Functional Metal-organic Frameworks Research Articles

Cobalt Formate, a Functional MOF: Electrocatalytic Water Oxidation

Cobalt formate dihydrate, Co(HCOO)[Formula: see text]H2O (Co-MOF), a known crystalline substance, is a three-dimensional coordination polymer. The compound crystallizes in a monoclinic space group [Formula: see text]. In this work, we are reporting a unique synthesis method to prepare this MOF without using any formate salt or formic acid. The source of formate in compound Co-MOF is dimethylformamide (DMF) solvent used in the relevant synthesis. This metal–organic framework (MOF) containing compound, which can be described as a functional MOF, has recently been exploited for hydrogen isotope separation. This functional material has never been used as an electrocatalyst. In this work, we have demonstrated that this water insoluble compound Co-MOF can be used as heterogeneous electrocatalyst for oxygen evolution reaction (OER) by water oxidation at neutral pH. An overpotential of 275.5[Formula: see text]mV versus RHE was required to attain a current density of 1[Formula: see text]mAcm[Formula: see text]. The relevant Faradic efficiency (89.93%) and the turn over frequency (TOF) of 0.277[Formula: see text]s[Formula: see text] make the material an efficient electrocatalyst for water oxidation to molecular oxygen at neutral pH.

Pd Clusters on Schiff Base–Imidazole-Functionalized MOFs for Highly Efficient Catalytic Suzuki Coupling Reactions

Subnanometer noble metal clusters have attracted much attention because of abundant low-coordinated metal atoms that perform excellent catalytic activity in various catalytic processes. However, the surface free energy of metals increases significantly with decreasing size of the metal clusters, which accelerates the aggregation of small clusters. In this work, new Schiff base–imidazole-functionalized MOFs were successfully synthesized via the postsynthetic modification method. Highly dispersed Pd clusters with an average size of 1.5 nm were constructed on this functional MOFs and behaved excellent catalytic activity in the Suzuki coupling of phenyboronic acid and bromobenzene (yield of biaryl >99%) under mild reaction conditions. Moreover, the catalyst can be reused six times without loss of activity. Such catalytic behavior is found to closely related to the surface functional groups that promote the formation of small Pd0 clusters in the metallic state.

Efficient oral insulin delivery enabled by transferrin-coated acid-resistant metal-organic framework nanoparticles.

Oral protein delivery is considered a cutting-edge technology to improve patients’ quality of life, offering superior patient compliance and convenience compared with injections. However, oral protein formulation has stagnated because of the instability and inefficient penetration of protein in the gastrointestinal tract. Here, we used acid-resistant metal-organic framework nanoparticles (UiO-68-NH2) to encapsulate sufficient insulin and decorated the exterior with targeting proteins (transferrin) to realize highly efficient oral insulin delivery. The UiO-68-NH2 nanocarrier with proper pore size achieved high insulin loading while protecting insulin from acid and enzymatic degradation. Through receptor-mediated transcellular pathway, the transferrin-coated nanoparticles realized efficient transport across the intestinal epithelium and controlled insulin release under physiological conditions, leading to a notable hypoglycemic effect and a high oral bioavailability of 29.6%. Our work demonstrates that functional metal-organic framework nanoparticles can protect proteins from the gastric environment and overcome the intestinal barrier, thus providing the possibility for oral biomacromolecule delivery.

Recent progress of functional metal–organic framework materials for water treatment using sulfate radicals

Human health is being threatened by the ever-increasing water pollution. Sulfate radical (SO4•−)-based advanced oxidation processes (SR-AOPs) are rapidly being developed and gaining considerable attention due to their high oxidation potential and selectivity as a way to purify water by degrading organic contaminants in it. Among the catalytic materials that can activate the precursor to generate SO4•−, metal–organic frameworks (MOFs) are the most promising heterogeneous catalytic material in SR-AOPs because of their various structure possibilities, large surface area, ordered porous structure, and regular activation sites. Herein, an in-depth overview of MOFs and their derivatives for water purification with SR-AOPs is provided. The latest studies on pristine MOFs, MOF composites, and MOF derivatives (metal oxides, metal–carbon hybrids, and carbon materials) are summarized. The mechanisms of decomposition of pollutants in water via radical and non-radical pathways are also discussed. This review suggests future research directions for water purification through MOF-based SR-AOP.

A fluorescence nanoplatform for the determination of hydrogen peroxide and adenosine triphosphate via tuning of the peroxidase-like activity of CuO nanoparticle decorated UiO-66.

A novel nanocomposite of CuO nanoparticle-modified Zr-MOF (CuO/UiO-66) was synthesized and developed as a fluorescence nanoplatform for H2O2 and adenosine triphosphate (ATP) via the "turn-on-off" mode in the presence of terephthalic acid (TA). The structure of CuO/UiO-66 was thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and other techniques. The CuO/UiO-66 with enhanced peroxidase-like (POD) activity obtained due to the Zr4+ in UiO-66 is beneficial to the aggregation of CuO NPs on its surface. As a result, the strengthened fluorescence at 425nm with the excitation of 300nm wasfound due to the highly fluorescent species of TAOH. This isproduced bythe oxidation of TA by ·OH that came from the catalysis of H2O2 via the peroxidase mimic of CuO/UiO-66. Hence the modification of CuO NPs on porous UiO-66 can provide a friendly and sensitive physiological condition for H2O2 detection. However, upon addition of ATP, the fluorescence intensity of TAOH at 425nm effectively declined owing to the formation of complexation of Zr4+-ATP and the interaction of CuO to ATP which hampers the catalytic reaction of CuO/UiO-66 to H2O2. Thespecific interaction induced "inhibition of the peroxide-like activity" endows the sensitive and selective recognition of ATP. The detection limits were 16.87 ± 0.2nM and 0.82 ± 0.1nM, and linear analytical ranges were 0.02-100μM and 0.002-30μM for H2O2 and ATP, respectively. The novel strategy was successfully applied toH2O2 and ATP determination in serum samples with recoveries of 97.2-103.8% for H2O2 and 97.6-101.7% for ATP, enriching the avenue to design functional MOFs and providing new avenue of multicomponent bioanalysis.

Rational Functionalization of UiO-66 with Pd Nanoparticles: Synthesis and In Situ Fourier-Transform Infrared Monitoring.

Functionalization of metal-organic frameworks (MOFs) with noble metal nanoparticles (NPs) is a challenging task. Conventional impregnation by metals often leads to agglomerates on the surface of MOF crystals. Functional groups on linkers interact with metal precursors and promote the homogeneous distribution of NPs in the pores of MOFs, but their uncontrolled localization can block channels and thus hinder mass transport. To overcome this problem, we created nucleation centers only in the defective pores of the UiO-66 MOF via the postsynthesis exchange. First, we have introduced defects into UiO-66 using benzoic acid as a modulator. Second, the modulator was exchanged for amino-benzoic acid. As a result, amino groups have decorated mainly the defective pores and attracted the Pd precursor after impregnation. The interaction of the metal precursor with amino groups and the growth of NPs were monitored by in situ infrared spectroscopy. Three processes were distinguished: the gaseous HCl release, NH2 reactivation, and growth of extended Pd surfaces. Uniform Pd NPs were located in the pores because of the homogeneous distribution of the precursor and pore diffusion-limited nucleation rate. Our work demonstrates an alternative approach of controlled Pd incorporation into UiO-66 that is of great importance for the rational design of heterogeneous catalysts.

Zwitterionic Luminescent 2D Metal-Organic Framework Nanosheets (LMONs): Selective Turn-On Fluorescence Sensing of Dihydrogen Phosphate.

While a plethora of organic linkers based on carboxylic acids have been utilized in the construction of MOFs, zwitterionic linkers that typify the attributes of naturally occurring amino acids have been exploited only scarcely to the best of our knowledge. Zwitterionic interior characteristics should be expected to impart unique properties to the resultant MOFs with a high potential to interact with guest species through electrostatic interactions. In our investigations with bis(p-carboxyphenyl)imidazolylarenes as a novel class of linkers for the development of functional MOFs, we have found that bisimidazole-tetracarboxylic acid H4DMBI undergoes metal-assisted self-assembly with Zn(NO3)2 to yield a layered MOF (Zn-DMBI). In the latter, the linker serves as a two-connecting linker with imidazoles and carboxylic acids behaving as zwitterions. The layers are offset stacked in the crystal structure and are bound firmly by hydrogen bonds between imidazolium and carboxylate ions. Such a packing precludes fluorescence from being observed due to self-quenching. However, exfoliation into zwitterionic 2D metal-organic nanosheets (MONs) by sonication in methanol for 1 h liberates palpable fluorescence. Furthermore, the suspension of luminescent MONs (LMONs) in methanol permits selective sensing of anions; in particular, dihydrogen phosphate (H2PO4-) that is complementary to the zwitterions in terms of hydrogen bond donor and acceptor sites is observed with fluorescence enhancement by 120%, leading to its detection at a sub-parts-per-million (0.13 ppm) level. Thus, access to zwitterionic 2D MONs and their application for selective anion sensing with "turn-on" fluorescence are demonstrated by a rational de novo bottom-up approach.

High-κ Polyimide-Based Dielectrics by Introducing a Functionalized Metal-Organic Framework.

In this work, novel metal-organic framework/polyimide (MOF/PI) composite films possessing dielectric properties were synthesized via a solution blending method. UiO-66 and UiO-66-NH2 nanoparticles were first prepared by a hydrothermal method and added into PI to obtain the composite films. Compared with pure PI, the dielectric properties of the MOF/PI composites were substantially enhanced. The amine functionalization gave UiO-66-NH2/PI composite films better dielectric properties in comparison with UiO-66/PI composite films because of improved interaction between PI and UiO-66-NH2. It showed that the dielectric constant of the PI composite film containing 20 wt% UiO-66-NH2 is 8.8 at 102 Hz, which was approximately 2.5 times that of the pure PI (3.5 at 102 Hz). The dielectric loss of the composite film was less than 0.034. Moreover, the breakdown strength of 20 wt% UiO-66-NH2/PI composite films was found to be 208 kV/mm. We describe this new perspective for the preparation of high-performance polymer-based dielectric materials and their application as electrical materials.

Signal synergistic amplification strategy based on functionalized CeMOFs for highly sensitive electrochemical detection of phenolic isomers

A novel functionalized metal–organic frameworks (MOFs) was synthesized by decorating gold nanoparticles (AuNPs), electrochemical-reduced graphene oxide (ERGO) and carbon nanotubes (CNTs) on CeMOFs. As an ultra-sensitive electrochemical sensing platform, it was first applied to detect phenolic isomers (catechol (CC) and hydroquinone (HQ)) simultaneously through the signal synergistic amplification strategy. The performance of the electrochemical sensor has been significantly enhanced by the signal synergy based on the ordered pore nano-restriction effect of MOFs, the good stability and high conductivity of AuNPs and the ultra-fast electron migration rate produced by ERGO/CNTs with three-dimensional (3D) network structure. The sensor exhibits wide linear ranges of 3–190 μM and 3–200 μM with low detection limits of 0.73 μM and 0.44 μM for CC and HQ, respectively. Impressively, the sensor possesses ultra-stability, good repeatability and superior anti-interferent ability due to the stable porous 3D structure of the MOFs nanocomposites. It has been successfully applied to the detection of phenolic isomers in real samples with the recoveries of 100.1%-104.5% for CC and 93.5%-104.0% for HQ. Therefore, the ERGO/CNTs/AuNPs@CeMOFs nanocomposites proved to be a very promising and highly sensitive sensing platform for phenolic isomers detection.

Multi-functional stimuli-responsive biomimetic flower assembled from CLCE and MOF-based pedals

Simulating the structures and behaviors of living organisms are of great significance to develop novel multi-functional intelligent devices. However, the development of biomimetic devices with complex deformable structures and synergistic properties is still on the way. Herein, we propose a simple and effective approach to create the multi-functional stimuli-responsive biomimetic devices with independently pre-programmable colorful visual patterns, complex geometries and morphable modes. The metal organic framework (MOF)-based composite film acts as a rigidity actuation substrate to support and mechanically guide the spatial configuration of the soft chiral nematic liquid crystal elastomer (CLCE) sheet. We can directly program the structural color of the CLCE sheet by adjusting the thickness distribution without tedious chemical modification. By using this coordination strategy, we fabricate an artificial flower, which exhibits a synergistic effect of both shape transformation and color change like paeonia ‘Coral Sunset’ at different flowering stages, and can even perform different flowering behaviors by bending, twisting and curling petals. The assembled bionic flower is innovatively demonstrated to respond to local stimuli of humidity, heat or ultraviolet irradiation. Therefore, the spatial assembly of CLCE combined with functional MOF materials has a wide range of potential application in multi-functional integrated artificial systems.

Superprotonic Conductivity of a Functionalized Metal-Organic Framework at Ambient Conditions.

Seeking fast proton transport pathways at ambient conditions is desirable but challenging. Here, we report a strategy to synthesize a composite material with a polyoxometalate (POM) and an ionic liquid (IL) confined in stable metal-organic framework (MOF) channels through electrostatic interaction. The obtained SO3H-IL-PMo12@MIL-101 possesses fast proton transfer, and its proton conductivity can reach 1.33 × 10-2 S cm-1 at ambient conditions (30 °C, 70% relative humidity (RH)), which is the highest value among the MOF-based proton conductors operated in an ambient environment. Therefore, it has the potential of becoming a room-temperature proton conductor without a humidifier. Importantly, the composite material is further fabricated into a composite membrane for proton-exchange membrane fuel cells (PEMFCs), which can deliver a power density of 0.93 mW cm-2 at 30 °C and 98% RH. This result can lay a fundamental basis for the application of MOF-based proton conductors in the area of electrochemical energy conversion.

Construction of multifunctional NH2-UiO-66 metal organic framework: sensing and photocatalytic degradation of ketorolac tromethamine and tetracycline in aqueous medium.

Existence of pharmaceutical residues in water has endangered environmental pollution worldwide, which makes it ineludible to develop prospective bifunctional materials which not only possess excellent fluorescence behaviour to monitor pharmaceuticals but also exhibit simultaneous photocatalytic removal efficiency. Strengthened by functionalized metal organic framework (MOF) materials, we present here an amine functionalized zirconium-based MOF NH2-UiO-66 which has been successfully synthesized using solvothermal approach. The as prepared MOF was subjected to numerous structural, morphological and compositional characterizations. Interestingly, featured by the excellent fluorescent intensity of MOF modulated by LMCT effect, NH2-UiO-66 was screened to detect pharmaceutical compounds with KTC and TC in aqueous solution. The prepared functionalized MOF showcased excellent sensing platform with magnificent response range (0‒3µM), lower limit of detection (160nM; KTC and 140nM; TC), excellent selectivity and influential anti-interference capability. More importantly, the practical utility of the proposed sensor was further explored for the determination of pharmaceutical drugs in real water samples with suitable recoveries. Simultaneously, the synthesized MOF also exhibited high photocatalytic efficiency towards the removal of KTC and TC under solar light irradiation. The degradation efficiency for KTC and TC was found to be 68.3% and 71.8% within 60 and 280min of solar light, respectively. Moreover, excellent recyclability was demonstrated by the current synthesized system over five cycles. Overall, this study presents a feasible route for the utilization of functionalized MOFs as potential dual functional materials towards the simultaneous detection and degradation of specific pharmaceuticals from aqueous medium.

Carbon Dioxide Capture Chemistry of Amino Acid Functionalized Metal-Organic Frameworks in Humid Flue Gas.

Metal-organic framework-808 has been functionalized with 11 amino acids (AA) to produce a series of MOF-808-AA structures. The adsorption of CO2 under flue gas conditions revealed that glycine- and dl-lysine-functionalized MOF-808 (MOF-808-Gly and -dl-Lys) have the highest uptake capacities. Enhanced CO2 capture performance in the presence of water was observed and studied by using single-component sorption isotherms, CO2/H2O binary isotherm, and dynamic breakthrough measurements. The key to the favorable performance was uncovered by deciphering the mechanism of CO2 capture in the pores and attributed to the formation of bicarbonate as evidenced by 13C and 15N solid-state nuclear magnetic resonance spectroscopy studies. On the basis of these results, we examined the performance of MOF-808-Gly in simulated coal flue gas conditions and found that it is possible to capture and release CO2 by vacuum swing adsorption. MOF-808-Gly was cycled at least 80 times with full retention of performance. This study significantly advances our understanding of CO2 chemistry in MOFs by revealing how strongly bound amine moieties to the MOF backbone create the chemistry and environment within the pores, leading to the binding and release of CO2 under mild conditions without application of heat.

Amino-functional metal–organic framework as a general applicable adsorbent for simultaneous enrichment of nine neonicotinoids

Four functional metal–organic frameworks (MOFs) were facilely synthesized through one-pot procedure or post-synthetic modification method and used as adsorbents for simultaneous enrichment of nine neonicotinoids (NEOs) for the first time. Of which, amino-functional MIL-101-NH2 showed general high affinity towards nine NEOs and performed the best due to extraordinarily high surface area and specific interaction with analytes. A simple, efficient and robust method was developed by dispersive solid phase extraction with MIL-101-NH2 combining with liquid chromatography mass spectrometry (dSPE-LC-MS). Key factors that affecting enrichment efficiency were investigated and the established method demonstrated high sensitivity with detection limits as low as 0.02–0.1 ng/mL and wide linearity range over 3–4 orders of magnitude. Besides, compared with commercial SPE cartridges, this adsorbent showed superior enrichment efficiency and could be used for at least five times without obvious recovery decrease. The method was also applied for determination of trace NEOs in aqueous environmental samples and NEO contamination was found. This study demonstrated the practicability of facile synthetic MOF in simultaneous enrichment of nine NEOs and offered a simple and general method for trace amount NEOs sample preparation in real samples.

Research progress in photo/electrocatalytic reduction of CO2 by functionalized metal–organic frameworks

Environmental pollution and energy shortage caused by climate change are the most serious non-traditional security problems facing the world today, seriously threatening the survival and development of human society.

Systematic Investigation of Pfos Adsorption from Water by Metal Organic Frameworks, Activated Carbon, Metal Organic Framework@Activated Carbon, and Functionalized Metal Organic Frameworks

Systematic Investigation of Pfos Adsorption from Water by Metal Organic Frameworks, Activated Carbon, Metal Organic Framework@Activated Carbon, and Functionalized Metal Organic Frameworks

An Eu3+-functionalized metal–organic framework (Eu@Zn-MOF) for the highly sensitive detection of rotenone in serum

An Eu3+-functionalized hybrid material (Eu@Zn-MOF) is successfully prepared by the postmodification of Eu3+ ions on its free sulfonic groups. Eu@Zn-MOF can serve as a visual probe to specifically recognize and detect rotenone on the basis of fluorescence quenching effect.

Efficient and selective removal of Pb2+ from aqueous solution by using an O- functionalized metal-organic framework.

Lead (Pb) is one of the most widespread and highly toxic heavy metals in the environment. The design and synthesis of adsorbent materials for the selective and efficient removal of Pb2+ from aqueous solution has received much attention. Herein, the ligand 4,4'-azoxydibenzoic acid with the O- group was elaborately selected to construct a novel Pr-based MOF for Pb2+ removal. The as-prepared MOF adsorbents with high stability exhibited ultra-high selectivity for Pb2+, even in the presence of various highly concentrated competitive ions (with the ratios from 1 : 5 to 1 : 50). Also, a high uptake capacity (560.26 mg g-1) can be achieved for the MOF material, due to the availability of sufficient adsorption sites. The strong electrostatic attraction and coordination interaction between the numerous active O- sites on MOF adsorbents and Pb2+ can account for the good adsorption performance for Pb2+, which was systematically verified by zeta potential, FT-IR and XPS studies.

Advances in antitumor nanomedicine based on functional metal-organic frameworks beyond drug carriers.

Nanoscale metal-organic frameworks (MOFs) have attracted widespread interest due to their unique properties including a tunable porous structure, high drug loading capacity, structural diversity, and outstanding biocompatibility. MOFs have been extensively explored as drug nanocarriers in biotherapeutics. However, by harnessing the functionality of ligands and metal ions or clusters in MOFs, the applications of MOFs can be extended beyond drug delivery vehicles. Based on the intrinsic properties of the components of MOFs (e.g. magnetic moments of metal ions and fluorescence of ligands), different imaging modes can be achieved with varied MOFs. With careful design of the composition of MOFs (e.g. modification of organic linkers), they can respond to tumor microenvironments to realize on-demand treatment. By incorporating porphyrin-based ligands (photosensitizers for photodynamic therapy) or high-Z metal ions (radiosensitizers for radiotherapy) into the scaffold of MOFs, MOFs themselves can act as anticancer therapeutic agents. In this review, we highlight the application of MOFs from the above-mentioned aspects and discuss the prospects and challenges for using MOFs in stimuli-responsive imaging-guided antitumor therapy.

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy.

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)4] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)4] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.