Ultrathin Chiral Metal-Organic-Framework Nanosheets for Efficient Enantioselective Separation

Recent progress in the design, synthesis and applications of chiral metal-organic frameworks

Chiral Metal-Organic Frameworks (CMOFs) are unique crystalline and porous class of materials which is composed of organic linkers and metal ions. CMOFs surpass traditional organic and inorganic porous materials because of their tunable shape, size, functional diversity, and selectivity. Specific applications of CMOFs may be exploited by introducing desired functional groups. CMOFs have chiral recognition abilities, making them unique for chiral compound synthesis and separation. The CMOFs can be synthesized through different approaches. Two main approaches have been discussed, i.e., direct and indirect synthesis. Synthetic strategies play an essential role in getting desired properties in MOFs. CMOFs find potential applications in adsorption, asymmetric catalysis, luminescence, degradation, and enantioselective separation. The MOFs' porosity, stability, and reusability make them an attractive material for these applications. The plethora of applications of CMOFs have motivated chemists to synthesize novel MOFs and number of MOFs have been ever-escalating. Herein, the synthetic methods of CMOFs and their various applications have been discussed.

Two-Dimensional Metal-Organic Framework Nanosheets: Synthesis and Applications in Electrocatalysis and Photocatalysis

Two-dimensional metal-organic nanosheets (2D MONs) are an emerging class of ultrathin, porous, and crystalline materials. The organic/inorganic hybrid nature offers MONs distinct advantages over other inorganic nanosheets in terms of diversity of organic ligands and metal notes. Compared to bulk three-dimensional metal-organic frameworks, 2D MONs possess merits of high density and readily accessible catalytic sites, reduced diffusion pathways for reactants/products, and fast electron transport. These features endow MONs with enhanced physical/chemical properties and are ideal for heterogeneous catalysis. In this Review, state-of-the-art synthetic methods for the fabrication of 2D MONs were summarized. The advances of 2D MONs-based materials for electrocatalysis and photocatalysis, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂ RR), and electro-/photocatalytic organic transformations were systematically discussed. Finally, the challenges and perspectives regarding future design and synthesis of 2D MONs for high-performance electrocatalysis and photocatalysis were provided.

Interplay of Polarity and Confinement in Asymmetric Catalysis with Chiral Rh Diene Complexes in Microemulsions

Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2 F2 ) were synthesized and characterized by X-ray crystallography and theoretical calculations. These complexes were used in Rh-catalyzed 1,2-additions of phenylboroxine to N-tosylimine in microemulsions stabilized either exclusively by n-octyl-β-D-glucopyranoside (C8 G1 ) or a C8 G1 -film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 % (90 %ee) for the surfactant film doped with 5 wt% of AOT as compared to 52 % (58 %ee) for neat C8 G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies.

Ultrathin two-dimensional metal-organic framework nanosheets-an emerging class of catalytic nanomaterials

The development of ultrathin two-dimensional metal-organic framework nanosheets (2D MONs) has recently attracted increasing interest due to their extensive application potential originating from their ultrathin thickness, large surface area, and innumerable accessible surface-active sites. In this frontier article, we highlight the recent developments of 2D MONs for highly efficient heterogeneous catalysts; they can be grouped into three sections in terms of their functions: (i) as catalysts capable of showing outstanding intrinsic reactivity, (ii) as support materials for prevalent catalysts, and (iii) as catalysts with multifunctional catalytic activity for diverse organic transformations. In addition, the present challenges and future opportunities in this field are also discussed.

The Renewable and Sustainable Conversion of Chitin into a Chiral Nitrogen-Doped Carbon-Sheath Nanofiber for Enantioselective Adsorption

Well-known hard-template methods for nitrogen (N)-doped chiral carbon nanomaterials require complicated construction and removal of the template, high-temperature pyrolysis, harsh chemical treatments, and additional N-doping processes. If naturally occurring chiral nematic chitin nanostructures [(C₈ H₁₃ NO₅ )_n ] in exoskeletons were wholly transformed into an N-doped carbon, this would be an efficient and sustainable method to obtain a useful chiral nanomaterial. Here, a simple, sacrificial-template-free, and environmentally mild method was developed to produce an N-doped chiral nematic carbon-sheath nanofibril hydrogel with a surface area >300 m²  g^-1 and enantioselective properties from renewable chitin biomass. Calcium-saturated methanol physically exfoliated bulk chitin and produced a chiral nematic nanofibril hydrogel. Hydrothermal treatment of the chiral chitin hydrogel at 190 °C produced an N-doped chiral carbon-sheath nanofibril hydrogel without N-doping. This material preferentially adsorbed d-lactic acid over l-lactic acid and produced 16.3 % enantiomeric excess of l-lactic acid from a racemic mixture.

DNAzyme-Loaded Metal-Organic Frameworks (MOFs) for Self-Sufficient Gene Therapy

DNAzymes have been recognized as potent therapeutic agents for gene therapy, while their inefficient intracellular delivery and insufficient cofactor supply precludes their practical biological applications. Metal-organic frameworks (MOFs) have emerged as promising drug carriers without in-depth consideration of their disassembled ingredients. Herein, we report a self-sufficient MOF-based chlorin e6-modified DNAzyme (Ce6-DNAzyme) therapeutic nanosystem for combined gene therapy and photodynamic therapy (PDT). The ZIF-8 nanoparticles (NPs) could efficiently deliver the therapeutic DNAzyme without degradation into cancer cells. The pH-responsive ZIF-8 NPs disassemble with the concomitant release of the guest DNAzyme payloads and the host Zn²⁺ ions that serve, respectively, as messenger RNA-targeting agent and required DNAzyme cofactors for activating gene therapy. The auxiliary photosensitizer Ce6 could produce reactive oxygen species (ROS) and provide a fluorescence signal for the imaging-guided gene therapy/PDT.

Novel approaches for biomolecule immobilization in microscale systems

This manuscript reviews novel approaches applied for biomolecule immobilization in microscale systems.

Molecular tectonics: enantiomerically pure chiral crystals based on trans-1,2-cyclohexanediol

Enantiomerically pure trans-1,2-cyclohexanediol ((R,R) or (S,S)) based organic ligands act self-complementary units. In addition, their coordination behavior towards Cd²⁺ cation in the presence of N donor ancillary ligand was investigated.

π-Conjugated Molecule Boosts Metal-Organic Frameworks as Efficient Oxygen Evolution Reaction Catalysts

To improve the efficiency of water electrolysis, developing efficient oxygen evolution reaction (OER) electrocatalysis is extremely important due to its four-electron transfer dynamics. In this work, a π-conjugated molecule (2,3,6,7,10,11-hexahydroxytriphenylene, HHTP), which can accelerate the electron transfer, is coated directly on pristine ZIF-67, resulting in a composite named HHTP@ZIF-67, via a simple one-step solvothermal method. The obtained HHTP@ZIF-67 possesses a Brunauer-Emmett-Teller surface area of 2013.9 m² g^-1 and displays microporous behavior, which can provide enough active sites for OER. The double-layer capacitance of HHTP@ZIF-67 is also enhanced, corresponding to an enlarged electrochemical active surface area. HHTP@ZIF-67 presents a quite low overpotential of 238 mV at 10 mA cm^-2 in 1.0 m KOH. This material synthesized via the simple coating strategy is promising in the application of energy conversion devices.