Fabrication of 2D Metal-Organic Framework Nanosheets with Highly Colloidal Stability and High Yield through Coordination Modulation

The facile preparation of mixed-valence metal-organic frameworks through reduction

A facile strategy was applied to prepare Cu(I)/Cu(II) MOFs, where the Cu(I) amount and porous structure can be adjusted by the degree of reduction and the size of HKUST-1, by partially reducing HKUST-1 with hydrazine hydrate at room temperature. The Cu(I)/Cu(II) MOFs exhibit excellent catalytic capability in the Cu(I)-catalyzed azide alkyne cycloaddition.

Polymer Brush-Grafted Metal-Organic Framework Nanoplates for Enhanced Catalysis of CO2 Cycloaddition with Epoxides

The catalytic performance of 2D nanostructure-based heterogeneous catalysts is highly dependent on their dispersibility and stability. This work presents the grafting of polymer brushes onto 2D Cu-based metal-organic framework (MOF) nanoplates via a versatile UV-induced radical polymerization approach for CO2 fixation through the cycloaddition reaction with epoxides. The polymer brushes are selectively anchored to the external surfaces of the nanoplates, which greatly improves the long-term dispersibility and stability in solvents while preserving their intrinsic porosity and crystalline structures. The poly(N,N-dimethylaminoethyl methacrylate)-grafted 2-aminoterephthalic acid copper (CuBDC-NH2@PDMAEMA) nanoplates, with an optimized polymer brush thickness, achieve a maximum yield of 81.2% in the cycloaddition of CO2 with styrene oxide, which is a 1.6-fold improvement over that of the unmodified CuBDC-NH2 nanoplates. The grafted nanoplates also exhibit excellent recyclability with a slight decrease in yield after multiple consecutive cycles. The versatility of this approach is further demonstrated by its application to other polymers and epoxide substrates, where polymer-grafted catalysts consistently outperform their pristine counterparts. Our strategy provides a feasible and efficient route to synthesize MOF/polymer hybrid nanomaterials for sustainable CO2 utilization and green chemistry applications.

Michael and Schiff-Base Reactions-Assisted Fluorescence Sensor Based on the MOF Nanosheet Microspheres for the Effective Discrimination and Detection of Hydroquinone and Catechol

A novel sensing platform was constructed for the recognition and identification of dihydroxybenzene isomers based on the MOF-0.02TEA fluorescence sensor with the morphology of nanosheet microspheres through coordination modulation. Based on the sensing principle that the amino group on the MOF-0.02TEA can make the Michael reaction with o-benzoquinone and p-benzoquinone, which were individually the oxidation intermediate of catechol and hydroquinone, the fluorescence intensity of MOF-0.02TEA could be quenched through the inner filter effect (IFE) without the interference from resorcinol. Besides, catechol and hydroquinone could be further distinguished with the assistance of the Schiff-base reaction by introducing o-phenylenediamine (OPD) to the detection system. The MOF-0.02TEA sensor exhibited good selectivity, and the detection limits for catechol and hydroquinone were 90.5 nmol/L and 0.52 μmol/L (S/N = 3), respectively. Moreover, the sensor could be used for the determination of dihydroxybenzene isomers in tap water and lake water.

Liquid exfoliation of a series of expanded layered Cu(II)-paddlewheel metal-organic frameworks to form nanosheets

Ultrasonic liquid exfoliation provides a convenient route for converting layered materials into nanosheets. However, the relationship between the structure and morphology of the bulk materials and the properties of the resulting nanosheets remains poorly understood. In this work, we prepare an isoreticular series of layered metal-organic frameworks (MOFs) based on linear aromatic dicarboxylate derivatives (L1, L2, L3) with three different linker lengths (L3 > L2 > L1) and using copper(II) nitrate and acetate as metal ion sources. Liquid exfoliation of large crystals of all three MOFs [Cu2(L)2](solvent)2, synthesised from Cu(NO3)2, produced monolayer nanosheets with longer linkers leading to larger lateral dimensions. Exfoliation of smaller MOF crystals, formed using the copper(II) acetate salt under identical conditions, produced a much higher concentration of multi-layer nanosheets with smaller lateral dimensions. These results indicate that the initial crystal size plays an important role in determining both the lateral dimensions and the thicknesses of nanosheets. Such insights contribute to a deeper understanding of the design principles governing metal-organic framework nanosheets (MONs) and other two-dimensional materials.

Superlong Metal-Organic Framework Nanowire Fabricated via Steam-Assisted Conversion

1D nanomaterials have attracted great attention due to their outstanding anisotropic and linear structures. A facile method is developed to fabricate 1D copper metal-organic framework nanowires (Cu-MOF-NW) through steam-assisted conversion from Cu-MOF precursors. During the steam-assisted conversion, Cu-MOF precursor gradually dissolves in methanol steam, and then recrystallized into Cu-MOF-NW, which shows high aspect ratio of about 600 and identical crystal structure of MOF-74. As-prepared Cu-MOF-NW with multiscale porous structure can effectively remove cationic dyes even in dye mixture. Moreover, Cu-MOF-NW, as an ideal template, is calcined to form Cu nanoparticle-doped carbon nanofiber with maintaining its 1D morphology, which shows excellent electrocatalytic activity for the non-enzymatic sensing of glucose.

Recent Advances in the Synthesis and Application of Monolayer 2D Metal-Organic Framework Nanosheets

Monolayer 2D metal-organic framework (MOF) nanosheets, characterized by abundant exposed active sites and tunable structure and function (such as altering the metal nodes or organic ligands), have emerged as a pivotal class of 2D materials, demonstrating irreplaceable applications across diverse research domains in materials and chemistry. This review provides a comprehensive survey of the latest research progress in the synthesis of monolayer 2D MOF nanosheets. Specifically, recent synthetic strategies, including top-down and bottom-up methods, are delved and their applications in gas separation, catalysis, sensing platforms, and energy storage are explored. Additionally, the challenges faced in the investigation of monolayer 2D MOF nanosheets are elucidated and future opportunities for these materials as a novel generation of 2D materials are outlined.

Toward Enhancing Low Temperature Performances of Lithium-Ion Transport for Metal-Organic Framework-Based Solid-State Electrolyte: Nanostructure Engineering or Crystal Morphology Controlling

Metal-organic frameworks (MOFs) have emerged as attractive candidates for Li+ conducting electrolytes due to their regular channels and controllable morphology, making their presence prominent in the field of solid-state lithium batteries. However, most MOF-based electrolytes are researched at or near room temperature, which poses a challenge to their practical applications at low temperatures. Herein, a thin layer flower-shaped 2D Cu-MOF (CuBDC-10)-based solid-state electrolytes (SSEs) for lithium-ion batteries (LIBs) are developed for facilitating Li+ transport at lower temperatures, which achieve an ion conductivity of 10-4 S cm-1 at -30 °C. The CuBDC-10-based SSE exhibits outstanding ionic conductivity over a wide temperature range of -40 to 100 °C (0.073-3.68 × 10-3 S cm-1). This work provides strategies for exploring MOF-based SSEs with high ionic transport performances at low temperatures.

Polar alcohol guest molecules regulate the stacking modes of 2-D MOF nanosheets

The modulation of two-dimensional metal-organic framework (2-D MOF) nanosheet stacking is an effective means to improve the properties and promote the application of nanosheets in various fields. Here, we employed a series of alcohol guest molecules (MeOH, EtOH and PrOH) to modulate Zr-BTB (BTB = benzene-1,3,5-tribenzoate) nanosheets and to generate untwisted stacking. The distribution of stacking angles was statistically analyzed from high-angle annular dark-field (HAADF) and fast Fourier transform (FFT) images. The ratios of untwisted stacking were calculated, such as 77.01% untwisted stacking for MeOH, 83.45% for EtOH, and 85.61% for PrOH. The obtained untwisted Zr-BTB showed good separation abilities for different substituted benzene isomers, superior para selectivity and excellent column stability and reusability. Control experiments of 2-D Zr-TCA (TCA = 4,4',4''-tricarboxytriphenylamine) and Zr-TATB (TATB = 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzoic acid) nanosheets with similar pore sizes and stronger polarity regulated by the alcohol guests exhibited moderate separation performance. The electron microscopy images revealed that polar alcohol regulation dominantly generated the twisted stacking of Zr-TCA and Zr-TATB with various Moiré patterns. Polar guest molecules, such as alcohols, provide strong host-guest interactions during the regulation of MOF nanosheet stacking, providing an opportunity to design new porous Moiré materials with application prospects.

Direct Synthesis of Metal-Organic Framework Sols: Advances and Perspectives

The intrinsic lack of processability in the conventional nano/microcrystalline powder form of metal-organic frameworks (MOFs) greatly limits their application in various fields. Synthesis of MOFs with certain flowability make them promising for multitudinous applications. The direct synthesis strategy represents one of the simplest and efficient method for synthesizing solution processable MOF sols/suspensions, compared with other approaches, for instance, the post-synthesis surface modification, the direct dispersion of MOFs in hindered ionic liquids, as well as the calcination method toward a few MOFs with melting behavior. This article reviews the recent direct synthesis strategies of solution processable MOF sols and their typical applications in different fields. The direct synthesis strategies of MOF sols can be classified into two categories: particle size reduction strategy, and selective coordination strategy. The synthesis mechanism of different strategies and the factors affecting the formation of sols are summarized. The application of solution processable MOF sols in different fields are introduced, showing great application potentials. Furthermore, the challenges faced by the direct synthesis of MOF sols and the main methods to deal with the challenges are emphasized, and the future development trend is prospected.

A Nitro Functionalized MOF with Multi-Enzyme Mimetic Activities for the Colorimetric Sensing of Glucose at Neutral pH

Benefiting from the advantages like large surface area, flexible constitution, and diverse structure, metal-organic frameworks (MOFs) have been one of the most ideal candidates for nanozymes. In this study, a nitro-functionalized MOF, namely NO₂-MIL-53(Cu), was synthesized. Multi-enzyme mimetic activities were discovered on this MOF, including peroxidase-like, oxidase-like, and laccase-like activity. Compared to the non-functional counterpart (MIL-53(Cu)), NO₂-MIL-53(Cu) displayed superior enzyme mimetic activities, indicating a positive role of the nitro group in the MOF. Subsequently, the effects of reaction conditions on enzyme mimetic activities were investigated. Remarkably, NO₂-MIL-53(Cu) exhibited excellent peroxidase-like activity even at neutral pH. Based on this finding, a simple colorimetric sensing platform was developed for the detection of H₂O₂ and glucose, respectively. The detection liner range for H₂O₂ is 1-800 μM with a detection limit of 0.69 μM. The detection liner range for glucose is linear range 0.5-300 μM with a detection limit of 2.6 μM. Therefore, this work not only provides an applicable colorimetric platform for glucose detection in a physiological environment, but also offers guidance for the rational design of efficient nanozymes with multi-enzyme mimetic activities.

Carbon dots@Cu metal-organic frameworks hybrids for ratiometric fluorescent determination of pesticide thiophanate-methyl

A dual-emission fluorescent (FL) probe was constructed by coordinating Cu²⁺ of copper metal-organic frameworks (Cu-MOFs) with - COO^- group of carbon dots (CDs) for pesticide thiophanate-methyl (TM) determination. TM was recognized by organic ligands (H₂BDC and H₂BDC-NH₂) of Cu-MOFs via π stacking. Due to the higher affinity of Cu²⁺ to TM than ligands and CDs, TM chelated with Cu²⁺ to form TM-Cu complex. Thus coordination of Cu-MOFs was damaged and the ligands were released resulting in the FL intensity increase of Cu-MOFs (F₄₃₀). And also CDs were released from CDs@Cu-MOFs hybrids and electron transfer from CDs to CuMOFs was inhibited, leading to the FL intensity increase of CDs (F₆₀₀). The FL intensity ratio (F₄₃₀/F₆₀₀) showed a good linear relationship with TM concentrations of 0.0307-0.769 μmol L^-1 with a limit of detection (LOD) of ~ 3.67 nmol L^-1. The probe was successfully applied to detect TM in spiked food samples with satisfactory recoveries of 93.1-113%. Additionally, visual detection of TM was achieved according to the fluorescence color variation from blue to carmine, indicating promising application of CDs@Cu-MOFs probe.

Fabrication of Hierarchical Quaternary Architectures of Metal-Organic Frameworks through Programmed Transformation

A new method to construct hierarchical architectures has been developed by programmed transformation of metal-organic frameworks (MOFs). A MOF precursor was fabricated by reaction of Cu(OAC)₂ and 2,5-dihydroxyterephthalic acid (H₄DOBDC), which could perform transformation in pure methanol solvent and the sodium dodecyl benzene sulfonate (SDBS) solution of methanol, respectively. Interestingly, two kinds of immersion solutions could induce the transformation of the MOF precursor into MOF-74, which resulted in different morphologies: nanoneedles for the methanol and nanosheets for the SDBS. Herein, nanosheets-mesorods-microcuboid hierarchical quaternary architectures of MOF have been successfully achieved by sequential immersion of the precursor in two kinds of transformation solutions, which demonstrates well-defined hierarchy from the nanoscale to mesoscale to microscale. A unique hierarchical architecture could be recognized as quaternary structures, taking the MOF unit cell as the primary structure, the nanosheets as the secondary structure, the mesorods as the tertiary structure, and the microcuboid as the quaternary structure. Our study indicated the potential of programmed transformation between MOFs in the construction of hierarchical architectures, offering a new approach to sophisticated materials.