Hollow Metal-Organic-Framework Micro/Nanostructures and their Derivatives: Emerging Multifunctional Materials

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.

Bimetallic metal-organic frameworks and their derivatives

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

CoSe2 hollow microspheres with superior oxidase-like activity for ultrasensitive colorimetric biosensing

As one of the transition metal dichalcogenide, CoSe₂ has received much attention because of its superior physicochemical properties. In this work, a self-templated approach was proposed for constructing CoSe₂ hollow microspheres by utilizing ZIF-67 hollow sphere as a template. In the followed selenylation process, selenium vapor reacts with cobalt ion in ZIF-67 to form CoSe₂ microspheres. The obtained CoSe₂ microspheres retain the cavity of the ZIF-67 and massive uniformly dispersed CoSe₂ nanoparticles are embedded throughout carbon walls. The hollow interior and porous structure of CoSe₂ microspheres provide an enhanced surface-volume ratio and short charge/mass transfer distance. The CoSe₂ microspheres show a typical oxidase-like property able to promote 3,3',5,5'-tetramethylbenzidine (TMB) oxidation by dissolved oxygen to produce an intensive color reaction. Reactive oxygen species trials demonstrate that ·OH, ¹O₂ and O₂^•- radicals coexist in the TMB-CoSe₂ system. Based on its inhibitive role, a rapid and ultrasensitive determination of glutathione was reached, showing four orders of magnitude linear range from 0.005 to 10 μM and a limit of detection of 4.62 nM (S/N = 3). The assay has been successfully used to glutathione determination in practical samples.

Facile Assembly of Multifunctional Antibacterial Nanoplatform Leveraging Synergistic Sensitization between Silver Nanostructure and Vancomycin

The emergence of antibiotic-resistant bacterial strains renders the conventional antibiotic therapy less efficient. The integration of two distinct bactericides into one compact platform provides a promising strategy to realize a combinational antimicrobial therapy. In this work, an efficient chemo-Ag nanohybrid antibacterial platform was facilely developed based on the integration of vancomycin-carrying polydopamine with silver nanoparticles (PDA@Van-Ag). The as-synthesized antibacterial nanohybrid inherited the intrinsic properties of both bactericides to achieve a synergistic antibacterial performance against both Staphylococcus aureus and Escherichia coli strains by attacking bacteria from two distinct fronts. Through this combinational therapy, the efficiency of antibiotic against S. aureus was significantly improved by reducing drug dosage with less opportunity for imparting drug resistance. In addition, this antibacterial nanohybrid, with innate photothermal properties, could achieve auxiliary hyperthermia-assisted bacterial inactivation in the meantime. Furthermore, the outstanding in vivo bacteria-killing activity and wound-healing acceleration were demonstrated in a S. aureus-infected mouse skin defect model. Taken together, this bactericidal nanohybrid could achieve sustained antibiotic release and wipe out bacteria more effectively in a synergistic way, thus reducing the emergence of antibiotic resistance. This work holds great potential to advance the development of novel antibacterial agents and combinational strategies as a promising supplement of antibiotics in the near future.

Reducing Oxygen Evolution Reaction Overpotential in Cobalt-Based Electrocatalysts via Optimizing the "Microparticles-in-Spider Web" Electrode Configurations

Sluggish kinetics of the multielectron transfer process is still a bottleneck for efficient oxygen evolution reaction (OER) activity, and the reduction of reaction overpotential is crucial to boost reaction kinetics. Herein, a correlation between the OER overpotential and the cobalt-based electrode composition in a "Microparticles-in-Spider Web" (MSW) superstructure electrode is revealed. The overpotential is dramatically decreased first and then slightly increased with the continuous increase ratio of Co/Co₃ O₄ in the cobalt-based composite electrode, corresponding to the dynamic change of electrochemically active surface area and charge-transfer resistance with the electrode composition. As a proof-of-concept, the optimized electrode displays a low overpotential of 260 mV at 10.0 mA cm^-2 in alkaline conditions with a long-time stability. This electrochemical performance is comparable and even superior to the most currently reported Co-based OER electrocatalysts. The remarkable electrocatalytic activity is attributed to the optimization of the electrochemically active sites and electron transfer in the MSW superstructure. Theoretical calculations identify that the metallic Co and Co₃ O₄ surface catalytic sites play a vital role in improving electron transport and reaction Gibbs free energies for reducing overpotential, respectively. A general way of boosting OER kinetics via optimizing the electrode configurations to mitigate reaction overpotential is offered in this study.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence

A novel viologen-based multifunctional Eu-MOF integrating photochromism, photomodulated fluorescence, and electrochromic and electroluminochromic properties was investigated.

Enhanced Chemisorption and Catalytic Effects toward Polysulfides by Modulating Hollow Nanoarchitectures for Long-Life Lithium-Sulfur Batteries

Hollow nanostructures with intricate interior and catalytic effects hold great promise for the construction of advanced lithium-sulfur batteries. Herein, a double-shelled hollow polyhedron with inlaid cobalt nanoparticles encapsulated by nitrogen-doped carbon (Co/NC) nanodots (Co-NC@Co₉ S₈ /NPC) is reported, which is acquired by using imidazolium-based ionic-polymer-encapsulated zeolitic imidazolate framework-67 as a core-shelled precursor. The Co/NC nanodots promote redox kinetics and chemical adsorbability toward polysulfides, while the interconnected double shells serve as a nanoscale electrochemical reaction chamber, which effectively suppresses the polysulfide shuttling and accelerates ion/electron transport. Benefiting from structural engineering and reaction kinetics modulation, the Co-NC@Co₉ S₈ /NPC-S electrode exhibits high cycling stability with a low capacity decay of 0.011% per cycle within 2000 cycles at 2 C. The electrode still shows high rate performance and cyclability over 500 cycles even in the case of high sulfur loading of 4.5 mg cm^-2 and 75 wt% sulfur content. This work provides one type of new hollow nanoarchitecture for the development of advanced Li-S batteries and other energy storage systems.

Synthesis of Well-Defined Internal-Space-Controllable UiO-66 Spherical Nanostructures Used as Advanced Nanoreactor

Controllable well-defined metal-organic frameworks (MOFs) spherical structures are expected to be potential good platforms for the heterogeneous catalysis, guest storage, and drug delivery. However, the synthesis faces a challenge. In this paper, a series of well-defined MOFs spherical structures including core-shell, yolk-shell, and hollow spheres were successfully constructed with colloidal carbon as template. Meanwhile, the shapes of MOFs unit (bricklike or octahedral) and sizes of the formed internal space can be controlled simultaneously by rationally changing the modulator and removing template in a good manner. The as-prepared structures could not only keep intact crystal textures but also be used as advanced nanoreactors for enhancing raw materials conversion in the heterogeneous reaction of synthesizing benzoin ethyl ether. Particularly, by our internal-space-controllable nanoreactors, for the first time, the quantifiable relation between nanoreactors' internal space sizes and raw material conversion was revealed.

Field-Effect Transistor Based on an in Situ Grown Metal-Organic Framework Film as a Liquid-Gated Sensing Device

Ni₃(HITP)₂, a novel and promising two-dimensional metal-organic framework (MOF) material, has been utilized in the areas of catalysis, sensing, and supercapacitors. It is very suitable for preparing field-effect transistor (FET) devices due to its good conductivity, porous structure, as well as easy film formation. Nevertheless, there is a challenge to transfer membrane materials undamaged to the substrates. Here, we reported a simple approach to fabricate the Ni-MOF-based FET with an in situ grown Ni₃(HITP)₂ membrane as the channel material of the FET. With this method, we obtained a large-area, dense, and uniform film composed of thin sheets, and the thickness and density of the MOF film were tunable through changing the reaction time. The as-prepared Ni-MOF-FET had a good mobility of 45.4 cm² V^-1 s^-1 and on/off current ratio of 2.29 × 10³. Moreover, this FET served as a liquid-gated device for the first time with bipolar behavior and good response to the gluconic acid at the range from 10^-6 to 10^-3 g/mL, verifying the potential of the Ni-MOF-FET as biosensors.

General Synthesis of Mixed Semiconducting Metal Oxide Hollow Spheres with Tunable Compositions for Low-Temperature Chemiresistive Sensing

Metal oxide hollow spheres (MOHSs) with multicomponent metal elements exhibit intriguing properties due to the synergistic effects of different components. However, it remains a great challenge to develop a general method to synthesize multicomponent MOHSs due to the different hydrolysis and condensation rates of precursors for different metal oxides. Herein, we demonstrate a general strategy for the controllable synthesis of MOHSs with up to five metal elements by decomposition of metal-phenolic coordination polymers (MPCPs), which are prepared by chelation of tannic acid with various metal ions. After calcination to burn out the organic component and induce heterogeneous contraction of MPCPs, a series of MOHSs with multishell structure, high specific surface area (55-171 m²/g), and crystalline mesoporous framework are synthesized, including binary (Fe-Co, Ni-Zn, and Ni-Co oxides), ternary (Ni-Co-Mn and Ni-Co-Zn oxides), and quinary (Ni-Co-Fe-Cu-Zn oxides) MOHSs. The gas sensing nanodevices based on quinary MOHSs show much higher response (10.91) than those based on single component toward 50 ppm of ethanol at 80 °C with the response/recovery time of 85/160 s. The quinary oxides sensor also displays high selectivity toward ethanol against other interfering gases (e.g., methanol, formadehyde, toluene, methane, and hydrogen) and long-term stability (∼94.0% after 4 weeks), which are extremely favorable for practical applications.

Structural Engineering of Low-Dimensional Metal-Organic Frameworks: Synthesis, Properties, and Applications

Low-dimensional metal-organic frameworks (LD MOFs) have attracted increasing attention in recent years, which successfully combine the unique properties of MOFs, e.g., large surface area, tailorable structure, and uniform cavity, with the distinctive physical and chemical properties of LD nanomaterials, e.g., high aspect ratio, abundant accessible active sites, and flexibility. Significant progress has been made in the morphological and structural regulation of LD MOFs in recent years. It is still of great significance to further explore the synthetic principles and dimensional-dependent properties of LD MOFs. In this review, recent progress in the synthesis of LD MOF-based materials and their applications are summarized, with an emphasis on the distinctive advantages of LD MOFs over their bulk counterparties. First, the unique physical and chemical properties of LD MOF-based materials are briefly introduced. Synthetic strategies of various LD MOFs, including 1D MOFs, 2D MOFs, and LD MOF-based composites, as well as their derivatives, are then summarized. Furthermore, the potential applications of LD MOF-based materials in catalysis, energy storage, gas adsorption and separation, and sensing are introduced. Finally, challenges and opportunities of this fascinating research field are proposed.

Bioengineering of Metal-organic Frameworks for Nanomedicine

Controlled structure, tunable porosity, and readily chemical functionalizability make metal-organic frameworks (MOFs) a powerful biomedical tool. Nanoscale MOF particles have been increasingly studied as drug carriers, bioimaging agents, and therapeutic agents due to their excellent physiochemical properties. In this review, we start with MOF as a nanocarrier for drug delivery, covering therapeutic MOF agents followed by a comprehensive discussion of surface bioengineering of MOF for improved biostability, biocompatibility, and targeted delivery. Finally, we detail the challenges and prospects of the future of MOF research for biomedical applications.

Magnetically Hollow Pt Nanocages with Ultrathin Walls as a Highly Integrated Nanoreactor for Catalytic Transfer Hydrogenation Reaction

Fabricating efficient and stable nanocatalysts for chemoselective hydrogenation of nitroaromatics is highly desirable because the amines hold tremendous promise for the synthesis of nitrogen containing chemicals. Here, a highly reactive and stable porous carbon nitride encapsulated magnetically hollow platinum nanocage is developed with subnanometer thick walls (Fe3O4@snPt@PCN) for this transformation. This well-controlled nanoreactor is prepared via the following procedures: the preparation of core template, the deposition of platinum nanocage with subnanometer thick walls, oxidative etching, and calcination. This highly integrated catalyst demonstrates excellent performance for the catalytic transfer hydrogenation of various nitroaromatics and the reaction can reach >99% conversion and >99% selectivity. With the ultrathin wall structure, the atom utilization of platinum atoms is highly efficient. The X-ray photoelectron spectroscopy results indicate that partial electrons transfer from the iron oxides to Pt nanowalls, and this increases the electron density of snPt nanoparticles, thus promoting the catalytic activity for the transfer hydrogenation of nitroaromatics. For the reduction of 4-nitrophenol, the reaction rate constant K app is 0.23 min-1 and the turnover frequency (TOF) is up to 3062 h-1. Additional reaction results illustrate that this magnetic nanoreactor can be reused more than eight times and it is a promising catalytic nanoplatform in heterogeneous catalysis.

Bi2S3 nanorod-stacked hollow microtubes self-assembled from bismuth-based metal-organic frameworks as advanced negative electrodes for hybrid supercapacitors

Bismuth sulfide (Bi2S3) with a lamellar structure has emerged as a promising negative electrode material for supercapacitors (SCs) due to its high theoretical specific capacity. Meanwhile, the improvement of electrochemical properties strongly depends on the size, shape and morphologies of Bi2S3 nanomaterials. Herein, the hierarchical Bi2S3 nanorod-stacked hollow microtubes are self-assembled through a facile self-sacrificing template strategy from bismuth-based metal-organic framework microprisms. Benefiting from the unique structures with a large specific surface area (54.3 m2 g-1), the as-prepared Bi2S3 exhibits an ultrahigh specific capacity (532 C g-1 at 1 A g-1) as a negative electrode for SCs, outperforming other reported Bi2S3 materials.

Hollow cobalt sulfide for highly efficient uranium adsorption from aqueous solutions

The preparation of hollow Co₃S₄nanostructures by ZIF-67 guarantees high uranium adsorption performance in aqueous solutions.

Interfacial engineering of metal–organic frameworks/graphene oxide composite membrane by polyethyleneimine for efficient H2/CH4 gas separation

Interfacial engineering has demonstrated a significant effect on fabricating highly efficient membranes for gas separation.