Article pubs.acs.org/Langmuir
Patchy Micelles Based on Coassembly of Block Copolymer Chains and Block Copolymer Brushes on Silica Particles Shuzhe Zhu,† Zhan-Wei Li,‡ and Hanying Zhao*,† †
Key Laboratory of Functional Polymer Materials, Ministry of Education, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *
ABSTRACT: Patchy particles are a type of colloidal particles with one or more well-defined patches on the surfaces. The patchy particles with multiple compositions and functionalities have found wide applications from the fundamental studies to practical uses. In this research patchy micelles with thiol groups in the patches were prepared based on coassembly of free block copolymer chains and block copolymer brushes on silica particles. Thiol-terminated and cyanoisopropyl-capped polystyrene-block-poly(N-isopropylacrylamide) block copolymers (PS-b-PNIPAM-SH and PS-b-PNIPAM-CIP) were synthesized by reversible addition−fragmentation chain transfer polymerization and chemical modifications. Pyridyl disulfide-functionalized silica particles (SiO2-SS-Py) were prepared by fourstep surface chemical reactions. PS-b-PNIPAM brushes on silica particles were prepared by thiol−disulfide exchange reaction between PS-b-PNIPAM-SH and SiO2-SS-Py. Surface micelles on silica particles were prepared by coassembly of PS-b-PNIPAMCIP and block copolymer brushes. Upon cleavage of the surface micelles from silica particles, patchy micelles with thiol groups in the patches were obtained. Dynamic light scattering, transmission electron microscopy, and zeta-potential measurements demonstrate the preparation of patchy micelles. Gold nanoparticles can be anchored onto the patchy micelles through S−Au bonds, and asymmetric hybrid structures are formed. The thiol groups can be oxidized to disulfides, which results in directional assembly of the patchy micelles. The self-assembly behavior of the patchy micelles was studied experimentally and by computer simulation.
1. INTRODUCTION With the rapid development of nanoscience and synthetic chemistry, many different anisotropic particles with anisotropy in size, shape, and composition have been produced.1−6 The anisotropic particles find wide applications in chemistry and materials science. Among the anisotropic particles, Janus particles and patchy particles are two types of particles which were widely investigated in these years. Janus particles, named after the two-faced Roman god Janus, have two hemispheres with dissimilar properties, and the two halves present different chemical or physical properties.7−10 Because of the lack of centrosymmetry in the structures, Janus particles present very interesting aggregation behaviors and are able to self-assemble at the liquid−liquid interface.11−14 Patchy particles are defined as anistropic particles with one or more well-defined patches on the surfaces. The patches display directional interactions,15,16 which enables well control over the assembly process of the particles. Janus particles can be regarded as a special type of patchy particles with only one patch occupying half of the surface. Like artificial atoms, patchy particles with controllable number of patches allow for formation of three-dimensional directional bonding, and the particles are able to perform directed assemblies.17,18 In the past decade, many methods were employed in the fabrication of patchy particles, including templating method, colloidal assembly method, particles © XXXX American Chemical Society
lithography, glancing-angle deposition, nanosphere lithography, and capillary fluid flow method.19 In order to fabricate advanced materials, new methods for the preparation of welldefined patchy particles in large scale are required. The self-assembly of block copolymers into micelles is one of the prevalent research subjects in polymer science. The polymeric micelles find practical and potential applications in nanoscience and biotechnology.20,21 In these years, Janus micelles and patchy micelles have been prepared based on self-assembly of block copolymers.22−24 In this paper, a novel method to prepare patchy micelles with thiol groups in the patches is proposed. Our strategy is based on formation of surface micelles coassembled by free diblock copolymer chains and block copolymer brushes on silica particles and cleavage of the surface micelles from the silica particles. Polystyrene-blockpoly(N-isopropylacrylamide) (PS-b-PNIPAM) was synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization. The trithiocarbonate moieties at the ends of PNIPAM blocks were reduced with hexylamine or removed by 2,2′-azo(bis)isobutyronitrile (AIBN), and thiol-terminated block copolymer (PS-b-PNIPAM-SH)25,26 or cyanoisopropyl Received: February 9, 2015 Revised: March 22, 2015
A
DOI: 10.1021/acs.langmuir.5b00526 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Schematic Outlines for (a) Preparation of Polystyrene-block-poly(N-isopropylacrylamide) Block Copolymer Brushes on Silica Particles and (b) Fabrication and Self-Assembly of Patchy Micelles with Thiol Groups in the Patches
excess Bu3P. 2,2′-Dipyridyl disulfide (9.0 mg, 0.041 mmol) was dissolved in 4 mL of methanol and degassed before use. The silica particles were dispersed into the solution of 2,2′-dipyridyl disulfide, and the dispersion turned into yellow immediately. The thiol− disulfide exchange reaction was conducted for 12 h at room temperature. After the reaction, pyridyl-S-S-SiO2 particles were collected and washed by methanol. Synthesis of PS-b-PNIPAM Block Copolymer. PS-b-PNIPAM block copolymer was synthesized by two-step RAFT polymerization. A typical approach was described as follows. Styrene (10.4 mL, 90.0 mmol), RAFT agent S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (0.11 g, 0.30 mmol), and AIBN (7.0 mg, 0.043 mmol) were dissolved in 15 mL of toluene in a 50 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 90 °C for 12 h and stopped by quenching the Schlenk flask in ice water. After being concentrated, PS was precipitated in methanol. Based on size exclusion chromatography (SEC) result, the apparent number-average molecular weight (Mn) and the molecular weight distribution of PS are 6.8K and 1.18. As determined by 1H NMR, the average repeat unit number of PS is 67. In the second step of RAFT polymerization, PS macro-RAFT agent (0.20 g, 0.029 mmol), NIPAM (0.66 g, 5.8 mmol), and AIBN (0.95 mg, 0.0058 mmol) were dissolved in 2.4 mL of DMF in a 10 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 70 °C for 7 h and stopped by quenching the Schlenk flask in ice water. After being concentrated, PS-b-PNIPAM block polymer was precipitated in ether. Synthesis of PS-b-PNIPAM Brushes on Silica Particles. The silica particles with PS-b-PNIPAM brushes on the surface were synthesized by the “grafting to” method. A typical approach was described as follows. Pyridyl-S-S-SiO2 (50 mg) and PS-b-PNIPAM block copolymer (60 mg) were dissolved in 1.5 mL of DMF in a 10 mL Schlenk flask. After being degassed by three freeze−pump−thaw cycles, hexylamine (1.2 μL, 0.0090 mmol) was added to reduce the RAFT agents at the ends of diblock copolymer chains to thiol groups. The diblock copolymer chains were grafted to the silica particles by thiol−disulfide exchange reaction. After 20 h reaction at room temperature, silica particles were collected by centrifugation. Synthesis of Cyanoisopropyl Group-Capped PS-b-PNIPAM Block Copolymer (PS-b-PNIPAM-CIP). AIBN was used to remove trithiocarbonate at the ends of PS-b-PNIPAM block copolymer chains. A typical approach was described as follows. PNIPAM-b-PS (50 mg, 0.0022 mmol) and AIBN (36.8 mg, 0.225 mmol) were dissolved in 3 mL of DMF in a 10 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The reaction was performed at 90 °C for 6 h. After being concentrated, the polymer was precipitated in
group-capped block copolymer (PS-b-PNIPAM-CIP) were obtained.26 Pyridyl disulfide-functionalized silica particles (SiO2-SS-Py) were prepared by surface chemical reactions. Original silica particles were reacted with (3-isocyanatopropyl)triethoxysilane and cystamine successively, and disulfides were introduced to the surfaces of silica particles. After cleavage of the disulfides with butylphosphine (Bu3P) and a subsequent chemical reaction with 2,2′-dipyridyl disulfide, SiO2-SS-Py were prepared. PS-b-PNIPAM diblock copolymer brushes on silica particles were prepared by the “grafting to” method. After thiol−disulfide exchange reaction between PS-b-PNIPAM-SH and SiO2-SS-Py, PS-b-PNIPAM diblock copolymer brushes on silica particles were prepared (Scheme 1a). Surface micelles on silica particles were prepared by coassembly of PS-b-PNIPAM diblock copolymer brushes and PS-b-PNIPAM-CIP free polymer chains in water, a solvent for PNIPAM and a precipitant for PS. In a surface micelle, PS blocks collapse forming the cores of the micelles, and PNIPAM form the corona. The surface micelles are anchored onto the surface of silica particles through disulfide bonds. Upon cleavage of the disulfide bonds, patchy micelles with thiol groups in the patches were prepared. The fabrication and self-assembly of patchy micelles on silica particles are illustrated in Scheme 1b.
2. EXPERIMENTAL METHODS Synthesis of Silica Particles with Disulfide Groups on the Surface. In a flask, 3.0 g of SiO2 particles was dispersed in 100 mL of dry toluene after sonication, and 3-isocyanatopropyltriethoxysilane (1.2 mL, 4.8 mmol) was added under an argon atmosphere. The solution was stirred at room temperature for 12 h. The particles were collected after centrifugation and redispersed in 50 mL of dry THF. Cystamine (0.75 g, 4.8 mmol) was added into the dispersion, and the reaction was performed at 90 °C for 12 h. After the reaction, aminomodified silica particles (H2N-S-S-SiO2) were collected, washed by ethanol, and dried under vacuum. Synthesis of Silica Particles with Pyridyl Disulfide Groups on the Surface (Pyridyl-S-S-SiO2). H2N-S-S-SiO2 particles (500 mg) were dispersed in 4 mL of methanol in a 10 mL Schlenk flask, and a catalytic amount of doubly distilled water was added into the dispersion. After three freeze−pump−thaw cycles, Bu3P solution (10 wt % in hexane, 0.25 mL) was added into the solution. The mixture was stirred at room temperature for 12 h. The silica particles were collected after centrifugation and washed with methanol to remove B
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Figure 1. (a) UV−vis spectrum of silica particles with PS67-b-PNIPAM141 brushes on the surface. The inset represents a standard curve of pyridine-2thione in DMF. (b) Thermogravimetric analysis of (1) original silica particles, (2) pyridyl disulfide-modified silica particles, (3) silica particles with diblock copolymer brushes and (4) silica particles with surface micelles. ether, and the cyanoisopropyl group-capped block copolymer was obtained. Coassembly of PS-b-PNIPAM Brushes and PS-b-PNIPAM-CIP. Silica particles with PS-b-PNIPAM brushes (20 mg) and PS-bPNIPAM-CIP block copolymer (5.0 mg) were dispersed (or dissolved) in 1.0 mL of DMF, and 5.0 mL of doubly distilled water was added into the mixture. After stirring for 2 h, the colloid solution was dialyzed against doubly distilled water with dialysis tubing (MWCO 7 kDa) to remove DMF. The micelles self-assembled by free polymers were removed by centrifugation. After 2 min of centrifugation at 7000 rpm, the silica particles settled at the bottoms of the vessels were collected, and the micelles formed by the free block copolymer chains were in the supernatant solution. Preparation of Patchy Micelles. 1,4-Dithio-DL-threitol (DTT, 2.0 mg) was added into 2 mL of suspension of silica particles with surface micelles (6.6 mg) after one pump−thaw cycle. The mixture was stirred at room temperature under an argon atmosphere. The silica particles were removed by centrifugation, and the cleaved surface micelles were obtained. The excess DTT was removed by dialysis of the micellar solution against doubly distilled water under an argon atmosphere.
TEM image and size distribution of SiO2 particles are shown in Figure S6. As determined by TEM, the average size of SiO2 particles is 860 nm. After a reaction with 3-isocyanatopropyltriethoxysilane, SiO2 particles with isocyanate groups on the surfaces were obtained, and after a reaction with excess cystamine, disulfide groups were introduced onto the silica particles (Scheme 1a). Upon cleavage of the disulfides with trin-butylphosphine (Bu3P), thiol groups were produced on the SiO2 particles. Pyridyl disulfides were prepared on the particles after a reaction of the thiols with 2,2′-dipyridyl disulfide. Besides pyridyl disulfides, pyridine-2-thione, which has an absorption at 360 nm in methanol solution,28,29 was also produced in the reaction. The UV−vis spectrum of SiO2 dispersion after thiol−disulfide exchange reaction is shown in Figure S7. The appearance of a peak at 360 nm indicates the production of pyridine-2-thione in the solution and the preparation of pyridyl disulfide modified SiO2 particles. The concentration of pyridine-2-thione in solution, as well as the surface content of thiol groups on SiO2 particles, can be determined based on a standard curve (inset of Figure S7). Our calculation result shows that the content of pyridyl disulfide groups on the silica particles is about 0.05 mmol/g. Polymer brushes on SiO2 particles were prepared by the “grafting to” method. In DMF solution, PS67-b-PNIPAM141-SH block copolymer chains were anchored onto the surface of SiO2-SS-Py by thiol−disulfide exchange reaction (Scheme 1a). In the exchange reaction, pyridine-2-thione was also produced. The absorption of the solution at 372 nm indicates the production of pyridine-2-thione (Figure 1a) and the grafting of block copolymer chains to the particles. The inset of Figure 1a is a standard curve of pyridine-2-thione in DMF. Based on the standard curve, the grafting density of block copolymer chains on SiO2 particles is deterimined to be about 0.48 chains/nm2. Thermogravimetric analysis (TGA) results of original, pyridyl disulfide modified and block copolymer modified SiO2 particles are shown in Figure 1b. If the weight retention of the residue at 800 °C is used as the reference, the weight retentions of the original SiO2 particles, SiO2-SS-Py, and diblock copolymermodified SiO2 particles are 95.5, 93.3, and 91.6 wt %, respectively. The grafting density of the block copolymer brushes determined by TGA is about 0.2 chains/nm2, which is lower than UV result. In the TGA measurement, carbonization of the polymer brushes may occur, and the residue at high temperature contains carbonized polymer,30,31 resulting in lower grafting density. Transmission electron microscopy
3. RESULTS AND DISCUSSION The diblock copolymer of PS and PNIPAM was prepared by RAFT polymerization. On the basis of 1H NMR results, the average repeating unit numbers of PS and PNIPAM are 67 and 141 (Figure S1 and S2 in the Supporting Information). The apparent number-average molecular weight and the dispersity of the diblock copolymer determined by SEC are 21.6K and 1.14 (Figure S3). Herein, the block copolymer is refereed as PS67-b-PNIPAM141. The trithiocarbonate moieties were reduced with hexylamine, and thiol-terminated block copolymer, PS67-b-PNIPAM141-SH, was obtained.25,26 PS67-b-PNIPAM141SH was used in the preparation of block copolymer brushes on silica particles. In order to synthesize PS67-b-PNIPAM141-CIP, the trithiocarbonate moieties on PS67-b-PNIPAM141 were removed in the presence of AIBN.27 PS67-b-PNIPAM141-CIP was used in the preparation of surface micelles by coassembly with block copolymer brushes on silica particles. Figure S4 shows UV−vis spectra of PS67-b-PNIPAM141 and PS67-bPNIPAM141-CIP. After a reaction with AIBN, the absorbance of the trithiocarbonate at 310 nm disappears, demonstrating the removal of the trithiocarbonate terminal moieties. 1H NMR spectra of PS67-b-PNIPAM141 and PS67-b-PNIPAM141-CIP are shown in Figure S5. After removal of the trithiocarbonate moieties, the peak at 3.23 ppm corresponding to the methane protons on RAFT agent disappears. C
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Langmuir (TEM) result indicates that the diblock copolymer brush forms a layer around the silica particles (Figure S8). Previous theoretical and experimental studies on surfacetethered block copolymer brushes have demonstrated that the polymer brushes have nanosized phase separation on solid surfaces upon treatment with selective solvents.32−38 In the presence of free block copolymer chains, block copolymer brushes and free block copolymer chains are able to coassemble into surface micelles, a type of micelle which is covalently anchored onto the solid surface.39 In this research, we prepared surface micelles on SiO2 particles by coassembly of PS67-bPNIPAM141 brushes and free PS67-b-PNIPAM141-CIP polymer chains in water, a good solvent for PNIPAM and a precipitant for PS, and fabricated patchy micelles by cleavage of the disulfide bonds between the surface micelles and the silica particles (Scheme 1b). Silica particles with PS67-b-PNIPAM141 brushes and PS67-b-PNIPAM141-CIP free polymer chains were dissolved in DMF, a solvent for PS-b-PNIPAM, and 5-fold of doubly distilled water was added into the solution. DMF was removed after dialysis of the solution against water. After lowspeed centrifugation, the silica particles with surface micelles on the surfaces were collected at the bottom of the sediment. TGA curve of the particles with surface micelles is presented in Figure 1b (curve d). After coassembly of the polymer brushes and free polymer chains, the weight retention of the hybrid particles reduces to 77.9 wt %. The weight ratio of free polymer to polymer brushes in the surface micelles was determined by UV−vis. Silica particles with surface micelles were dispersed in DMF, and after centrifugation, silica particles were removed and free polymer chains were left in the supernatant fluid. The content of free block polymer in DMF was determined by UV− vis based on a standard curve made by the absorption of PS at 270 nm. In this research, surface micelles with two different weight ratios of free polymer to polymer brushes were prepared on the silica particles. Figure 2a shows the scanning electron microscopy (SEM) image of SiO2-SS-Py. The small-molecule-modified particles have very smooth surfaces. However, the coassembly of free block copolymer chains and block copolymer brushes results in the formation of surface micelles and rough surfaces (Figure 2b). The surface micelles were cleaved from the silica particles by 1,4-dithio-DL-threitol (DTT) in aqueous solution, and patchy micelles were obtained after centrifugation. TEM images of patchy micelles prepared at two different weight ratios of free polymer to polymer brushes are shown in Figure 2c,d. In both cases spherical micelles are observed. The sizes of the patchy micelles at dry state can be calculated based on TEM results. When the weight ratios of free polymer to polymer brushes are 4/1 and 7.7/1, the average sizes of the micelles are 32 and 57 nm, respectively, indicating an increases in the size of the patchy micelles with the weight ratio. At higher weight ratio, more “free” polymer chains are involved in the coassembly, resulting in the formation of the surface micelles with bigger sizes. Dynamic light scattering (DLS) curves of the patchy micelles are shown in Figure 2e. Both of the two different patchy micelles possess unimodal size distributions. The average hydrodynamic diameters (DH) and polydispersity indexes of the patchy micelles prepared at 4/1 and 7.7/1 are 90 nm, 0.094 and 130 nm, 0.117, also demonstrating the size of the surface micelles increases with the weight ratio. Figure 2f shows SEM image of silica particles after cleavage of surface micelles. In comparison to the silica particles with surface micelles (Figure 2b), after removal of surface micelles the silica
Figure 2. SEM images of (a) pyridyl disulfide-modified SiO2 and (b) SiO2 with surface micelles on the surfaces. (c, d) TEM images of surface micelles cleaved from the surface of SiO2 particles. In the surface micelles, the weight ratios of “free” polymer to polymer brushes are 4/1 (c) and 7.7/1 (d). (e) DLS curves of cleaved surface micelles prepared at weight ratios of 4/1 (curve 1) and 7.7/1 (curve 2). (f) SEM image of silica particles after cleavage of surface micelles from the surfaces.
particles have smooth surface, which indicates that the cleavage reaction is efficient and this method can be used to produce patchy micelles in large quantity. The patches on the micelles are occupied by thiols, so quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) chains with pyridyl disulfide terminal groups can be anchored onto the micelles through thiol−disulfide exchange reaction. q-PDMAEMA was synthesized by a combination of RAFT polymerization and quaternization reaction.14 The terminal dithiocarbonate moieties were reduced with hexylamine, and after a reaction with 2,2′-dipyridyl disulfide, q-PDMAEMA with pyridyl disulfide terminal groups was prepared. 1H NMR characterizations of q-PDMAEMA with pyridyl disulfide and dithiocarbonate moieties can be found in the Supporting Information (Figure S9). Figure 3a shows TEM image of patchy micelles with q-PDMAEMA in the patches. Because q-PDMAEMA chains are quaternized by iodomethane, and iodine on the polymer chains enhances the contrast, qPDMAEMA domains on the micelles can be observed on a TEM without staining. In the TEM image, the eyeball-like structures represent patchy micelles with q-PDMAEMA in the patches. A typical micelle is indicated by an arrow, where the dark dot in the structures is q-PDMAEMA domain, and the gray part represents a surface micelle cleaved from the silica surface. Figure 3b shows DLS curves of patchy micelles before and after grafting of q-PDMAEMA chains. After grafting of qD
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Figure 3. (a) TEM image of patchy micelles after grafting of quaternized poly(2-(dimethylamino)ethyl methacrylate) (q-PDMAEMA) to the patches. (b) DLS curves of patchy micelles before and after grafting of q-PDMAEMA to the patches through thiol−disulfide exchange reaction. The insets of (b) show a TEM image and a cartoon picture of a patchy micelle with q-PDMAEMA chains. The scale bars in (a) and (b) represent 100 nm.
Thiol groups are efficiently coupled back to the disulfides under oxidation conditions.41,42 Oxidation-induced aggregation of the patchy micelles was investigated in this research. Figures 5a−c show TEM images of the patchy micelles after standing by in air for different time. TEM results indicate that the size of the micelles increases with time. Upon cleavage from the silica particles, the size of the patchy micelles is around 32 nm. After standing by in air for 1, 3, and 5 days, the average sizes of the patchy micelles are 34, 46, and 80 nm, respectively. The change in the size of the patchy micelles is attributed to the oxidation of thiol groups in the patches and aggregation of the micelles (Scheme 1). The size change of the patchy micelles was also monitored by DLS. DLS curves of the surface micelles standing by in air for different time are shown in Figure 5d. Exposing the micellar solution to air for 1, 3, 5, and 7 days, the average DHs of the patchy micelles are 73, 304, 403, and 412 nm, respectively, also indicating the oxidation of the thiol groups and aggregation of the patchy micelles. A plot showing the change of DH with exposing time is presented in the inset of Figure 5d. Within the first 2 days, the average DH of the patchy micelles is around 73 nm, and after that the size increases with time due to the aggregation of the patchy micelles. It is noted that the size of the aggregates is kept almost unchanged after 4day storage. In a previous research, we prepared spherical micelles with thiol groups randomly distributed in the coronae and found that the sizes of the isotropic micelles increased with time.43 After 6-day oxidation, large aggregates with sizes above 1 μm as well as precipitates were observed. In another research, the fusion of polymer membrane self-assembled by cleaved diblock copolymer brushes in selective was investigated.44 The fusion process was driven by the oxidation of thiol groups into disulfides. In this research, the anisotropic nature of the patchy micelles results in directional assembly of the patchy micelles. In order to understand the oxidation-induced directional assembly of the patchy micelles, computer simulation was conducted by using soft Janus particle model.45,46 In the model, the surface of a particle is simply divided into one green repulsive and one red attractive region. The red attractive regions refer to the patches with reactive thiols. The driving force for the self-assembly of the patchy particles is the oxidation of thiols and the formation of disulfides. Because the “free” PS67-b-PNIPAM141-CIP polymer chains have the same molecular structure as the brush polymer with only the difference in the terminal groups and the weight percentage of brush polymer in a surface micelle is about 20%, it is reasonable to calculate that 20% of a micellar surface is occupied by thiol groups. In the simulation, 20% of a patchy particle surface is the
PDMAEMA, the average hydrodynamic diameter increases from 90 to 137 nm, and the polydispersity index increase from 0.094 to 0.182. A TEM image and a cartoon picture showing the structure of a patchy micelle are presented in the inset of Figure 3b. The ξ-potential of the patchy micelles was measured. After grafting of q-PDMAEMA, the ξ-potential of the micelles changes from −6.3 to 20 mV, demonstrating the anchoring of q-PDMAEMA onto the patches. Thiol groups are produced in the patches upon cleavage of the disulfides with DTT, so gold nanoparticles (AuNPs) are able to be anchored onto the patchy micelles through S−Au bonds. Herein, AuNPs with an average size of 15 nm were employed.40 Figure 4a shows TEM image of patchy micelles
Figure 4. (a) TEM image of patchy micelles with AuNPs in the patches. (b) A cartoon picture showing the structure of the aggregated patchy micelles with AuNPs. The scale bar in the inset of (a) represents 100 nm.
labeled with AuNPs. The TEM specimen was stained by exposing to OsO4 vapor, and PNIPAM phases were stained. In the image, asymmetrical particles in the range from 46 to 120 nm were observed. In a particle the gray part represents stained patchy micelle, and the deep dark part represents AuNPs aggregated in the patches. Because one AuNP may bind to several patchy micelles, the sizes of the AuNPs-labeled micelles are bigger than the patchy micelles cleaved from the silica particles (Figure 2c). A magnified TEM image showing the structure of a typical aggregated micelle is shown in the inset of Figure 4a. The asymmetric structure of the micellar aggregate demonstrates AuNPs-induced assembly of the patchy micelles. A cartoon picture showing the structure of the aggregated patchy micelles is presented in Figure 4b. In an aggregated structure, AuNPs binding to different patches hold the structure together. E
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Figure 5. (a−c) TEM images of surface micelles after standing by in air for (a) 1, (b) 3, and (c) 5 days. (d) DLS curves of surface micelles after standing by in air for 1 day (curve 1), 3 days (curve 2), 5 days (curve 3), and 7 days (curve 4). The inset shows a plot of hydrodynamic diameter of the patchy micelles vs storage time. The scale bars in the TEM images represent 200 nm.
Figure 6. Relative distribution of neighboring patchy micelles in self-assembled structures. The distributions have been normalized so that ∑sN(s) = 1. The insets show equilibrium self-assembly structures of the patchy particles (right side) and a TEM image of the aggregates of patchy micelles (left side).
4. CONCLUSIONS In conclusion, we have demonstrated that patchy micelles with thiol groups in the patches can be prepared by cleavage of the surface micelles coassembled by free polymer chains and block copolymer brushes on silica particles. Upon cleavage of the surface micelles from the silica particles, patchy micelles with thiol groups in the patches are produced. AuNPs can be anchored onto the patchy micelles through Au−S bonds, forming asymmetrical aggregates or isolated micelles with AuNPs. The thiol groups can be oxidized to disulfides, which results in directional assembly of the patchy micelles. This research provides a novel method to prepare asymmetric structures, and the structures will find applications in many scientific communities from the fundamental studies to commercial applications.
attractive region. Based on our computer simulation result, the distribution N(s) of the number of patchy micelles (s) in an aggregated structure is shown in Figure 6a. The value of s is mainly in the range of 2−5. A typical equilibrium self-assembled structures is shown in Figure 6b. The patchy particles aggregate together through the interaction between the attractive regions, and most of the aggregated structures are composed of 2−5 particles. A magnified TEM image of a typical aggregated structure is shown in the inset of Figure 6a. As indicated by the red lines, the structure is composed of three patchy micelles, which demonstrates that the simulation result agrees well with our experimental result. A model and simulation details can be found in the Supporting Information. F
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(17) Wang, Y.; Wang, Y.; Breed, D. R.; Manoharan, V. N.; Feng, L.; Hollingsworth, A. D.; Pine, D. J. Colloids with valence and specific directional bonding. Nature 2012, 491, 51−55. (18) Ejima, H.; Richardson, J. J.; Caruso, F. Multivalent directed assembly of colloidal particles. Angew. Chem., Int. Ed. 2013, 52, 3314− 3316. (19) Pawar, A. B.; Kretzschmar, I. Fabrication, assembly, and application of patchy particles. Macromol. Rapid Commun. 2010, 31, 150−168. (20) Nicolas, J.; Miguel, V. S.; Mantovani, G.; Haddleton, D. M. Fluorescently tagged polymer bioconjugates from protein derived macroinitiators. Chem. Commun. 2006, 45, 4697−4699. (21) Okada, M. Chemical syntheses of biodegradable polymers review article. Prog. Polym. Sci. 2002, 27, 87−133. (22) Erhardt, R.; Zhang, M.; Böker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; Müller, A. H. E. Amphiphilic Janus micelles with polystyrene and poly(methacrylic acid) hemispheres. J. Am. Chem. Soc. 2003, 125, 3260−3267. (23) Fang, B.; Walther, A.; Wolf, A.; Xu, Y.; Yuan, J.; Müller, A. H. E. Undulated multicompartment cylinders by the controlled and directed stacking of polymer micelles with a compartmentalized corona. Angew. Chem., Int. Ed. 2009, 48, 2877−2880. (24) Schacher, F.; Betthausen, E.; Walther, A.; Schmalz, H.; Pergushov, D. V.; Müller, A. H. E. Interpolyelectrolyte complexes of dynamic multicompartment micelles. ACS Nano 2009, 3, 2095−2102. (25) Willcock, H.; O’Reilly, R. K. End group removal and modification of RAFT polymers. Polym. Chem. 2010, 1, 149−157. (26) Wang, Z.; He, J.; Tao, Y.; Yang, L.; Jiang, H.; Yang, Y. Controlled chain branching by RAFT-based radical polymerization. Macromolecules 2003, 36, 7446−7452. (27) Perrier, S.; Takolpuckdee, P.; Mars, C. A. Reversible additionfragmentation chain transfer polymerization: end group modification for functionalized polymers and chains transfer agent recovery. Macromolecules 2005, 38, 2033−2036. (28) Ryu, J. H.; Jiwpanich, S.; Chacko, R.; Bickerton, S.; Thayumanavan, S. Surface-functionalizable polymer nanogels with facile hydrophobic guest encapsulation capabilities. J. Am. Chem. Soc. 2010, 132, 8246−8247. (29) Duong, H. T. T.; Marquis, C. P.; Whittaker, M.; Davis, T. P.; Boyer, C. Acid degradable and biocompatible polymeric nanoparticles for the potential codelivery of therapeutic agents. Macromolecules 2011, 44, 8008−8019. (30) Fu, Q.; Rama Rao, G. V.; Ista, L. K.; Wu, Y.; Andrzejewski, B. P.; Sklar, L. A.; Ward, T. L.; Lopez, G. P. Control of molecular transport through stimuli-responsive ordered mesoporous materials. Adv. Mater. 2003, 15, 1262. (31) Du, P.; Yang, H.; Zeng, J.; Liu, P. Folic acid-conjugated temperature and pH dual-responsive yolk/shell microspheres as a drug delivery system. J. Mater. Chem. B 2013, 1, 5298. (32) Zhulina, E. B.; Singh, C.; Balazs, A. C. Self-assembly of tethered diblocks in selective solvents. Macromolecules 1996, 29, 8254−8259. (33) Yin, Y.; Sun, P.; Li, B.; Chen, T.; Jin, Q.; Ding, D.; Shi, A. A simulated annealing study of diblock copolymer brushes in selective solvents. Macromolecules 2007, 40, 5161−5170. (34) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. AFM study of tethered polystyrene-b-poly(methyl methacrylate) and polystyreneb-poly(methyl acrylate) brushes on flat silicate substrates. Macromolecules 2000, 33, 8821−8827. (35) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. Nanopattern formation from tethered PS-b-PMMA brushes upon treatment with selective solvents. J. Am. Chem. Soc. 2000, 122, 2407−2408. (36) Zhao, B.; Brittain, W. J. Synthesis, characterization, and properties of tethered polystyrene-b-polyacrylate brushes on flat silicate substrates. Macromolecules 2000, 33, 8813−8820. (37) Yang, Y.; Liu, L.; Zhang, J.; Li, C.; Zhao, H. PMMA colloid particles stabilized by layered silicate with PMMA-b-PDMAEMA block copolymer brushes. Langmuir 2007, 23, 2867−2873. (38) Chen, K.; Liang, D.; Tian, J.; Shi, L.; Zhao, H. In-situ polymerization at the interfaces of micelles: a “grafting from” method
ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra, UV−vis spectra, SEC curves of polymers, and computer simulation details. 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] (H.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (NSFC) under Contracts 21074058 and 51473079 and the National Basic Research Program of China (973 Program, 2012CB821500).
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REFERENCES
(1) Walther, A.; Müller, A. H. E. Janus particles: synthesis, selfassembly, physical properties, and applications. Chem. Rev. 2013, 113, 5194−5261. (2) Hu, J.; Zhou, S.; Sun, Y.; Fang, X.; Wu, L. Fabrication, properties and applications of Janus particles. Chem. Soc. Rev. 2012, 41, 4356− 4378. (3) Jiang, S.; Chen, Q.; Tripathy, M.; Luijten, E.; Schweizer, K. S.; Granick, S. Janus particle synthesis and assembly. Adv. Mater. 2010, 22, 1060−1071. (4) Damasceno, P. F.; Engel, M.; Glotzer, S. C. Predictive selfassembly of polyhedra into complex structures. Science 2012, 337, 453−457. (5) Du, J.; O’Reilly, R. K. Anisotropic particles with patchy, multicompartment and Janus architectures: Preparation and application. Chem. Soc. Rev. 2011, 40, 2402−2416. (6) Glotzer, S. C.; Solomon, A. M. J. Anisotropy of building blocks and their assembly into complex structures. Nat. Mater. 2007, 6, 557− 562. (7) Loget, G.; Kuhn, A. Bulk synthesis of Janus objects and asymmetric patchy partices. J. Mater. Chem. 2012, 22, 15457−15474. (8) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. Design and synthesis of Janus micro- and nanoparticles. J. Mater. Chem. 2005, 15, 3745−3760. (9) Zhou, T.; Dong, B.; Qi, H.; Mei, S.; Li, C. Y. Janus hybrid hairy nanoparticles. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1620−1640. (10) Yan, J.; Sung, C. B.; Granick, S. Colloidal superstructures programmed innto magnetic Janus particles. Adv. Mater. 2015, 27, 874−879. (11) Liu, B.; Wie, W.; Qu, X.; Yang, Z. Janus colloids formed by biphasic grafting at a Pickering emulsion interface. Angew. Chem., Int. Ed. 2008, 47, 3973−3975. (12) Takahara, Y. K.; Ikeda, S.; Ishion, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. Asymmetrically modified silica particles: a simple particulate surfactant for stabilization of oil droplets in water. J. Am. Chem. Soc. 2005, 127, 6271−6275. (13) Liang, F.; Shen, K.; Qu, X.; Zhang, C.; Wang, Q.; Li, J.; Liu, J.; Yang, Z. Inorganic Janus nanosheets. Angew. Chem., Int. Ed. 2011, 50, 2379−2382. (14) Liu, J.; Liu, G.; Zhang, M.; Sun, P.; Zhao, H. Synthesis and selfassembly of amphiphilic Janus laponite disks. Macromolecules 2013, 46, 5974−5984. (15) Rodríguez-Fernández, D.; Liz-Marzán, L. M. Metallic Janus and patchy particles. Part. Part. Syst. Charact. 2013, 30, 46−60. (16) Zhang, Z.; Glotzer, S. C. Self-assembly of patchy particles. Nano Lett. 2004, 4, 1407. G
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Langmuir to prepare micelles with mixed coronal chains. J. Phys. Chem. B 2008, 112, 12612−12617. (39) Li, Y.; Yang, D.; Adronov, A.; Gao, Y.; Luo, J.; Li, H. Covalent functionalization of single-walled carbon nanotubes with thermoresponsive core cross-linked polymeric micelles. Macromolecules 2012, 45, 4698−4706. (40) Liu, G.; Tian, J.; Zhang, X.; Zhao, H. Amphiphilic Janus god nanoparticles prepared by interface-directed self-assembly: synthesis and self-assembly. Chem.Asian J. 2014, 9, 2597−2603. (41) Little, C.; O’Brien, P. J. Products of oxidation of a protein thiol group after reaction with various oxidizing agents. Arch. Biochem. Biophys. 1967, 122, 406−410. (42) McKillop, A.; Koyuncu, D.; Krief, A.; Dumont, W.; Renier, P.; Trabelsi, M. Efficicient, high yield, oxidation of thiols and selenols to disulphides and diselenides. Tetrahedron Lett. 1990, 31, 5007−5010. (43) Sun, L.; Liu, J.; Zhao, H. Reactive polymeric micelles with disulfide groups in the coronae. Polym. Chem. 2014, 5, 6584−6592. (44) Sun, L.; Zhao, H. Cleavage of diblock copolymer brushes in a selective solvent and fusion of vesicles self-assembled by pinned micelles. Langmuir 2015, 31, 1867−1873. (45) Li, Z.; Lu, Z.; Sun, Z.; An, L. Model, self-assembly structures, and phase diagram of soft Janus particles. Soft Matter 2012, 8, 6693− 6697. (46) Li, Z.; Lu, Z.; Zhu, Y.; Sun, Z.; An, L. A simulation model for soft triblock Janus particles and their ordered packing. RSC Adv. 2013, 3, 813−822.
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DOI: 10.1021/acs.langmuir.5b00526 Langmuir XXXX, XXX, XXX−XXX
Patchy Micelles Based on Coassembly of Block Copolymer Chains and Block Copolymer Brushes on Silica Particles Shuzhe Zhu,† Zhan-Wei Li,‡ and Hanying Zhao*,† †
Key Laboratory of Functional Polymer Materials, Ministry of Education, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *
ABSTRACT: Patchy particles are a type of colloidal particles with one or more well-defined patches on the surfaces. The patchy particles with multiple compositions and functionalities have found wide applications from the fundamental studies to practical uses. In this research patchy micelles with thiol groups in the patches were prepared based on coassembly of free block copolymer chains and block copolymer brushes on silica particles. Thiol-terminated and cyanoisopropyl-capped polystyrene-block-poly(N-isopropylacrylamide) block copolymers (PS-b-PNIPAM-SH and PS-b-PNIPAM-CIP) were synthesized by reversible addition−fragmentation chain transfer polymerization and chemical modifications. Pyridyl disulfide-functionalized silica particles (SiO2-SS-Py) were prepared by fourstep surface chemical reactions. PS-b-PNIPAM brushes on silica particles were prepared by thiol−disulfide exchange reaction between PS-b-PNIPAM-SH and SiO2-SS-Py. Surface micelles on silica particles were prepared by coassembly of PS-b-PNIPAMCIP and block copolymer brushes. Upon cleavage of the surface micelles from silica particles, patchy micelles with thiol groups in the patches were obtained. Dynamic light scattering, transmission electron microscopy, and zeta-potential measurements demonstrate the preparation of patchy micelles. Gold nanoparticles can be anchored onto the patchy micelles through S−Au bonds, and asymmetric hybrid structures are formed. The thiol groups can be oxidized to disulfides, which results in directional assembly of the patchy micelles. The self-assembly behavior of the patchy micelles was studied experimentally and by computer simulation.
1. INTRODUCTION With the rapid development of nanoscience and synthetic chemistry, many different anisotropic particles with anisotropy in size, shape, and composition have been produced.1−6 The anisotropic particles find wide applications in chemistry and materials science. Among the anisotropic particles, Janus particles and patchy particles are two types of particles which were widely investigated in these years. Janus particles, named after the two-faced Roman god Janus, have two hemispheres with dissimilar properties, and the two halves present different chemical or physical properties.7−10 Because of the lack of centrosymmetry in the structures, Janus particles present very interesting aggregation behaviors and are able to self-assemble at the liquid−liquid interface.11−14 Patchy particles are defined as anistropic particles with one or more well-defined patches on the surfaces. The patches display directional interactions,15,16 which enables well control over the assembly process of the particles. Janus particles can be regarded as a special type of patchy particles with only one patch occupying half of the surface. Like artificial atoms, patchy particles with controllable number of patches allow for formation of three-dimensional directional bonding, and the particles are able to perform directed assemblies.17,18 In the past decade, many methods were employed in the fabrication of patchy particles, including templating method, colloidal assembly method, particles © XXXX American Chemical Society
lithography, glancing-angle deposition, nanosphere lithography, and capillary fluid flow method.19 In order to fabricate advanced materials, new methods for the preparation of welldefined patchy particles in large scale are required. The self-assembly of block copolymers into micelles is one of the prevalent research subjects in polymer science. The polymeric micelles find practical and potential applications in nanoscience and biotechnology.20,21 In these years, Janus micelles and patchy micelles have been prepared based on self-assembly of block copolymers.22−24 In this paper, a novel method to prepare patchy micelles with thiol groups in the patches is proposed. Our strategy is based on formation of surface micelles coassembled by free diblock copolymer chains and block copolymer brushes on silica particles and cleavage of the surface micelles from the silica particles. Polystyrene-blockpoly(N-isopropylacrylamide) (PS-b-PNIPAM) was synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization. The trithiocarbonate moieties at the ends of PNIPAM blocks were reduced with hexylamine or removed by 2,2′-azo(bis)isobutyronitrile (AIBN), and thiol-terminated block copolymer (PS-b-PNIPAM-SH)25,26 or cyanoisopropyl Received: February 9, 2015 Revised: March 22, 2015
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Scheme 1. Schematic Outlines for (a) Preparation of Polystyrene-block-poly(N-isopropylacrylamide) Block Copolymer Brushes on Silica Particles and (b) Fabrication and Self-Assembly of Patchy Micelles with Thiol Groups in the Patches
excess Bu3P. 2,2′-Dipyridyl disulfide (9.0 mg, 0.041 mmol) was dissolved in 4 mL of methanol and degassed before use. The silica particles were dispersed into the solution of 2,2′-dipyridyl disulfide, and the dispersion turned into yellow immediately. The thiol− disulfide exchange reaction was conducted for 12 h at room temperature. After the reaction, pyridyl-S-S-SiO2 particles were collected and washed by methanol. Synthesis of PS-b-PNIPAM Block Copolymer. PS-b-PNIPAM block copolymer was synthesized by two-step RAFT polymerization. A typical approach was described as follows. Styrene (10.4 mL, 90.0 mmol), RAFT agent S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (0.11 g, 0.30 mmol), and AIBN (7.0 mg, 0.043 mmol) were dissolved in 15 mL of toluene in a 50 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 90 °C for 12 h and stopped by quenching the Schlenk flask in ice water. After being concentrated, PS was precipitated in methanol. Based on size exclusion chromatography (SEC) result, the apparent number-average molecular weight (Mn) and the molecular weight distribution of PS are 6.8K and 1.18. As determined by 1H NMR, the average repeat unit number of PS is 67. In the second step of RAFT polymerization, PS macro-RAFT agent (0.20 g, 0.029 mmol), NIPAM (0.66 g, 5.8 mmol), and AIBN (0.95 mg, 0.0058 mmol) were dissolved in 2.4 mL of DMF in a 10 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 70 °C for 7 h and stopped by quenching the Schlenk flask in ice water. After being concentrated, PS-b-PNIPAM block polymer was precipitated in ether. Synthesis of PS-b-PNIPAM Brushes on Silica Particles. The silica particles with PS-b-PNIPAM brushes on the surface were synthesized by the “grafting to” method. A typical approach was described as follows. Pyridyl-S-S-SiO2 (50 mg) and PS-b-PNIPAM block copolymer (60 mg) were dissolved in 1.5 mL of DMF in a 10 mL Schlenk flask. After being degassed by three freeze−pump−thaw cycles, hexylamine (1.2 μL, 0.0090 mmol) was added to reduce the RAFT agents at the ends of diblock copolymer chains to thiol groups. The diblock copolymer chains were grafted to the silica particles by thiol−disulfide exchange reaction. After 20 h reaction at room temperature, silica particles were collected by centrifugation. Synthesis of Cyanoisopropyl Group-Capped PS-b-PNIPAM Block Copolymer (PS-b-PNIPAM-CIP). AIBN was used to remove trithiocarbonate at the ends of PS-b-PNIPAM block copolymer chains. A typical approach was described as follows. PNIPAM-b-PS (50 mg, 0.0022 mmol) and AIBN (36.8 mg, 0.225 mmol) were dissolved in 3 mL of DMF in a 10 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The reaction was performed at 90 °C for 6 h. After being concentrated, the polymer was precipitated in
group-capped block copolymer (PS-b-PNIPAM-CIP) were obtained.26 Pyridyl disulfide-functionalized silica particles (SiO2-SS-Py) were prepared by surface chemical reactions. Original silica particles were reacted with (3-isocyanatopropyl)triethoxysilane and cystamine successively, and disulfides were introduced to the surfaces of silica particles. After cleavage of the disulfides with butylphosphine (Bu3P) and a subsequent chemical reaction with 2,2′-dipyridyl disulfide, SiO2-SS-Py were prepared. PS-b-PNIPAM diblock copolymer brushes on silica particles were prepared by the “grafting to” method. After thiol−disulfide exchange reaction between PS-b-PNIPAM-SH and SiO2-SS-Py, PS-b-PNIPAM diblock copolymer brushes on silica particles were prepared (Scheme 1a). Surface micelles on silica particles were prepared by coassembly of PS-b-PNIPAM diblock copolymer brushes and PS-b-PNIPAM-CIP free polymer chains in water, a solvent for PNIPAM and a precipitant for PS. In a surface micelle, PS blocks collapse forming the cores of the micelles, and PNIPAM form the corona. The surface micelles are anchored onto the surface of silica particles through disulfide bonds. Upon cleavage of the disulfide bonds, patchy micelles with thiol groups in the patches were prepared. The fabrication and self-assembly of patchy micelles on silica particles are illustrated in Scheme 1b.
2. EXPERIMENTAL METHODS Synthesis of Silica Particles with Disulfide Groups on the Surface. In a flask, 3.0 g of SiO2 particles was dispersed in 100 mL of dry toluene after sonication, and 3-isocyanatopropyltriethoxysilane (1.2 mL, 4.8 mmol) was added under an argon atmosphere. The solution was stirred at room temperature for 12 h. The particles were collected after centrifugation and redispersed in 50 mL of dry THF. Cystamine (0.75 g, 4.8 mmol) was added into the dispersion, and the reaction was performed at 90 °C for 12 h. After the reaction, aminomodified silica particles (H2N-S-S-SiO2) were collected, washed by ethanol, and dried under vacuum. Synthesis of Silica Particles with Pyridyl Disulfide Groups on the Surface (Pyridyl-S-S-SiO2). H2N-S-S-SiO2 particles (500 mg) were dispersed in 4 mL of methanol in a 10 mL Schlenk flask, and a catalytic amount of doubly distilled water was added into the dispersion. After three freeze−pump−thaw cycles, Bu3P solution (10 wt % in hexane, 0.25 mL) was added into the solution. The mixture was stirred at room temperature for 12 h. The silica particles were collected after centrifugation and washed with methanol to remove B
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Figure 1. (a) UV−vis spectrum of silica particles with PS67-b-PNIPAM141 brushes on the surface. The inset represents a standard curve of pyridine-2thione in DMF. (b) Thermogravimetric analysis of (1) original silica particles, (2) pyridyl disulfide-modified silica particles, (3) silica particles with diblock copolymer brushes and (4) silica particles with surface micelles. ether, and the cyanoisopropyl group-capped block copolymer was obtained. Coassembly of PS-b-PNIPAM Brushes and PS-b-PNIPAM-CIP. Silica particles with PS-b-PNIPAM brushes (20 mg) and PS-bPNIPAM-CIP block copolymer (5.0 mg) were dispersed (or dissolved) in 1.0 mL of DMF, and 5.0 mL of doubly distilled water was added into the mixture. After stirring for 2 h, the colloid solution was dialyzed against doubly distilled water with dialysis tubing (MWCO 7 kDa) to remove DMF. The micelles self-assembled by free polymers were removed by centrifugation. After 2 min of centrifugation at 7000 rpm, the silica particles settled at the bottoms of the vessels were collected, and the micelles formed by the free block copolymer chains were in the supernatant solution. Preparation of Patchy Micelles. 1,4-Dithio-DL-threitol (DTT, 2.0 mg) was added into 2 mL of suspension of silica particles with surface micelles (6.6 mg) after one pump−thaw cycle. The mixture was stirred at room temperature under an argon atmosphere. The silica particles were removed by centrifugation, and the cleaved surface micelles were obtained. The excess DTT was removed by dialysis of the micellar solution against doubly distilled water under an argon atmosphere.
TEM image and size distribution of SiO2 particles are shown in Figure S6. As determined by TEM, the average size of SiO2 particles is 860 nm. After a reaction with 3-isocyanatopropyltriethoxysilane, SiO2 particles with isocyanate groups on the surfaces were obtained, and after a reaction with excess cystamine, disulfide groups were introduced onto the silica particles (Scheme 1a). Upon cleavage of the disulfides with trin-butylphosphine (Bu3P), thiol groups were produced on the SiO2 particles. Pyridyl disulfides were prepared on the particles after a reaction of the thiols with 2,2′-dipyridyl disulfide. Besides pyridyl disulfides, pyridine-2-thione, which has an absorption at 360 nm in methanol solution,28,29 was also produced in the reaction. The UV−vis spectrum of SiO2 dispersion after thiol−disulfide exchange reaction is shown in Figure S7. The appearance of a peak at 360 nm indicates the production of pyridine-2-thione in the solution and the preparation of pyridyl disulfide modified SiO2 particles. The concentration of pyridine-2-thione in solution, as well as the surface content of thiol groups on SiO2 particles, can be determined based on a standard curve (inset of Figure S7). Our calculation result shows that the content of pyridyl disulfide groups on the silica particles is about 0.05 mmol/g. Polymer brushes on SiO2 particles were prepared by the “grafting to” method. In DMF solution, PS67-b-PNIPAM141-SH block copolymer chains were anchored onto the surface of SiO2-SS-Py by thiol−disulfide exchange reaction (Scheme 1a). In the exchange reaction, pyridine-2-thione was also produced. The absorption of the solution at 372 nm indicates the production of pyridine-2-thione (Figure 1a) and the grafting of block copolymer chains to the particles. The inset of Figure 1a is a standard curve of pyridine-2-thione in DMF. Based on the standard curve, the grafting density of block copolymer chains on SiO2 particles is deterimined to be about 0.48 chains/nm2. Thermogravimetric analysis (TGA) results of original, pyridyl disulfide modified and block copolymer modified SiO2 particles are shown in Figure 1b. If the weight retention of the residue at 800 °C is used as the reference, the weight retentions of the original SiO2 particles, SiO2-SS-Py, and diblock copolymermodified SiO2 particles are 95.5, 93.3, and 91.6 wt %, respectively. The grafting density of the block copolymer brushes determined by TGA is about 0.2 chains/nm2, which is lower than UV result. In the TGA measurement, carbonization of the polymer brushes may occur, and the residue at high temperature contains carbonized polymer,30,31 resulting in lower grafting density. Transmission electron microscopy
3. RESULTS AND DISCUSSION The diblock copolymer of PS and PNIPAM was prepared by RAFT polymerization. On the basis of 1H NMR results, the average repeating unit numbers of PS and PNIPAM are 67 and 141 (Figure S1 and S2 in the Supporting Information). The apparent number-average molecular weight and the dispersity of the diblock copolymer determined by SEC are 21.6K and 1.14 (Figure S3). Herein, the block copolymer is refereed as PS67-b-PNIPAM141. The trithiocarbonate moieties were reduced with hexylamine, and thiol-terminated block copolymer, PS67-b-PNIPAM141-SH, was obtained.25,26 PS67-b-PNIPAM141SH was used in the preparation of block copolymer brushes on silica particles. In order to synthesize PS67-b-PNIPAM141-CIP, the trithiocarbonate moieties on PS67-b-PNIPAM141 were removed in the presence of AIBN.27 PS67-b-PNIPAM141-CIP was used in the preparation of surface micelles by coassembly with block copolymer brushes on silica particles. Figure S4 shows UV−vis spectra of PS67-b-PNIPAM141 and PS67-bPNIPAM141-CIP. After a reaction with AIBN, the absorbance of the trithiocarbonate at 310 nm disappears, demonstrating the removal of the trithiocarbonate terminal moieties. 1H NMR spectra of PS67-b-PNIPAM141 and PS67-b-PNIPAM141-CIP are shown in Figure S5. After removal of the trithiocarbonate moieties, the peak at 3.23 ppm corresponding to the methane protons on RAFT agent disappears. C
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Langmuir (TEM) result indicates that the diblock copolymer brush forms a layer around the silica particles (Figure S8). Previous theoretical and experimental studies on surfacetethered block copolymer brushes have demonstrated that the polymer brushes have nanosized phase separation on solid surfaces upon treatment with selective solvents.32−38 In the presence of free block copolymer chains, block copolymer brushes and free block copolymer chains are able to coassemble into surface micelles, a type of micelle which is covalently anchored onto the solid surface.39 In this research, we prepared surface micelles on SiO2 particles by coassembly of PS67-bPNIPAM141 brushes and free PS67-b-PNIPAM141-CIP polymer chains in water, a good solvent for PNIPAM and a precipitant for PS, and fabricated patchy micelles by cleavage of the disulfide bonds between the surface micelles and the silica particles (Scheme 1b). Silica particles with PS67-b-PNIPAM141 brushes and PS67-b-PNIPAM141-CIP free polymer chains were dissolved in DMF, a solvent for PS-b-PNIPAM, and 5-fold of doubly distilled water was added into the solution. DMF was removed after dialysis of the solution against water. After lowspeed centrifugation, the silica particles with surface micelles on the surfaces were collected at the bottom of the sediment. TGA curve of the particles with surface micelles is presented in Figure 1b (curve d). After coassembly of the polymer brushes and free polymer chains, the weight retention of the hybrid particles reduces to 77.9 wt %. The weight ratio of free polymer to polymer brushes in the surface micelles was determined by UV−vis. Silica particles with surface micelles were dispersed in DMF, and after centrifugation, silica particles were removed and free polymer chains were left in the supernatant fluid. The content of free block polymer in DMF was determined by UV− vis based on a standard curve made by the absorption of PS at 270 nm. In this research, surface micelles with two different weight ratios of free polymer to polymer brushes were prepared on the silica particles. Figure 2a shows the scanning electron microscopy (SEM) image of SiO2-SS-Py. The small-molecule-modified particles have very smooth surfaces. However, the coassembly of free block copolymer chains and block copolymer brushes results in the formation of surface micelles and rough surfaces (Figure 2b). The surface micelles were cleaved from the silica particles by 1,4-dithio-DL-threitol (DTT) in aqueous solution, and patchy micelles were obtained after centrifugation. TEM images of patchy micelles prepared at two different weight ratios of free polymer to polymer brushes are shown in Figure 2c,d. In both cases spherical micelles are observed. The sizes of the patchy micelles at dry state can be calculated based on TEM results. When the weight ratios of free polymer to polymer brushes are 4/1 and 7.7/1, the average sizes of the micelles are 32 and 57 nm, respectively, indicating an increases in the size of the patchy micelles with the weight ratio. At higher weight ratio, more “free” polymer chains are involved in the coassembly, resulting in the formation of the surface micelles with bigger sizes. Dynamic light scattering (DLS) curves of the patchy micelles are shown in Figure 2e. Both of the two different patchy micelles possess unimodal size distributions. The average hydrodynamic diameters (DH) and polydispersity indexes of the patchy micelles prepared at 4/1 and 7.7/1 are 90 nm, 0.094 and 130 nm, 0.117, also demonstrating the size of the surface micelles increases with the weight ratio. Figure 2f shows SEM image of silica particles after cleavage of surface micelles. In comparison to the silica particles with surface micelles (Figure 2b), after removal of surface micelles the silica
Figure 2. SEM images of (a) pyridyl disulfide-modified SiO2 and (b) SiO2 with surface micelles on the surfaces. (c, d) TEM images of surface micelles cleaved from the surface of SiO2 particles. In the surface micelles, the weight ratios of “free” polymer to polymer brushes are 4/1 (c) and 7.7/1 (d). (e) DLS curves of cleaved surface micelles prepared at weight ratios of 4/1 (curve 1) and 7.7/1 (curve 2). (f) SEM image of silica particles after cleavage of surface micelles from the surfaces.
particles have smooth surface, which indicates that the cleavage reaction is efficient and this method can be used to produce patchy micelles in large quantity. The patches on the micelles are occupied by thiols, so quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) chains with pyridyl disulfide terminal groups can be anchored onto the micelles through thiol−disulfide exchange reaction. q-PDMAEMA was synthesized by a combination of RAFT polymerization and quaternization reaction.14 The terminal dithiocarbonate moieties were reduced with hexylamine, and after a reaction with 2,2′-dipyridyl disulfide, q-PDMAEMA with pyridyl disulfide terminal groups was prepared. 1H NMR characterizations of q-PDMAEMA with pyridyl disulfide and dithiocarbonate moieties can be found in the Supporting Information (Figure S9). Figure 3a shows TEM image of patchy micelles with q-PDMAEMA in the patches. Because q-PDMAEMA chains are quaternized by iodomethane, and iodine on the polymer chains enhances the contrast, qPDMAEMA domains on the micelles can be observed on a TEM without staining. In the TEM image, the eyeball-like structures represent patchy micelles with q-PDMAEMA in the patches. A typical micelle is indicated by an arrow, where the dark dot in the structures is q-PDMAEMA domain, and the gray part represents a surface micelle cleaved from the silica surface. Figure 3b shows DLS curves of patchy micelles before and after grafting of q-PDMAEMA chains. After grafting of qD
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Figure 3. (a) TEM image of patchy micelles after grafting of quaternized poly(2-(dimethylamino)ethyl methacrylate) (q-PDMAEMA) to the patches. (b) DLS curves of patchy micelles before and after grafting of q-PDMAEMA to the patches through thiol−disulfide exchange reaction. The insets of (b) show a TEM image and a cartoon picture of a patchy micelle with q-PDMAEMA chains. The scale bars in (a) and (b) represent 100 nm.
Thiol groups are efficiently coupled back to the disulfides under oxidation conditions.41,42 Oxidation-induced aggregation of the patchy micelles was investigated in this research. Figures 5a−c show TEM images of the patchy micelles after standing by in air for different time. TEM results indicate that the size of the micelles increases with time. Upon cleavage from the silica particles, the size of the patchy micelles is around 32 nm. After standing by in air for 1, 3, and 5 days, the average sizes of the patchy micelles are 34, 46, and 80 nm, respectively. The change in the size of the patchy micelles is attributed to the oxidation of thiol groups in the patches and aggregation of the micelles (Scheme 1). The size change of the patchy micelles was also monitored by DLS. DLS curves of the surface micelles standing by in air for different time are shown in Figure 5d. Exposing the micellar solution to air for 1, 3, 5, and 7 days, the average DHs of the patchy micelles are 73, 304, 403, and 412 nm, respectively, also indicating the oxidation of the thiol groups and aggregation of the patchy micelles. A plot showing the change of DH with exposing time is presented in the inset of Figure 5d. Within the first 2 days, the average DH of the patchy micelles is around 73 nm, and after that the size increases with time due to the aggregation of the patchy micelles. It is noted that the size of the aggregates is kept almost unchanged after 4day storage. In a previous research, we prepared spherical micelles with thiol groups randomly distributed in the coronae and found that the sizes of the isotropic micelles increased with time.43 After 6-day oxidation, large aggregates with sizes above 1 μm as well as precipitates were observed. In another research, the fusion of polymer membrane self-assembled by cleaved diblock copolymer brushes in selective was investigated.44 The fusion process was driven by the oxidation of thiol groups into disulfides. In this research, the anisotropic nature of the patchy micelles results in directional assembly of the patchy micelles. In order to understand the oxidation-induced directional assembly of the patchy micelles, computer simulation was conducted by using soft Janus particle model.45,46 In the model, the surface of a particle is simply divided into one green repulsive and one red attractive region. The red attractive regions refer to the patches with reactive thiols. The driving force for the self-assembly of the patchy particles is the oxidation of thiols and the formation of disulfides. Because the “free” PS67-b-PNIPAM141-CIP polymer chains have the same molecular structure as the brush polymer with only the difference in the terminal groups and the weight percentage of brush polymer in a surface micelle is about 20%, it is reasonable to calculate that 20% of a micellar surface is occupied by thiol groups. In the simulation, 20% of a patchy particle surface is the
PDMAEMA, the average hydrodynamic diameter increases from 90 to 137 nm, and the polydispersity index increase from 0.094 to 0.182. A TEM image and a cartoon picture showing the structure of a patchy micelle are presented in the inset of Figure 3b. The ξ-potential of the patchy micelles was measured. After grafting of q-PDMAEMA, the ξ-potential of the micelles changes from −6.3 to 20 mV, demonstrating the anchoring of q-PDMAEMA onto the patches. Thiol groups are produced in the patches upon cleavage of the disulfides with DTT, so gold nanoparticles (AuNPs) are able to be anchored onto the patchy micelles through S−Au bonds. Herein, AuNPs with an average size of 15 nm were employed.40 Figure 4a shows TEM image of patchy micelles
Figure 4. (a) TEM image of patchy micelles with AuNPs in the patches. (b) A cartoon picture showing the structure of the aggregated patchy micelles with AuNPs. The scale bar in the inset of (a) represents 100 nm.
labeled with AuNPs. The TEM specimen was stained by exposing to OsO4 vapor, and PNIPAM phases were stained. In the image, asymmetrical particles in the range from 46 to 120 nm were observed. In a particle the gray part represents stained patchy micelle, and the deep dark part represents AuNPs aggregated in the patches. Because one AuNP may bind to several patchy micelles, the sizes of the AuNPs-labeled micelles are bigger than the patchy micelles cleaved from the silica particles (Figure 2c). A magnified TEM image showing the structure of a typical aggregated micelle is shown in the inset of Figure 4a. The asymmetric structure of the micellar aggregate demonstrates AuNPs-induced assembly of the patchy micelles. A cartoon picture showing the structure of the aggregated patchy micelles is presented in Figure 4b. In an aggregated structure, AuNPs binding to different patches hold the structure together. E
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Figure 5. (a−c) TEM images of surface micelles after standing by in air for (a) 1, (b) 3, and (c) 5 days. (d) DLS curves of surface micelles after standing by in air for 1 day (curve 1), 3 days (curve 2), 5 days (curve 3), and 7 days (curve 4). The inset shows a plot of hydrodynamic diameter of the patchy micelles vs storage time. The scale bars in the TEM images represent 200 nm.
Figure 6. Relative distribution of neighboring patchy micelles in self-assembled structures. The distributions have been normalized so that ∑sN(s) = 1. The insets show equilibrium self-assembly structures of the patchy particles (right side) and a TEM image of the aggregates of patchy micelles (left side).
4. CONCLUSIONS In conclusion, we have demonstrated that patchy micelles with thiol groups in the patches can be prepared by cleavage of the surface micelles coassembled by free polymer chains and block copolymer brushes on silica particles. Upon cleavage of the surface micelles from the silica particles, patchy micelles with thiol groups in the patches are produced. AuNPs can be anchored onto the patchy micelles through Au−S bonds, forming asymmetrical aggregates or isolated micelles with AuNPs. The thiol groups can be oxidized to disulfides, which results in directional assembly of the patchy micelles. This research provides a novel method to prepare asymmetric structures, and the structures will find applications in many scientific communities from the fundamental studies to commercial applications.
attractive region. Based on our computer simulation result, the distribution N(s) of the number of patchy micelles (s) in an aggregated structure is shown in Figure 6a. The value of s is mainly in the range of 2−5. A typical equilibrium self-assembled structures is shown in Figure 6b. The patchy particles aggregate together through the interaction between the attractive regions, and most of the aggregated structures are composed of 2−5 particles. A magnified TEM image of a typical aggregated structure is shown in the inset of Figure 6a. As indicated by the red lines, the structure is composed of three patchy micelles, which demonstrates that the simulation result agrees well with our experimental result. A model and simulation details can be found in the Supporting Information. F
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra, UV−vis spectra, SEC curves of polymers, and computer simulation details. 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] (H.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (NSFC) under Contracts 21074058 and 51473079 and the National Basic Research Program of China (973 Program, 2012CB821500).
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