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Self-Assembly of Vesicles from Amphiphilic Aromatic Amide-Based Oligomers Yun-Xiang Xu, Gui-Tao Wang, Xin Zhao,* Xi-Kui Jiang, and Zhan-Ting Li* State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China ReceiVed October 15, 2008. ReVised Manuscript ReceiVed December 15, 2008 A novel class of linear arylamide oligomers has been designed and synthesized from naphthalene-2,7-diamine and benzene-1,3,5-tricarboxylic acid segments. The molecules carry two (tert-butoxycarbonylamino) groups at the ends and one to three hydrophilic N,N-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)amino groups at one side of the backbone. The oligomers self-assembled into vesicular structures in methanol as a result of ordered stacking of the oligomeric amide backbones, which were evidenced by SEM, AFM, TEM, and fluorescent micrography experiments. It was also found that the tert-butoxycarbonylamino groups at the ends played an important role in promoting the ordered stacking of the backbones. Structural factors that affected the self-assembly of the oligomers were investigated. A two-layer model that was supported by TEM has been proposed for the formation of the vesicular structures, which was driven by both the hydrogen bonding and aromatic stacking.
Introduction Due to their quantum size effect and surface effect, nanosized structures usually exhibit unique properties that are different from those of their bulk counterparts. Among various nanoscaled particles, vesicles, microscopic capsules that enclose a volume with thin membranes consisting of a bilayer or multilayer of specific molecules or polymers, have drawn great attention because of their potential applications in drug and gene delivery,1-4 as nanoreactors,5-8 and as artificial cell membranes.9,10 Since their discovery several decades ago, many efforts have been devoted to fabricating particles with vesicular structures. Most of the early examples focused on various lipid molecules,11-14 surfactants,15-20 and block copolymers.21-27 More recently, vesicles generated from well-defined molecular * Corresponding authors. Phone: 0086-21-54925122. Fax: 0086-2164166128. E-mail:
[email protected] (X.Z.),
[email protected] (Z.T.L.). 60.
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architectures such as calixarene,28-32 cucurbituril,33-35 fullerene,36-39 cyclodextrin,40-43 and cyclophane44,45 have also been reported. One of the principles for self-assembling vesicular structures concerns the utilization of the hydrophobically driven π-π stacking of rationally designed aromatic systems in aqueous or (20) Park, J.; Rader, L. H.; Thomas, G. B.; Danoff, E. J.; English, D. S.; DeShong, P. Soft Matter 2008, 4, 1916–1921. (21) Harada, A.; Kataoka, K Science 1999, 283, 65–67. (22) Disher, D. E.; Eisenberg, A. Science 2002, 297, 967–973, and references therein. (23) Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2004, 43, 4896–4899. (24) Checot, F.; Brulet, A.; Oberdisse, J.; Gnanou, Y.; Mondain-Monval, O.; Lecommandoux, S. Langmuir 2005, 21, 4308–4315. (25) Du, J.; Tang, Y.; Lewis, A. L.; Armes, S. P. J. Am. Chem. Soc. 2005, 127, 17982–17983. (26) Li, Y. T.; Lokitz, B. S.; McCormick, C. L. Angew. Chem., Int. Ed. 2006, 45, 5792–5795. (27) Morishima, Y. Angew. Chem., Int. Ed. 2007, 46, 1370–1372. (28) Tanaka, Y.; Mayachi, M.; Kobuke, Y. Angew. Chem., Int. Ed. 1999, 38, 504–506. (29) Lee, M.; Lee, S.-J.; Jiang, L.-H. J. Am. Chem. Soc. 2004, 126, 12724– 12725. (30) Houmadi, S.; Coquiere, D.; Legrand, L.; Faure, M. C.; Goldmann, M.; Reinaud, O.; Remita, S. Langmuir 2007, 23, 4849–4855. (31) Zhou, J.-L.; Chen, X.-J.; Zheng, Y.-S. Chem. Commun. 2007, 5200– 5202. (32) Guan, B.; jiang, M.; Yang, X.; Liang, Q.; Chen, Y. Soft Matter 2008, 4, 1393–1395. (33) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S. W.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474–4476. (34) Lee, H.-K.; Park, K. M.; Jeon, Y. J.; Kim, D.; Oh, D. H.; Kim, H. S.; Park, C. K.; Kim, K. J. Am. Chem. Soc. 2005, 127, 5006–5007. (35) Kim, K.; Selvapalam, N.; Ko, Y. H.; Park, K. M.; Kim, D.; Kim, J. Chem. Soc. ReV. 2007, 36, 267–279. (36) Zhou, S. Q.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944–1947. (37) Charvet, R.; Jiang, D.-L.; Aida, T. Chem. Commun. 2004, 2664–2665. (38) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga, K. Chem. Commun. 2005, 5982–5984. (39) Gayathri, S. S.; Patnaik, A. Langmuir 2007, 23, 4800–4808. (40) Ravoo, B. J.; Darcy, R. Angew. Chem., Int. Ed. 2000, 39, 4324–4326. (41) Falvey, P.; Lim, C. W.; Darcy, R.; Revermann, T.; Karst, U.; Giesbers, M.; Marcelis, A. T. M.; Lazar, A.; Coleman, A. W.; Reinhoudt, D. N.; Ravoo, B. J. Chem. Eur. J. 2005, 11, 1171–1180. (42) Lim, C. W.; Crespo-Biel, O.; Stuart, M. C. A.; Reinhoudt, D. N.; Huskens, J.; Ravoo, B. J. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6986–6991. (43) Dong, D.; Baigl, D.; Cui, Y.; Sinay, P.; Sollogoub, M.; Zhang, Y. Tetrahedron 2007, 63, 2973–2977. (44) Murakami, Y.; Kikuchi, J.; Ohno, T.; Hayashida, O.; Kojima, M. J. Am. Chem. Soc. 1990, 112, 7672–7681. (45) Seo, S. H.; Chang, J. Y.; Tew, G. N. Angew. Chem., Int. Ed. 2006, 45, 7526–7530.
10.1021/la8034243 CCC: $40.75 2009 American Chemical Society Published on Web 02/03/2009
Self-Assembly of Vesicles from Oligomers
Langmuir, Vol. 25, No. 5, 2009 2685 Scheme 1. Synthetic Route for Compounds T1-T3
other polar media. Discrete hydrophilic aliphatic chains, especially hydrophilic oligoglycols, are usually incorporated into the rigid backbones to provide solubility and also control the order of stacking. In this context, Schenning and Meijer et al. had reported a class of straight oligo(p-phenylene vinylene)-based vesicles in water.46 Recently, we also found that several hydrogen-bonded, folded oligomeric aromatic hydrazides could stack to generate vesicles in methanol.47 These linear molecules are all rigid, which guaranteed ordered stacking of their aromatic backbones. To explore the possibility of assembling vesicular structures from relatively flexible aromatic systems, we have designed a new class of amphiphilic aromatic amide oligomers. Herein, we describe the characterization of the new vesicles assembled from them.
Results and Discussion Design and Synthesis. Linear aromatic amide polymers or oligomers may adopt extended or folded conformation, which stack in polar solvents. We therefore designed and synthesized three molecules of this kind, that is, T1-T3, in which one to three hydrophilic glycol moieties were introduced on the benzene units to produce amphiphilicity. The large naphthalene units were expected to enhance intermolecular stacking and therefore related self-assembly.
For the synthesis of T1, diamine 1 was first treated with ditert-butyl dicarbonate to afford 2 (Scheme 1). With 2 available, dimethyl 1,3,5-benzenetricarboxylate 3 was coupled with amine 4 to yield 5. Compound 5 could be hydrolyzed with lithium hydroxide to give diacid 9 or monoacid 6 by controlling the (46) Hoeben, F. J. M.; Shklyarevskiy, I. O.; Pouderoijen, M. J.; Engelkamp, H.; Schenning, A. P. H. J.; Christianen, P. C. M.; Maan, J. C.; Meijer, E. W. Angew. Chem., Int. Ed. 2006, 45, 1232–1236. (47) Cai, W.; Wang, G.-T.; Xu, Y.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2008, 130, 6936–6937.
amount of the base. Compound 6 was then reacted with amine 2 to afford 7, which was further hydrolyzed to yield acid 8. With those intermediates available, T2 was obtained by treating compound 8 with diamine 1 in DMF using HATU as condensation agent, while T1 was generated from the reaction of diacid 9 with amine 2 in the presence of EDCI and HOBt. Compound T1 was then further treated with trifluoroacetic acid in dichrloromethane to afford diamine 10, which was then coupled with acid 8 in DMF to generate T3. Vesicle Self-Assembly. The morphology of the aggregates of T1-T3 in methanol was first investigated by tapping-mode AFM. The AFM images are provided in Figure 1. The images were prepared by depositing their methanol solutions on silicon surface. All the images show the formation of spherical aggregates. Crosssection analysis (bottom ones) of the typical structures revealed a large ratio of diameter to height for all the three samples (approximately 11.4, 7.5, and 25.8, respectively), which indicated that they were of flattened shape.48 The result also supports that the spherical structures were hollow and contained solvent molecules, which were evaporated after being transferred from the solution to the silicon surface.36,49 SEM imaging provided further evidence for the formation of the vesicular structures (Figure 2). The diameters of the vesicles of T3 ranged approximately from 2 to 5 µm (Figure 2c) and pores could be observed on the surface of some of the vesicles. Similar vesicular structures were also exhibited from the samples of T1 and T2 (Figure 2a,b), even though the sizes of those of T2 were notably smaller. Twin, triplet, or small-large vesicles could also be observed. This result implies that the larger ones should be generated from the fusion of smaller ones (see Figure 4, vide infra). Notably, some vesicles formed by T1 showed collapsed structures on the silicon surface, suggesting that these spherical structures were not as stable as those of T2 and T3. The fluorescence micrographs provide the first direct evidence for the hollow feature of the vesicles (Figure 3). There is a large difference between the luminance of the outer rings of the spots (48) Yang, M.; Wang, W.; Yuan, F.; Zhang, X.; Li, J.; Liang, F.; He, B.; Minch, B.; Wegner, G. J. Am. Chem. Soc. 2005, 127, 15107–15111. (49) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S. Angew. Chem., Int. Ed. 2006, 45, 3261–3264.
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Figure 1. Tapping-mode AFM images and cross-section analysis of (a) T1 (0.02 mM), (b) T2 (0.1 mM), and (c) T3 (0.1 mM) in methanol on silicon plate after the solvent was evaporated.
Figure 2. SEM images of the samples (4 mg/mL) of (a) T1 in methanol, (b) T2 in methanol, (c) T3 in methanol, (d) T3 in methanol-chloroform (1:1), (e) T3 in methanol-chloroform (9:1), and (f) T2 in methanol-water (9:1) on a silicon surface after the solvent was evaporated.
and that of their inner part, which clearly shows that the vesicles are centrally hollow, although the wall thickness of the vesicles could not be evaluated from the images. Dye-encapsulation experiments were also performed. In the presence of rhodamine B, the vesicles could entrap the dye. After dialysis, the dye in the solution could be removed. However, the entrapped dye could be kept in the cavity of the vesicles,33,47 which not only supported their hollow feature but also indicated that the assembling structures were quite stable. TEM was also used to investigate the vesicular nature of the aggregates. The results provided further evidence for their hollow feature (Figure 4), as is revealed by the clear contrast between the peripheral and central areas of the spherical aggregates, which
is typically produced by the projection of hollow spheres.50 The images also show that the vesicles may overlap (Figure 4a), contact without fusion (Figure 4b), or partially fuse (Figure 4c), which reflects their good stability. Due to the interference of halos surrounding the vesicles, accurate evaluation of their wall thickness could not be performed. Therefore, TEM images of T2 at lower concentration (0.01 mM) were recorded (Figure 4d). It was estimated that the wall thickness of the vesicles was approximately 2.4 nm. The result appears to suggest that the wall of the vesicles has an interdigitated bilayer structure (Figure (50) (a) Ajayaghosh, A.; Chithra, P.; Varghese, R. Angew. Chem., Int. Ed. 2007, 46, 230–233. (b) Xie, D.; Jiang, M.; Zhang, G. Z.; Chen, D. Y. Chem. Eur. J. 2008, 13, 3346–3353.
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Figure 3. Fluorescence micrographs of vesicles of (a) T1, (b) T2, and (c) T3 formed in methanol (2 mg/mL) and rhodamine B-entrapped vesicles of (d) T2 and (e) T3 after being purified by dialysis.
5, vide infra),50 although other stacking patterns cannot be excluded. Effect of the Solvent Systems on the Self-Assembly. In order to get insight into the assembling mechanism, the SEM images were also obtained for the samples produced from their solutions in chloroform and methanol mixtures (vide supra, Figure 2). No spherical aggregates were observed for the compounds in pure chloroform. With the increasing addition of methanol, vesicular structures began to generate for all the three compounds. Moreover, the density and size of the vesicles increased with the
percentage of methanol (Figure 2d, 2e). For T2, adding 10% of water to the solution could cause the size of the vesicles to increase substantially (approximately 5-fold, compared to that in methanol, figure 2f). These results indicate that intermolecular stacking of the backbones, which was enhanced in polar media, was the important driving force for the formation of the vesicles. Effect of the End Groups on the Self-Assembly. To evaluate the structure-morphology relationship of this type of oligomer, compounds 11a-c were also prepared. SEM investigations showed that both 11a and 11b could not give rise to vesicular aggregates in methanol, chloroform-methanol, or methanol-water mixtures. It was also the case for compound 10. These results implied that the Boc groups of T1-T3 played a crucial role in promoting the formation of the vesicular structures. It has been recently reported that Boc improves the assembling selectivity of artificial duplexes from linear hydrazide-based monomers by inhibiting the intermolecular head-to-tail contact.51 It might be reasonable to propose that the Boc groups in T1-T3 weakened similar intermolecular head-to-tail stacking through simple steric hindrance. As a result, the formation of linear supramolecular polymers was avoided and their backbones stacked in the faceto-face style to generate column assemblies, which further assembled to form the bilayered structures of the vesicles (Figure 5, vide infra).
Interestingly, SEM showed that compound 11c, which bears twopyreneunitsattheends,couldformvesiclesinmethanol-chloroform solvent, but it did not possess such capacity in pure methanol (see Supporting Information, Figure S1). The detailed reason is yet unclear, although intermolecular stacking between the pyrene Figure 4. TEM images of the samples of (a) T1, (b) T2, (c) T3 (0.5 mg/mL), and (d) T2 (0.01 mM) in methanol after the solvent was evaporated (200 kV).
(51) (a) Yang, Y.; Xiang, J.-F.; Chen, C.-F. Org. Lett. 2007, 9, 4355–4357. (b) Yang, Y.; Xiang, J.-F.; He, M.; Hu, H.-Y.; Chen, C.-F. J. Org. Chem. 2008, 73, 6369–6377.
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Figure 5. Tentative model for the formation of the vesicular structures from the linear molecules (with T2 as example). The green part represents polyethylene glycol chains, the purple part represents the aromatic backbone, and the yellow part represents Boc groups.
units might play a role in enhancing the aggregation. This result shows that the formation of the vesicles from this class of oligomers might be further tuned by modifying the appended groups. Self-Assembling Mechanism. 1H NMR dilution studies in methanol-d4 showed that the signals of the aromatic units of T1-T3 shifted downfield upon decreasing their concentration, which supported that intermolecular stacking existed for these compounds in the polar solvent (see Supporting Information, Figures S2-S4).52 It is well-known that the amide units form intermolecular hydrogen bonding in less polar solvent such as chloroform. However, this interaction might also occur in methanol for T1-T3 because extensive stacking would shield the aromatic backbones from the competitive solvent molecules, as exhibited by DNA or synthetic systems.53 The above TEM study has shown that the vesicles had a wall with a thickness of approximately 2.4 nm. CPK modeling revealed that the backbones of the oligomers have a width of approximately 0.8 nm, while the length of the extended chains is about 1.1 nm. Therefore, we propose that the vesicles possess a wall of interdigitated bimolecular layers,48 which consisted of alternately stacked molecules of the oligomers, as shown in Figure 5. (52) Wang, W.; Li, L.-S.; Helms, G.; Zhou, H.-H.; Li, A. D. Q. J. Am. Chem. Soc. 2003, 125, 1120–1121. (53) Yoshikawa, I.; Sawayama, J.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 1038–1041.
Conclusions In this study, we describe a new series of aromatic amidebased oligomers that self-assemble into vesicular structures in methanol. Stacking interaction and hydrogen bonding have been proposed to be the major driving forces, whereas steric Boc groups at the ends of the oligomers are utilized to inhibit intermolecular head-to-tail stacking and to help to generate column-styled aggregates. Previous self-assembly of vesicles from aromatic organic molecules has focused on the utilization of rigid backbones. The present study indicates that relative flexible molecular systems can also be used for this purpose. In the next coming steps, we foresee performing structural modifications aimed at preparing vesicles that are responsive to external photo- or electric stimuli. Acknowledgment. We thank the National Science Foundation of China (Nos. 20732007, 20621062, 20425208, 20572126, 20672137),theNationalBasicResearchProgram(2007CB808000), and Chinese Academy of Sciences (KJCX2-YW-H13) for financial support. Supporting Information Available: General methods, the synthetic procedures and characterizations of compounds T1-T3 and 11a-11c, SEM images of 11c obtained from different solvents, and spectra of 1H NMR dilution studies of T1-T3 in methanol-d4. This material is available free of charge via the Internet at http://pubs.acs.org. LA8034243
