Nanoproteases: Alternatives to Natural Protease for Biotechnological Applications

Some nanomaterials with intrinsic protease-like activity have the advantages of good stability, biosafety, low price, large-scale preparation and unique property of nanomaterials, which are promising alternatives for natural proteases in various applications. An especial term, "nanoprotease", has been coined to stress the intrinsic proteolytic property of these nanomaterials. As a new generation of artificial proteases, they have become a burgeoning field, attracting many researchers to design and synthesize high performance nanoproteases. In this review, we summarize recent progress on all types of nanoproteases with regard of their activity, mechanism and application and introduce a new and effective strategy for engineering high-performance nanoproteases. In addition, we discuss the challenges and opportunities of nanoprotease research in the future.

Pectin-Coated Iron-Based Metal-Organic Framework Nanoparticles for Enhanced Foliar Adhesion and Targeted Delivery of Fungicides

Conventional agrochemicals are underutilized due to their large particle sizes, poor foliar retention rates, and difficult translocation in plants, and the development of functional nanodelivery carriers with high adhesion to the plant body surface and efficient uptake and translocation in plants remains challenging. In this study, a nanodelivery system based on a pectin-encapsulated iron-based MOF (TF@Fe-MOF-PT NPs) was constructed to enhance the utilization of thifluzamide (TF) in rice plants by taking advantage of the pectin affinity for plant cell walls. The prepared TF@Fe-MOF-PT NPs exhibited an average particle size of 126.55 nm, a loading capacity of 27.41%, and excellent dual-stimulus responses to reactive oxygen species and pectinase. Foliar washing experiments showed that the TF@Fe-MOF-PT NPs were efficiently adhered to the surfaces of rice leaves and stems. Confocal laser scanning microscopy showed that fluorescently labeled TF@Fe-MOF-PT NPs were bidirectionally delivered through vascular bundles in rice plants. The in vitro bactericidal activity of the TF@Fe-MOF-PT NPs showed better inhibitory activity than that of a TF suspension (TF SC), with an EC50 of 0.021 mg/L. A greenhouse test showed that the TF@Fe-MOF-PT NPs were more effective than TF SC at 7 and 14 d, with control effects of 85.88 and 78.59%, respectively. It also reduced the inhibition of seed stem length and root length by TF SC and promoted seedling growth. These results demonstrated that TF@Fe-MOF-PT NPs can be used as a pesticide nanodelivery system for efficient delivery and intelligent release in plants and applied for sustainable control of pests and diseases.

Leveraging Diffusion Kinetics to Reverse Propane/Propylene Adsorption in Zeolitic Imidazolate Framework-8

The separation challenge posed by propylene/propane mixtures arises from their nearly identical molecular sizes and physicochemical properties. Metal-organic frameworks (MOFs) have demonstrated potential in addressing this challenge through the precision tailoring of pore sizes and surface chemistry. However, introducing modifications at the molecular level remains a considerable hurdle. This work presents an approach to reversibly tune the propylene/propane adsorption preference in zeolitic imidazolate framework-8 (ZIF-8) by manipulating the particle size and gas flow rate. Systematically increasing the ZIF-8 crystals from 9 to 224 μm restricts propane diffusion, thereby reversing its preferential adsorption over propylene. Furthermore, raising the gas flow rate of mixed propylene/propane shifts the rate-determining breakthrough step from thermodynamic equilibrium to kinetics, again reversing the adsorption preference in a particular ZIF-8 sample. We propose "dynamic selectivity (Sd(t))" as a concept that incorporates both thermodynamic and kinetic factors to elucidate these unexpected findings. Moreover, the driving force equation, grounded on the concept of Sd(t), has improved the precision and stability of the computational simulation for fixed-bed adsorption processes. This work underscores the potential of diffusion-based modulation, implemented through manageable external changes, as a viable strategy to optimize separation performance in porous adsorbent materials.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

Electrochemiluminescent bioassay based on Ru@Zr-BTC-MOFs nanoparticles for determination of let-7a miRNA using the hybridization chain reaction

Carboxyl-rich tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) ([Ru(dcbpy)3]2+) and 1,3,5-phenyl tricarboxylic acid (H3BTC) were used as the organic ligand to synthesize the metal-organic frameworks by a simple one-pot hydrothermal method with ZrCl4 as metal ion source. Subsequently, the excellent electrochemiluminescence (ECL) luminophore (denoted as Ru@Zr-BTC-MOFs) was obtained. The Ru@Zr-BTC-MOFs displayed outstanding ECL properties, and a sensitive ECL bioassay based on Ru@Zr-BTC-MOFs was designed for the detection of let-7a microRNA (miRNA) using hybrid chain reaction (HCR). Under the optimal experimental conditions, the proposed bioassay exhibited a good linear relationship in the range from 50.0 fM to 5.00 × 102 pM with a detection limit of 3.71 fM. Besides, the proposed sensor exhibited satisfactory performance in real samples. The recovery was 91 ~ 108%, and the relative standard deviation was less than 5.6%. It might have potential clinical applications for detecting miRNA in biomedical research and clinical diagnosis. The schematic diagram of the preparation of Ru@Zr-BTC-MOFs (a) and ECL sensor for detecting let -7a (b).

Preparation of lysozyme-imprinted mesoporous Zr-based metal-organic frameworks with remarkable specific recognition

The development of high-performance protein-imprinted materials remains challenging due to defects concerning high mass transfer resistance and non-specific binding, which are crucial for protein purification and enrichment. In this paper, lysozyme-imprinted mesoporous Zr-based MOF (mesoUiO-66-NH2@MIPs) with specific and selective recognition of lysozyme (Lyz) were prepared by surface imprinting technology. In particular, the excellent hydrophilicity mesoporous MOFs (mesoUiO-66-NH2) with a pore size of 10 nm was prepared as a carrier for Lyz immobilization by an auxiliary modulation strategy to regulate the microporous structure of UiO-66-NH2 with the propionic acid solution, enabling massive loading of the macromolecular protein Lyz. The mesoUiO-66-NH2@MIPs reached a maximum saturation adsorption of 206.54 mg g-1 on Lyz in 20 min at 25 °C with an imprinting factor of 2.57 and selection factors of 2.02, 2.34, and 2.45 for cytochrome c (Cyt c), bovine serum albumin (BSA) and bovine hemoglobin (BHb), respectively. More importantly, the mesoUiO-66-NH2@MIPs could specifically recognize Lyz from the mixed protein system. The adsorption capacity of Lyz could still reach 78.55% after 5 cycles with good cyclic regeneration performance. This provides a new research option for developing and applying novel porous MOF in biomolecule imprinting technology and the specific separation of biomolecules.

Plastic Avalanches in Metal-Organic Framework Crystals Due to the Dynamic Phase Mixing

The compressive properties of metal-organic framework (MOF) crystals are not only crucial for their densification process but also key in determining their performance in many applications. We herein investigated the mechanical responses of a classic crystalline MOF, HKUST-1, using in situ compression tests. A serrated flow accompanied by the unique strain avalanches was found in individual and contacting crystals before their final flattening or fracture with splitting cracks. The plastic flow with serrations is ascribed to the dynamic phase mixing due to the progressive and irreversible local phase transition in HKUST-1 crystals, as revealed by molecular dynamics and finite element simulations. Such pressure-induced phase coexistence in HKUST-1 crystals also induces a significant loading-history dependence of their Young's modulus. The observation of plastic avalanches in HKUST-1 crystals here not only expands our current understanding of the plasticity of MOF crystals but also unveils a novel mechanism for the avalanches and plastic flow in crystal plasticity.

Dual Functional Ultrasensitive Point-of-Care Clinical Diagnosis Using Metal-Organic Frameworks-Based Immunobeads

Sepsis is an acute systemic infectious syndrome with high fatality. Fast and accurate diagnosis, monitoring, and medication of sepsis are essential. We exploited the fluorescent metal-AIEgen frameworks (MAFs) and demonstrated the dual functions of protein detection and bacteria identification: (i) ultrasensitive point-of-care (POC) detection of sepsis biomarkers (100 times enhanced sensitivity); (ii) rapid POC identification of Gram-negative/positive bacteria (selective aggregation within 20 min). Fluorescent lateral flow immunoassays (LFAs) are convenient and inexpensive for POC tests. MAFs possess a large surface area, excellent photostability, high quantum yield (∼80%), and multiple active sites serving as protein binding domains for ultrasensitive detection of sepsis biomarkers (IL-6/PCT) on LFAs. The limit of detection (LOD) for IL-6/PCT is 0.252/0.333 pg/mL. Rapid appraisal of infectious bacteria is vital to guide the use of medicines. The dual-functional fluorescent MAFs have great potential in POC tests for the clinical diagnosis of bacterial infections.

Manganese-Based Immunostimulatory Metal-Organic Framework Activates the cGAS-STING Pathway for Cancer Metalloimmunotherapy

Metal-organic frameworks (MOFs) show tremendous promise for drug delivery due to their structural and functional versatility. However, MOFs are usually used as biologically inert carriers in most cases. The creation of intrinsically immunostimulatory MOFs remains challenging. In this study, a facile and green synthesis method is proposed for the preparation of a manganese ion (Mn2+)-based immunostimulatory MOF (ISAMn-MOF) for cancer metalloimmunotherapy. ISAMn-MOF significantly facilitates the activation of cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) related genes and signaling pathways in bone-marrow-derived dendritic cells (BMDCs). BMDCs treated with ISAMn-MOF secrete 4-fold higher type I interferon and 2- to 16-fold higher proinflammatory cytokines than those treated with equivalent MnCl2. ISAMn-MOF alone or its combination with immune checkpoint antibodies significantly suppresses tumor growth and metastasis and prolongs mouse survival. Mechanistic studies indicate that ISAMn-MOF treatment facilitates the infiltration of stimulatory immune cells in tumors and lymphoid organs. This study provides insight into the design of bioactive MOFs for improved cancer metalloimmunotherapy.

Stimuli-Responsive Design of Metal-Organic Frameworks for Cancer Theranostics: Current Challenges and Future Perspective

Scientific fraternity revealed the potential of stimuli-responsive nanotherapeutics for cancer treatment that aids in tackling the major restrictions of traditionally reported drug delivery systems. Among stimuli-responsive inorganic nanomaterials, metal-organic frameworks (MOFs) have transpired as unique porous materials displaying resilient structures and diverse applications in cancer theranostics. Mainly, it demonstrates tailorable porosity, versatile chemical configuration, tunable size and shape, and feasible surface functionalization, etc. The present review provides insights into the design of stimuli-responsive multifunctional MOFs for targeted drug delivery and bioimaging for effective cancer therapy. Initially, the concept of cancer, traditional cancer treatment, background of MOFs, and approaches for MOFs synthesis have been discussed. After this, applications of stimuli-responsive multifunctional MOFs-assisted nanostructures that include pH, light, ions, temperature, magnetic, redox, ATP, and others for targeted drug delivery and bioimaging in cancer have been thoroughly discussed. As an outcome, the designed multifunctional MOFs showed an alteration in properties due to the exogenous and endogenous stimuli that are beneficial for drug release and bioimaging. The several reported types of stimuli-responsive surface-modified MOFs revealed good biocompatibility to normal cells, promising drug loading capability, target-specific delivery of anticancer drugs into cancerous cells, etc. Despite substantial progress in this field, certain crucial issues need to be addressed to reap the clinical benefits of multifunctional MOFs. Specifically, the toxicological compatibility and biodegradability of the building blocks of MOFs demand a thorough evaluation. Moreover, the investigation of sustainable and greener synthesis methods is of the utmost importance. Also, the low flexibility, off-target accumulation, and compromised pharmacokinetic profile of stimuli-responsive MOFs have attracted keen attention. In conclusion, the surface-modified nanosized design of inorganic diverse stimuli-sensitive MOFs demonstrated great potential for targeted drug delivery and bioimaging in different kinds of cancers. In the future, the preference for stimuli-triggered MOFs will open a new frontier for cancer theranostic applications.

Dual-ligand two-dimensional terbium-organic frameworks nanosheets for ratiometric fluorescence detection of phosphate

Here, we reported a ratiometric fluorescence strategy for the detection of phosphate (Pi) in artificial wetland water. The strategy was based on dual-ligand two-dimensional terbium-organic frameworks nanosheets (2D Tb-NB MOFs). 2D Tb-NB MOFs were prepared through blending 5-boronoisophthalic acid (5-bop), 2-aminoterephthalic acid (NH₂-BDC) and Tb³⁺ ions at room temperature in the presence of triethylamine (TEA). The dual-ligand strategy realized dual emission originated from ligand NH₂-BDC and Tb³⁺ ions at 424 and 544 nm, respectively. Pi could compete with ligands to coordinate Tb³⁺ due to the strong binding ability between Pi and Tb³⁺, resulting in structural destruction of 2D Tb-NB MOFs, so static quenching and antenna effect between ligands and metal ions were interrupted, and emission at 424 nm was enhanced and emission at 544 nm was weakened. This novel probe had excellent linearity with Pi concentrations from 1 to 50 μmol/L; the detection limit was 0.16 μmol/L. This work revealed that mixed ligands improved sensing efficiency of MOFs by enhancing the sensitivity of the coordination between the analyte and MOFs.

DUCNP@Mn-MOF/FOE as a Highly Selective and Bioavailable Drug Delivery System for Synergistic Combination Cancer Therapy

Heterostructures comprising lanthanide-doped upconversion nanoparticles (DUCNPs) and metal-organic frameworks (MOFs) are emerging as promising nanosystems for integrating medical diagnosis and treatment. Here, the DUCNP@Mn-MOF nanocarrier was developed, which showed good efficiency for loading and delivering a cytotoxic antitumor agent (3-F-10-OH-evodiamine, FOE). The combined advantages of the pH-responsive and peroxidase-like properties of Mn-MOF and the unique optical features of DUCNPs granted the DUCNP@Mn-MOF/FOE system synergistic chemodynamic and chemotherapeutic effects. The DUCNP@Mn-MOF nanocarrier effectively overcame the intrinsic limitations of FOE, such as its unfavorable physicochemical properties and limited in vivo potency. This complexed nanosystem was responsive to the tumor microenvironment and showed excellent tumor targeting capability. Thus, DUCNP@Mn-MOF/FOE exhibited highly selective and bioavailable drug delivery properties and is promising for cancer therapy. In a mouse breast cancer model, DUCNP@Mn-MOF/FOE inhibited tumor growth without significant toxicity. Therefore, the proposed nanosystem represents a promising theragnostic platform for multimodal combination diagnosis and therapy of tumors.

Crystallization Kinetics of a Liquid-Forming 2D Coordination Polymer

We investigated a mechanism of crystal melting and crystallization behavior of a two-dimensional coordination polymer [Ag₂(L1)(CF₃SO₃)₂] (1, L1 = 4,4'-biphenyldicarbonitrile) upon heating-cooling processes. The crystal showed melting at 282 °C, and the following gentle cooling induced the abrupt crystallization at 242 °C confirmed by DSC. A temperature-dependent structural change has been discussed through calorimetric, spectroscopic, and mechanical measurements. They indicated that the coordination-bond networks are partially retained in the melt state, but the melt showed a significantly low viscosity of 9.8 × 10^-2 Pa·s at T_m which is six orders lower than that of ZIF-62 at T_m (435 °C). Rheological studies provided an understanding of the fast relaxation dynamics for the recrystallization process, along with that the high T_m provides enough thermal energy to crossover the activation energy barrier for the nucleation. The isothermal crystallization kinetics through calorimetric measurements with applying the Avrami equation identified the nature of the nuclei and its crystal growth mechanism.

Photothermal Hydrogel Encapsulating Intelligently Bacteria-Capturing Bio-MOF for Infectious Wound Healing

Chronic wounds are characterized by long-term inflammation and persistent infection, which make them difficult to heal. Therefore, an urgent desire is to develop a multifunctional wound dressing that can prevent wound infection and promote wound healing by creating a favorable microenvironment. In this study, a curcumin-based metal-organic framework (QCSMOF-Van), loaded with vancomycin and coated with quaternary ammonium salt chitosan (QCS), was prepared. Multifunctional composite hydrogels were conveniently synthesized by combining methacrylic anhydride modified gelatin and methacrylic anhydride modified oxidized sodium alginate with QCSMOF-Van through radical polymerization and Schiff base reaction. It is important to note that the QCSMOF-Van could capture bacteria through the positive charges on the surface of QCS. In this process, due to the synergistic effect of broad-spectrum antibacterial Zn²⁺ and vancomycin, the metabolism of bacteria was well inhibited, and the efficient capturing and rapid killing of bacteria were achieved. The QCSMOF-Van hydrogels could precisely regulate the balance of M1/M2 phenotypes of macrophages, thereby promoting the regeneration of nerves and blood vessels, which promotes the rapid healing of chronic wounds. This advanced cascade management strategy for tissue regeneration highlights the potential of multifunctional composite hydrogels in chronic wound dressings.

Copper-olsalazine metal-organic frameworks as a nanocatalyst and epigenetic modulator for efficient inhibition of colorectal cancer growth and metastasis

Despite the extensive explorations of nanoscale metal-organic frameworks (nanoMOFs) in drug delivery, the intrinsic bioactivity of nanoMOFs, such as anticancer activity, is severely underestimated owing to the overlooked integration of the hierarchical components including nanosized MOFs and molecular-level organic ligands and metal-organic complexes. Herein, we propose a de novo design of multifunctional bioactive nanoMOFs ranging from molecular to nanoscale level, and demonstrate this proof-of-concept by a copper-olsalazine (Olsa, a clinically approved drug for inflammatory bowel disease, here as a bioactive linker and DNA hypomethylating agent) nanoMOF displaying a multifaceted anticancer mechanism: (1) Cu-Olsa nanoMOF-mediated redox dyshomeostasis for enhanced catalytic tumor therapy, (2) targeting downregulation of cyclooxygenase-2 by the organic complex of Cu²⁺ and Olsa, and (3) Olsa-mediated epigenetic regulation. Cu-Olsa nanoMOF displayed an enzyme-like catalytic activity to generate cancericidal species ·OH and ¹O₂ from rich H₂O₂ in tumors, improved the expression of tumor suppressors TIMP3 and AXIN2 by epigenetic modulation, and fulfilled selective inhibition of colorectal cancer cells over normal cells. The hyaluronic acid-modified nanoMOF further verified the efficient suppression of CT26 colorectal tumor growth and metastasis in murine models. Overall, these results suggest that Olsa-based MOF presents a platform of epigenetic therapy-synergized nanomedicine for efficient cancer treatment and provides a powerful strategy for the design of intrinsically bioactive nanoMOFs. STATEMENT OF SIGNIFICANCE: Metal-organic frameworks (MOFs) with intrinsic bioactivities such as anticancer and antibacterial activity are of great interest. Herein, we reported a bioactive copper-olsalazine (Cu-Olsa) nanoMOF as a nanodrug for colorectal cancer treatment. This nanoMOF per se displayed enzyme-like catalytic activity to generate cancericidal species ·OH and ¹O₂ from rich H₂O₂ in tumors for nanocatalytic tumor therapy. Upon dissociation into small molecular copper-organic complex and olsalazine in cancer cells, COX-2 inhibition and epigenetic modulation were fulfilled for selective inhibition of colorectal cancer growth and metastasis.

Quantum Buckling in Metal-Organic Framework Materials

Metal-organic frameworks are porous materials composed of metal ions or clusters coordinated by organic molecules. As a response to applied uniaxial pressure, molecules with a straight shape in the framework start to buckle. At sufficiently low temperatures, this buckling has a quantum nature described by a superposition of degenerate buckling states. Buckling states of adjacent molecules couple in a transverse field Ising type behavior. Based on the example of the metal organic framework topology MOF-5, we derived the phase diagram under applied strain, showing a normal phase, a parabuckling phase, and a ferrobuckling phase. At zero temperature, quantum phase transitions between the three phases can be induced by strain. This novel type of order opens a new path toward strain induced quantum phases.

Machine Learning Meets with Metal Organic Frameworks for Gas Storage and Separation

The acceleration in design of new metal organic frameworks (MOFs) has led scientists to focus on high-throughput computational screening (HTCS) methods to quickly assess the promises of these fascinating materials in various applications. HTCS studies provide a massive amount of structural property and performance data for MOFs, which need to be further analyzed. Recent implementation of machine learning (ML), which is another growing field in research, to HTCS of MOFs has been very fruitful not only for revealing the hidden structure-performance relationships of materials but also for understanding their performance trends in different applications, specifically for gas storage and separation. In this review, we highlight the current state of the art in ML-assisted computational screening of MOFs for gas storage and separation and address both the opportunities and challenges that are emerging in this new field by emphasizing how merging of ML and MOF simulations can be useful.

Giant Negative Compressibility by Liquid Intrusion into Superhydrophobic Flexible Nanoporous Frameworks

Materials or systems demonstrating negative linear compressibility (NLC), whose size increases (decreases) in at least one of their dimensions upon compression (decompression) are very rare. Materials demonstrating this effect in all their dimensions, negative volumetric compressibility (NVC), are exceptional. Here, by liquid porosimetry and in situ neutron diffraction, we show that one can achieve exceptional NLC and NVC values by nonwetting liquid intrusion in flexible porous media, namely in the ZIF-8 metal-organic framework (MOF). Atomistic simulations show that the volumetric expansion is due to the presence of liquid in the windows connecting the cavities of ZIF-8. This discovery paves the way for designing novel materials with exceptional NLC and NVC at reasonable pressures suitable for a wide range of applications.

Metal-Organic Frameworks Derived Functional Materials for Electrochemical Energy Storage and Conversion: A Mini Review

With many apparent advantages including high surface area, tunable pore sizes and topologies, and diverse periodic organic-inorganic ingredients, metal-organic frameworks (MOFs) have been identified as versatile precursors or sacrificial templates for preparing functional materials as advanced electrodes or high-efficiency catalysts for electrochemical energy storage and conversion (EESC). In this Mini Review, we first briefly summarize the material design strategies to show the rich possibilities of the chemical compositions and physical structures of MOFs derivatives. We next highlight the latest advances focusing on the composition/structure/performance relationship and discuss their practical applications in various EESC systems, such as supercapacitors, rechargeable batteries, fuel cells, water electrolyzers, and carbon dioxide/nitrogen reduction reactions. Finally, we provide some of our own insights into the major challenges and prospective solutions of MOF-derived functional materials for EESC, hoping to shed some light on the future development of this highly exciting field.

Uniformly bimetal-decorated holey carbon nanorods derived from metal-organic framework for efficient hydrogen evolution

The hydrogen evolution reaction (HER) as a fundamental process in electrocatalysis plays a significant role in clean energy technologies. For an energy-efficient HER, it demands an effective, durable, and low-cost catalyst to trigger proton reduction with minimal overpotential and fast kinetics. Here, we successfully fabricate a highly efficient HER catalyst of N-C/Co/Mo₂C holey nanorods with Co/β-Mo₂C nanoparticles uniformly embedded in nitrogen-doped carbon (N-C/Co/Mo₂C) by pyrolyzing the molybdate-coordinated zeolitic imidazolate framework (ZIF-67/MoO₄^2-) holey nanorods, which result from the reaction between CoMoO₄ and MeIM in a methanol/water/triethylamine mixed solution. The uniform distribution of MoO₄^2- in the ZIF-67/MoO₄^2- enables Co/β-Mo₂C nanoparticles to be well-distributed within nitrogen-doped carbon holey nanorods. This synthetic strategy endows the N-C/Co/Mo₂C catalyst with uniformly decorated bimetal, thus attaining excellent HER electrocatalytic activities with a small overpotential of 142.0 mV at 10 mA cm^-2 and superior stability in 1.0 mol L^-1 KOH aqueous solution.

Near-Field Infrared Nanospectroscopy Reveals Guest Confinement in Metal-Organic Framework Single Crystals

Metal-organic frameworks (MOFs) can provide exceptional porosity for molecular guest encapsulation useful for emergent applications in sensing, gas storage, drug delivery, and optoelectronics. Central to the realization of such applications, however, is the successful incorporation of a functional guest confined within the host framework. Here, we demonstrate, for the first time, the feasibility of scattering-type scanning near-field optical microscopy (s-SNOM) and nano-Fourier transform infrared (nanoFTIR) spectroscopy, in concert with density functional theory (DFT) calculations to reveal the vibrational characteristics of the Guest@MOF systems. Probing individual MOF crystals, we pinpoint the local molecular vibrations and, thus, shed new light on the host-guest interactions at the nanoscale. Our strategy not only confirms the successful encapsulation of luminescent guest molecules in the porous host framework in single crystals but also further provides a new methodology for nanoscale-resolved physical and chemical identification of wide-ranging framework materials and designer porous systems for advanced applications.

Molecular Rotors in a Metal-Organic Framework: Muons on a Hyper-Fast Carousel

Using muon-spin spectroscopy, we study the exceptional dynamical properties of rotating molecular struts engineered within a Zn-based metal-organic framework at cryogenic temperatures, where the internal motions of almost any other organic substance are quenched. Muon-spin spectroscopy is particularly suited for this aim, as the experimental evidence suggests several implantation sites for the muons, among which at least one directly onto the rotating moiety. The dynamics of the molecular rotors are characterized by the exceptionally low activation energy EA ∼ 30 cal mol-1. At the same time, we evidence a highly unusual temperature dependence of the dipolar interaction of muons with nuclear magnetic moments on the rotors, suggesting a complex influence of the rotations on the muon implantation and diffusion.

Highly Conductive Polybenzimidazole Membranes at Low Phosphoric Acid Uptake with Excellent Fuel Cell Performances by Constructing Long-Range Continuous Proton Transport Channels Using a Metal-Organic Framework (UIO-66)

Phosphoric acid (PA)-doped polybenzimidazoles generally require high PA doping levels to achieve high conductivity as high-temperature proton exchange membranes. However, high PA doping levels result in a significant decrease in the mechanical properties of and PA leaching from the membranes. Herein, a Zr-based metal-organic framework material (UIO-66) was introduced into poly[2,2'-(p-oxydiphenylene)-5,5'-benzimidazole] (OPBI) membranes. The composite membranes exhibited long-range continuous proton transport channels when the mass ratio of UIO-66 to OPBI was ≥30 wt %. The long-range continuous proton transport channels endowed the composite membranes with high proton conductivity at low PA doping levels. When the doping of UIO-66 in the OPBI membrane reached 40 wt %, the membrane exhibited the highest proton conductivity (0.092 S cm^-1, at 160 °C) at a low PA uptake (73.25%), while the conductivity of the pristine OPBI membrane was 0.050 S cm^-1 with a high PA uptake (217.43%). Additionally, in the oxyhydrogen fuel cell test, 40%UIO-66@OPBI membranes exhibited excellent fuel cell performance with a peak power density of 583 mW cm^-2 at 160 °C, which is 50% higher than that of the pristine OPBI membrane (374 mW cm^-2). A single cell based on 40%UIO-66@OPBI also demonstrated good durability and could remain at about 600 mV after 500 h of operation under a constant load of 200 mA cm^-2.

Heavy metal elimination based on metal organic framework highly loaded on flexible nanofibers

Efficient adsorbents for removal heavy metals are extensively urgent in modern society. Metal-organic frameworks (MOFs) with abundant porosity and tunable structure make it potential to access the advantages of high permeability and adsorbability in water pollutant control. However, MOFs nanoparticles inconvenient to recycle in solution hinder its application in water pollutant treatment. Herein, we report an in-situ growth and large-scalable manufacturing method to fabricate ZIF-8 nanoparticles on electrospun polyacrylonitrile (PAN) nanofibers membrane (ZIF-8/PAN NF) by hot pressing. Consequently, the prepared ZIF-8/PAN NF possesses high loading, uniform dispersion and large-scalable area as well as good flexibility. The fabricated ZIF-8/PAN NF exhibits excellent performance with fast flux (12,000 L/(m²h)) and high filtration efficiency (96.5%) for Cu²⁺ in dynamic adsorption. Additionally, adsorption and electrochemistry are introduced simultaneously. The Cu²⁺ removal rate of ZIF-8/PAN NF reaches 34.1% in 4 min with combination of adsorption and electrochemistry. While it is 29.2% for Cu²⁺ elimination in adsorption. Given the outstanding performance and easy manufacture, this study might bring MOFs powder to eliminate water pollution into practical application.

Anti-Biofouling and Water-Stable Balanced Charged Metal Organic Framework-Based Polyelectrolyte Hydrogels for Extracting Uranium from Seawater

Metal-organic frameworks (MOFs) are diffusely defined as a promising class of porous material for uranium extraction from seawater, but there are still challenges in their stability and anti-biofouling performance. Herein, a water-stable and anti-biofouling ZIF-67/SAP_0.45 composite hydrogel was reported by the sequential processes of electrostatic interactions between the oppositely charged polymer, ionic gelation, and template growth of ZIF-67 crystals. Entanglement of positively charged polyethyleneimine (PEI) and negatively charged sodium alginate (SA) polymer chains provided external porosities, anti-biofouling properties, and mechanical support for the hydrogels and further reduced the possibility of ZIF-67 aggregation. The neutral composite hydrogel possessed the least Nitzschia on the surface after 7 days contact, which endows the adsorbent with a high uranium uptake capacity of 2107.87 ± 41.64 μg g^-1 at 1 mg L^-1 uranium-containing seawater with 8.6 × 10⁵ mL^-1 Nitzschia. Additionally, this adsorbent showed water stability with an uranium uptake capacity of 232.88 ± 8.02 mg g^-1 even after five adsorption-desorption cycles because of the excellent preparation method. Benefitting from the distinctive hierarchical structure and large accessible surface area, the resultant adsorbent achieved a high uranium capacity of 6.99 ± 0.26 mg g^-1 in real seawater. This flexible and scalable approach made the MOF/SAP composite hydrogel a highly desirable uranium adsorbent.

A differential photoelectrochemical method for glucose determination based on alkali-soaked zeolite imidazole framework-67 as both glucose oxidase and peroxidase mimics

A differential photoelectrochemical (PEC) method for glucose determination is reported using a nanocomposite with double mimic enzymes of glucose oxidase (GOx) and peroxidase. The nanocomposite was prepared by soaking zeolite imidazole framework-67 (ZIF-67) in 0.1 M NaOH solution at room temperature for 30 min, abbreviated as CoxOyHz@ZIF-67. The Michaelis-Menten constant of CoxOyHz@ZIF-67 to H2O2 and glucose is 121 μM and 3.95 mM, respectively. Using the photoelectrode of CoxOyHz@ZIF-67/TiO2 nanotubes (NTs), glucose was oxidized firstly by dissolved oxygen to generate H2O2 under the catalysis of CoxOyHz film as the mimics of GOx. The product of H2O2 enhanced the photocurrent of TiO2 NTs under the catalysis of ZIF-67 as the mimics of peroxidase. The molecular sieve effect of ZIF-67 frameworks reduces the interferences from molecules with size larger than the apertures in ZIF-67. Under the excitation of a 150 W xenon lamp with full spectrum, the photocurrent was measured in a two-electrode system without external additional potential. By using the photocurrent difference between two photocells, i.e CoxOyHz@ZIF-67/TiO2 NTs and Pt electrode, ZIF-67/TiO2 NTs and Pt electrode, as the signal, the selectivity for glucose determination is improved further. The differential PEC method was applied to the determination of glucose with a linear range 0.1 μM~1 mM and a detection limit of 0.03 μM. Graphical abstract.

Epitaxial Growth of Highly Transparent Metal-Porphyrin Framework Thin Films for Efficient Bifacial Dye-Sensitized Solar Cells

Bifacial dye-sensitized solar cells (DSSCs) are regarded as promising solar energy conversion devices with high efficiency and less resource consumption. In this work, a highly transparent and efficient counter electrode (CE) is fabricated by introducing highly dispersed single Pt atoms doped into the van der Waals layer-by-layer epitaxially grown Zn-TCPP thin film (Zn-TCPP-Pt). The resulting Zn-TCPP-Pt CE has similar catalytic activity to commercial Pt CE but shows a better light transmission capacity in the range of visible light. The bifacial DSSC with Zn-TCPP-Pt thin film CE achieves high power conversion efficiencies of 5.48 and 4.88% under front-side and rear-side irradiation, respectively. With maximized atomic efficiency, excellent performance was obtained with about 1% Pt content and highly transparent CEs. Therefore, the light energy resource utilization rate of such less Pt and transparence CE is greatly improved in bifacial dye-sensitized solar cells, making it a promising candidate to replace Pt CE.

Metal organic framework assisted in situ complexation for miniaturized solid phase extraction of organic mercury in fish and Dendrobium officinale

Zirconium-based metal-organic frameworks, namely Zr-based MOF, was employed as adsorbent material in the miniaturized solid phase extraction of organic mercury compounds in food prior to capillary electrophoresis-diode array detector analysis. The synthesized adsorbent was characterized by different spectroscopic techniques. Parameters influencing the extraction and complexation of methylmercury chloride, ethylmercury chloride and phenylmercury chloride such as type of eluent solvent, type and amount of adsorbent were investigated. In addition, linear ranges contained 2.00-300.00 ng mL^-1 for MeHg⁺, 5.00-500.00 ng mL^-1 for EtHg⁺ and PhHg⁺, and the established method presented good linearity (R² ≥ 0.998). Under the optimized experimental conditions, the ranges of detection limit and quantitation limit were 0.022-0.067 ng mL^-1 and 0.073-0.220 ng mL^-1, respectively. The relative standard deviations of intra- and inter-day analysis were less than 3.2 and 3.1%, respectively. Trueness of the present method was successfully accomplished by means of the recovery assays (81.4-98.5%) in the blank samples with two concentration levels. The repeatability %RSD of the method was lower than 2.7%. Overall, the developed approach proved to have the latent capability to be utilized in routine analysis of organic mercury compounds in fish and Dendrobium officinale.

Electrostatic Interaction-Induced Formation of Enzyme-on-MOF as Chemo-Biocatalyst for Cascade Reaction with Unexpectedly Acid-Stable Catalytic Performance

Combining biocatalytic and chemocatalytic reactions in a one-pot reaction not only avoids the tedious isolation of intermediates during the reactions but also provides a desirable alternative to extend the range of catalytic reactions. Here, we report a facile strategy to immobilize an enzyme, glucose oxidase (GOx), on PCN-222(Fe) induced by electrostatic interaction in which PCN-222(Fe) serves as both a support and chemocatalyst. The immobilization was confirmed through ζ potential measurement, confocal laser scanning microscopy, Fourier transform infrared spectrometry, and UV-vis spectroscopy. This chemo-biocatalyst was applied to a cascade reaction to catalyze glucose oxidation and ABTS (ABTS = 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (or pyrogallol) oxidation. The catalytic kinetics studies show that these chemo-biocatalytic cascade reactions obey the Michaelis-Menten equation, which indicates that the cascade reactions follow the typical enzymatic dynamic regulation process. Interestingly, GOx/PCN-222(Fe) exhibits an exceptional acid-stable catalytic performance as evidenced by circular dichroism spectroscopy where no significant structure change was observed toward acidic solutions with different pH values. GOx/PCN-222(Fe) also displays desirable recyclability since no significant loss of conversion rates was found after six repeated reactions. This work presents a convenient strategy to construct metal-organic framework based chemo-biocatalysts, which may find potential applications in sensing and nanomachines.

Size-Dependent Mechanical Properties of a Metal-Organic Framework: Increase in Flexibility of ZIF-8 by Crystal Downsizing

Size engineering is an emerging strategy to modulate the mechanical properties of crystalline materials. Herein, micro- and nanodimensional single crystals of the prototypical metal-organic framework (MOF) ZIF-8 are generated using solvothermal and solution methods, respectively. Atomic force microscopy-based nanoindentation technique was used to measure the Young's modulus values of micro- and nanodimensional individual ZIF-8 crystals. We demonstrate that crystal downsizing to nanoscale dimensions results in a 40% reduction in crystal stiffness. The change is attributed to a greater contribution of surface effects to the physical properties of nanocrystalline ZIF-8. The observed change in the mechanical properties may be used to explain reported size-dependent changes in gas adsorption of ZIF-8, thought to be a result of differences in framework flexibility at the nanoscale. Our work provides an important example on how downsizing of crystalline metal-organic materials can give rise to specific and tunable physical properties.

Preparation of polydopamine-modified zeolitic imidazolate framework-8 functionalized electrospun fibers for efficient removal of tetracycline

This work shows a simple and environmental friendly methodology to obtain a kind of polydopamine coating assisted preparation of zeolitic imidazolate framework-8 (ZIF-8) functionalized composite electrospun fiber (ZIF-8/PDA/PAN fibers) adsorbent. Characterization of the composite electrospun fiber was carried out and the tetracycline (TC) adsorption properties from water were also studied in detail. At the same time, principle adsorption mechanisms were thoroughly studied. The results show that the pseudo-second-order model can simulate sorption kinetics well, while sorption isotherms are able to significantly conform to the Freundlich model, and the adsorption capacity of the fibers can reach 478.18 mg/g at 298 K. In addition, the Weber-Morris model indicates that the processes of adsorption of ZIF-8/PDA/PAN fibers for TC involve surface adsorption as well as intraparticle diffusion, and the limit rate step is not only the intraparticle diffusion but also the binding of the sorbate to the sorbent. Moreover, the adsorption efficiency toward TC by ZIF-8/PDA/PAN fibers still reached over 85% of its initial adsorption capacity after five adsorption/desorption cycles, which signified that the adsorbents is stable and recyclable. This work indicates that the obtained ZIF-8/PDA/PAN fibers have practical application prospects in the field of antibiotic adsorption.

Duet of Acetate and Water at the Defects of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are porous crystalline materials with promising applications in molecular adsorption, separation, and catalysis. It has been discovered recently that structural defects introduced unintentionally or by design could have a significant impact on their properties. However, the exact chemical composition and structural evolution under different conditions at the defects are still under debate. In this study, we performed multidimensional solid-state nuclear magnetic resonance (SSNMR) coupled with computer simulations to elucidate an important scenario of MOF defects, uncovering the dynamic interplay between residual acetate and water. Acetate, as a defect modulator, and water, as a byproduct, are prevalent defect-associated species, which are among the key factors determining the reactivity and stability of defects. We discovered that acetate molecules coordinate to a single metal site monodentately and pair with water at the neighboring position. The acetates are highly flexible, which undergo fast libration as well as a slow kinetic exchange with water through dynamic hydrogen bonds. The dynamic processes under variable temperatures and different hydration levels have been quantitatively analyzed across a broad time scale from microseconds to seconds. The integration of SSNMR and computer simulations allows a precision probe into defective MOF structures with intrinsic dynamics and disorder.