Orthogonal Approach to Construct Cell-Like Vesicles via Pillar[5]arene

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Orthogonal Approach to Construct Cell-Like Vesicles via Pillar[5]arene-Based Amphiphilic Supramolecular Polymers Leilei Rui,† Lichao Liu,† Yong Wang,‡ Yun Gao,† and Weian Zhang*,† †

Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China S Supporting Information *

ABSTRACT: A reduction-responsive supramolecular diblock copolymer was successfully constructed by the orthogonal assembly of two homopolymers in aqueous solution via pillar[5]arene-based host−guest interactions for the first time. The homopolymers, namely, polystyrene modified with ethyl viologen as the terminal group (PS-EV) and polyethylene glycol bearing pillar[5]arene (PEG-P[5]A), were synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization and coupling reaction, respectively. The supramolecular diblock copolymer can self-assemble into stable binary cell-like vesicles in aqueous solution, where a spherical micelle as the cell nucleus was embedded in a clear outer vesicle as the cell membrane. The cell-like vesicles can be disassembled into spherical aggregates through addition of reducing agent and further used for encapsulation and controlled release of model molecules.

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Among some noncovalent interactions,17−20 host−guest recognition has been widely applied in supramolecular chemistry due to their high directionality, cooperative manner, and tunable strength.21 To date, a variety of macrocyclic hosts have been used extensively to construct supramolecular amphiphilies via host−guest recognition and further to form interesting assembled morphologies such as vesicles and micelles.22−31 Recently, Scherman, Woisel, Zhang, Zhou, and Kim have fabricated a variety of supramolecular amphiphiles formed by macrocyclic host-containing polymers through host−guest recognition.23,24,28,29,32−34As a new kind of macrocyclic hosts, pillar[n]arenes composed of hydroquinone units linked by methylene bridges at the para positions have been widely studied due to their symmetrical pillar architecture, facile and high-yield synthesis, and accessible derivatizations.35−40 Nowadays, pillar[n]arene-based host−guest chemistry has also been explored in the fabrication of supramolecular amphiphiles, which could provide a useful platform for the applications of supramolecular amphiphiles such as explosive detectors, drug delivery systems, and transmembrane channels.41−45 For example, Huang et al. constructed pH-responsive supramolecular vesicles via host−guest complexation between a water-soluble pillar[n]arene and an amphiphilic guest bearing a hydrophilic methyl viologen unit.46 However, to our best knowledge, all above pillar[n]arene-based supramolecular

ytomimetic chemistry as described by the cell-mimetic behaviors has attracted significant attention from scientists. They have put great effort into understanding the chemical composition, structure, and function of the materials based on cells and further constructed the artificial cells using the various materials to mimic the formation of cells and living organisms. In the past research, vesicles have been widely used to mimic cellular processes including birth, fission, replication, metabolism, etc., which could be easily fabricated by a single amphiphilic compartment such as lipid, surfactant, and block copolymer.1−7 For example, Menger et al. had developed synthetic vesicles to mimic the structural features and dynamic processes of cells.4,5 In contrast to the above single-compartment vesicles, the development of multicompartmentalized vesicles could expand the scope of current vesicular morphologies and produce diverse functions from a prokaryotic to more of an eukaryotic level of sophistication since multicompartmentalization could provide the spatial segregation and assembly of biomolecules in different compartments and the exquisite control over metabolic reactions in eukaryotic cells. Thus, mimicking the compartmentalization of eukaryotic cells using multicompartmentalized vesicles would be presented as a great anticipation and challenge. Recently, Lecommandoux, Hest, Chiu, Weitz, Schiller, and Nallani have done a lot of excellent work on multicompartmentalized vesicles for cell mimicry.8−13 Supramolecular assemblies constructed by small molecules or polymers via orthogonal noncovalent interactions have aroused great interest owing to the resultant unique properties.14−16 © XXXX American Chemical Society

Received: December 10, 2015 Accepted: December 28, 2015

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Scheme 1. Orthogonal Construction, Self-Assembly, and Application for the Calcein Release of Supramolecular Amphiphilic Diblock Copolymer Based on Host−Guest Interactions between PEG-P[5]A and PS-EV

Figure 1. Partial 1H NMR spectra (CDCl3, 400 MHz) of PS29-EV at a fixed concentration of 4 mM with different concentrations of PEG-P[5]A: (a) 0 mM, (b) 1 mM, (c) 1.4 mM, (d) 2 mM, (e) 2.4 mM, (f) 4 mM, (g) 8 mM. (h) Partial 1H NMR spectrum (CDCl3, 400 MHz) of pure PEG-P[5]A at a concentration of 2 mM.

(Na2S2O3) for the controlled release of a water-soluble model drug molecule, calcein. Before cytomimetic vesicles were constructed using the amphiphilic supramolecular copolymer, PEG-P[5]A⊃PS29-EV, the complexation of PEG-P[5]A with PS-EV was first investigated to form the supramolecular diblock copolymer by 1H NMR, fluorescence spectroscopy, and UV−vis spectroscopy, respectively. Figure 1 showed the 1H NMR titration results of PS29-EV with PEG-P[5]A. The remarkable upfield chemical shifts of proton resonances H1 in PS29-EV could be observed upon addition of PEG-P[5]A. Moreover, the signal of protons H2 on the EV group of PS29-EV changed from two peaks to one peak. Additionally, the resonance peak related to protons H4 disappeared, and protons H1 and H2 of PS29-EV became broader after the complexation between PS29-EV and PEG-P[5]A.46,47 The possible reason is the shielding effect of the electron-rich cavities of P[5]A moieties for EV moieties, where the EV moieties of PS29-EV were included into the cavities of P[5]A moieties of PEG-P[5]A. On the other hand, the protons on PEG-P[5]A also exhibited slight chemical shift changes.46 Additionally, we have performed the 1H NMR

vesicles have been well fabricated from pillar[n]arene-based small molecules, and pillar[n]arene-containing polymers have not been reported for constructing smart supramolecular vesicles yet. Herein, we reported for the first time an orthogonal supramolecular approach to construct multicompartmentalized vesicles for mimicking the architectural arrangement of eukaryotic cells using an amphiphilic supramolecular diblock copolymer, PEG-P[5]A⊃PS29-EV, via host−guest interaction between P[5]A and EV moieties (Scheme 1). To fabricate reduction-responsive supramolecular vesicles, polyethylene glycol-functionalized pillar[5]arene (PEG-P[5]A) was synthesized by attaching pillar[5]arene onto the terminal group of PEG (Scheme S1 and Figures S1−S6), and a polystyrene homopolymer modified with an ethyl viologen unit (PS-EV) was prepared via reversible addition−fragmentation chain transfer (RAFT) polymerization using S-1-dodecyl-S′-(R, R′dimethyl-R″-acetic acid) trithiocarbonate (DDAT) bearing EV as a chain transfer agent (CTA) (Scheme S2 and Figures S7− S11). The cytomimetic vesicles can be disassembled into spherical aggregates through addition of a reducing agent 113

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Figure 2. (A) Fluorescence spectra of P[5]A-PEG (2.50 × 10−5 M) in THF at room temperature with different concentrations of PS29-EV: 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.5, 4.5, 5.5, 6.5, 8.5, and 10.5 × 10−5 M in THF. (B) UV−vis spectra of (a) P[5]A-PEG, (b) PS29-EV, and (c) PS29-EV in the presence of 1 equiv of P[5]A-PEG (2.50 × 10−5 M) in THF.

Figure 3. TEM images: (a) PEG-P[5]A:PS29-EV = 1:1 cell-like vesicles, (b) enlarged image of the cell-like vesicle with scale bar of 100 nm, (d) the spherical aggregates from PEG-P[5]A⊃PS29-EV vesicles after the addition of sodium thiosulfate, (e) enlarged image of the spherical aggregates. DLS results: (c) PEG-P[5]A⊃PS29-EV vesicles, (f) the spherical aggregates from PEG-P[5]A⊃PS29-EV vesicles after the addition of sodium thiosulfate.

binding between the host P[5]A cavities and the guest EV moieties.41,46 Further evidence showed that PEG-P[5]A and PS29-EV could form the complex PEG-P[5]A⊃PS29-EV by UV−vis absorption spectroscopy. As shown in Figure S16, it was found that upon gradual addition of PS29-EV a notable red-shift of the absorption peak of PEG-P[5]A was observed, indicating the formation of a typical charge-transfer complex.46 Moreover, as shown in Figure 2B, the absorption spectrum of the mixture showed a new absorption band (from 360 to 450 nm) which can be attributed to the charge transfer between electron-rich host P[5]A cavities and electron-poor guest EV moieties.46,48,49 Therefore, based on the results of 1H NMR, the molar ratio plot, the fluorescence titration, and the UV−vis absorption, we can conclude that PEG-P[5]A and PS29-EV formed a 1:1 [2]pseudorotaxane in THF, which is mainly driven by chargetransfer (CT) effects, hydrogen bonds, and π−π stacking interactions in THF. The high binding affinity of this host− guest system should be attributed to the cooperativity of the above noncovalent interactions.

measurement in DMSO to verify the complexation between PS29-EV and PEG-P[5]A in a high polar solvent (Figure S12). Moreover, from the 2D NOESY spectrum (Figure S13), intermolecular correlations were also observed between protons H1, H2 on the PS29-EV and protons Hc of P[5]A, respectively, indicating that the EV moieties of PS29-EV threaded into the cavity of P[5]A.46 The association constant for the complexation between PEG-P[5]A and PS-EV was further evaluated by the fluorescence titration experiments of PEG-P[5]A at varying PS29-EV concentrations at room temperature in THF. As shown in Figure 2A, upon gradual addition of PS29-EV into PEG-P[5]A, the fluorescence intensity was quenched significantly, indicating the occurrence of host−guest interaction between PEG-P[5]A and PS29-EV. The molar ratio plot of the fluorescence titration demonstrated that the complexation between PEG-P[5]A and PS29-EV had a 1:1 stoichiometry (Figure S14).41,46,47 Moreover, the association constant (Ka) was calculated to be (6.43 ± 0.41) × 104 M−1 by using a nonlinear curve-fitting method (Figure S15), revealing a strong 114

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in Figure 3d and Figure 3e, the assemblies underwent remarkable morphological change from a cell-like vesicle to a spherical aggregate upon addition of Na2S2O3. Moreover, the size of the aggregates showed a dramatic decrease from 440 to 240 nm after being exposed to Na2S2O3 solution (0.5 mg mL−1) (Figure 3c and Figure 3f). Here, it is a surprise to find that the spherical aggregates have a similar size with the inner aggregate in the cell-like vesicles (Figure 3b). On the basis of the above TEM and DLS results, we found that the membrane of the cell-like vesicles could be destroyed due to the decomplexation of PEG-P[5]A⊃PS29-EV to form spherical aggregates. Thus, we could ensure that the outer vesicular membrane formed only from supramolecular copolymer PEGP[5]A⊃PS29-EV. To further confirm our assumption, the selfassembly experiment of PEG-P[5]A:PS29-EV = 1:1 with Na2S2O3 was also performed in water. Figure S19 showed only spherical aggregates could be obtained with an average diameter of 200 nm, which is similar to that from cell-like vesicles treated with Na2S2O3. Therefore, we can conclude that the inner spherical aggregates should be formed by the coassembly of homopolymers PEG-P[5]A and PS29-EV. Furthermore, UV−vis spectroscopy also showed there was no significant CT band at the wavelength at around 450 nm after the aggregates of the PEG-P[5]A:PS29-EV = 1:1 treated with Na2S2O3 (Figure S20), which further revealed that the host− guest complexation of PEG-P[5]A and PS29-EV could be disrupted by addition of the reducing agent, Na2S2O3.52 Additionally, a clear formation mechanism of cell-like vesicles from supramolecular copolymer PEG-P[5]A:PS29-EV = 1:1 was also illustrated in Scheme 1. Moreover, the effect of polymer composition on the self-assembly behavior of the supramolecular copolymer was also systematically studied by varying the ratio of PEG-P[5]A to PS29-EV (Figures S21− S23), revealing that the average size and morphology of the stable aggregates are highly dependent on the ratio of PEGP[5]A to PS29-EV. The reduction-responsive cell-like vesicles based on PEGP[5]A:PS29-EV = 1:1 can be further applied in controlled release of small molecules such as calcein. As a result, the calcein-loaded vesicular solution turned to orange compared with the colorless calcein-unloaded vesicular solution after removing unloaded calcein molecules by dialysis, implying that calcein had been successfully encapsulated into the supramolecular cell-like vesicles (Figure 4). Subsequently, the release

After the establishment of the PEG-P[5]A⊃PS29-EV recognition motif in THF, UV−vis spectroscopy was also utilized to confirm the complexation process in aqueous solution. As shown in Figure S17, the binary complex (PEGP[5]A:PS29-EV = 1:1) exhibited a charge-transfer adsorption band (400−500 nm), and there was no absorption spectrum of EV at near 260 nm compared with the binary complex with the ratio of PEG-P[5]A:PS29-EV = 1:2. Here, the disappearance of the absorption spectrum of EV moieties is mainly because they were embedded in the internal of the aggregates, and their mobility was restricted.50 Thus, UV−vis results convincingly confirmed the formation of host−guest complexation between PEG-P[5]A and PS29-EV in aqueous solution. The self-assembly behavior of the amphiphilic supramolecular copolymer PEG-P[5]A⊃PS29-EV in water was further investigated. The critical vesicle concentration (CVC) of PEG-P[5]A⊃PS29-EV (P[5]A:PS29-EV = 1:1) was about 5.1 × 10−3 mg/mL monitored by the fluorescence absorbance with pyrene as a hydrophobic fluorescent probe (Figure S18). The assembled morphologies of PEG-P[5]A⊃PS29-EV (1:1) were studied by TEM and DLS, respectively. Here, it is surprising to find that the amphiphilic supramolecular copolymer can selfassemble into vesicles embedded by a spherical aggregate (Figure 3a), which are very similar to a eukaryotic vesicular structure with a clear outer vesicle as the cell membrane (cytoderm) and an inner spherical aggregate as the cell nucleus. The average diameter of outer vesicles was around 420 nm, which is consistent with the hydrodynamic diameter (Dh) of approximately 440 nm determined by DLS as shown in Figure 3c. The membrane thickness of the cell-like vesicles was around 19 nm as shown in the enlarged image (Figure 3b), which is in line with the interdigitated molecular length of the pseudocopolymer PEG-P[5]A⊃PS29-EV (9.7 nm),26 indicating the orthogonal assembly in the bilayer membrane of the vesicles formed by PEG-P[5]A⊃PS29-EV was in an antiparallel packing pattern (Scheme 1). Additionally, ζ-potential results showed these vesicles were quite stable in water (Figure S23c).41,42 Moreover, it is very interesting that each vesicle has a spherical aggregate in its inner cavity, with an average diameter of about 220 nm. These unusual cell-like morphologies were first fabricated, and a probable formation mechanism was also discussed. Here, the formation of cell-like vesicles should be also related to the self-assembly process. In the selfassembly process of PEG-P[5]A⊃PS29-EV, the homopolymers of PEG-P[5]A and PS29-EV were first dissolved in THF, respectively. Then, the THF solution of PS29-EV was added dropwise into the THF solution of PEG-P[5]A to form supramolecular copolymer, PEG-P[5]A⊃PS29-EV. Finally, the THF solution was dialyzed into water to obtain self-assembled aggregates in water. Due to the incomplete complexation in organic solvent, we could not obtain pure supramolecular copolymer. Thus, it also involved the self-assembly of the precursors, PEG-P[5]A and PS29-EV, in the following selfassembly process of PEG-P[5]A⊃PS29-EV. On the basis of the results of TEM and DLS, we assumed that the outer membrane of the cell-like vesicles would form from the assembly of PEGP[5]A⊃PS29-EV, while the inner spherical aggregates could be from the coassembly of homopolymers, PEG-P[5]A and PS29EV. To further elucidate the formation mechanism of the above cell-like vesicles, the assembled morphologies could be destroyed by the decomplexation of the supramolecular copolymer using the reduced agent (Na2S2O3).51 As shown

Figure 4. Release kinetics of calcein from the PEG-P[5]A:PS29-EV = 1:1 cell-like vesicles. Inset images: (a) calcein-unloaded vesicular solution and (b) calcein-loaded vesicular solution. 115

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(8) Chiu, H.-C.; Lin, Y.-W.; Huang, Y.-F.; Chuang, C.-K.; Chern, C.S. Angew. Chem., Int. Ed. 2008, 47, 1875. (9) Marguet, M.; Bonduelle, C.; Lecommandoux, S. Chem. Soc. Rev. 2013, 42, 512. (10) Peters, R. J. R. W.; Marguet, M.; Marais, S.; Fraaije, M. W.; van Hest, J. C. M.; Lecommandoux, S. Angew. Chem., Int. Ed. 2014, 53, 146. (11) Kim, S.-H.; Shum, H. C.; Kim, J. W.; Cho, J.-C.; Weitz, D. A. J. Am. Chem. Soc. 2011, 133, 15165. (12) Huber, M. C.; Schreiber, A.; von Olshausen, P.; Varga, B. R.; Kretz, O.; Joch, B.; Barnert, S.; Schubert, R.; Eimer, S.; Kele, P.; Schiller, S. M. Nat. Mater. 2015, 14, 125. (13) Fu, Z.; Ochsner, M. A.; de Hoog, H.-P. M.; Tomczak, N.; Nallani, M. Chem. Commun. 2011, 47, 2862. (14) Lehn, J.-M. Science 2002, 295, 2400. (15) Aida, T.; Meijer, E. W.; Stupp, S. I. Science 2012, 335, 813. (16) Daskalaki, E.; Le Droumaguet, B.; Gerard, D.; Velonia, K. Chem. Commun. 2012, 48, 1586. (17) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601. (18) Li, Y.; Park, T.; Quansah, J. K.; Zimmerman, S. C. J. Am. Chem. Soc. 2011, 133, 17118. (19) Mugemana, C.; Guillet, P.; Hoeppener, S.; Schubert, U. S.; Fustin, C.-A.; Gohy, J.-F. Chem. Commun. 2010, 46, 1296. (20) Mugemana, C.; Guillet, P.; Fustin, C.-A.; Gohy, J.-F. Soft Matter 2011, 7, 3673. (21) Hu, J.; Liu, S. Acc. Chem. Res. 2014, 47, 2084. (22) Bügler, J.; Sommerdijk, N. A. J. M.; Visser, A. J. W. G.; van Hoek, A.; Nolte, R. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1999, 121, 28. (23) Jin, H.; Zheng, Y.; Liu, Y.; Cheng, H.; Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2011, 50, 10352. (24) Tao, W.; Liu, Y.; Jiang, B.; Yu, S.; Huang, W.; Zhou, Y.; Yan, D. J. Am. Chem. Soc. 2012, 134, 762. (25) Stadermann, J.; Komber, H.; Erber, M.; Däbritz, F.; Ritter, H.; Voit, B. Macromolecules 2011, 44, 3250. (26) Yan, Q.; Yuan, J.; Cai, Z.; Xin, Y.; Kang, Y.; Yin, Y. J. Am. Chem. Soc. 2010, 132, 9268. (27) Zhao, Q.; Wang, Y.; Yan, Y.; Huang, J. ACS Nano 2014, 8, 11341. (28) Bigot, J.; Charleux, B.; Cooke, G.; Delattre, F.; Fournier, D.; Lyskawa, J.; Sambe, L.; Stoffelbach, F.; Woisel, P. J. Am. Chem. Soc. 2010, 132, 10796. (29) Sambe, L.; Stoffelbach, F.; Poltorak, K.; Lyskawa, J.; Malfait, A.; Bria, M.; Cooke, G.; Woisel, P. Macromol. Rapid Commun. 2014, 35, 498. (30) Guo, D.-S.; Liu, Y. Acc. Chem. Res. 2014, 47, 1925. (31) Nalluri, S. K. M.; Ravoo, B. J. Angew. Chem., Int. Ed. 2010, 49, 5371. (32) Zheng, Y.; Yu, Z.; Parker, R. M.; Wu, Y.; Abell, C.; Scherman, O. A. Nat. Commun. 2014, 5, 5772. (33) Wang, Y.; Ma, N.; Wang, Z.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823. (34) Baek, K.; Hwang, I.; Roy, I.; Shetty, D.; Kim, K. Acc. Chem. Res. 2015, 48, 2221. (35) Kanai, S.; Nojiri, Y.; Konishi, G.; Nakamoto, Y. Polym. Prepr. Jpn. J. 2006, 55, 303. (36) Ogoshi, T.; Kanai, S.; Fujinami, S.; Yamagishi, T.-a.; Nakamoto, Y. J. Am. Chem. Soc. 2008, 130, 5022. (37) Xue, M.; Yang, Y.; Chi, X.; Zhang, Z.; Huang, F. Acc. Chem. Res. 2012, 45, 1294. (38) Strutt, N. L.; Forgan, R. S.; Spruell, J. M.; Botros, Y. Y.; Stoddart, J. F. J. Am. Chem. Soc. 2011, 133, 5668. (39) Yu, G.; Han, C.; Zhang, Z.; Chen, J.; Yan, X.; Zheng, B.; Liu, S.; Huang, F. J. Am. Chem. Soc. 2012, 134, 8711. (40) Yu, G.; Ma, Y.; Han, C.; Yao, Y.; Tang, G.; Mao, Z.; Gao, C.; Huang, F. J. Am. Chem. Soc. 2013, 135, 10310.

behavior of calcein was investigated in the absence and presence of Na2S2O3, respectively. When the calcein-loaded vesicular solution was exposed to 0.5 mg mL−1 of Na2S2O3, the cumulative drug release in 14 h was more than 50%. However, for the control without a trigger (Na2S2O3), only about 10% of calcein molecules were released within 14 h. On the basis of the above results, we can conclude that the reducing agent resulted in the collapse of the cell-like vesicles with concomitant release of the encapsulated functional molecules, indicating these celllike vesicles have the potential application in drug release systems. In conclusion, we first constructed a supramolecular copolymer PEG-P[5]A⊃PS-EV based on PEG-P[5]A and PS-EV via host−guest interaction, which could further form unusual cell-like vesicles where the spherical aggregates were embedded in vesicles. In these cell-like vesicles, the outer membrane and the inner spherical aggregates formed from the self-assembly of supramolecular copolymer PEG-P[5]A⊃PS29EV and coassembly of amphiphilic homopolymers PEG-P[5]A and PS29-EV, respectively. Owing to the reduction responsiveness of EV, the dye molecule calcein could be controlledreleased by adding Na2S2O3. All the results presented that these supramolecular cell-like vesicles would be of great interest and importance in application of biotechnology and biomedicine.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00900. Experimental details and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64253033. Fax: +86 21 64253033. E-mail: [email protected] (W. Zhang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51173044 and 21574039), Research Innovation Program of SMEC (No.14ZZ065), Shanghai Pujiang Program under 14PJ1402600, and the State Key Laboratory of Materials-Oriented Chemical Engineering (KL14-03). W. Z. also acknowledges the support from the Fundamental Research Funds for the Central Universities.

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