Emerging Multifunctional Metal-Organic Framework Materials

Metal organic framework-derived CoPS/N-doped carbon for efficient electrocatalytic hydrogen evolution

Electrocatalytic hydrogen evolution has attracted a great deal of attention due to the urgent need for clean energy. Herein, we demonstrate the synthesis of ternary pyrite-type cobalt phosphosulphide (CoPS) nanoparticles supported on a nitrogen-doped carbon matrix, CoPS/N-C, through carbonization and subsequent phosphosulfurization of Co-based zeolitic imidazolate frameworks (ZIF-67), as promising hydrogen evolution reaction (HER) electrocatalysts in both acidic and alkaline solutions. The polyhedral structure of ZIF-67 can be well maintained in the as-prepared CoPS/N-C nanocomposites. In particular, CoPS/N-C provides a geometric catalytic current density of -10 mA cm-2 at overpotentials of -80 and -148 mV vs. a reversible hydrogen electrode (RHE) and a Tafel slope of 68 and 78 mV dec-1 in 0.5 M H2SO4 and 1 M KOH, respectively, which is superior to most of the transition metal phosphosulfide materials. This MOF-derived synthesis of a transition metal phosphosulfide supported heteroatom-doped carbon matrix provides a promising opportunity for the development of highly efficient electrocatalysts for renewable energy devices.

Encapsulation of Crabtree's Catalyst in Sulfonated MIL-101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL-101(Cr) metal-organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non-functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well-defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

A facile modular approach to the 2D oriented assembly MOF electrode for non-enzymatic sweat biosensors

The preparation of ordered metal organic frameworks (MOFs) will be a critical process for MOF-based nanoelectrodes in the future. In this work, we develop a novel approach to fabricating a type of MOF electrode based on flexible amino-functionalized graphene paper modified with 2D oriented assembly of Cu3(btc)2 nanocubes via facile interfacial synthesis and an effective dip-coating method. One interesting finding is that 2D arrays of Cu3(btc)2 nanocubes at oil-water interfaces can be transferred on amino-functionalized graphene paper, leading to a densely packed monolayer of Cu3(btc)2 nanocubes with a uniform size loaded on the paper electrode. The electrode demonstrates a variety of excellent sensing performances toward sweat lactate and glucose and has been applied in a non-enzymatic electrochemical biosensing platform for the first time. The modular nature of this approach to assembling MOF nanocrystals will provide new insight into the design of MOF-based electrodes for a wide range of applications in biosensing instruments, wearable electronics, and lab-on-a-chip devices.

A label-free fluorescence assay for hydrogen peroxide and glucose based on the bifunctional MIL-53(Fe) nanozyme

A label-free nanozyme MIL-53(Fe) with the dual-function of catalyzing and emitting fluorescence was utilized for turn-on fluorescence detection of hydrogen peroxide and glucose. The proposed strategy provides a cost-effective, safe and sensitive method for the design and development of multiple enzyme cascade assays for various biomolecules.

Functionalization of Metal-Organic Frameworks for Photoactive Materials

Metal-organic frameworks (MOFs) are intriguing platforms with multiple functionalities. Additional functionalization of MOFs generates novel materials for various applications. Here, three main topics are examined regarding the functionalization of MOFs for use as photoactive materials. The first is chemical approaches for postsynthetic modification of the metal clusters and organic linkers in MOFs; that is, sites on pore surfaces and chemical trapping of photoactive moieties within the pores, which create materials with chemical functionalities for water splitting and CO₂ reduction by light. The second topic focuses on the functionalization of MOFs for photochemical response and the versatile applications of such materials. State-of-the-art research on functionalizing MOFs through photochemical reactions on the pore surface and within the pores as guests is also summarized. The third topic introduces the functionalization of MOFs for photofunctional materials, including photoluminescent tuning and integration, photoluminescent LED devices and barcodes, and photophysical applications for chemical sensing. Finally, conclusions and perspectives on the fields are given.

Design and Development of Fluorescent Sensors with Mixed Aromatic Bicyclic Fused Rings and Pyridyl Groups: Solid Mediated Selective Detection of 2,4,6-Trinitrophenol in Water

For a strategic incorporation of both π-electron-rich moieties and Lewis basic moieties acting as hydrogen bonding recognition sites in the same molecule, two new fluorescent sensors, N,N'-bis(anthracen-9-ylmethyl)-N,N'-bis(pyridin-2-ylmethyl)butane-1,4-diamine (banthbpbn, 1) and N,N'-bis(naphthalen-1-ylmethyl)-N,N'-bis(pyridin-2-ylmethyl)butane-1,4-diamine (bnaphbpbn, 2), have been developed for the selective detection of highly explosive 2,4,6-trinitrophenol (TNP) in water. Each of the two identical ends of these sensors that are linked with a flexible tetra-methylene spacer contains a mixed aromatic bicyclic fused ring (anthracene or naphthalene) and a pyridyl group. These are synthesized via the simple reduced Schiff base chemistry, followed by the nucleophilic substitution reaction under basic conditions in high yields. Both 1 and 2 were characterized by Fourier transform infrared, UV-vis, and NMR (¹H and ¹³C) spectroscopy, and high-resolution mass spectrometry. The bulk phase purity of 1 and 2 and their stability in water were confirmed by powder X-ray diffraction (PXRD). Utilizing the effect of solvents on their emission spectra as determined by fluorescence spectroscopy, spectral responses for 1 and 2 toward various nitro explosives were recorded to determine a detection limit of 0.6 and 1.6 ppm, respectively, for TNP in water via the "turn-off" quenching response. Also, the detailed mechanistic investigation for their mode of action through spectral overlap, lifetime measurements, Stern-Volmer plots, and density functional theory calculations reveals that resonance energy transfer and photoinduced electron transfer processes, and electrostatic interactions are the key aspects for the turn-off response toward TNP by 1 and 2. In addition, the selectivity for TNP has been found to be more in 1 compared to 2. Both exhibit good recyclability and stability after sensing experiments, which is confirmed by PXRD and field-emission scanning electron microscopy.

Ligand modification of UiO-66 with an unusual visible light photocatalytic behavior for RhB degradation

A series of isostructural UiO-66-X (X = H, NH₂, Br, (OH)₂, (SH)₂) catalysts have been successfully synthesized by modifying different functional groups on the ligand. The effects of the ligand modification of UiO-66 were investigated for their photocatalytic activity of Rhodamine B degradation under visible light. Surprisingly, UiO-66-NH₂ and UiO-66-(OH)₂ which have narrow bandgaps and excellent visible light absorption do not show outstanding photocatalytic performances compared to UiO-66 and UiO-66-Br. Electrochemical test results indicated that the conduction band potential of UiO-66-X and the separation efficiency of electrons were quite important in these photocatalytic reactions, other than the electronic effect as reported. Similar photocatalytic degradation behaviors were found for Congo red and methyl orange. Herein, we firstly reported different mechanisms of selective degradation in the case of UiO-66, which subverted the previous understanding of photodegradation behavior.

Beyond Crystal Engineering: Significant Enhancement of C2H2/CO2 Separation by Constructing Composite Material

Different from the established crystal engineering method for enhancing gas-separation performance, we demonstrate herein a distinct approach. In contrast to the pristine MOF (metal-organic framework) material, the C₂H₂/CO₂ separation ability for the resultant Ag NPs (nanoparticle)@Fe₂O₃@MOF composite material, estimated from breakthrough calculations, is greatly enhanced by 2 times, and further magnified up to 3 times under visible light irradiation.

Metal-organic-framework-supported immunostimulatory oligonucleotides for enhanced immune response and imaging

We have demonstrated the ability of iron carboxylate metal-organic frameworks to efficiently deliver unmethylated cytosine-phosphate-guanine oligonucleotides. The nanoconjugates induced a stronger immune response than did free cytosine-phosphateguanine oligonucleotides and showed T₂-magnetic resonance imaging ability both in vitro and in vivo.

Supramolecular scaffolds enabling the controlled assembly of functional molecular units

To assemble functional molecular units into a desired structure while controlling positional and orientational order is a key technology for the development of high-performance organic materials that exhibit electronic, optoelectronic, biological and even dynamic functions. For this purpose, we cannot rely simply on the inherent self-assembly properties of the target functional molecular units, since it is difficult to predict, based solely on the molecular structure, what structure will be achieved upon assembly. To address this issue, it would be useful to employ molecular building blocks with self-assembly structures that can be clearly predicted and defined, to make target molecular units assemble into a desired structure. To date, various motifs of molecular assemblies, polymers, discrete and/or three-dimensional metal-organic complexes, nanoparticles and metal/metal oxide substrates have been developed to create materials with particular structures and dimensionalities. In this perspective, we define such assembly motifs as "supramolecular scaffolds". The structure of supramolecular scaffolds can be classified in terms of dimensionality, and they range in size from nano- to macroscopic scales. Functional molecular units, when attached to supramolecular scaffolds either covalently or non-covalently, can be assembled into specific structures, thus enabling the exploration of new properties, which cannot be achieved with the target molecular units alone. Through the classification and overview of reported examples, we shed new light on supramolecular scaffolds for the rational design of organic and polymeric materials.

A Robust 3D Cage-like Ultramicroporous Network Structure with High Gas-Uptake Capacity

A three-dimensional (3D) cage-like organic network (3D-CON) structure synthesized by the straightforward condensation of building blocks designed with gas adsorption properties is presented. The 3D-CON can be prepared using an easy but powerful route, which is essential for commercial scale-up. The resulting fused aromatic 3D-CON exhibited a high Brunauer-Emmett-Teller (BET) specific surface area of up to 2247 m²  g^-1 . More importantly, the 3D-CON displayed outstanding low pressure hydrogen (H₂ , 2.64 wt %, 1.0 bar and 77 K), methane (CH₄ , 2.4 wt %, 1.0 bar and 273 K), and carbon dioxide (CO₂ , 26.7 wt %, 1.0 bar and 273 K) uptake with a high isosteric heat of adsorption (H₂ , 8.10 kJ mol^-1 ; CH₄ , 18.72 kJ mol^-1 ; CO₂ , 31.87 kJ mol^-1 ). These values are among the best reported for organic networks with high thermal stability (ca. 600 °C).

Copper Metal-Organic Framework Nanoparticles Stabilized with Folic Acid Improve Wound Healing in Diabetes

The successful treatment of chronic nonhealing wounds requires strategies that promote angiogenesis, collagen deposition, and re-epithelialization of the wound. Copper ions have been reported to stimulate angiogenesis; however, several applications of copper salts or oxides to the wound bed are required, leading to variable outcomes and raising toxicity concerns. We hypothesized that copper-based metal-organic framework nanoparticles (Cu-MOF NPs), referred to as HKUST-1, which are rapidly degraded in protein solutions, can be modified to slowly release Cu²⁺, resulting in reduced toxicity and improved wound healing rates. Folic acid was added during HKUST-1 synthesis to generate folic-acid-modified HKUST-1 (F-HKUST-1). The effect of folic acid incorporation on NP stability, size, hydrophobicity, surface area, and copper ion release profile was measured. In addition, cytotoxicity and in vitro cell migration processes due to F-HKUST-1 and HKUST-1 were evaluated. Wound closure rates were assessed using the splinted excisional dermal wound model in diabetic mice. The incorporation of folic acid into HKUST-1 enabled the slow release of copper ions, which reduced cytotoxicity and enhanced cell migration in vitro. In vivo, F-HKUST-1 induced angiogenesis, promoted collagen deposition and re-epithelialization, and increased wound closure rates. These results demonstrate that folic acid incorporation into HKUST-1 NPs is a simple, safe, and promising approach to control Cu²⁺ release, thus enabling the direct application of Cu-MOF NPs to wounds.

Direct evidence of charge separation in a metal-organic framework: efficient and selective photocatalytic oxidative coupling of amines via charge and energy transfer

For the first time, the photoexcited charge separation in a metal–organic framework has been evidenced with clear ESR signals, based on efficient and selective photocatalytic oxidative coupling of amines.

The selective aerobic oxidative coupling of amines under mild conditions is an important laboratory and commercial procedure yet a great challenge. In this work, a porphyrinic metal–organic framework, PCN-222, was employed to catalyze the reaction. Upon visible light irradiation, the semiconductor-like behavior of PCN-222 initiates charge separation, evidently generating oxygen-centered active sites in Zr-oxo clusters indicated by enhanced porphyrin π-cation radical signals. The photogenerated electrons and holes further activate oxygen and amines, respectively, to give the corresponding redox products, both of which have been detected for the first time. The porphyrin motifs generate singlet oxygen based on energy transfer to further promote the reaction. As a result, PCN-222 exhibits excellent photocatalytic activity, selectivity and recyclability, far superior to its organic counterpart, for the reaction under ambient conditions via combined energy and charge transfer.

MOF based fluorescent assay of xanthine oxidase for rapid inhibitor screening with real-time kinetics monitoring

The activity assay of xanthine oxidase (XO) is of great application value in clinical diagnosis because the abnormal level of this enzyme is related to a series of pathological states. In this work, a Zr based metal-organic framework (BTB-MOF) with stable photoluminescence in pure water and buffer solution was synthesized. The examination about the fluorescent responses of this material to xanthine and its oxidation product, uric acid, showed that, although both of them affected the emission of BTB-MOF in quenching form, the efficiencies presented much difference. Taking advantage of this feature, a fluorescent method was developed for the activity assay of XO, that is, BTB-MOF was added to the enzymatic oxidation system as a sensor to transduce the proceeding of the reaction real-timely to the signal of fluorescent intensity change. Our method can work under the interference of normal biologically related species, and precisely reflect XO activity in the range of 0.2-40 U L^-1 (detection limit = 0.004 U L^-1). With consecutive fluorescence intensity scan, this assay could be applied as a high speed screening method of XO inhibitors with the testing time of 1 min. This work shows the wide potential application of MOFs in enzyme analysis.

High Rate Magnesium-Sulfur Battery with Improved Cyclability Based on Metal-Organic Framework Derivative Carbon Host

Mg batteries have the advantages of resource abundance, high volumetric energy density, and dendrite-free plating/stripping of Mg anodes. However the injection of highly polar Mg²⁺ cannot maintain the structural integrity of intercalation-type cathodes even for open framework prototypes. The lack of high-voltage electrolytes and sluggish Mg²⁺ diffusion in lattices or through interfaces also limit the energy density of Mg batteries. Mg-S system based on moderate-voltage conversion electrochemistry appears to be a promising solution to high-energy Mg batteries. However, it still suffers from poor capacity and cycling performances so far. Here, a ZIF-67 derivative carbon framework codoped by N and Co atoms is proposed as effective S host for highly reversible Mg-S batteries even under high rates. The discharge capacity is as high as ≈600 mA h g^-1 at 1 C during the first cycle, and it is still preserved at ≈400 mA h g^-1 after at least 200 cycles. Under a much higher rate of 5 C, a capacity of 300-400 mA h g^-1 is still achievable. Such a superior performance is unprecedented among Mg-S systems and benefits from multiple factors, including heterogeneous doping, Li-salt and Cl^- addition, charge mode, and cut-off capacity, as well as separator decoration, which enable the mitigation of electrode passivation and polysulfide loss.

Single Pt Atoms Confined into a Metal-Organic Framework for Efficient Photocatalysis

It is highly desirable yet remains challenging to improve the dispersion and usage of noble metal cocatalysts, beneficial to charge transfer in photocatalysis. Herein, for the first time, single Pt atoms are successfully confined into a metal-organic framework (MOF), in which electrons transfer from the MOF photosensitizer to the Pt acceptor for hydrogen production by water splitting under visible-light irradiation. Remarkably, the single Pt atoms exhibit a superb activity, giving a turnover frequency of 35 h^-1 , ≈30 times that of Pt nanoparticles stabilized by the same MOF. Ultrafast transient absorption spectroscopy further unveils that the single Pt atoms confined into the MOF provide highly efficient electron transfer channels and density functional theory calculations indicate that the introduction of single Pt atoms into the MOF improves the hydrogen binding energy, thus greatly boosting the photocatalytic H₂ production activity.

Metal-Organic Framework-Based Selective Sensing of Biothiols via Chemidosimetric Approach in Water

Selective detection of biothiols holds prime importance owing to the role of varied concentrations of biothiols in various diseases, thus demanding extensive research for developing materials toward selective sensing. In this study, we targeted postsynthetic modification (PSM) approach for imparting desired functionality to chemically stable UiO-66-NH₂ metal-organic framework, which exhibits highly selective sensing toward biothiols. The appended dinitrobenzenesulfonyl group reacts with biothiols via chemidosimetric approach. As a consequence, the probe exhibits a turn on response, which holds immense importance for facile biological studies.

ZnO Nanorod-Induced Heteroepitaxial Growth of SOD Type Co-Based Zeolitic Imidazolate Framework Membranes for H2 Separation

Up to now, the fabrication of well-intergrown Co-based zeolitic imidazolate framework (ZIF) membranes on porous tubular supports is still a major challenge. We report here a heteroepitaxial growth for preparing well-intergrown Co-based ZIFs (ZIF-67 and ZIF-9) tubular membranes with high performance and excellent thermal stability by employing a thin layer of ZnO nanorods acting as both nucleation centers and anchor sites for the growth of metal-organic framework membranes. The results show that well-intergrown Co-ZIF-67 and Co-ZIF-9 membranes are successfully achieved on the ZnO nanorod-modified porous ceramic tubes. This highly active heteroepitaxial growth may be attributed to the fact that the (Zn,Co) hydroxy double salt intermediate produced in situ from ZnO nanorods acts as heteroseeds and enables the uniform growth of Co-based membranes. The H₂/CO₂ selectivity of the as-prepared Co-ZIF-9 tubular membrane could reach about 23.8 and the H₂/CH₄ selectivity of Co-ZIF-67 tubular membrane is as high as 45.4. Moreover, the membranes demonstrate excellent stability because of the ZnO nanorods as linkers between the membrane and substrate.

Insights into the Use of Metal-Organic Framework As High-Performance Anticorrosion Coatings

Metal-organic frameworks (MOFs) have shown great potential in gas storage and separation, energy storage and conversion, vapor sensing, and catalysis. Nevertheless, rare attention has been paid to their anticorrosion performances. At present, substantial hydrophobic and water stable MOFs (like ZIF-8), which are potentially favorable for their applications in anticorrosion industry, have been successfully designed and prepared. In this study, a facile ligand-assisted conversion strategy was employed to fully convert ZnAl-CO₃ layered double hydroxide (LDH) precursor buffer layers to well intergrown ZIF-8 coatings. DC Polarization tests indicated that prepared ZIF-8 coatings showed the corrosive current 4 orders of magnitude lower than that of bare Al substrates, demonstrating that MOF materials were superb candidates for high-performance anticorrosion coatings.

Photoelectron Transfer at ZnTPyP Self-Assembly/TiO2 Interfaces for Enhanced Two-Photon Photodynamic Therapy

Two-photon (TP) absorption nanomaterials are highly desirable for deep-tissue clinical diagnostics and orthotopic disease treatment. Here, a well-designed core/shell nanostructure was successfully synthesized with a ZnTPyP self-assembly nanocrystal (ZSN) inner core coated by a homogeneous TiO₂ layer outside (ZSN-TO). The ZSN is a good photosemiconductor, showing both one-photon (OP) and TP absorption properties for red fluorescence emission and electron-hole pair generation; TiO₂ with good biocompatibility acts as the electron acceptor, which can transfer photoelectron from ZSN to TiO₂ for highly effective electron-hole separation, favoring the production of long-life superoxide anion (O₂^•-) by electrons and oxygen and strong oxidizing hydroxyl radical (•OH) by holes and surrounding H₂O. Once pretreated with ZSN-TO, the simultaneous OP-405 nm or TP-800 nm laser stimulation and fluorescent imaging of reactive oxygen species (ROS) showed dynamical and continuous generation of ROS in HeLa cells, with cytotoxicity significantly increasing via the type-1-like photodynamic therapy process. The results demonstrated that the combination of organic ZSN with inorganic TiO₂ has great applications as an excellent photosensitizer for deep-tissue fluorescent imaging and noninvasive disease treatment via TP photodynamic therapy.