Bioinspired Molecular Scalpel for Two-Dimensional Metal-Organic Nanosheet: Design Strategies and Recent Progress.

Ultrathin two-dimensional (2D) metal-organic nanosheets (MONs) have attracted continued attention in the field of advanced functional materials. Their nanoscale thickness, high surface-to-volume ratio, and abundant accessible active sites, are superior advantages compared with their 3D bulk counterparts. Bioinspired molecular scalpel strategy is a promising method for the creation of 2D MONs, and may solve the current shortcomings of MONs synthesis. This review aims to provide a state-of-the-art overview of molecular scalpel strategies and share the results of current development to provide a better solution for MONs synthesis. Different types of molecular scalpel strategies have been systematically summarized. Both mechanisms, advantages and limitations of multiform molecular scalpel strategies have been discussed. Besides, the challenges to be overcome and the question to be solved are also introduced.

Small-size and well-dispersed Fe nanoparticles embedded in carbon rods for efficient oxygen reduction reaction.

The preparation of ultra-small and well-dispersed metal nanoparticles (NPs) is of great importance for promoting oxygen reduction. Here, a metal (Fe and Zn) NP (7 nm) based catalyst derived from a Zn-based metal-organic framework was obtained by a vapor adsorption strategy, demonstrating a high half-wave potential (0.868 V) and power density (196 mW cm-2).

Customized Synthesis of MOF Nanoplates via Molecular Scalpel Strategy for Efficient Oxygen Reduction in Zn-Air Batteries.

The production of metal-organic framework (MOF) nanoplates with well-defined geometric morphology is remarkable for expanding their applications. Herein, the cobalt-based MOF nanoplates with hexagonal channels from a layer-pillared MOF are accomplished, via a molecular scalpel strategy, utilizing monodentate pyridine to replace the bidentate 4,4'-bipyridine. The morphology can be modified from nanorods to nanoplates with controllable thickness tuned by the amounts of pyridine. Succeeding carbonization treatment transforms the MOF nanoplates into Co particles homogeneously encapsulated in the nitrogen-doped carbon layers. The prepared catalyst with a unique platelike morphology displays a high half-wave potential of 0.88 V in oxygen reduction reaction. When used in primary Zn-air batteries, it delivers a high peak power density of 280 mW cm-2 . This work clarifies the structure-morphology-reactivity connection of MOF nanoplates.

A solvent-responsive terbium-organic framework for photocatalytic CO2 reduction.

A Tb-MOF with a 5,5,6-c 3-nodal net, {[Tb(DTDA)1.5·2H2O]·DMF·8H2O}n (AQNU-4), has been synthesized (DTDA = 2',5'-diethoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid). AQNU-4 contains a 1-periodic secondary building unit (rod SBU) and has an unusual topological type with the point symbol {32·44·62·7·8}{32·46·5·6}{32·46·52·65}. Efficient light harvesting and electron-hole separation make AQNU-4 an effective photocatalyst for use in the CO2 reduction reaction (CO2RR). Interestingly, as a photocatalyst, AQNU-4 shows an obvious solvent dependence during the catalytic process. In cyclohexanone in particular, it shows the highest CO2 to CO conversion, reaching 138.65 µmol g-1 h-1, making AQNU-4 a good solvent-responsive MOF material.

Regulation of Chirality in Metal-Organic Frameworks (MOFs) Based on Achiral Precursors through Substituent Modification.

The generation and regulation of chirality are closely related to the origin of life. Using achiral precursors to spontaneously build chiral MOFs remains a major challenge. Here, a method to synthesize chiral MOFs from achiral precursors by utilizing chiral fragments was achieved. The transformation from chiral fragments of 1 to chiral frameworks of 2 and 3 was realized by modifying the substituents, and the enantiomer resolution of 3-P41212 and 3-P43212 was achieved by d/l camphoric acid. 3 was then further studied in applications.

Interfacing MnO and FeCo alloy inside N-doped carbon hierarchical porous nanospheres derived from metal-organic framework to boost high-performance oxygen reduction for Zn-air batteries.

Efficient and strong non-precious metal catalysts are urgently needed for the oxygen reduction reaction (ORR). Here, a facile hydrothermal-pyrolysis process was implemented to engineer CoFe-MnO heterointerfaces encapsulated in N-doped carbon (CFM-NC) nanospheres with a metal-organic framework (MOF) as the precursor. Due to heterointerfaces and hierarchical porosity, CFM-NC-800 exhibited superior ORR activity (half-wave potential of 0.86 V) and durability (30 000 s). Importantly, CFM-NC-800-based Zn-air batteries (ZABs) were capable of extending the outstanding performance, with a high power density of 260 mW cm-2 and a specific capacity of 812 mA h g-1. Furthermore, the CFM-NC-800-RuO2-based ZABs showed remarkable stability (480 cycles), outperforming Pt/C-RuO2 (360 cycles). This work highlights the effects of CoFe-MnO heterointerfaces and hierarchical porosity in ORR electrocatalysis, thereby providing a new avenue for energy conversion and storage.

A stable Cd-MOF as a dual-responsive luminescent biosensor for the determination of urinary diphenyl phosphate and hippuric acid as biomarkers for human triphenyl phosphate and toluene poisoning.

Rapid and accurate determination of biomarkers of human poisoning in real urine is of great significance for the assessment of health status. Herein, a luminescent urea-functionalized metal-organic framework (MOF), {[Cd(L)0.5(bpbix)]·x(solv)}n (1) (H4L = 5,5'-(((naphthalene-1,5-diylbis(azanediyl))bis(carbonyl))bis(azanediyl))diisophthalic acid; bpbix = 4,4'-bis((1H-imidazol-1-yl)methyl)biphenyl), has been successfully synthesized, and exhibits good stability in aqueous solutions in the normal urinary pH range and real urine. Complex 1 can serve as a dual-responsive luminescent biosensor for the detection of diphenyl phosphate (DPP) and hippuric acid (HA) as biomarkers of flame retardant triphenyl phosphate and toluene poisoning, and shows the advantages of high sensitivity, rapid response, good anti-interference capability, and reversibility. More significantly, complex 1 is successfully applied to the sensitive and accurate detection of DPP and HA in real urine with satisfactory recoveries. This work presents a dual-responsive luminescent MOF-based biosensor for simple, rapid, accurate, and reversible determination of urinary DPP and HA, which has promising application potential for the diagnosis of diseases related to triphenyl phosphate and toluene poisoning.

Stable Cd Metal-Organic Framework as a Multiresponsive Luminescent Biosensor for Rapid, Accurate, and Recyclable Detection of Hippuric Acid, Nucleoside Phosphates, and Fe3+ in Urine and Serum.

Detecting biomarkers associated with diseases has significant meaning for early prevention, diagnosis, and treatment of diseases. The development of luminescent biosensors for rapid and accurate detection in real urine and serum is urgently desired for human health monitoring. Herein, a luminescent cadmium metal-organic framework, {[Cd(L)(bpbix)]·x(solv)}n (1), was successfully prepared by using a urea-functionalized dicarboxylate ligand, 5-(3-(pyridin-4-yl)ureido)isophthalic acid (H2L), 4,4'-bis((1H-imidazol-1-yl)methyl)biphenyl (bpbix), and the Cd2+ ion. The structure of 1 presents a 2-fold interpenetrating three-dimensional pillared-layer framework. The complex 1 exhibits good stability in different-pH aqueous solutions and physiological fluids. Strikingly, the complex 1 shows quick response, high sensitivity, good anti-interference performance, and a recyclable ability for simultaneous sensing of hippuric acid (HA), nucleoside phosphates, and Fe3+ in water. More significantly, this sensor can realize the sensitive and accurate detection of HA, nucleoside phosphates, and Fe3+ in real urine and serum and meet the practical detection needs in clinical diagnosis. These results indicate that the complex 1 as a multiresponsive luminescent biosensor possesses great potential for practical detection of HA, nucleoside phosphates, and Fe3+ in biological samples.

From Hydrogen Bond to van der Waals Force: Molecular Scalpel Strategy to Exfoliate a Two-Dimensional Metal-Organic Nanosheet.

The facile exfoliation of a two-dimensional metal-organic nanosheet of {[Co(HL)(H2O)(Py)3/4]·1/2H2O·DMF}n [1-Py; H3L = 5-(1H-pyrazol-4-yl)isophthalic acid and Py = pyridine] was achieved, via a molecular scalpel strategy, by weakening intermolecular forces between adjacent layers. The resulting 1-Py/KB40 (KB = Ketjen black) shows an increased oxygen evolution reaction (OER) performance with an overpotential of 370 mV at a current density of 10 mA cm-2 and a Tafel slope of 58 mV dec-1. This work sheds light on the structure-morphology-reactivity relationship of such materials in OER.

Response to the Temperature and Solvent Stimulation of MOF Material in a Single-Crystal to Single-Crystal Manner.

Under solvothermal conditions, a three-dimensional mononuclear crystal AQNU-1, {[Co(H2L)(DPD)(H2O)2]·2DMA}n (H2L = 5-(bis(4-carboxybenzyl)amino)isophthalic acid, DPD = 4,4'-(2,5-diethoxy-1,4-phenylene)dipyridine) has been synthesized. The transformations of AQNU-1 to binuclear {[Co2(L)(DPD)1.5(H2O)3]·DMA·H2O}n (AQNU-2) and pentanuclear {[Co5(L)2(DPD)2(OH)2]·2H2O}n (AQNU-3) were realized by double stimulation of temperature and solvent, which were accomplished by single-crystal to single-crystal (SC-SC) reaction.

MOF-derived CoNi,CoO,NiO@N-C bifunctional oxygen electrocatalysts for liquid and all-solid-state Zn-air batteries.

Metal-organic framework (MOF)-derived carbon composites with embedded metal alloy/metal oxides have attracted much attention due to their low-cost and excellent electrochemical reactivity. However, the drawback of this concept is the severe carbon evaporation during their synthesis, resulting in a reduction of active sites and catalytic durability. To solve this problem, this study proposes the possibility of using Ketjen black (KB) to replenish the carbon content. Impressively, MOF-derived bimetal and oxide N-doped porous carbon (CoNi-CoO-NiO@NC-800) exhibits extremely high catalytic activity with an oxygen reduction reaction (half-wave potential: 0.83 V) and oxygen evolution reaction (overpotential: 352 mV @ 10 mA cm-2) potential gap of 0.75 V due to the virtue of the hierarchically porous structure and sufficient active sites. By combining theoretical studies, a strong synergetic coupling of the CoNi dual metal is proposed in decreasing the overall reaction barriers and promoting the reversible oxygen reactions. Moreover, the assembled liquid- and all-solid-state Zn-air batteries (ZABs) with the bifunctional catalyst as the air cathode demonstrate superior discharging (223 mW cm-2 at 310 mA cm-2) and charging-discharging performance and long lifetime (450 cycles, 75 h). This work will provide guidance for the rational design of metal/carbon hybrid catalysts and cut-price reproducible energy systems.

A Water-Stable Tb-MOF As a Rapid, Accurate, and Highly Sensitive Ratiometric Luminescent Sensor for the Discriminative Sensing of Antibiotics and D2O in H2O.

The design and development of self-calibrating ratiometric luminescent sensors for the fast, accurate, and sensitive discrimination and determination of pollutants in wastewater is highly desirable for public and environmental health. Herein, a 3D porous Tb(III)-based metal-organic framework (MOF), {[Tb(HL)(H2O)2]·x(solv)}n (1), was facilely synthesized using a urea-functionalized tetracarboxylate ligand, 5,5'-(((1,4-phenylenebis(azanediyl))bis(carbonyl))bis(azanediyl))diisophthalic acid (H4L). The activated framework showed a good water stability in both aqueous solutions at a wide pH range of 2-14 and simulated antibiotic wastewaters. Interestingly, this Tb-MOF exhibited dual luminescence owing to the partial energy transfer from the antenna H4L to Tb3+. More importantly, activated 1 (1a) that was dispersed in water showed a fast, accurate, and highly sensitive discrimination ability toward antibiotics with a good recyclability, discriminating three different classes of antibiotics from each other via the quenching or enhancement of the luminescence and tuning the emission intensity ratio between the H4L ligand and the Tb3+ center for the first time. Simultaneously, 1a is a ratiometric luminescent sensor for the rapid, accurate, and quantitative discrimination of D2O from H2O. Furthermore, this complex was successfully used for the effective determination of antibiotics and D2O in real water samples. This work indicates that 1a represents the first ever MOF material for the discriminative sensing of antibiotics and D2O in H2O and promotes the practical application of Ln-MOF-based ratiometric luminescent sensors in monitoring water quality and avoiding any major leak situation.

The difference in the CO2 adsorption capacities of different functionalized pillar-layered metal-organic frameworks (MOFs).

The excessive use of fossil energy has caused the CO2 concentration in the atmosphere to increase year by year. MOFs are ideal CO2 adsorbents that can be used in CO2 capture due to their excellent characteristics. Studies of the structure-activity relationship between the small structural differences in MOFs and the CO2 adsorption capacities are helpful for the development of efficient MOF-based CO2 adsorbents. Therefore, a series of pillar-layered MOFs with similar structural and different functional groups were designed and synthesized. The CO2 adsorption tests were carried out at 273 K to explore the relationship between the small structural differences in MOFs caused by different functional groups and the CO2 adsorption capacities. Significantly, compound 6 which contains a pyridazinyl group has a 30.9% increase in CO2 adsorption capacity compared to compound 1 with no functionalized group.

Mixed matrix membranes containing fluorescent coordination polymers for detecting Cr2O72- with high sensitivity, stability and recyclability.

Three coordination polymers (CPs) were synthesized by using CdII, fluorescent 9,10-di(4-pyridyl)anthracene (dpa), and cyclohexane-1,4-dicarboxylic acid (H2cda), and they are [Cd2(dpa)2(cda)Cl2]n (1), [Cd(dpa)2(cda)]n (2) and [Cd(dpa)(cda)(H2O)]n (3). Both 1 and 2 are fluorescent and contain nonporous layers. 3 is an isomer of 2 and contains a porous diamondoid network. Fluorescent mixed matrix membranes were prepared by dispersing the particles of 1 or 2 within the matrix of polymethyl methacrylate, and showed high sensitivity and selectivity for detecting Cr2O72- in water. Both stability and recyclability of the MMMs were remarkably higher than those of the CP powders.

Molecular engineering in a family of pillared-layered metal-organic frameworks for tuning gas adsorption behavior.

In this work, inspired by a water-assisted three-dimensional supramolecular structure 1, we use a mixed-ligand strategy to form a 3D pillared-layered matrix by the introduction of linear ligands to compete against the water molecules. The resulting analogue microporous MOFs of 2-H, 2-F and 2-N, decorated with different functional groups, similarly show the CO2 uptake. Thanks to the negligible N2 adsorption capacity, enhanced selective adsorption towards CO2 is achieved in compound 2-N. That is, we present here an alternative plan for the high CO2 selective adsorption performance. In addition, the structure stability and moderate affinity for CO2 of these microporous MOFs endow them with excellent reusability.

A Europium-based MOF Fluorescent Probe for Efficiently Detecting Malachite Green and Uric Acid.

Lanthanide (such as Tb and Eu) metal-organic frameworks (MOFs) have been widely used in fluorescent probes because of their multiple coordination modes and brilliant fluorescence characteristic. Many lanthanide MOFs were applied in detecting metal ions, inorganic anions, and small molecules. However, it's rarely reported that Ln-MOF was devoted to detecting malachite green (MG) and uric acid (UA). We prepared a europium-based metal-organic framework (Eu-TDA) (TDA = 2,5-thiophenedicarboxylic acid group). Luminescence studies demonstrated that Eu-TDA can rapidly detect MG and UA with excellent selectivity and sensitivity, where individual quenching efficiency Ksv (MG: 5.8 × 105 M-1; UA: 4.15 × 104 M-1) and detection limit (MG: 0.0221 µM; UA: 0.689 µM) were regarded as the excellent MOF sensors for detecting MG and UA. The quenching of Eu-TDA's fluorescence emission by MG and UA was likely due to the spectral overlap, energy transfer, and competition. Among 11 metal cations and 14 anions, Eu-TDA can quickly and effectively recognize MG and UA with highly selective and sensitive properties. Our method possesses potential application in detecting UA in human blood and MG in the fishpond.

Three metal-organic framework isomers of different pore sizes for selective CO2 adsorption and isomerization studies.

Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) with tunable pore sizes, shapes and functionalities have excellent prospects in many applications, such as carbon capture. Molecular sieving can usually enable very high CO2 adsorption selectivity but has rarely been achieved, because it is difficult to precisely control the pore size in the range of 3-4 Å. We report here three MOF isomers built from CdII, terephthalic acid and 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine with the same stoichiometric ratio, among which 1 and 2 are framework-catenation isomers and 2 and 3 are framework-topological isomers. 1 contains 2-fold interpenetrated networks (topology of pcu) and 1D ultra-micropores and shows highly selective adsorption of CO2 over N2 and CH4, which is mainly ascribed to the molecular sieving effect of the framework. 2 contains a pcu network with 3D interconnected micropores, and 3 contains a kag network with much larger pores of 15 Å. Framework isomerization, in this case, was shown to be a feasible way of tuning the pore size of a MOF for selective CO2 adsorption. The effects of hydrothermal reaction conditions and additives on the structures and the formation of the MOF isomers were also studied.

Improving the Stability and Visualizing the Structural Transformation of the Stimuli-Responsive Metal-Organic Frameworks (MOFs).

New metal-organic frameworks (MOFs) based on flexible tetra-carboxylate ligands and Cu(II) are designed to gain stimuli-responsive materials. Unstable MOFs can be more stable with unabated flexibility by replacing coordinated solvent molecules with auxiliary N-based ligands. Two of them are intensively studied by in situ single-crystal X-ray diffraction (SCXRD) analysis and the unit cell parameters during transformations have been observed in detail. They undergo exceptional structural transformations which can be divided into two processes: the thermal-responsive phase transition and the solvent-responsive phase transition. The thermal-responsive phase transition takes place in a narrow temperature interval reversibly. However, the solvent-responsive phase transition is a gradual and irreversible process. The stimuli-responsive mechanism has also been explored by comparing the parameters of the crystal structures under different temperatures. Fascinatingly, their exceptional structural transformations correlate with the flexibility of the ligand fragments and the [Cu2(RCOO)4] clusters.

Tetrazole-based porous metal-organic frameworks for selective CO2 adsorption and isomerization studies.

Tetrazole-based molecules have numerous bridging coordination modes which afford great synthetic possibilities for the preparation of porous metal-tetrazolate architectures for many applications, such as carbon capture. We reported here three tetrazole-based MOFs: 1, {[Cu12(ttz)8/3Cl5(H2O)16]11+·11Cl-}n (H3ttz = N2,N4,N6-tris(4-(1H-tetrazol-5-yl)phenyl)-1,3,5-triazine-2,4,6-triamine), contains highly positively charged Cu12 clusters and the largest mesopores (32 Å) among the reported MOFs based on a tri-topic tetrazole ligand. 2 and 3 are two MOF isomers built by using CuII and 2-(1H-tetrazol-5-yl)pyrimidine. 3 contains nonporous layers, while 2 contains 1D channels and showed high selectivity for adsorbing CO2, which should be attributed to the high density of free nucleophilic tetrazole N atoms on the pore surfaces. We found that the isomerization between 2 and 3 was caused by the diverse coordination modes of tetrazole-based ligands and can be controlled in synthesis processes.

Three Anionic Indium-Organic Frameworks for Highly Efficient and Selective Dye Adsorption, Lanthanide Adsorption, and Luminescence Regulation.

Through solvothermal reaction of InCl3 and tetracarboxylate ligands with different substituent groups on diphenyl ethers, three new anionic indium-organic frameworks have been successfully prepared. They are {[(CH3)2NH2]In(G-1)(H2O)}·9DMF (1), {[(CH3)2NH2]In(G-2)}·15DMF (2), and {[(CH3)2NH2]2In2(G-3)2}·16DMF (3) {DMF = N, N'-dimethylformamide; H4(G-1) = 5',5″″-oxybis(2'-methoxy[1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid); H4(G-2) = 5',5″″-oxybis(2'-amino[1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid); H4(G-3) = 5',5″″-oxybis([1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid)}. Compounds 1-3 can be simplified as unimodal 4-connected frameworks with different topological types: lon, cag, and dia, respectively. Compounds 1 and 3 display 2-fold interpenetrating nets, while compound 2 is non-interpenetrating. Compounds 1 and 3 can adsorb cationic methylene blue (MB) with good capacity and a high adsorption rate due to their anionic frameworks and channel-type voids. In particular, compound 1 exhibits great selectivity for cationic MB in the mixtures of MB and methyl orange. In addition, the adsorption behavior of rare earth ions (Eu3+ and Tb3+) on compounds 1 and 3 has also been studied. Due to the different structural features and channel sizes of compounds 1 and 3 caused by different substituents on the ligands, the adsorption properties of rare earth ions on the two compounds are different.