The γ-cyclodextrin (γ-CD) metal–organic frameworks (CD-MOF-1) consist of γ-CD and potassium (K+) ions through coordinating an eight-coordinated K+ ion with two C5-linked oxygen and C6-linked hydroxyl (C5–O/C6–OH) groups in the primary faces of adjacent γ-CD units and two C2- and C3-linked hydroxyl (C2–OH/C3–OH) groups in the secondary faces. Herein, we found polysaccharide gels with only C2–OH/C3–OH or C5–O/C6–OH groups in pyranoid rings can form four-coordinated K+ ions and then coordinate γ-CD in a KOH solution for CD-MOF-1 growth. Exposure of C2–OH/C3–OH or C5–O/C6–OH groups in polysaccharide gels is important to form active four-coordinated K+ ions. Mechanism supporting this work is that four-coordinated K+ ion sites are first formed after coordinating C2–OH/C3–OH groups in pectin and then coordinating C5–O/C6–OH groups in the primary faces of γ-CD units. Alternatively, four-coordinated K+ ions with C5–O/C6–OH groups in chitosan can coordinate the C2–OH/C3–OH groups in the secondary faces of γ-CD units. Mechanism of CD-MOF-1 growing on pectin and chitosan gels through the proposed four-coordinated K+ ions is also universally applicable to other polysaccharide gels with similar C2–OH/C3–OH or C5–O/C6–OH groups such as alginate gel. Based on this mechanism, we developed pectin and chitosan gel-based CD-MOF-1 composites and exemplified applications of them in antibacterial and organic dye removal. To help future research and applications of this mechanism, we share our theoretical assumption for further investigations that any matrices with an ortho-hydroxyl carbon chain or ortho-hydroxyl ether structures may form four-coordinated K+ ions for CD-MOF-1 growth. The proposed mechanism will broaden the development of novel CD-MOF-1 composites in various fields.
He Wanjun,Lv Danyu,Wu Qingxiao,Xu Huanyu,Guan Yongguang,Zeng Xin-An
Litchi
( L. chinensis Sonn.) is a
perishable fruit that is vulnerable to oxidative browning and fungal
infection. Herein, ferulic acid and natamycin were chosen as the antioxidant
and antimycotic agents, respectively. To improve their water solubility,
we used CD-MOF-1 to encapsulate ferulic acid. Meanwhile, natamycin
was encapsulated with gum arabic and pullulan. The most homogeneous
and stable natamycin nanocapsules were fabricated through stirring
0.5 mg/mL natamycin using 2.5% (w/v) gum arabic-pullulan (1:1, w/w,
1.25% each) in water. The main encapsulation mechanisms of ferulic
acid and natamycin were electrostatic interactions between the carboxyl
groups of ferulic acid and K + ions from CD-MOF-1 and hydrophobic
interactions between the macrocyclic lactone ring of natamycin and
the protein part of gum arabic, respectively. In addition, pullulan
was indispensable for stabilizing natamycin. The antioxidant and antifungal
composite sol presented 99.85% oxidation inhibition and 79.06% mycelial
growth inhibition and effectively delayed the oxidative browning and
fungal decay on litchi pericarps.
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