Polymer Vesicles as Robust Scaffolds for the Directed Assembly of nanocristais altamente cristalinos

Polymer Vesicles as Robust Scaffolds for the Directed Assembly of nanocristais...

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DOI: 10.1021/la900523s 13703Langmuir 2009, 25(24), 13703–13711 Published on Web 05/18/2009 pubs.acs.org/Langmuir ©2009 American Chemical Society

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 =7 0-150 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

Rapidadvancesinnanotechnologyrequiresimpleand 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 materials with ordered structures using either synthetic molecules (e.g., surfactants10-13 and block copolymers14-19) or biomolecules (e.g., proteins, polypeptides, and DNA)1,2,20-2 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 inthe micelle coronaconsistingofP4 VP chainsthat 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

†Part of the “Langmuir 25th Year: Molecular and macromolecular self- assemblies” special issue. *Corresponding authors. E-mail: gscholes@chem.utoronto.ca; mwinnik@ chem.utoronto.ca. (1) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128–4158. (2) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (3) Tang, Z. Y.; Kotov, N. A. Adv. Mater. 2005, 17, 951–962. (4) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Phys. Chem. 2008, 9, 20–42. (5) Schuler, D.; Frankel, R. B. Appl. Microbiol. Biotechnol. 1999, 52, 464–473. (6) Schuler, D. J. Mol. Microbiol. Biotechnol. 1999, 1, 79–86. (7) Sundstrom, V.; Pullerits, T.; van Grondelle, R. J. Phys. Chem. B 1999, 103, 2327–2346. (8) Roszak, A. W.; Howard, T. D.; Southall, J.; Gardiner, A. T.; Law, C. J.;

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Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 5–59. (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,

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13704 DOI: 10.1021/la900523s Langmuir 2009, 25(24), 13703–13711

Article Wang et al.

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, theexactlocationoftheQDsrelativeto the lipid vesicles is not yet clear because of the thin, mobile nature of the lipid membrane.Thismakestheexperimentalcharacterizationofthese hybrid lipid vesicles a significant challenge.

Polymer vesicles show remarkably higher stability and toughnessthanthevesiclesformedbylipidsorbylow-molecular-weight surfactants.26,32,3 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.Onthe 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

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-b-

PAA62, where the subscripts refer to the number average degrees of polymerization), in mixtures of tetrahydrofuran (THF)/diox- ane/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 usto 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. Herewe believe thatthe binding ofthe 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 thenanocrystalsalsocontributetodifferentsynergisticassemblies 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

TheblockcopolymerofPS404-b-PAA62(Figure1)employedin 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 whichwas cappedby oleicacid,as wellas CdSe/ ZnS core/shell QDs capped by trioctylphosphine oxide (TOPO).

All of the nanocrystals used here were synthesizedby the thermal decomposition ofprecursorsathightemperatureinthepresenceof alkyl ligands (seeExperimental Section).Nanocrystalssynthesized in this way are highly crystalline and monodisperse in size and shape. Previous studies from our laboratory47 as well as from others48demonstratedthatPAAisabletodisplacethealkylligands fromthesurfacesofnanocrystalssuchasPbSandTiO2 andrender them dispersibleinto polar media. Here, we take a further step by usinga PAA-basedamphiphilicblock copolymer,PS404-b-PAA62, as botha multidentateligand (i.e., the PAA block) anda structure- directing 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)

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Wang et al. Article

results for the self-assembly of PS404-b-PAA62 in solution without nanocrystals.Thenwesuccessivelydescribethecoassemblyofthis polymer in solution in the presence of each type of nanocrystal. 2.1. Preparation and Characterization of PS-b-PAA Ve- sicles. Westart byexamining 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 quenchedbyaddinga largeamountofwater, followedbydialysis againstwater.Figure2Ashowsadark-fieldTEMimageofPS404- b-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, andhollowsphericalaggregateswithhigherelectrontransmission 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 protrude into the solvent-filled interior cavity as well as into the external medium (Figure 2C). Whereas the sizes of the whole vesicles (d =6 0-150 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 ( 1 nm.

These results show that PS404-b-PAA62 is able to form vesicles under the experimental conditions described above. Nevertheless, with this polymer, wewere 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 resultsare shown in Supporting Information (Figures S1- S4). Under all of the conditions that we examined, we observed a mixture of micelles and vesiclesthat coexisted in dilute solution. 2.2. Organization of PbS QDs on Scaffolds of PS-b-

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