Glutamic Acid-Based Amphiphilic Random Copolymers - American

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ARTICLE pubs.acs.org/Langmuir

Self-Assembly and Morphology Control of New L-Glutamic Acid-Based Amphiphilic Random Copolymers: Giant Vesicles, Vesicles, Spheres, and Honeycomb Film Xuewang Zhu†,‡ and Minghua Liu*,† †

CAS Key Laboratory of Colloids, Interfaces and Chemical Thermodynamics Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China ‡ School of Science, Hebei University of Science and Technology, Shijiazhuang, 050018, China

bS Supporting Information ABSTRACT: New amphiphilic random copolymers containing hydrophobic dodecyl (C12) chain and hydrophilic L-glutamic acid were synthesized, and their self-assembly in solution as well as on the solid surfaces was investigated. The self-assembly behavior of these polymers are largely dependent on their hydrophilic and hydrophobic balances. The copolymer with a more hydrophobic alkyl chain (∼90%) selfassembled into giant vesicles with a diameter of several micrometers in a mixed solvent of ethanol and water. When the hydrophobic ratio decreased to ca. 76%, the polymer self-assembled into conventional vesicles with several hundred nanometers. The giant vesicles could be fused in certain conditions, while the conventional vesicles were stable. When the content of the hydrophilic part was further increased, no organized structures were formed. On the other hand, when the copolymer solutions were directly cast on solid substrates such as silicon plates, films with organized nanostructures could also be obtained, the morphology of which depended on solvent selection. When ethanol or methanol was used, spheres were obtained. When dichloromethane was used as the solvent, honeycomb-like morphologies were obtained. These results showed that through appropriate molecular design, random copolymer could self-assemble into various organized structures, which could be regulated through the hydrophobic/hydrophilic balance and the solvents.

1. INTRODUCTION The self-assembly of various building blocks into organized nano/microstructures as well as their morphological control is one of the central issues in colloid and nanosciences. Some building blocks such as amphiphiles and polymers could show versatile morphologies using only one kind of species under different conditions, which is very important in considering application of the nanostructured materials. Among possible polymers showing such properties, block polymers and dendrimers are highly focused due to their unique and excellent assembly behaviors.1 5 In contrast, the random copolymers are rather less studied due to their ill-defined properties in terms of structure control. However, compared with block polymers, random polymers are easy to be synthesized and their selfassemblies are also expected if appropriate design of the polymers were considered. For example, Yang et al. reported the continuous morphological transitions from spherical micelles through hollow tubes and wormlike rods to large vesicles for the designed photoresponsive amphiphilic random copolymer.6 Wang et al. studied colloidal sphere formation, H aggregation, and photoresponsive properties of amphiphilic random copolymers functionalized with branched azobenzene side chains.7,8 Liu et al.9 obtained bowl-shaped aggregates, large-compound micelles, and porous spherical micelles from self-assembly of amphiphilic random copolymers of poly(styrene-co-methacrylic acid) under r 2011 American Chemical Society

different conditions. Other researchers, such as Zhou,10 Yan,11 and Yusa,12 reported self-assembly of amphiphilic random copolymers in solution. In this paper, we designed a series of new random copolymers from N-acroloyl-L-glutamic acid (NALGA) and N-acroloyl-dodecyl amine (NADA) units through radical polymerization. Various organized nanostructures ranged from giant vesicles, vesicles, spheres, and honeycomb-like films were obtained using such polymers. Giant vesicles (GVs) are cell-sized membrane systems having diameters larger than 1 5 μm. Since the structure and dynamic behaviors are similar to those of biological cell membranes, GVs have attracted great attention for their potential applications in encapsulation and release of drugs, model systems as biological membranes.13 21 Thus far, various molecules such as lipids,22 25 synthetic surfactants,26 31 and amphiphilic copolymers32 36 have been used to construct GVs. Among various vesicles, polymer vesicles, also named polymersomes, have been attracting increasing interest due to their large hydrophilic reservoir and thick membrane, which could have a large storage capacity for both water-soluble and insoluble substances.37 40 In addition, compared to the vesicles from lipids with low thickness, the Received: July 13, 2011 Revised: September 26, 2011 Published: September 26, 2011 12844

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Figure 1. Synthetic route and molecular structures of monomers and copolymers.

polymersomes generally have a thicker wall with good colloidal stability.41,42 However, the GVs of the polymers are mainly from the well-defined block polymers,43 46 although it has been shown that ill-defined hyperbranch polymer could also form the GVs.47 50 Random copolymer51,52 is scarcely reported in forming the giant vesicles. Therefore, it is important to see if the simply designed random copolymers could facilitate formation of the giant vesicles. Besides the GVs, ordinary vesicles or hollow sphere structures could show potential applications in drug delivery systems and are receiving much attention. Besides the solution, the solid surfaces can also provide an important mean to control the supramolecular assembly behaviors. We further found that some of these copolymers could directly self-assemble into spheres or honeycomb-like film just by casting, the morphology of which depended on the solvents. The honeycomb-like ordered porous materials with pore sizes in the micrometer to submicrometer range have elicited much interest owing to their applications in separation, catalysis, optoelectronic devices, and so forth.53 58 Generally, these porous patterns could be obtained through a “breath figure” (BF) method59 67 in a higher humidity environment. However, in our case, simple casting of the CH2Cl2 solution of the polymer could produce such honeycomb-like structures. We used a simple random copolymer and realized versatile structures from small vesicles to giant vesicles and to the honeycomb-like film.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemical structure and synthetic steps of the random copolymers are shown in Figure 1. Acryloyl chloride was purchased from Alfa Aesar and vacuum distilled before use. L-Glutamic acid diethyl ester hydrochloride was from Yangzhou Baosheng BioChemical Co. Ltd. Dodecylamine, azodiisobutyronitrile (AIBN), triethylamine (TEA), dichloromethane, hydrochloric acid, ethanol, magnesium sulfate (MgSO4), and puerarin were from Beijing Chemical Reagent Co. AIBN was recrystallized from ethanol. All other chemicals were analytically pure and used without further purification. In the experimental process, deionized Millipore-Q water (18.2 MΩ 3 cm) was used. 2.2. Instruments. 1H NMR spectra were recorded on a BRUKER AVANCE 400 MHz spectrometer. Molecule weight was determined by Water 201 gel permeation chromatography (GPC). Transmission electron microscope (TEM) micrographs were obtained with a

JEM-2011 TEM (working voltage of 200 kV). A drop of the solution was placed onto a carbon-supported copper grid for 5 min. The excess liquid was sucked away by filter paper. Cryogenic transmission electron microscope (cryo-TEM) samples were prepared in a controlled environment vitrification system (CEVS). A small drop of sample was placed on a carbon-supported lacey TEM grid, and a thin film was produced by blotting off the redundant liquid with filter paper. This thin film was then quickly dipped into liquid ethane, which was cooled by liquid nitrogen. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined by a JEM-2200FS TEM (200 kV) at about 174 °C. Scanning electron microscope (SEM) images of samples were investigated using HITACHI S-4300 and S-4800 electron microscopes. Light microscopy, differential interference contrast microscopy, and confocal laser scanning microscopy (CLSM) were recorded using FV1000-IX81 confocal laser scanning microscopy. The excitation wavelength of CLSM is 488 nm with puerarin as a fluorescent probe. Puerarin was added in the solution of copolymer 1 before formation of the organized structures. The content of puerarin was 1 wt % of the copolymer. The solution was dropped on a 0.17 mm glass slide surface and air dried. The sample was then washed with water to remove free puerarin. FTIR spectra of copolymers were measured using a JASCO 660.

2.2. Synthesis of N-Acroloyl-L-glutamic Acid Diethyl Ester.

L-Glutamic

acid diethyl ester hydrochloride (0.01 mol) was dissolved in dichloromethane (40 mL), and TEA (3 mL) was added. The mixture was stirred for 30 min at room temperature. Acryloyl chloride (0.01 mol) dissolved in dichloromethane (30 mL) was added dropwise to the solution in 30 60 min and continued to stir for 3 h. The resulting solution was washed with 0.1 mol 3 L 1 hydrochloric acid and water 3 times. The organic phase was separated, dried over MgSO4, filtered, and evaporated nearly dry in a rotary evaporator at 40 °C. 1 H NMR (400 MHz, CDCl3): 6.52 (1H, s), 6.29 6.33 (1H, d, J = 16 Hz), 6.12 6.19 (1H, m), 5.67 5.70 (1H, d, J = 12 Hz), 4.67 4.72 (1H, m), 4.21 4.25 (2H, m), 4.10 4.15 (2H, m), 2.33 2.47 (2H, m), 2.22 2.27(1H, m), 2.02 2.09 (1H, m), 1.23 1.31 (6H, m). 2.3. Synthesis of N-Acroloyl-dodecyl Amine (NADA). The methods of preparation and separation were similar to those in section 2.2. 1 H NMR (400 MHz, CDCl3): 6.25 6.29 (1H, d, J = 16 Hz), 6.06 6.13 (1H, m), 5.75 (1H, br), 5.60 5.63 (1H, d, J = 12 Hz), 3.29 3.34 (2H, m), 1.50 1.57 (2H, m), 1.26 1.34 (18H, t), 0.86 0.70(3H, t). 2.4. Synthesis of Copolymers. Copolymerization was carried out with N-acroloyl-L-glutamic acid diethyl ester (0.78 g, 0.003 mol) and 12845

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Table 1. Details of Random Copolymers (poly(NALGA-coNADA)) reaction ratio copolymers

(M1:M2, mol)

m:n

Mn, g/mol

Mw/Mn

1 2

3:7 5:5

9:91 24:76

7100 8200

1.70 1.96

3

7:3

46:54

6400

2.00

4

9:1

77:23

4200

2.38

N-acroloyl-dodecyl amine (NADA) (1.80 g, 0.007 mol) as monomers, AIBN (0.0492 g, 0.0003 mol) as initiator, and ethanol (20 mL) as solvent at 80 °C for 10 h. The copolymer was precipitated when plenty of water was added in, washed with water, and dried by the infrared lamp. The pure copolymer was obtained after being dissolved in ethanol and precipitated by water two times. Poly(NALGA-co-NADA) was prepared after alkaline hydrolysis and acidification treatment. Poly(N-acroloyl-Lglutamic acid diethyl ester) (2 g) was dissolved in 50 mL of methanol. Alkaline hydrolysis (NaOH, 4 g) dissolved in 10 mL of water was added to the solution dropwise and continued to stir for 3 days at room temperature. White precipitate was obtained after the solution was adjusted to acidity with 0.1 mol/L 1 hydrochloric acid. A series of copolymers with different ratios of the hydrophobic part and L-glutamic acid component was obtained by changing the initial molar ratio of N-acroloyl-L-glutamic acid diethyl ester to N-acroloyldodecyl amine. Four initial ratios of 3:7, 5:5, 7:3, and 9:1 were used, and the real ratio in the polymers was analyzed by 1H NMR.

3. RESULTS AND DISCUSSION 3.1. Characterization of Copolymers. Four copolymers were synthesized by changing monomer ratios. Details of copolymers are listed in Table 1. These copolymers were synthesized using different ratios of the M1 (L-glutamic acid) and M2 (alkyl chains) units, which are hydrophilic and hydrophobic, respectively. Since M1 and M2 have a different polymerization speed, the final ratio of M1 and M2 units in the produced copolymer was different. Their relative ratios were determined by analyzing the ratio of the characteristic peak area of M1 and M2 by 1H NMR. The characteristic peaks of M1 and M2 are the hydrogen in the chiral carbon of L-glutamic acid (δ = 4.3) and the methyl group of lauryl amine (δ = 0.83). Although the final ratio of M1 to M2 is different from that of the starting units, there is a tendency that with the increase of the initial reaction ratio of M1 the content of the hydrophilic unit was also increased. The molecular weights of the formed polymers are in a range of 4000 8000 with a dispersity around 2. This showed that a random copolymer was formed. The solubility of the formed copolymers changed depending on their components. Copolymers 1 and 2 are soluble in alcohol but insoluble in water. On the contrary, copolymers 3 and 4 are soluble in water but insoluble in many organic solvents. 3.2. Self-Assembly of the Compounds in Mixed Ethanol/ Water Solvents—Vesicle Formation. Self-assembly properties of copolymers were performed in the mixed solvents of ethanol and water. In the experiments, copolymer 1 was first dissolved in ethanol and then an appropriate amount of water was added into the solution. The ethanol solutions of copolymer 1 turned turbid after adding 10% water (volume fraction), indicating aggregation of the copolymer in the solvents. Aggregation of the polymer can be visualized even using the optical microscope. Figure 2a and 2b shows the optical microscope and differential interference contrast micrographs (DICM) of the solution; larger circular

structures ranging from 1 to 20 μm can be seen. The size distribution of these circular structures is shown in Figure S1, Supporting Information, and most of them ranged from 2 to 3 μm after being prepared for 10 min. The details of these organized structures were further investigated using confocal laser scanning microscopy (CLSM) and TEM. Figure 2c shows the CLSM micrograph of the droplet on glass plates. It was revealed that vesicle structure was formed. The vesicle consisted of the bright edges and less fluorescent central regions, which supported their hollow feature. The size is about several micrometers, indicating formation of the giant vesicle (GV). Solid evidence for giant vesicular structures was obtained by cryo-TEM observation, as shown in Figure 2d and 2e. The vesicle structures are clearly seen. The wall thickness of vesicles measured by TEM is about 18 nm, as shown in the enlarged Figure 2f and 2g. Considering the ill-defined structure of the copolymer, we could suppose that the structure might be random, with the more hydrophilic segments near or at the interface with the solvent and the more hydrophobic segments in the interior of the vesicle wall. When copolymer 2 was used and self-assembled in the mixed ethanol/water solvents, a similar phenomenon was observed. However, in this case, the size of the vesicles is very small, which is several hundred nanometers, as shown in Figure 3. The wall thickness of vesicles measured by TEM is about 6 nm, which is thinner than that formed from copolymer 1. This might be due to the increase of L-glutamic acid groups that more hydrophilic groups on the vesicles surface can stabilize a larger surface area and produced a smaller size. In addition, in the SEM pictures we could see that most of these vesicles are connected to each other and some of them formed a string, as shown in Figure 3b and Figure S2, Supporting Information. Generally, there are hydrophobic patches on the surface of random copolymer assemblies, which could connect the neighboring vesicles. In the case of copolymer 2, there are more L-glutamic acid groups, which could form a hydrogen bond. These H bonds could also connect the neighboring vesicles. In the case of copolymers 3 and 4, neither vesicles nor other organized structures were observed. This might be due to the higher water solubility of the polymer with a higher content of the glutamic acid part. These results suggested that the balance between the hydrophobic alkyl chains and the hydrophilic glutamic acid is very important in forming the GVs or vesicles. 3.3. Fusion of the Vesicles. The fusion property is an important issue in the research on vesicles, which is frequently observed for vesicles from lipids and other molecules. Although our GVs have a thicker layer than usually GVs, they could also fuse into larger GVs under certain conditions. Using DICM we did a real-time monitoring of the fusion process for vesicles of copolymer 1 (Figure 4 and Figure S3, Supporting Information). Figure 4 records the fusion process of two vesicles. One vesicle close to the other vesicle gradually met and intimately contacted fused into one finally. The diameters of two vesicles were about 5 and 9 μm and became about 9.5 μm. The speed of fusion was on a time scale of 1 s, and some could be in a less short time (Figure S3, Supporting Information). It is similar to membrane fusion in biology. Therefore, the system could be used as a model to mimic some biological processes. When the water content was increased to 20 vol %, the GVs were still formed and become stable. Fusions of vesicles were scarcely observed (Figure S4, Supporting Information). Some 12846

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Figure 2. Optical (a) and differential interference contrast (b) micrographs, LSCM image (c), Cryo-TEM images (d g) of GVs from copolymer 1 at 10 vol % water content. The initial copolymer concentration in alcohol is 2 mg/mL. The scale bar is 20 μm in b. The vesicle size has a distribution from 1 to 20 μm. In the LSCM and TEM observations, different samples were used. Therefore, their sizes are not the same.

Figure 3. SEM (a, b) and TEM images (c, d) of vesicles from copolymer 2 at 30 vol % water content.

vesicles contacted with each other but did not fuse. Therefore, by adjusting the content of the water in the mixed solvents, the properties of the GVs can be regulated. 3.4. Self-Assembly of the Copolymers on Solid Substrate. Solid substrate can provide an important environment for selfassembly of the polymers. Evaporative self-assembly of random copolymers was studied by dissolving copolymer 1 in various solvents with different concentrations and drop cast on a silicon chip. The copolymer solutions were subjected to slow evaporation about 1 h at 20 °C in air. SEM and TEM pictures of copolymer 1 are shown in Figure 5 and Figure S5, Supporting Information. It is very clear from the SEM pictures that polymer 1 produced sphere structures for the films prepared from ethanol. When copolymer 1 concentrations were no more than 1 mg/mL, the size of spheres was about 200 300 nm and relatively uniform. When the concentration reached 2 mg/mL, the structures were not uniform spheres and sheet structures appeared; the size of some spheres were found to increase from nanometers to micrometers. Structures of evaporative self-assembly had large changes when copolymer 1 was dissolved in different solvents and drop cast on a silicon chip, as shown in Figure 6. The morphology from

methanol solution formed the same sphere structures as from ethanol solution. Irregular particles formed a film with a small amount pores from acetone and 1, 4-dioxane solution. Interestingly, the polymers produced typical microporous membranetype morphology for films prepared from dichloromethane on an indoor environment with 10 20% humidity. The porous morphology is not confined to small domains or edges, and it appeared throughout the substrate. The pore size and distance between each pore were 2 ( 0.5 and 0.8 ( 0.5 μm. Honeycombpatterned microporous films have potential applications in several fields. The breath figure method is a common method for fabricating honeycomb structures from a variety of materials, including polymers, but a high relative humidity is necessary. For our polymer, it is easier to obtain this honeycomb structure only by selecting appropriate solvent and casting on solid substrate. If the casting was performed at a higher humidity (90%, 50%) (Figure S6, Supporting Information), similar honeycomb-like structures could be obtained. This suggested that microporous structures formed due to the dewetting and phase separation mechanism.68 However, the size of the pores became less uniform, and the surfaces were rough, which was due to the hydrophilic nature of L-glutamic acid and H-bond ability at a higher humidity. Evaporative self-assembly of copolymer 2 in ethanol and methanol was similar to copolymer 1 (Figure S7, Supporting Information). Copolymer 2 is not soluble in some organic solvents, such as dichloromethane, because of the more polar group. Thus, evaporative self-assembly of copolymer 2 is limited by solvents. 3.5. FT-IR Spectra. These copolymers contained two components. One is the hydrophobic long alkyl chain connected through the amide group to the polymer chain. The other is the L-glutamic acid moiety attached to the polymer backbone. These polymers showed different solubility in water or organic solvents depending on their ratios. When polymers selfassembled in the mixed water/ethanol solvent, both the hydrophobic interactions between the alkyl chain and the hydrogen bond between the amide and carboxylic acid played an important roles. Such hydrophobic interactions or the hydrogen bond 12847

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Figure 4. Fusion images of GVs from copolymer 1 at 10 vol % water content. The initial copolymer concentration in alcohol is 2 mg/mL content: (a) 0, (b) 1, (c) 2, (d) 3, (e) 4, (f) 5 s. The time interval for each images was 1 s.

Figure 7. FT-IR spectra of copolymer 1 and copolymer 2 vesicles. Figure 5. SEM images of copolymer 1 structures prepared from ethanol with different concentrations: (a) 0.25, (b) 0.5, (c) 1, and (d) 2 mg/mL.

Figure 6. SEM images of copolymer 1 structures prepared from different solvents (1 mg/mL): (a) methanol, (b) acetone, (c) 1,4dioxane, (d f), dichloromethane.

(H bond) can be verified from the FT-IR spectra. Figure 7 shows the FT-IR spectra of the self-assembled structures from copolymer 1 and copolymer 2. Strong vibrations were observed at 3330, 2925, 2855, 1712, 1644, and 1550 cm 1 for copolymer 2, which

were assigned to the N H stretching vibration, asymmetric and symmetric vibrations of CH2, H bond between carboxylic acid, amide I, and amide II, respectively. The position of these bands indicated that both the amide groups and the carboxylic acid groups formed the H bond in the assemblies. In addition, the alkyl chain showed some gauche conformations since these vibration bands appeared at larger wavenumbers. 3.5. Discussion. Block polymers were largely reported to show excellent self-assembly behaviors, and many interesting nanostructures could be formed. In the present case, a random copolymer containing L-glutamic acid showed a similar good selfassembly manner. It was suggested that the H bond in the side chains and the hydrophobic interaction between the long alkyl chains played important roles. Vesicle formation and fusion are illustrated in Scheme 1. When water was added into a polymer solution of ethanol, hydrophobic groups tended to aggregate together by the chain segment movement and hydrophilic groups were located in the exterior of hydrophobic groups facing the outside water. Thus, vesicles formed. Both sides of vesicle wall are in the hydrophilic region; the interior is in the hydrophobic region. It is suggested that there is no obvious demarcation line between the hydrophilic and the hydrophobic region. L-Glutamic acid groups are mainly located in hydrophilic region, while some hydrophobic groups could penetrate each other. Vesicles from copolymer 1 have a thicker hydrophilic region compared to copolymer 2 because of the higher content of hydrophobic groups. Thus, the wall thickness of copolymer 1 vesicles is greater than copolymer 2. 12848

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Langmuir Scheme 1. Schematic Structure of Vesicles Wall of Copolymer 1 and Copolymer 2a

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molecular design easily synthesized copolymers could also show controllable and versatile organized nanostructures.

’ ASSOCIATED CONTENT

bS

Supporting Information. Size distribution, SEM images of vesicles from copolymer 2, other fusion images, fusion images achieved at a faster rate, GVs from copolymer 1 at 20 vol % water content, TEM images of copolymer 1 structures prepared from ethanol, honeycomb structures prepared in high humidity, and SEM images of copolymer 2 in ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-10-82615803. E-mail: [email protected].

a

The blue part represents hydrophilic groups, and the red part represents hydrophobic groups. There is a strong H bond between amide groups and the carboxylic acid groups. Upon formation of the vesicles, the hydrophobic part tended to aggregate inside while the hydrophilic part aggregated on the inner- and outer-sphere surface.

These giant vesicles showed fusion behavior. When two vesicles encountered each other, the walls of vesicles can be interpenetrated because both the packing of the alkyl chains was relative loose and the H bond between the layers was not so strong. Since the surface areas of vesicles decrease and the average density of hydrophilic groups on vesicles surfaces increase after fusing, the stability of vesicles increased. Thus, these vesicles can be fused. However, when copolymer 2 was used, although the packing of the alkyl chain decreased (76% alkyl chain), the H bond become stronger, which forbids the vesicles to be fused. Instead, we observed the alignment of the vesicles like a string of beads, which was suggested to be due to the H bond. In the case of casting, it is suggested similar interactions between the alkyl chains and the H bonds played an important role in forming these organized structures. In addition, evaporation of the interaction between the solvents and the polymers functioned also. In the case of ethanol or methanol, due to the possible H bond between the solvents and the polymers, they formed sphere structures.

4. CONCLUSIONS Four amphiphilic random copolymers with different hydrophilic and hydrophobic fractions were synthesized though radical polymerization. The copolymer containing 9% L-glutamic acid could form GVs in the mixed alcohol/water solvents. We further observed the fusion process of the GVs. When the content of the alkyl chain decreased, the stable vesicles were formed due to the strong H bond. Besides the solutions, these copolymers formed ordered structures in a large area on solid substrate, which was formed just by casting. By changing the solvents, the morphologies of the cast films can be regulated from spheres to honeycomb films. Our work showed that through an appropriate

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 91027042, 21021003), the Basic Research Development Program (2007CB808005 and 2009CB930802), and the Fund of the Chinese Academy of Sciences. ’ REFERENCES (1) Blanazs, A.; Armes, S. P.; Ryan, A. J. Macromol. Rapid Commun. 2009, 30, 267–277. (2) Smith, D. K.; Hirst, A. R.; Love, C. S.; Hardy, J. G.; Brignell, S. V.; Huang, B. Q. Prog. Polym. Sci. 2005, 30, 220–293. (3) Segalman, R. A. Mater. Sci. Eng. R 2005, 48, 191–226. (4) Soo, P. L.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 923–938. (5) Hamley, I. W. Nanotechnology 2003, 14, 39–54. (6) Tian, F.; Yu, Y. Y.; Wang, C. C.; Yang, S. Macromolecules 2008, 41, 3385–3388. (7) Deng, Y. H.; Li, Y. B.; Wang, X. G. Macromolecules 2006, 39, 6590–6598. (8) Li, Y. B.; Deng, Y. H.; Tong, X. L.; Wang, X. G. Macromolecules 2006, 39, 1108–1115. (9) Liu, X. Y.; Kim, J. S.; Wu, J.; Eisenberg, A. Macromolecules 2005, 38, 6749–6751. (10) Zhou, J. F.; Wang, L.; Wang, C. L.; Chen, T.; Yu, H. J.; Yang, Q. Polymer 2005, 46, 11157–11164. (11) Yan, L. F.; Zhang, G. Z.; Liu, G. M.; Ji, J.; Li, W. J. Appl. Polym. Sci. 2006, 100, 3718–3726. (12) Yusa, S.; Sakakibara, A.; Yamamoto, T.; Morishima, Y. Macromolecules 2002, 35, 5243–5249. (13) Ahmed, F.; Discher, D. E. J. Controlled Release 2004, 96, 37–53. (14) Luisi, P. L.; Walde, P.; Oberholzer, T. Curr. Opin. Colloid Interface Sci. 1999, 4, 33–39. (15) Szostak, J. W.; Bartel, D. P.; Luisi, P. L. Nature 2001, 409, 387–390. (16) Dimova, R.; Riske, K. A.; Aranda, S.; Bezlyepkina, N.; Knorr, R. L.; Lipowsky, R. Soft Matter 2007, 3, 817–827. (17) Voskuhl, J.; Ravoo, B. J. Chem. Soc. Rev. 2009, 38, 495–505. (18) Dobereiner, H.-G. Curr. Opin. Colloid Interface Sci. 2000, 5, 256–263. (19) Menger, F. M.; Angelova, M. I. Acc. Chem. Res. 1998, 31, 789–797. (20) Antonietti, M.; F€orster, S. Adv. Mater. 2003, 15, 1323–1333. (21) Kita-Tokarczyk, K.; Grumelard, J.; Haefele, T.; Meier, W. Polymer 2005, 46, 3540–3563. (22) Toyota, T.; Tsuha, H.; Yamada, K.; Takakura, K.; Yasuda, K.; Sugawara, T. Langmuir 2006, 22, 1976–1981. (23) Tanaka, T.; Sano, R.; Yamashita, Y.; Yamazaki, M. Langmuir 2004, 20, 9526–9534. 12849

dx.doi.org/10.1021/la202680j |Langmuir 2011, 27, 12844–12850

Langmuir (24) Tamba, Y.; Yamazaki, M. Biochemistry 2005, 44, 15823–15833. (25) Li, Y.; Lipowsky, R.; Dimova, R. J. Am. Chem. Soc. 2008, 130, 12252–12253. (26) Tsumotoa, K.; Matsuob, H.; Tomitaa, M.; Yoshimura, T. Colloids Surf., B 2009, 68, 98–105. (27) Takakura, K.; Toyota, T.; Sugawara, T. J. Am. Chem. Soc. 2003, 125, 8134–8140. (28) Ryhnen, S. J.; Sily, V. M. J.; Parry, M. J.; Luciani, P.; Mancini, G.; Alakoskela, J.-M. I.; Kinnunen, P. K. J. J. Am. Chem. Soc. 2006, 128, 8659–8663. (29) Roy, S.; Khatua, D.; Dey, J. J. Colloid Interface Sci. 2005, 292, 255–264. (30) Morigaki, K.; Walde, P. Langmuir 2002, 18, 10509–10511. (31) Xu, Y.-X.; Wang, G.-T.; Zhao, X.; Jiang, X.-K.; Li, Z.-T. Langmuir 2009, 25, 2684–2688. (32) Sun, J.; Chen, X.; Deng, C.; Yu, H.; Xie, Z.; Jing, X. Langmuir 2007, 23, 8308–8315. (33) Breitenkamp, K.; Emrick, T. J. Am. Chem. Soc. 2003, 125, 12070–12071. (34) Gao, K.-J.; Li, G.; Lu, X.; Wu, Y. G.; Xu, B.-Q.; Fuhrhopb, J.-H. Chem. Commun. 2008, 1449–1451. (35) Tsuda, K.; Dol, G. C.; Gensch, T.; Hofkens, J.; Latterini, L.; Weener, J. W.; Meijer, E. W.; De Schryver, F. C. J. Am. Chem. Soc. 2000, 122, 3445–3452. (36) Mu, M.; Ning, F.; Jiang, M.; Chen, D. Langmuir 2003, 19, 9994–9996. (37) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143–1146. (38) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967–973. (39) Agut, W.; Br^ulet, A.; Schatz, C.; Taton, D.; Lecommandoux, S. Langmuir 2010, 26, 10546–10554. (40) Sanson, C.; Schatz, C.; Le Meins, J.-F.; Br^ulet, A.; Soum, A.; Lecommandoux, S. Langmuir 2010, 26, 2751–2760. (41) Lee, J. C-M; Bermudez, H.; Discher, B. M.; Sheehan, M. A.; Won, Y. Y.; Bates, F. S.; Discher, D. E. Biotechnol. Bioeng. 2001, 73, 135–145. (42) Bermudez, H.; Brannan, A. K.; Hammer, D. A.; Bates, F. S.; Discher, D. E. Macromolecules 2002, 35, 8203–8208. (43) Kros, A.; Jesse, W.; Metselaar, G. A.; Cornelissen, J. J. L. M. Angew. Chem., Int. Ed. 2005, 44, 4349–4352. (44) Kita-Tokarczyk, K.; Grumelard, J.; Haefele, T.; Meier, W. Polymer 2005, 46, 3540–3563. (45) Battaglia, G.; Ryan, A. J. J. Am. Chem. Soc. 2005, 127, 8757–8764. (46) Kros, A.; Jesse, W.; Metselaar, G. A.; Cornelissen, J. J. L. M. Angew. Chem., Int. Ed. 2005, 44, 4349–4352. (47) Zhou, Y.; Yan, D. J. Am. Chem. Soc. 2005, 127, 10468– 10469. (48) Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2004, 43, 4896–4899. (49) Mai, Y.; Zhou, Y.; Yan, D. small 2007, 3, 1170–1173. (50) Zhou, Y.; Yan, D.; Dong, W.; Tian, Y. J. Phys. Chem. B 2007, 111, 1262–1270. (51) Ilhan, F.; Galow, T. H.; Gray, M.; Clavier, G.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 5895–5896. (52) Uzun, O.; Xu, H.; Jeoung, E.; Thibault, R. J.; Rotello, V. M. Chem.—Eur. J. 2005, 11, 6916–6920. (53) Kubo, S.; Gu, Z. Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314–8319. (54) Li, L.; Chen, C.; Li, J.; Zhang, A.; Liu, X.; Xu, B.; Gao, S.; Jin, G.; Ma, Z. J. Mater. Chem. 2009, 19, 2789–2796. (55) Min, E. H.; Wong, K. H.; Stenzel, M. H. Adv. Mater. 2008, 20, 3550–3556. (56) Zander, N. E.; Orlicki, J. A.; Karikari, A. S.; Long, T. E.; Rawlett, A. M. Chem. Mater. 2007, 19, 6145–6149. (57) Kuncicky, D. M.; Prevo, B. G.; Velev, O. D. J. Mater. Chem. 2006, 16, 1207–1211. (58) Kwak, E. S.; Lee, W.; Park, N. G.; Kim, J.; Lee, H. Adv. Funct. Mater. 2009, 19, 1093–1099. (59) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387–389.

ARTICLE

(60) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79–83. (61) Kojima, M.; Nakanishi, T.; Hirai, Y.; Yabu, H.; Shimomura, M. Chem. Commun. 2010, 46, 3970–3972. (62) Zhao, H.; Shen, Y.; Zhang, S.; Zhang, H. Langmuir 2009, 25, 11032–11037. (63) Kim, J. H.; Seo, M.; Kim, S. Y. Adv. Mater. 2009, 21, 4130–4133. (64) Fan, D.; Jia, X.; Tang, P.; Hao, J.; Liu, T. Angew. Chem., Int. Ed. 2007, 46, 3342–3345. (65) Lee, S. H.; Park, J. S.; Lim, B. K.; Mo, C. B.; Lee, W. J.; Lee, J. M.; Hong, S. H.; Kim, S. O. Soft Matter 2009, 5, 2343–2346. (66) Saunders, A. E.; Shah, P. S.; Sigman, M. B., Jr.; Hanrath, T.; Hwang, H. S.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2004, 4, 1943–1948. (67) Nurmawati, M. H.; Ajikumar, P. K.; Renu, R.; Valiyaveettil, S. Adv. Funct. Mater. 2008, 18, 3213–3218. (68) Wang, Y.; Liu, Z.; Huang, Y.; Han, B.; Yang, G. Langmuir 2006, 22, 1928–1931.

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dx.doi.org/10.1021/la202680j |Langmuir 2011, 27, 12844–12850