Article pubs.acs.org/Langmuir
Directional Self-Assembly of Ligand-Stabilized Gold Nanoparticles into Hollow Vesicles through Dynamic Ligand Rearrangement Robert J. Hickey,† Myungjoo Seo,‡ Qingjie Luo,† and So-Jung Park*,‡ †
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States Department of Chemistry and Nano Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul, 120-750, Korea
‡
S Supporting Information *
ABSTRACT: Here we report a novel approach to prepare all-nanoparticle vesicles using ligand-stabilized gold particles as a building block. Hydroxyalkylterminated gold nanoparticles were spontaneously organized into well-defined hollow vesicle-like assemblies in water without any template. The unusual anisotropic self-assembly was attributed to the ligand rearrangement on nanoparticles, which leads to increased hydroxyl group density at the nanoparticle/water interface. One-dimensional strings were formed instead of vesicles with increasing surface ligand density, which supports the hypothesis. The size and the wall thickness of vesicles were controlled by adjusting the concentration of nanoparticles or by adding extra surfactants. The work presented here highlights the dynamic nature of surface ligands on gold particles and demonstrates that the combination of ligand rearrangement and the hydrophobic effect can be used as a versatile tool for anisotropic self-assembly of nanoparticles.
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INTRODUCTION Vesicles composed of a thin, amphiphilic membrane and a hollow interior are of great interest owing to their cellmembrane-mimicking structure and their ability to encapsulate, protect, and transport both hydrophobic and hydrophilic components.1 Various types of artificial amphiphiles as well as natural phospholipids have been used as building blocks for vesicle formation. Examples include small molecules, block copolymers, dendrimers, and peptides.2−6 Recently, several strategies have been developed to combine the useful characteristics of vesicles and unique physical properties of inorganic nanoparticles. In one approach, we and others have used simultaneous assembly of nanoparticles and amphiphilic polymers to form polymer vesicles loaded with nanoparticles.7−12 In another approach, polymer-grafted nanoparticles were used as amphiphilic building blocks to form hollow assembly structures.13−18 For example, Duan and coworkers showed that gold nanoparticles decorated with a mixture of a hydrophobic polymer and a hydrophilic polymer can organize into vesicle-like assemblies composed of a monolayer of nanoparticles embedded in a matrix of grafted polymers.17 In a similar method, Nie and co-workers reported that gold nanoparticles immobilized with amphiphilic polymers can assemble into hollow vesicles or tubes.14,15 Herein, we report that ligand-stabilized “quasi-hydrophobic” gold nanoparticles can organize into well-defined vesicle-like assemblies in water and demonstrate that all-nanoparticle vesicles can be formed without any template or polymeric additives that guide the self-assembly. The self-assembly of hydroxyalkyl-terminated nanoparticles was investigated under various conditions (i.e., length and density of surface ligands, concentration) to understand what controls the self-assembly behavior in water. Unlike polymer-grafted particles, the © XXXX American Chemical Society
membrane of nanoparticle vesicles formed here from ligandstabilized nanoparticles is typically composed of multiple nanoparticle layers, where the wall thickness can be readily controlled by adjusting various parameters, including the concentration of nanoparticles, composition of surface ligands, and the addition of surfactants. The unique, directional selfassembly was attributed to the dynamic ligand rearrangement on nanoparticles, which leads to increased hydroxyl group density at the interface. Very recently, Zhang and co-workers reported that dodecanethiol-stabilized nanoparticles can assemble into vesicle-like structures in toluene/ketone mixture upon photo-oxidation of thiolated ligands on nanoparticles.19 Our work described here with hydroxyl-terminated particles presents a new mechanism for hollow assembly formation from ligand-stabilized nanoparticles and shows that vesicle-like assemblies can be formed purely from as-synthesized nanoparticles without any additives or chemical changes of surface ligands. Furthermore, the choice of ligands used in our study allowed for the fabrication of nanoparticle vesicles in water. The water dispersity along with the unique properties of gold nanoparticles makes our nanoparticle vesicles useful in biological and medical applications.
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EXPERIMENTAL SECTION
Materials. Gold(III) chloride trihydrate (HAuCl4·3H2O, >99.9%), 11-mercapto-1-undecanol (MUL, >97%), 6-mercapto-1-hexanol (MHL, >97%), tetraoctylammonium bromide (TOAB, 98%), and sodium borohydride (NaBH4, 99%) were purchased from SigmaAldrich. All solvents (ethanol, tetrahydrofuran, and toluene) were Received: October 3, 2014 Revised: December 6, 2014
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purchased from Fischer Scientific. Purified water (Millipore Milli-Q grade) with resistivity of 17.9 MΩ was used in all experiments. Characterization. Conventional TEM images were taken using a JEOL 1400 electron microscope, and STEM images were acquired using a JEOL 2010F electron microscope. Cryo-TEM samples were prepared using lacey carbon grids stabilized with Formvar (200 mesh, Ted Pella, part number 01890-F). The grids were prepared by removing the Formvar coating by submerging the grids in chloroform (30 s). After drying, both sides of the grid were coated with carbon using a carbon coater (Quorum Q150T ES). Both sides of the grid were subsequently treated (15 s) with a plasma cleaner (Gatan, Solarus Advanced Plasma System model 950) in the presence of hydrogen and oxygen gas. Then, 2 μL of samples was applied to the grid and blotted away. After 4−6 s of the initial plotting of the sample, the grid was rapidly plunged into liquid ethane (Gatan Cp3 cryo plunger) and then transferred to a liquid nitrogen cooled cryo-TEM holder (Gatan CT3500TR) for imaging. Cryo-TEM images were taken using a JEOL 2010 or a FEI-Tecnai T12 instrument. Synthesis and Purification of Gold Nanoparticles. Gold nanoparticles were synthesized using 11-mercapto-1-undecanol as the stabilizing agent following a literature method.20,21 The synthesized nanoparticles were washed four times with ethanol/ toluene solvent mixtures (3 mL of ethanol and 20 mL of toluene) and dispersed in ethanol. To study the effect of MUL density on nanoparticle surface, three batches of particles were synthesized with 12, 24, and 48 mg of MUL for 34 mg of HAuCl4·3H2O. The same procedure was used to prepare MHL-modified particles, except that MHL was used in place of MUL. Self-Assembly of Gold Nanoparticle Vesicles. To generate AuNP vesicles, 150 μL of AuNPs (3.0 μM) in ethanol was slowly added (10 μL/30 s) to 1.5 mL of tetrahydrofuran (THF, anhydrous) with stirring. Then, 450 μL of water was added dropwise (10 μL/30 s) to the solution with stirring to initiate the assembly. The solution was stirred overnight (15 h), and then 1.5 mL of water was added over 15 min. Finally, the solution was dialyzed against water (17.9 MΩ) for 24 h. It was then concentrated by centrifugation (14 000 rpm, 30 min) and subsequent redispersion in 1.5 mL of water.
Figure 1. (a) Pictorial description of vesicle formation from MULmodified AuNPs. (b−d) Cryo-TEM and (e) TEM images of nanoparticle vesicles assembled in water. The initial concentration of nanoparticles in 150 μL of ethanol was 3.0 μM (b, c, and e) or 0.6 μM (d).
resembles that of typical vesicles made of molecular amphiphiles such as amphiphilic polymers. The diameter of vesicles was determined to be 395 nm by DLS, which is similar to that of polymersomes successfully used in in vivo imaging and drug delivery applications.14 The concentration-dependent self-assembly behavior of MUL-stabilized AuNPs was similar to that of typical molecular amphiphiles (Figure 2). TEM images for assemblies formed at a series of different nanoparticle concentrations showed that the minimum concentration to form well-defined vesicles was about 0.3 μM (initial particle concentration in ethanol) at room temperature. Most uniform vesicles were formed at the low nanoparticle concentration range between 0.3 and 0.6 μM (Figure 2a), similar to molecular surfactants forming uniform assemblies at the critical micelle concentration (cmc).22 As nanoparticle concentration increased to 3.0 μM, nonhollow spherical assemblies appeared along with vesicles (Figure 2b). A further increase of nanoparticle concentration to 15.0 μM generated a mixture of vesicles and large irregular aggregates (Figure 2c). The aggregation number in each vesicle was increased with increasing concentration, resulting in vesicles with thicker membranes (Figure 2b,c). This behavior provides a means to control the membrane thickness and properties. The spontaneous formation of well-defined nanoparticle vesicles observed here without any template is surprising, as the vesicle formation requires a directional assembly of nanoparticles into two-dimensional membranes. Typically, the solvophobic effect of spherical nanoparticles leads to compact three-dimensional ordered assemblies or disordered aggregates,23 as anisotropic assembly requires some type of structural anisotropy, such as anisotropic particle shape, particle faceting, ligand patchiness, or mixed ligands;24 the MUL-modified AuNPs used in this study do not have apparent anisotropy when they are dispersed in a good solvent such as ethanol. We attribute the unusual self-assembly behavior to the rearrange-
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RESULTS AND DISCUSSION In typical experiments, hydroxyl-terminated gold nanoparticles (AuNPs) were synthesized by a modified Brust method11,20,21 using MUL as a stabilizing ligand. The size of nanoparticles was determined to be 2.3 ± 0.4 nm (Supporting Information). In a typical self-assembly procedure, 150 μL of synthesized AuNPs (0.3 μM) in ethanol was slowly added to 1.5 mL of tetrahydrofuran (THF) at the rate of 10 μL/30 s. Then, 450 μL of water (17.9 MΩ) was added dropwise (10 μL/30 s) to the solution to initiate the self-assembly of MUL-stabilized AuNPs. After overnight mixing (15 h), 1.5 mL of water was added to the solution over 15 min, and the solution was dialyzed against water. The nanoparticle assemblies were characterized by TEM, cryogenic TEM (cryo-TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS), and UV−vis spectroscopy. Cryo-TEM images (Figure 1b−d) showed that MULmodified AuNPs spontaneously organized into hollow vesiclelike structures in water, as illustrated in Figure 1a. Highermagnification cryo-TEM images (Figure 1d) and EDS analysis (Figure S1, Supporting Information) confirmed that the “vesicles” are indeed made of gold particles. Collapsed vesicles were observed in conventional dry TEM images (Figure 1e), consistent with the hollow structure seen in cryo-TEM images. The vesicle formation was accompanied by the increase of scattering intensity, as expected (Figure S2, Supporting Information). The appearance of nanoparticle vesicles B
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Figure 2. Cryo-TEM images of AuNP vesicles prepared at varying nanoparticle concentration. The initial particle concentration in ethanol was 0.6 μM (a), 3.0 μM (b), and 15.0 μM (c). The vesicle shell thickness of the three samples in panels a, b, and c were measured to be 21 ± 5, 32 ± 9, and 55 ± 19 nm, respectively, by measuring over 50 vesicles in TEM images for each sample.
ment19,25,26 of surface ligands in response to the addition of water, which is a marginal solvent for MUL-modified AuNPs (Scheme 1). As the water content increases and particles cluster
nanoparticle. Therefore, the vesicle formation should be favored over micelle-type assemblies for low ligand density particles. To test this hypothesis, three different batches of nanoparticles were synthesized with varying amounts of ligands. Figure 3 presents cryo-TEM images of assemblies formed from nanoparticles synthesized at [HAuCl4]:[MUL] ratios of 1:0.5, 1:1, and 1:2. The diameters of synthesized AuNPs were determined to be 2.2 ± 0.6, 2.3 ± 0.4, and 2.2 ± 0.6 nm by TEM for the three samples, and all three particles showed similar UV−vis absorption spectra (Supporting Information). As shown in Figure 3, AuNPs with the lowest MUL density resulted in the most uniform assemblies (Figure 3a). For those nanoparticles, vesicle structure is necessary to sufficiently reduce the contact between water and the hydrocarbon backbone of MUL. With increasing the ligand density (Figure 3b), a small amount of compact spherical assemblies was observed along with vesicles. With further increase of ligand density (Figure 3c), strings of nanoparticles were formed instead of vesicles, as the ligand density became sufficiently high for one-dimensional assembly. A similar ligand density dependence has been observed in a polymer-grafted nanoparticle system, quantum dots modified with poly(ethylene oxide) (PEO).18 In that study, a reduction of PEO density first induced a morphology change from isolated particles to nanoparticle strings, and a further reduction of PEG density resulted in a mixture of vesicles and one-dimensional nanoparticle networks. Our results with MUL-stabilized nanoparticles reported here show that a similar mechanism is applicable for ligand-stabilized nanoparticles without flexible polymer chains. Gold nanoparticles stabilized with a shorter ligand, MHL, showed similar self-assembly behavior as MUL-stabilized particles (Figure 4), demonstrating the generality of the
Scheme 1. Simplified Pictorial Description Showing How Two-Dimensional Assembly of Nanoparticles and the Ligand Rearrangement on the Particle Surface Can Lead to Increased Hydroxyl Group (Blue Dots) Density at the Interface
up, surface ligands reorganize to minimize the contact between water and the hydrocarbon backbone of MUL and to maximize the hydroxyl group density at the nanoparticle/water interface (Scheme 1). High hydroxyl group density at the particle/water interface can be achieved by the combination of ligand density redistribution (i.e., concentration of ligands at the water/ particle interface) and ligand bending.27 If the ligand redistribution plays an important role for the vesicle formation as hypothesized, the ligand density on nanoparticles should affect the self-assembly of MUL-stabilized AuNPs. Compared to small spherical micelles and rodlike micelles, vesicles have smaller interfacial area, and thus, the hydroxyl group density at the water/nanoparticle interface should be higher in the vesicle structure for a given
Figure 3. Cryo-TEM images of self-assembled AuNPs with varying surface ligand density. AuNPs were prepared with [HAuCl4]:[MUL] molar ratios of (a) 1:0.5, (b) 1:1, and (c) 1:2. The initial concentration of AuNPs was 0.3 μM for all samples. C
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Figure 4. TEM images of assemblies formed from MHL-stabilized AuNPs at initial nanoparticle concentrations of 0.6 μM (a), 2.0 μM (b), and 4.0 μM (c).
Figure 5. Effect of additional surfactants (MUL and OA) on the self-assembly of MUL-stabilized AuNPs. Cryo-TEM images of nanoparticle vesicles assembled without additional surfactants (a), nanoparticle vesicles assembled in 0.1 mg/mL MUL dispersion (b), and nanoparticle vesicles assembled in 0.1 mg/mL OA solution (c). (d) A TEM image of nanoparticle assemblies formed in 1 mg/mL OA solution. Inset in part d is a highermagnification image showing individual AuNPs (scale bar is 25 nm). (e) DLS data for samples shown in panels a (black solid), b (black dashed), c (red), and d (green). (f) Pictorial description showing how the addition of OA affects the size of nanoparticle vesicles.
typically form vesicles composed of multiple nanoparticle layers (Scheme 1). Considering that the particle diameter is about 5.8 nm, including the ligand layer, the number of nanoparticle layers in the vesicle wall is estimated to be around three to six for typical vesicles shown in Figure 1b. The distribution in ligand density among nanoparticles might contribute to the formation of multilayer nanoparticle membranes. Particles with small numbers of surface ligands should partition in the middle part of the membrane, as they are less soluble in water. On the other hand, particles with higher MUL density should preferentially decorate the outer and inner interfaces of the membrane, as depicted in Scheme 1. In addition, the particles occupying the outer interface should possess higher ligand density than the particles at the inter layer, as the outer interface has a larger interfacial area. A similar mechanism was shown to be responsible for the vesicle formation from a mixture of two different length amphiphilic polymers, where polymers with a longer hydrophilic block preferentially occupy the outer layer.29 The size and the wall thickness of nanoparticle vesicles were also controllable by coassembling nanoparticles with additional surfactants (Figure 5). The addition of excess MUL during the self-assembly process generated vesicles with thinner and flexible membranes (Figure 5b). When nanoparticles were assembled in the presence of another type of surfactant, oleic acid (OA), the vesicle size became significantly larger (Figure 5c) compared to the typical vesicles formed without OA
approach. This result indicates that the ligand density redistribution should play an important role for the anisotropic self-assembly observed here. While ligand bending28 can contribute to the vesicle formation, MHL is too short to explain the self-assembly behavior solely with the ligand bending. The MHL-stabilized particles showed similar concentration dependence as MUL-stabilized particles. However, the minimum concentration for the vesicle formation was somewhat higher than that of MUL-stabilized particles, which is expected on the basis of the solubility of MHL and MUL in water. For MHL ligands, vesicles with very thin flexible membranes started to emerge at 0.6 μM nanoparticle concentration (Figure 4a). At 0.6 μM, MUL-stabilized particles formed uniform, well-defined vesicles with membranes composed of multiple nanoparticle layers (Figure 2a). For MHL, such an assembly structure was formed at 2.0 μM concentration (Figures 4b and S5, Supporting Information). With a further increase of nanoparticle concentration (over 3.0 μM), spherical nanoparticle aggregates were formed along with vesicles (Figure 4c), similar to the behavior of MUL-stabilized particles. It is worth noting that the membrane structure and the mechanism of vesicle formation from these as-synthesized ligand-stabilized particles is distinct from vesicles formed from polymer-grafted particles,14−18 where vesicles are always made of a nanoparticle monolayer. Unlike the polymer-grafted particles,14,17,18 ligand-stabilized nanoparticles studied here D
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(4) Percec, V.; Wilson, D. A.; Leowanawat, P.; Wilson, C. J.; Hughes, A. D.; Kaucher, M. S.; Hammer, D. A.; Levine, D. H.; Kim, A. J.; Bates, F. S.; Davis, K. P.; Lodge, T. P.; Klein, M. L.; DeVane, R. H.; Aqad, E.; Rosen, B. M.; Argintaru, A. O.; Sienkowska, M. J.; Rissanen, K.; Nummelin, S.; Ropponen, J. Self-Assembly of Janus Dendrimers into Uniform Dendrimersomes and Other Complex Architectures. Science 2010, 328 (5981), 1009−1014. (5) Vargo, K. B.; Parthasarathy, R.; Hammer, D. A. Self-Assembly of Tunable Protein Suprastructures from Recombinant Oleosin. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 11657−11662. (6) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Spherical Bilayer Vesicles of Fullerene-Based Surfactants in Water: A Laser Light Scattering Study. Science 2001, 291 (5510), 1944−1947. (7) Mai, Y.; Eisenberg, A. Selective Localization of Preformed Nanoparticles in Morphologically Controllable Block Copolymer Aggregates in Solution. Acc. Chem. Res. 2012, 45 (10), 1657−1666. (8) Mai, Y.; Eisenberg, A. Controlled Incorporation of Particles into the Central Portion of Vesicle Walls. J. Am. Chem. Soc. 2010, 132 (29), 10078−10084. (9) Sanson, C.; Diou, O.; Thévenot, J.; Ibarboure, E.; Soum, A.; Brûlet, A.; Miraux, S.; Thiaudière, E.; Tan, S.; Brisson, A.; Dupuis, V.; Sandre, O.; Lecommandoux, S. Doxorubicin Loaded Magnetic Polymersomes: Theranostic Nanocarriers for MR Imaging and Magneto-Chemotherapy. ACS Nano 2011, 5 (2), 1122−1140. (10) Hickey, R. J.; Haynes, A. S.; Kikkawa, J. M.; Park, S.-J. Controlling the Self-Assembly Structure of Magnetic Nanoparticles and Amphiphilic Block-Copolymers: From Micelles to Vesicles. J. Am. Chem. Soc. 2011, 133 (5), 1517−1525. (11) Hickey, R. J.; Luo, Q.; Park, S.-J. Polymersomes and Multicompartment Polymersomes Formed by the Interfacial SelfAssembly of Gold Nanoparticles and Amphiphilic Polymers. ACS Macro Lett. 2013, 2, 805−808. (12) Hickey, R. J.; Koski, J.; Meng, X.; Riggleman, R. A.; Zhang, P.; Park, S.-J. Size-Controlled Self-Assembly of Superparamagnetic Polymersomes. ACS Nano 2014, 8 (1), 495−502. (13) Moffitt, M. G. Self-Assembly of Polymer Brush-Functionalized Inorganic Nanoparticles: From Hairy Balls to Smart Molecular Mimics. J. Phys. Chem. Lett. 2013, 4, 3654−3666. (14) He, J.; Huang, X.; Li, Y.-C.; Liu, Y.; Babu, T.; Aronova, M. A.; Wang, S.; Lu, Z.; Chen, X.; Nie, Z. Self-Assembly of Amphiphilic Plasmonic Micelle-Like Nanoparticles in Selective Solvents. J. Am. Chem. Soc. 2013, 135 (21), 7974−7984. (15) He, J.; Liu, Y.; Babu, T.; Wei, Z.; Nie, Z. Self-Assembly of Inorganic Nanoparticle Vesicles and Tubules Driven by Tethered Linear Block Copolymers. J. Am. Chem. Soc. 2012, 134 (28), 11342− 11345. (16) Niikura, K.; Iyo, N.; Higuchi, T.; Nishio, T.; Jinnai, H.; Fujitani, N.; Ijiro, K. Gold Nanoparticles Coated with Semi-Fluorinated Oligo(ethylene glycol) Produce Sub-100 nm Nanoparticle Vesicles without Templates. J. Am. Chem. Soc. 2012, 134 (18), 7632−7635. (17) Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. Plasmonic Vesicles of Amphiphilic Gold Nanocrystals: Self-Assembly and External-Stimuli-Triggered Destruction. J. Am. Chem. Soc. 2011, 133 (28), 10760−10763. (18) Nikolic, M. S.; Olsson, C.; Salcher, A.; Kornowski, A.; Rank, A.; Schubert, R.; Frömsdorf, A.; Weller, H.; Förster, S. Micelle and Vesicle Formation of Amphiphilic Nanoparticles. Angew. Chem., Int. Ed. 2009, 48 (15), 2752−2754. (19) Bian, T.; Shang, L.; Yu, H.; Perez, M. T.; Wu, L.-Z.; Tung, C.H.; Nie, Z.; Tang, Z.; Zhang, T. Spontaneous Organization of Inorganic Nanoparticles into Nanovesicles Triggered by UV Light. Adv. Mater. 2014, 26 (32), 5613−5618. (20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatised Gold Nanoparticles in a Two-Phase Liquid−Liquid System. J. Chem. Soc., Chem. Commun. 1994, 7, 801− 802. (21) Luo, Q.; Hickey, R. J.; Park, S.-J. Controlling the Location of Nanoparticles in Colloidal Assemblies of Amphiphilic Polymers by
(Figure 5a). With further increase of OA concentration, large vesicles and sheets were observed by TEM (Figure 5d); the AuNP sheets appear to be large vesicles that are burst open due to the TEM sample preparation conditions. DLS data presented in Figure 5e are consistent with TEM observations. These results suggest that the size and the membrane property of nanoparticle vesicles can be controlled by the coassembly with a large library of surfactants.
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CONCLUSIONS In summary, we report that gold nanoparticles stabilized with quasi-hydrophobic ligands can be organized into well-defined all-nanoparticle vesicles in water without any template. The vesicles stayed well-dispersed in water and stable over months. The unusual directional assembly was attributed to the dynamic ligand rearrangement in response to the changes in solvent compositions. The water addition to hydroxyl-terminated AuNPs causes surface ligands to reorganize to minimize the unfavorable interaction between water and hydrocarbon backbone of surface ligands and to maximize the hydroxyl group density at the interface. The wall thickness and the size of nanoparticle vesicles were controllable by adjusting nanoparticle concentration or through the coassembly with additional surfactants. It is well-known that one- or twodimensional self-assembly of nanoparticles can result in interesting collective properties.14,23,30 The work presented here provides a new pathway for directional anisotropic selfassembly of ligand-stabilized nanoparticles.
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ASSOCIATED CONTENT
S Supporting Information *
STEM images and EDS analyses of nanoparticle vesicles, UV− vis extinction spectra of MUL-stabilized AuNPs in different solvent mixtures, TEM images of MUL-stabilized AuNPs and their assemblies, and a cryo-TEM image of vesicles formed from MHL-stabilized AuNPs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS S.J.P. acknowledges the support from the Camille Dreyfus teacher scholar award and the NSF MRSEC seed award. The STEM, EDS, and cryo-TEM measurements were carried out using instrumentation at the Penn Regional Nanotechnology Facility and the Penn Electron Microscopy Resource Laboratory. The authors thank Dr. Dewight R. Williams for help with cryo-TEM imaging.
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REFERENCES
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