Radical-ionic metal-organic frameworks (MOFs) have unique optical, magnetic, and electronic properties. These radical ions, forcibly formed by external stimulus-induced redox processes, are structurally unstable and have short radical lifetimes. Here, we report two naphthalenediimide-based (NDI-based) Ca-MOFs: DGIST-6 and DGIST-7. Neutral DGIST-6, which is generated first during solvothermal synthesis, decomposes and is converted into radical-anionic DGIST-7. Cofacial (NDI)₂^•- and (NDI)₂^2- dimers are effectively stabilized in DGIST-7 by electron delocalization and spin-pairing as well as dimethylammonium counter cations in their pores. Single-crystal x-ray diffractometry was used to visualize redox-associated structural transformations, such as changes in centroid-to-centroid distance. Moreover, the unusual rapid reduction of oxidized DGIST-7 into the radical anion upon infrared irradiation results in effective and reproducible photothermal conversion. This study successfully illustrated the strategic use of in situ prepared cofacial ligand dimers in MOFs that facilitate the stabilization of radical ions.

Data-driven efficient synthetic exploration of anionic lanthanide-based metal-organic frameworks

The synthesis of lanthanide metal-organic frameworks with terephthalate (Ln-BDC-MOFs) was investigated using a data-driven approach. Visually mapping the previously reported synthetic conditions suggested the existence of unexplored search spaces for novel Ln-BDC-MOFs. By focusing on the unexplored chemical reaction space, we successfully synthesized a series of new anionic Ln-BDC-MOFs, KGF-15, which demonstrated potential as luminescent sensors for Cu²⁺ ions. This synthetic exploration approach can significantly reduce the experimental effort required to discover new materials.

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.

Ferric iron loaded porphyrinic zirconium MOFs on corncob for the enhancement of diuretics extraction

Herein, corncob waste was used as a scaffold for the fabrication of effective adsorbents. Porphyrinic zirconium metal-organic frameworks (MOFs) PCN-223 and PCN-224 constructed by different numbers of Zr6 cluster nodes were grown on the surface of the corncob. Fe (Ш) ions were implanted in the porphyrin ring by post-synthesis modification. The results showed that the extraction capacity of diuretics on PCN-224@corncob containing suitable pore size was larger than that of PCN-223@corncob. The adsorption of diuretics was further enhanced because of the electrostatic effect caused by implantation of Fe (Ш) ions. PCN-224-Fe@corncob was recyclable and selective for the extraction of furosemide (Fur) and bumetanide (Bum). Coupled in-syringe solid phase extraction (IS-SPE) with ultra-performance liquid chromatography (UPLC), an efficient, sensitive, and stable method was established. With a sensitivity between 0.6 and 1.0 μg/L and a recovery between 83.2% and 119.2%, it is used for the analysis of trace amounts of Fur and Bum in weight loss products and environmental water. The functionalized corncob has potential application for the adsorption of diuretics, and the metal ions implantation in MOFs provides a promising strategy for enhancing extraction capacity.

Rapid Generation of Metal-Organic Framework Phase Diagrams by High-Throughput Transmission Electron Microscopy

Metal-organic frameworks (MOFs) constructed from Zr₆ nodes and tetratopic carboxylate linkers display high structural diversity and complexity in which various crystal topologies can result from identical building units. To determine correlations between MOF topologies and experimental parameters, such as solvent choice or modulator identity and concentration, we demonstrate the rapid generation of phase diagrams for Zr₆-MOFs with 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene linkers under a variety of conditions. We have developed a full set of methods for high-throughput transmission electron microscopy (TEM), including automated sample preparation and data acquisition, to accelerate MOF characterization. The use of acetic acid as a modulator yields amorphous, NU-906, NU-600, and mixed-phase structures depending on the ratio of N,N-dimethylformamide to N,N-diethylformamide solvent and the quantity of the modulator. Notably, the use of formic acid as a modulator enables direct control of crystal growth along the c direction through variation of the modulator quantity, thus realizing aspect ratio control of NU-1008 crystals with different catalytic hydrolysis performance toward a nerve agent simulant.

Exchange of coordinated carboxylates with azolates as a route to obtain a microporous zinc-azolate framework

Metal-organic frameworks (MOFs) containing open metal sites are advantageous for wide applications. Here, carboxylate linkers are replaced with triazolate coordination in pre-formed Zn-MOF-74 via solvent-assisted linker exchange (SALE) to prepare the novel NU-250, within the known hexagonal channel-based MAF-X25 series that has not previously been synthesized de novo.

Identification of a metastable uranium metal–organic framework isomer through non-equilibrium synthesis

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal–organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier's principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Identifying the relationship between structure and energetics in a uranium MOF isomer system reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs.

Photosensitizer-based metal-organic frameworks for highly effective photodynamic therapy

Photodynamic therapy (PDT) uses a photosensitizer, molecular oxygen, and visible light as an alternative clinical protocol against located malignant tumors and other diseases. More recently, PDT has been combined to immunotherapy as a promising option to treat metastatic cancer. However, previous generations of photosensitizers (PSs) revealed clinical difficulties such as long-term skin photosensitivity (first generation), the need for drug delivery vehicles (second generation), and intracellular self-aggregation (third generation), which have generated a somewhat confusing scenario in PDT approaches and evolution. Recently, metal-organic frameworks (MOFs) with exceptionally high PS loading as a building unit of MOF framework have emerged as fourth-generation PS and presented outstanding outcomes under pre-clinical studies. For PS-based MOFs, the inorganic building unit (metal ions/clusters) plays an important role as a coadjuvant in PDT to alleviate hypoxia, to decrease antioxidant species, to yield ROS, or to act as a contrast agent for imaging-guided therapy. In this review, we intend to carry out a broad update on the recent history and the characteristics of PS-based MOFs from basic chemistry to the structure relationship with biological application in PDT. The details and variables that result in different photophysics, size, and morphology, are discussed. Also, we present an overview of the achievements on the pre-clinical assays in combination with other strategies, including alleviating hypoxia in solid tumors, chemotherapy, and the most recent immunotherapy for cancer.

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.

Unveiling the Unique Roles of Metal Coordination and Modulator in the Polymorphism Control of Metal-Organic Frameworks

Polymorphism control of metal-organic frameworks is highly desired for elucidating structure-property relationships, but remains an empirical process and is usually done in a trial-and-error approach. We adopted the rarely used actinide cation Th⁴⁺ and a ditopic linker to construct a series of thorium-organic frameworks (TOFs) with a range of polymorphs. The extraordinary coordination versatility of Th⁴⁺ cations and clusters, coupled with synthetic modulation, gives five distinct phases, wherein the highest degree of interpenetration (threefold) and porosity (75.9 %) of TOFs have been achieved. Notably, the O atom on the capping site of the nine-coordinated Th⁴⁺ cation can function as a bridging unit to interconnect neighboring secondary building units (SBUs), affording topologies that are undocumented for other tetravalent-metal-containing MOFs. Furthermore, for the first time HCOOH has been demonstrated as a bridging unit of SBUs to further induce structural complexity. The resulting TOFs exhibit considerably different adsorption behaviors toward organic dyes, thus suggesting that TOFs represent an exceptional and promising platform for structure-property relationship study.

Screening and Discrimination of Perfluoroalkyl Substances in Aqueous Solution Using a Luminescent Metal-Organic Framework Sensor Array

The extensive production and large-scale use of perfluoroalkyl substances (PFASs) have raised their presence in aquatic environments worldwide. Thus, the facile and reliable screening of PFASs in aqueous systems is of great significance. Herein, we designed a novel fluorescent sensor array for the rapid screening and discrimination of multiple PFASs in water. The sensor array comprised three highly stable zirconium porphyrinic luminescent metal-organic frameworks (i.e., PCNs) with different topological structures. The sensing mechanism was based on the static fluorescence quenching of PCNs by PFASs upon their adsorptive interactions. The fluorescence response patterns were characteristic for each PFAS because of their different adsorption affinities toward different PCNs. Through the interpretation of response patterns by statistical methods, the proposed PCN array successfully discriminated six different kinds of PFASs, each PFAS at different concentrations and PFAS mixtures at different molar ratios. The practicability of this array was further verified by effectively discriminating PFASs in two real water samples. Remarkably, the PCN sensors exhibited a very short response time toward PFASs (within 10 s) due to the ordered pore structure allowing fast PFAS diffusion. This study not only provides a facile method for rapid PFAS screening in waters but also broadens the application of luminescent metal-organic frameworks and array techniques in sensing fields.

Mechanistic Investigations into and Control of Anisotropic Metal-Organic Framework Growth

The porphyrinic metal-organic framework, PCN-222, exhibits anisotropic growth behavior to form nanorods and microrods with aspect ratios 3 < x < 94. Control of microrod aspect ratios has been demonstrated through the identification of several factors that dictate crystal growth, particularly the concentrations of a ligand, a modulator, and an exogenous base. An increase in the local concentration of a deprotonated ligand, which is proportional to the nucleation rate, is associated with smaller crystals, while increased modulator concentration leads to longer microrods. Addition of a deprotonating agent not only contributes to higher aspect ratios but also results in an improvement to particle dispersity. Here, we report acid-base co-modulation methods with difluoroacetic acid and triethylamine to effectively tune PCN-222 aspect ratios. A series of mechanisms is identified for the growth of PCN-222: (1) ligand deprotonation, (2) nucleation, (3) oriented attachment, (4) Ostwald ripening, and (5) dissolution-recrystallization. Time trials of co-modulated samples revealed three separate ripening growth events, with each resulting in larger and more monodisperse crystals. With an understanding of these crystal growth factors and mechanisms, the highest aspect ratio, non-templated metal-organic frameworks were synthesized (94 ± 9).

Modulating the Stacking Model of Covalent Organic Framework Isomers with Different Generation Efficiencies of Reactive Oxygen Species

The development of a facile synthesis and controllable layer stacking approach for covalent organic frameworks (COFs) is an important issue for modulating their properties and realizing their application diversity. Herein, three COF isomers with different stacking models (eclipsed AA, staggered AB, and ABC stacking) were obtained by modulating the reaction temperature and solvent medium. Experimental and theoretical calculations show that the ABC stacking isomer obtained at room temperature is the kinetic product, while the AA stacking isomer prepared by the solvothermal method is a thermodynamic product. Owing to the tautomerism involved in the reaction process, these isomers possess different ratios of enol and keto forms. Thus, they exhibit different generation efficiencies of Type I and Type II reactive oxygen species (ROS). The ABC stacking isomers could be employed as metal-free heterogeneous photocatalysts for visible-light-induced oxidation of amines to imines, owing to the highest generation efficiency of Type I ROS.

Understanding disorder and linker deficiency in porphyrinic zirconium-based metal-organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221

Porphyrin-based metal–organic frameworks (MOFs), exemplified by MOF-525, PCN-221, and PCN-224, are promising systems for catalysis, optoelectronics, and solar energy conversion. However, subtle differences between synthetic protocols for these three MOFs give rise to vast discrepancies in purported product outcomes and description of framework topologies. Here, based on a comprehensive synthetic and structural analysis spanning local and long-range length scales, we show that PCN-221 consists of Zr₆O₄(OH)₄ clusters in four distinct orientations within the unit cell, rather than Zr₈O₆ clusters as originally published, and linker vacancies at levels of around 50%, which may form in a locally correlated manner. We propose disordered PCN-224 (dPCN-224) as a unified model to understand PCN-221, MOF-525, and PCN-224 by varying the degree of orientational cluster disorder, linker conformation and vacancies, and cluster–linker binding. Our work thus introduces a new perspective on network topology and disorder in Zr-MOFs and pinpoints the structural variables that direct their functional properties.

Zirconium-based metal–organic frameworks have defective structures that are useful in catalysis and gas storage. Here, the authors study the interplay between cluster disorder and linker vacancies in PCN-221 and propose a new structure model with tilted Zr₆O₄(OH)₄ clusters rather than Zr₈O₆ clusters.

Continuous Fluidic Techniques for the Precise Synthesis of Metal-Organic Frameworks

The continuous fluidics-based synthesis of metal-organic frameworks (MOFs) has attracted considerable attention, resulting in advancements in the reaction efficiency, a continuous production of complex structures, and access to products that are difficult or impossible to attain by bulk synthetic routes. This Minireview discusses the continuous fluidics-based synthesis of MOFs in terms of reaction process control, and is divided into three chapters dealing with the efficient synthesis of high-quality MOFs, the confined-space synthesis of MOF composites with diverse morphologies, and the selective synthesis of metastable products. The products of continuous fluidic synthetic process are introduced (e. g., uniform products, composites, fibers, membranes, and metastable products with advantageous properties that cannot be obtained by bulk synthesis), and their usefulness is demonstrated by referencing representative examples.

Ultramicrotomy preparation of magnetic nanoparticles for transmission electron microscopy

Transmission electron microscopy (TEM) is one of the most important methods for the morphological characterization and structure analysis of nanomaterials. However, the characterization of magnetic materials has always been a challenge due to limitations arising from the design of electron microscopes. To tackle this problem, advanced sample preparation technology is needed, especially for magnetic materials. Here in this work ultrathin sectioning technology (ultramicrotomy) is used for the sample preparation of magnetic Fe₃O₄ nanoparticles embedded into a resin, where the loaded resin can be sliced into nanoscale sheets. By the optimization of the embedding method and the slicing process, nano-sheets with uniform thickness and exceptional flatness were prepared, where the nanoparticles exhibited uniform dispersion. It is shown that this technology also helps reducing the degree of pollution of the electron microscope by the magnetic nanoparticles under different electron beam irradiation intensities. Generally, the magnetic nanoparticles are more resistant to electron beam bombardment when embedded into a resin.

Physical properties of porphyrin-based crystalline metal‒organic frameworks

Metal‒organic frameworks (MOFs) are widely studied molecular assemblies that have demonstrated promise for a range of potential applications. Given the unique and well-established photophysical and electrochemical properties of porphyrins, porphyrin-based MOFs are emerging as promising candidates for energy harvesting and conversion applications. Here we discuss the physical properties of porphyrin-based MOFs, highlighting the evolution of various optical and electronic features as a function of their modular framework structures and compositional variations.

Electrolytic synthesis of porphyrinic Zr-metal-organic frameworks with selective crystal topologies

The thermodynamic (PCN-222) and kinetic (PCN-224) products of porphyrinic Zr-metal-organic frameworks (MOFs) were synthesized via an anodic dissolution approach for the first time. To the best of our knowledge, this is the first report of MOF polymorphs being controlled by electrolysis. The selective formation of PCN-222 requires an amorphous component to be present on the electrode during the initial reaction process.

Copper-alloy catalysts: structural characterization and catalytic synergies

Recent progress in the development of copper-alloy catalysts is highlighted, focusing on the structural and mechanistic characterizations of the catalysts in different catalytic reactions, and challenges and opportunities in future research.

Metal Organic Frame-Upconverting Nanoparticle Assemblies for the FRET Based Sensor Detection of Bisphenol A in High-Salt Foods

To resolve the occurrence of unfulfillable detection in high-salts foods, we used fluorescence resonant energy transfer (FRET) sensors based on nanoparticle upconversion. In this study, we developed a novel FRET sensor for the detection of bisphenol A (BPA) in high-salt foods. We based this approach on the assembly of aptamer modified upconversion nanoparticles (DNA1-UCNPs) and complementary DNA modified metal organic frames (DNA2-MOFs), which possessed corresponding wavelength absorption. Targeting BPA signal transduction was performed using the BPA aptamer, via competitive recognition between the BPA analyte and complementary DNA sequences in a high-salt solution. Sensor adaption in high-salt samples was attributed to functional hydrophilic groups, modified in the MOFs, and the enhanced colloidal stability of these MOFs. The MOF-UCNP assembly displayed considerable analytical performance in terms of BPA detection, with a linear range of 0.1-100 nM, and a limit of detection (LOD) of 0.02 nM, in a 340 mM NaCl food sample (the energy drink, Gatorade). Thus, this method provides a solid basis for small molecules detection in high-salt foods.

Structural Diversity of Zirconium Metal-Organic Frameworks and Effect on Adsorption of Toxic Chemicals

While linkers with various conformations pose challenges in the design and prediction of metal-organic framework (MOF) structures, they ultimately provide great opportunities for the discovery of novel structures thereby enriching structural diversity. Tetratopic carboxylate linkers, for example, have been widely used in the formation of Zr-based MOFs due to the ability to target diverse topologies, providing a promising platform to explore their mechanisms of formation. However, it remains a challenge to control the resulting structures when considering the complex assembly of linkers with unpredicted conformations and diverse Zr₆ node connectivities. Herein, we systematically explore how solvents and modulators employed during synthesis influence the resulting topologies of Zr-MOFs, choosing H₄TCPB-Br₂ (1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene) as a representative tetratopic carboxylate linker. By modulating the reaction conditions, the conformations of the linker and the connectivities of the Zr₆ node can be simultaneously tuned, resulting in four types of structures: a new topology (NU-500), she (NU-600), scu (NU-906), and csq (NU-1008). Importantly, we have synthesized the first 5-connected Zr₆ node to date with the (4,4,4,5)-connected framework, NU-500. We subsequently performed detailed structural analyses to uncover the relationship between the structures and topologies of these MOFs and demonstrated the crucial role that the flexible linker played to access varied structures by different degrees of linker deformation. Due to a variety of pore structures ranging from micropores to hierarchical micropores and mesopores, the resulting MOFs show drastically different behaviors for the adsorption of n-hexane and dynamic adsorption of 2-chloroethyl ethyl sulfide (CEES) under dry and humid conditions.

Structure-directing role of immobilized polyoxometalates in the synthesis of porphyrinic Zr-based metal-organic frameworks

We evidence the structure-directing role of the PW12O403- polyoxometalate in porphyrinic MOF synthesis whereby it promotes the formation of the kinetic topology. Its immobilization into the MOF is successfully achieved at a high temperature yielding the kinetic MOF-525/PCN-224 phases, while prohibiting the formation of the thermodynamic MOF-545 product. A combined experimental/theoretical approach uses differential PDF and DFT calculations along with solid-state NMR to show the structural integrity of the POM and its location next to the Zr-based nodes.

Metal-Organic Frameworks for the Exploitation of Distance between Active Sites in Efficient Photocatalysis

Discoveries of the accurate spatial arrangement of active sites in biological systems and cooperation between them for high catalytic efficiency are two major events in biology. However, precise tuning of these aspects is largely missing in the design of artificial catalysts. Here, a series of metal-organic frameworks (MOFs) were used, not only to overcome the limit of distance between active sites in bio-systems, but also to unveil the critical role of this distance for efficient catalysis. A linear correlation was established between photocatalytic activity and the reciprocal of inter active-site distance; a smaller distance led to higher activity. Vacancies created at selected crystallographic positions of MOFs promoted their photocatalytic efficiency. MOF-525-J33 with 15.6 Å inter active-site distance and 33 % vacancies exhibited unprecedented high turnover frequency of 29.5 h^-1 in visible-light-driven acceptorless dehydrogenation of tetrahydroquinoline at room temperature.