CD-MOFs Crystal Transformation from Dense to Highly Porous Form

May 31, 2019 - Transformation of a dense metal–organic framework (MOF) to a highly porous ... and characterizations of powder X-ray diffraction (PXR...
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Article Cite This: Cryst. Growth Des. 2019, 19, 3888−3894

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CD-MOFs Crystal Transformation from Dense to Highly Porous Form for Efficient Drug Loading Huanyu Ding,†,‡,# Li Wu,‡,# Tao Guo,‡,# Zaiyong Zhang,‡ Bello Mubarak Garba,‡ Ge Gao,‡ Siyu He,‡ Wei Zhang,‡ Yizhi Chen,∥ Yangjing Lin,∥ Hewen Liu,*,⊥ Jamshed Anwar,*,§ and Jiwen Zhang*,‡ †

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China Center for Drug Delivery Systems, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China ∥ Hainan Hualon Pharmaceutical Co., Ltd, Haikou 570311, China ⊥ CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China § Chemical Theory and Computation, Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom

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S Supporting Information *

ABSTRACT: Transformation of a dense metal−organic framework (MOF) to a highly porous form can radically improve its applications in drug loading. In this study, an environmentally friendly synthesis of potassium acetate γcyclodextrin metal−organic framework (γ-CD-MOF) in water was identified as a dense crystal form. Importantly, the molecular arrangement of the dense γ-CD-MOF was confirmed by single crystal X-ray diffraction and other characterizations. If the dense γ-CD-MOF was directly dried after separation from the mother solution, it is incapable of loading a model drug. However, the fresh dense crystal could be transformed into a highly porous form by introducing ethanol. The crystal transformation was demonstrated by enhanced drug loading capability and characterizations of powder X-ray diffraction (PXRD), small-angle X-ray scattering (SAXS), and the N2 adsorption isotherm. In all, the crystal transformation from dense to highly porous form could significantly facilitate the applications of γ-CD-MOFs in drug loading and other potential fields.

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INTRODUCTION As emerging versatile materials, metal organic frameworks (MOFs) are well recognized for their chemical properties of coordination crystal1−3 and molecular architecture features of high porosity.4 With highly ordered cellular-like porous structure, MOFs have been applied in catalalysis,5 drug delivery,6 environmental fields,7 and so on. It has been reported that there are three types of crystal transformation for MOFs, namely, rotation of bond angles,8 transformation through vapor sorption/desorption,9 and substitution of metal ions.10 These transformations caused alterations reflecting in extensiveness like coordination number, space group, color, and magnetism.9 Different combinations based on the same metal ions and organic struts brought varied structures and functions. For example, both generated by tetratopic carboxylate linkers and Zr6 cuboctahedral SBUs, NU-901 and NU-1000 had different network topologies and showed a difference in nitrogen isotherms and pore size distributions.11 As a biocompatible material, potassium hydroxide-γ-cyclodextrin-MOFs (KOH-γ-CD-MOFs) was first defined as a porous, body-centered cubic (BCC) array of (γ-CD)6 units, built up by γ-CD and K+ ions in 2010.12 In recent years, © 2019 American Chemical Society

applications of KOH-γ-CD-MOFs have drawn some attention, including gas storage13 like formaldehyde,14 carbon dioxide,15,16 and sulfur hexafluoride,17 adsorption of nongaseous drugs/materials of lansoprazole,18 ibuprofen,19 fluorescein, and rhodamine B.20 Although types of CDs, e.g., α-CD, β-CD, and γ-CD, coordinated with different metal ions like potassium and sodium could form different structural arrangements, only BCC array of (γ-CD)6 units for KOH-γ-CD-MOFs has been reported at present.21 This study will provide another arrangement of γ-CD and potassium acetate (KAc) as KAc-γ-CD-MOFs, which is densely packed. Through ethanol induction, the KAc-γ-CDMOFs could transform to a highly porous form, the same as KOH-γ-CD-MOFs with BCC array of (γ-CD)6 units. The original preparation strategy, namely, the vapor-diffusion method reported by Stoddart et al.,12 has been applied to produce KOH-γ-CD-MOFs with long crystal growth period (∼7 days), extensive involvement of methanol, and low yield. In our previous studies, the preparation was optimized by Received: March 11, 2019 Revised: April 29, 2019 Published: May 31, 2019 3888

DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894

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and the energy was calculated by Amber 12 in which the force field was constructed by Gaussian 09. Drug Loading Investigation on Dense KAc-γ-CD-MOFs and Porous KAc-γ-CD-MOFs. Valsartan (6.25 g) was placed in a 50 mL conical flask with 25 mL absolute ethanol, to make a solution of 250 mg/mL concentration, which was then sonicated in ultrasonic bath. Two groups were formed (dense KAc-γ-CD-MOFs and porous KAcγ-CD-MOFs), each containing 1000 mg of dense/porous KAc-γ-CDMOFs dispersed in ethanolic solution of valsartan (molar ratio of valsartan:KAc-γ-CD-MOFs was set as 22:1). The solution was stirred at 400 r/min and 40 °C for 24 h. After the reaction, the solution was filtered by a Brinell funnel, using a microporous membrane of 0.22 μm. Filter cake was collected by suction filtration and dried at 60 °C for 4 h in a precision blast dryer, and the powder of valsartan/MOFs was obtained. The loading was processed in triplicate. Determination of Valsartan Loadings in KAc-γ-CD-MOFs. Valsartan loaded KAc-γ-CD-MOFs powder of 5 mg was placed in a 50 mL volumetric flask, and the mixed solvent (ethanol:water = 1:1, v/v) was added to make constant volume to the tick mark. After filtration by a 0.45 μm microporous membrane, a ultraviolet spectrophotometer was used to determine drug loading concentrations at a wavelength of 250 nm. Drug loading was defined as the amount of drug (mg) in 100 mg of γ-CD-MOFs, as determined by UV. Characterizations. The scanning electron microscope (SEM, S3400, Hitachi) was used to determine morphology of KAc-γ-CDMOFs crystals. The samples were fixed on double-sided adhesive tape which had been pasted at a metal stub and coated with gold. Polarization properties of specimens were observed by a polarizing microscope (DYP-990). A little crystal in mother solution was dripped onto a glass slide and dispersed by a coverslip. After adjusting the focal length, light intensity, and polaroid angle, the polarized images were captured. The crystallinity of the specimens was characterized by powder Xray diffraction (PXRD). All the prepared γ-CD-MOFs crystals were irradiated with monochromatized Cu Kα radiation and their diffraction patterns were detected with a Bruker D8 Advance diffractometer (Bruker, Germany) under tube voltage of 40 kV and tube current of 40 mA in a stepwise scan mode (8°/min). Eligible crystals were selected through an optical microscope for Xray crystallography. A Bruker D8 Venture single-crystal diffractometer (Cu Kα, λ = 1.54178 Å) was employed to collect all data. The corresponding data were gathered by Bruker APEX3 program and simplified by Bruker SAINT program. Absorption correction was executed by multiscan method which was implemented in SADABS. The structure was figured out by direct methods with SHELXT program and refined by least-squares methods with the SHELXL2014 program contained in the OLEX2 suite. The non-hydrogen atoms were identified by the SHELXT program directly and refined with anisotropic displacement parameters. The hydrogen atoms were located from the difference Fourier map inspection and refined by the formula: Uiso(H) = 1.5Ueq(N, O). Nitrogen adsorption−desorption isotherm was measured by a porosimeter (Micromeritics ASAP 2020, USA) with a liquid nitrogen bath (−196 °C). The samples were activated by soaking in dichloromethane for 3 days and dried under vacuum at 50 °C for 12 h to remove the interstitial impurities.

microwave method22 and solvothermal method17 that both shortened the reaction time (∼20 min). Yet, the preparations still have drawbacks in view of environmental protection23,24 and production efficiency; e.g., all methods used methanol to help crystal growth and separation in the mother solution. The discovery of hydrothermal synthesis in this study provided a novel production strategy of CD-MOFs. Herein, the new method included two steps, namely, (1) hydrothermal synthesis via crystallization caused by fast temperature decrease using a controlled ratio of γ-CD and potassium acetate. The fresh crystal KAc-γ-CD-MOFs was dense. (2) Crystal transformation from dense status to highly porous triggered by washing with ethanol, which had never been reported in the literature before (Figure S1). In comparison to reported strategies, this new strategy enhanced the yield to several folds in water with pH of 6.5−7.5 without using surfactant. The discovery of the crystal transformation of CD-MOFs under ethanol induction is of interest, as it unmasked the mechanism for reshaping the molecular coordination of the crystal building blocks.

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EXPERIMENTAL SECTION

Materials. Potassium acetate (KAc), methanol (MeOH), ethanol (EtOH), acetone (CH3COCH3), ethyl acetate (CH3COOC2H5), and hexane (C6H14) were all analytical grade and provided by Sinopharm Chemical Reagent Co., Ltd. (China). The γ-cyclodextrin (γ-CD) of pharmaceutical grade was purchased from Maxdragon Biochem Ltd. (China). Water was purified by passing through a reverse osmosis unit and then a Milli-Qs reagent water system. All other chemicals were of analytical grade and used without further purification. Synthesis of Dense KAc-γ-CD-MOFs and Porous KAc-γ-CDMOFs. The γ-CD of 10240 mg and KAc of 6200 mg were dissolved in 40 mL distilled water at 70 °C and filtered through a 0.45 μm microporous membrane into a conical flask. After incubating at 25 °C for 6 h, dense KAc-γ-CD-MOFs precipitated. When 40 mL ethanol was added into the precipitate of dense KAc-γ-CD-MOFs, crude porous KAc-γ-CD-MOFs formed. After washing two times with 40 mL ethanol and drying at 60 °C in ventilated drying oven for 4 h, pure porous KAc-γ-CD-MOFs was collected (the latest technology of hydrothermal synthesis could produce 422 g porous K-γ-CD-MOFs per liter of mother solution, and its yield increased by 19 times compared with solvothermal synthesis,17,25 for which the productivity was only 22.5 g/L). Observation of Crystal Transformation. Because the process of transformation was too rapid to adjust focal distance under the microscope, video coverage was used for capturing a pictorial cascade of events that occurred during the transformation process from dense KAc-γ-CD-MOFs to porous KAc-γ-CD-MOFs. Briefly, a drop of mother solution containing a small number of particles of dense KAcγ-CD-MOFs was added onto a glass slide. It was then covered by a coverslip to help disperse and immobilize the crystal particles, while the video started to record. Ethanol was dropwise added onto the slide and allowed to permeate through the space between microslide and coverslip gap. Synthesis of Dense KAc-γ-CD-MOFs Single Crystal. The single crystal of dense KAc-γ-CD-MOFs was obtained by reacting γCD (10240 mg) with KAc (6200 mg) in 40 mL distilled water, then heated at 70 °C in water bath until it was dissolved. The mother solution was filtered into the culture dish using a 0.45 μm microporous membrane, then frozen at −80 °C for 20 min. After incubation at 25 °C for 30 min, the single crystal was harvested. Calculation of Electrostatic Potential (ESP) Charge and Energy. The crystal structure of porous KAc-γ-CD-MOFs model was extracted from the reported single crystal structure of γ-CD-MOFs21 and the crystal structure of dense KAc-γ-CD-MOFs was extracted from our crystal data. The ESP charge was calculated by Gaussian 09

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RESULTS AND DISCUSSION Crystal Transformation. Pictures of the events were captured in different time intervals (Figure 1). Initially, the crystals with polarization (appearing as yellow and light blue) were about 50 μm × 5 μm columnar form (Figure 1a). After 10 s, the crystals that first contact ethanol transformed to diminutive cubic form rapidly (Figure 1b, as shown by a red box). At 20 s, a large proportion of columnar crystals had disappeared; however, more cubic crystals formed (Figure 1c). Approximately 30 s later, no columnar crystals could be found, 3889

DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894

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drying might result in some problems in which dense KAc-γCD-MOFs will dissolve in the remaining water adsorbed in wet particles and bring some changes in the detection result. To keep its complete structure, freeze-drying was used to remove residual moisture. Afterward, the sample was measured for its capacity for N2 adsorption. Figure 2b,c shows very low N2 uptake and incremental volume, which implied that dense KAc-γ-CD-MOFs was nonporous, quite different from the reported γ-CD-MOFs. Influence of Drying on Dense KAc-γ-CD-MOFs. Interestingly, a fascinating phenomenon was observed during the drying process of dense KAc-γ-CD-MOFs, indicating the role of water molecules in shaping the crystal molecular arrangement. After removing the water molecules by freezedrying, dense KAc-γ-CD-MOFs was transformed from nicepolarized-light columnar crystal to dim-amorphous shape (amorphism KAc-γ-CD-MOFs), and this disordering process (Figure 3a,b) was confirmed by XRD analysis in Figure 3c. If amorphic KAc-γ-CD-MOFs was dispersed in the aqueous mother solution, it could transform back to dense KAc-γ-CDMOFs (Figure S3). Even if dense KAc-γ-CD-MOFs was unstable, it can be stored in its amorphous state for a long time. Using hot stage polarized light microscopy to observe, there was a detailed event in the drying process (Figure 3d,f) in which dense KAc-γ-CD-MOFs changed into helix-form shaped crystal under 40 °C. This instability of dense KAc-γ-CD-MOFs could bring new functions like moisture sensitivity sensors or preparations for specific shapes. Furthermore, amorphism KAc-γ-CD-MOFs could also transform to porous KAc-γ-CDMOFs by induction of ethanol which will be proven as follows. Confirmation of Crystalline Transformation. The structure of crystals transformed from dense KAc-γ-CDMOFs was confirmed by XRD, SEM, small-angle X-ray scattering (SAXS), and the N2 adsorption isotherm. In these experimental results, KOH-γ-CD-MOFs were synthesized according to Liu’s article,22 and its single-crystal information obtained from Cambridge crystallography database (CCDC number 773709). It was obvious that all XRD characteristic peaks of the crystals (porous KAc-γ-CD-MOFs, KOH-γ-CDMOFs, and simulated KOH-γ-CD-MOFs) were on the same angle which confirmed they possess identical crystal form (Figure 4a). The SAXS spectrum illustrated that both KAc-γCD-MOFs and KOH-γ-CD-MOFs had the identical indices of crystal faces, which meant K+ ions and γ-CD in the two types of crystals were arrayed according to the same molecular arrangement.25 SEM image of porous KAc-γ-CD-MOFs (Figure 4c) showed the same cubic shape with KOH-γ-CDMOFs (Figure 4d). The N2 uptake ability and pore width confirmed porous KAc-γ-CD-MOFs to be a highly porous material similar to KOH-γ-CD-MOFs (Figure 4e,f,g,h). All evidence pointed to one fact, that porous KAc-γ-CD-MOFs should be the BCC array of (γ-CD)6 units. Mechanism of Crystal Transformation. To ascertain how the matters (crystal molecules and organic phase) interact at transformation step, different washing strategies were adopted to deduce the mechanism. Aqueous dense KAc-γCD-MOFs (referred to as group dense KAc-γ-CD-MOFs) and dried particles of dense KAc-γ-CD-MOFs (referred to as group amorphism KAc-γ-CD-MOFs) were washed by methanol, ethanol, acetone, ethyl acetate, and hexane, respectively. As shown in Figure 5, crystal transformation occurred after washing with methanol, ethanol, and acetone for group dense KAc-γ-CD-MOFs. However, in group amorphism KAc-γ-CD-

Figure 1. Microscope snapshots of columnar crystals transformed into cubic crystals with introduction of ethanol. a, b, c and d were obtained after interval of every nearly 10 s.

and the particle size of the formed cubic crystals gradually increased (Figure 1d), for which the size was measured to be about 5−25 μm. Interestingly, when the cubic crystals were returned into the aqueous mother solution of γ-CD and potassium acetate, it could transform back to the columnar form. Affirmation of dense KAc-γ-CD-MOFs. For exploring the nature of transformation, a single crystal X-ray diffraction analysis of dense KAc-γ-CD-MOFs was carried out. Upon comparing simulated XRD results of dense KAc-γ-CD-MOFs and KOH-γ-CD-MOFs in Figure 2a, it was obvious that dense KAc-γ-CD-MOFs was different from KOH-γ-CD-MOFs.12,21 A similar preparation method without KAc or γ-CD was adopted to check which ingredient was not required for crystal formation, and both had no products, which meant the new crystal must consist of K+ ions and γ-CD. Normal heating in

Figure 2. Confirmation of new crystals with no porosity by XRD and BET results. (a) XRD results of dense KAc-γ-CD-MOFs’ single crystal and KOH-γ-CD-MOFs’ single crystal proved that they were different crystals. (b) Nitrogen adsorption isotherm and (c) the corresponding pore size distribution showed nonporosity of dense KAc-γ-CD-MOFs. 3890

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Figure 3. Loss of moisture leads to destruction of crystal. (a) Microscope image showed polarization property of dense KAc-γ-CD-MOFs crystal. (b) Polarization was destroyed after drying. (c) XRD crystallinity patterns verified the destructive effect of drying on dense KAc-γ-CD-MOFs. (d), (e), and (f) showed the degree of damage over time to crystal after being dried.

bonds with average distribution on four γ-CDs, which were subordinate to three lattices. It was indicated that the transformation from dense KAc-γ-CD-MOFs to porous KAcγ-CD-MOFs was the result of structure re-establishment at the molecular level. Figure 6b illustrated the packing arrangement of dense KAc-γ-CD-MOFs. γ-CD and K+ ions constituted a wavy plane by the coordinate bond, and the planes overlapped together to form a columnar crystal without any linkage. This peculiar structure confirmed that close packing resulted in nonporous structure; on the other hand, the nonlinking between the planes ensured that every plane could be separated, implying a possibility to craft another form of KAc-γ-CD-MOFs based on the bond coordination arrangement as shown in Figure 5b. Molecular simulation showed that the single point energy of dense KAc-γ-CD-MOFs was −401 kcal/mol, higher than that of the porous KAc-γ-CD-MOFs (−488 kcal/mol). It was obvious that the dense KAc-γ-CD-MOFs was crystalline in transition state, and more energy had been released to form the porous KAc-γ-CD-MOFs. On the other hand, the calculation results showed that the electrostatic potential of −OH group in C2 for γ-CD was −0.48 (-O) and +0.34 (-H), respectively, and in C6 was −0.57 (-O) and +0.41 (-H), respectively. And the electrostatic potential of −OH group in ethanol was −0.59 (-O) and +0.36 (-H), respectively. The crystal structure in this study showed that it mainly depended on the electrostatic interaction of −OH group between C2 and C6 of γ-CD to form the dense KAc-γ-CD-MOFs. The ethanol molecule would occupy the interaction sites of −OH groups in C2 and C6. Therefore, the existence of ethanol in this system would make it difficult to form the dense KAc-γ-CD-MOFs. Drug Loading Capacity of Dense KAc-γ-CD-MOFs and Porous KAc-γ-CD-MOFs. The drug loading efficiency of

MOFs, there was no crystal transformation after washing with acetone. Intuitively, the dissolubility of the KAc-γ-CD-MOFs’ building substances was checked in Table S1, and acetone should be focused as the key point of contrast because it had different results in different groups; then the underlying logic emerged. In group dense KAc-γ-CD-MOFs, these organic solvents, which possessed the same result as acetone, showed similar solubility pattern to acetic acid (CH3COOH), γ-CD, and water. Similarly, in group amorphism KAc-γ-CD-MOFs, these organic solvents, which possessed the same result as acetone, showed the same solubility as CH3COOH, γ-CD and KAc. By removing identical properties which could not bring different results, the solubility of KAc in organic solution (miscible with water), with or without water, became the key to realizing transformation. However, considering the influence coming from different sizes of the organic reagent molecule or other factors like the acetate radical being able to react with water to generate CH3COOH, it was difficult to affirm that elution of KAc or acetate radical by organic solvent resulted in transformation. Therefore, one possible deduction is that the organic molecule broke part of the coordinate bond, activated K+ ions metal ligand, changed the link mode, and resulted in the formation of a new molecular arrangement. Molecular Analysis. From the result of single crystal XRD, the lattice of dense KAc-γ-CD-MOFs belonged to the orthorhombic system with 90° vertex angles, and the lengths of the three sides were 10.906, 22.920, and 30.793 Å, respectively. In dense KAc-γ-CD-MOFs (Figure 6a), it was illustrated that every K+ ion had eight coordinate bonds distributed on three γ-CDs by three, three, and two links, respectively. Porous KAc-γ-CD-MOFs’ data was substituted by KOH-γ-CD-MOFs owing to the same BCC array of (γ-CD)6 units, and it showed that every K+ ion had eight coordinate 3891

DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894

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Figure 4. Crystal transformation from nonporous to highly porous was confirmed by XRD, SEM, SAXS, and BET. (a) XRD results of γ-CD-MOFs produced by three kinds of process paths. (b) SAXS results of porous KAc-γ-CD-MOFs and KOH-γ-CD-MOFs. (c,d) SEM images of porous KAcγ-CD-MOFs and KOH-γ-CD-MOFs. Nitrogen gas adsorption isotherms of (e) porous KAc-γ-CD-MOFs and (f) KOH-γ-CD-MOFs. Corresponding pore size distributions of (g) porous KAc-γ-CD-MOFs and (h) KOH-γ-CD-MOFs.

Figure 5. PXRD profiles showed different crystal transformations for dense KAc-γ-CD-MOFs and amorphism KAc-γ-CD-MOFs.

dense KAc-γ-CD-MOFs and porous KAc-γ-CD-MOFs was investigated for their applications. Based on the cavity from structure, it was unquestionable that the carrier ability has existed in porous KAc-γ-CD-MOFs, as well as in dense KAc-γCD-MOFs, because it would transform to porous KAc-γ-CDMOFs in ethanol at drug loading step even if dense KAc-γ-CDMOFs were nonporous. To verify whether this crystal transformation process influenced the capability of loading drugs, valsartan was selected as the model drug because of its

high solubility in ethanol, poor solubility in water, suitable molecular size, and functional groups. The similar drug loadings of dense KAc-γ-CD-MOFs as 33.5 ± 1.5 wt % and porous KAc-γ-CD-MOFs as 30.2 ± 1.3 wt % indicated that the drug loading capacity of dense KAc-γ-CD-MOFs in crystal transformation process was higher than that of porous KAc-γCD-MOFs. Considering conversion efficiency and moisture content, it was obvious that dense KAc-γ-CD-MOFs might possess lower carrier ability compared to porous KAc-γ-CD3892

DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894

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Accession Codes

CCDC 1902341 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-551-63607780. *E-mail: [email protected]. Tel: +44-1524-592392. *E-mail: [email protected]. Tel: +86-21-20231980. ORCID

Hewen Liu: 0000-0002-4596-7798 Jamshed Anwar: 0000-0003-1721-0330 Jiwen Zhang: 0000-0001-8478-8621 Author Contributions #

J.Z., J.A. and L.W. designed experiments and manuscript. H.D. and M.G.B. performed experiments and drafted the manuscript. All the coauthors discussed and prepared the manuscript. H.D., L.W., and T.G. contributed equally to the manuscript.

Figure 6. (a) Crystal transformation from dense to highly porous structure (octagon represented γ-CD, and gray annulus were out of lattice. Balls represented K+ ions). (b) Spatial arrangement of dense KAc-γ-CD-MOFs (annulus represented γ-CD, different colorsblue, red, and greenmeant different layers without linking. K+ ions were represented by blue balls).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the Project funded by the National Science and Technology Major Projects for the Major New Drugs Innovation and Development (2018ZX09721002-009), Strategic Priority Research Program of Chinese Academy of Sciences (XDA12050307), and National Natural Science Foundation of China (81430087, 81803441).

MOFs in speculation, but the reverse in fact. That might be from the following two reasons: For porous KAc-γ-CD-MOFs, cavities were occupied by air at drying step that completely stopped the touch of valsartan on crystal and impeded medicine carrying. For dense KAc-γ-CD-MOFs, the molecular arrangement was destroyed and recombined to form porous KAc-γ-CD-MOFs at the transformation step to ensure full contact of drug molecules on crystal and to be enclosed in γCD-MOFs unit.

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CONCLUSIONS This study highlighted the interesting technology of harvesting KAc-γ-CD-MOFs through saturation−precipitation properties of the building blocks in water, which offered additional safety by discarding the pernicious liquid waste disposal steps and increased productivity yield by dozen times more compared to the reported γ-CD-MOFs synthesis methods. From the point of transformation, this process was a discovery in the aqueous state of KAc-γ-CD-MOFs and revealed its fast turnability with insights into transforming from nonporous dense KAc-γ-CDMOFs into highly porous KAc-γ-CD-MOFs for drug loading and other applications.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.9b00319. Three production strategies of γ-CD-MOFs, SEM of samples after washing, transformation process, and PXRD of porous KAc-γ-CD-MOFs for stability investigation; different solubilities of relevant substances in different reagents (PDF) 3893

DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894

Crystal Growth & Design

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cages of CD-MOF: dramatic enhancement of solubility and bioavailability of azilsartan. Acta Pharm. Sin. B 2019, 9, 97−106.

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DOI: 10.1021/acs.cgd.9b00319 Cryst. Growth Des. 2019, 19, 3888−3894