Polymer Vesicles as Robust Scaffolds for the Directed Assembly of

May 18, 2009 - Department of Chemistry, University of Toronto, 80 St. George Street ... E-mail: [email protected]; [email protected]. ...
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Polymer Vesicles as Robust Scaffolds for the Directed Assembly of Highly Crystalline Nanocrystals† Mingfeng Wang, Meng Zhang, Conrad Siegers, Gregory D. Scholes,* and Mitchell A. Winnik* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, M5S 3H6 Ontario, Canada Received February 12, 2009. Revised Manuscript Received March 30, 2009 We report the incorporation of various inorganic nanoparticles (NPs) (PbS, LaOF, LaF3, and TiO2, each capped by oleic acid, and CdSe/ZnS core/shell QDs capped by trioctylphosphine oxide) into vesicles (d = 70-150 nm) formed by a sample of poly(styrene-b-acrylic acid) (PS404-b-PAA62, where the subscripts refer to the degree of polymerization) in mixtures of tetrahydrofuran (THF), dioxane, and water. The block copolymer formed mixtures of crew-cut micelles and vesicles with some enhancement of the vesicle population when the NPs were present. The vesicle fraction could be isolated by selective sedimentation via centrifugation, followed by redispersion in water. The NPs appeared to be incorporated into the PAA layers on the internal and external walls of the vesicles (strongly favoring the former). NPs on the exterior surface of the vesicles could be removed completely by treating the samples with a solution of ethylenediaminetetraacetate (EDTA) in water. The triangular nanoplatelets of LaF3 behaved differently. Stacks of these platelets were incorporated into solid colloidal entities, similar in size to the empty vesicles that accompanied them, during the coassembly as water was added to the polymer/LaF3/THF/ dioxane mixture.

1. Introduction Rapid advances in nanotechnology require simple and efficient methodologies to organize materials into specifically defined assemblies with dimensions on the nanometer scale.1-4 An attractive approach utilized by nature is the directed organization of functional objects via the support of proteins or cell membranes. One such example is the linear alignment of magnetic nanocrystals bound with the cell membranes of magnetotactic bacteria, which enables the bacteria to orient themselves and swim along the lines of a magnetic field.5,6 Another example is the intricate circular organization of chromophores in the protein scaffolds of natural light harvesting complexes.7-9 The finely adjusted distance and orientation between the chromophores are essential for the highly efficient energy transfer in photosynthesis. The principle of hierarchical organization in nature has provided scientists with the inspiration to explore functional † Part of the “Langmuir 25th Year: Molecular and macromolecular selfassemblies” special issue. *Corresponding authors. E-mail: [email protected]; mwinnik@ chem.utoronto.ca.

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materials with ordered structures using either synthetic molecules (e.g., surfactants10-13 and block copolymers14-19) or biomolecules (e.g., proteins, polypeptides, and DNA)1,2,20-22 as the structure-directing agents. Recently, we reported that spherical micelles of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4 VP) formed in alcohols can serve as scaffolds for the spatially defined organization of semiconductor nanocrystals (i.e., quantum dots (QDs)) and poly(3-hexylthiophene) (P3HT).23 The nanocrystals self-organize in the micelle corona consisting of P4 VP chains that bind as a multidentate ligand to the nanocrystals, whereas the P3HT molecules are hydrophobically incorporated into the core of the PS block. Here we explore whether this concept is applicable to other morphologies of polymer self-assemblies such as vesicles. Vesicles are hollow, spherical self-assemblies normally formed by amphiphilic molecules such as lipids, surfactants, and amphiphilic block copolymers in a liquid medium. The insoluble part of the molecule forms the vesicular wall, which serves as the anchor for soluble moieties protruding both into the solvent-filled interior cavity as well as into the external medium.24 The compartmentalized structure of vesicles allows the encapsulation of (14) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195–217. (15) Hamley, I. W. Angew. Chem,. Int. Ed. 2003, 42, 1692–1712. (16) Duxin, N.; Liu, F. T.; Vali, H.; Eisenberg, A. J. Am. Chem. Soc. 2005, 127, 10063–10069. (17) Lin, Y.; Boker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55–59. (18) Bockstaller, M. R.; Michiewicz, R. A.; Thomas, E. L. Adv. Mater. 2005, 17, 1331–1349. (19) Nie, Z. H.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Nat. Mater. 2007, 6, 609–614. (20) Aldaye, F. A.; Palmer, A. L.; Sleiman, H. F. Science 2008, 321, 1795–1799. (21) Lamm, M. S.; Sharma, N.; Rajagopal, K.; Beyer, F. L.; Schneider, J. P.; Pochan, D. J. Adv. Mater. 2008, 20, 447–451. (22) Lin, C. X.; Liu, Y.; Rinker, S.; Yan, H. ChemPhysChem 2006, 7, 1641–1647. (23) Wang, M. F.; Kumar, S.; Lee, A.; Felorzabihi, N.; Shen, L.; Zhao, F.; Froimowicz, P.; Scholes, G. D.; Winnik, M. A. J. Am. Chem. Soc. 2008, 130, 9481– 9491. (24) Segota, S.; Tezak, D. Adv. Colloid Interface Sci. 2006, 121, 51–75.

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various functional objects such as fluorescent dyes,25 inorganic nanoparticles,26-29 and biomolecules (DNA, proteins, etc.),30,31 thus making them intriguing carriers for applications in bionanotechnology and medicine. For example, CdSe QDs capped by trioctylphosphine oxide (TOPO) have been encapsulated into lipid vesicles via hydrophobic interactions for the controlled targeting of live cells.27 In addition, Beaune et al.28 recently reported giant lipid vesicles containing maghemite NPs and CdSe/ZnS core/shell QDs. Lipid vesicles are intrinsically fluidic and physically soft, however, resulting in relatively low stability and short circulation times under physiological conditions. In addition, the exact location of the QDs relative to the lipid vesicles is not yet clear because of the thin, mobile nature of the lipid membrane. This makes the experimental characterization of these hybrid lipid vesicles a significant challenge. Polymer vesicles show remarkably higher stability and toughness than the vesicles formed by lipids or by low-molecular-weight surfactants.26,32,33 Thus, they have served as robust carriers to encapsulate both hydrophilic26,34,35 and hydrophobic36-41 species. On one hand, water-soluble nanocrystals have been incorporated into the inner aqueous pool of block copolymer vesicles.26 This approach, however, requires tedious surface modification of the nanocrystals to make them hydrophilic. Moreover, the efficiency of incorporating nanocrystals into the vesicles remains low because a significant number of nanocrystals remain in the aqueous media outside the vesicles. This, in turn, makes it difficult to isolate the vesicles from these nonencapsulated nanocrystals. On the other hand, hydrophobic alkyl-capped nanocrystals that are normally synthesized by high-tempera ture thermolysis of metal precursors42-45 have been encapsulated into polymer vesicles formed by amphiphilic block copolymers such as polyisoprene-block-poly(ethylene oxide) (PI-b-PEO)37 and polybutadiene-block-poly(ethylene oxide) (PB-b-PEO).40,41 (25) Ghoroghchian, P. P.; Frail, P. R.; Susumu, K.; Blessington, D.; Brannan, A. K.; Bates, F. S.; Chance, B.; Hammer, D. A.; Therien, M. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 2922–2927. (26) Antonietti, M.; Forster, S. Adv. Mater. 2003, 15, 1323–1333. (27) Gopalakrishnan, G.; Danelon, C.; Izewska, P.; Prummer, M.; Bolinger, P. Y.; Geissbuhler, I.; Demurtas, D.; Dubochet, J.; Vogel, H. Angew. Chem., Int. Ed. 2006, 45, 5478–5483. (28) Beaune, G.; Dubertret, B.; Clement, O.; Vayssettes, C.; Cabuil, V.; Menager, C. Angew. Chem., Int. Ed. 2007, 46, 5421–5424. (29) Martina, M. S.; Fortin, J. P.; Menager, C.; Clement, O.; Barratt, G.; Grabielle-Madelmont, C.; Gazeau, F.; Cabuil, V.; Lesieur, S. J. Am. Chem. Soc. 2005, 127, 10676–10685. (30) Walde, P.; Ichikawa, S. Biomol. Engi. 2001, 18, 143–177. (31) Guo, X.; Szoka, F. C. Acc. Chem. Res. 2003, 36, 335–341. (32) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967–973. (33) Discher, B. M.; Hammer, D. A.; Bates, F. S.; Discher, D. E. Curr. Opin. Colloid Interface Sci. 2000, 5, 125–131. (34) Choucair, A.; Soo, P. L.; Eisenberg, A. Langmuir 2005, 21, 9308–9313. (35) Borchert, U.; Lipprandt, U.; Bilang, M.; Kimpfler, A.; Rank, A.; PeschkaSuss, R.; Schubert, R.; Lindner, P.; Forster, S. Langmuir 2006, 22, 5843–5847. (36) Lecommandoux, S. B.; Sandre, O.; Checot, F.; Rodriguez-Hernandez, J.; Perzynski, R. Adv. Mater. 2005, 17, 712–718. (37) Krack, M.; Hohenberg, H.; Kornowski, A.; Lindner, P.; Weller, H.; Forster, S. J. Am. Chem. Soc. 2008, 130, 7315–7320. (38) Ghoroghchian, P. P.; Lin, J. J.; Brannan, A. K.; Frail, P. R.; Bates, F. S.; Therien, M. J.; Hammer, D. A. Soft Matter 2006, 2, 973–980. (39) Nardin, C.; Thoeni, S.; Widmer, J.; Winterhalter, M.; Meier, W. Chem. Commun. 2000, 1433–1434. (40) Binder, W. H.; Sachsenhofer, R.; Farnik, D.; Blaas, D. Phys. Chem. Chem. Phys. 2007, 9, 6435–6441. (41) Mueller, W.; Koynov, K.; Fischer, K.; Hartmann, S.; Pierrat, S.; Basche, T.; Maskos, M. Macromolecules 2009, 42, 357–361. (42) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (43) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (44) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414– 3439. (45) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660.

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In aqueous solutions of these systems, the hydrophobic nanocrystals become incorporated into the hydrophobic layer of the vesicle wall (composed of the insoluble block of these block copolymers, i.e., the PI block for PI-b-PEO or the PB block for PB-b-PEO). These experimental results are to some extent consistent with theoretical predictions46 on the self-coassembly of block copolymer and nanoparticles in dilute solution. Here, we show that polymer vesicles formed by an amphiphilic diblock copolymer, polystyrene-block-poly(acrylic acid) (PS404-bPAA62, where the subscripts refer to the number average degrees of polymerization), in mixtures of tetrahydrofuran (THF)/dioxane/water can capture a variety of preformed colloidal nanocrystals in the PAA-enriched hydrophilic shell of the vesicles. The robustness of the PS404-b-PAA62 vesicles allows us to examine the localization of various nanocrystals on the motifs of the vesicles. Our results indicate that these nanocrystals do not enter the vesicle wall composed of the insoluble PS block, despite the hydrophobic nature of the alkyl ligands originally bound to the nanocrystal surfaces. Here we believe that the binding of the PAA block as a multidentate ligand47,48 to the surface of the nanocrystals plays an essential role in terms of the localization of the nanocrystals on both the interior and the exterior PAA shells of the vesicles. In addition, the size, shape, and surface chemistry of the nanocrystals also contribute to different synergistic assemblies of the polymer/nanocrystal composites in dilute solution. Our approach to functionalizing polymer vesicles with various nanocrystals does not require any premodification of the surfaces of the as-synthesized nanocrystals, thus enabling a simple and efficient way to prepare multicomponent complexes with organized structures and integrated functions.

2. Results and Discussion The block copolymer of PS404-b-PAA62 (Figure 1) employed in our experiments contains a PS block that is much longer than the PAA block. In dilute solution in a selective solvent for the PAA block, such asymmetric block copolymers form a variety of aggregates such as spherical micelles, cylindrical micelles, vesicles, and large compound micelles.49,50 In the work discussed here, we look at solvent compositions that lead to vesicle formation, often accompanied by crew-cut micelles. The nanocrystals that we examined include PbS, LaOF, LaF3, and TiO2, each of which was capped by oleic acid, as well as CdSe/ ZnS core/shell QDs capped by trioctylphosphine oxide (TOPO). All of the nanocrystals used here were synthesized by the thermal decomposition of precursors at high temperature in the presence of alkyl ligands (see Experimental Section). Nanocrystals synthesized in this way are highly crystalline and monodisperse in size and shape. Previous studies from our laboratory47 as well as from others48 demonstrated that PAA is able to displace the alkyl ligands from the surfaces of nanocrystals such as PbS and TiO2 and render them dispersible into polar media. Here, we take a further step by using a PAA-based amphiphilic block copolymer, PS404-b-PAA62, as both a multidentate ligand (i.e., the PAA block) and a structuredirecting agent for various preformed nanocrystals in order to prepare organized complex nanostructures in a simple way. In the following sections, we first present transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (46) Zhang, L.; Lin, J.; Lin, S. Macromolecules 2007, 40, 5582–5592. (47) Lin, W.; Fritz, K.; Guerin, G.; Bardajee, G. R.; Hinds, S.; Sukhovatkin, V.; Sargent, E. H.; Scholes, G. D.; Winnik, M. A. Langmuir 2008, 24, 8215–8219. (48) Zhang, T. R.; Ge, J. P.; Hu, Y. P.; Yin, Y. D. Nano Lett. 2007, 7, 3203–3207. (49) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728–1731. (50) Zhulina, E. B.; Adam, M.; LaRue, I.; Sheiko, S. S.; Rubinstein, M. Macromolecules 2005, 38, 5330–5351.

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Figure 1. Molecular structure (upper) of the PS-b-PAA block copolymer and schematic presentation of the variety of nanocrystal building blocks.

Figure 2. (A) TEM (dark field) and (B) SEM images of PS404-b-PAA62 vesicles and micelles formed in dioxane/THF/water at a weight ratio of 1:0.2:1. (C) Schematic structures of the micelles and the vesicles.

results for the self-assembly of PS404-b-PAA62 in solution without nanocrystals. Then we successively describe the coassembly of this polymer in solution in the presence of each type of nanocrystal. 2.1. Preparation and Characterization of PS-b-PAA Vesicles. We start by examining the self-assembly of PS404-b-PAA62 in dilute solution in the absence of nanocrystals. The preparation of PS-b-PAA vesicles followed a method developed by Eisenberg and co-workers.49 Briefly, PS404-b-PAA62 was first dissolved in a mixed solvent of dioxane/THF, a common good solvent for both PS and PAA blocks. Then deionized water as a selective solvent for the PAA block was added dropwise to the polymer solution under vigorous stirring. After the mixture was stirred continuously for 20 min, the resulting polymer self-assemblies were quenched by adding a large amount of water, followed by dialysis against water. Figure 2A shows a dark-field TEM image of PS404b-PAA62 self-assemblies formed in a tertiary mixture of dixoane/ THF/water (1:0.2:1 w/w/w). One can see the coexistence of homogeneous spheres, which correspond to crew-cut micelles, and hollow spherical aggregates with higher electron transmission in the center than around their periphery. The hollow nature of these structures cannot be discerned by SEM (Figure 2B). This result establishes that these are vesicles and not toroidal micelles.49 The walls of the vesicles consist of the long, insoluble PS chains from which the relatively short, soluble PAA chains Langmuir 2009, 25(24), 13703–13711

protrude into the solvent-filled interior cavity as well as into the external medium (Figure 2C). Whereas the sizes of the whole vesicles (d = 60-150 nm) and their internal cavities (several to tens of nanometers) are polydisperse, the thickness of the hydrophobic wall formed by PS blocks remained relatively uniform (29 ( 3 nm). The average diameter measured from TEM images of the crew-cut micelles was 45 ( 11 nm. These results show that PS404-b-PAA62 is able to form vesicles under the experimental conditions described above. Nevertheless, with this polymer, we were never able to obtain vesicles exclusively nor were we able to obtain vesicles that were uniform in size and shape by changing the ratio of dioxane to THF, the content of water, the addition rate of water, or the polymer concentration. These results are shown in Supporting Information (Figures S1S4). Under all of the conditions that we examined, we observed a mixture of micelles and vesicles that coexisted in dilute solution. 2.2. Organization of PbS QDs on Scaffolds of PS-bPAA Vesicles. In this section, we examine whether these PS-bPAA vesicles can serve as scaffolds for the organization of oleic acid-coated PbS quantum dots (denoted as PbS/OA). Figure 3A shows a dark-field TEM image of the PbS/OA QDs on a carboncoated TEM grid. A droplet of a chloroform solution (dispersion) was placed on the grid and allowed to dry. Size analysis of the images gave an average diameter of d = 3.0 ( 0.5 nm for these DOI: 10.1021/la900523s

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Figure 3. Dark-field TEM images of (A) PbS/OA QDs (d = 3.0 ( 0.5 nm) from chloroform and (B, C) PS404-b-PAA62 self-assemblies formed in the presence of different amounts of these PbS/OA QDs: (B) 50 μg of QDs/mg of block copolymer in dioxane/THF/water (1:0.2:1 w/w/w) and (C) 100 μg of QDs/mg of block copolymer in dioxane/THF/water (1:0.4:1 w/w/w). (D) High-magnification dark-field TEM image of the sample shown in C. (E) Dark-field TEM image with high magnification of the PS404-b-PAA62 vesicles after treatment with sodium ethylenediaminetetraacetate (EDTA) followed by dialysis against deionized water. (F) Schematic illustration of the selective removal of the PbS QDs on the external surface of the PS404-b-PAA62vesicles by exposure to EDTA. The QDs in the vesicular cavity remained intact during the treatment. We note that each of the TEM images shown in B-E represents a projection of the 3D vesicles into the 2D imaging plane. As a consequence, some of the QDs that adsorbed onto the exterior surface of the vesicles appear to be trapped in the PS wall of the vesicles.

nanocrystals. Figure 3B shows a dark-field TEM image of PS404b-PAA62 self-assemblies formed in the presence of 50 μg of PbS/ OA QDs/mg of polymer. The coexistence of two types of polymer self-assembled structures (i.e., micelles and vesicles) is discerned in TEM images of this sample. An inspection of the low-magnification TEM images (not shown) suggests a higher fraction of PS404b-PAA62 vesicles in the presence of the QDs in comparison to the polymer self-assemblies without QDs, and other conditions were kept the same. However, it was difficult to quantify the number ratio of vesicles to micelles by TEM. In addition, one can see tiny bright spots adsorbed onto each vesicle in Figure 3B. These bright spots correspond to PbS QDs because of their high electron density relative to the polymer motifs. Some QDs align along the interior side of the PS wall of the vesicles, and some of the nanocrystals are localized on the exterior surface of the vesicles. At the same time, one can also see the adsorption of some PbS particles on the surface of the micelles. This adsorption is much less significant than that on the vesicles. The mechanism for this selective adsorption, though interesting, is not yet clear. Figure 3C,D shows dark-field TEM images of PbS/PS404b-PAA62 self-assemblies in dioxane/THF/water (1:0.4:1 w/w/w) in a ratio of 100 μg of QDs/mg of polymer. Compared to the results shown in Figure 3B, one can see that the average number of QDs adsorbed onto the vesicles increased with 13706 DOI: 10.1021/la900523s

increasing QD/polymer ratios. These QDs were adsorbed on both the interior and exterior walls of the vesicles. The highmagnification TEM image (Figure 3D) shows that most of the QDs remain separated after adsorption to the vesicles and micelles, despite the relatively large ratio of QD/polymer. The mechanism for the formation of these hybrid QD/polymer vesicles described above is not clear. We speculate that the nanocrystals interact first with the PAA block of the PS-PAA block copolymer in the mixture of THF/dioxane. Then the slow addition of water as the selective solvent induces the co-organization of the polymer and the nanocrystals into hybrid vesicles. In contrast, we observed a black precipitate when an aliquot (0.2 mL in THF) of PbS/OA QD dispersion (ca. 0.5 mg/mL) was added to a solution of the preformed polymer vesicles in water. This result suggests that, under these circumstances, the OAcapped QDs aggregated in water by themselves before they were able to bind to the PAA domains of the vesicles. 2.3. Selective Removal of PbS QDs from the Exterior Surface of PS-b-PAA Vesicles. In this section, we describe experiments to examine whether the QDs captured on the exterior surface of the vesicles could be selectively removed by exposure to a strong metal chelator such as ethylenediaminetetraacetate (EDTA). The sample of PbS/PS404-b-PAA62 vesicles containing 100 μg of QDs/mg of polymer (Figure 3C) was chosen as an example. This sample (2 mL, ca. 1 mg/mL) was first dialyzed Langmuir 2009, 25(24), 13703–13711

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Figure 4. (A) Dark-field TEM image of PS404-b-PAA62 vesicles formed in dioxane/THF/water (1:0.5:1 w/w/w) in the presence of 60 μg of LaOF/OA nanocrystals (d = 2.9 ( 0.4 nm)/mg of the block copolymer. The final polymer concentration was 8 mg/mL. The tiny bright spots correspond to the LaOF nanocrystals. (B) Dark-field TEM image of the hybrid vesicles shown in A after the treatment with EDTA. The LaOF nanocrystals on the exterior surface of the vesicles were selectively removed by the EDTA treatment, whereas those nanocrystals in the interior cavity of the vesicles remained in place.

against deionized water to remove any organic solvents (dioxane, THF), followed by addition to an aliquot (2 mL) of an aqueous solution of NaOH and EDTA (pH 9, 2 mg/mL (6.8  10-2 M) EDTA). The mixture was stirred at room temperature for 2 days, followed by dialysis against water. Figure 3E shows a highmagnification TEM image of this sample after being dried on a copper grid coated with carbon film. In this case, only the QD particles on the inside of the vesicles can be discerned, whereas the exterior surface of the vesicles is bare and clean. Figure 3E shows that after the EDTA treatment the PbS QDs on the exterior surfaces of the vesicles were completely removed, whereas the QDs in the interior cavity of the vesicles remained intact. An unanswered question concerns the fate of the PbS/PAA QDs on exposure to EDTA. One possibility is that EDTA digests the QDs, and Pb2+/EDTA complexes passed through the dialysis membrane bag. Alternatively, PbS/EDTA QDs may form colloidal solutions that can pass through the pores of the dialysis bag. This mater needs further investigation. Figure 3F presents a schematic structure of the hybrid vesicles before and after the EDTA treatment. 2.4. Organization of LaOF and TiO2 Nanocrystals on PS-b-PAA Vesicles. To examine the ability of PS404-b-PAA62 vesicles to serve as scaffolds for organizing other presynthesized alkane-capped nanocrystals, we investigated oleic acid-coated LaOF and TiO2 nanocrystals. The preparation of these hybrid vesicles followed the same procedure used to prepare the PbS/ PS404-b-PAA62 vesicles as described above. The polymer and the nanocrystals were suspended in a dioxane/THF mixture, followed by the slow addition of water under stirring. The mixture was continuously stirred for 3 days. Finally, an aliquot (0.6 mL) of the mixture was added to 10 mL of deionized water to quench the selfassemblies. Figure 4A shows a dark-field TEM image of the air-dried PS404-b-PAA62 self-assemblies formed in dioxane/THF/water (1:0.5:1 w/w/w) in the presence of 60 μg of LaOF/OA nanocrystals (d = 2.9 ( 0.4 nm)/mg of the block copolymer. As in the case of PbS/PS404-b-PAA62 self-assemblies, both micellar and vesicular structures coexisted in the self-assembly of LaOF/PS404-bPAA62. Again, the average number of LaOF nanocrystals adsorbed to the polymer vesicles was significantly higher than that of the nanocrystals adsorbed to the polymer micelles. We also examined whether the LaOF nanocrystals could be selectively removed from the exterior surface of the vesicles. In this experiment, an aqueous dispersion (2 mL) of the dialyzed LaOF/PS404-b-PAA62 vesicles shown in Figure 4A was exposed to an aqueous solution (2 mL) of NaOH and EDTA (2 mg/mL) Langmuir 2009, 25(24), 13703–13711

at pH 11. The mixture was stirred for 2 days, followed by centrifugation at 14 000 rpm for 10 min to remove excess EDTA and NaOH. The sediment was washed once with water, followed by redispersion in 2 mL of water. Figure 4B shows a dark-field TEM image of this sample after being dried in air on a grid. One can see that the LaOF nanocrystals from the exterior surface of the polymer vesicles were completely removed after the EDTA treatment, whereas the nanocrystals inside the vesicles remained intact. At the same time, these vesicles appear to have irregular inner surfaces compared to the untreated vesicles (Figure 4A). These intermediate structures are presumably a consequence of vesicle fusion and fission. This topic will be discussed in more detail when we present results for the system of CdSe-ZnS(485) QDs and PS404-b-PAA62 (section 2.5). We obtained similar results with TiO2 nanocrystals, despite their relative high polydispersity in both the size and the shape compared to the PbS QDs and LaOF nanocrystals described above, on the motif of PS404-b-PAA62 vesicles (Figure S6, Supporting Information). The similarity of the self-organization of the PbS, LaOF, and TiO2 nanocrystals on PS404-b-PAA62 vesicles is not surprising on the basis of their common original ligands (i.e., OA) and the fact that PAA can bind strongly to each of these nanocrystals. 2.5. Organization CdSe-ZnS Core-Shell QDs on PS-bPAA Vesicles. In this section, we turn to the organization of CdSe-ZnS core-shell QDs on the scaffolds of the PS404-bPAA62 vesicles. We explore whether the distinctive binding strength of -COOH groups with ZnS in comparison with PbS, TiO2, or LaOF would lead to different structures for QD/PS-bPAA self-assemblies. Figure 5A shows a dark-field TEM image of the TOPO-coated CdSe-ZnS(605) QDs on grids prepared from a toluene solution of the QDs. The size analysis of the images gives a diameter of 5.7 ( 0.8 nm for the inorganic core of these nanocrystals. Figure 5B shows a dark-field TEM image of the CdSe-ZnS (605)/PS404-b-PAA62 self-assemblies at a relatively low QD/polymer ratio (i.e., 35 μg of QDs/mg of block copolymer). Interestingly, one can see that most of the vesicles contain only one QD at the inner edge of the vesicle wall. In addition, the larger QDs prefer to localize within the vesicle with a larger inner cavity, whereas smaller QDs enter the vesicles with smaller cavities. In contrast, there is no adsorption at all of the CdSe-ZnS(605) QDs on the surface of PS404-b-PAA62 micelles. Figure 5C shows that when the weight ratio of CdSe-ZnS(605) QDs to PS404-b-PAA62 was increased to 140 μg of QDs/mg of polymer the average number of QDs incorporated into the DOI: 10.1021/la900523s

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Figure 5. Dark-field TEM images of (A) TOPO-coated CdSe/ZnS(605) core/shell QDs (d = 5.7 ( 0.8 nm) from toluene. (B, C) PS404-bPAA62 self-assemblies formed in dioxane/THF/water (1:0.5:1 w/w/w) in the presence of different numbers of core/shell CdSe/ZnS QDs followed by dialysis against deionized water: (B) 35 μg of QDs/mg of block copolymer and (C) 140 μg of QDs/mg of block copolymer. (D) Photoluminescence spectra of the original QDs from toluene and vesicle-incorporated QDs in water.

polymer vesicles increased significantly. Interestingly, most of the QDs align along the interior wall of the vesicles, whereas the number of QDs adsorbed on the exterior surface of the vesicles is not significant. There are a few clusters of QDs on the surfaces of the micelles. These may have formed during drying of the sample. Figure 5D shows that the TOPO-coated CdSe-ZnS(605) QDs in toluene show a band-edge photoluminescence (PL) peak centered at 617 nm and a PL quantum yield of 0.24. The maximum PL of the QDs after incorporation into the PS404-bPAA62 vesicles shifted to 605 nm, accompanied by a slight broadening of the PL spectrum. The measurement of the actual PL quantum yield of the QDs inside the vesicles is difficult to make because of the turbidity of the sample. In general, it is difficult to explain solvent-induced shifts of QD emission spectra. In one of our previous publications,47 we described the formation of poly(acrylic acid) (PAA)-stabilized PbS QDs by the ligand exchange of oleic acid-coated QDs. We reported a similar blue shift and showed by TEM that ligand exchange was accompanied by a small decrease in QD diameter. To know whether the size of the core-shell QDs plays a role in their selective localization on the inner wall of the PS404-b-PAA62 vesicle, we examined smaller CdSe-ZnS core-shell QDs with a band-edge absorbance at 485 nm. Figure 6A shows a dark-field TEM image of the TOPO-coated QDs, referred to as CdSe/ZnS (485). The average diameter of the inorganic core of these QDs is 3.1 ( 0.6 nm (as measured from TEM images), which is smaller than that of CdSe-ZnS(605) (d = 5.7 ( 0.8 nm). Figure 6B shows a representative dark-field TEM image of PS404-b-PAA62 selfassemblies formed in dioxane/THF/water (1:0.5:1 w/w/w) in the presence of 60 μg of CdSe-ZnS(485) QDs/mg of block copolymer, followed by dialysis against deionized water. One can see that a small portion of the QD particles adsorb on the exterior surface of the vesicles, although most of the QDs appear to accumulate in the inner cavity of the vesicles. These results imply that, compared to larger QDs, smaller QDs show less 13708 DOI: 10.1021/la900523s

spatial selectivity for their localization on the inner wall of the PS-b-PAA vesicle. The mechanism for this high spatial selectivity of the organization of the core/shell CdSe/ZnS(605) QDs to the inner wall of the PS404-b-PAA62 vesicles is not yet clear. Eisenberg et al.52 found that shorter PAA chains protrude primarily into the inside of the PS-b-PAA vesicles, whereas longer PAA chains remain on the outside of the vesicles. This asymmetric distribution of PAA chains may contribute to the selective adsorption of the QDs into the interior surface of the vesicles. In addition, it is known that the outside surface of these vesicles has a positive curvature and the part inside (the vesicular cavity) has a negative curvature. This curvature difference might also play a role in the selective localization of the QDs in these vesicles. Other parameters such as the size and surface chemistry of the QDs might play a role in their interaction with the polymer vesicles. The size-selective organization of nanocrystals in thin films of block copolymers has been demonstrated before both experimentally and theoretically. For example, Wiesner and coworkers51 observed similar size-dependent segregation in a selfassembled tertiary system of poly(isoprene-b-ethylene oxide) with small silica-type nanoparticles and larger silica-coated gold nanoparticles. Pasyuk et al.53 studied by neutron reflection the effect of nanoparticle size on the internal structure of symmetric d-PSPBMA (where d stands for a deuterated copolymer) diblock copolymer. They found that small nanoparticles of γ-Fe2O3 (4 nm in diameter) concentrate close to d-PS-PBMA interfaces whereas larger nanoparticles (6 nm in diameter) localize in the center of PS domains. Similar size-selective organization was observed by Bockstaller et al.54 in ternary systems consisting of symmetric (52) Luo, L. B.; Eisenberg, A. Langmuir 2001, 17, 6804–6811. (51) Warren, S. C.; Disalvo, F. J.; Wiesner, U. Nat. Mater. 2007, 6, 248–248. (53) Lauter-Pasyuk, V.; Lauter, H. J.; Ausserre, D.; Gallot, Y.; Cabuil, V.; Hamdoun, B.; Kornilov, E. I. Physica B 1998, 248, 243–245. (54) Bockstaller, M. R.; Lapetnikov, Y.; Margel, S.; Thomas, E. L. J. Am. Chem. Soc. 2003, 125, 5276–5277.

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Figure 6. Dark-field TEM images: (A) TOPO-coated CdSe-ZnS(485) core-shell QDs (d = 3.1 ( 0.6 nm) from toluene. (B) PS404-b-PAA62

self-assemblies formed in dioxane/THF/water (1:0.5:1 w/w/w) in the presence of 60 μg of CdSe-ZnS(485) QDs/mg of block copolymer followed by dialysis against deionized water. (C) CdSe-ZnS(485)/PS404-b-PAA62 self-assemblies after centrifugation (8000 rpm, 10 min) and redispersion of the sediment into water. The arrows label the complex structures formed by the fission and fusion of the vesicles. The inset shows a high-magnification image of a single QD/polymer vesicle. (D) CdSe-ZnS(485)/PS404-b-PAA62 vesicles after treatment with EDTA, followed by centrifugation and redispersion of the sediment into water.

poly(styrene-b-ethylene propylene) (PS-PEP) and two differently sized nanocrystal species (dcore = 3.5 ( 1 nm in the case of gold and dcore = 21.5 ( 2.5 nm for silica nanoparticles). Using a mean-field approach, Balazs and co-workers55 studied a 2D system representing a mixture of nanoparticles within a lamellar block copolymer in the intermediate segregation limit. They predicted that interfacial segregation of particles is expected to occur for particle sizes of R/R0 < 0.2, whereas the concentration of particles at the center of the domain is expected for R/R0 > 0.3, where R is the radius of the particle and R0 is the natural size of the polymer. Figure 6B also shows the presence of some PS404-b-PAA62 micelles with a few QDs adsorbed. Considering that these micelles have an overall smaller size than the vesicles and that the adsorption of QDs on the micellar structures is less significant than that on the vesicles, one can imagine separating these two types of polymer self-assemblies by centrifugation. To examine this possibility, an aqueous dispersion of CdSe-ZnS(485)/PS404-bPAA62 vesicles (shown in Figure 6B) was subjected to centrifugation at 8000 rpm for 10 min. The sediment was isolated and then redispersed in water. Figure 6C shows that essentially all of the micelles were removed by the centrifugation process, consistent with their smaller size and lower density compared to those of the QD-containing vesicles. In Figure 6C, one can also see some complex structures (labeled by arrows). Such intermediate structures can be attributed to the fission and fusion of the vesicles. Eisenberg and co-workers52 observed similar structures by TEM when examining PS-b-PAA vesicles formed in solutions of dioxane/THF/water. They proposed a mechanism for the fusion and fission process and illustrated this mechanism by carefully arranging several images (55) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Science 2001, 292, 2469–2472.

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selected from a large number of TEM micrographs. The first step in the fusion process appears to involve the contact and adhesion of two vesicles, followed by the coalescence and formation of a center wall. The latter is then destabilized and breaks, and the structures evolve to give uniform vesicles via smoothing of the outer walls. However, the mechanism of fission of vesicles involves in the elongation of the vesicles, followed by the formation of an internal waist, narrowing of the external waist, and finally complete separation. More direct evidence for vesicle fusion and fission was obtained by real-time detection of giant vesicles formed by lipids56 and by copolymers with a hyperbranched poly(3-ethyl-3-oxetanemethanol) core that was grafted with a large population of polyethylene oxide arms.57,58 Next we examined whether the CdSe/ZnS(485) core/shell QDs binding to the exterior periphery of the PS404-b-PAA62 vesicles (Figure 6B) could also be selectively removed by the EDTA treatment that we used for PbS/PS-b-PAA and LaOF/PS-b-PAA vesicles. In this experiment, 1 mL of an aqueous dispersion of CdSe-ZnS(485)/PS404-b-PAA62 (Figure 6B) was mixed with 1 mL of EDTA (pH 9, 6.8  10-2 M) and stirred for 4 h. Then the excess EDTA was removed by centrifugation (13 000 rpm, 10 min). The sediment consisting of the hybrid vesicles was separated and washed with deionized water twice and finally redispersed in 1 mL of deionized water. A dark-field TEM image of this sample is shown in Figure 6D. Again, one can see that the exterior periphery of the purified vesicles is free of any QD binding whereas the QDs localized in the inner cavity of the vesicles remain intact. 2.6. Coassembly of PS-b-PAA and Nonspherical Nanocrystals: An Example of Triangular LaF3 Nanoplates. In this section, we take triangular LaF3 nanoplates as an example to (56) Kas, J.; Sackmann, E. Biophys. J. 1991, 60, 825–844. (57) Zhou, Y. F.; Yan, D. Y. J. Am. Chem. Soc. 2005, 127, 10468–10469. (58) Zhou, Y. F.; Yan, D. Y. Angew. Chem., Int. Ed. 2005, 44, 3223–3226.

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Figure 7. (A) Dark-field TEM image of oleic acid-coated LaF3 triangular nanoplates in THF. (B) Schematic presentation showing the dimensions of individual triangular nanoplates and their stacks (labeled with a white rectangle). (C) Low-magnification dark-field TEM image of the coassemblies of LaF3 triangular nanoplates and PS404-b-PAA62 in dioxane/THF/water (1:0.5:1 w/w/w). (D) High-magnification dark-field TEM image of the area labeled with the white rectangle in C.

examine the effect of nanocrystal shape on the coassembly of PS404-b-PAA62/nanocrystals in solution. Figure 7A shows a dark-field TEM image of oleic acid-stabilized triangular LaF3 nanoplates dried on grids from a THF dispersion. A size analysis of these images gives dimensions of (15.9 ( 1.2) nm  (2 ( 0.3) nm (Figure 7B) for these nanoplates. Figure 7C,D shows different magnifications of the TEM images for the coassemblies of LaF3 nanoplates and PS404-b-PAA62 in dioxane/THF/water (1:0.5:1 w/w/w). In the absence of the block copolymer, most of the LaF3 nanoplates appear to be separated, although a few stacklike aggregates (labeled with a white rectangle in Figure 7A) were observed on grids prepared from THF solution. In contrast, the coassembly of these nanoplates with PS404-b-PAA62 in dioxane/ THF/water (1:0.5:1 w/w/w) led to 1D stack aggregates with the solid polymer aggregates as the supporting structures (Figure 7C,D). In some of these aggregates, the nanoplates stand vertically and pack horizontally on the polymer scaffold so that the thickness (2.0 ( 0.3 nm) of the LaF3 can be measured. These solid structures are larger than the crew-cut micelles formed in the absence of nanoparticles and provide a clear indication that the presence of the inorganic component can influence the selfassembly process. Surprisingly, there is no indication of the incorporation of these nanoplates into the polymer vesicles; all of the vesicles seen in these TEM images remain empty. This result is in contrast to the preferable localization of the spherical nanocrystals (e.g., PbS, LaOF, TiO2, and CdSe/ZnS core/shell QDs) in the PAA domains of the vesicles as described above.

3. Summary The methodology described in this article represents a simple, straightforward way to construct complex, robust, and functionalized vesicles via the self-assembly of amphiphilic block copolymers with alkane-capped colloidal nanocrystals that are synthesized independently. The experimental results demonstrate 13710 DOI: 10.1021/la900523s

that PbS, LaOF, and TiO2 nanocrystals localize at both the interior and exterior surfaces of the PS404-b-PAA62 vesicles. A primary driving force for this localization can be attributed to the binding of the PAA block as a multidentate ligand to the surfaces of these nanocrystals. Interestingly, CdSe-ZnS(605) core-shell QDs originally coated with TOPO have a highly selective affinity for the inner wall of the PS404-b-PAA62 vesicles, whereas the same type of core-shell QDs with a smaller size shows less spatial selectivity. In addition, the shape of the nanocrystals also has a significant effect on the coassembly with the block copolymer. Triangular LaF3 nanoplates become incorporated into dense, micelle-like polymer structures and have little interaction with the PS404-b-PAA62 vesicles that are formed in parallel. The robustness of the PS404-b-PAA62 vesicles allows the selective removal of the nanocrystals adsorbed in the exterior surface of the vesicles while leaving the QDs inside the vesicular cavity intact. This result opens the opportunity to modify the exterior surface with other functional materials. For example, Wang et al.59 recently reported the use of Coulombic interactions to adsorb negatively charged dextran-coated magnetic nanoparticles onto the positively charged surface of cylindrical micelles. Caruso and co-workers60 have used layer-by-layer techniques to modify particle surfaces, for example, by attaching a layer of LaPO4 nanoparticles onto the surfaces of polystyrene beads.61 Thus, one can imagine the possibility in the future to incorporate other nanoparticles onto the exterior surface of polymer vesicles that contain different nanoparticles on the inside. In addition, one could incorporate hydrophobic materials such as organic dyes and conjugated polymers within the walls of the vesicles. These types of robust hybrid polymer vesicles with structural and (59) Wang, H.; Patil, A. J.; Liu, K.; Petrov, S.; Mann, S.; Winnik, M. A.; Manners, I., Adv. Mater. 2009, in press. (60) Caruso, F. Adv. Mater. 2001, 13, 11–22. (61) Schuetz, P.; Caruso, F. Chem. Mater. 2002, 14, 4509–4516.

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functional hierarchies may find applications in areas such as light harvesting, biological labeling, and diagnostics.

4. Experimental Section 4.1. Materials and Methods. The PS404-b-PAA62 polymer (Mn = 46 500 and Mw/Mn = 1.18, where the subscripts refer to the number-average degree of polymerization) was purchased from Polymer Source, Inc. (Dorval, Quebec). Titanium(IV) isopropoxide (TTIP, Aldrich, 99.99%) and oleic acid (OA) (Aldrich, tech. 90%) were used as received. All of the colloidal nanocrystals were synthesized by the traditional organometallic approach at high temperature in the presence of oleic acid (OA) for PbS,62 LaOF,63 LaF3,63 TiO2,64 and trioctylphosphine oxide (TOPO) for CdSe/ZnS core/shell QDs.65 These species become the major surface ligands of the nanoparticles. The band-edge absorbance of the PbS QD sample was located at 1160 nm. Its average diameter as measured by TEM was 3.0 ( 0.5 nm. Two samples of TOPO-capped CdSe/ZnS core/shell QDs were employed in this study. Both of them were provided by Evident Technology. One has a band-edge absorbance at 605 nm corresponding to an average size of 5.7 ( 0.8 nm. This sample is denoted as CdSe-ZnS(605). The other, denoted as CdSe-ZnS(485), has a band-edge absorbance at 485 nm corresponding to an average size of 3.1 ( 0.6 nm. The average diameter of the LaOF spherical nanocrystals was 2.9 ( 0.4 nm, which was measured by TEM. All of the size analyses were performed using Image J, an image-processing program developed at the National Institutes of Health. The TEM and SEM images were taken using a Hitachi HD2000 STEM instrument. Photoluminescence (PL) spectra were measured using a SPEX Fluorolog-3 spectrofluorometer (Jobin Yvon/SPEX, Edison, NJ). Rhodamine 6G in ethanol was used as a standard for PL quantum yield measurement for CdSeZnS(605) QDs.

4.2. Preparation of PS-b-PAA/Nanocrystal Hybrid Vesicles. The preparation of the polymer/nanocrystal hybrid vesicles followed a method developed by Eisenberg and his colleagues.49,66 PS404-b-PAA62 (1.0 mL, 20 mg/mL in 1,4-dioxane) was mixed with each aliquot of the following (62) Hines, M. A.; Scholes, G. D. Adv. Mater. 2003, 15, 1844–1849. (63) Sun, X.; Zhang, Y. W.; Du, Y. P.; Yan, Z. G.; Si, R.; You, L. P.; Yan, C. H. Chem. Eur. J. 2007, 13, 2320–2332. (64) Joo, J.; Kwon, S. G.; Yu, T.; Cho, M.; Lee, J.; Yoon, J.; Hyeon, T. J. Phys. Chem. B 2005, 109, 15297–15302. (65) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1996, 100, 468–471. (66) Choucair, A.; Lavigueur, C.; Eisenberg, A. Langmuir 2004, 20, 3894–3900.

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nanocrystals: PbS/OA (0.2 or 0.4 mL, 5.2 mg/mL in THF), LaOF/OA (0.5 mL, 2.4 mg/mL in THF), LaF3/OA (0.5 mL, 3.0 mg/mL in THF), TiO2/OA (0.1 or 0.5 mL, 16 mg/mL in THF), CdSe/ZnS(605)/TOPO (0.5 mL, 1.4 or 5.6 mg/mL), or CdSe/ZnS (485)/TOPO (0.5 mL, 2.4 mg/mL). The mixture remained clear at this stage. After stirring for 20 min, 1.0 mL of deionized water was added with a 1 mL syringe to the mixture slowly (ca. 0.02 mL/s) under vigorous stirring. The addition rate was judged roughly by counting the drops over time. Upon addition of water until ca. 0.3 mL, the mixture became turbid, a sign of the formation of polymer assemblies. After continuously stirring overnight, ca. 4 mL of water was added to the mixture. The organic solvents (i.e., THF and dioxane) in the resulting polymer assemblies were removed by dialysis using a cellulose membrane (Spectra/Por@, molecular weight cut off of 1000, 4.6 mL/cm) against 2 L of deionized water over 3 days.

4.3. Selective Removal of PbS QDs from the Exterior Surface of a PbS/PS-b-PAA Hybrid Vesicles. To remove the QDs on the outer surface of the vesicles, the dialyzed solution (2 mL, ca. 1 mg/mL of PS404-b-PAA62) of PbS/PS404-b-PAA62 was mixed with an ethylenediaminetetraacetic acid (EDTA) solution in water (6.8  10-2 M, 2 mL, pH 9 (adjusted with 1.0 M of NaOH)) and stirred vigorously for 24 h. Finally, the mixture was dialyzed against deionized water over 2 days. TEM samples were prepared by drying one droplet of a dialyzed sample on a carbon-coated copper grid in air at room temperature.

Acknowledgment. We are grateful to Dr. Karolina Fritz for the synthesis of PbS QDs and to Dr. Margret Hines at Evident Technology for providing the CdSe/ZnS core/shell QDs. We thank NSERC Canada for the support of this research. C.S. is grateful to the King Abdullah University of Science and Technology (KAUST) for a postdoctoral research grant (Award No. KUS-F1-030-21). Supporting Information Available: Experimental details for turbidity and TEM measurements. Synthesis of TiO2 nanocrystals. Turbidity and TEM results of PS404-b-PAA62 self-assemblies in the absence of nanocrystals. TEM image of LaOF/OA nanocrystals prepared from a solution in THF. TEM images of TiO2/OA nanocrystals in THF and TiO2/PS404-b-PAA62 self-assemblies formed in dioxane/THF/water (1:0.45:1 by weight). This material is available free of charge via the Internet at http://pubs.acs.org.

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