Precise Localization of Inorganic Nanoparticles in Block Copolymer

Dec 19, 2014 - ... in Block Copolymer Micellar Aggregates: From Center to Interface ... from the center to the interface in polystyrene-block-poly(4-v...
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Precise Localization of Inorganic Nanoparticles in Block Copolymer Micellar Aggregates: From Center to Interface Ruijing Liang,† Jiangping Xu,†,‡ Weikun Li,† Yonggui Liao,† Ke Wang,† Jichun You,§ Jintao Zhu,*,† and Wei Jiang‡ †

Key Laboratory for Large-Format Battery Materials and System of the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Chuangchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, China § College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China ABSTRACT: Localization of inorganic nanoparticles (NPs) into polymer matrix plays a crucial role in determining the performance of the hybrid materials. Herein, we employed a simple, yet versatile hydrogen bonding directed supramolecular assembly (HBSA) strategy to control the localization of Au NPs from the center to the interface in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) cylindrical micellar aggregates by tailoring the selectivity of polymer brushes on the surface of Au NPs. PS-b-P4VP(PDP)x comb− coil supramolecules were constructed by PS-b-P4VP and npentadecylphenol (PDP) via the hydrogen bonding between the pyridine of P4VP and the hydroxyl of PDP. PS-tethered Au NPs (PS-Au NPs) were selectively localized in the PS domain while a binary mixture of PS and P4VP-tethered Au NPs (PS/P4VP-Au NPs) were adsorbed to the interface between PS and P4VP(PDP)x domains presumably due to the redistribution of PS and P4VP ligands on the Au NPs surface, triggering the formation of amphiphilic surfactant-like NPs. Isolated cylindrical micellar aggregates with controlled localization of Au NPs were obtained by the selective disassembly of the supramolecules. Moreover, supramolecular block domain size, composition of the supramolecules, size of the Au NPs, and volume fraction of the Au NPs were carefully investigated to reveal their effects on the morphology of hybrid micellar aggregates and localization of NPs. These hybrid micellar aggregates with controlled localization and loading efficiency of Au NPs may extend the applications of the hybrid micellar aggregates and find more potential uses in optical/electronic devices, catalysis, and drug delivery system.

1. INTRODUCTION Inorganic nanoparticles (NPs) have attracted much attention due to their intrinsic functionality, controlled size, and surface chemistry, enabling them acting as building blocks for constructing advanced materials with built-in functionalities.1−3 Recently, many studies have focused on the incorporation of NPs into block copolymer (BCP) micellar aggregates, since these hybrid aggregates can combine the advantages from both components, resulting in new performance capable of meeting the requirements in applications, such as surface-enhanced Raman scattering (SERS) probes,4,5 catalysts,6 biotechnology and nanomedicine,7 etc. Generally, the most noteworthy is onedimensional cylindrical hybrid micellar aggregates. When acting as drug carriers, cylindrical micellar aggregates offer additional advantages to control the biodistribution, targeting, and release profile of therapeutic agents due to their large core volume, elongated structure, and relative stability.8,9 Spatial localization of NPs (e.g., localized in the core, corona, or at interface) in the BCP micelles plays a significant role in determining the integral properties and applications of these hybrid materials.2 For example, the incorporation of quantum © XXXX American Chemical Society

dots (QDs) into the core of the BCP micelles can not only improve the stability of the NPs and reduce their toxicity to the body but also preserve the unique optical performance of QDs.10 Meanwhile, micellar scaffolds with specific domains can be used to partition different species within the micelles. Recently, Farinha et al. incorporated QDs and Au NPs in the core and corona of the micelles, respectively, with a precise distance of ∼20 nm between QDs and Au NPs. The precise spacing between QDs and Au NPs prevented any quenching of QDs photoluminescence (PL) by the metal NP and led to an apparent enhancement of QD PL emission (∼8 times) relative to QD emission from micelles without Au NPs.11 Generally, coassembly of preformed NPs and BCP (e.g., ex situ method) has been considered to be an efficient way of producing hybrid aggregates with controlled morphology and NPs distribution. In this case, precise localization of NPs into the central portion of BCP micelles has been achieved by several methods, e.g., Received: September 4, 2014 Revised: November 15, 2014

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coprecipitation,12,13 interfacial instabilities of emulsion droplets,14,15 templated cooperative assembly,16 heating−cooling process,17,18 and hydrogen bonding directed supramolecular assembly (HBSA),19,20 among others. To localize NPs in cylindrical micellar core, Eisenberg et al. used Au NPs modified with amphiphilic polystyrene-block-poly(acrylic acid) (PS-bPAA) to coassemble with the same copolymers to obtain hybrid cylindrical micelles.13 Interfacial instabilities of emulsion droplets containing amphiphilic PS-block-poly(ethylene oxide) (PS-b-PEO) have also been employed to encapsulate hydrophobic NPs into the core of cylindrical micelles.15 Yet, the localization and loading efficiency of NPs remained difficult to control. Recently, we employed a simple HBSA method to precisely localize PS-tethered Au NPs along the centerline of PS core of PS-block-poly(4-vinylpyridine) (PS-b-P4VP) cylindrical micellar aggregates.19,20 This approach allows one to control the localization, loading efficiency, and interparticle distance of the NPs within the hybrid micellar aggregates. In addition, electrostatic and coordinative interactions between NPs and the corona of BCP micelles were applied to place the NPs on the corona of the micelles.21,22 For example, poly(ferrocenylsilane)-block-P2VP (PFS-b-P2VP) cylindrical micelles with Au and PbS NPs incorporated in the corona can be generated through this method.21 It is also important to precisely localize NPs at the core− corona interface of cylindrical micellar aggregates to achieve tubular NP arrays, which have different physical and chemical properties from that of the micelles with centrally localized NPs. For example, the confinement of the NPs to the narrow interfacial region results in high local particle filling fractions and gives rise to electromagnetic coupling upon light irradiation, accompanied by a pronounced increase in absorbance.23 Therefore, such hybrid systems hold great promise for generating novel optical, acoustic, electronic, and magnetic materials.24 Recently, Park and co-workers employed Au NPs modified with mixed hydrophobic dodecanethiol (DT) and hydrophilic mercaptoundecanol (MUL) to control the surface chemistry of NPs and thus their localization in PS-bPAA micelles.25 Depending on the ratio between DT and MUL on the NPs surface, NPs either localized at the interface between PS and PAA blocks or aggregated at the center of the micelles. For nonspherical assemblies, the preformed rodlike micelles were decorated with 100% MUL-coated Au NPs through hydrogen-bonding interaction between carboxyl and hydroxyl groups. Yet, due to the limited compatibility of the inorganic NPs and BCP, it is still difficult to prepare hybrid cylindrical micelles with NPs localized at the interface by coassembly of amphiphilic BCP and NPs in selective solvents. For bulk and film composite systems, some progresses have been achieved to control the localization of NPs at the interfaces of different domains. Strategies such as varying the areal chain density (Σ) of ligands on the surface of NPs,26 size of NPs (d),27 volume fractions of NPs (φNP),28 molecular weight of polymer ligands,29 component of NPs surface ligands,30−32 thermal annealing,33 and others have been employed, which markedly influenced the interactions between NPs and polymers and the spatial distribution of NPs.34 In general, the most straightforward way of tailoring the surface properties of NPs is to modify the NPs surface with different ligands such as small molecules,35,36 mixture of homopolymers,30−32 random copolymers,31,37 or BCPs.38−41 Once achieving surfactant-like or “neutral” properties, NPs segregate to the interface between two domains of the BCPs and decrease

their interfacial tension. Meanwhile, the NPs distribution in nanocomposites has also been studied and predicted through theory and computer simulation.42−46 For example, Feng et al. employed self-consistent field theory (SCFT) to gain the insight into the position transitions of polymer-tethered NPs in BCP nanocomposites.42,43 By varying the chain length and density of the grafted polymer, NPs concentration, and size and selectivity of the NPs, it is possible to produce self-assembled nanostructures with various morphologies and modulate the spatial localization of the NPs. Usually, theoretical investigation provides an insight into the rational design of self-assembled composites with desired nanostructures and enhanced properties. Nowadays, it is still a challenge to generate hybrid cylindrical micellar aggregates with tunable NPs localization and controlled NPs loading efficiency for various applications in optical devices, catalysts, and drug delivery systems. Herein, we employed HBSA, a facile and versatile method, to generate hybrid cylindrical micellar aggregates with controlled NPs localization from the center to the interface. Au NPs coated with a single homopolymer ligand (PS-Au NPs) or binary mixed ligands (PS/P4VP-Au NPs or PS/PEO-Au NPs) were employed to precisely control the localization of NPs in the micellar aggregates. Polymer-tethered Au NPs were mixed with comb−coil supramolecule PS-b-P4VP(PDP)x (the subscript x represents the ratio of n-pentadecylphenol (PDP) to 4VP units), and hybrid films with hierarchical structures were obtained after the removal of the organic solvent. PS-Au NPs were selectively localized in the PS domain while PS/P4VP-Au NPs were adsorbed to the interface of the block due to a redistribution of PS and P4VP ligands on Au NPs surface. Isolated cylindrical micellar aggregates with controlled localization of Au NPs can be obtained by the selective disassembly of the supramolecules (Figure 1).47 One of the key advantages

Figure 1. Illustration showing the strategy for preparing the hybrid micellar aggregates with controlled localization of Au NPs. Au NPs with tailored surface selectivity were mixed with comb−coil supramolecule PS-b-P4VP(PDP)x and self-assembled into hierarchically structured films. Isolated hybrid cylindrical micellar aggregates were finally obtained by removal of the small molecule PDP and selective disassembly of the hybrid films.

of using supramolecular assemblies for preparing hybrid micellar aggregates is that the composition of the blocks can be easily adjusted by changing x while using the same copolymer, thus allowing effective control of the structure of the hybrid film and the morphology of resulting micellar aggregates (such as spheres, cylinders, and plates). Another key advantage is that hydrogen bonds within the supramolecules allow controlled cleavage and the small molecule additives (i.e., PDP) can be removed easily by dialysis against selective solvent, which endows this strategy with the potential B

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Table 1. Characteristics of PS-Au, PS/P4VP-Au, and PS/PEO-Au NPs Formed with Various Initial Input Mole Ratio of Au Atoms to Polymer Ligandsa PS grafting density ΣPS (chains/nm2)

P4VP grafting density ΣP4VP (chains/nm2)

f PS

3.23 2.83 3.17 2.57 PS grafting density ΣPS (chains/nm2)

0.50

5.32

gold core diameter (nm)

Au:PS:P4VP (input mole ratio)

4.4 8.0 11.4 8.0 gold core diameter (nm)

1:0.30:0.30 1:0.30:0.30 1:0.30:0.30 1:0.30:0 Au:PS:PEO (input mole ratio)

0.50 0.50 0.50 1.00

4.4

1:0.30:0.30

f PS

FPS

FP4VP

0.70 0.92 0.82 0 PEO grafting density ΣPEO (chains/nm2)

0.82 0.75 0.79 1.00

0.18 0.25 0.21 0

FPS

FPEO

3.83

0.58

0.42

a

f PS represents the initial mole fraction of PS2K-SH in the synthesis solution, with varied initial mole ratio of PS2K-SH and P4VP2.5K-SH or PEO2KSH. FPS, FP4VP, and FPEO indicate the actual mole fraction of ligands on Au NPs surface, e.g., FPS is equal to the ratio of ΣPS to Σ. procedure was repeated 4−5 times in order to separate PS/P4VP-Au NPs from unbound ligands and residual surfactant. Subsequently, the resulting NPs were redispersed in chloroform and purified by filtration with a poly(vinylidene fluoride) (PVDF) membrane (pore diameter: 0.22 μm). A similar preparation and purification procedure was employed to prepare PS-Au NPs and PS/PEO-Au NPs. For PS-Au NPs, Au aqueous solution was added to an equal volume of THF containing PS2K-SH (input mole ratio Au:PS = 1:0.3, i.e., f PS = 1.00), followed by ultrasonication for 1 h, incubation for 24 h, and precipitation− centrifugation procedure as described above. For PS/PEO-Au NPs, Au NPs aqueous solution was added to an equal volume of THF containing PS2K-SH and PEO2K-SH (input mole ratio Au:PS:PEO = 1:0.3:0.3, i.e., f PS = 0.50), followed by ultrasonication for 1 h, incubation for 24 h, and precipitation−centrifugation procedure. 2.4. Preparation of PS-b-P4VP/NPs Hybrid Micellar Aggregates. PS-b-P4VP(PDP)x supramolecules were prepared by dissolving PS-b-P4VP and PDP separately in chloroform to form 1 wt % stock solutions. Then, the solutions were mixed together with desired amount of PDP, followed by stirring for 24 h to complete the hydrogen bonding. Subsequently, PS-Au or PS/P4VP-Au NPs in chloroform was added to the PS-b-P4VP(PDP)x solution. Chloroform was then allowed to evaporate slowly to form the hybrid film, followed by solvent annealing in a saturated chloroform atmosphere at room temperature for 24 h and dried in vacuum for another 3 h. Subsequently, a small amount of hybrid film (∼0.05 mg) was dispersed in ethanol by gentle ultrasonication and placed in a dialysis tubing (molecular weight cutoff: 12−14 kDa) and dialyzed against ethanol for 1 week to remove PDP. Finally, the isolated hybrid micellar aggregates were obtained. 2.5. Preparation of PS-b-PEO/NPs Hybrid Micellar Aggregates. A similar procedure was employed to prepare the supramolecular hybrid film of PS-b-PEO(DBSA)x encapsulated with PS/ PEO-Au NPs. The hybrid films were then dialyzed against water to obtain the isolated hybrid micellar aggregates. 2.6. Characterization. The internal structure of hybrid micellar aggregates was characterized by using a FEI Tecnai G2 20 transmission electron microscope (TEM) operated at an accelerated voltage of 200 kV. After dialysis, to selectively disassemble the supramolecular structures and remove PDP, the hybrid micellar solution was dropped onto a copper grid precoated with a carbon thin film. The samples were negatively stained with 1 wt % phosphotungstic acid (PTA) aqueous solution before TEM observation. The dried hybrid film was embedded in the epoxy resin (Electron Microscopy Sciences, Inc.) and cured in an oven at 50 °C for 48 h. Thin cross sections with a thickness of ∼70 nm were cut with a diamond knife (Electron Microscopy Sciences, Inc.) using a Leica ultramicrotome (UCT-GAD/E-1/00, Germany). The cross sections were stained by exposing them to iodine vapor, which selectively stained the P4VP domain. Morphology of the hybrid micellar aggregates were also investigated by scanning electron microscope (SEM, Sirion 200). A drop of the dilute PDP-removed micellar aggregates solution was dropped onto the clean silicon wafer, without coated with conducting materials.

applications in drug delivery and controlled release. Supramolecular block domain size (L), composition of the supramolecules, size of the Au NPs (d), and volume fraction of the Au NPs (φNP) were carefully investigated to determine their effects on hybrid micellar morphology and NPs localization. These hybrid micellar aggregates with tunable NPs localization and controlled NPs loading efficiency can potentially be used in various applications such as optical/ electronic devices, sensors, catalysts, and drug delivery systems.

2. EXPERIMENTAL SECTION 2.1. Materials. PS20K-b-P4VP17K (Mw/Mn = 1.08), PS51K-bP4VP18K (Mw/Mn = 1.15), PS110K-b-P4VP107K (Mw/Mn = 1.15), PS38K-b-PEO11K (Mw/Mn = 1.06), PS2K-SH (Mw/Mn = 1.15), P4VP2.5K-SH (Mw/Mn = 1.2), and PEO2K-SH (Mw/Mn = 1.05) were purchased from Polymer Source, Inc., Canada. Sodium borohydride (NaBH4, purity ≥98%), trisodium citrate (purity ≥99%), 3-npentadecylphenol (PDP, purity ≥90%, recrystallized twice from nhexane before use), dodecylbenzenesulfonic acid (DBSA, purity ≥96%), and cetyltrimethylammonium bromide (CTAB, purity ≥98%) were obtained from Sigma-Aldrich. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, purity ≥99.99%) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. All of the glassware used were cleaned by aqua regia and rinsed with deionized water prior to the experiments. 2.2. Synthesis of Au NPs with Various Sizes. Monodisperse Au NPs with different core diameter (4.4 ± 0.8, 8.0 ± 0.9, and 11.4 ± 0.8 nm) were synthesized following the previously reported seeding growth approach.48 The core diameter of the Au NP was obtained by measuring ∼100 NPs from TEM images. All the Au NPs solutions were used as the starting materials for the ligand-exchange reaction within 1 h after preparation. Au NPs with a core diameter of 4.4 nm were stabilized by citrate, and those with core diameters of 8.0 and 11.4 nm were coated with CTAB. 2.3. Surface Modification of Au NPs.19 Before synthesis, CTABstabilized Au NPs were centrifuged (15 000 rpm, 15 min) to remove excess CTAB and redispersed in deionized water. Citrate-stabilized Au NPs and CTAB-stabilized Au NPs were used as the starting material in the ligand-exchange reaction. In a typical experiment for preparing PS/ P4VP-Au NPs, Au NPs aqueous solution (50 mL, ∼12.5 μmol) was added to the organic solution containing 10 mL of chloroform and 40 mL of THF with dissolved PS2K-SH (7.5 mg, 3.75 μmol) and P4VP2.5K-SH (9.5 mg, 3.75 μmol) (input mole ratio Au:PS:P4VP = 1:0.3:0.3, i.e., the initial mole fraction of PS2K-SH in the synthesis solution f PS = 0.50). Followed by ultrasonication for 1 h and incubation for 24 h, a dark reddish-violet stratified solution was obtained. The lower red organic layer contained PS/P4VP-Au NPs while the upper colorless layer was discarded. Following the addition of methanol to the organic layer to induce the NP precipitation, Au NPs were separated by centrifugation (15 000 rpm, 15 min), and the supernatant was removed. Subsequently, PS/P4VP-Au NPs were redispersed in 0.5 mL of chloroform. The precipitation−centrifugation C

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Ligands grafting density (Σ), fractions of PS chains (FPS, equal to the ratio of ΣPS to Σ), and P4VP or PEO chains (FP4VP and FPEO, equal to 1 − FPS) on the Au NPs surface were estimated based on the average core diameter of Au NPs from TEM images and the elemental analysis (Vario Micro cube, Elementar), as shown in Table 1. The weight fraction of carbon, hydrogen, oxygen, nitrogen, and sulfur from pyrolysis of PS-Au, PS/P4VP-Au, or PS/PEO-Au NPs was confirmed, and the weight fraction of residue was assumed to be that of the Au core. The weight fraction of the polymer chains were converted into volume fractions using the density of PS (∼1.05 g/cm3), P4VP (∼1.00 g/cm3), and Au NPs (∼19.3 g/cm3).19 The Au core diameter obtained from TEM image analysis was used to calculate the average surface area per Au NP. The number of polymer ligands per gold particle for various core−shell types of NPs, divided by the average surface area of the Au NP, gave Σ of polymer ligands on the NP surface. The volume fraction of Au NPs (φNP) in hybrid supramolecular assemblies refers to the ratio of the volume of PS/P4VP-Au NPs or PS/PEO-Au NPs to that of the corresponding hybrid supramolecular films.19 Hybrid micellar aggregates after dialysis in ethanol solution were placed in quartz sample cell with a 0.7 cm cell path length. UV−vis spectra were recorded by UV 1801 spectrophotometer (Beijing Rayleigh Analytical Instrument Corp. China) at 25 °C with reference spectrum of anhydrous ethanol.

Figure 3. TEM images of (a) cylindrical micellar aggregates formed from PS38K-b-PEO11K(DBSA)2.0 and (b) spherical micellar aggregates formed from PS38K-b-PEO11K(DBSA)3.0. 4.4 nm PS/PEO-Au NPs with fixed φNP of 12% were added to the supramolecules.

chain deformation induced by the insertion of NPs. In this case for neutral polymer-grafted NPs, the NPs surface ligands and volume fractions of NPs (φNP) (i.e., enthalpic effects), size of NPs (d), and supramolecular block domain size (L) (i.e., entropic effects) were all tailored to investigate their influence on the localization of the NPs and morphology of the resulting hybrid micellar aggregates. 3.1. Effect of the Ligand Properties. Surface modification of the NPs is not only necessary to stabilize them against aggregation in the BCP matrix but also to tune their entropy and enthalpy interactions within each BCP domain.31 On the basis of this strategy, we successfully controlled the NPs localization within PS-b-P4VP(PDP)x domains, and hybrid micellar aggregates with controlled NPs localization were obtained by the selective disassembly of the supramolecules. Two kinds of Au NPs with different ligands, PS-Au and PS/ P4VP-Au NPs, were synthesized via a ligand-exchange method.19 Then, the Au NPs were mixed with PS51K-bP4VP18K(PDP)2.0 supramolecules with φNP of 12%. After the complete removal of the organic solvent (e.g., chloroform) by evaporation, a hybrid nanocomposite film with cylindrical PS domains and continuous P4VP(PDP)x phases was obtained (Figure 1).19 Cross-sectional structures of the composite film are shown in Figure 2a,b, where the dark phase is P4VP owing to the selectively staining of the P4VP domain by iodine. PS-Au NPs were found predominately at the center of the PS phase (Figure 2a), whereas PS/P4VP-Au NPs were localized at the interface between the block domains (Figure 2b). After rupturing the hydrogen bonding and selective disassembly of the composite films with ethanol, isolated hybrid micellar aggregates with controlled NP localization were obtained (Figure 2c,d). PS-Au NPs were clearly observed at the center of the cylindrical micellar aggregates due to the preferential interaction of the Au NPs with the PS block and unfavorable interaction with the P4VP(PDP)x domain (Figure 2c). In contrast, PS/P4VP Au NPs were localized at the PS/P4VP interface of the hybrid micellar aggregates (Figure 2d). Notably, SEM investigation revealed similar cylindrical micellar structures, where micellar samples were prepared without coating with any conducting materials. In SEM image (Figure 2f), the bright dots on the surface of the micellar aggregates originate from the Au NPs, confirming that the Au NPs were localized at the PS/P4VP interface. Thus, Au NPs coated by a binary mixture of PS and P4VP ligands acted as neutral NPs, which were less selective for both PS and P4VP domains.

3. RESULTS AND DISCUSSION Generally, the localization of NPs in the BCP matrix and the morphology of resulting hybrid BCP assembly are primarily influenced by the enthalpy from the compatibility of the Au NPs with polymer and entropy resulting from the polymer

Figure 2. (a, b) TEM images of cross sections of PS51K-bP4VP18K(PDP)2.0 films incorporated with (a) PS-Au NPs and (b) PS/P4VP-Au NPs, Au NPs with a core size of 8.0 nm, φNP = 12%. (c, d) TEM and (e, f) SEM images of isolated hybrid cylindrical micellar aggregates with NPs localized at the center (c, e) and at the interface (d, f), obtained from the removal of PDP by ethanol. D

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Figure 4. TEM images of the hybrid cylindrical micellar aggregates encapsulated with 4.4 nm PS/P4VP-Au NPs formed from (a) PS20K-bP4VP17K(PDP)1.0, (b) PS51K-b-P4VP18K(PDP)2.0, and (c) PS110K-b-P4VP107K(PDP)1.0 (with fixed φNP = 12%).

changed dramatically with NPs being directed toward the interface between PS and P2VP domains. When the FPS was further decreased to 0.80, all of the NPs were localized at the interface. As FPS approached 0, NPs moved to the P2VP domain. Therefore, for the PS/P4VP-Au NPs in the current PSb-P4VP(PDP)x supramolecule system, there may also be a rearrangement of PS and P4VP ligands on the surface of Au NPs as proposed by Kramer and co-workers,31 which triggered strong adsorption of NPs to the interface of PS and P4VP(PDP)x domains in a similar fashion. To test the generality of our method, we employed another supramolecular pair, PS38K-b-PEO11K with dodecylbenzenesulfonic acid (DBSA), which can also form hydrogen bonding with the PEO block. Similarly, hybrid cylindrical micellar aggregates with NPs localized at the interface were obtained from PS38K-bPEO11K(DBSA)2.0 encapsulated with 4.4 nm PS/PEO-Au NPs (φNP = 12%) (Figure 3a). By increasing the volume ratio of DBSA and employing the supramolecule PS 3 8 K -bPEO11K(DBSA)3.0, spherical micellar aggregates were also obtained (Figure 3b). The changes of the composition of supramolecule did not cause a dramatic variation to the NPs distribution. As stated above, hybrid cylindrical micellar aggregates could be obtained from different BCPs, hydrogenbonding agents, and Au NPs, confirming the generality of our method to prepare hybrid micellar aggregates which will extend their use in the applications of nanomedicine and biotechnology. 3.2. Effect of the Supramolecular Block Domain Size L. For NPs preference to one domain in the bulk or micellar aggregate, both theoretical simulations46,50 and experimental results19,29,51,52 suggest that the localization of particles within a BCP domain can be controlled simply by varying the size d of the particles relative to the size L of the polymer domain in which they reside. To investigate the localization of NPs for various d/L ratios, we first fixed the PS/P4VP-Au NPs core size at 4.4 nm and φNP of 12% and varied the supramolecular block domain size L by using three different kinds of supramolecules: PS 20K-b-P4VP 17K(PDP) 1.0 , PS 51K-b-P4VP 18K(PDP)2.0 , and PS110K-b-P4VP107K(PDP)1.0. Interestingly, with an increase in the molecular weight of the supramolecule, the PS/P4VP-Au NPs were confined at the interface between the PS and P4VP(PDP)x domains (Figure 4a−c). Thus, the confinement effect made no significant difference on the localization of PS/ P4VP-Au NPs which remained at the interface between the block acting as the surfactant. Our result is inconsistent with the previous reports29 that for PS-tethered Au NPs in BCP films, PS-Au NPs resided at or near the center of the PS domain for low molecular weight diblock, while PS-Au NPs

Figure 5. TEM images of hybrid cylindrical micellar aggregates encapsulated with different kinds of PS/P4VP-Au NPs with a core size of (a) 4.4 nm, (b) 8.0 nm, and (c) 11.4 nm NPs formed from PS51K-bP4VP18K(PDP)2.0 with φNP of 12%. (d) Corresponding UV−vis spectra of the hybrid PS51K-b-P4VP18K micellar aggregates incorporation of PS/P4VP-Au NPs with different sizes. Inset in upper right is the enlarged peak of the spectra. Generally, the size of PS/P4VP-Au NPs d is determined not only by the core size of the metal NPs but also the polymer ligand molecular weight and grafting density Σ. In our case, enthalpic effect (mainly reflecting on Σ and FPS) played a dominant role in localization of PS/P4VP-Au NPs with a high Σ at the interface between BCP domains, whereas entropic effect (mainly represented by d) was less important.

The mole fractions of PS (FPS) and P4VP (FP4VP) grafted on the particle surfaces were measured by combining TEM and elemental analysis. To avoid the exposure of Au NPs surface and maintain a high grafting density (Σ),49 a low initial mole ratio of gold atoms to polymer ligands (input mole ratio Au:PS:P4VP = 1:0.3:0.3) was applied. As shown in Table 1, Au NPs have almost the same FPS and Σ. Notably, PS/P4VP-Au with quite a high FPS of ∼0.80 can be precisely localized at the interface of PS and P4VP domains, which is consistent with the previous results reported by Kramer’s group.31 In their case, PS1.5K/P2VP1.5K-Au NPs with a FPS of 0.92 were localized near the center of the PS domains in the lamellar PS-b-P2VP films. As the FPS was decreased to 0.90, the distribution of NPs E

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Figure 6. TEM (a−c, g) and SEM (d−f, h) images of the hybrid micellar aggregates formed from PS51K-b-P4VP18K(PDP)2.0 encapsulated with 8.0 nm PS/P4VP-Au NPs with various φNP: (a, d) 12%, (b, e) 24%, (c, f) 36%, and (g, h) 48%. (i) UV−vis spectra of the hybrid PS51K-b-P4VP18K micellar aggregates with 8.0 nm PS-Au NPs at the center (φNP = 12%) and hybrid PS51K-b-P4VP18K micellar aggregates with 8.0 nm PS/P4VP-Au NPs at the interface with various φNP. Inset in upper right is the enlarged peak of the spectra.

were less confined to the center with a broader distribution for high molecular weight diblock as proposed. This result was interpreted in terms of the relative entropic contributions of NPs and the diblock copolymers. In addition, our previous study indicated that large PS-tethered Au NPs can be found at the center of specific domains of the BCP aggregates, whereas small ones have a broader distribution to maximize their entropy because of the greater translational entropy relative to that of large NPs given a constant value of φNP. The inconsistency between these investigations and the results in this study implied that enthalpy has a more significant effect on the distribution of NPs than entropy for the neutral PS/P4VPAu NPs in the current case, resulting in localization of Au NPs to the interface when increasing the supramolecular block domain size L with the fixed size of NPs and φNP. 3.3. Effect of d and φNP of Au NPs. Besides the surface chemistry of NPs, the size d and volume fraction φNP of NPs also play an important role in controlling the localization of NPs in the matrix, based on the previous experimental27,51,52 and simulative43,45,46 results for NP/BCP composite system. In general, sufficiently small particles can disperse freely within the polymer matrix, since the stretching effect induced by smallsized particles is less significant than that by large-sized particles.46

In order to investigate NPs size effect on their distribution in the micellar aggregates, we first synthesized PS/P4VP-Au NPs with three different core sizes (4.4, 8.0, and 11.4 nm) by a ligand-exchange method (Table 1).48 These three kinds of Au NPs with a fixed φNP of 12% were encapsulated in cylindrical micellar aggregates formed from the selective disassembly of the supramolecule PS51K-b-P4VP18K(PDP)2.0. Interestingly, we found that Au NPs with various d were also localized at the interface of the hybrid micellar aggregates (Figure 5a−c), which was different from the previous reports focused on the NPs preference of one domain in the polymer matrix.19,27,51 As first predicted by Balazs et al.,46,50 increasing d results in the NPs migration from the interface to the center of one domain due to the entropy penalty induced by the stretching of polymer chains. Yet, in the present study, the increase in the NPs size from 4.4 to 11.4 nm did not result in the significant variation of the localization of the NPs in the micellar aggregates. Our results indicate that the NPs neutral surface produced the strong adsorption energythe enthalpy attraction between the ligands and the matrix overcame the entropic repulsion related to the size of NPs. In addition, when d was increased, FPS remained almost the same (Table 1). A previous report indicated that small variations in particle size had negligible effect on NPs localization, as compared to that caused by Σ.28 F

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domains. With the strong adsorption energy and the redistribution of PS/P4VP ligands on the surface of Au NPs, the neutral PS/P4VP-Au NPs acted as a surfactant which tailored the interfacial tension between the supramolecule blocks. With the increase of φNP, interfacial tension between PS and P4VP(PDP)x domains decreased, which resulted in a dramatic change in the morphology of supramolecule thin film. These results showed that the segregation of neutral NPs to the interfaces played an important role in controlling the structure of the hybrid composite system and the resulting micellar morphology. With the variation of micellar morphology from cylinder to sphere, the specific surface area of hybrid micellar aggregates increased, maintaining a nearly constant interparticle distance between the NPs. Thus, almost no shift was detected in the SPR peak in UV−vis spectra (Figure 6i). To further explore the effect of φNP on the morphology of hybrid micellar aggregates incorporated with Au NPs, we employed another supramolecule with a higher molecular weight, PS110K-b-P4VP107K(PDP)1.0. The increase in molecular weight of BCP may decrease the conformational entropy loss of the BCP resulting from BCP chain deformation induced by the insertion of NPs, leading to a larger critical filling φNP to trigger the morphology transition of hybrid micellar aggregates. As shown in Figure 7a,c, the cylindrical micellar aggregates changed to a mixture of cylindrical and spherical micellar aggregates when φNP increased to 36%. Further increase of φNP to 48% led to a large number of spherical micellar aggregates along with small fraction of the cylindrical micellar aggregates (Figure 7b,d). These hybrid micellar aggregates with high NPs loading may find potential applications in sensors, catalysts, data storage, and biomedicine.

Figure 7. TEM (a, b) and SEM (c, d) images of the hybrid micellar aggregates from high molecular weight of PS110K-b-P4VP107K(PDP)1.0 encapsulated with 8.0 nm PS/P4VP-Au NPs with various φNP: (a, c) 36% and (b, d) 48%.

In our case, though d was varied, PS/P4VP-Au NPs with similar FPS and Σ possessed almost the same chemical surface and bore the same strong adsorption. Thus, the small difference of d did not affect the localization of the NPs and the morphology of the hybrid micellar aggregates. On the other hand, Au NPs with different core size have characteristic surface plasmon resonance (SPR) peaks. As a consequence, UV−vis spectra of hybrid micellar aggregates incorporated with different size of Au NPs indicated that there was a 10 nm red-shift due to the increase of d (Figure 5d). Thus, our method is proved to be an efficient way of producing hybrid micellar aggregates with controlled NPs size and localization. Furthermore, φNP plays another important role in controlling the morphology and inner structure of the BCP aggregates. Usually, surfactant-like Au NPs can be employed to control interfacial interactions when simply varying φNP, which induce a dramatic change in shape and morphology for particles.53 In addition, varying φNP can control the interparticle distance of NPs and the resulting optical properties of the hybrid micellar aggregates.19 To explore the effect of φNP on the morphology of hybrid micellar aggregates and interparticle distance of Au NPs, PS51K-b-P4VP18K(PDP)2.0 was chosen to encapsulate 8.0 nm PS/P4VP-Au NPs with varied φNP from 12% to 48% (Figure 6a−h). When φNP was increased from 12% to 24%, the morphology of the cylindrical micellar aggregates remained nearly constant (Figure 6b,e). But the Au NPs were still confined at the interface of the hybrid micellar aggregates, resulting in a slight decrease in the interparticle distance and a 3 nm red-shift of the SPR peak (Figure 6i).54 Surprisingly, once φNP was increased beyond 24%, the cylindrical micellar aggregates changed to a mixture of cylindrical and spherical micellar aggregates (Figure 6c,f, φNP = 36%). A further increase of φNP to 48% resulted in all of the cylindrical micellar aggregates changing to spherical micellar aggregates (Figure 6g,h). This interesting result can be attributed to the change in interfacial interactions between the PS and P4VP(PDP)x

4. CONCLUSIONS We employed a HBSA approach to control the localization of Au NPs in cylindrical BCP micellar aggregates by tailoring the surface chemistry of Au NPs. Selective PS-Au NPs can be localized at the center of hybrid cylindrical micellar aggregates while neutral PS/P4VP-Au NPs can be localized to the interface. Our results suggest that there is a rearrangement of PS and P4VP ligands on the Au NPs surface, leading to a strong adsorption of NPs to the interface of PS and P4VP(PDP)x domains. Strong adsorption energy showed the competition between enthalpy and entropy. By increasing the supramolecule confined space L and the size d of Au NPs while keeping FPS and Σ the same, enthalpy had a more significant effect on the distribution of NPs than entropy, making the NPs locate firmly to the interface of the hybrid micellar aggregates. The generality of our approach makes it widely useful in different supramolecular systems with controlled size and size distribution of NPs. Besides that, the neutral surfactant-like PS/P4VP-Au NPs with various φNP can tailor the interfacial tension between the supramolecule blocks, triggering a morphology transition from cylindrical micellar aggregates to spherical micellar aggregates. These functional hybrid micellar aggregates with tunable localization of NPs may broaden the applications of hybrid materials in optical/microelectronic field and drug delivery system.55

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Fax (+86) 27 875-43632, Tel (+86) 27 877-93240 (J.Z.). G

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding for this work provided by the National Basic Research Program of China (973 Program, 2012CB821500) and the National Natural Science Foundation of China (51173056 and 91127046). We also thank the HUST Analytical and Testing Center for allowing us to use its facilities.

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