Well-Defined Metal-Organic Framework Hollow Nanocages

Engineering Crystallinity Gradients for Tailored CaO2 Nanostructures: Enabling Alkalinity-Reinforced Anticancer Activity with Minimized Ca2+/H2O2 Production

CaO2 nanoparticles (CNPs) can produce toxic Ca2+ and H2O2 under acidic pH, which accounts for their intrinsic anticancer activity but at the same time raises safety concerns upon systemic exposure. Simultaneously realizing minimized Ca2+/H2O2 production and enhanced anticancer activity poses a dilemma. Herein, we introduce a "crystallinity gradient-based selective etching" (CGSE) strategy, which is realized by creating a crystallinity gradient in a CNP formed by self-assembled nanocrystals. The nanocrystals distributed in the outer layer have a higher crystallinity and thus are chemically more robust than those distributed in the inner layer, which can be selectively etched. CGSE not only leads to CNPs with tailored single- and double-shell hollow structures and metal-doped compositions but more surprisingly enables significantly enhanced anticancer activity as well as tumor growth inhibition under limited Ca2+/H2O2 production, which is attributed to an alkalinity-reinforced lysosome-dependent cell death pathway.

Metal Organic Framework Cubosomes

We demonstrate a general strategy for the synthesis of ordered bicontinuous-structured metal organic frameworks (MOFs) by using polymer cubosomes (PCs) with a double primitive structure (Im 3 ‾ ${\bar{3}}$ m symmetry) as the template. The filling of MOF precursors in the open channel of PCs, followed by their coordination and removal of the template, generates MOF cubosomes with a single primitive topology (Pm 3 ‾ ${\bar{3}}$ m) and average mesopore diameters of 60-65 nm. Mechanism study reveals that the formation of ZIF-8 cubosomes undergoes a new MOF growth process, which involves the formation of individual MOF seeds in the template, their growth and eventual fusion into the cubosomes. Their growth kinetics follows the Avrami equation with an Avrami exponent of n=3 and a growth rate of k=1.33×10^-4 , indicating their fast 3D heterogeneous growth mode. Serving as a bioreactor, the ZIF-8 cubosomes show high loading of trypsin enzyme, leading to a high catalytic activity in the proteolysis of bovine serum albumin.

Hollow terbium metal-organic-framework spheres: preparation and their performance in Fe3+ detection

Hollow metal–organic framework (MOF) micro/nanostructures have been attracting a great amount of research interest in recent years. However, the synthesis of hollow metal–organic frameworks (MOFs) is a great challenge. In this paper, by using 1,3,5-benzenetricarboxylic acid (H₃BTC) as the organic ligand and 2,5-thiophenedicarboxylic acid (H₂TDC) as the competitive ligand and protective agent, hollow terbium MOFs (Tb-MOFs) spheres were synthesized by a one-pot solvothermal method. By comparing the morphology of Tb-MOFs in the presence and absence of H₂TDC, it is found that H₂TDC plays a key role in the formation of the hollow spherical structure. Single crystal analyses and element analysis confirm that H₂TDC is not involved in the coordination with Tb³⁺. Interestingly, Tb-MOFs can be used as the luminescent probes for Fe³⁺ recognition in aqueous and N,N-dimethylformamide (DMF) solutions. In aqueous solution, the quenching constant (K_SV) is 5.8 × 10⁻⁴ M⁻¹, and the limit of detection (LOD) is 2.05 μM. In DMF, the K_SV and LOD are 9.5 × 10⁻⁴ M⁻¹ and 0.80 μM, respectively. The sensing mechanism is that the excitation energy absorption of Fe³⁺ ions reduces the energy transfer efficiency from the ligand to Tb³⁺ ions.

(a) Pictures of Tb-MOFs suspension (left) and Fe³⁺ (right) under 365 nm illumination. (b) Pictures of Fe³⁺ with (left) and without (right) Tb-MOFs. (c) Pictures of Tb-MOFs powder before (left) and after (right) Fe³⁺ adsorption.

Architecture and Preparation of Hollow Catalytic Devices

Since pioneering work done in the late 1990s, synthesis of functional hollow materials has experienced a rapid growth over the past two decades while their applications have been proven to be advantageous across many technological fields. In the field of heterogeneous catalysis, the development of micro- and nanoscale hollow materials as catalytic devices has also yielded promising results, because of their higher activity, stability, and selectivity. Herein, the architecture and preparation of these catalysts with tailorable composition and morphology are reviewed. First, synthesis of hollow materials is introduced according to the classification of template mediated, template free, and combined approaches. Second, different architectural designs of hollow catalytic devices, such as those without functionalization, with active components supported onto hollow materials, with active components incorporated within porous shells, and with active components confined within interior cavities, are evaluated respectively. The observed catalytic performances of this new class of catalysts are correlated to structural merits of individual configuration. Examples that demonstrate synthetic approaches and architected configurations are provided. Lastly, possible future directions are proposed to advance this type of hollow catalytic devices on the basis of our personal perspectives.

Hollow Polypyrrole Nanospindles for Highly Effective Cancer Therapy

Polypyrrole (PPy) hollow nanostructures continue to attract the interest researchers because of their good biocompatibility, high photothermal conversion efficiency, and excellent stability. The preparation of PPy hollow nanostructures by the hard templating method without complicated post-synthetic treatment and additional oxidizing agents remains a challenge. In this work, we report a facile and novel hard templating method to fabricate hollow PPy nanospindles in which MIL-88(Fe) serves as the template. Fe³⁺ centers in MIL-88(Fe) could induce the polymerization of pyrrole to construct the shell, and MIL-88(Fe) would be decomposed by solvent water. This method did not require any extra oxidizing agents and post-synthetic treatment. Hollow PPy nanospindles exhibit excellent photothermal and drug loading ability, and the therapy effect of cancer was significant. This method provides a new hard templating approach for the synthesis of polymer hollow nanostructures.

Fabrication of tubular braid reinforced PMIA nanofiber membrane with mussel-inspired Ag nanoparticles and its superior performance for the reduction of 4-nitrophenol

A novel tubular braid reinforced (TBR) PMIA/CA-PEI/Ag nanofiber membrane for application in dynamic catalysis was introduced in this study. The preparation method of the TBR PMIA/CA-PEI/Ag nanofiber membrane was facile and efficient. The TBR PMIA/CA-PEI/Ag nanofiber membrane was characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The mechanical properties were evaluated by a universal material testing machine. The tensile strength of TBR nanofiber membrane exceeded 500 MPa, whereas that of the nanofiber membrane without reinforcement was merely 10 MPa. Besides, the compressive strength of the TBR nanofiber membrane was also reinforced, which indicated that the TBR nanofiber membrane could withstand a higher operating pressure. The reduction of 4-NP to 4-AP was selected as the model reaction to evaluate the catalytic property of TBR PMIA/CA-PEI/Ag nanofiber membrane. The apparent rate constant of dynamic catalysis was 34.58 times higher than that of static catalysis. After 10 cycles, the conversion of 4-NP was still higher than 95.3%. This indicated that the TBR PMIA/CA-PEI/Ag nanofiber membrane had superior stability and recyclability. Besides, the TBR PMIA/CA-PEI/Ag nanofiber membrane also showed superior catalytic performance when it was used for catalyzing other environmental pollutants.

Elution-free ultra-sensitive enrichment for glycopeptides analyses: Using a degradable, post-modified Ce-metal-organic framework

In this work, we presented a facile elution-free method for ultrasensitive enrichment of glycopeptides using two kinds of novel Ce-metal-organic frameworks (Ce-MOF) post-modified with hyaluronic acid (Ce-MOF@HA) and glutamic acid (Ce-MOF@Glu). Both of the synthesized materials remained stable in the loading buffer to enrich glycopeptides selectively and degrade in the eluent to release captured glycopeptides. Due to the dissolution of materials, the elution step of the enrichment process is omitted, resulting in an extremely high sensitivity (detection limit, 0.5 fmol/μL). Meanwhile, Ce-MOF@HA and Ce-MOF@Glu also possessed excellent selectivity with molar ratios of IgG and BSA digests being 1:1000 and 1:500, respectively. Noticeably, the practical applicability of the obtained materials was inspected by analyzing the glycopeptides enriched from human serum (2 μL) by nano-LC-MS, in which 434 N-glycopeptides from 182 N-glycoproteins (by Ce-MOF@HA) and 328 N-glycopeptides from 135 N-glycoproteins (by Ce-MOF@Glu) were detected, respectively. This work provides a new method to simplify the process of glycopeptides enrichment and also paves a novel way for the enrichment of trace targets from complex matrices.

Encapsulation of Dual-Emitting Fluorescent Magnetic Nanoprobe in Metal-Organic Frameworks for Ultrasensitive Ratiometric Detection of Cu2

An effective dual-emission fluorescent metal-organic framework (MOF)-based nanoprobe has been established for ultrasensitive and rapid ratiometric detection of Cu²⁺ . Such a nanoprobe was prepared by encapsulating fluorescein isothiocyanate (FITC), and Eu(III) complex-functionalized Fe₃ O₄ into the zeolitic imidazolate framework material (ZIF-8). In this nanoprobe, FITC was used as a reference signal, thus improving the influence of external uncertainties. The Eu-complex signal could be quenched after adding an amount of Cu²⁺ . The ZIF-8 could enrich the target analytes, which can amplify the fluorescence signal due to the good adsorption properties of the ZIF-8. Based on above structural and compositional features, the detection limit of the nanoprobe is 0.1 nm for Cu²⁺ , almost 2×10⁴ times lower than the maximum allowable amount of Cu²⁺ in drinking water, which constructed a platform for effective detection of Cu²⁺ . Using the nanoprobe to detect Cu²⁺ in aqueous solution is rapid and the probe still remained stable. Additionally, this sensor for the ratiometric fluorescence imaging of copper ions was also certified in real samples and live cells.

Multi-shelled Hollow Metal-Organic Frameworks

Hollow metal-organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.

Regulating the spatial distribution of metal nanoparticles within metal-organic frameworks to enhance catalytic efficiency

Composites incorporating metal nanoparticles (MNPs) within metal-organic frameworks (MOFs) have broad applications in many fields. However, the controlled spatial distribution of the MNPs within MOFs remains a challenge for addressing key issues in catalysis, for example, the efficiency of catalysts due to the limitation of molecular diffusion within MOF channels. Here we report a facile strategy that enables MNPs to be encapsulated into MOFs with controllable spatial localization by using metal oxide both as support to load MNPs and as a sacrificial template to grow MOFs. This strategy is versatile to a variety of MNPs and MOF crystals. By localizing the encapsulated MNPs closer to the surface of MOFs, the resultant MNPs@MOF composites not only exhibit effective selectivity derived from MOF cavities, but also enhanced catalytic activity due to the spatial regulation of MNPs as close as possible to the MOF surface.

Nanocaged platforms: modification, drug delivery and nanotoxicity. Opening synthetic cages to release the tiger

Nanocages (NCs) have emerged as a new class of drug-carriers, with a wide range of possibilities in multi-modality medical treatments and theranostics. Nanocages can overcome such limitations as high toxicity caused by anti-cancer chemotherapy or by the nanocarrier itself, due to their unique characteristics. These properties consist of: (1) a high loading-capacity (spacious interior); (2) a porous structure (analogous to openings between the bars of the cage); (3) enabling smart release (a key to unlock the cage); and (4) a low likelihood of unfavorable immune responses (the outside of the cage is safe). In this review, we cover different classes of NC structures such as virus-like particles (VLPs), protein NCs, DNA NCs, supramolecular nanosystems, hybrid metal-organic NCs, gold NCs, carbon-based NCs and silica NCs. Moreover, NC-assisted drug delivery including modification methods, drug immobilization, active targeting, and stimulus-responsive release mechanisms are discussed, highlighting the advantages, disadvantages and challenges. Finally, translation of NCs into clinical applications, and an up-to-date assessment of the nanotoxicology considerations of NCs are presented.

Competitive coordination strategy for the synthesis of hierarchical-pore metal-organic framework nanostructures

We demonstrate a competitive coordination strategy for the synthesis of H-MOF nanostructures, such as two-dimensional H-MOF nanosheets and H-MOF nanocubes, evolving through an etching process tuned by a competitive ligand.

Metal–organic frameworks (MOFs) usually have micropores smaller than 2 nm, which may restrict their applications in some cases. Hierarchical-pore MOFs (H-MOFs) are a new family of MOF materials, possessing both micro- and mesopores to address this problem. Here we demonstrate a competitive coordination strategy for the synthesis of H-MOF nanostructures, such as two-dimensional (2D) H-MOF nanosheets and H-MOF nanocubes, evolving through an etching process tuned by a competitive ligand. The as-synthesized 2D H-MOF nanosheets can serve as a substrate to in situ immobilize Pd nanoparticles to achieve a surfactant-free Pd catalyst, by means of a simple soaking method of Pd²⁺ precursors. Combined with the unique structure and gas adsorption capacity of H-MOF-5, the Pd-H-MOF-5 catalyst exhibits superior catalytic performance.

Template-Directed Synthesis of Porous and Protective Core-Shell Bionanoparticles

Metal-organic frameworks (MOFs) are promising high surface area coordination polymers with tunable pore structures and functionality; however, a lack of good size and morphological control over the as-prepared MOFs has persisted as an issue in their application. Herein, we show how a robust protein template, tobacco mosaic virus (TMV), can be used to regulate the size and shape of as-fabricated MOF materials. We were able to obtain discrete rod-shaped TMV@MOF core-shell hybrids with good uniformity, and their diameters could be tuned by adjusting the synthetic conditions, which can also significantly impact the stability of the core-shell composite. More interestingly, the virus particle underneath the MOF shell can be chemically modified using a standard bioconjugation reaction, showing mass transportation within the MOF shell.

An alternative synthetic approach for macro–meso–microporous metal–organic frameworks via a “domain growth” mechanism

A nanoscale “domain growth” mechanism was proposed based on experimental facts to describe the formation process of macro–meso–microporous HKUST-1 with 3-dimensional networks.

Facile preparation of a hierarchically porous metal–organic nanocomposite with excellent catalytic performance

A hierarchical porous MOF nanocrystal, hpCuL (L = 2,4,6-tris(3,5-dicarboxylatephenylamino)-1,3,5-triazine) was prepared via a facile gel-aging process. This nanocomposite exhibits high catalytic activity and stability for the reduction of 4-nitrophenol.

Metal-Organic Frameworks Derived Porous Core/Shell Structured ZnO/ZnCo2O4/C Hybrids as Anodes for High-Performance Lithium-Ion Battery

Metal-organic frameworks (MOFs) derived porous core/shell ZnO/ZnCo2O4/C hybrids with ZnO as a core and ZnCo2O4 as a shell are for the first time fabricated by using core/shell ZnCo-MOF precursors as reactant templates. The unique MOFs-derived core/shell structured ZnO/ZnCo2O4/C hybrids are assembled from nanoparticles of ZnO and ZnCo2O4, with homogeneous carbon layers coated on the surface of the ZnCo2O4 shell. When acting as anode materials for lithium-ion batteries (LIBs), the MOFs-derived porous ZnO/ZnCo2O4/C anodes exhibit outstanding cycling stability, high Coulombic efficiency, and remarkable rate capability. The excellent electrochemical performance of the ZnO/ZnCo2O4/C LIB anodes can be attributed to the synergistic effect of the porous structure of the MOFs-derived core/shell ZnO/ZnCo2O4/C and homogeneous carbon layer coating on the surface of the ZnCo2O4 shells. The hierarchically porous core/shell structure offers abundant active sites, enhances the electrode/electrolyte contact area, provides abundant channels for electrolyte penetration, and also alleviates the structure decomposition induced by Li(+) insertion/extraction. The carbon layers effectively improve the conductivity of the hybrids and thus enhance the electron transfer rate, efficiently prevent ZnCo2O4 from aggregation and disintegration, and partially buffer the stress induced by the volume change during cycles. This strategy may shed light on designing new MOF-based hybrid electrodes for energy storage and conversion devices.

Well-Dispersed and Size-Controlled Supported Metal Oxide Nanoparticles Derived from MOF Composites and Further Application in Catalysis

Supported metal oxide nanoparticles are important in heterogeneous catalysis; however, the ability to tailor their size, structure, and dispersion remains a challenge. A strategy to achieve well-dispersed and size-controlled supported metal oxides through the manageable growth of a metal organic framework (Cu-BTC) on TiO2 followed by pyrolysis is described.

Metal-organic framework derived ZnO/ZnFe2O4/C nanocages as stable cathode material for reversible lithium-oxygen batteries

Tremendous efforts have been devoted to exploring various Li-O2 cathode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, most of the high-activity ORR/OER catalysts can also accelerate side-reactions, such as electrolyte degradation on cycling. To address this issue, we change our strategy from pursuing highly active catalysts to developing stable cathodes that are compatible with the electrolyte. In this work, hierarchical mesoporous ZnO/ZnFe2O4/C (ZZFC) nanocages are synthesized from the templates of metal-organic framework (MOF) nanocages. Such ZZFC nanocages have lower ORR/OER catalytic activity as compared with the widely used catalysts for fuel cells, but they do not catalyze the degradation of organic electrolyte during operation. Furthermore, the optimized porosity and conductivity can fit well the needs of the Li-O2 cathode. When employed in a Li-O2 battery, the ZZFC cathode delivers a primary discharge/charge capacity exceeding 11 000 mAh g(-1) at a current density of 300 mA g(-1) and an improved cyclability with capacity of 5000 mAh g(-1) for 15 cycles. The superior electrochemical performance is ascribed to the hierarchical porosity and little degradation of the organic electrolyte.

Universal strategy for homogeneously doping noble metals into cyano-bridged coordination polymers

Coordination polymers with large surface areas and uniform but tunable cavities have attracted extensive attention because of their unique properties and potential applications in numerous fields. The introduction of noble metal into coordination polymers, which may enhance or display new behaviors beyond their parent counterparts, presents great challenges in maintaining the fragile coordination structures and meeting the compatibility. Here, cyano-bridged coordination polymers are robust and show very nice compatibilities with a series of noble metals, such as Pd, Pt, Au, Ag. Those noble elements partially take the place of the transition metal ions under room temperature (for Au and Ag) or a mild hydrothermal environment (for Pd and Pt) without damaging the framework. By using this universal simple synthetic procedure, we prepared a series of noble metal containing metal hexacyanoferrate (MHCF) with various morphologies and structures, including Pd/Pt/Ag/Au-MnHCF, Pd/Pt/Ag/Au-CoHCF, and Pd/Pt/Ag/Au-NiHCF. Among them, Pd-MnHCF demonstrates the control of morphologies by adjusting operational details, and notably, it shows very unique, enhanced catalytic performance, reflecting the superiority of cyano-connected positive-valent Pd as a single-atom catalyst.

Light-mediated cascade transformation of activated alkenes: BiOBr nanosheets as efficient photocatalysts for the synthesis of α-aryl-β-trifluoromethyl amides

A facile light induced, BiOBr nanosheet promoted one-pot consecutive trifluoromethylation/aryl migration/desulfonylation and N–H bond formation of activated alkenes is proposed.