Green Synthesis of Cyclodextrin-Based MetalâOrganic Frameworks

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Green synthesis of cyclodextrin-based metal–organic frameworks through the seed-mediated method for the encapsulation of hydrophobic molecules Chao Qiu, Jinpeng Wang, Yang Qin, Haoran Fan, Xueming Xu, and Zhengyu Jin J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00400 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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Journal of Agricultural and Food Chemistry

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Green synthesis of cyclodextrin-based metal–organic frameworks

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through the seed-mediated method for the encapsulation of

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hydrophobic molecules

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Chao Qiu,†,‡,§ Jinpeng Wang,†,‡,§ Yang Qin,†,‡ Haoran Fan,†,‡ Xueming Xu,†,‡,§ Zhengyu

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Jin*,†,‡,§

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†State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi,

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Jiangsu 214122, China

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‡School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122,

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China

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§Synergetic Innovation Center of Food Safety and Nutrition, Jiangnan University,

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Wuxi 214122, China

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*Corresponding

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[email protected] (Z. Jin)).

author:

Zhengyu

Jin

(Tel:/Fax:

86-51085913299;

Email:

1

ACS Paragon Plus Environment


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ABSTRACT: Metal–organic frameworks (MOFs) are attracting considerable

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attention because of their unique structural properties, such as a high surface area,

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highly porous topology, and tunable size and shape, which enable them to have

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potential applications as a new class of carriers for functional agent or drug delivery.

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However, most of the MOFs and the polymers used are not pharmaceutically

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acceptable. For the first time, this study successfully conducted the rapid synthesis of

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cyclodextrin metal–organic frameworks (CD-MOFs) through a facile and green

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seed-mediated method. The size control, crystal structure, and thermal properties of

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CD-MOFs with and without seeds were investigated. When 1 mg/mL seed was added,

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the size of γ-CD-MOF crystals decreased from 6.2±0.8 µm to 1.8±0.4 µm. The

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CD-MOFs synthesized though the seed-mediated method had higher crystallinity and

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thermal stability than those that were not. Furthermore, the CD-MOFs could

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encapsulate hydrophobic molecules, such as nile red (NR), which was chosen as a

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model, and the interaction mechanism between γ-CD-MOFs and NR was investigated.

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Results showed the formation of a 1:1 complex between NR and CD-MOFs,

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demonstrating the potential of these polymers as carriers for hydrophobic drug

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delivery applications.

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KEYWORDS: γ-cyclodextrin, delivery, interaction, cytotoxicity, thermal stability

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INTRODUCTION

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Metal–organic frameworks (MOFs) have attracted much attention because of

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their unique properties, such as a high surface area, highly porous topology, and

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tunable size and shape.1 The success in controlling the functionality and the structure

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of MOFs has led to numerous applications, including gas adsorption,2-4 separation,5

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catalysis,6,7 molecular recognition,8,9 and drug delivery.10-14 MOFs are constructed

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from metal ions and multitopic organic ligands through coordination bonds. However,

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most of the MOFs reported are composed of organic subunits derived from

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non-renewable petrochemical feedstocks and transition metals.15 The prospect of

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large-scale MOF applications also raises environmental concerns related to their

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disposal, that is, the quantities of metal released upon framework decomposition.16

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Estimated from the toxicology data LD50, some metals such as Ca, Fe, K, and Na are

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considered to be biologically acceptable.17 MOFs based on transition metals, such as

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Co, Ni, Cr, and Gd, among others, and/or non-food-grade organic linkers considered

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to be toxic are not acceptable for biological, food, and medicine applications.10 The

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search for bio-compatible MOFs for biological or drug delivery applications has

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driven the research toward nontoxic, renewable linkers.18 Herein, we report a strategy

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to overcome this problem using less toxic alkaline earth metals and γ-cyclodextrin

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(γ-CD), a food-grade oligosaccharide enzymatically synthesized from starch.19

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The size and the shape of MOF materials are critical for their various applications.

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Therefore, much effort has been made to shorten the synthesis time and to create

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smaller crystals using microwave-assisted, mechanochemical, and sonochemical

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methods.16,20 In Liu et al.,21 micron and nanometer-sized CD-MOFs were prepared by

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the microwave method for rapid and facile synthesis. However, the method is difficult

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in practice in cost-effective and scale-up production. At the same time, several

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strategies have been adopted for controlling the size and the morphology of MOFs by

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altering the synthetic parameters, such as solvent, molar ratio of reactants,

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temperature, and duration.12,21 However, these strategies do not provide MOFs with

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sufficient quality in relation to particle size and size distributions in a controlled

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manner. To obtain small crystals, a modified method with the addition of

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cetyltrimethylammonium bromide (CTAB) was reported to produce smaller-sized

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CD-MOFs.22 However, CTAB is toxic and has a risk of sample contamination.12 The

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emergent MOF commercialization is creating a need for developing innovative and

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green approaches for MOF synthesis.

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In this study, for the first time, the short linear chain nanoparticles (SNPs) from

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debranched starch were used as seeds to fabricate CD-MOFs because of their natural

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abundance in starch and their safety. To our knowledge, no reports have been made on

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the preparation of CD-MOFs based on SNP seeds by the seed-mediated method.

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Furthermore, the ultimate goal of a significant segment of research is to create

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CD-MOFs for the encapsulation of hydrophobic active ingredients or drugs. Thus, the

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hydrophobic molecule Nile Red (NR) was chosen as a model, and the interaction

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mechanism between γ-CD-MOFs and NR was investigated.

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MATERIALS AND METHODS

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Materials. γ-CD and NR were purchased from Sigma (Shanghai, China). Waxy

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cornstarch (WCS) were purchased from Sinopharm Chemical Reagent Co. Ltd.

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(Shanghai, China). All other chemicals were reagent grade.

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Synthesis of starch nanoparticle (SNP) seeds. The seeds of short linear chain

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SNPs were prepared according to the method described by Sun et al.23 Briefly, the

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10% gelatinized WCS were debranched with pullulanase (3.09×10-6 katal/g of dry

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starch) in pH 4.8 phosphate/citrate buffer solutions at 58 °C for 8 h. The obtained SNP

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solution was frozen at −80 °C and then freeze dried.

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Synthesis of γ-CD-MOFs. The γ-CD-MOF crystals were obtained by the

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modified vapor diffusion method. A mother solution was prepared by mixing γ-CD

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(324 mg) and KOH (112 mg) in pure water (10 mL), which was placed in a glass

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vessel. Then, a fixed quantity of SNP seeds (0, 5, 10, and 15 mg, respectively) was

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added. After ultrasonic dispersion, the solution was filtered through a 0.45 µm organic

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filter membrane into another glass tube, and 5 mL of absolute ethanol as an emulsifier

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was added. About 5 mL of MeOH was allowed to vapor-diffuse into this solution at

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50 °C for 6 h. The supernatant was then transferred into another glass tube, and a

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fixed quantity of MeOH was added drop-wise into the supernatant to trigger the

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precipitation of crystalline material, which was incubated at room temperature for 1 h.

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The MOF crystals were collected after separation, washed with 15 mL absolute

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MeOH twice, and dried overnight at 45 °C under vacuum.

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Characterization. The surface topography of samples was observed by scanning

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electron microscopy (SEM) using the method of Mijung et al.24 Samples were

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observed under a Geol scanning electron microscope (JEOL, 7500F, Japan). The

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particle size of γ-CD-MOF obtained from SEM images were measured using Image J

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1.45 software, accurately by choosing at least 100 particles from different SEM

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micrographs. The crystalline structures of samples were analyzed using a Bruker

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X-ray diffractometer (D2 PHASER, Bruker AXS Inc., Germany) following the

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method of Kiatponglarp et al.25 at 40 kV and 30 mA Cu-Ka radiations. Diffractograms

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were collected at room temperature from 3° to 40° (2θ) at a scan rate of 2°/min. TGA

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was conducted by means of synchronous thermal analysis (STA449C/4/G, Netzsch,

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Germany). Samples weighing 3–6 mg were heated from 30 °C to 600 °C at a heating

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rate of 10 °C min−1. FTIR spectra were recorded using a Nicolet IS10 spectrometer

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(ThermoNicolet Inc., U.S.A) equipped with a ATR accessory was used in the present

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work. The resolution was 4 cm–1 and the total number of scans was 32. The

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cytotoxicity of the γ-CD-MOF was investigated through cell viability tests using

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undifferentiated Caco-2 cells (Shanghai Institute of Cell Biology) following the

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method of Ji et al.26

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Encapsulation of NR. The inclusion experiments were conducted following the

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method described by Indirapriyadharshini et al.27 with some modifications. Briefly, a

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concentrated stock solution of NR was prepared in methanol. Small aliquots from this

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stock solution were added to Tris–HCl buffer solutions (pH 7.4) to give a final

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fluorophore concentration of 3.14 × 10-5 M. Different amounts of γ-CD-MOF crystals

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(0, 0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL, respectively) were added and sonicated to

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achieve a good dispersion, and then the samples were continuously stirred using a

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magnetic stirrer for 1 h at a constant rate of 500 rpm in a dark place. The interactions

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between γ-CD-MOFs and NR were measured using a fluorescence spectrophotometer

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(F-7000, Hitachi, Japan) at room temperature. The emission spectra were recorded

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between 575 nm and 750 nm at an excitation wavelength of 560 nm and a scanning

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speed of 1200 nm/min. The excitation and emission slit widths were 2.5 nm.

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Statistical analysis. All experiments were conducted at least thrice, and the mean

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values and standard deviations were determined. The experimental data were analyzed

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using analysis of variance (ANOVA) and were expressed as mean values ± standard

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deviations. Differences were considered at a significance level of 95% (P﹤0.05).

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Pearson’s correlation coefficients among the parameters were calculated using the

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Statistical Package for the Social Sciences (SPSS) version 17.0 software.

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RESULTS AND DICUSSION

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Morphology and particle size of SNP seeds and γ-CD-MOF. Morphology and

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particle size of the SNP seeds are presented in Figure S1. The synthesized SNP seeds

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displayed uniform spherical shapes with diameters of approximately 80 nm. The

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polydispersity index (PDI) value measured by dynamic light scattering (DLS) was

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0.168, indicating good polydispersion of SNP seeds in aqueous solutions. The

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morphological scanning electron microscopy (SEM) images and the size of the

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γ-CD-MOFs with and without SNP seeds are presented in Figure 1. The synthesized

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γ-CD-MOFs without SNP seeds exhibited uniform cubic shapes, and the mean

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diameter of the granules was 6.2±0.8 µm based on the SEM images, consistent with

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that reported in the literature with a 4–7 µm particle size distribution.12,14 A decrease

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in particle size was observed with the addition of SNP seeds, indicating that the

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crystal growth was regulated. With the increase in concentration of SNP seeds from

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0.5 mg/mL to 1.5 mg/mL, the size of the γ-CD-MOF crystals decreased from 6.2±0.8

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µm to 3.9±1.1 µm (Figure 1B), 1.8±0.4 µm (Figure 1C), and 2.6±0.6 µm (Figure 1D),

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respectively. To our knowledge, this work is the first to report on the synthesis of

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γ-CD-MOFs by the seed-mediated method with SNPs as an efficient size modulator.

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Schematic representation of the size-controlled synthesis of γ-CD-MOFs is

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presented in Scheme 1. The formation of the γ-CD-MOF crystals should experience

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three stages: appearance of a nucleus by the self-assembly of γ-CD, growth and

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coagulation of the crystal nucleus to reach an appreciable size, and aggregation of

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γ-CD-MOF. When SNP seeds were added, γ-CDs were easily adsorbed onto the

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surface of SNPs due to the interactions between SNPs and CD-MOFs. In this way,

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γ-CDs would be arranged in order more easily, which could accelerate the formation

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of the nucleus. Moreover, the aggregation of γ-CD-MOFs was reduced because of the

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steric effect of the SNP seeds, thus forming ultra-fine γ-CD-MOFs. Controlling the

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nucleation and the crystal growth rates is the key factor in modifying the size of

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γ-CD-MOFs. Liu et al.12 reported that the size of γ-CD-MOFs of 1–3 µm modulated

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by PEG 20000 was recorded as small in comparison with that obtained with CTAB

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through the vapor diffusion method. They suggested that this small particle size could

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be due to the high number of nucleation sites.

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X-Ray diffraction (XRD) and N2 adsorption isotherms analysis. The XRD

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patterns of the γ-CD-MOFs with and without SNP seeds are presented in Figure 2.

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The major characteristic peaks of γ-CD-MOFs occurred at 4.0, 5.7, 7.0, 13.2, and

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16.9°, which indicated the high crystallinity of γ-CD-MOFs, consistent with the

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literature.12,22 Compared with that of γ-CD-MOFs without SNP seeds, the crystallinity

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of γ-CD-MOFs synthesized though the seed-mediated approach was higher as

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evidenced by the peak intensity in the XRD patterns, and the peak intensity of the

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XRD patterns was more than three times higher than others when 1 mg/mL seed was

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added. Similar results were also reported by Xu et al.,13 who found that the Zr- and

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Ni-MOFs synthesized by the seed-mediated method had high crystallinity.

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Furthermore, the γ-CD-MOFs synthesized though the seed-mediated approach

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afforded more crystals than the control experiment of the synthesis of the traditional

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approach, whereas the pure SNP seeds remained clear (Figure S2). Many studies have

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demonstrated that the production of γ-CD-MOF crystals at a short period of time (< 6

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h) is challenging. In our experiment, the preparation period was shortened from over

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24 h to 6 h, and the crystals retained their good porous and crystalline properties,

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indicating that the seed-mediated synthesis offers a more efficient and faster method

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than the previous approach.

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The nitrogen adsorption and desorption isotherms were measured using the

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Micrometrics ASAP2010 system. The typical isotherm exhibited a high N2 uptake

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(Figure 3), thus validating the porous characteristics of these materials. The Brunauer

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Emmett Teller (BET) specific surface area, Langmuir surface area, and mean pore

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diameters of the γ-CD-MOFs with and without SNP seeds are summarized in Table 1.

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As indicated in Table 1, the maximize BET (Langmuir) surface areas of γ-CD-MOFs

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were estimated to be 846.2 (1132.3) m2/g when 1.0 mg/mL SNP seed was added,

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which is significantly higher than that of γ-CD-MOFs without SNP seeds, although

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slightly lower than the theoretical BET surface areas of 1030 m2/g reported in a recent

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study.12 This coverage corresponds to a measured pore diameter of 2.45 nm, which

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indicates a typical characteristic of mesoporous materials. The γ-CD-MOFs

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synthesized by the seed-mediated method produced more compact crystallization than

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the control experiment of the synthesis through the traditional approach. The seeds

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introduced before the commencement of crystallization enabled the crystal growth

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process to bypass the most energy-intensive step, that is, the dynamic dissociation

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growth equilibrium of crystal clusters.13

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Thermogravimetric (TG) analysis. Thermal stability is an important factor in

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the development of practical γ-CD-MOFs materials. As illustrated in Figure 4, the

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γ-CD-MOFs without SNP seeds started to decompose at about 210 °C, and the main

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mass loss was almost finished at about 320 °C. For the γ-CD-MOFs with added seeds,

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the thermal degradation temperature significantly increased with the increase in added

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amount, thus indicating a perfect crystalline structure consistent with the results on

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crystallinity. Thus, they require more energy to complete degradation during TG.

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However, when the addition of SNP seeds reached 1.5 mg/mL, the thermal stability of

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CD-MOFs decreased. This reduction could be explained in terms of the decreased

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ordering of crystallization, which could be due to the aggregation of SNP seeds and

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the resultant blocking of the crystallization.

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Fourier transformation infrared (FTIR) spectroscopy analysis. The infrared

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spectra of the SNP seeds and the γ-CD-MOF crystals with and without added SNP

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seeds are presented in Figure 5. The FTIR spectra for the SNP seeds showed a strong

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absorption peak at 3311 cm−1, which corresponded to those of the -OH group of

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SNP.28 In the presence of SNP seeds, as the content of the SNP seeds increased, the

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peak of the hydrogen bonds shifted to a shorter wavelength, from 3311 cm–1 (without

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seeds) to 3296, 3287, and 3303 cm-1, respectively, indicating a possible interaction

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between the OH group of the γ-CD-MOFs and the SNP seeds. This result indicated

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that the hydrogen bonds between the molecular chains in the γ-CD-MOF crystals

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synthesized by the seed-mediated method were stronger, consistent with the XRD

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results. Moussa et al.29 proposed that the hydrogen bonding greatly contributed to an

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H-bond type of interaction. Similarly, Zhang et al.30 reported that the bands at about

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3400 cm−1 shifted to a shorter wavelength, suggesting the strengthening of the

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hydrogen bonding.

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Cytotoxicity. The cell viability of CD-MOFs prepared with different

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concentrations of SNP seeds is shown in Figure 6. The CD-MOFs with and without

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SNP seeds exhibited a high cell viability (> 98%) in a low concentration (< 250

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µg/mL). When the concentrations ranged at 250–1000 µg/mL, cell viability decreased

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slightly but remained above 90%, indicating that the CD-MOFs synthesized by the

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seed-mediated method were safe and nontoxic. Therefore, the seed-mediated method

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provides facile and green chemistry techniques to fabricate CD-MOFs.

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Interaction of NR with γ-CD-MOF crystals. The γ-CD or γ-CD-MOF crystals

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with and without SNP seeds had a strong binding interaction with NR, which is

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reflected from its fluorescence spectrum (Figure 7). The fluorescence spectrum of the

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NR exhibited a maximum fluorescence emission at about 630 nm. For the γ-CD

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system, the fluorescence intensity of NR increased with an increase in the

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concentration of γ-CD, but no shift occurred in the fluorescence maximum

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wavelength. Similar results were reported in a recent investigation31. For the

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γ-CD-MOF without SNP seeds, the fluorescence spectrum showed an enhancement in

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intensity along with a slightly blue shift. When the γ-CD-MOF with SNP seeds was

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added, an obvious increase in the fluorescence intensity was observed with an

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increase in the concentration of γ-CD-MOF. Furthermore, the fluorescence spectrum

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showed a drastic blue shift of about 6 nm of the fluorescence maximum. The

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spectacular increase in the fluorescence of NR along with an appreciable blue shift in

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the band maximum indicates that an interaction between γ-CD-MOF and NR occurred.

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Similarly, Jana et al.31 pointed out the fact that an interaction occurred between γ-CD

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and NR, which could be due to NR experiencing a reduced polarity inside the γ-CD

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cavity.

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Fluorescent images of the inclusion complexes of NR and γ-CD-MOFs with 1

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mg/mL SNP seeds were observed with a fluorescence microscope (Olympus Co.,

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Tokyo, Japan). The initial visualization of the images (Figure 8) showed that NR had

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a strong fluorescence inside the γ-CD-MOFs, indicating the incorporation of NR

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within the cavity of the CD-MOF crystals. A few minutes later, the fluorescence was

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quenched (Figure 8B). This result confirmed the occurrence of the interaction

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between NR and γ-CD-MOFs.

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The fluorescence data of NR in the γ-CD-MOF crystals are analyzed by the

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Benesi–Hildebrand equation:32 1 1 1 , = + F − F0 K ( F∞ − F0 )[ S ] F∞ − F0

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where K is the binding constant, [S] is the concentration of γ-CD-MOF or γ-CD, F0 is

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the fluorescence intensity of NR without S, F is that with a certain concentration of S,

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and F∞ is that in the highest concentration of S.

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The linearity in the plot revealed the validity of the above equation and confirmed

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a one-to-one interaction between the NR and γ-CD or γ-CD-MOFs, consistent with

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previous reports31. This finding implied the formation of 1:1 inclusion complexes of

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γ-CD or γ-CD-MOFs with NR, resulting in the enhancement of their fluorescence

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emission spectra. The binding constant (K) and the correlation coefficients (R) from

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the Benesi–Hildebrand equation are presented in Table 2. The good fit between the

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experimental data and the Benesi–Hildebrand equations was indicated by high a

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correlation coefficient (R2 > 0.98). Compared with the γ-CD and CD-MOFs without

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seeds, the CD-MOFs synthesized by the seed-mediated method had a higher K value,

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which suggested that the CD-MOFs with seeds exhibited stronger inclusive ability.

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The results agreed with the reported fluorescence data of CD inclusion complexation,

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which showed that strong inclusion complexes are usually produced as indicated by

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the higher values of complex binding constants.32,33

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In summary, we first reported the synthesis of γ-CD-MOFs through the

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seed-mediated method, which provided facile and green techniques, as specialized

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equipment and complex operating conditions are not required and the risk of sample

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contamination is usually significantly reduced. To shorten the synthesis time and to 13



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create smaller crystals, the SNP seeds from debranched starch were selected as a

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template because of their natural abundance of starch and their safety. The size

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variation could be controlled by the addition of SNP seeds. Unlike in γ-CD-MOFs

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without seeds (6.2±0.8 µm), a smaller particle size (1.8±0.4 µm) and higher BET

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surface areas (846.2 m2/g) could be obtained by the seed-mediated method because of

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the high number of nucleation sites. The CD-MOFs synthesized though the

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seed-mediated method showed higher crystallinity and thermal stability, indicating

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that the seed-mediated synthesis offers a more efficient and faster method than the

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previous approach. The cytotoxicity results confirmed that the CD-MOFs synthesized

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by the seed-mediated method were safe and nontoxic. The fluorescence data revealed

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the formation of 1:1 complexes between NR and CD-MOFs. The visualization of

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fluorescence microscope images showed the incorporation of NR within the cavity of

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the CD-MOF crystals. The newly fabricated CD-MOFs could have a potential

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application as delivery carriers of active factors or drugs in the pharmaceutical and

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food fields.

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Conflict of interest

291 292

No conflicts of interest are declared for any of the authors.

Acknowledgements

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This project was supported by the National Natural Science Foundation of China

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(Grant No. 31401524, 31230057), the Natural Science Foundation of Jiangsu

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Province (BK20140143).

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Table 1 Surface parameters of the γ-CD-MOFs with and without added SNP seeds. Samples

BET surface area (m2/g)

Langmuir surface area (m2/g)

Pore diameters (nm)

γ-CD-MOF without seeds

165.3

352.6

3.31

γ-CD-MOF with 0.5 mg/mL SNP seeds

764.4

962.4

2.52


846.2

1132.3

2.45


465.5

612.6

3.15

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Table 2 Inclusion constants (K) and correlation coefficients (R) of NR/γ-CD or NR/γ-CD-MOFs inclusion complex from the Benesi–Hildebrand equation. Samples

K

R

NR/γ-CD

0.322±0.007c

0.989

NR/γ-CD-MOF without seeds

0.493±0.012b

0.995

NR/γ-CD-MOF with seeds

0.562±0.018a

0.997

Values expressed are mean ± standard error (n = 3). Values followed by the different letter in the same column are significantly different (P < 0.05).

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Figure Captions Figure 1 SEM morphology images of γ-CD-MOF crystals prepared with different concentrations of SNP seeds: 0 mg/mL (A), 0.5 mg/mL (B), 1 mg/mL (C), and 1.5 mg/mL (D), respectively.

Figure 2 XRD of the γ-CD-MOF crystals with and without added SNP seeds. Figure 3 Nitrogen adsorption–desorption isotherm of γ-CD-MOF crystals with and without added SNP seeds.

Figure 4 TG analysis curves of the γ-CD-MOF crystals with and without added SNP seeds.

Figure 5 FTIR spectroscopy curves of the SNP seeds and the γ-CD-MOF crystals with and without added SNP seeds.

Figure 6 In vitro cell viability of Caco-2 cells against the γ-CD-MOF crystals with and without added SNP seeds.

Figure 7 Fluorescence spectra of NR in different concentrations of γ-CD (A), γ-CD-MOF without SNP seeds (B), and γ-CD-MOF with 1 mg/mL SNP seeds (C), and the Benesi–Hildebrand plots for the complexation of NR and γ-CD-MOFs. 0→5: the γ-CD-MOF content was 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL, respectively.

Figure 8 Fluorescent microscope images of NR/γ-CD-MOFs with 1 mg/mL SNP seeds. Scale bars are 10 µm.

Figure S1 Transmission electron microscopic image and the particle size distribution as determined DLS of the SNP seeds.

22


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Figure S2 Image of pure SNP seeds (A), γ-CD-MOF crystals (B), and γ-CD-MOF crystals with added SNP seeds (C) obtained after 12 h.

23



Add seeds

Controlled

Small

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C

growth

Nile red Hydrophobic model

Without seeds

Growth

Large

A

Nile red loaded γ-CD-MOFs

Scheme 1. Schematic representation of the size-controlled synthesis of γ-CD-MOFs through facile and green seed-mediated method.

24


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A

B

10 µm

C

10 µm

D

10 µm

10 µm 1 µm

Figure 1 SEM morphology images of γ-CD-MOF crystals prepared with different concentrations of SNP seeds: 0 mg/mL (A), 0.5 mg/mL (B), 1 mg/mL (C), and 1.5 mg/mL (D), respectively.

25



5

10

15

4000

20

25

30

35

Page 26 of 33

40

CD-MOF+1.5 m g/m L Seeds

2000 0 15000

CD-MOF+1 m g/mL Seeds

Intensity

10000 5000 0 3000

CD-MOF+0.5 m g/m L Seeds

2000 1000 0 600 CD-MOFs without Seeds 400 200 0 5

10

15

20

25

30

35

40

2? (° )

2θ (º)

Figure 2 XRD of the γ-CD-MOF crystals with and without added SNP seeds.

26


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CD-MOF+1 mg/mL Seeds

Quantity Adsorbed (cm3/g STP)

250 CD-MOF+1.5 mg/mL Seeds

200

CD-MOF+0.5 mg/mL Seeds

150

100 CD-MOFs without Seeds

50 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 3 Nitrogen adsorption–desorption isotherm of γ-CD-MOF crystals with and without added SNP seeds.

27



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100 CD-MOF+1 mg/mL Seeds 80

Weight (%)

CD-MOF+1.5 mg/mL Seeds CD-MOF+0.5 mg/mL Seeds

60

CD-MOFs without Seeds 40

20

0 100

200

300

400

500

600

O

Temperature ( C)

Figure 4 TG analysis curves of the γ-CD-MOF crystals with and without added SNP seeds.

28


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SNP Seeds

Relative Intensity (%)

3280

3303 CD-MOF+1.5 mg/mL Seeds

3287

CD-MOF+1 mg/mL Seeds

3296 CD-MOF+0.5 mg/mL Seeds 3311 CD-MOFs without Seeds 0

4000

3500

3000

2500

2000

1500

1000

500

-1

cm

Figure 5 FTIR spectroscopy curves of the SNP seeds and the γ-CD-MOF crystals with and without added SNP seeds.

29



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CD-MOFs without Seeds CD-MOF+0.5 mg/mL Seeds CD-MOF+1.0 mg/mL Seeds CD-MOF+1.5 mg/mL Seeds

Cell viability (%)

100

80

60

40

20

0

0

125

250

500

1000

Concentration (µg/mL) Figure 6 In vitro cell viability of Caco-2 cells against the γ-CD-MOF crystals with and without added SNP seeds.

30


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A

800

5

600

Fluorescence Intensity


800

0

400

200

B

5

600

0

400

200

0

0 575

600

625

650

675

700

725

750

575

600

625

650

675

700

725

750

Wavelength (nm)

Wavelength (nm) 0.026 1000

C

0.024

CD-MOF+1 mg/mL Seeds CD-MOFs without Seeds CD

D

0.022 0.020

800

0.018 0 600

1/F-F0


5

400

0.016 0.014 0.012 0.010 0.008

200

0.006 0.004

0 575

0.002 600

625

650

675

700

725

0.000 750 0.5

1.0

1.5

Wavelength (nm)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

-1 1/[S] (mg/mL)

Figure 7 Fluorescence spectra of NR in different concentrations of γ-CD (A), γ-CD-MOF without SNP seeds (B), and γ-CD-MOF with 1 mg/mL SNP seeds (C), and the Benesi–Hildebrand plots for the complexation of NR and γ-CD-MOFs. 0→5: the γ-CD-MOF content was 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL, respectively.

31



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A

B

Figure 8 Fluorescent microscope images of NR/γ-CD-MOFs with 1 mg/mL SNP seeds. Scale bars are 10 µm.

32


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Graphical Abstract

33


Green Synthesis of Cyclodextrin-Based MetalâOrganic Frameworks

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