Effect of potassium salts on the structure of γ‐cyclodextrin MOF and the encapsulation properties with thymol

Abstract

BACKGROUND: Thymol is a natural essential oil with strong volatility, low solubility, poor dispersion, strong irritation, and an unpleasant smell, which often requires appropriate porous materials to encapsulate thymol during the application process. However, the encapsulation efficiency of thymol in inclusion complexes is low, and new methods of encapsulation need to be developed. In the present study, the encapsulation capacity, storage stability, and antibacterial activity of thymol were investigated using γ-cyclodextrin (γ-CD) metal-organic frameworks (MOFs) by cocrystallization and high-temperature adsorption methods. The effect of different potassium salts (i.e. KOH, KCl, and KAc) on the structure and complexation of γ-CD-MOFs was also analyzed.RESULTS: Compared with γ-CD, the thymol encapsulation capacity of γ-CD-MOFs was increased by two-to three-fold, with the encapsulation content following the order: KAc-γ-CD-MOF (293.8 mg g -1 ) > KOH-γ-CD-MOF (287.7 mg g -1 ) > KCl-γ-CD-MOF (249.3 mg g -1 ). The anions in the solution participate in the coordination and influence the symmetry relationship between atoms and ions. This explains the differences in both the three-dimensional γ-CD-MOF structure and the thymol encapsulation amount, as well as the high storage stability of thymol. CONCLUSION: The in vitro release kinetics and antibacterial experiments showed that the inclusion complexes prepared by γ-CD-MOFs had higher stability, sustainability, and antibacterial activity, which suggests that it is an excellent complex material for industrial and agricultural applications.

Controlled Release of Thymol by Cyclodextrin Metal-Organic Frameworks for Preservation of Cherry Tomatoes

“…Presumably, γ-CD-MOF is more suitable for thymol encapsulation due to its matched cavity diameter with the dimension of this aroma molecule. Our previous study has found that γ-CD-MOF (prepared by KOH) showed regular cubic crystalline structures with smooth and particle surfaces [ 31 ]. Generally, the hydrophobic cavity of cyclodextrin and newly formed pores between cyclodextrin molecules are two types of cavity structures that exist in CD-MOF crystals.…”

“…The cavity size of α-, β-, and γ-CDs increases gradually, and the newly formed cavity size is closely related to the length of the K−O bond and the arrangement of CD molecules [ 42 ]. Studies have shown that the monoclinic space group crystallizes α-CD-MOF and β-CD-MOF, and the diameter of the intermolecular cavity is about 0.5 nm [ 29 , 42 ], while γ-CD-MOF is crystallized in a cubic space group with high symmetry [ 31 ], and cavities are found in the structure with diameters of 1.7 nm, 1.0 nm, and 0.4 nm [ 31 , 41 ]. Thymol binds to CD-MOF through hydrophobic interactions and van der Waals forces [ 30 ].…”

“…Thymol binds to CD-MOF through hydrophobic interactions and van der Waals forces [ 30 ]. CDs with larger cavities provide the aroma with enough space to enter in, which increases the binding probability of thymol combining with CD-MOF [ 31 ]. In contrast, the smaller cavities of CDs hinder thymol molecules from entering the hydrophobic cavity.…”

“…Moreover, compared with γ-CD, the thymol encapsulation capacity of γ-CD-MOFs generated by potassium salts was increased by two- to three-fold. It was considered that anions in the preparation solution led to different three-dimensional CD-MOF structures by affecting the symmetry relationship between ions and atoms, thus resulting in different thymol encapsulation contents [ 31 ]. Further investigations on release kinetics also revealed that β-CD-MOF provided menthol with better controlled-release behaviors [ 30 ].…”

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

“…Hydrothermal synthesis 2. Crystal transformation Valsartan 33.5% 30.2% Ding et al ( 2019 ) CD-MOF γ-CD KOH K + Vapor diffusion Scutellarin 30.42% Zhao et al ( 2020b ) γ-CD-MOF γ-CD KNO 3 K + Vapor diffusion Menthol ~ 11% Hu et al ( 2021 ) KAc-γ-CD-MOF-THY-HT KOH-γ-CD-MOF-THY-HT KCl-γ-CD-MOF-THY-HT γ-CD KAc KOH KCl K + Hydrothermal Thymol 293.8 mg/g 287.7 mg/g 249.3 mg/g Pan et al ( 2022 ) β-CD-MOF β-CD Na 2 C 2 O 4 Na + Solvothermal 5-Fluorouracil 23.02% Lu et al ( 2015 ) β-CD-MOF-1 β-CD-MOF-2 β-CD CsCl Cs + Template-induced approach 5-Fluorouracil Methotrexate 5-Fluorouracil Methotrexate 1.379 g/g 0.689 g/g 1.510 g/g 1.217 g/g Liu et al ( 2017c ) β-CD-MOF β-CD KOH K + …”