Nanoparticle Vesicles with Controllable Surface Topographies

Nov 11, 2015 - The silica NPVs gain different surface topographies, such as raspberry- and brain coral-like topographies, under controlled heat treatm...
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Nanoparticle Vesicles with Controllable Surface Topographies through Block Copolymer-Mediated Self-Assembly of Silica Nanospheres Shujun Zhou, Ayae Sugawara-Narutaki, Sachio Tsuboike, Junzheng Wang, Atsushi Shimojima, and Tatsuya Okubo* Department of Chemical System Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Silica nanoparticle vesicles (NPVs) with encapsulating capability and surface permeability are highly attractive in nanocatalysis, biosensing, and drug delivery systems. Herein, we report the facile fabrication of silica NPVs composed of a monolayer of silica nanospheres (SNSs, ca. 15 nm in diameter) through the block copolymer-mediated self-assembly of SNSs. The silica NPVs gain different surface topographies, such as raspberry- and brain coral-like topographies, under controlled heat treatment conditions. The vesicular assembly of SNSs is successful with a series of poly(propylene oxide)-poly(ethylene oxide)poly(propylene oxide) block copolymers, and the size of NPVs can be tuned by changing their molecular weight. The polymer is easily extracted from the NPVs with their colloidal dispersibility and structural integrity intact. The polymer-free silica NPVs further serve as a reaction vessel and host for functional materials such as tin oxide nanoparticles.

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INTRODUCTION Nanoparticle vesicles (NPVs) have attracted considerable attention for their encapsulation capability and surface permeability, which lead to applications in catalysis,1 energy storage,2 sensing,3 and drug delivery.4 NPVs can be prepared with templating and nontemplating methods. Templating methods include the layer-by-layer assembly of nanoparticles on sacrificial latex templates5,6 and the assembly of nanoparticles on emulsion-droplet templates.7,8 With nontemplating methods, polymer plays an important role as the mediator of nanoparticle assembly. Polyelectrolytes can drive nanoparticles to assemble into robust NPVs through a charge-driven flocculation mechanism, giving NPVs with thick shells.9−11 Block copolymer provides a powerful tool to mediate the vesicular assembly of nanoparticles.12−14 Hydration-induced coassembly of nanoparticles and amphiphilic block copolymers in organic solvents led to polymeric vesicles with the nanoparticles embedded in the vesicle wall.15,16 Nanoparticles with well-defined surface-grafted block copolymers, i.e., colloidal amphiphiles, were created, and they underwent amphiphile-like self-assembly to give vesicles with a monolayer shell.14,17−21 Nanoparticles with surface-grafted homopolymers similarly assembled into NPVs through amphiphile-like assembly behavior.22−24 © XXXX American Chemical Society

Colloidal silica NPVs are a prominent class of silica hollow nanomaterials that have low densities, high surface areas, mechanical stability, ease of functionalization, low toxicity, and good biocompatibility.25 Silica NPVs were mostly prepared with polymer latex5,6 or emulsion-droplet templates.26−28 In emulsion-droplet methods, the surfaces of silica NPs should be hydrophobized for the stabilization of the emulsion, and subsequent cross-linking between the NPs is required to obtain robust NPVs.27,28 Recently, silica NPVs assembled from silica nanoparticles modified with cyclodextrin and adamantyl groups were reported, and their assembly was driven by supramolecular host−guest interactions of the surface functional groups.29 To date, there is still no simple polymer-mediated self-assembly approach to fabricating stable colloidal silica NPVs having a monolayer shell. We previously reported the self-assembly of silica nanospheres (SNSs) into one-dimensional (1D) chainlike30,31 and two-dimenstional (2D) ringlike32 nanostructures using amphiphilic block copolymers as the mediators, demonstrating that block copolymers allow facile and flexible control over the Received: September 11, 2015 Revised: November 10, 2015

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DOI: 10.1021/acs.langmuir.5b03424 Langmuir XXXX, XXX, XXX−XXX

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Industries. The block copolymers used in this study are shown in Table S1. 25R4, 25R2, 17R4, P65, and P123 were obtained from BASF Corporation. F127 was purchased from Sigma-Aldrich Corporation. L121 and L64 were received from Adeka Corporation as gifts. All chemicals were used without further purification. Deionized water was used as the solvent in all experiments. Synthesis of SNSs. A colloidal suspension of SNSs of ca. 15 nm in diameter was prepared as previously reported.33−36 Typically, L-lysine (0.037 g) was dissolved in water (34.80 g), and then TEOS (2.60 g) was added. The mixture was allowed to react at 60 °C for 24 h using a water bath with magnetic stirring at 500 rpm. Finally, a homogeneous and optically clear suspension of SNSs (2 wt % SiO2, pH ∼9.4) was obtained. A colloidal suspension of SNSs of ca. 30 nm diameter was prepared via the seed-regrowth method. Briefly, the as-prepared suspension of SNSs of ca. 15 nm diameter (6.86 g) was added to a lysine aqueous solution (6.7 mM, 27.44 g), followed by the addition of TEOS (2.05 g). The mixture was allowed to react at 60 °C for 24 h using a water bath with magnetic stirring at 500 rpm, giving a suspension of SNSs (2 wt % SiO2, pH ∼9.3). Vesicular Assembly of SNSs. In a typical procedure, 25R4 was dissolved at 2 wt % in the as-prepared SNS suspension at room temperature to give a homogeneous 25R4−SNS suspension. Subsequently, the pH of the suspension was adjusted to pH 9.0 using a 0.1 M HCl aqueous solution. The suspension was transferred to a Teflon container, sealed in an autoclave, and statically treated under hydrothermal conditions. Polymer Extraction. The as-prepared silica NPVs were washed four times with ethanol and then three times with water to remove the polymer. During the washing procedures, centrifugation at 10 000 rpm for 10 min was applied to sediment the NPVs, and redispersion of the NPVs in ethanol or water was achieved through sonication or shaking. Finally, the NPVs were redispersed in a proper amount of water to contain ca. 2 wt % silica. Preparation of Silica NPVs Loaded with Tin Oxide Nanoparticles. SnO2 NPs were synthesized through the hydrolysis of SnCl4 with arginine according to the literature.40 As-prepared NPVs (pH 9.0, 150 °C, 24 h) was washed three times with ethanol and three times with water to extract 25R4. The polymer-free NPVs were then redispersed in water to reach a concentration of 2 wt % silica. SnCl4· 5H2O (0.4 g) was added to the polymer-free NPV suspension (10 g) in a glass vial, and then water was added to reach a total mass of 20 g. After being stirred at room temperature for 2 h, L-arginine (2.2 g) was added to the suspension, and it was stirred at room temperature for 24 h. The suspension was washed three times with water by centrifugation (10 000 rpm, 10 min), the water was decanted, and the suspension was redispersed in water with shaking. The arginine and residual ions were removed by dialysis against water. The suspension was then autoclaved at 140 °C for 24 h to increase the crystallinity of SnO2. Characterization. SEM observations were conducted on a Hitachi S-900 microscope operating at 6 kV. TEM observations were performed on a JEOL JEM 2000EXII microscope and a Hitachi H800 operated at 200 kV. Nitrogen adsorption−desorption measurements were conducted at 77 K using an Autosorb-iQ instrument (Quantachrome). The vesicles were calcined at 500 °C for 8 h and degassed at 400 °C for 6 h prior to nitrogen adsorption−desorption measurements. The BET specific surface area was calculated from adsorption data in the relative pressure range of 0.02−0.2. The total pore volume was evaluated from the adsorbed amount of nitrogen at a relative pressure of 0.99. CHN elemental analysis was conducted on a CE-440 CHN analyzer. TG-DTA analysis was conducted on a Thermoplus TG8120 (Rigaku) analyzer at 5 K/min using a mixture of 10% O2−90% He as the carrier gas. FTIR was performed using an FT/ IR 6100 spectrometer (Jasco).

colloidal self-assembly behaviors of silica nanoparticles. Nanochains of SNSs were formed in the presence of pluronic polymers consisting of poly(ethylene oxide) and poly(propylene oxide) (PEO-PPO-PEO).30,31 Nanorings of SNSs were obtained using block copolymers poly[(2-ethoxyethyl vinyl ether)-block-(2-methoxyethyl vinyl ether)] (EOVEMOVE).32 Both PEO-PPO-PEO and EOVE-MOVE are temperature-responsive, and each block segment has a distinct lower critical solution temperature (LCST). The 1D and 2D assembly of SNSs took place at the temperature where these temperature-responsive block copolymers were amphiphilic. Herein, we show that SNSs of ca. 15 nm diameter assemble three-dimensionally into NPVs composed of a monolayer shell with exotic surface topographies with reverse pluronic polymers (PPO-PEO-PPO) (Scheme 1). The NPVs were obtained under Scheme 1. Block Copolymer-Mediated Self-Assembly of Silica Nanospheres into Nanoparticle Vesicles with Raspberry- and Brain Coral-like Surface Topographies

hydrothermal conditions (≥110 °C) with both PEO and PPO segments being hydrophobic. The SNSs are synthesized through the hydrolysis and condensation reactions of tetraethyl orthosilicate in water using L-lysine, a basic amino acid, as the catalyst.33−36 The as-prepared SNS suspension has a pH of ∼9.4, at which the SNS surface carries deprotonated silanol groups (Si−O−); meanwhile, a portion of lysine molecules adsorb to the SNS surface through electrostatic interactions with the Si−O− groups.37 The SNSs are well dispersed by electrostatic repulsions between the Si−O− groups and hence provide ideal building units for self-assembly into NPVs. The block copolymer, reverse pluronic 25R4 (Table S1), has an average molecular weight of 3600 and 40 wt % EO content. 25R4 can adsorb to the silica surface through hydrogen bonding between the ether oxygens of EO and the silanol groups of silica.38 The polymer-mediated vesicular assembly of SNSs was conducted under hydrothermal conditions, which promoted the fusion of SNSs through the Gibbs−Thomson effect to strengthen the NPV structure and reconstruct the NPV surface.39 The surface reconstruction leads to brain corallike surface topography of the NPVs. The polymer was easily extracted while retaining the structural integrity and dispersibility of the NPVs, which were further exploited as a reaction vessel and host for functional materials such as SnO 2 nanoparticles.

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RESULTS AND DISCUSSION Silica NPVs with Controllable Surface Topographies. The NPVs were prepared by dissolving polymer 25R4 in the SNS (ca. 15 nm) suspension, followed by pH adjustment and hydrothermal treatment of the suspension. NPVs with well-

EXPERIMENTAL SECTION

Materials. Tetraethyl orthosilicate (TEOS) was purchased from Tokyo Chemical Industry Co., Ltd. L-Lysine, L-arginine, tin(IV) chloride, and ethanol were purchased from Wako Pure Chemical B

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Langmuir defined shapes were typically obtained with 2 wt % 25R4 and 2 wt % SiO2 at pH 9.0 after hydrothermal treatment at 110−190 °C for 24 h. Figure 1 shows the transmission electron

Figure 2. SEM images of the products obtained from the 25R4−SNS suspension (2 wt % 25R4 and SNSs, pH 9.0) after 24 h of heat treatment at (a) 60, (b) 80, (c) 100, (d) 110, (e) 150, and (f) 190 °C. The inset in (e) is an enlarged SEM image of the networklike structure on the NPV surface.

The NPVs formed at a low temperature have a type IV isotherm (Figure 3(a)), which is indicative of mesopores. The

Figure 1. TEM images of NPVs composed of SNSs ca. 15 nm in diameter after hydrothermal treatment at (a) 110 °C and (b) 150 °C. SEM images of (c) polymer-free NPVs (150 °C) composed of SNSs ca. 15 nm in diameter and (d) NPVs (150 °C) composed of SNSs ca. 30 nm in diameter. The insets in (a) and (d) show broken NPVs in the corresponding samples.

microscopy (TEM) and scanning electron microscopy (SEM) images of the NPVs. The contrast observed in the TEM image indicates the hollow interiors (Figure 1(a),(b)). The NPVs are composed of a monolayer of SNSs, as indicated by the SEM image of the broken NPV (Figure 1(a), inset). The NPVs are robust and retain their structural integrity after intense washing and redispersion procedures (Figure 1(c)). The removal of polymer after washing has been confirmed with thermogravimetric−differential thermal analysis (TG-DTA) and Fourier transform infrared (FTIR) measurements (Figure S1). In addition, larger SNSs (ca. 30 nm in diameter) assemble into NPVs composed of several layers of SNSs after hydrothermal treatment (Figure 1(d)). Well-defined NPVs form at temperatures higher than 110 °C, whereas irregular aggregates of SNSs are obtained at lower temperatures (Figure 2). The mean diameter of the NPVs ranges from 218 to 280 nm (Figure S2), showing little dependence on the treatment temperatures within 110−190 °C. Meanwhile, the polydispersity of the particle size tends to decrease with increasing temperature (Figure S2). The surface topographies of the NPVs can change significantly with the hydrothermal treatment temperature. The NPVs prepared at 110 °C have rough, bumpy shells that resemble the surface of raspberries (Figure 2(d)). At 150 °C, the NPVs have an exotic surface topography resembling that of the brain corals (Figure 2(e)). A close examination of the brain coral-like topography suggests that it is a meandering silica network confined to a curved spherical surface. At an even higher temperature of 190 °C, large irregular pores form and the NPV surface is very smooth (Figure 2(f)). The pore characteristics of the NPVs are examined with nitrogen adsorption−desorption measurements.

Figure 3. Nitrogen adsorption−desorption isotherms of NPVs formed at (a) 110, (b) 150, and (c) 190 °C. The isotherms in (b) and (c) are vertically offset by 1000 and 2000 cm3 g−1, respectively. The inset is a table of BET surface area and total pore volume of the NPVs.

hysteresis loop disappears with increasing temperature (Figure 3(b),(c)). The NPVs exhibit total pore volumes (0.57−1.71 cm3 g−1) higher than that of the solid silica spheres having a similar diameter.41 The actual pore volumes of the NPVs should be even higher because nitrogen adsorption−desorption measurements cannot completely probe the volume of the NPV interiors, which are larger than 200 nm. The NPVs hence promise an extremely high encapsulation capacity. The Brunauer−Emmett−Teller (BET) surface area of the NPVs decreases with increasing temperature (Figure 3, inset), which is consistent with increasing surface smoothness of the NPVs observed by SEM (Figure 2(d)−(f)). The change in surface topography was also induced by continuous hydrothermal treatment of the NPVs at a constant temperature. The NPVs prepared after different heating times at 150 °C are shown in Figure 4 as an example. The NPVs obtained after 2 h have a rough raspberry-like surface in which many individual SNSs can be distinguished (Figure 4(a)). The brain coral-like topography develops after 15 h (Figure 4(b)), but prolonged hydrothermal treatment inevitably leads to C

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at a low pH of ∼8.5 indicates a lack of sufficient electrostatic repulsions to balance the interparticle attractions, which may be hydrophobic interactions induced by surface-adsorbed polymer 25R4. Such pH-dependent dispersion/aggregation behavior of SNSs due to the changed electrostatic repulsion strength has also been observed in their one-dimensional assembly mediated by PEO-PPO-PEO block copolymers.30,31,42 Effects of the 25R4-to-Silica Ratio on the Vesicular Assembly of SNSs. The proper concentration of 25R4 is necessary to induce the vesicular assembly of SNSs. In the absence of 25R4, the SNS suspension remains transparent after 24 h of hydrothermal treatment at 150 °C. The close-packed structure of the SNSs upon drying on a silicon wafer (Figure S3(a)) suggests that the SNSs are well dispersed in the liquid phase.33−37 With 2 wt % SNSs, 1.8−2.2 wt % 25R4 is optimal for the assembly of SNSs into NPVs; otherwise, the SNSs form ill-defined aggregates (Figure S3(b)−(d)). Moreover, a series of suspensions with varied 25R4 and SNS concentrations were prepared and subjected to hydrothermal treatment at optimal temperature and pH. They yielded either NPVs (Figure S3(e)− (i)) or irregular aggregates. On the basis of SEM observations, a morphology diagram was charted as a function of 25R4 and SNS concentration (Figure 6). The diagram shows that NPVs

Figure 4. SEM images of NPVs prepared from the 25R4−SNS suspension (2 wt % 25R4 and SNSs, pH 9.0) after (a) 2 h, (b) 15 h, and (c) 12 days of hydrothermal treatment at 150 °C.

smooth NPV surfaces with irregular pores (Figure 4(c)). The tendency for NPVs to have a smoother surface with increasing hydrothermal temperature or treatment time can be explained by the Gibbs−Thomson effect, which states that surface curvature differences drive mass transfer from convex to concave for a minimized total chemical potential.39 The silica solubility and diffusivity remarkably increase under hydrothermal conditions.39 The silica NPVs have different surface curvatures, which promote silica dissolution from the SNS surfaces and silica redeposition at the SNS−SNS contact points.39 Effects of pH on Vesicular Assembly of SNSs. Under optimal concentration and temperature conditions, the favorable pH range for vesicular assembly is found to be 8.8−9.4 (Figure 5). The SNSs remain dispersed at a high pH of

Figure 6. Morphology diagram of the products obtained from 25R4− SNS suspensions with varied compositions. ○ and △ represents NPVs and aggregates, respectively.

can be obtained at an approximate 25R4-to-silica mass ratio of 1:1. We speculate that there may be some cooperation between SNSs and 25R4 to achieve the vesicular assembly. The asprepared NPVs generally contain 15−20 wt % 25R4 according to CHN elemental analysis, whereas by calculation at least 55− 71 wt % 25R4 is required if the vesicle pores are completely filled (Table S2, note [c]). Therefore, the NPV chamber should contain 25R4 as well as a large quantity of water in the liquid phase. Effects of Polymer Structure on Vesicular Assembly of SNSs. A series of block copolymers composed of PEO and PPO segments (Table S1) are used instead of 25R4 to investigate the effects of polymer structure on SNS assembly. Two other PPO-PEO-PPO block copolymers, i.e., 25R2 (Mw 3100, 20 wt % EO) and 17R4 (Mw 2650, 40 wt % EO), successfully lead to the formation of silica NPVs (Figure 7). The NPVs formed with 25R2 and 17R4 have mean diameters of ∼164 and ∼64 nm, respectively. It appears that the size of

Figure 5. SEM images of the products obtained from the 25R4−SNS suspension at (a) pH 8.5, (b) pH 8.8, (c) pH 9.4, and (d) pH 10.0 after 24 h of hydrothermal treatment at 150 °C. The inset in (d) is a photograph of the corresponding suspension after hydrothermal treatment. All of the suspensions contain 2 wt % 25R4 and SNS.

∼10.0 (Figure 5(d)) while they form irregular aggregates at a low pH of ∼8.5 (Figure 5(a)). Colloidal silica has an isoelectric point at pH 2 to 3,39 above which silanol groups on the silica surface are deprotonated into negatively charged Si−O− groups. Raising the pH to above the isoelectric point of silica increases the number of Si−O− groups, hence enhancing the electrostatic repulsions between silica surfaces. The SNS dispersion at a high pH of ∼10 suggests the existence of considerably strong electrostatic repulsions that prevent SNS assembly, whereas the tendency of SNSs to randomly aggregate D

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conditions. Consequently, the initially formed polymer amphiphile−SNS coassembled structure can be preserved to some extent. Because the polymer block sequence significantly affects the association behaviors of polymers in their amphiphilic states45 and the molecular configurations of polymers at the polymer/particle interface,46 the final assembled structures of SNSs differ with the polymer block sequence. Silica NPVs as a Reaction Vessel and Host for Tin Oxide. The silica NPVs have many attractive features, including structural robustness, hydrothermal stability, redispersibility in water and ethanol, easy retrieval from solvent, high surface permeability, and availability of functionalization, which render them particularly promising materials for use as nanoreactors.47−49 As a demonstration, the polymer-free silica NPVs were employed as a reaction vessel and host for SnO2 NPs. The as-prepared silica NPVs (pH 9.0, 150 °C, 24 h) were vigorously washed to extract polymer 25R4 and resuspended in water. SnO2 NPs were synthesized by using SnCl4 and Larginine in the presence of polymer-free NPVs. Here the reactants can go through the NPV’s nanopores; therefore, the reaction proceeds both inside and outside the NPVs. The SnO2 NP are trapped at the NPVs because of their larger size or interaction with the silica substrate. The loading of SnO2 NPs onto silica NPVs after vigorous washing is confirmed with TEM. SnO2 NPs ca. 1 to 2 nm in size can be seen on the NPVs (Figure 8), and the stereoscopic viewing of the TEM image demonstrates that the SnO2 NPs are located both inside and outside the NPV (Figure S6).

Figure 7. SEM images of NPVs prepared in (a) a 25R2−SNS suspension (2 wt % 25R2 and SNS) at pH 9.0 and (b) a 17R4−SNS suspension (2 wt % 17R4 and SNS) at pH 8.8 after 24 h of hydrothermal treatment at 150 °C.

the NPVs decreases with decreasing molecular weight of the PPO-PEO-PPO block copolymers (25R4 > 25R2 > 17R4), implicating possible NPV size control through using different PPO-PEO-PPO block copolymers. In contrast, the PEO-PPOPEO block copolymers (i.e., L121, P123, L64, P65, and F127) result in irregular aggregates (Figure S4). Notably, L64 is comparable to 25R4 in terms of chemical composition and molecular weight except that it has a reversed block sequence (Table S1). The PPO-PEO-PPO and PEO-PPO-PEO block copolymers should behave as hydrophobic polymers at temperatures above 110 °C where the NPVs are successfully obtained because the PEO and PPO segments are thermoresponsive and become hydrophobic at temperatures above around 80 and 20 °C, respectively.43 A possible situation above 110 °C is such that these polymers hydrophobically aggregate into oil droplets, around which the hydrophilic SNSs accumulate to reduce the polymers’ exposure to water. However, this model cannot explain why the assembled structures of SNSs differ depending on the polymer block sequence. In our experiments, the SNSs and block copolymers are typically mixed at room temperature and then heated to the target temperature. We assume that the assembly process during temperature elevation affects the final assembled structures of SNSs. Because the block copolymers are amphiphilic up to around 80 °C, they can form supramolecular aggregates in water during temperature elevation. For example, the 25R4−water phase diagram shows that aqueous solution containing 0−60 wt % 25R4 has a water phase and a lamellar lyotropic liquid-crystal phase composed of densely packed vesicles within 55−75 °C.44 In fact, we observed that the aqueous solution containing 2 wt % 25R4 and 6.7 mM lysine (the base catalyst of SNS synthesis) macroscopically separated into an oil phase and a water phase at 60 °C (Figure S5), as is consistent with the descriptions of the 25R4−water phase diagram.44 When SNSs (SiO2 2 wt %) were introduced into the above solution, a white turbid suspension rather than a phase-separated solution was obtained at 60 °C, indicating that 25R4 and SNSs coassemble to form aggregates. Such coassembly of 25R4 and SNSs is enabled by hydrogen bonding between the EO and silanol groups. Further increasing the temperature to above the LCST of PEO will cause the hydrophobic collapse of the polymers to form oil-rich phases. The SNSs preferentially locate at the oil/water interface rather than being trapped in the oil phase because hydrogen bonding between the polymer and SNS is weakened at higher temperatures. Meanwhile, the SNS motion is restricted because the SNS−SNS contact points are simultaneously fixed through silica dissolution and redeposition under hydrothermal

Figure 8. TEM image of NPVs containing SnO2 NPs. Some SnO2 NPs are indicated by arrows.

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CONCLUSIONS We presented a facile polymer-mediated self-assembly approach to fabricating colloidally stable silica NPVs composed of a monolayer of tiny SNSs (ca. 15 nm in diameter). The vesicular assembly of SNSs took place in the presence of PPO-PEO-PPO block copolymer 25R4 under hydrothermal conditions. Silica NPVs with different surface topographies such as raspberryand brain coral-like were prepared through controlling the hydrothermal treatment temperature and duration. Although the mechanism of vesicular assembly is not yet completely understood, we found that block copolymers having a PPOPEO-PPO block sequence were favorable to inducing the vesicular assembly of SNSs. The polymer 25R4 could be easily extracted from the silica NPVs with their structural integrity and dispersibility intact. The polymer-free silica NPVs with E

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(6) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mö hwald, H. Electrostatic Self-Assembly of Silica Nanoparticle−Polyelectrolyte Multilayers on Polystyrene Latex Particles. J. Am. Chem. Soc. 1998, 120, 8523−8524. (7) Duan, H.; Wang, D.; Sobal, N. S.; Giersig, M.; Kurth, D. G.; Möhwald, H. Magnetic Colloidosomes Derived from Nanoparticle Interfacial Self-Assembly. Nano Lett. 2005, 5, 949−952. (8) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles. Science 2002, 298, 1006− 1009. (9) Wong, M. S.; Cha, J. N.; Choi, K.-S.; Deming, T. J.; Stucky, G. D. Assembly of Nanoparticles into Hollow Spheres Using Block Copolypeptides. Nano Lett. 2002, 2, 583−587. (10) Murthy, V. S.; Cha, J. N.; Stucky, G. D.; Wong, M. S. ChargeDriven Flocculation of Poly(L-lysine)−Gold Nanoparticle Assemblies Leading to Hollow Microspheres. J. Am. Chem. Soc. 2004, 126, 5292− 5299. (11) Bagaria, H. G.; Wong, M. S. Polyamine−Salt Aggregate Assembly of Capsules as Responsive Drug Delivery Vehicles. J. Mater. Chem. 2011, 21, 9454−9466. (12) Haryono, A.; Binder, W. H. Controlled Arrangement of Nanoparticle Arrays in Block-Copolymer Domains. Small 2006, 2, 600−611. (13) Schacher, F. H.; Rupar, P. A.; Manners, I. Functional Block Copolymers: Nanostructured Materials with Emerging Applications. Angew. Chem., Int. Ed. 2012, 51, 7898−7921. (14) Hu, J.; Wu, T.; Zhang, G.; Liu, S. Efficient Synthesis of Single Gold Nanoparticle Hybrid Amphiphilic Triblock Copolymers and Their Controlled Self-Assembly. J. Am. Chem. Soc. 2012, 134, 7624− 7627. (15) 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, 495−502. (16) 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. (17) Liu, Y.; Li, Y.; He, J.; Duelge, K. J.; Lu, Z.; Nie, Z. EntropyDriven Pattern Formation of Hybrid Vesicular Assemblies Made from Molecular and Nanoparticle Amphiphiles. J. Am. Chem. Soc. 2014, 136, 2602−2610. (18) 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, 7974−7984. (19) Huang, P.; Lin, J.; Li, W.; Rong, P.; Wang, Z.; Wang, S.; Wang, X.; Sun, X.; Aronova, M.; Niu, G.; Leapman, R. D.; Nie, Z.; Chen, X. Biodegradable Gold Nanovesicles with an Ultrastrong Plasmonic Coupling Effect for Photoacoustic Imaging and Photothermal Therapy. Angew. Chem., Int. Ed. 2013, 52, 13958−13964. (20) He, J.; Wei, Z.; Wang, L.; Tomova, Z.; Babu, T.; Wang, C.; Han, X.; Fourkas, J. T.; Nie, Z. Hydrodynamically Driven Self-Assembly of Giant Vesicles of Metal Nanoparticles for Remote-Controlled Release. Angew. Chem., Int. Ed. 2013, 52, 2463−2468. (21) 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, 11342−11345. (22) Song, J.; Zhou, J.; Duan, H. Self-Assembled Plasmonic Vesicles of SERS-Encoded Amphiphilic Gold Nanoparticles for Cancer Cell Targeting and Traceable Intracellular Drug Delivery. J. Am. Chem. Soc. 2012, 134, 13458−13469. (23) Song, J.; Pu, L.; Zhou, J.; Duan, B.; Duan, H. Biodegradable Theranostic Plasmonic Vesicles of Amphiphilic Gold Nanorods. ACS Nano 2013, 7, 9947−9960. (24) Song, J.; Fang, Z.; Wang, C.; Zhou, J.; Duan, B.; Pu, L.; Duan, H. Photolabile Plasmonic Vesicles Assembled from Amphiphilic Gold Nanoparticles for Remote-Controlled Traceable Drug Delivery. Nanoscale 2013, 5, 5816−5824.

sufficient mechanical stability, high surface permeability, and ease of processability were further employed as a host for other functional materials such as SnO2 nanoparticles. The silica NPVs presented here show great potential as an ideal platform for the design of multifunctional nanoreactors and biomedical systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03424. List of block copolymers, TG-DTA curve and IR spectrum of NPVs after washing, statistical data of the size of NPVs, SEM images of NPVs synthesized under various conditions, contents of 25R4 in NPVs, digital photographs of the sample, and TEM stereopair of an NPV containing SnO2 NPs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses

(A.S.-N.) Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. (A.S.) Department of Applied Chemistry, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-0072, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Fusion Materials” (area no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and by a Grant-in-Aid for Scientific Research (B) (23350098) from the Japan Society for the Promotion of Science. We thank Adeka Corporation for freely offering the block copolymers. Part of this work was conducted at the Center for Nano Lithography & Analysis, The University of Tokyo, and the High Voltage Electron Microscope Laboratory, Nagoya University. S.Z. is grateful for financial support from the China Scholarship Council (CSC) and the Global Center of Excellence for Mechanical Systems Innovation (GMSI, The University of Tokyo).

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DOI: 10.1021/acs.langmuir.5b03424 Langmuir XXXX, XXX, XXX−XXX