Article Cite This: Chem. Mater. 2019, 31, 5264−5273
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Symmetry Transitions of Polymer-Grafted Nanoparticles: Grafting Density Effect Hongseok Yun,† Ji Woong Yu,‡ Young Jun Lee,† Jin-Seong Kim,† Chan Ho Park,† Chongyong Nam,‡ Junghun Han,† Tae-Young Heo,§ Soo-Hyung Choi,§ Doh C. Lee,† Won Bo Lee,*,‡ Gila E. Stein,*,∥ and Bumjoon J. Kim*,†
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Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea § Department of Chemical Engineering, Hongik University, Seoul 04066, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: We examined the packing structure of polystyrene-coated gold nanoparticles (Au@PS) as a function of grafting density. A series of Au@PS nanoparticles with grafting densities in the range of 0.51−1.94 chains nm−2 were prepared by a ligand exchange process using thiol-terminated PS and then selfassembled at a liquid−air interface. We observed a transition from disordered to bodycentered cubic (bcc) to face-centered cubic (fcc) arrangements with increasing grafting density, even though the ligand length-to-core radius ratio (λ) was as high as 3.0, a condition that typically favors nonclose-packed bcc symmetry in the self-assembly of hard nanoparticles. To explain this phenomenon, we define λeff to include the concentrated polymer brush regime as part of the “hard core”, which predicts that the softness of Au@PS nanoparticles is reduced from 1.53 to 0.14 in a theta solvent as the grafting density increases from 0.51 to 1.94 chains nm−2. This new definition of λ can also predict the effective radii of nanoparticles using the established optimal packing model. The experimental findings are supported by a combination of coarse-grained molecular dynamics simulation and adaptive common neighbor analysis, which show that changes in grafting density can drive the observed transitions in nanoparticle packing. These studies provide new insights for controlling the selfassembled symmetries of polymer-coated nanocrystals using a simple ligand exchange process to tune particle softness.
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INTRODUCTION Ligand-coated colloidal nanoparticle (NP) superlattices have novel collective properties that are not equal to the mere sum of the constituent properties.1−5 The ability to modulate the position, orientation, and interparticle spacing of NPs, as well as the symmetry of the self-assembled structures, is of great interest for applications in electronics,6,7 photonics,8−11 and magnetics.1,12−14 One of the critical factors that control the self-assembled structure of NPs is the softness parameter, λ, which is the ratio of the extended ligand length (L) to the core radius (R).15−17 When λ is less than 0.6−0.7, the self-assembly behavior of NPs is like “hard spheres” which favors the formation of close-packed structures with face-centered cubic (fcc) or hexagonal close-packed (hcp) symmetries. By contrast, when λ is greater than 0.6−0.7, NPs become more like “soft spheres” and prefer to adopt a nonclose-packed body-centered cubic (bcc) symmetry via extension and compression of soft organic ligands, particularly when the ligand shells can interpenetrate. Recently, polymeric ligands have been used to not only promote new functions, such as biocompatibility,18,19 stimuliresponsiveness,20−22 and physicochemical stability,23−25 but © 2019 American Chemical Society
also to provide the benefit of expanding the structural diversity of NP superlattices. Alivisatos and Xu et al. reported the use of polymeric ligands for building binary NP superlattices with tunable symmetries and interparticle distances.26 These authors controlled the effective diameter of polymer-grafted NPs by changing the molecular weight (Mn) of the polymeric ligands.26−28 However, in addition to Mn, the effective diameter of polymer-coated NPs could be controlled by changing the grafting density (∑), as ∑ controls the penetrability of the polymeric shell. It is well-known that there are three categories of ∑-dependent polymer conformations: (i) “dilute brush” regime at low ∑, where polymer chains are contracted into the form of a “mushroom”; (ii) “semidilute polymer brush (SDPB)” regime, where polymer chains are stretched away from the surface to reduce steric hindrance between the chains; and (iii) “concentrated polymer brush (CPB)” regime, where polymer chains are highly extended.29−32 Thus, it is important to investigate the effects Received: April 30, 2019 Revised: June 27, 2019 Published: June 27, 2019 5264
DOI: 10.1021/acs.chemmater.9b01699 Chem. Mater. 2019, 31, 5264−5273
Article
Chemistry of Materials of ∑ on the self-assembly of polymer-coated NPs, as this parameter could offer another handle for precise control of the superstructure and related properties. Herein, we examine the self-assembly of polystyrene-coated Au NPs (Au@PS NPs) as a function of ∑. A series of Au@PS NPs were prepared by a simple ligand exchange process using thiol-terminated PS (PS−SH). The ∑ of PS was varied from 0.51 to 1.94 chains nm−2 by changing the Au-to-PS feed ratio during the ligand exchange process. Using a combination of transmission electron microscopy (TEM) and X-ray scattering, we detected a transition from disordered to ordered NP superlattices as ∑ was increased from 0.51 to 0.71 chains nm−2. Surprisingly, a transition from bcc to fcc symmetry was observed as ∑ was increased from 1.10 to 1.53 chains nm−2, even though λ was as high as 3.0. This outcome stands in contrast to the cases of NPs coated with short alkyl chains, where the bcc structure is favored when λ exceeds 0.6−0.7. This phenomenon can be explained by considering the CPB regime of the polymer shell as a part of the “hard core” and the rest of the polymer chains as a “soft shell”. This approach showed not only decreasing softness upon the increase of ∑ but also successfully predicted the effective radii of NPs using the optimal packing model (OPM). To aid in the interpretation of experimental data, molecular simulations were performed with a coarse-grained (CG) model. Using the adaptive common neighbor analysis (CNA) method, we showed that increases in ∑ significantly enhance the potential energy between NPs, which drives the observed structural transition from disordered to bcc and to fcc. Our findings provide key insights into the role of ∑ on interparticle interactions and self-assembled symmetries of polymer-grafted inorganic NPs. Importantly, this approach allows us to design materials consisting of NPs with precisely tuned packing symmetries and interparticle distances in the regime that cannot be accessed by hard NPs. This can be used to manipulate interparticle interactions such as magnetic,1,33 plasmonic,34,35 and electronic coupling.36,37
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added to the solution for initiating polymerization. Subsequently, styrene (15 g) was poured into the solution. After 8 h, ethylene sulfide (0.392 mL) was injected into the solution. When the ethylene sulfide was injected, the color of the solution gradually turned from red to orange. When the color of the solution stopped changing, anhydrous methyl alcohol (5 mL) was added for terminating the reaction. After the solution was precipitated by excess methanol, PS−SH was obtained as a white powder (14 g). The number-average molecular weight (Mn) and dispersity (Đ) were 1.8 kg mol−1 and 1.1, respectively. Mn and Đ of PS−SH were analyzed by size exclusion chromatography (waters 2414) using UV and refractive index detectors, which was calibrated by PS standards with THF in 40 °C. Ligand Exchange with PS−SH. The surface ligands on the Au NPs were replaced by a simple ligand exchange process. For the ligand exchange, as-synthesized Au NPs were dissolved in THF to a concentration of 5 mg mL−1. A PS−SH solution in THF was prepared separately with a polymer concentration of 0.31 mg mL−1. The Au NP solution and PS−SH solution were mixed at a 1:1 volume ratio and magnetically stirred for 12 h at room temperature. The PS-coated NPs were precipitated by ethanol and centrifuged at 8000 rpm for 10 min. The supernatant was removed and then the Au@PS NPs were redispersed in toluene. This procedure was repeated 5−6 times to get rid of excess unbound PS−SH. The NPs were finally dispersed in toluene at a concentration of 3 mg mL−1. The resulting Au@PS NPs had the grafting density (∑) of 0.51 chains nm−2. To further increase ∑ to 0.71, 1.10, 1.53, and 1.94 chains nm−2, PS solutions with PS concentrations of 0.625, 1.25, 2.5, and 5 mg mL−1 were used, respectively. Liquid−Air Interface Assembly. In a typical assembly process for Au@PS, Au@PS NP of 3 mg mL−1 in toluene was used. 15 μL of the solution was dropped on an ethylene glycol (EG) surface in a home-made Teflon well (1.5 × 1.5 × 1.5 cm3). The well was covered by a glass and solvent was slowly evaporated (overnight). A pink solid film was formed on top of the EG surface. A substrate (i.e., TEM grid or silicon wafer) was placed under the film using tweezers and then gently lifted upward to transfer the film to the substrate. The film was dried under vacuum overnight to get rid of residual EG from the substrate. Samples on TEM grids and silicon wafers were used for TEM analysis and grazing incidence X-ray scattering (GIXS) measurements, respectively. Each sample showed the same packing symmetry (fcc or bcc) every time the self-assembly experiment was performed (each self-assembly experiment was repeated at least three times). Characterization. TEM images were obtained using a JEM-3011 HR microscope operating at 300 kV. The samples for GIXS measurements were prepared on a silicon substrate via liquid−air interface assembly. The X-ray scattering measurements were performed on beamline 9A in the Pohang Accelerator Laboratory (South Korea). The samples were irradiated with an 11.055 keV (λ = 0.112 nm) X-ray beam at a sample-to-detector distance of 2.5 m. The off-specular scattering was recorded using a MAR-CCD area detector (1920 × 1920 pixels, 0.0886 nm pixel size). The incidence angle of the X-ray beam was varied in the range of 0.05−0.25°. Detailed procedures for GIXS data analysis are included in the Supporting Information.
EXPERIMENTAL SECTION
Chemicals. HAuCl4·3H2O (99%), oleylamine (OAm, technical grade), tetralin (technical grade), tert-butylamine-borane complex (TBAB), styrene (99%), ethylene sulfide (98%), anhydrous cyclohexane, anhydrous methyl alcohol, and sec-butyllithium solution (secBuLi, 1.6 M in hexane) were purchased from Sigma-Aldrich. Styrene and ethylene sulfide were dried over calcium hydride and then distilled at reduced pressure. Distilled styrene, ethylene sulfide, and cyclohexane were degassed with three cycles of freeze−pump−thaw. sec-BuLi and methyl alcohol were used as received. Synthesis of Au NPs. An orange-colored precursor solution of tetralin (5.0 mL), OAm (5.0 mL), and HAuCl4·3H2O (50 mg) was prepared in air at 22 °C and magnetically stirred under N2 for 10 min. A solution containing 0.25 mmol of TBAB, tetralin (500 μL), and OAm (500 μL) was sonicated for mixing and injected into the precursor solution for reducing the Au precursor. The solution color changed from orange to deep purple within 5 s, indicating the reduction of the precursor. After 1 h, 20 mL of hexane was added and the solution was precipitated by adding 60 mL of ethanol to obtain the Au NPs. The Au NPs were collected by centrifugation (8000 rpm, 2 min) and washed with ethanol again and redispersed in tetrahydrofuran (THF). The average diameter of the Au NPs was 3.10 ± 0.24 nm. Synthesis of Thiol-Terminated Polystyrene (PS−SH). PS−SH was synthesized by living anionic polymerization as previously reported.27 A 1000 mL three-neck round-bottomed flask containing cyclohexane (250 mL) was heated to 35 °C. sec-BuLi (10.03 mL) was
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RESULTS AND DISCUSSION Au NPs were synthesized using a previously reported method38 and a representative TEM image is presented in Figure S1. The average diameter of the Au NPs was 3.10 nm with a very narrow size distribution (
