Triple‑, Double‑, and Single-Shelled Hollow Spheres of SulfonatedMicroporous Organic Network as Drug Delivery MaterialsJune Young Jang,†,‡ Thai Minh Duy Le,∞,‡ Ju Hong Ko,† Yoon-Joo Ko,∂ Sang Moon Lee,§

Hae Jin Kim,§ Ji Hoon Jeong,Δ Thavasyappan Thambi,*,∞ Doo Sung Lee,*,∞

and Seung Uk Son*,†

†Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea∞School of Chemical Engineering, Theranostic Macromolecules Research Center, Sungkyunkwan University, Suwon 16419, Korea∂Laboratory of Nuclear Magnetic Resonance, NCIRF, Seoul National University, Seoul 08826, Korea§Korea Basic Science Institute, Daejeon 34133, KoreaΔSchool of Pharmacy, Theranostic Macromolecules Research Center, Sungkyunkwan University, Suwon 16419, Korea

*S Supporting Information

Recently, various microporous materials have been used todeliver drugs.1−11 Because of their microporosity (poresize < 2 nm), molecular drugs could be incorporated into thematerials. However, the possible clogging of micropores by theincorporated drugs can hinder the accommodation and releaseof drugs. To improve the drug delivery performance ofmicroporous materials, hollow materials have been engi-neered.12 Although more efficient loading and release ofdrugs could be achieved with the hollow microporousmaterials, most drugs actually were loaded on the microporesin shells because of the molecular interaction of shell materialswith the drugs. Thus, to improve the loading efficiency ofdrugs, multishelled hollow microporous materials can beengineered.13−17 Multishelled hollow materials are expectedto load more drugs per particle than single-shelled hollowmaterials. Moreover, the intershell space of multishelled hollowmaterials can accommodate additional drugs.Recently, microporous organic network (MON) materials

have been prepared by the networking of organic buildingblocks.18−24 Because of their high surface areas, chemicalstability, and facile postmodification, the MON materials havegreat potential as drug delivery materials. However, the studiesof MON materials as drug delivery systems are in an earlystage.25−30 Our research group has studied the morphologicalengineering of MON materials31−33 using various templatessuch as silica spheres, metal−organic frameworks, or zeoliticimidazolate frameworks (ZIFs).34 Recently, we applied hollowMON materials to deliver drugs.25 However, there have beenno reports on the preparation of triple- or double-shelledhollow MON materials because of the limitation of a syntheticapproach.35−37 In this work, we report on the synthesis oftriple- and double-shelled hollow sulfonated MON spheres(TH-SMON and DH-SMON) and their drug deliveryperformance, compared with that of single-shelled hollowsulfonated MON spheres (H-SMON).Figure 1 shows a synthetic scheme for the H-SMON, DH-

SMON, and TH-SMON. Single-shelled hollow MON (H-MON) was prepared through the formation of MON on thesurface of silica spheres by the Sonogashira coupling of tetra(4-ethynylphenyl)methane38 with 1,4-diiodobenzene and thesuccessive etching of the inner silica.

We had tried to introduce silica and ZIF materials34 into thesurface of H-MON, but we failed to obtain good qualitymaterials. When we used the sulfonated H-MON (H-SMON),we could introduce the ZIF-8 materials into the H-SMON toform H-SMON@ZIF-8. The polar sulfonate groups might

Received: November 6, 2018Revised: December 21, 2018Published: December 24, 2018

Figure 1. A synthetic scheme for H-/DH-/TH-SMONs.

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induce coordination of Zn2+ ions to the surface of H-SMON.In addition, sulfonic acid groups might direct basic 2-methylimidazole. The second MON layer was formed on thesurface of the ZIF-8 layer through the Sonogashira coupling oftetra(4-ethynylphenyl)methane with 1,4-diiodobenzene. Afterthe ZIF-8 was etched by treatment with acetic acid, theresultant H-SMON@MON was sulfonated to form water-compatible DH-SMON. For the preparation of TH-SMON,the ZIF-8 was incorporated on the DH-SMON. Afterformation of a MON layer on the DH-SMON@ZIF-8, theetching of ZIF-8, followed by sulfonation, resulted in the TH-SMON.As shown in Figure 2, the morphological structures of H-

SMON, DH-SMON, TH-SMON, and intermediate materialswere investigated by scanning (SEM) and transmissionelectron microscopy (TEM).

H-SMON showed an average diameter of 220 nm and auniform shell thickness of 15−20 nm (Figure 2a,d,k). After theH-SMON was coated with ZIF-8, the diameter of H-SMON@ZIF-8 was increased to ∼270 nm, indicating that the thicknessof ZIF-8 coating is ∼25 nm (Figure 2b,e). DH-SMON showedan average diameter of 300 nm, a shell thickness of 15−20 nm,and an intershell thickness of ∼25 nm (Figure 2c,f,k),indicating that the etching of ZIF-8 coating created a voidintershell space. Further coating of DH-SMON with ZIF-8

increased the diameter of DH-SMON@ZIF-8 to ∼350 nm(Figure 2g,i). Finally, the TH-SMON had a diameter of 380nm, a shell thickness of 15−20 nm, and an intershell thicknessof ∼25 nm (Figure 2h,j,k). The contents of sulfonic acids inthe H-SMON, DH-SMON, and TH-SMON were analyzed tobe 1.84, 1.98, and 1.92 mmol/g, respectively, by the elementalanalysis of sulfur contents (5.91, 6.34, and 6.14 wt % sulfur,respectively).The H-SMON, DH-SMON, TH-SMON, and intermediate

materials were further characterized by various spectroscopies(Figures 3 and S1 in the Supporting Information).Homogeneous distribution of sulfonic acids in the hollowSMONs was confirmed through the elemental mapping ofcarbon, sulfur, and oxygen by electron energy loss spectroscopy(EELS) (Figure 3a).

Figure 2. SEM images of (a) H-SMON, (b) H-SMON@ZIF-8, (c)DH-SMON, (g) DH-SMON@ZIF-8, and (h) TH-SMON. TEMimages of (d) H-SMON, (e) H-SMON@ZIF-8, (f) DH-SMON, (i)DH-SMON@ZIF-8, and (j) TH-SMON. (k) Diameter and intershellthickness distribution diagrams of H-SMON, DH-SMON, and TH-SMON.

Figure 3. (a) EELS elemental mapping images (scale bars = 100 nm);(b) N2 isotherm curves obtained at 77 K; and (c) pore sizedistribution diagrams (the DFT method) of H-SMON, DH-SMON,and TH-SMON. (d) PXRD patterns and (e) IR absorption spectra ofH-SMON, DH-SMON, TH-SMON, and intermediate materials.

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According to the analysis of N2 isotherm curves obtained at77 K, surface areas (SA) and micropore volumes (Vmic) weredecreased from 954 m2/g and 0.26 cm3/g to 515 m2/g and0.15 cm3/g, respectively, through the sulfonation of H-MONto H-SMON, matching well with the trends observed inpostsynthetic modification of MONs in the literature (Figures3b and S1).11 By adding MON shell to H-SMON, the SA andVmic were increased to 778 m

2/g and 0.22 cm3/g, respectively.The sulfonation of H-SMON@MON to DH-SMON de-creased SA and Vmic to 484 m

2/g and 0.12 cm3/g, respectively.The same trends were observed in the synthesis of TH-SMONfrom DH-SMON. The SA and Vmic were increased to 553 m

2/g and 0.13 cm3/g by adding MON shell to DH-SMON. Thesulfonation of DH-SMON@MON to TH-SMON decreasedSA and Vmic to 453 m

2/g and 0.11 cm3/g, respectively. In thepore size distribution analysis based on density functionaltheory (DFT), while H-SMON had exclusively micropores(pore sizes < 2 nm), the DH-SMON and TH-SMON hadsignificant mesopores at 7−50 nm with Vmeso of 0.22 and 0.26cm3/g, respectively, which resulted from the void intershellspace (Figure 3c).While MONs in the hollow SMONs and intermediate

materials were amorphous,42,43 the ZIF-8 materials in H-SMON@ZIF-8 and DH-SMON@ZIF-8 showed their uniquecrystalline X-ray diffraction (XRD) peaks, indicating thesuccessful formation of ZIF-8 (Figure 3d). While the IRspectrum of H-MON showed major peaks of aromatic C=Cand C−H vibrations at 1503 and 825 cm−1, respectively, thehollow SMONs showed new vibration peaks at 1215 and 3448cm−1, corresponding to the S=O and O−H vibrations ofsulfonic acid groups, respectively (Figure 3e).39,40 The solid-state 13C NMR spectrum of H-MON showed the 13C peaks ofbenzyl carbon and internal alkynes at 64 and 91 ppm,respectively (Figure S2). While the 13C peaks of H-MONappeared at 146, 136, 131, and 122 ppm, the sulfonated H-MON showed a significant change with two major aromatic13C peaks at 146 and 128 ppm.39,40 The intermediate materialscontaining both MON and SMON showed the mixed 13CNMR patterns of sulfonated and nonsulfonated aromaticspecies. Thermogravimetric analysis (TGA) showed that H-SMON, DH-SMON, and TH-SMON are thermally stable upto 279, 206, and 192 °C, respectively (Figure S2).The H-SMON, DH-SMON, and TH-SMON were dispersed

well in aqueous media, and the dispersions were stable evenafter 2 days (Figure S3). Their dispersibility increasedgradually from H-SMON to DH-SMON and TH-SMON. Inwater contact angle measurements, water drops wereimmediately absorbed into the materials (Figure 4a).Considering the excellent water compatibility, we studied thehollow SMONs as drug delivery materials (Figure 4b−d). Atfirst, we tested with rhodamine B (RhB) as a model moleculebecause of the high cost of our target drug, doxorubicin(DOX). When we treated H-SMON, DH-SMON, and TH-SMON with RhB (feed ratio of 10:1 of materials:RhB), theloading amounts of RhB were increased from 5.7 ± 0.3 wt % to6.8 ± 0.2 and 7.3 ± 0.2 wt %, respectively, corresponding toloading efficiencies of 61 ± 3, 73 ± 3, and 78 ± 2%,respectively. The released amount of RhB in pH 7.4 PBSsolution at 37 °C increased gradually from RhB/H-SMON toRhB/DH-SMON and RhB/TH-SMON (Figure 4b). After 7days, 41, 53, and 60% of the RhB in the H-SMON, DH-SMON, and TH-SMON were released, respectively. When wetreated H-SMON, DH-SMON, and TH-SMON with DOX

(feed ratio of 10:1 of materials:DOX), the loading amounts ofDOX were measured to be 7.6 ± 0.1, 8.2 ± 0.2, and 8.7 ± 0.1wt %, respectively, corresponding to loading efficiencies of 83± 1, 89 ± 2, and 95 ± 1%, respectively. The released amountsof DOX in pH 7.4 PBS solution at 37 °C were increasedgradually from DOX/H-SMON to DOX/DH-SMON andDOX/TH-SMON. After 1 day, 44, 62, and 73% of the DOX inthe H-SMON, DH-SMON, and TH-SMON were released,respectively. After 7 days, 62, 80, and 92% of the DOX in theH-SMON, DH-SMON, and TH-SMON were released,respectively. The inferior loading and release of RhB,compared with DOX, might result from its ionic naturewhich hindered the efficient loading and release through thestrong interaction with sulfonic acids in the SMON materials.40

When we conducted the release tests of DOX/TH-SMON atpH 5.0, the release rate increased, possibly because of theimproved solubility of DOX in the acidic medium (Figure S4).

Figure 4. (a) Water contact angles and (b) DOX/RhB releasebehaviors (3 mg of drug-loaded materials, pH 7.4 PBS solution, 30mL, 37 °C) of H-SMON (7.6 wt % DOX, 5.7 wt % RhB), DH-SMON (8.2 wt % DOX, 6.8 wt % RhB), and TH-SMON (8.7 wt %DOX, 7.3 wt % RhB). (c) CLSM images after treating cancer cells(MCF7 cell line) with the DOX-loaded H-SMON, DH-SMON, andTH-SMON for 1 h (scale bar = 10 μm). (d) Cell viabilities of humanbreast cancer cells (MCF7 cell line) after treatment with DOX-loadedH-SMON, DH-SMON, and TH-SMON for 2 days.

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It is noteworthy that the microenvironment of cancer cells isknown to be acidic, with pH in the range of 5.0−6.8.41Next, we conducted in vitro cell tests using human breast

cancer cells for the DOX delivery performance of H-SMON,DH-SMON, and TH-SMON (Figure 4c,d). According toconfocal laser scanning microscopy (CLSM), the strongfluorescence of DOX was observed in the cancer cells(MCF7 lines) (Figure 4c). The fluorescence intensities ofDOX in the cells increased gradually from the case of DOX/H-SMON to the cases of DOX/DH-SMON and DOX/TH-SMON. In the case of DOX/TH-SMON, the distorted shapesof cancer cells were significantly observed. The MTT assayshowed that the H-SMON, DH-SMON, and TH-SMONwithout DOX loading were inert at least up to 100 μg/mL forthe cancer cells (Figure S5). When we used the same amountof DOX-loaded materials, DOX delivery efficiencies graduallyincreased from DOX/H-SMON to DOX/DH-SMON andDOX/TH-SMON (Figure 4d). Cancer cell viabilities (MCF7cell line) decreased from 74% to 67% and 59% after 2 dayswith 10 μg/mL of DOX/H-SMON, DOX/DH-SMON, andDOX/TH-SMON, respectively. When the DOX-loadedmaterials increased to 100 μg/mL, cancer cell viabilitiesfurther decreased to 54, 44, and 38% after 2 days for DOX/H-SMON, DOX/DH-SMON, and DOX/TH-SMON, respec-tively. It is noteworthy that the DOX/TH-SMON of 10 and100 μg/mL showed normal cell (293T cell line) viabilities of76 and 62%, respectively (Figure S6).Overall observations indicate that as the shell number of

hollow spheres increases, the drug loading efficiency increases.We speculate that drugs can be loaded additionally in theintershell space of DH-SMON and TH-SMON. In addition,because of the increased amounts of sulfonic acids per particle,the water-compatibility of multishelled SMON increased,resulting in the efficient release of drugs. Supposing that therelease kinetics are similar among the SMOM materials, theenhanced DOX loading and release efficiencies make themultishelled SMONs promising for drug delivery. In controlstudies, nonhollow SMOM materials with a broad sizedistribution in the range of 0.7−1.6 μm were obtained in theabsence of silica templates and showed a poor DOX loadingefficiency of 77 ± 3% (feed ratio of 10:1 of material:DOX) andpoor release performance due to their large sizes (Figure S7).According to the pore analysis of RhB/TH-SMON, theoriginal micro- and mesopore volumes were significantlyreduced from 0.11 and 0.26 cm3/g to 0.01 and 0.18 cm3/g,respectively, with a surface area of 43 m2/g, indicating that theshells and intershell space of TH-SMON accommodate drugs(Figure S8). Interestingly, the DOX release behaviors matchedwell with the intershell space trends of H-SMON, DH-SMON,and TH-SMON (Figure S9).In conclusion, this work shows that multishelled spheres of

microporous organic networks can be engineered byalternately applying MON chemistry and ZIF-8 chemistry.As the shell number increased, the drug loading and releaseefficiency increased. We suggest that this trend is attributableto the enhanced intershell space of multishelled spheres andmore facile diffusion of aqueous solution into the multishelledmaterials. We believe that the chemical surrounding and theshell number can be further optimized by a predesignedbuilding block approach and further shell engineering.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.8b04674.

Experimental procedures; additional characterizationdata of H-SMON, DH-SMON, TH-SMON, nonhollowSMON, intermediate materials, and RhB/TH-SMON;and additional DOX release behavior of DOX-loadedmaterials (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*S. U. Son. E-mail: sson@skku.edu.*D. S. Lee. E-mail: dslee@skku.edu.*T. Thambi. E-mail: thambi@skku.edu.ORCIDHae Jin Kim: 0000-0002-1960-0650Ji Hoon Jeong: 0000-0002-4836-3551Doo Sung Lee: 0000-0002-7979-7459Seung Uk Son: 0000-0002-4779-9302Author Contributions‡J.Y.J. and T.M.D.L. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by Basic Science Research Programs(2016R1E1A1A01941074, NRF-2016R1A6A3A11931661,NRF-2017R1D1A1B03028061, MEST-20100027955) throughthe National Research Foundation of Korea (NRF) funded bythe Ministry of Science, ICT and Future Planning and theMinistry of Education, Science and Technology (MEST).

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DOI: 10.1021/acs.chemmater.8b04674Chem. Mater. 2019, 31, 300−304

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http://dx.doi.org/10.1021/acs.chemmater.8b04674