Molecular Mechanism of Loading Sulfur Hexafluoride in γ-Cyclodextrin Metal–Organic Framework

Cyclodextrin-based metal-organic frameworks transforming drug delivery

Cyclodextrin-based metal-organic frameworks (CD-MOFs) are gaining traction in the realm of drug delivery due to their inherent versatility and potential to amplify drug efficacy, specificity, and safety. This article explores the predominant preparation techniques for CD-MOFs, encompassing methods like vapor diffusion, microwave-assisted, and ultrasound hydrothermal approaches. Native CD-MOFs present compelling advantages in drug delivery applications. They can enhance drug loading capacity, stability, solubility, and bioavailability by engaging in diverse interactions with drugs, including host-guest, hydrogen bonding, and electrostatic interactions. Beyond their inherent properties, CD-MOFs can be customized as drug carriers through two primary strategies: co-crystallization with functional components and surface post-modifications. These tailored modifications pave the way for controlled release manners. They allow for slow and sustained drug release, as well as responsive releases triggered by various factors such as pH levels, glutathione concentrations, or specific cations. Furthermore, CD-MOFs facilitate targeted delivery strategies, like pulmonary or laryngeal delivery, enhancing drug delivery precision. Overall, the adaptability and modifiability of CD-MOFs underscore their potential as a versatile platform for drug delivery, presenting tailored solutions that cater to diverse biomedical and industrial needs.

Synthesis of Nanosized γ-Cyclodextrin Metal-Organic Frameworks as Carriers of Limonene for Fresh-Cut Fruit Preservation Based on Polycaprolactone Nanofibers

To address the issue of bacterial growth on fresh-cut fruits, this paper reports the synthesis of nanosized γ-cyclodextrin metal-organic frameworks (CD-MOFs) using an ultrasound-assisted method and their application as carriers of limonene for antibacterial active packaging. The effects of the processing parameters on the morphology and crystallinity of the CD-MOFs are investigated, and the results prove that the addition of methanol is the key to producing nanosized CD-MOFs. The limonene loading content of the nanosized CD-MOFs can reach approximately 170 mg g-1. The sustained-release behaviors of limonene in the CD-MOFs are evaluated. Molecular docking simulations reveal the distribution and binding sites of limonene in the CD-MOFs. CD-MOFs are deposited on the surfaces of polycaprolactone (PCL) nanofibers via an immersion method, and limonene-loaded CD-MOF@PCL nanofibers are prepared. The morphology, crystallinity, thermal stability, mechanical properties, and antibacterial activity of the nanofibers are also studied. The nanofiber film effectively inhibits bacterial growth and prolongs the shelf life of fresh-cut apples. This study provides a novel strategy for developing antibacterial active packaging materials based on CD-MOFs and PCL nanofibers.

New opportunities for cyclodextrins in supramolecular assembly: metal organic frameworks, crystalline self-assembly, and catalyzed assembly

Cyclodextrins (CDs) are widely used macrocycles in supramolecular assembly due to their easy availability, versatile functionality and excellent biocompatibility. Although they are well-known for forming host-guest complexes with a wide range of guests and this host-guest chemistry has long been utilized in industry and academia, new opportunities have arisen in recent years, particularly in supramolecular assembly. In the present review, we will first provide a basic introduction to CDs and then summarize their emerging roles in the fields of supramolecular chemistry and materials. This includes their involvement in hybrid frameworks with inorganic components such as metal ions and polyoxometalates, crystalline self-assembly with amphiphilic molecules, and their new possibility of "catassembly" and induced chiral supramolecular structures that have previously been overlooked. Finally, we will comment on the future perspectives of CDs to inspire more ideas and efforts, with the aim of promoting diverse applications of CDs in supramolecular materials.

Synthesis and potential applications of cyclodextrin-based metal-organic frameworks: a review

Metal-organic frameworks are porous polymeric materials formed by linking metal ions with organic bridging ligands. Metal-organic frameworks are used as sensors, catalysts for organic transformations, biomass conversion, photovoltaics, electrochemical applications, gas storage and separation, and photocatalysis. Nonetheless, many actual metal-organic frameworks present limitations such as toxicity of preparation reagents and components, which make frameworks unusable for food and pharmaceutical applications. Here, we review the structure, synthesis and properties of cyclodextrin-based metal-organic frameworks that could be used in bioapplications. Synthetic methods include vapor diffusion, microwave-assisted, hydro/solvothermal, and ultrasound techniques. The vapor diffusion method can produce cyclodextrin-based metal-organic framework crystals with particle sizes ranging from 200 nm to 400 μm. Applications comprise food packaging, drug delivery, sensors, adsorbents, gas separation, and membranes. Cyclodextrin-based metal-organic frameworks showed loading efficacy of the bioactive compounds ranging from 3.29 to 97.80%.

Organo-macrocycle-containing hierarchical metal-organic frameworks and cages: design, structures, and applications

Developing hierarchical ordered systems is challenging. Using organo-macrocycles to construct metal-organic frameworks (MOFs) and porous coordination cages (PCCs) provides an efficient way to obtain hierarchical assemblies. Macrocycles, such as crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, can be incorporated within MOFs/PCCs and they also endow the resultant composites with enhanced properties and functionalities. This review summarizes recent developments of organo-macrocycle-containing hierarchical MOFs/PCCs, emphasizing applications and structure-property relationships of these hierarchically porous materials. This review provides insights for future research on hierarchical self-assembly using macrocycles as building blocks and functional ligands to extend the applications of the composites.

Encapsulating quercetin in cyclodextrin metal-organic frameworks improved its solubility and bioavailability

BACKGROUND

Quercetin (Que) has many pharmacological activities, such as anticancer, antioxidant, cardiovascular protection, antihypertensive and lipid-lowering activities. However, its poor water solubility greatly limits its application in medicine and food. γ-Cyclodextrin metal-organic frameworks (γ-CD-MOFs) are novel porous carriers for loading functional products. In this study, Que was successfully loaded into γ-CD-MOFs, and the new compound (Que-CD-MOFs) was characterised by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy.

RESULTS

The apparent solubility of Que-CD-MOFs was enhanced by 100-fold compared with that of pure Que. The free radical scavenging ability of the encapsulated Que was significantly improved. The cytotoxicity of Que-CD-MOFs to HK-2 cells was decreased, and their inhibition on HT-29 tumour cells was maintained, as confirmed by CCK-8 assays. Flow cytometry of HT-29 cells showed that Que-CD-MOFs can inhibit G2 phase cells. Based on molecular modelling, Que molecules were preferentially located inside the cavities of γ-CD pairs in γ-CD-MOFs.

CONCLUSION

γ-CD-MOFs are promising carriers for bioactive agents in food and pharmaceutical applications. © 2021 Society of Chemical Industry.

Chaotropic Effect as an Assembly Motif to Construct Supramolecular Cyclodextrin-Polyoxometalate-Based Frameworks

In aqueous solution, low-charged polyoxometalates (POMs) exhibit remarkable self-assembly properties with nonionic organic matter that have been recently used to develop groundbreaking advances in host-guest chemistry, as well as in soft matter science. Herein, we exploit the affinity between a chaotropic POM and native cyclodextrins (α-, β-, and γ-CD) to enhance the structural and functional diversity of cyclodextrin-based open frameworks. First, we reveal that the Anderson-Evans type polyoxometalate [AlMo₆O₁₈(OH)₆]^3- represents an efficient inorganic scaffold to design open hybrid frameworks built from infinite cyclodextrin channels connected through the disk-shaped POM. A single-crystal X-ray analysis demonstrates that the resulting supramolecular architectures contain large cavities (up to 2 nm) where the topologies are dictated by the rotational symmetry of the organic macrocycle, generating honeycomb (bnn net) and checkerboard-like (pcu net) networks for α-CD (C₆) and γ-CD (C₈), respectively. On the other hand, the use of β-CD, a macrocycle with C₇ ideal symmetry, led to a distorted-checkerboard-like network. The cyclodextrin-based frameworks built from an Anderson-Evans type POM are easily functionalizable using the molecular recognition properties of the macrocycle building units. As a proof of concept, we successfully isolated a series of compartmentalized functional frameworks by the entrapment of polyiodides or superchaotropic redox-active polyanions within the macrocyclic host matrix. This set of results paves the way for designing multifunctional supramolecular frameworks whose pore dimensions are controlled by the size of inorganic entities.

Solidification of volatile D-Limonene by cyclodextrin metal-organic framework for pulmonary delivery via dry powder inhalers: In vitro and in vivo evaluation

D-Limonene (D-Lim), a volatile oil extracted from citrus fruits, has therapeutic effects on lung inflammation and cancer, whilst the deep delivery of D-Lim was challenging due to its physical instability for a long period of time. To prevent the volatilization of D-Lim and achieve efficient pulmonary delivery, herein, D-Lim was loaded into biodegradable γ-cyclodextrin metal-organic framework (γ-CD-MOF) with optimal loading efficiency achieving 13.79 ± 0.01% (molar ratio of D-Lim and γ-CD-MOF was 1.6:1), which possessed cubic shape with controllable particle size (1-5 μm). The experimental results indicated that γ-CD-MOF could improve the stability of D-Lim. A series of characterizations and molecular docking were used to reveal the interaction between D-Lim and γ-CD-MOF. The solidification of D-Lim by γ-CD-MOF played a crucial role in the exploitation of its inhalable dosage form, dry powder inhaler (DPI). Specifically, the aerosolization of D-Lim@γ-CD-MOF for inhalation was satisfactory with a fine particle fraction (FPF) of 33.12 ± 1.50% at 65 L/min of flow rate. Furthermore, in vivo study had shown a 2.23-fold increase in bioavailability of D-Lim solidified by γ-CD-MOF for inhalation compared to D-Lim for oral administration. Therefore, it is considered that γ-CD-MOF could be an excellent carrier for pulmonary drug delivery to realize solidification and lung therapeutic effects of volatile oils.

Recent progress in the synthesis, structural diversity and emerging applications of cyclodextrin-based metal-organic frameworks

Inorganic-organic hybrid materials, especially metal-organic frameworks (MOFs) composed of metals and organic linkers, have emerged as a new class of versatile materials owing to their tunable structure and controllable functionality. As typical biocompatible MOFs, cyclic oligosaccharide cyclodextrin-based carbohydrate metal-organic frameworks (CD-MOFs) have recently attracted considerable attention due to their edible, renewable and biodegradable nature. Herein, we focus on the latest advances concerning these materials. First, the synthesis methods and structural diversity of CD-MOFs are introduced and summarized. Besides, the synthetic strategies of moisture-resistant CD-MOFs are also emphasized. Moreover, their applications, including gas adsorption, separation, sensing, memristor fabrication, as templates in nanoparticle synthesis, light emission and especially drug delivery, are systematically discussed and highlighted. Finally, to conclude the review, some insights and current challenges that need to be addressed for the further development of these materials are proposed. We anticipate that this review will result in a better understanding of CD-MOFs and will help maximize the potential functions of these materials.

Metal-Organic Framework Based on α-Cyclodextrin Gives High Ethylene Gas Adsorption Capacity and Storage Stability

Two metal-organic framework (MOF) materials, that is, α-cyclodextrin (α-CD)-MOF-Na and α-CD-MOF-K, were successfully synthesized and exhibited excellent adsorption capacity and storage stability for ethylene gas. The ethylene encapsulation capacity of α-CD-MOF-Na and α-CD-MOF-K reached 47.4 and 52.9% (w/w), respectively, which was significantly higher than those of other materials reported such as α-CD and V-type starch. The release characteristics of ethylene inclusion complexes (ICs) were determined under different temperatures and relative humidity conditions. The ethylene gas could be stably encapsulated in α-CD-MOF-ethylene ICs at 25 °C for up to 30 days. The crystal structure of α-CD-MOFs was determined to explain their high capacity and stability for ethylene storage. Molecular simulation was used to model the location of ethylene molecules in α-CD-MOFs. Alpha-CD-MOF-Na and α-CD-MOF-K showed "8"-shaped and spindle-shaped cavity, respectively, which effectively adsorbed and stored the ethylene gas. Accelerated ripening experiments showed that 5 mg of α-CD-MOF ICs could ripen bananas within 4 days, with an effect similar to that of free ethylene gas. We suggest that α-CD-MOF materials are an excellent material for ethylene storage with potential application in industrial and agricultural areas.

The Study of Release Mechanisms for Drug in Cyclodextrin Metal-Organic Frameworks

γ-Cyclodextrin metal-organic frameworks (γCDMOF) recently emerged as biofriendly, highly porous, and crystalline materials with potential applications in drug delivery. However, little is known about their drug entrapment and release characteristics, which are key parameters in the design of drug carriers. The macroscopic properties of a material are determined by its microstructure. Thus, the characteristics of the constitutive units of the cubic crystalline γCDMOF determine their drug loading and release behaviors. In this study, the release profile of prednisolone (PNS) form γCDMOF was predicted, and the mechanism was analyzed based on the γCDMOF molecular structure. For the first time, experimental, molecular simulation, and mathematical modeling methods were combined to gain insights into the drug distribution in cubic porous crystals of γCDMOF as well as on drug release kinetics. The predicted release profile was in good agreement with the experimental results, showing that the modeling method was reliable. The methodology developed here could provide a reference for further investigations of drug penetration and release in supramolecular systems.

Long-term entrapment and temperature-controlled-release of SF6 gas in metal-organic frameworks (MOFs)

In this work, a metal-organic framework (MOF), namely MFU-4, which is comprised of zinc cations and benzotriazolate ligands, was used to entrap SF₆ gas molecules inside its pores, and thus a new scheme for long-term leakproof storage of dangerous gasses is demonstrated. The SF₆ gas was introduced into the pores at an elevated gas pressure and temperature. Upon cooling down and release of the gas pressure, we discovered that the gas was well-trapped inside the pores and did not leak out - not even after two months of exposure to air at room temperature. The material was thoroughly analyzed before and after the loading as well as after given periods of time (1, 3, 7, 14 or 60 days) after the loading. The studies included powder X-ray diffraction measurements, thermogravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, ¹⁹F nuclear magnetic resonance spectroscopy and computational simulations. In addition, the possibility to release the gas guest by applying elevated temperature, vacuum and acid-induced framework decomposition was also investigated. The controlled gas release using elevated temperature has the additional benefit that the host MOF can be reused for further gas capture cycles.

Crystal Transformation of β-CD-MOF Facilitates Loading of Dimercaptosuccinic Acid

The β-cyclodextrin-metal-organic framework (β-CD-MOF), a potential drug delivery carrier, presents a densely packed laminated crystal structure (CCDC number 1041782) that prevents drug from entering inside the molecular voids in most CD units. In this paper, it was demonstrated that dimercaptosuccinic acid (DMSA), an instable small molecule chemical drug, was successfully loaded in β-CD-MOF with a high molar ratio of 1:1.35 (β-CD-MOF:DMSA) determined by high-performance liquid chromatography. The drug loading mechanism of β-CD-MOF/DMSA was supported by a series of experimental characterizations and molecular simulations. The morphology observations revealed that crystalline particles of β-CD-MOF transformed to reticular microstructure after drug loading evidenced by powder X-ray diffraction (PXRD), scanning electron microscope (SEM), synchrotron radiation Fourier transform infrared spectroscopy (SR-FTIR), and etc. It is of interest to note that the stability of DMSA was well improved by β-CD-MOF, but decreased by γ-CD-MOF, indicating different protective capacities between the two types of CD-MOFs. Thus, it is hypothesized that the transformation from laminated molecular arrangement of β-CD-MOF to reticular microstructure leads to an enhanced drug-loading capability for delivery of specific drugs.

MOF Capacitates Cyclodextrin to Mega-Load Mode for High-Efficient Delivery of Valsartan

PURPOSE

To investigate the mechanism of enhancing solubility and bioavailability of water-insoluble drug, valsartan (VAL), with being mega-loaded by cyclodextrin metal organic framework (CD-MOF).

METHODS

VAL was successfully mega-loaded into CD-MOF by magnetic agitation of VAL in ethanolic solution. Characterizations including powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), synchrotron radiation-based Fourier transform-infrared spectroscopy (SR-FTIR) ¹³C solid-state nuclear magnetic resonance spectroscopy ( ¹³C SS-NMR), nitrogen gas adsorption, and small-angle X-ray scattering (SAXS) were carried out to confirm the mechanism and incorporation behavior of VAL in CD-MOF. Ball milling process combined with molecular modeling was also used to confirm the mechanism. Improvement of bioavailability in vivo was confirmed by pharmacokinetic experiment in beagles.

RESULTS

As a carrier with payload 150% higher than conventional CD complexation, CD-MOF included molecules of VAL as complexations in the chambers of (γ-CD)₂, and nanoclusters in the confined spherical cages of (γ-CD)₆ confirmed by SAXS and ¹³C SS-NMR. Ball milling combined with molecular modeling inferred that the reduced release rate of the milled CD-MOF with ultrahigh drug payload was mainly due to the partial aggregation of the VAL nanoclusters. The molecules of VAL as nanoclusters in the cages of (γ-CD)₆ are critical in dramatically improving the apparent solubility (39.5-fold) and oral bioavailability (1.9-fold) of VAL in contrast to γ-CD inclusion.

CONCLUSIONS

The new understanding of drug nanoclusters in CD-MOF will help to design more efficient drug delivery systems using CD-MOF carrier with nanocavities.

Synthesis of catalytically active bimetallic nanoparticles within solution-processable metal–organic-framework scaffolds

Bimetallic alloy nanoparticles are synthesized by in situ reduction of mixed metal ions inside CD-MOFs.