Nano-sized metal-organic frameworks: Synthesis and applications

A General Strategy for Instantaneous and Continuous Synthesis of Ultrasmall Metal-Organic Framework Nanoparticles

Ultrasmall metal-organic frameworks (MOFs) may generate unique properties to expand the scope of applications. However, the synthesis is still a great challenge. Herein, we propose a strategy to synthesize ultrasmall MOFs by high gravity technology. With the aid of tremendous intensification of molecular mixing and mass transfer in high-gravity field, six typical MOFs were obtained instantaneously in a continuous way. These samples are monodispersed with sub-5 nm in size, smaller than the previously reported values and even close to the length of one crystal unit cell. As a proof-of-concept, catalytic activity for Knoevenagel reaction can be significantly enhanced using ultrasmall ZIF-8. Conversion time of benzaldehyde was decreased by 94 % or 75 % compared to those using conventional or hierarchically porous ZIF-8. More importantly, this approach is readily scalable with the highest space-time yield for nano-MOFs, which may promote the convenient synthesis and practical applications of ultrasmall MOFs in large-scale.

Recent Progress in Phthalocyanine-Polymeric Nanoparticle Delivery Systems for Cancer Photodynamic Therapy

This perspective article summarizes the last decade’s developments in the field of phthalocyanine (Pc)-polymeric nanoparticle (NP) delivery systems for cancer photodynamic therapy (PDT), including studies with at least in vitro data. Moreover, special attention will be paid to the various strategies for enhancing the behavior of Pc-polymeric NPs in PDT, underlining the great potential of this class of nanomaterials as advanced Pcs’ nanocarriers for cancer PDT. This review shows that there is still a lot of research to be done, opening the door to new and interesting nanodelivery systems.

From Co-MOF to CoNi-MOF to Ni-MOF: A Facile Synthesis of 1D Micro-/Nanomaterials

Controlling the growth of metal-organic frameworks (MOFs) at the micro-/nanoscopic scale will result in new physical properties and novel functions into the materials without changing the chemical identities and the characteristic features of the MOFs themselves. Herein, we report a facile approach to synthesize a series of MOFs [Co-MOF, Co_xNi_y-MOFs (x and y represent the molar ratio of Co²⁺ and Ni²⁺ and x/y = 1:1, 1:5, 1:10, 1:15, and 1:20), and Ni-MOF] with a one-dimensional micro-/nanoscaled rod-like architecture. From Co-MOF to Co_xNi_y-MOFs to Ni-MOF, the diameters of the rods turn to be spindly with the increase of Ni²⁺ content which will facilitate the supercapacitor performances. Interestingly, Co₁Ni₂₀-MOF exhibits a highest specific capacity of 597 F g^-1 at 0.5 A g^-1 and excellent cycle performance (retained 93.59% after 4000 cycles) among these MOF materials owing to its micro-/nanorod structure with a smaller diameter and the synergy effect between the optimum molar ratio of Co²⁺ and Ni²⁺.

High adsorption of sodium diclofenac on post-synthetic modified zirconium-based metal-organic frameworks: Experimental and theoretical studies

Water pollution by pharmaceuticals is currently a great concern due to their ecological risks. In this study, zirconium-based metal-organic frameworks (UiO-66-(COOH)₂) were used for removal of the nonsteroidal anti-inflammatory drug (NSAID) diclofenac sodium (DCF). They have been synthesized using a hydrothermal method. Copper and iron metal ions were incorporated in the framework using post-synthetic modification techniques to produce UiO-66-(COOCu)₂ and UiO-66-(COOFe)₂. The resulted MOFs were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning/transmission electron microscopy, and Brunauer-Emmett-Teller. The effects of the pH, initial concentration, and adsorption time on the adsorption process of diclofenac were studied. The maximum adsorption capacities obtained on UiO-66-(COOH)₂, UiO-66-(COOCu)₂, and UiO-66-(COOFe)₂ were 480.5, 624.3, and 769.1 mg/g, respectively. The adsorption of diclofenac was found to be better fitted with Langmuir isotherm and pseudo-second-order kinetic models. The adsorption mechanism was investigated using XRD, FT-IR, density functional theory and Monte Carlo simulation, in which the latter method was used to calculate the adsorption energies and determine the possible interactions between diclofenac and the adsorbents. UiO-66-(COOH)₂, UiO-66-(COOCu)₂, and UiO-66-(COOFe)₂ exhibited good recyclability for diclofenac removal, which confirms the sustainability of these materials.

Assembly of Colloidal Clusters Driven by the Polyhedral Shape of Metal-Organic Framework Particles

Control of the assembly of colloidal particles into discrete or higher-dimensional architectures is important for the design of myriad materials, including plasmonic sensing systems and photonic crystals. Here, we report a new approach that uses the polyhedral shape of metal–organic-framework (MOF) particles to direct the assembly of colloidal clusters. This approach is based on controlling the attachment of a single spherical polystyrene particle on each face of a polyhedral particle via colloidal fusion synthesis, so that the polyhedral shape defines the final coordination number, which is equal to the number of faces, and geometry of the assembled colloidal cluster. As a proof of concept, we assembled six-coordinated (6-c) octahedral and 8-c cubic clusters using cubic ZIF-8 and octahedral UiO-66 core particles. Moreover, we extended this approach to synthesize a highly coordinated 12-c cuboctahedral cluster from a rhombic dodecahedral ZIF-8 particle. We anticipate that the synthesized colloidal clusters could be further evolved into spherical core–shell MOF@polystyrene particles under conditions that promote a higher fusion degree, thus expanding the methods available for the synthesis of MOF–polymer composites.

Design of metal-organic framework composites in anti-cancer therapies

Metal-organic frameworks are a class of new and promising anti-cancer materials. MOFs with adjustable pore size, large specific surface area, diverse structure, and excellent chemical and physical properties make them a class of effective protection carriers for anti-cancer substances. This review is centered on the core point of "anti-cancer" and discusses MOFs' research progress in anti-cancer therapies. Firstly, we provided readers with the different types of MOFs, their preparation strategies and the resulting structures. Then, different MOF composites and their biological applications were systematically presented. The specificity of biomolecules endows MOFs with broader anti-cancer applications, while MOFs can protect the drugs and biomolecules to make the best of a challenging situation. Finally, we elucidated a comprehensive overview of the biological applications of MOFs, including research hotspots as drug delivery and biomolecule carriers. Besides, we looked forward to the future developments of MOFs in the field of anti-cancer therapies. As a class of novel materials, the anti-cancer applications of MOFs are extended through the combination of different materials and different methods to improve their efficacy.

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.

Understanding entrapped molecular photosystem and metal-organic framework synergy for improved solar fuel production

Artificial photosystems assembled from molecular complexes, such as the photocatalyst fac-ReBr(CO)3(4,4'-dcbpy) (dcbpy = dicarboxy-2,2'-bipyridine) and the photosensitiser Ru(bpy)2(5,5'-dcbpy)Cl2 (bpy = 2,2'-bipyridine), are a wide-spread approach for solar fuel production. Recently metal-organic framework (MOF) entrapping of such complexes was demonstrated as a promising concept for catalyst stabilisation and reaction environment optimisation in colloidal-based CO2 reduction. Building on this strategy, here we examined the influence of MIL-101-NH2(Al) MOF particle size, the electron donor source, and the presence of an organic base on the photocatalytic CO2-to-CO reduction performance, and the differences to homogeneous systems. A linear relation between smaller scaffold particle size and higher photocatalytic activity, longer system lifetimes for benign electron donors, and increased turnover numbers (TONs) with certain additive organic bases, were determined. This enabled understanding of key molecular catalysis phenomena and synergies in the nanoreactor-like host-guest assembly, and yielded TONs of ∼4300 over 96 h of photocatalysis under optimised conditions, surpassing homogeneous TON values and lifetimes.

In Situ Porphyrin Substitution in a Zr(IV)-MOF for Stability Enhancement and Photocatalytic CO2 Reduction

Despite numerous inherent merits of metal-organic frameworks (MOFs), structural fragility has imposed great restrictions on their wider involvement in many applications, such as in catalysis. Herein, a strategy for enhancing stability and enabling functionality in a labile Zr(IV)-MOF has been proposed by in situ porphyrin substitution. A size- and geometry-matched robust linear porphyrin ligand 4,4'-(porphyrin-5,15-diyl)dibenzolate (DCPP^2- ) is selected to replace the 4,4'-(1,3,6,8-tetraoxobenzo[lmn][3,8]phenanthroline-2,7(1H,3H,6H,8H)-diyl)dibenzoate (NDIDB^2- ) ligand in the synthesis of BUT-109(Zr), affording BUT-110 with varied porphyrin contents. Compared to BUT-109(Zr), the chemical stability of BUT-110 series is greatly improved. Metalloporphyrin incorporation endows BUT-110 MOFs with high catalytic activity in the photoreduction of CO₂ , in the absence of photosensitizers. By tuning the metal species and porphyrin contents in BUT-110, the resulting BUT-110-50%-Co is demonstrated to be a good photocatalyst for selective CO₂ -to-CO reduction, via balancing the chemical stability, photocatalytic efficiency, and synthetic cost. This work highlights the advantages of in situ ligand substitution for MOF modification, by which uniform distribution and high content of the incoming ligand are accessible in the resulting MOFs. More importantly, it provides a promising approach to convert unstable MOFs, which mainly constitute the vast MOF database but have always been neglected, into robust functional materials.

Acute Toxicity of Cu-MOF Nanoparticles (nanoHKUST-1) towards Embryos and Adult Zebrafish

Metal-organic frameworks (MOFs) demonstrate unique properties, which are prospective for drug delivery, catalysis, and gas separation, but their biomedical applications might be limited due to their obscure interactions with the environment and humans. It is important to understand their toxic effect on nature before their wide practical application. In this study, HKUST-1 nanoparticles (Cu-nanoMOF, Cu₃(btc)₂, btc = benzene-1,3,5-tricarboxylate) were synthesized by the microwave (MW)-assisted ionothermal method and characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) techniques. The embryotoxicity and acute toxicity of HKUST-1 towards embryos and adult zebrafish were investigated. To gain a better understanding of the effects of Cu-MOF particles towards Danio rerio (D. rerio) embryos were exposed to HKUST-1 nanoparticles (NPs) and Cu²⁺ ions (CuSO₄). Cu²⁺ ions showed a higher toxic effect towards fish compared with Cu-MOF NPs for D. rerio. Both forms of fish were sensitive to the presence of HKUST-1 NPs. Estimated LC₅₀ values were 2.132 mg/L and 1.500 mg/L for zebrafish embryos and adults, respectively. During 96 h of exposure, the release of copper ions in a stock solution and accumulation of copper after 96 h were measured in the internal organs of adult fishes. Uptake examination of the major internal organs did not show any concentration dependency. An increase in the number of copper ions in the test medium was found on the first day of exposure. Toxicity was largely restricted to copper release from HKUST-1 nanomaterials structure into solution.

Efficient chemosensors for toxic pollutants based on photoluminescent Zn(ii) and Cd(ii) metal-organic networks

Optical sensors with high sensitivity and selectivity, as important analytical tools for chemical and environmental research, can be realized by straightforward synthesis of luminescent one-, two- and three-dimensional Zn(ii) and Cd(ii) crystalline coordination arrays (CPs and MOFs). In these materials with emission centers typically based on charge transfer and intraligand emissions, the quantitative detection of specific analytes, as pesticides or anions, is probed by monitoring real-time changes in their photoluminescence and color emission properties. Pesticides/herbicides have extensive uses in agriculture and household applications. Also, a large amount of metal salts of cyanide is widely used in several industrial processes such as mining and plastic manufacturing. Acute or chronic exposure to these compounds can produce high levels of toxicity in humans, animals and plants. Due to environmental concerns associated with the accumulation of these noxious species in food products and water supplies, there is an urgent and growing need to develop direct, fast, accurate and low-cost sensing methodologies. In this critical frontier, we discuss the effective strategies, chemical stability, luminescence properties, sensitivity and selectivity of recently developed hybrid Zn(ii)/Cd(ii)-organic materials with analytical applications in the direct sensing of pesticides, herbicides and cyanide ions in the aqueous phase and organic solvents.

Cooperative Interplay of Brønsted Acid and Lewis Acid Sites in MIL-101(Cr) for Cross-Dehydrogenative Coupling of C-H Bonds

Cross-dehydrogenative coupling (CDC) is an effective tool for carbon-carbon bond formation in chemical synthesis. Herein, we report a metal-organic framework (MOF) possessing dual Lewis acidic Cr sites and sulfonic acid sites (MIL-101(Cr)-SO₃H) as an efficient catalytic material for direct cross-coupling of xanthene and different nucleophiles using O₂ as the oxidant. The highly porous structure of MIL-101(Cr)-SO₃H enables the free access of reactants to the catalytic active sites inside MOF pores. Kinetic studies indicated that the Cr sites of MOF accelerate the rate-limiting autoxidation reaction of xanthene, which synergistically work with the sulfonic acid group on MOF ligands in promoting the CDC reactions. Besides, the catalytic system shows excellent functional group compatibility, and a variety of valuable xanthene derivatives were synthesized with satisfactory yields. Furthermore, MIL-101(Cr)-SO₃H can be reused and its catalytic activity and crystal structure remain after six consecutive runs.

Click Modification of a Metal-Organic Framework for Two-Photon Photodynamic Therapy with Near-Infrared Excitation

The exploitation of effective strategies to develop materials bearing deep tissue focal fluorescence imaging capacity and excellent reactive oxygen species (ROS) generation ability is of great interest to address the high-priority demand of photodynamic therapy (PDT). Therefore, we use a rational strategy to fabricate a two-photon-active metal-organic framework via a click reaction (PCN-58-Ps). Moreover, PCN-58-Ps is capped with hyaluronic acid through coordination to obtain cancer cell-specific targeting properties. As a result, the optimized composite PCN-58-Ps-HA exhibits considerable two-photon activity (upon laser excitation at a wavelength of 910 nm) and excellent light-triggered ROS (¹O₂ and O₂^•-) generation ability. In summary, the interplay of these two critical factors within the PCN-58-Ps-HA framework gives rise to near-infrared light-activated two-photon PDT for deep tissue cancer imaging and treatment, which has great potential for future clinical applications.

Metal-Organic-Framework-Supported Molecular Electrocatalysis for the Oxygen Reduction Reaction

Synthesizing molecule@support hybrids is appealing to improve molecular electrocatalysis. We report herein metal-organic framework (MOF)-supported Co porphyrins for the oxygen reduction reaction (ORR) with improved activity and selectivity. Co porphyrins can be grafted on MOF surfaces through ligand exchange. A variety of porphyrin@MOF hybrids were made using this method. Grafted Co porphyrins showed boosted ORR activity with large (>70 mV) anodic shift of the half-wave potential compared to ungrafted porphyrins. By using active MOFs for peroxide reduction, the number of electrons transferred per O₂ increased from 2.65 to 3.70, showing significantly improved selectivity for the 4e ORR. It is demonstrated that H₂ O₂ generated from O₂ reduction at Co porphyrins is further reduced at MOF surfaces, leading to improved 4e ORR. As a practical demonstration, these hybrids were used as air electrode catalysts in Zn-air batteries, which exhibited equal performance to that with Pt-based materials.

A historical perspective on porphyrin-based metal-organic frameworks and their applications

Porphyrins are important molecules widely found in nature in the form of enzyme active sites and visible light absorption units. Recent interest in using these functional molecules as building blocks for the construction of metal-organic frameworks (MOFs) have rapidly increased due to the ease in which the locations of, and the distances between, the porphyrin units can be controlled in these porous crystalline materials. Porphyrin-based MOFs with atomically precise structures provide an ideal platform for the investigation of their structure-function relationships in the solid state without compromising accessibility to the inherent properties of the porphyrin building blocks. This review will provide a historical overview of the development and applications of porphyrin-based MOFs from early studies focused on design and structures, to recent efforts on their utilization in biomimetic catalysis, photocatalysis, electrocatalysis, sensing, and biomedical applications.

Bilayer pH-sensitive colorimetric films with light-blocking ability and electrochemical writing property: Application in monitoring crucian spoilage in smart packaging

Bilayer colorimetric films were developed for monitoring fish spoilage by using gelatin (GN) incorporated with ZnO nanoparticles as the upper layer (GN-ZnO), and gellan gum (GG) incorporated with mulberry anthocyanins (MBA) as the lower layer (GG-MBA). The color stability of the bilayer colorimetric films under visible and ultraviolet light was improved with the increase of ZnO nanoparticles content. Meanwhile, the bilayer films had good NH3 sensitivity. The limit of detection of the GG-MBA/GN-2.0% ZnO film to NH3 was 0.01 mM. The electrochemical writing ability of the bilayer films was also identified, indicating the feasibility of inks-free printing on biopolymer films. Finally, the GG-MBA/GN-2.0% ZnO film with an electrochemical writing pattern was used to monitor crucian spoilage. The GG-MBA/GN-2.0% ZnO film with electrochemical writing pattern showed visible color changes with the crucian spoilage. In conclusion, the bilayer colorimetric film was expected to be a good fish spoilage indicator in smart packaging.

Luminescence response mode and chemical sensing mechanism for lanthanide-functionalized metal–organic framework hybrids

This comprehensive review systematically summarizes the luminescence response mode and chemical sensing mechanism for lanthanide-functionalized MOF hybrids (abbreviated as LnFMOFH).

A hierarchically multifunctional integrated catalyst with intimate and synergistic active sites for one-pot tandem catalysis

As a typical process-intensive strategy, a tandem reaction driven by a multifunctional catalyst is a paragon of the green catalytic process.

Novel H-Bonded Synthons in Copper Supramolecular Frameworks with Aminoethylpiperazine-Based Ligands. Synthesis, Structure and Catalytic Activity

New Schiff base complexes [Cu2(HL1)(L1)(N3)3]∙2H2O (1) and [Cu2L2(N3)2]∙H2O (2) were synthesized. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. The HL1 ligand results from the condensation of salicylaldehyde and 1-(2-aminoethyl)piperazine, while a new organic ligand, H2L2, was formed by the dimerization of HL1 via a coupling of two piperazine rings of HL1 on a carbon atom coming from DMF solvent. The dinuclear building units in 1 and 2 are linked into complex supramolecular networks through hydrogen and coordination bondings, resulting in 2D and 1D architectures, respectively. Single-point and broken-symmetry DFT calculations disclosed negligible singlet-triplet splittings within the dinuclear copper fragments in 1 and 2. Catalytic studies showed a remarkable activity of 1 and 2 towards cyclohexane oxidation with H2O2 in the presence of nitric acid and pyridine as promoters and under mild conditions (yield of products up to 21%). Coordination compound 1 also acts as an active catalyst in the intermolecular coupling of cyclohexane with benzamide using di-tert-butyl peroxide (tBuOOtBu) as a terminal oxidant. Conversion of benzamide at 55% was observed after 24 h reaction time. By-product patterns and plausible reaction mechanisms are discussed.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Luminescent lanthanide metal-organic framework nanoprobes: from fundamentals to bioapplications

Metal-organic frameworks (MOFs), a unique type of porous material characterized by high porosity, large internal surface area and remarkable structural tunability, have emerged as very attractive functional materials for a variety of applications. As a promising subclass of MOFs, lanthanide metal-organic frameworks (Ln-MOFs) integrate the unique advantages of MOFs and the intrinsic features of lanthanide ions, such as sharp emission bands, long luminescent lifetimes, large Stokes shifts, high color purity and high resistance to photobleaching. In this minireview, we provide a brief overview of the most recent advances in luminescent Ln-MOF nanoprobes, which covers from their chemical and physical fundamentals to bioapplications, including their synthetic strategies, optical properties and promising bioapplications in biodetection, bioimaging and therapy. Finally, some of the most important emerging trends and future efforts toward this rapidly evolving field are also envisioned.

Ionic liquid induced highly dense assembly of porphyrin in MOF nanosheets for photodynamic therapy

A facile fabrication of porphyrin-integrated MOF nanosheets as efficient photosensitizers for photodynamic therapy (PDT) is presented.