Size Selective Incorporation of Gold Nanoparticles in Diblock

The 1-dodecanethiol-coated AuNPs (AuxR) with D0/dw0 < 0.3 will stably ..... When Au3.5S are added, the main morphology of the aggregates is vesicle (c...
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Size Selective Incorporation of Gold Nanoparticles in Diblock Copolymer Vesicle Wall Jiangping Xu,†,‡ Yuanyuan Han,† Jie Cui,† and Wei Jiang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: A systematic study is conducted to reveal how far the polymeric vesicle wall can embed gold nanoparticles (AuNPs) with different sizes by combining experiments and self-consistent field simulations. Both the experimental and simulative results indicate that the location of AuNPs in vesicle wall or in spherical micelle is heavily size dependent. Whether the AuNPs enter the vesicle wall or not is determined by a ratio of the diameter of AuNPs (D0) to the thickness of the vesicle wall (dw0). The 1dodecanethiol-coated AuNPs (AuxR) with D0/dw0 < 0.3 will stably disperse in the vesicle walls. For polystyrene-coated AuNPs (AuxS), a criterion of D0/dw0 is proposed based on the phase diagram; i.e., the AuxS with D0/dw0 < 0.5 can be located in the vesicle wall. Otherwise, the AuxR and the AuxS prefer to locate in spherical micelles. Moreover, the contributions of enthalpy and entropy to the total free energy of the system are respectively calculated to reveal the mechanism of the size selective distribution of AuNPs. The results demonstrate that the escape of AuNPs from vesicle walls and their selective distribution in spherical micelles is an entropy-driven process. Our study provides an important guideline for fabricating nanoparticle/block copolymer hybrid vesicles in dilute solution.

1. INTRODUCTION Cooperative self-assembly of inorganic nanoparticles (NPs) and block copolymers (BCPs) has been perceived as a promising route to fabricate functional hybrid materials which takes advantage of the unique physical properties of NPs and the excellent processability of polymers.1−4 Since the properties of such composites depend not only on the intrinsic properties of each component but also on the spatial distribution of the NPs in the polymer matrix, precise control of NPs arrangement in the substrate remains an important and difficult issue in this field.5−15 In solution, cooperative self-assembly of NPs and amphiphilic BCPs has been explored as a powerful route for constructing hybrid colloids. Several groups have worked in this field and made significant progress.16−34 Taton and co-workers have developed a simple way to encapsulate various types of NPs in the core of spherical micelles of polystyrene-bpoly(acrylic acid) (PS-b-PAA).22−24 The NPs were regarded as simple solutes to swell the micelles or surface templates to guide the absorption of polymers. However, Park et al.25−27 showed that NPs could play an active role in the self-assembly process rather than being passively incorporated as a solute. They obtained unique cavity-like structures in core−shell type hybrid colloids. Their results indicated that both the enthalpic interaction and the polymer stretching energy were important factors in the formation of coassemblies. © 2013 American Chemical Society

Among the diverse morphologies of the aggregates, vesicles are regarded as effective carriers which have potential applications in the cosmetic, medical, and catalysis fields as well as the academic interest.35−37 While most of the potential applications rely on the presence of hydrophilic cavities, which offer an opportunity for the encapsulation of various hydrophilic substances, the hydrophobic vesicle wall can also be used to incorporate with functional organic molecules and inorganic NPs, such as quantum dots and noble metal NPs. In recent years, encapsulation of NPs in vesicle walls has attracted considerable interest.26,30,38−41 Both in situ and ex situ approaches have been applied to incorporate NPs in the vesicle walls.42−46 Mai and Eisenberg have reported a novel method to incorporate NPs into the central part of the vesicle walls.17 The key point was that the NPs were decorated with the polymers that were similar to those forming the vesicles. Recently, Mueller et al. successfully embedded quantum dots (5.7 nm) coated with oleicacid/oleylamine in polybutadiene-bpoly(ethylene oxide) (PB-b-PEO) vesicle wall (16 nm) using different preparation approaches.28 Förster et al. found that the magnetic Fe3O4 nanoparticles (14.1 nm) could not be located in the center of the polyisoprene-b-poly(ethyl oxide) vesicle Received: June 5, 2013 Revised: July 18, 2013 Published: July 22, 2013 10383

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glassware were cleaned by aqua regia and rinsed with deionized water prior to the experiments. 2.2. Synthesis of Ligand-Coated AuNPs. 1-Dodecanethiolcoated AuNPs (AuxR, R represents 1-dodecanethiol, x is the diameter of the AuNP core) and PS2K-SH-coated AuNPs (AuxS, S represents polystyrene) were used in this study. For Au2.0R, Au5.5R, and Au1.9S, the Brust two-phase method was used.47 Au3.5S, Au6.2S, and Au9.0S were synthesized by the seeding growth approach and a following ligand exchange process.48,49 The graft density of the ligands and the diameter of the AuNPs were measured by thermogravimetric analysis (TGA, TA Co., Q50) and transmission electron microscope (TEM, JEOL, JEM-1011), respectively. The details of the synthesis and characterization of the NPs are shown in the Supporting Information. The synthesized AuxR and AuxS were dispersed in chloroform. 2.3. Preparation and Characterization of Hybrid Micelles. Generally, the incorporation of NPs in the micelles was carried out by dropwise addition of water into the combined solution of NPs and BCPs. Taking the PS144-b-PAA22/AuNP in THF/water system as an example, the requisite amount of the AuNPs chloroform solution was added to a vial and dried in nitrogen flow. THF solution of PS144-bPAA22 (4 mg mL−1, 0.5 mL) was then added to the vial. After being stirred for 10 min, 0.125 mL of water was dropwise added to the mixture. The mixture was stirred at 500 rpm for 3 days (or for varying times for the kinetic studies) at room temperature. After quenching in water, the hybrid micellar solution was dialyzed against water for 4 days to remove the organic solvents. The aqueous solution of micelle was used for TEM characterization.

wall (11.7 nm) but rather at the periphery decorating the hydrophobic/hydrophilic interface, which leads to bridging of adjacent bilayers and the formation of oligolamellar vesicles.39 The Korgel group has focused on the incorporation of AuNPs in the bilayer of lipid vesicle. They found the diameter of the NPs, the length of the ligands, solvent annealing, and preparation method could seriously affect the accumulation of NPs in the lipid bilayer.40,41 When hydrophobic particles are inserted into polymersome membrane, the question arises about the NP size limit allowing their insertion. There is probably an upper limit in the diameter of the NP relatively to the wall thickness above which the insertion of NP is not possible, but this has not been established yet. A systematic study with different NP sizes and wall thickness is necessary to reveal how far the membrane can curve to embed NPs. Lin’s group32 and Li’s group31 have worked on the coassembly of NP/BCP by combining the self-consistent field theory (SCFT) method and density functional theory (DFT) method. The ratio of NP radius to BCP chain gyration radius (R0/Rg), the volume fraction of the NPs, and the surface selectivity of NPs were demonstrated to play important role in the cooperative self-assembly of NP/BCP. However, these studies do not figure out the criterion between the diameter of NPs (D0, including the ligand thickness) and the thickness of the vesicle wall (dw0), which determines the possibility of inserting NPs in the vesicle walls. Furthermore, although simulations and experiments have demonstrated that the localization of NPs in the domain of BCP thin film can be precisely controlled by tuning the enthalpic and entropic interaction between the NPs and the polymers,7,9,12,13 however, current knowledge of what controls the coassembly structure in solution is at the primitive stage, and the key factors that govern the assembly process have not yet been identified. Herein, we studied the cooperative self-assembly of ligandcoated AuNPs and diblock copolymer in selective solvents by combining experiments and SCFT simulations. The results indicate that the location of AuNPs in vesicle wall or spherical micelle is heavily size dependent. Small AuNPs can be inserted into the vesicle wall, whereas large AuNPs will be excluded out of the vesicle wall and selectively locate in spherical micelle. A threshold based on the ratio of the diameter of NP to the thickness of vesicle wall (D0/dw0) has been proposed. Moreover, we demonstrated that the selective distribution of AuNPs was an entropically driven process. Our results provide an important guideline for fabricating NP/BCP hybrid vesicles in dilute solution.

3. SIMULATION METHOD AND MODEL In the present work, we followed the theory framework developed by Fredrickson et al.50 and later used by Xu et al.15,51 The numerical simulations are carried out in 3D cubic lattice. The amphiphilic AB diblock copolymers, NPs grafted by short homopolymer A, and selective solvent S are dissolved in volume V. Each diblock copolymer consists of N segments, including a hydrophobic block A with NA segments and a hydrophilic block B with NB segments, i.e., N = NA + NB. The length fraction of block A in one diblock copolymer chain is fA = NA/N. For the grafted NPs, each particle consists of σ short homopolymers A with β segments, where β is the chain length ratio of the grafted homopolymer to the diblock copolymer. The size of the NPs is defined by the volume ratio of a nanoparticle to a diblock copolymer chain which is denoted by α = 4πRP3/(3Nρ0−1), where Rp is the radius of the NPs and ρ0−1 is the volume of one segments of tethered chains and of the diblock copolymers. The volume fractions of diblock copolymers and grafted nanoparticles are f D and f G , respectively. As a result, the volume fraction of solvent is f S = 1 − f D − f G. We set the chain length of AB diblock copolymer N = 17 with the length fraction of hydrophobic block A being fA = 0.8823, which is close to the experimental value of length fraction of PS in PS144-b-PAA22 (approximately 0.8675). Each NP is tethered by three homopolymer chains (i.e., σ = 3). And each homopolymer chain consists of two segments (i.e., β = 0.1176), the nature of which is the same as block A. To mimic dilute solution, the volume fraction of the AB diblock copolymer/NPs mixture was fixed at f D + f G = 0.1 with f D = 0.095 and f G = 0.005. A detailed description about the SCFT model for the mixture of amphiphilic linear AB diblock copolymer/NPs in dilute solution is given in the Supporting Information.

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, purity 99.99%) and 1-dodecanethiol (purity 98%) were purchased from Alfa Aesar. Sodium borohydride (NaBH4) and trisodium citrate were obtained from Sinopharm Chemical Reagent. Cetyltrimethylammonium bromide (CTAB, purity 98%) and tetraoctylammonium bromide (TOAB, purity 98%) were supplied by Aladdin. Amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid) (PS144-b-PAA22, Mn = 15 000 g mol−1 for PS block and Mn = 1600 g mol−1 for PAA block, Mw/Mn = 1.10), polystyrene-bpoly(ethylene oxide) (PS356-b-PEO148, Mn = 37 000 g mol−1 for PS block and Mn = 6500 g mol−1 for PEO block, Mw/Mn = 1.06), and thiol-terminated polystyrene (PS2K-SH, Mn = 2000 g mol−1, Mw/Mn = 1.15, thiol functionality >95%) were purchased from Polymer Source. Other chemicals were supplied by Beijing Chemical Factory. All of the materials were used after receiving without further purification. The

4. RESULTS AND DISCUSSION 4.1. Cooperative Self-Assembly of AuxR with PS144-bPAA22. The neat PS144-b-PAA22 (4 mg/mL) can self-assemble 10384

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Figure 1. TEM images of the morphologies of AuxR/PS144-b-PAA22 hybrid aggregates in THF/water. The AuNPs in (a, b) and (c−f) are Au2.0R and Au5.5R, respectively. The volume fractions of AuxR are 5.2% in (a, b), 1.5% in (c, d), and 4.7% in (e, f).

into vesicles (wall thickness dw0 = 22.4 nm) in THF/water (4:1, v/v) binary mixed solvent (Figure S1). Hydrophobically functionalized AuxR was added to the polymer solution before water addition. The water simultaneously desolvated both the PS block and the AuxR and induced the cooperative selfassembly. We found that the location of the AuxR heavily depended on their sizes, which was tested by two types of AuNPs with different diameters, Au2.0R and Au5.5R. For Au2.0R, most of them can uniformly disperse in the vesicle walls and enlarge the thickness of the vesicle wall from 22.4 to 35.5 nm (Figure 1a,b). Occasionally, a few vesicles are uneven in wall thickness. Au2.0R aggregates can be observed in the thicker bumps. This can be ascribed to the incompatibility between the 1-dodecanethiol and the PS matrix; thus, the Au2.0R may aggregate in the vesicle walls. Only a small part, ca. 5.3% (number fraction), of spherical micelles can be found. But for the Au5.5R, the results shown in Figure 1c−f demonstrate that the AuNPs selectively distribute in spherical micelles, while the vesicles are devoid of AuNPs at all and the thickness of vesicle wall is nearly the same as that of the vesicle of neat PS144-bPAA22 (Table 1). This result indicates that the incorporation of Au5.5R induces the emergence of more spherical micelles (ca. 39.9% number fraction). A question that we are concerned about is that if the vesicles can vanish and only spherical micelles exist when the concentration of AuNPs is sufficiently high. As the concentration of Au5.5R increases, the proportion of hybrid spherical micelles and the average number of AuNPs per micelle indeed increase (Figure 1c,e). However, when the volume fraction of Au5.5R increases to 9.0%, macroscopic precipitation occurs. It is a common phenomenon in NP/BCP self-assembly,27,29 in which the hybrid colloids are unstable when too many NPs are encapsulated in the micelles. This means that the amount of NPs that can be encapsulated is limited. The concentration-induced morphological change generally consistent with the results of simulation study accomplished by Lin and co-workers.32 They found the

Table 1. Characteristic of AuNPs and Hybrid Micelles of PS144-b-PAA22 AuNP

fspherea (%)

NAub

dwc (nm)

Dcored (nm)

D0e (nm)

D0/dw0f

position of NPsg

blankh Au2.0R Au5.5R Au1.9S Au3.5S Au6.2S Au9.0S

0 5.3 39.9 14.9 31.1 83.3 94.8

− −i 4.4 −i 5.5 1.7 1.0

22.4 35.5 21.2 33.7 40.1 24.6 25.5

− 2.0 5.5 1.9 3.5 6.2 9.0

− 3.4 6.9 6.7 10.4 12.8 16.2

− 0.15 0.31 0.30 0.46 0.57 0.72

− V S V V+S S S

a

The number fraction of spherical micelle in the sphere/vesicle blends. The average number of AuNPs in the cores of spherical micelles. c The thickness of the vesicle wall. dThe diameter of the AuNP core. e The diameter of the AuNP core and the ligands shell. fdw0 is the thickness of vesicle wall of neat PS144-b-PAA22, dw0 = 22.4 nm. gV is short for vesicle, S for sphere. hThe control experiment, neat PS144-bPAA22 self-assemble in THF/water (4:1, v/v). iThe average number of AuNPs in spherical micelles in these two cases are not given since the AuNPs are too small to be precisely counted. b

morphologies of NP/BCP aggregates could experience a transition from vesicles to a mixture of circle-like and rod micelles as the increase of NP radius and/or NP volume fraction. In our experiments, vesicle-to-sphere transition was identified; however, the rod micelles could not be observed (but could be found in another system; see Figure S4f), and the AuNPs escaped from the vesicle wall. This can be ascribed to the fact that the incompatibility between 1-dodecanethiol and PS leads to an enthalpic repulsion between Au5.5R and the matrix.25 This is an important difference between our experiments and the work of Lin et al. As a consequence, the rod micelles cannot be observed in this case. Moreover, besides the enthalpic repulsion, the incorporation of NPs in micelles can result in a entropic penalty.12 Both the entropic and enthalpic repulsion force the AuxR out of the vesicles and 10385

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Figure 2. TEM images of the morphologies of AuxS/PS144-b-PAA22 hybrid aggregates in THF/water. The AuxS are Au1.9S in (a, b), Au3.5S in (c, d), Au6.2S in (e, f), and Au9.0S in (g, h). The top row of low magnification shows the number of spherical micelles increases as the increasing size of the AuNPs. The bottom row shows (b) Au1.9S distributed in the vesicle wall, (d) Au3.5S located in both the vesicle wall and the spherical micelles (indicated by arrows in c and d), (f) most of Au6.2S are embedded in spheres and few of them stay in the vesicle wall, and (h) all of Au9.0S located in the spheres while the vesicle walls are free of AuNPs.

(Figure 2e,f). Few NPs can be found in vesicle walls, and the thickness reduced to 24.6 nm. When Au9.0S are added, 94.8% of the assemblies are spherical micelles (Figure 2g,h). AuNPs can hardly be found in vesicles, and the average thickness of the vesicle wall was 25.5 nm. Since the Au6.2S and the Au9.0S cannot be inserted in the vesicle walls, the thickness of the wall reduces to a value that is close to the thickness of pure vesicles without AuNPs. To further investigate the effect of the size on the selective distribution of NPs, the SCFT simulation was employed to study the cooperative self-assembly behavior of AB diblock copolymer/NPs mixture. As done in experiments, the volume fraction of NPs in simulation is kept constant when we change the size of the NPs (α). The simulative results obtained at four typical NPs sizes are shown in Figure 3. Only hydrophobic blocks A are drawn in light blue. The vesicles are plotted transparently so that the NPs (represented by red) can be seen conveniently. As shown in Figure 3a, the small NPs with size α < 0.03 can stably distribute in the vesicle wall. When the NPs size α > 0.03, spherical micelles emerge and the NPs are distributed in the core of the spheres (Figure 3b). This result indicates that the NPs cannot be completely incorporated in the vesicle wall when the size of NPs is large enough. Furthermore, from Figure 3c,d we can see that as the size of the NPs increases, more hybrid spherical micelles appear, meaning that the bigger the NPs, the easier they are excluded out of the vesicle wall. The simulative results shown in Figure 3 are in good accordance with the experimental results shown in Figure 2. To reveal the detailed microstructure of the hybrid vesicle and to calculate the vesicle wall thickness, density profiles of blocks A, B, and NPs are respectively plotted in Figure 4a. The inset is a hybrid vesicle formed at α = 0.02. The arrow shows

selectively locate in spherical micelles to reduce the total free energy of the system. This will be further discussed in detail in section 4.4. 4.2. Cooperative Self-Assembly of AuxS with PS144-bPAA22. As mentioned above, the incorporation of AuxR in PS144-b-PAA22 assemblies causes both the enthalpic and entropic repulsion. In order to minimize the influence of the enthalpic repulsive interaction between 1-dodecanethiol and PS block, the AuNPs were chemically modified with PS2K-SH. The AuxS are enthalpically compatible with the PS core of the aggregates. Thus, the enthalpic repulsion between the AuNPs and the PS matrix is minimized.9 In this case, the volume fraction of AuxS to the sum of AuxS and PS block was kept at 5.4%. As shown in Figure 2a,b, for Au1.9S, they uniformly locate in the wall of the vesicles and enlarge the wall thickness to 33.7 nm (Table 1). About 14.9% of the aggregates are spheres. Even when the stirring time extends to 5 days, the hybrid vesicular structure can be retained. In this case, no obvious aggregation of the AuNPs can be observed in the vesicle wall since the NPs densely grafted by PS chains are enthalpically miscible with the PS matrix. When Au3.5S are added, the main morphology of the aggregates is vesicle (ca. 31.1% of them are spherical micelles, Figure 2c,d). Meanwhile, the Au3.5S exist in both the vesicles and the spheres. Because of the incorporation of Au3.5S, the wall thickness of the vesicles increases to 40.1 nm. However, it is obvious that the density of AuNPs (number of AuNPs in unit volume of PS) in spherical micelles is much larger than that in vesicles. This result indicates that the Au3.5S object to locate in the vesicle walls. Compared with the case of AuxR/PS144-bPAA22, the reducing enthalpic repulsion causes a part of Au3.5S cannot be excluded out of the vesicle wall due to the insufficient repulsive force. When Au6.2S are employed, the aggregates are composed with 16.7% vesicles and 83.3% spherical micelles 10386

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we can also see that the dw still increases with the increase of NPs size even when α > 0.03, which is different from the results obtained in experiments. This is because the SCFT is a density distribution-based simulation method. The densities of each component on each lattice site cannot be zero. Therefore, NPs always have possibilities to locate in the vesicle wall and hence cannot be completely squeezed out of the vesicle and placed into the spherical micelles. As a result, though most of the NPs locate in the spherical micelles, the NPs left in the vesicle wall still enlarge the thickness of the wall. 4.3. Kinetic Study of the Escape of Au5.5R from the Vesicles. The cooperative self-assembly of Au5.5R (volume fraction: 1.5%) and PS144-b-PAA22 was selected for the kinetic study. The results are shown in Figure 5. At the first stage, the main morphology is spherical micelles encapsulated with 1−2 Au5.5R (average 1.4 per micelle, Figure 5a). At the last stage, vesicles free of AuNPs and spherical micelles encapsulated with 3−5 Au5.5R (average 4.4 per micelle, Figure 5c) are the main morphologies. The intermediate states between the early and final stage were trapped by quenching the micellar solution in large amount of water and ex situ examined by TEM (Figure 5b). As the stirring time extends, cavities appear in the core of the hybrid spherical micelles (Figure 5d,e). It is the early stage of the swell pathway of sphere-to-vesicle transition, which was described in details by He and co-workers.52,53 The polymer chains flip-flop when the volumes of spherical micelles exceed a critical value. Then the cores of the micelles become hydrophilic and the water enter the semivesicles to swell them to vesicles. The cavities gradually grow up, and the Au5.5R are eccentric in the semivesicles leading to bumps on the surfaces (Figure 5f). Then the bumps go on growing up and finally detach from the mother vesicles (Figure 5g). This process is similar to the budding reproduction of cell.54 The Au5.5R are repelled out of the vesicle through this process. The newborn hybrid spherical micelles fuse each other to bigger spheres with more Au5.5R in the cores (Figure 5h). Figure 5 displays the snapshots showing the escaping process of Au5.5R from vesicle wall. Among these snapshots, vesicles with bumps on the surfaces (Figure 5f) due to the incorporation of NPs have also been reported by Ma et al.31 In their SCFT simulation, this particular morphology was an equilibrium state of the system due to the equilibrium requirement of the SCFT simulation. However, in our experiment, we find that the vesicle

Figure 3. SCFT simulation of coassembly of homopolymer A grafted NPs and AB diblock copolymers. (a) NPs size α = 0.02; the NPs stay in the vesicle wall. (b−d) The number fraction of spherical micelles increases as α increases from 0.04 to 0.09.

the direction along which the density profiles are obtained. From Figure 4a we can see two higher peaks on the density profile of the hydrophobic block A. The peak positions represent the positions of the vesicle walls along X-direction. Meanwhile, it can be seen that the peak position of the hydrophobic block A and the peak position of NPs distribution are almost at the same value of X, meaning that the NPs are uniformly distributed in the vesicle wall, which agrees with the experimental result shown in Figure 2b. In addition, since the hydrophilic block B is located on the outer and inner surface of the vesicle wall, two lower peaks can be seen at both sides of the peaks of block A. The vesicle wall thickness dw can be represented by the distance between the two adjacent lower peaks on the density profile of block B, which is shown in Figure 4a. The vesicle wall thickness obtained at different NP sizes is shown in Figure 4b. For comparison, the wall thickness of the pure vesicle (dw0) self-assembled from neat dilock copolymer is also shown in Figure 4b (denoted by black square at α = 0). From Figure 4b we can see that the wall thicknesses of hybrid vesicles dw are higher than that of the pure diblock copolymer vesicle. When α < 0.03, it is clear that dw increases as the NPs sizes increases, which is in good agreement with the experiments results shown in Figure 2 and Table 1. However,

Figure 4. (a) Density profiles of blocks A and B. The inset is a hybrid vesicle formed at α = 0.02. The arrow shows the direction along which the density profiles are obtained. The vesicle wall thickness dw can be represented by the distance between the two adjacent lower peaks on the density profile of block B. (b) Variation of hybrid vesicle wall thickness with NPs size. Black square represents the wall thickness of the vesicle self-assembled from neat diblock copolymer. 10387

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Figure 5. Snapshots of morphological transition from sphere to vesicle/sphere blend. The TEM images were obtained at different times after water addition: (a) 3 h, spheres with average 1.4 Au5.5R in the cores; (b) 1 day, vesicle/sphere blend with some intermediate structures indicated by arrows; (c) 3 days, vesicle/sphere blend, the vesicle wall is free of Au5.5R and the number of Au5.5R in spheres increases to 4.4 per micelle; (d)−(h) show the dynamic process of spherical micelles swelling to vesicles and the escape of Au5.5R from the vesicles; (h) shows the fusion of two spherical micelles to increase the number of Au5.5R in the cores. The scale bars in (d)−(g) are 25 nm.

Recently, Kao and co-workers found in thin films of supramolecular nanocomposites the NPs were expelled to the surface when D0/L > 0.28.7 In our work, the vesicle wall is a curve surface and the membrane is not really a bilayer because entanglement and interdigitation can occur between hydrophobic blocks.37 Moreover, the ratio of the diameter of Au5.5R (including the ligands) to vesicle wall thickness is D0/dw0 = 0.31 (Table 1). As a consequence, the vesicle wall is not thick enough to load the large NPs when D0/dw0 > 0.3. During the swelling process in which the hybrid spherical micelles change to vesicles, the NPs in the wall cause bumps on the surface of the vesicles. The bumps will detach from the vesicles due to both the enthalpic and entropic repulsion of the vesicle wall, which reduces the total free energy of the system. Since no AuNPs entered the vesicle wall, its thickness was nearly the same as that of the pure vesicle (Table 1). For Au2.0R (D0/dw0 = 0.15), the NPs can be stably embedded in the wall of the vesicles, as is shown in Figure 1a,b. This is because the conformation entropy loss of PS blocks caused by Au2.0R is obviously lower than that caused by Au5.5R. The previous simulative and experimental studies reported that the NPs with D0/L < 0.2 (L is the respective domain dimension) can locate at the interface of incompatible domains.5,12 However, in the present study, Figure 1b shows that most of the Au2.0R uniformly dispersed in the vesicle wall. This is because both the enthalpic and entropic effects in our experiments are neither sufficient enough to exclude the Au2.0R out of the vesicle nor to organize the Au2.0R to an ordered array. Actually, for the smaller Au2.0R, the decrease in polymer chain conformational entropy loss is outweighed by the NPs’ translational entropy gain due to the uniform dispersion, while for the larger Au5.5R, the enthalpic repulsion, conformational entropy, and translational entropy loss yield a bias driving the NPs out of the vesicle wall.5 Thus, an enthalpic compatibility between the ligands and the matrix is urgent to keep the large NPs in the vesicle wall (see below). On the basis of the experiment and discussion above, we concluded that the size of AuxR played a critical role in the incorporation of NPs in the vesicle wall.

with spherical bumps is an intermediate state in the NPs escaping process. 4.4. Effect of Enthalpy and Entropy. Encapsulation of NPs in the polymer matrix results in both enthalpic and entropic interactions. The enthalpic effect depends on the compatibility between the ligands of NPs and the polymer matrix. The entropic effect mainly originates from the polymer chain conformation entropy and the NPs’ translational entropy.12 The enthalpic repulsion originated from the incompatibility between alkane and PS and the entropic repulsion coming from the conformation entropy loss were regarded as the driving force that push the NPs to the central part of the PS lamella in thin film5 or to the core−shell interface of a compound spherical micelles in solution phase.25 In the present study, as described in section 4.3, the PS144-b-PAA22 and Au5.5R coassembled into spherical micelles at the early stage. Although the 1-dodecanethiol is slightly unfavorable with the PS core, it is much more unfavorable with water. As a consequence, the Au5.5R prefer to stay in the hydrophobic PS core. Meanwhile, the incorporation of NPs in the PS core results in a large conformation entropy loss of the PS chains that distribute around the NPs. Therefore, an entropic repulsion originates and pushes the AuNPs to the free end of the PS chains, i.e., the center of the spherical micelle, as shown in Figure 5a. As the stirring time extend, these hybrid spherical micelles shown in Figure 5a will finally evolve to vesicles because vesicle is the equilibrium morphology at the preset condition (Figure S1). The stretching degree of PS blocks decreases as the morphology changes from sphere to vesicle,55 which means that the wall thickness of the vesicle is thinner than the core diameter of the spherical micelle (Figure 5g, vesicle wall thickness is 20.1 nm, while the diameter of the spherical bump is 42.3 nm). Thus, the vesicle wall may not be able to encapsulate the large NPs in the wall.39,40 Bockstaller et al.5 have demonstrated that in the thin film of polystyrene-bpoly(ethylene propylene) (PS-b-PEP), where the thickness of PEP phase is ∼80 nm, NPs with D0/L > 0.3 (L is the respective domain dimension) will be pushed to the center of PEP phase. 10388

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Figure 6. Encapsulation of AuxS in the vesicle wall of PS356-b-PEO148 in DMF/water mixture. TEM images show (a) the vesicles without AuxS, dw0 = 33.0 nm, (b) the vesicles with Au1.9S, D0/dw0 = 0.20, and (c) the vesicles and spheres with Au9.0S, D0/dw0 = 0.49.

For the case of coassembly of AuxS and PS144-b-PAA22, the enthalpic repulsion, which is unfavorable for the AuxR loading in vesicle wall, was reduced by replacing the 1-dodecanethiol with short PS chain. As summarized in Table 1, when D0/dw0 < 0.5, the AuxS can be stably stay in vesicle walls and enlarge the thickness of the walls. But for AuxS with D0/dw0 > 0.5, the NPs will be excluded out of the vesicle walls and the wall thickness reduces to a value close to that of the pure vesicle. Comparing with the case of AuxR/PS144-b-PAA22, we find that the chemically modification of AuNPs with PS shifts the threshold of D0/dw0 from 0.3 to 0.5, meaning that the vesicle wall are more capable of loading AuxS. Based on this, increasing the thickness of the vesicle wall indicates that vesicle is capable of encapsulating larger AuxS. We attempted to load AuxS in the vesicle wall of PS356-b-PEO148 (in DMF/water), whose thickness was 33.0 nm (details can be found in Figure S2). As shown in Figure 6, Au1.9S can uniformly disperse in the vesicle walls. However, the Au9.0S, which cannot enter the wall of PS144-b-PAA22 vesicle, can be embedded in the wall of PS356b-PEO148 vesicle. Notably, the ratio of Au9.0S diameter to thickness of PS356-b-PEO148 vesicle wall is D0/dw0 = 0.49, which is less than 0.5. In order to confirm the D0/dw0 = 0.5 is the threshold above which the NPs are selectively located in the spheres, we attempt to insert AuxS in PS144-b-PAA22 or PS356-bPEO148 vesicles in dioxane/water system since the thickness of vesicle wall is different in different solvents.55 For PS144-bPAA22/dioxane/water system, dw0 = 23.9 nm; both Au6.2S (D0/ dw0 = 0.54) and Au9.0S (D0/dw0 = 0.68) cannot be inserted into the vesicle wall (Figure S3). For the PS356-b-PEO148/dioxane/ water system, dw0 = 27.9 nm, Au6.2S (D0/dw0 = 0.46) locate in both vesicle and spherical micelle. But almost all of Au9.0S (D0/ dw0 = 0.58) selectively locate in spherical micelle (Figure S4). The results above are summarized in a phase diagram, as shown in Figure 7. Each point in the phase diagram corresponds to a experimental result of AuxS coassembling with PS144-b-PAA22 or PS356-b-PEO148 in different solvents. The dashed line (D0/ dw0 = 0.5) is drawn to identify the resulting phase boundary, which divides the phase diagram into two characteristic zones. The bottom right zone is the stable region for inserting AuxS in vesicle wall, while the top left zone for AuxS selectively locating in spherical micelle. These results confirm that D0/dw0 = 0.5 is indeed the threshold to determine whether the NPs can be inserted in the vesicle wall or not. Furthermore, we conducted other experiments in which the Au1.9S and Au9.0S (volume ratio 1:3, 1:1, and 3:1, total 5.4%) simultaneously self-assemble with PS144-b-PAA22. The results show that the ratio of Au1.9S to Au9.0S does not significantly affect the dynamic process and the final morphology. At first, spherical micelles with Au1.9S and

Figure 7. Phase diagram of the location of AuxS: (○) in vesicle wall; (◑) in both vesicle wall and spherical micelle; (●) in spherical micelle. Each point in the phase diagram corresponds to a experimental result. The dashed line (D0/dw0 = 0.5) is drawn to identify the resulting phase boundary.

Au9.0S in the cores are the predominant morphology (Figure 8a). After being stirred for 3 days, the spheres evolve to vesicles

Figure 8. Simultaneous self-assembly of two types of PS-terminated AuNPs, Au1.9S and Au9.0S (1:1, total 5.4%), with PS144-b-PAA22 in THF/water: (a) spherical micelles with both Au1.9S and Au9.0S in the cores obtained 3 h after water addition; (b) sphere/vesicle blends obtained 3 days after water addition; the Au1.9S stay in the vesicle wall, while the Au9.0S selectively locate in the spheres (indicated by arrows).

with Au1.9S in the walls but free of Au9.0S. The Au9.0S selectively locate in the core of spherical micelles (Figure 8b). These results described above indicate that the location of AuxS is size selective. As the size of the AuNPs increases, the entropic repulsion between the NPs and the polymer increases. The increasing entropic repulsive force excludes the NPs out of the vesicles through the budding reproduction pathway to reduce the entropy loss of the polymer chains. NPs selectively 10389

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critical role in the selective distribution of AuNPs. These results presented in this paper not only provide an a new insight into the size selective coassembly behavior but also offer an important guideline for researchers to incorporate functional NPs in vesicle walls in solution.

distribute in the central part of the spherical micelles to reduce the total free energy of the system. As stated above, we deduce that the entropic effect plays a critical role in the selective location of larger AuNPs in spherical micelles. To confirm our deduction and to further reveal the mechanism of the selective distribution of NPs in vesicle wall or in the core of spherical micelle, we calculated the contributions of enthalpy and entropy to the total free energy of the system as the increasing NP size according to eq S1 in the Supporting Information. The results are shown in Figure 9.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of the ligand-coated AuNPs, experiments details, theory, and simulation details, and Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-431-85262151; Fax +86-431-85262126; e-mail [email protected] (W.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China for Youth Science Funds (21104078) and General Program (51173056). The authors thank Prof. Jintao Zhu and Ruijing Liang for help in synthesis of polystyrene-coated gold nanoparticles.

Figure 9. Total free energy and its entropic and enthalpic components at different NP sizes.

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For the convenience of comparison, the total free energy has been shifted by −0.6. Figure 9 shows that the energy from entropy increases with increasing NP size, whereas the energy from enthalpy decreases. Meanwhile, it can be seen that the total free energy (i.e., the sum of the first two energies) of the system increases with increasing NP size. This means that the entropy plays a more important role in the distribution of NPs. From Figure 9, we see that the entropic contribution to the free energy increases with the increase of the NPs size. This means that the entropy of the system decreases when the NPs size is increased. Balazs et al.12 reported that large NPs will cause the hydrophobic block stretch to get around the dispersed NPs, which results in a loss of conformational entropy. Therefore, we conclude that the selective distribution of NPs is an entropically driven process.

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