Applications of Cyclodextrins Modified/Coated Metal–Organic Frameworks

The introduction of cyclodextrins (CDs) via noncovalent bonds could improve various physiochemical properties of metal-organic frameworks (MOFs) and expand their practical applications in aqueous solutions, for example, biocompatible CDs could improve the solubility of CDs-coated MOFs as a hybrid delivery system. And external-stimuli responsiveness of CDs-coated MOFs is key for widening applications in controlled drug releasing system, catalysis, detection, extraction and separation.

could further combine with MOFs, such as loading CDs on the outer surface of MIL-100(Fe) via the phosphate-Fe bonding [2,3] and leading to a significant decrease of macrophage uptake ( Figure 1). Particularly, the tetraethylene glycol-bearing β-CDs phosphate (H1) modified MOFs exhibit remarkably enhanced binding affinity towards the specific mannose receptor, i.e., Concanavalin A, due to the possession of mannose moieties, reduced macrophage internalization and multivalent effects based on the scaffold of β-CDs. MOFs, indicating that hybrid materials have penetrated into macrophase and affected its functions. Cells were stained in green with calcein, the cell nucleus is in black, and the rhodamine-B-labeled hybrid materials exhibited red signals. The differences of images (a,b,c) of the same cells caused by three different heights above the glass slide including 2.3, 4.9 and 7.6 µm, respectively. Scale bar = 5 µm. Reproduced from Reference [2] with permission from the Nature Publishing Group, copyright 2015.
CDs-modified MOFs can be used for delivering and releasing drugs. In the first step, CDs mainly perform as a cap to stop the release of loaded drugs, for example, tetraethylene glycol functionalized β-CDs (H2) can further be coated on the outer surface of DOX-loaded MIL-100(Fe) without affecting the payload of the nanocomposite [3]. In the second step, removing CDs from hybrid materials will lead to the release of loaded drugs, and different CDs coated on the surface of inorganic materials have different performances, for example, the effect of releasing the tritium-labeled antiretroviral drug, AZT-PT by CDs-coated MOFs is estimated by scintillation counting [4], and reveals that polymeric β-CDs (such as H3) modified MOFs afford a better control of drug release over monomeric β-CDs (H4 and H5) coated ones, due to the presence of nonmodified CDs moieties in the structure and interacting with drugs. Although it is known that AZT has interactions with native and primary face-modified β-CDs, polymeric β-CDs-modified MOFs can increase drug retention in 13%.
Particularly, the external stimuli-responsive release of drugs can be achieved by the employment of CDs-coated MOFs.
For example, external stimuli-responsive molecules such as azobenzene can be introduced into the hybrid materials such as the water-stable, biocompatible and degradable Zr-MOF, then act as the switch tool to associate/disassociate with coated β-CDs in accordance with different light irradiation conditions and, finally, control the performance of thus obtained integrated hybrid organic-inorganic materials, which have already stored drug models such as rhodamine B [5].
Additionally, the "gatekeeper", β-CDs, can also be removed to release cargos inside the UiO-68-azo-bearing Zr-MOFs upon the addition of competitive guests, e.g., the drug for Parkinson's disease, amantadine [5]. Thus, this kind of β-CDscoated Zr-MOFs might be used as a platform for on-command drug delivery systems (Figure 2). Additionally, the organic-inorganic hybrid material, β-CDs/Fe O @SiO @MIL-100(Fe) [6], can be used as a pHresponsive drug delivery system due to the possession of pH-sensitive β-CDs, which is unstable and easily hydrolyzed at lower pH solutions. It is found that the drug model, cephalexin is released by this system slower under physiological conditions than the acidic buffer solution, indicating that the release happens in a controlled manner. Furthermore, it is observed that increasing temperature enhanced the drug release percentage, due to the shrinking of temperaturesensitive polymers inside hybrid materials ( Figure 3). However, the coating of the organic layer onto the surface of Fe O @SiO @MIL-100(Fe) results in a decrease in magnetic responsivity, and the magnetic responsiveness of this hybrid material was not explored directly upon releasing drugs. The employment of CDs is a very good control strategy for tuning MOFs to apply in the catalysis area. For example, with the assistance of host-guest complex between β-CDs and L1 as assembly agents, UiO-66-NH loaded polypropylene performs well in catalytic degradation of dimethyl 4-nitrophenyl phosphate, a chemical warfare agent simulant, with a halflife of less than 5 min [8].
Additionally, the CDs-modified MOFs hybrid materials also reveal good catalytic behavior, due to the possession of the large surface area of MOFs as well as the synergistic effect between MOFs and CDs, for example, the nanocomposite, MOF-235/β-CDs has catalytic activity for the hydrogen peroxide-luminol system, and can enhance the chemiluminescence response with more than 30-fold in comparison with that of hydrogen peroxide-luminol system