A simple route to improve inorganic nanoparticles loading efficiency in

Mar 12, 2013 - Department of Chemistry and Biochemistry, University of Maryland, College ... including film rehydration,10 solution precipitation,11,1...
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A Simple Route To Improve Inorganic Nanoparticles Loading Efficiency in Block Copolymer Micelles Weikun Li,†,‡ Shanqin Liu,† Renhua Deng,† Jianying Wang,† Zhihong Nie,*,‡ and Jintao Zhu*,† †

Key Laboratory of Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: The formation of well-defined polymer/inorganic nanoparticles (NPs) hybrid micelles with high loading of the NPs is critical to the development of nanomaterials with desired optical, electric, magnetic, and mechanical properties. Herein, we introduce a simple strategy to encapsulate monodisperse polystyrene (PS)-grafted Au NPs into the PS core of PS-b-poly(4-vinylpyridine) (PS-b-PVP) micelles through block copolymer (BCP)-based supramolecular assembly. We demonstrate that selective incorporation of gold NPs into the PS cores during the assembly process can induce the formation of well-ordered hybrid micelles with spherical, cylindrical, or nanosheet morphologies. The number of NPs in each micelle can be effectively increased by simply increasing the content of NPs and adjusting the ratio of 3-npentadecylphenol (PDP) to the P4VP units accordingly. The balance between the NP loading (increasing the volume fraction of PS domain) and the PDP addition (increasing the volume fraction of PVP(PDP) domain) maintains the same micellar morphology while achieving high NP loading. Moreover, strong enthalphic attraction of H-bonding between PDP and P4VP can increase the effective interaction parameter of the system to maintain the strong segregation, leading to the formation of ordered structures. The mass density of NPs in the hybrid micelles was further enhanced after removal of the added PDP from the supramolecules. No macrophase separation or order−order morphological transition was observed even when the volume fraction of PS-grafted NPs (φNP‑M) in the hybrid micelles reached 84.1 vol % (or 68 wt % on the ligand free NPs basis). Furthermore, we show that ordered clusters of NPs were generated within the spherical micelles when the φNP‑M reached 72.5 vol %. This directed supramolecular assembly provides an easy means to tailor the interactions between BCPs and NPs, thus generating ordered structures which can only be achieved when the loading of NPs is high enough. This approach is versatile and applicable to different types of NPs and different micellar aggregates and supramolecular pairs. It offers a new route for preparing hybrids with applications in the fields of molecular electronic devices, high-density data storage, nanomedicine, and biosensors.

1. INTRODUCTION The incorporation of inorganic nanoparticles (NP) into block copolymer (BCP) matrices (i.e., lamellae or films)1−4 has been intensively studied to create novel functional hybrid materials with applications in high-performance catalysis, sensors, optics, and electronic devices.5,6 Particularly, discrete micelles,7 such as spherical or cylindrical micelles,8 vesicles,9 and many others, can effectively template the fabrication of hybrid assemblies with desirable properties. A number of strategies have been developed for encapsulating various NPs into BCP micelles, including film rehydration,10 solution precipitation,11,12 interfacial instabilities of emulsion droplets,13,14 heating−cooling processing,15,16 and directed supramolecular assembly,17 among others.18 For example, Taton and co-workers reported that amphiphilic BCPs polystyrene-b-poly(acrylic acid) (PS-b-PAA) can self-assemble around hydrophobic gold, magnetic, and semiconductor NPs to encapsulate the particles within the micelles in aqueous solution.12 The hydrophobic PS cores © 2013 American Chemical Society

physically sequester the NPs from the aqueous solution while the hydrophilic PAA shell can make the micelles disperse well in the solution and provide anchor points where biomolecules or target ligands can anchor. In general, it is critical to control both the distribution and loading of the NPs within the target domains in order to achieve collective properties of ensembles of NPs. One grand challenge is to prepare well-ordered hybrid NP/BCP systems that contain NPs at sufficient concentrations for practical device applications. The presence of well-ordered, high-content NPs within BCP micelles offers the hybrid nanomaterials with enhanced collective properties which do not present in individual NPs and bulk materials.19,20 For instance, the assembly of plasmonic NPs into secondary structures21 may Received: December 7, 2012 Revised: February 5, 2013 Published: March 12, 2013 2282

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create “hot spots” for surface-enhanced Raman scattering (SERS)16 due to the plasmon coupling between adjacent particles. In addition, due to the collective properties, polymer micelles packed with magnetic NP clusters are more responsive to an external magnetic field and show a significant increase in transverse relaxivity in magnetic resonance imaging (MRI) than individually dispersed magnetic NPS.22,23 Berret and co-workers further demonstrated that transverse relaxivity rate of composite aggregates can be improved by increasing the size of NP clusters by using longer polymers.24 Moreover, when employed as the memory storage device, high concentration of NPs in the BCPs usually results in a broad memory window for the devices.25 However, in most previous reports, the NPs density in polymer hybrid micelles was usually low. The reason is that the incorporation of NPs with high loading in host polymer domain leads to high chain conformation entropic penalties and drastically affects the self-assembly structure by changing the relative volume ratio between the two different blocks.26 Generally, NPs can disperse well in the BCP micelles at low NPs volume fraction (φNP‑M). However, the high φNP‑M triggers the swelling of the host domains, the order−order morphological transitions of BCP, or even macrophase separation. Several groups have demonstrated that the incorporation of NPs can induce morphological changes of BCP assemblies,17,20 indicating that NPs can play an active role in the assembly process rather than being passively incorporated in the polymer matrix. Several great efforts have been made to achieve high NP loading in BCP matrices.27,28 For example, Warren and coworkers prepared mesoporous metals in which NPs coated with an ionic liquid organic shell and complementary BCPs.29 While their system employed somewhat complex processing, it did yield ordered assemblies with high NPs density. More recently, Watkins and co-workers demonstrate that, for the polymer bulk or films, very high φNP (∼36 vol %) can be obtained through strong enthalphic attraction of H-bonds between one block of the copolymer and small molecules which can significantly suppress the macrophase separation of NPs and favor microphase separation.20 Yet, morphological transition occurred from lamellar to cylindrical morphology when the concentration of Au NPs was increased to 30 wt %. This additive-induced ordering transition was driven by both an increase in the effective interaction parameter and the changes in the overall volume fraction between the two microphasesegregated domains. Nowadays, it still remains a challenge to encapsulate NPs within micellar cores with high NPs density and well-defined structures. This report describes a simple strategy to encapsulate PSgrafted Au NPs into the PS core of PS-b-poly(4-vinylpyridine) (PS-b-P4VP) micelles through BCP-based supramolecular assembly. In this way, relatively high NP loadings (84.1 vol %) can be achieved by manipulating the addition of NPs and PDP. Micellar morphology can be regulated through varying the amount of the small molecule 3-n-pentadecylphenol (PDP) which can form hydrogen bonds with P4VP block and alter the volume fraction of P4VP(PDP) domains in the hierarchically structured film. Prior study has developed a simple route for the encapsulation of PS-grafted Au NPs within wormlike micelle cores through the directed supramolecular assembly.17 We have shown that once φNP beyond a critical content, loading of NPs drives the supramolecular system to from a new phase, such as nanosheets and perforated nanosheets. Herein, in order to achieve high NPs loading while maintaining the micellar

morphology, we describe a new approach through adding NPs in the PS domains along with the addition of PDP to P4VP domain. Hybrid micelles such as wormlike micelles or spherical micelles can be obtained after removal of the added PDP by breaking up the hydrogen bonding. The mass density of NPs in the hybrid micelles was thus further enhanced because of removal of the PDP from the supramolecules. In the case of spherical micelle, the number of the encapsulated NPs and the size of each micelle can be well controlled by manipulating the amount of added NPs and PDP. No macrophase separation and order−order morphological transitions were observed even when φNP in the hybrid micelles reached up to 84.1%. Furthermore, we demonstrate that ordered NPs clusters were achieved within the spherical micelles when φNP beyond 72.5%. This represents a new way to obtain NPs clusters with welldefined size and structure, which will offer new possibilities of tuning the coupling of the physical properties of hybrids.21

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium borohydride (NaBH4, purity ≥98%), Lascorbic acid (purity ≥99%), PDP (purity 90%, recrystallized twice from n-hexane before used), and cetyltrimethylammonium bromide (CTAB, purity ≥99%) were obtained from Sigma-Aldrich. PS20K-bP4VP17K (PDI = 1.08), PS17K-b-P4VP49K (PDI = 1.15), and PS2K-SH (PDI = 1.15) were purchased from Polymer Source, Inc., Canada. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, purity 99.99%) was purchased from Sinopharm Chemical Reagent. All of the materials were used after receiving without further purification. All the glassware were cleaned by aqua regia and rinsed with deionized water prior to the experiments. 2.2. Au NPs Synthesis. Monodisperse Au NPs with size of 7.5 nm were synthesized by seeding growth approach (see the Supporting Information Figure S1).30 Gold NPs solutions were used as starting materials for ligand-exchange reaction within 1 h after preparation. 2.3. Synthesis of PS-Coated Au NPs. PS2K-coated Au NPs were synthesized through two-step ligand-exchange approach.17 CTABstabilized NPs were used as the starting material. Typically, 4 mL of chloroform was added to ∼40 mL of aqueous solution (∼0.037 mg/ mL) of gold NPs, and the resulting solution was standing for 1 h to sufficiently extract excess CTAB in the aqueous solution. After organic phase was removed, the resulting aqueous solution (80 mL) was added to 80 mL of PS2K-SH (15 mg, 7.5 μmol) solution in THF. Followed by sonication for 1 h and incubation for 24 h, dark reddish-violet precipitate was thus obtained. Subsequently, Au NPs were separated by centrifugation (10 000 rpm, 10 min) and removal of the supernatant. Repeated precipitation−centrifugation procedure (3−4 times) was conducted until no unbound PS-SH and residual surfactant were left. Furthermore, 8 mg of PS2K-SH in chloroform was added to the resulting PS-coated Au NPs solution, followed by the same sonication, incubation, and purification process as carried out above. Au NPs with high grafting density were thus obtained. NPs core diameter obtained from transmission electron microscopy (TEM) image analysis was used to calculate the average surface area per NP. Weight fractions of polymer ligands relative to gold were measured by thermal gravimetric analysis (TGA). Grafting density of PS on the NP surface was estimated based on average particle size from TEM images and the TGA results, assuming ρgold = 19.3 g/cm3 and ρPS = 1.05 g/ cm3.17 2.4. Preparation of PS-b-P4VP/NPs Hybrid Micelles. PS-bP4VP and PDP were separately dissolved in chloroform to from 1 wt % solutions. Then, the solutions were mixed together at desired amount of PDP, followed by stirring overnight to complete the hydrogen-bonding formation. Subsequently, PS-coated Au NPs in chloroform was added to the PS-b-P4VP(PDP) solution. The resulting solution was then kept stirring overnight. Slow evaporation of chloroform gave rise to hybrid sample film, followed by solvent annealing under saturated chloroform atmosphere at room temperature over 48 h before drying in vacuum for 24 h. BCPs reassemble 2283

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and form more ordered assemblies, and NPs rearrange and adjust their location during the solvent vapor annealing process. Finally, ∼0.5 mg of the hybrid sample was dispersed in ethanol and dialyzed against ethanol to remove PDP by using dialysis tubing (DM27 EI9004; cutoff: 12 000−14 000).31 2.5. Characterization. Transmission Electron Microscopy (TEM). TEM investigation was performed on a FEI Tecnai G2 20 microscope operated at an acceleration voltage of 200 kV, equipped with a CCD camera (Gatan USC4000, Gatan). A drop of PS-coated Au NPs solution in chloroform was placed on TEM copper grids precoated with a carbon thin film. After dialyzing to disassemble the supramolecular structures and remove PDP, one drop of the hybrid micellar solution was placed onto TEM grid. The samples were negatively stained with 1 wt % phosphotungtic acid (PTA) aqueous solution before TEM observation.13,32 Thermogravimetric Analysis (TGA). Dry PS-coated NPs powders were placed in ceramic crucible and analyzed over the temperature range of room temperature to 800 °C at the rate of 10 °C/min under dry flow of N2 at a rate of 30 mL/min. UV−vis Spectroscopy. Hybrid micelles in ethanol solution after dialysis was placed in quartz sample cell with a 0.7 cm cell path length. Absorption spectra were recorded by using UV 1801 spectrophotometer (Beijing Rayleigh Analytical Instrument Corporation) at 25 °C with reference spectrum of anhydrous ethanol. Proton Nuclear Magnetic Resonance Spectra (1H NMR). 1H NMR were recorded on a Bruker AV400 (400 MHz) spectrometer. Tetramethylsilane (TMS) is used as reference. In the 1H NMR measurements, samples were first dried in a vacuum oven at 100 °C for 24 h. Then, PS-b-P4VP (10 mg/mL), PDP (10 mg/mL), and PS-bP4VP(PDP)x (x = 0.5, 1.0, 2.0, and 3.0) (10 mg/mL PS-b-P4VP with varied PDP) were dissolved in CDCl3, respectively.

formation of hydrogen bonding between phenol group of PDP and pyridine group of PS-b-P4VP can be confirmed by 1H NMR investigation (see Figure S2).33−35 Monodisperse Au NPs with size of 7.5 nm was synthesized through a seeding growth approach and functionalized with thiol-terminated PS by ligand exchange procedure (see Figure S1).17 The grafting density of PS on the surface of the gold NPs was estimated to be 2.5 chain/nm2, and the Au content in the PS-grafted Au NPs was 72 wt % (or volume fraction of 13 vol %, shown in Figure S3). As shown in Scheme 1a, PS-grafted Au NPs were selectively incorporated into PS domains during the assembly process due to the enthalpically preferential interaction of PS brush on surface of NPs with PS block. Because of the reversible nature of the hydrogen bonding between PDP and P4VP, the formed hierarchical microstructures can be disassembled into isolated spherical micelles with PS/NP core and P4VP shell when P4VP(PDP) phase forms the matrix. P4VP shell of the hybrid micelles can be identified from TEM images after iodine vapor staining, as shown in Figure S4. The core−shell structure protects NPs from the surrounding environment and the hybrid micelles can thus disperse well in alcohol or acidic aqueous system (Figure S5). The grafting density of polymer on NPs plays an important role in dispersing NPs in the micellar core and achieving high NP loading. The NPs with high grafting density can provide complete miscibility even at high φNP.36 If the areal chain density on the NP is less than a critical value, there is enough bare gold NP surface that interacts with P4VP block to direct the transformation of NPs to the interface of the micelles (Figure S6a), which is consistent with the result reported by Kramer and co-workers.37 And some of the NPs aggregate since the ligands provide not enough steric hindrance to overcome the NPs aggregate tendency due to the interparticle van der Waals force (Figure S6b). On the other hand, the high grafting density makes the PS stretch away from the interface to avoid overlapping, forming a polymer brush.38 And due to high curvature on their surface, it provides sufficient space for the matrix polymers close to the NP surface, and the matrix polymers can more readily penetrate into the PS brush of the NPs. In this way, the matrix provides favorable interaction with brush on the NPs to overcome the polymer conformational entropic loss. In our case, the grafting density of PS on the surface of NP is 2.5 chains/nm2, which is high enough for dispersing the NP in polymer matrix. After solvent annealing, highly ordered hierarchical structures can be obtained through the coassembly of NPs and the BCP-based supramolecules, which can be considered as a synergistic interaction between the NP assembly and BCP ordering process. Before demonstrating the strategy of improving metal NP loading efficiency, we first test the NP loading effect on the hybrid aggregate morphology based on the directed supramolecular assembly. In our case, the ratio of PDP to 4VP unit was fixed at 2.0 while varying the φNP from 14.2% to 84.1%. Clearly, with the increase of φNP, the mean number of NP in the spherical micelles was increased from 0.5 ± 0.6 (Figure 1a) to 1.2 ± 0.7 (Figure 1b). The TEM image and the inset histogram in Figure 1b show that ∼75% spherical micelles contained only one NP. However, the number of NP within the micelles cannot increase continuously due to the morphological transition from spherical micelles (φNP < 33%, Figure 1a,b) to cylindrical micelles (φNP = 49.7%, Figure 1c) and eventually to nanosheet (φNP = 56.9%, Figure 1d).20 We attribute the NPdriven sphere-to-cylinder and cylinder-to-nanosheet morpho-

3. RESULTS AND DISCUSSION 3.1. Morphological Transition Occurrence with Solely Increasing of NP Content. The strategy for preparation of the hybrid micelles with high loading of NPs is illustrated in Scheme 1. Briefly, we used PS20K-b-P4VP17K (volume fraction of PS φPS = 56%) and PDP to form PS-b-P4VP(PDP)x (x represents ratio of PDP to 4VP unit) comb−coil supramolecules, which can further assemble into sphere-withinlamellae morphology in the bulk when x is set at 2. The Scheme 1. Illustration Shows the Strategy for the Formation of Hybrids Micelles Encapsulated with Gold NPs through Directed Supramolecular Assembly of Comb−Coil PS-bP4VP(PDP)x: (a) Isolated Hybrid Micelles Were Obtained through Disassembly of the Supramolecules via Removal of PDP; (b) Number of the Encapsulated NPs Can Be Increased through Simply Adding NPs and PDP Simultaneously

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Figure 1. Bright-field TEM images of hybrid assemblies formed from PS20K-b-P4VP17K(PDP)2.0 encapsulated of gold NPs (size 7.5 nm) with content of (a) φNP‑M = 14.2%, φNP‑S = 4%, spherical micelles with ∼1 NP; (b) φNP‑M = 33.1%, φNP‑S = 11.3%, spherical micelles with ∼2 or more NPs; (c) φNP‑M = 49.7%, φNP‑S = 20.3%, cylindrical micelles and small amount of spherical micelles; (d) φNP‑M = 56.9%, φNP‑S = 25.4%, dominating nanosheets along with cylindrical micelles. We define φNP‑M and φNP‑S as the volume fraction of the NPs in the micelles and the volume fraction of NPs in the BCP-based supramolecules, respectively. NP size and the interparticle distance were obtained from the analysis of TEM images while volume fraction of loaded NPs was obtained based on the results of TGA analysis and TEM images.

NPs located within the center of the host domain as the content of NPs was beyond 15 vol %.42 The inherent 3D nature of the self-assembled BCP-based supramolecules suggests their possible use in fabricating 3D structured devices in a single step rather than the layer-by-layer deposition.43 Because of the external pressure and confinement caused by space constrained condition, the particles form ultrahigh-density ordered 1D nanostructured arrays and partial ringlike structures. Our approach provides a new avenue to control spatial distribution of NPs and form the highly ordered structures with applications in sensing, drug delivery, and medical diagnosis. Yet, the results also indicate that it is hard to achieve high NPs density while keeping the same assembly structure by solely increasing the NP contents due to the unavoidable morphological transition. 3.2. New Strategy for Achieving High Loading of NPs. To obtain the dispersed PS spherical domains (say for example) in the P4VP(PDP) matrix, one has to tune the PS volume fraction ( f) in the range of f < 0.16 according to the phase diagram.44 Dispersing NPs within BCP microdomain alters the BCP chain configuration, where the entropic penalty depends on the polymer chain stiffness, architecture, and NP size. By attaching PDP to P4VP, P4VP changes from a random coil to a P4VP(PDP)x comb that orders within the P4VP(PDP)x microdomain. Increasing the stoichiometry of PDP to P4VP stiffens the P4VP(PDP)x block. Free PDP can also be intercalated between the hydrogen-bonded PDP, further increasing the stiffness of the comb-block. This increases the entropic penalty arising from the polymer chain deformation

logical transitions to the relative volume change between the PS and P4VP(PDP)2.0 phase of the supramolecules.9,39 The incorporation of NPs increases the effective volume taken up by PS phase, while the relative volume ratio of P4VP(PDP) microdomain decreases,39 favoring the formation of lamellae. Similar phenomena have been observed for the coassembly of NPs and BCPs in solutions processing.9 More interestingly, the NP is found to locate in the center of the micelles for spherical and cylindrical micelles. Isolated hybrid micelles with uniformly dispersed Au NPs in the center of the micelles can be obtained when D/R0 > 1.0, where D (14 ± 0.7 nm) is the size of NP core and PS brush and R0 (∼9.5 nm) is root-mean-square end-toend distance of PS block, assuming that PS blocks are freely jointed chains.17 It has been demonstrated that localizing polymer-coated NPs near the center of the compatible polymer domain sacrifices the translational entropy of the particles but avoids an even larger chain stretching penalty incurred by distributing particles throughout the domain.17,40 Meanwhile, our result represents on the formation of highly ordered, close-packed two-dimensional lattices in sandwiched BCPs by directed supramolecule assembly. The hexagonally ordered NP monolayer array in the nanosheets with average interparticle distance of 4.0 nm in Figure 1d is similar to the DNA mediated NPs superlattice sheets in which interparticle distance can be rationally controlled by adjusting DNA length.41 Similar structure has also been predicted by Balazs and co-workers, in which, for larger NP loaded, the system organizes into well ordered “core−shell” lamellar structure with 2285

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Figure 2. (a−f) Bright-field TEM images of spherical hybrid micelles formed from PS20K-b-P4VP17K (PDP)x encapsulated of gold NPs (size 7.5 nm) in each micelle (N is the number of NPs in each micelle): (a) N = 1.8 ± 1.3, φNP‑M = 24.8%, x = 2.0; (b) N = 3.4 ± 1.3, φNP‑M = 39.8%, x = 2.0; (c) N = 6.9 ± 2.9, φNP‑M = 56.9%, x = 2.5; (d) N = 27.8 ± 11.3, φNP‑M = 72.5%, x = 2.5; (e) N = 48.9 ± 27.3, φNP‑M = 79.8%, x = 3.0; (f) N = 76.7 ± 45.2, φNP‑M = 84.1%, x = 3.5. (g) Plot shows the average number of NPs per micelle as a function of φNP‑M. N1 (black curve) was obtained from the measurement based on TEM images, and each point in (g) was obtained by averaging over 100 micelles. N2 (red curve) was obtained from the estimation, as described in the text; (h) UV−vis spectra for the spherical micelles encapsulated with varied density of NPs in ethanol.

upon the incorporation of the NPs. Moreover, strong enthalphic attraction of hydrogen-bonding interaction between PDP and P4VP can increase the effective interaction parameter of the system to maintain the system within strong segregation region. Thus, macrophase separation will be highly suppressed, and ordered structures can be formed through favorable microphase separation.45 The loading of Au NPs in the micelles can be readily adjusted by increasing the amount of the NPs, while the hybrid micellar morphology can be adjusted by added small molecule PDP in the supramolecules to balance the volume fraction ratio of PS(NP)/P4VP(PDP). More importantly, small molecule can be easily washed away by rupture the H-bonding using appropriate solvents. Thus, by simply adding the small molecules, the interactions between the BCP and NPs, enthalphic attraction, the polymer chain configuration, stiffness, and packing can be readily tailored to manipulate the enthalpic and entropic contributions to the NPs assembly process. The balance between the NP loading and the PDP addition keeps the ratio of PS(NP) and P4VP(PDP) in the same morphology region of the phase diagram. Consequently,

we can obtain the expected hybrid micellar morphology with controllable loading density of NPs. As shown in Figure 2, 7 nm Au NPs and PS20K-bP4VP17K(PDP)x supramolecule system were employed. With x = 2, free PDP can be intercalated between H-bonded PDP, giving φP4VP(PDP) = 85%, and PS spherical domain embedded in P4VP(PDP)2.0 matrix was formed.17 TEM image in Figure 2a shows that ∼75% of the spherical micelles contain only one NP along with some empty spherical micelles when the volume fraction of NP in the supramolecules (φNP‑S) is 7.8%. After removal of the PDP, the φNP‑M was consequently increased to 24.8%. A key advantage of our method is that the φNP‑M in each of micelles can be easily adjusted; the increase in the volume of PS domain can be balanced by adding PDP, thus enabling us to achieve a high φNP while maintaining the micellar morphology. With the increase of φNP‑M to 56.9% while the ratio of PDP to 4VP unit was 2.5 in order to keep the spherical micellar shape, the TEM image in Figure 2c indicated that most of the micelles have been incorporated with 6.9 ± 2.9 NPs. The simultaneous increase in NPs and PDP content led to the increase of NP 2286

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(modified with small random segments,46 solvent vapor annealing,17,47 and sapphire wafers to guide the self-assembly48) ) or precise separation of the clusters through density gradient centrifugation.16 3.3. NP Clusters with Lennard-Jones Geometries.49 Furthermore, we found that NPs tends to form NPs clusters within the spherical micelles when the φNP‑M beyond 72.5%, as shown in Figure 3. TEM images demonstrate that spherical

number inside the micelles from 6.9 (Figure 2c) to 27.8 (Figure 2d), 48.9 (Figure 2e), and to 76.7 (Figure 2f). Once φNP‑S reached 46%, the ratio of PDP to 4VP unit was 3.5 to keep the spherical shape, nearly ∼80 NPs were incorporated into the core of spherical micelles (Figure 2f). In this case, the φNP‑M reached up to 84.1% in the hybrid micelles. We note that the φNP‑M refers to the volume fraction of NPs together with the grafted PS in the hybrid micelles, and the volume fraction of bare gold NPs is lowered to 11 vol % (or 68 wt %) due to the high grafting density of PS on the NPs surface. Further increase φNP‑M of NPs beyond 84.1%, macrophase separation occurs, giving rise to irregular structures with aggregation of NPs as displayed in Figure S7. The NPs confined in the center of PS domains swell the PS domains correspondingly. The overall dimension of the spherical micelles increased from 27 ± 5 to 82 ± 22 nm when increasing the NPs content from 0 to 84.1 vol % (Figure 2a−f and Figure S8). Notably, the size distribution of the hybrid micelles became much broader upon increasing the loadings of NPs. We attribute the broader size distribution of hybrid micelles at higher loading of NPs and PDP to the following reasons: (i) The incorporation of high-content NPs disrupts the order of BCP structures due to the loss of the polymer conformational entropy. (ii) The formation of larger NP domains cause further excessive stretching of polymer chains. (iii) The high PDP content required to achieve high NP loading leads to nonhomogeneous distribution of PDP in the P4VP(PDP)x domain. The relationship between NP numbers in the micelles as a function of φNP‑M was measured from the TEM images and plotted in Figure 2h. The number of NPs as a function of loaded NPs in the supramolecules can be estimated through a simple equation:

(1)

Figure 3. Bright-field TEM images of hybrid spherical micelles formed from PS20K-b-P4VP17K (PDP)x encapsulated of gold NPs (size: 7.5 nm): (a) φNP‑M = 72.5%, x = 2.5; (b) φNP‑M = 75.6%, x = 3.0; (c) φNP‑M = 79.8%, x = 3.0; (d) φNP‑M = 84.1%, x = 3.5. Clearly, NPs with ordered structures are formed inside the spherical micelles.

where DNPs is the NP diameter including the gold NP core and ligand shell and Dmicelle is the mean diameter of the hybrid micelle. The mean value of the DNPs and Dmicelle, obtained from TEM measurement, was used for the estimation. Figure 2g shows the number of NPs in each micelle calculated by using eq 1. Clearly, the experimental results are in a good agreement with theoretical calculations. The small difference between the experimental results and the calculation ones arises from the measurement from TEM images. The number of the NPs counted from TEM images is originated from the projection of the NPs on the supporting carbon film of TEM grid; thus, the overlap of the NPs cannot be counted. It is hard to precisely count the NPs inside the micelles especially when the number of NPs increases and forms the clusters. Therefore, the number of the NPs obtained from measurement of TEM images is usually lower than that obtained from the calculation. The micelles brought large amount of NPs to close with each other, leading to the red-shift of surface plasmon resonance (SPR) due to the plasmon coupling. Figure 2h shows that the SPR spectra of the hybrid micelles is red-shifted slightly (∼11 nm) when the loading of NPs is increased, while the intensity of the spectra for the hybrid micelles is increased obviously at the same wavelength number (Figure S5). We note that the size of the micelles and the number of NPs in each micelle are not quite uniform after the preparation. This can be improved through the optimization of the sample preparation strategy

micelles are packed with NPs and that the NPs density increases with the increase of NPs mass percent. From Figure 3a, we can see clearly that ring patterns are formed inside the spherical micelles when the number of NPs in the micelles reaches ∼40 (see Figure S9). With the number of NPs increased to ∼100, ∼200, and ∼250, ordered hexagonally50 (Figure 3b), “zipper”-like lines on the surface (Figure 3c) and multiring patterns (Figure 3d) formed within the spherical micelles. The 2D projection simulations described by Kraus and co-workers are capable of replicating a large number of the NPs 2D projection in the experiment49 (see Figure S9). Although our result is not sufficient to establish a strict mapping at this stage, this strategy provides a general means to design and fabricate NPs cluster with tunable number of NPs. The collective properties of 3D NPs assemblies can be easily tuned by varying the interparticle distance or the length of grafting homopolymer on the surface of NPs. 3.4. PDP Amount Effect on the Aggregate Morphologies. We have shown that the morphological transition occurred from spherical micelles to wormlike micelles and then to nanosheets by manipulating the volume fraction of the PS microdomain in the supramolecules by adding NPs (see Figure 2). A key advantage of using supramolecular assembly for preparing hybrid micelles is that composition of the blocks can be easily adjusted by changing the amount of side chains while using the same copolymer, allowing reversible change in

N=

3 4 ⎛ Dmicelle ⎞ ⎟ ϕ π⎜ 3 ⎝ 2 ⎠ NP

3 4 ⎛ D NPs ⎞ ⎟ π⎜ 3 ⎝ 2 ⎠

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changed from nanosheets (Figure 4a, x = 1.0) to wormlike micelles (Figure 4b, x = 1.5; Figure 4c, x = 2.0) and to spherical micelles (Figure 4d, x = 2.5) when the ratio of PDP to 4VP unit was increased from 1.0 to 2.5 while keeping the Au NPs loading constant (φNP‑M = 62.3%). In all these cases, the loading density of the NPs in the isolated hybrid micelles was the same (62.3%) although the NPs in the supramolecules was varied due to the varied PDP contents to induce the morphological transition. In the case of cylindrical micelles, NPs was dispersed uniformly in the center of the cylinders and formed multiple lines, as shown in Figure 4b,c. In the case of x = 2.5, free PDP can be intercalated between the hydrogen-bonded PDP, which can not only change the volume fraction of the block P4VP(PDP)x but also stretch the P4VP chains, triggering the morphology transition. At high PDP content, PDP does not distribute homogeneously in the P4VP(PDP) domain and even disperse in PS blocks. Thus, NPs distributed broader within the PS spherical and wormlike microdomains when PDP content was high in the supramolecules, as shown in Figure 4b−d. 3.5. Superstructures Formation. The stability of hybrid micelles in various environments is very important in the potentials of biomedical application such as drug delivery and imaging.51 The hybrid micelles’ stability mainly depends on the grafting density of polymer hairs and length of the corona segment in the hybrid micelles, which both can be tailored by our method. At x < 3.0, the hybrid micelles remain stable in ethanol after preparation for several months. At x > 4.0, the spherical micelles with high NP loading (φNP‑M > 56%) aggregated in an uncontrollable and irreversible manner and formed giant spherical solid or hollow aggregates through secondary self-assembly after the complete removal of PDP by dialysis. Figure 5a−c shows the individual spherical micelles encapsulated with multiple NPs without dialysis. Only slight aggregation between the micelles occurred with the increase of PDP or NP content (Figure 5a−c). After the removal of PDP by dialysis, solid or hollow aggregates with size of ∼100−600

morphology and its characteristic size. In this study, we also investigated the effect of PDP content on the hybrid micellar morphology while keeping the NP loading constant compared to BCPs. Figure 4 shows TEM images that assembly structures

Figure 4. Bright-field TEM images of hybrid assemblies formed from PS20K-b-P4VP17K (PDP)x encapsulated of gold NPs (size 7.5 nm) with content of (a) φNP‑M = 62.3%, φNP‑S = 40.4%, x = 1.0, nanosheets and perforated nanosheets; (b) φNP‑M = 62.3%, φNP‑S = 34.4%, x = 1.5, cylindrical micelles with ∼2 NPs lines; (c) φNP‑M = 62.3%, φNP‑S = 29.9%, x = 2.0, cylindrical micelles with two or three NPs lines and tendency to aggregate after dialysis; (d) φNP‑M = 62.3%, φNP‑S = 26.4%, x = 2.5, spherical micelles with ∼21 NPs.

Figure 5. Bright-field TEM images of hybrid spherical micelles (a−c) and the corresponding aggregates after dialysis (d−f), which were formed from PS20K-b-P4VP17K(PDP)x encapsulated of gold NPs (size 7.5 nm) with content of (a, d) φNP‑M = 56.9%, x = 4.0, spherical micelles containing ∼4 NPs; (b, e) φNP‑M = 72.5%, x = 5.0, spherical micelles containing ∼8 NPs; (c, f) φNP‑M = 72.5%, x = 6.0, spherical micelles containing ∼6 NPs. 2288

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Figure 6. (a−g) Bright-field TEM images of spherical hybrid micelles formed from PS17K-b-P4VP49K (PDP)x encapsulated of gold NPs (size 7.5 nm) in each micelle with different φNP‑M and PDP contain (a) N ∼ 0.5, φNP‑M = 7.6%, x = 1.0; (b) N ∼ 2.5, φNP‑M = 14.2%, x = 1.0; (c) N ∼ 4, φNP‑M = 24.8%, x = 1.0; (d) N ∼ 15, φNP‑M = 33.1%, x = 1.5; (e) N ∼ 20, φNP‑M = 49.7%, x = 1.5; (f) N ∼ 40, φNP‑M = 56.9%, x = 2.0; (g) N ∼ 80, φNP‑M = 62.3%, x = 2.0; (h) irregular aggregates, φNP‑M = 72.5%, x = 2.0.

nm were observed, as shown in Figure 5d−f. Clearly, the aggregates were formed from the secondary assembly of the spherical hybrid micelles (Figure 5e,f, indicated by arrows). The reasons for the accumulation of the spherical micelles can be attributed to three main factors. First, stable micellar polymer chains form a dense spherical brush to provide steric stabilization. Thus, surface density of corona chains of single micelle becomes lower during the dialysis since mass space between the P4VP block is exposed which is preoccupied by PDP. The micelles thus aggregate into superstructures where the corona chains reorganize to form a dense brush to prevent further aggregation.52 Second, with the increase of the NP loading, the φNP‑M in single micelle and micellar diameter increase while the φP4VP will decrease. The P4VP brushes are not long enough to stabilize the large AuNPs/PS cores. Third, in order to control the morphology of hybrid micelles, more PDP is needed in the supramolecules. Consequently, free PDP can act as stabilizer together with BCPs to disperse Au NPs and the corresponding micelles. In this case, a small portion of micellar core might be only covered with PDP. Removal of the stabilizer PDP by dialysis reduces the stability of the micellar cores, thus leading to the formation of aggregates. Nevertheless, our approach provides a clue to fabricate giant hybrid hollow or solid spheres through the secondary self-assembly of the hybrid micelles. This process resembles the construction of a super “house” using micelle “bricks”. In order to test our assumption and improve the stabilization of the hybrid micelles, we used another BCP which has a longer polymer brush P4VP49K but similar core-forming block PS17K compared to PS20K-b-P4VP17K. Similarly, the number of NPs encapsulated into the core of spherical micelles can be increased gradually from 1 to ∼80 with the increase of NP and PDP content, as shown in the Figure 6. Irregular aggregates can also be obtained when the NP content beyond 72.5 vol %, as displayed in Figure 6h. In contrast, the formed hybrid

micelles are more stable and can be kept for 6 weeks after removal of PDP via dialysis. Therefore, our approach is general and can be used to different supramolecular pair and NPs with different size, shape, and functionalities. The stability of the hybrid micelles can be highly improved by using higher molecular weight of P4VP brushes since longer polymers can solubilize larger amount of NPs without morphological change and aggregation. 3.6. Phase Diagram of the Hybrid Micelles. A comprehensive understanding of the coassembly of the BCPbased supramolecules/NPs blends is important from the fundamental point of view. So far, only a few reports have been made relative to the investigation of the phase behaviors of BCPs at high φNP.20,42 The effect of the amount of PDP and φNP‑M on the hybrid micellar morphology is summarized in a phase diagram in Figure 7. Generally, spherical and cylindrical micelles and nanosheets can be obtained when the PDP content and NP loading density were controlled in appropriate regions in the phase diagram. It is worth noting that at x > 4.0 irregular aggregates are always observed in φNP‑M < 40% region. We speculate that excessive PDP comes out from the bulk, crystallizes on the surface of the bulk during annealing, and hampers the order of the microdomains.53 When 40% < φNP‑M < 80%, the PDP acts as surfactant around the hybrid micelles and the NPs to stabilize the spherical hybrid morphology. When φNP‑M >80%, macrophase separation will occur because NPs form the continuous phase. In this case, the micelles formation can be considered as the process of adding supramolecules to the aggregates of NPs where NP is the primary phase. As the supramolecules concentration increases, PS blocks in the supramolecules and the NPs tend to cluster together to form micellar core, leaving the P4VP(PDP) block outside, thus reducing the unfavorable contact between P4VP(PDP) and NPs. In this case, the preferred micelle shape is mainly determined by the ratio of PDP to PVP units. 2289

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for NPs with various functional cores (magnetic, catalytic, or fluorescence) to produce multifunctional composites materials with well-defined nanostructures and high NP density.

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ASSOCIATED CONTENT

S Supporting Information *

Additional photographs, TEM images, plot, 1H NMR spectra, and UV−vis spectra of the hybrid micelles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.T.Z.); [email protected] (Z.N.). Notes

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, 2012CB812500, 2012CB932500), the National Natural Science Foundation of China (51173056, 91127046), Chinese Ministry of Education (NCET-10-0398), Excellent Youth Foundation of Hubei Scientific Committee (2012FFA008), and Program of Chutian Scholars in Hubei Province. We also thank HUST Analytical and Testing Center for allowing us to use its facilities. Z.N. acknowledges the support of startup funds and the Research and Scholarship Award from the University of Maryland.

Figure 7. Phase diagram summarizing the effect of PDP content and the φNP‑M on the hybrid PS20K-b-P4VP17K micellar morphology. The dashed lines indicate the superimposed boundaries between different morphological regions of hybrid micelles: cylindrical micelles (□); nanosheets (○); spherical micelles (△); irregular aggregates (▽); perforated nanosheets (◁). We note that, in some cases, two symbols existed in one data point indicate that two primary micellar structures coexist in the same experimental condition. We, therefore, consider this point as the part of the boundary of the different phases.

We presume this is because the PDP change the supramolecule length ratio and the interaction between NPs and PS block. When x is greater than 4, nonuniform distribution of PDP triggers the macrophase separation and the formation of irregular structures. The phase diagram given here will provide guidance for the design and synthesis of well-ordered hybrid micellar aggregates with desired structure and NPs density.

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REFERENCES

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4. CONCLUSION In summary, we demonstrate a facile and effective approach to prepare hybrid PS-b-P4VP assemblies encapsulated with monodisperse gold NPs. Control over the spatial distribution of the NPs and high loading of up to 84.1 vol % has been achieved. The hybrid materials can be disassembled into welldefined discrete nano-objects, which are otherwise difficult to obtain. Generally, the addition of NPs grafted with PS which is miscible with the core-forming block of the micelles increases the volume fraction of the PS domain. The addition of PDP to the P4VP domain not only balances the increased volume fraction resulted from the NP addition but also increases the effective interaction parameter of the system to maintain the strong segregation. The number of NPs in each micelle can be effectively enhanced from ∼1 to 80 by simultaneously adding NPs and PDP to the supramolecules. Through this facile route, the loading number of NPs can be effectively improved while maintaining the spherical micellar morphology. Moreover, we show that the NPs can form well-defined structural superlattices that are difficult to obtain with low NPs density. The findings in the study can be extended to different types of commonly synthesized NPs, supramolecular pairs, and different micellar aggregates (cylindrical micelles or sheetlike structures). The hybrid micelles developed in this study possess the high NP density. We believe that this may offer new opportunities in the fields of nanomedicine, high-density data storage, and biosensors. These results clearly demonstrate the opportunities 2290

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