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Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions Jae-Min Ha, Sung-Hwan Lim, Jahar Dey, Sang-Jo Lee, MinJae Lee, Shin-Hyun Kang, Kyeong Sik Jin, and Sung-Min Choi Nano Lett., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions Jae-Min Ha, † Sung-Hwan Lim, † Jahar Dey, † Sang-Jo Lee, † Min-Jae Lee, † Shin-Hyun Kang, † Kyeong Sik Jin, ‡ and Sung-Min Choi †*

†

Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea

‡

Pohang Accelerator Laboratory, Pohang, Gyeongbuk 37673, Republic of Korea

* Correspondence to [email protected]

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ABSTRACT Nanoparticle superlattices (NPSLs) are of great interests as materials with designed emerging properties depending on the lattice symmetry as well as composition. The symmetry transition of NPSLs depending on environmental conditions can be an excellent ground for making new stimuli-responsive functional materials. Here, we report a spherical micelle-assisted method to form exceptionally ordered NPSLs which are inherently sensitive to environmental conditions. Upon mixing functionalized gold nanoparticles (AuNPs) with a nonionic surfactant spherical micellar solution, NPSLs of different symmetries such as NaZn13, MgZn2, and AlB2-type are formed depending on the size ratio between micelles and functionalized AuNPs, and composition. The NPSLs formed by the spherical micelle-assisted method show thermally reversible order-order (NaZn13–AlB2) and order-disorder (MgZn2–isotropic) symmetry transitions, which are consistent with the Gibbs free energy calculations for binary hard-sphere model. This approach may open up new possibilities for NPSLs as stimuli-responsive functional materials.


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KEYWORDS: nanoparticle superlattice, spherical micelle, stimuli-responsive symmetry transition, small-angle x-ray scattering.


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The NPSLs can exhibit new emerging properties through collective coupling between nanoparticles, which is dependent on the lattice symmetry as well as particle composition, providing new opportunities in a broad range of potential applications including plasmonic1,2, optoelectronic3, and catalytic4–6 applications. Single-component, binary or multi-component NPSLs of different symmetries have been successfully synthesized by using an interplay of entropy, van der Waals, electrostatic, hydrogen bonding, and/or other interactions.7–15 Recently, the structural symmetry transitions of NPSLs depending on external stimuli, which can be used as a new route for stimuli-responsive functional devices, has attracted great attention. The symmetry transitions of NPSLs were observed upon heating the NPSLs prepared by the slow evaporation method16,17 or introducing reprogramming strands into the NPSLs prepared by the DNA-mediated method18, which are irreversible. The reversible symmetry transitions of NPSLs depending on temperature19, solvent vapor pressure20,21, cyclic hydration-dehydration of ligand molecules22 or dynamic modulation of DNA bonding behavior by specific chemical stimuli23 were also reported. However, reports on the stimuli-responsive symmetry transitions of NPSLs are still very limited. To explore this more systematically and extensively, a new method to fabricate NPSLs which are inherently sensitive to stimuli is essential. Amphiphilic molecules such as surfactants and block copolymers show a wide spectrum of highly ordered nanostructures and exhibit a rich phase behavior depending on external stimuli such as temperature24, pressure25 and others26. The amphiphilic molecular systems have been extensively used as excellent templates to form 1D, 2D, and 3D nanoparticle assemblies, in which the selective affinity of functionalized nanoparticles with specific domains or interfaces is typically used.27–29 While this approach provides well-ordered or nano-patterned hybrid nanoparticle systems, the degree of ordering is not quite comparable to the typical NPSLs. The


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spherical micellar systems at high concentration exhibit highly ordered crystalline structures such as body-centered cubic (bcc), face-centered cubic (fcc) and hexagonally close-packed (hcp). Considering that the spherical micellar system at the crystalline phase is essentially similar to the single-component spherical NPSLs, it can be an excellent template or matrix to form NPSLs, possibly providing the stimuli-responsive nature to the NPSLs. However, the spherical micelletemplated or assisted synthesis of NPSLs has not been reported yet. Here, we report for the first time the spherical micelle-assisted formation of highly ordered gold nanoparticle (AuNP) superlattices and their thermally reversible symmetry transition. When AuNPs grafted with thiolated poly(ethylene glycol) (SH-PEG) are mixed with the non-ionic surfactant pentacosa(ethylene glycol) monododecyl ether (C12E25) spherical micellar system, exceptionally ordered binary NPSLs with different symmetries (such as AlB2, NaZn13, and MgZn2-type) are formed depending on the size ratio between micelles and PEG-functionalized AuNPs. All the structures observed in this study are well explained by the binary hard-sphere models. The NPSLs formed by the spherical micelle-assisted method show thermally reversible order-order (NaZn13–AlB2) and order-disorder (MgZn2–isotropic) symmetry transitions. This transition is consistent with the phase diagram predicted by Gibbs free energy calculations for binary hard-sphere models,30,31 showing that the spherical micelle-assisted method is an equilibrium method.


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Figure 1. Schematics of experimental design. 3.7 nm AuNPs functionalized with SH-PEG of different molecular weight 1 or 2 kDa (denoted as Au-PEG-1 or -2) are mixed with C12E25 micellar solution at the bcc phase. The mass ratio of C12E25 and water was kept constant at (40/60) and a different amount of Au-PEG-1 or -2 was added to each sample. Solid core with a semi-transparent shell is also used to present micelle and functionalized AuNPs in a simplified form. The molecular structures of C12E25 and SH-PEG (1 and 2kDa) are shown in the box.

Nonionic surfactants C12E25 in water form a highly ordered bcc phase at room temperature when the C12E25/water mixing ratio is 40/60 by weight. Monodisperse 3.7 nm AuNPs functionalized with thiolated PEG of two different molecular weights 1kDa and 2kDa, respectively (denoted as Au-PEG-1 and Au-PEG-2), were prepared (Supporting Information,


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Figures S1, S2, S3, and S4). Here, PEG which is same as the hydrophilic part of C12E25, pentacosa(ethylene glycol), was selected as ligand molecules to make the interaction of functionalized AuNPs with micelles simple. Different amounts of Au-PEG-1 and Au-PEG-2 were added to a solution of C12E25/water at a fixed mixing ratio (40/60 by weight) (Figure 1). All mixtures showed no visual aggregation after 1 hour of vortex-mixing and more than 50 times of alternative centrifugation. Small-angle x-ray scattering (SAXS) measurement of the C12E25/water (40/60 by weight) was performed to understand the C12E25 micellar system which is used as a templating matrix in this study. The SAXS intensities of the C12E25/water mixture show that the mixture exists in a highly ordered bcc phase at room temperature and makes a transition to the isotropic phase at 45 °C (Supporting Information, Figure S5), which is consistent with a previous study.32 The lattice parameter estimated from the SAXS intensity at 25 °C is 9.0 nm. Assuming that the micelles in the bcc phase just touch each other, the diameter of C12E25 spherical micelles in the bcc phase was estimated to be 7.8 nm. The SAXS intensities of Au-PEG-1/water (40/60 by volume) and Au-PEG-2/water (40/60 by volume) at 25 °C show that the mixtures exist in the fcc phase (Supporting Information, Figure S6). Here, the volume fraction of Au-PEG-1 and -2 was set to the volume fraction of C12E25 in the C12E25/water (40/60 by weight. The mass density of C12E25 is close to 1 g/cm3). In a similar way, the diameters of the Au-PEG-1 and -2 were estimated to be 10.7 nm and 13.2 nm, respectively. Therefore, the size ratios (γ) of C12E25 micelle to Au-PEG-1 and -2 are 0.73 and 0.59, respectively.


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Figure 2. SAXS measurements of Au-PEG/C12E25/water mixture at different mixing ratios. SAXS intensities of (a) Au-PEG-1/C12E25/water (mp/40/60, mp = 6, 8, 10, 15, 31 for which the number ratios, Au-PEG-1 : C12E25 micelle, are 1:40, 1:30, 1:24, 1:16, and 1:7.5, respectively) and (b) Au-PEG-2/C12E25/water (mp/40/60, mp = 9, 11, 15, 23, 46 for which the number ratios, Au-PEG-2 : C12E25 micelle, are 1:33, 1:26, 1:19, 1:13, and 1:6.3, respectively) at 25°C. The SAXS intensity of C12E25/water (40/60) is shown at the bottom of each graph for comparison. SAXS intensities are shifted vertically for visual clarity. 2-D SAXS patterns of (c) Au-PEG1/C12E25/water

(31/40/60),

(d)

Au-PEG-2/C12E25/water

(15/40/60),

and

(e)

Au-PEG-

2/C12E25/water (46/40/60) are shown on the right.


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SAXS measurements of the Au-PEG-1/C12E25/water and Au-PEG-2/C12E25/water at 25 °C were performed with different amounts of Au-PEG-1 (mp/40/60 by weight, mp = 6, 8, 10, 15, 31), and Au-PEG-2 (mp/40/60 by weight, mp = 9, 11, 15, 23, 46) (Figure 2). Here, the mass ratio of C12E25 and water was kept constant at (40/60) and a different amount of Au-PEG-1 or -2 (mp) was added to each sample. It is remarkable that the SAXS intensities of the mixtures show new sharp peaks with high-order peaks up to about 40th order, which is rarely observed for NPSLs or any other ordered colloidal systems. This clearly indicates that exceptionally highly ordered NPSLs are formed in the binary mixtures. The SAXS patterns of Au-PEG1/C12E25/water and Au-PEG-2/C12E25/water are clearly different from each other, indicating that superlattices of two different symmetries are formed depending on the molecular weight of PEGfunctionalized on AuNPs, i.e. the size ratio between C12E25 micelle and Au-PEG. While the SAXS intensities of Au-PEG-1/C12E25/water show the same symmetry at all the compositions measured in this study, those of Au-PEG-2/C12E25/water change as mp is increased to 46, indicating a symmetry transition. Here, it should be noted that since the electron density difference between AuNPs and micelles are very large (more than 10 times difference), the SAXS intensities are dominated by the AuNP-AuNP correlations, making the scattering from the micelle-micelle correlation not observable.


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Figure 3. Structure factors of Au-PEG/C12E25/water mixtures. Structure factors of (a) Au-PEG1/C12E25/water (31/40/60) and (b) Au-PEG-2/C12E25/water (15/40/60) are well-indexed by P63/mmc and Fm-3c space group, respectively. (c) Au-PEG-2/C12E25/water (46/40/60) shows a mixed phase which is well-indexed by Fm-3c and P6/mmm space group (for clarity, only the indexing by P6/mmm space group is shown). The measured structure factors were obtained by dividing the measured SAXS intensity by the particle form factor of AuNP. The calculated structure factors using the FullProf program33 are shown at the bottom of each figure for comparison. In the schematics for the crystalline structures (on the right), the simplified notations given in Figure 1 are used to present Au-PEG-1, -2, and C12E25 micelles.


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The structure factors of the Au-PEG-1/C12E25/water mixtures are well-indexed with the space group P63/mmc (Figure 3a). The lattice parameters for the Au-PEG-1/C12E25/water (31/40/60) mixture are a = 16.0 nm and c = 26.0 nm and the domain size estimated from Scherrer’s equation is ca. 420 nm (Supporting Information, Table S1 for the lattice parameters and the domain size of other compositions). To identify the crystalline structure of the Au-PEG-1/C12E25/water mixture, the measured structure factor was compared with the simulated ones33 for different crystalline structures which belong to the space group P63/mmc. The simulated structure factor for MgZn2type (here, AuNPs and C12E25 micelles are placed at the positions of Mg and Zn, respectively, and the measured lattice parameters are used) reproduces the relative intensity distribution as well as the peak positions of the measured structure factor (Figure 3a). However, other crystalline structures cannot reproduce the relative intensity distribution of the measured structure factor. The validity of the MgZn2-type structure is checked with packing geometry consideration. When Au-PEG-1 (with the diameter of 10.7 nm) and C12E25 micelle (with the diameter of 7.8 nm) are placed at the positions of Mg and Zn, respectively and the measured lattice parameters are used, all the nearest neighboring components touch each other, overlapping within 0.5 nm or less. Considering the uncertainty of the particle size estimation in the concentrated condition, the small overlap between neighboring components is understandable. The relative scattering peak intensity and the packing geometry consideration strongly indicate that Au-PEG-1/C12E25/water mixture investigated in this study form the MgZn2-type structure.

The structure factors of the Au-PEG-2/C12E25/water mixtures (mp/40/60, mp = 9, 11, 15, 23) shows a peak position ratio of 1:√2:√3:√4:√5:√6:√8:√9:√10, up to about √40. This clearly indicates that the Au-PEG-2 particles form a highly ordered simple cubic lattice. The nearest


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neighboring center-to-center distance between Au-PEG-2 particles estimated from the SAXS pattern is 18.3 nm for Au-PEG-2/C12E25/water (15/40/60) mixture (Figure 3b). Considering that the nearest neighboring particle distance is ca. 5 nm larger than the diameter of Au-PEG-2 (13.2 nm), it is clear that the simple cubic structure cannot be formed with Au-PEG-2 only. To understand the crystalline structure of Au-PEG-2/C12E25/water, we should consider binary superlattices which contain Au-PEG-2 at the four corners of cubes and C12E25 micelles at some positions within the cubes. Since the Pm-3m or Fm-3c space group can reproduce the measured scattering pattern when Au-PEG-2 particles are placed at the corner of cubic units regardless of the C12E25 micelle positions (due to the very low electron density of micelles compared with AuNP), different crystalline structures which belong to the Pm-3m space group (CsCl, AuCu3, and CaB6) or the Fm-3c space group (NaZn13) are considered. When Au-PEG-2 (with a diameter of 13.2 nm) and C12E25 micelle (with a diameter of 7.8 nm) are placed at the positions of Na and Zn, respectively, in the NaZn13-type structure (with the measured nearest neighboring distance between Au-PEG-2 particles), all the nearest neighboring components touch each other within 0.2 nm or less. In this case, the packing density is estimated to be 0.72, which is much higher than the packing densities estimated for other crystal symmetries. The packing densities for CsCl, AuCu3 and CaB6-type structures estimated in a similar way are 0.24, 0.32, and 0.44, respectively. The packing geometry consideration together with the SAXS patterns strongly indicates that Au-PEG-2/C12E25/water mixture (mp/40/60, mp = 9, 11, 15, 23) form the NaZn13type structure.

When the mixing ratio of the Au-PEG-2/C12E25/water mixture is 46/40/60, a set of new peaks appear in addition to the NaZn13–type correlation peaks, indicating the co-existence of different


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crystalline structures. Since the number ratio (Au-PEG-2:C12E25 micelle) in the mixture is 1:6.3, the number of C12E25 micelles is insufficient to form the NaZn13 structure, and other structure should appear. The new peaks are well-indexed with the space group P6/mmm and the lattice parameters are a = 13.3 nm and c = 13.6 nm (Figure 3c). Since the lattice parameters are very close to the diameter of Au-PEG-2 (13.2 nm) (i.e., all Au-PEG-2 particles are almost touching each other), the AlB2-type structure is only possible structure among the space group P6/mmm which can accommodate C12E25 micelles (7.8 nm) within the unit cell. When Au-PEG-2 and C12E25 micelles are placed at the positions of Al and B, respectively, in the AlB2-type structure (with the measured lattice parameters), the neighboring Au-PEG-2 and C12E25 micelle touch each other within 0.3 nm or less. This indicates that the AlB2 and NaZn13-type structures co-exist in the Au-PEG-2/C12E25/water (46/40/60) mixture.


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Figure 4. Space-filling curves for different crystalline symmetries. The packing density of binary hard-sphere system versus size ratio is calculated for AlB2, NaZn13, and MgZn2-type symmetries, respectively. The three colored bands (with width marked by arrows) correspond to the range of stability for each symmetry predicted by computer simulations.

The entropy-driven particle packing to maximize the free-volume available to individual particles has been widely employed to explain the crystallization mechanism of binary superlattices of nanoparticles with two different sizes, although all other interactions need to be considered for a full understanding.7,12,13,30 Simulation studies using the hard-sphere model show that AlB2, NaZn13, and Laves (MgZn2, MgNi2, and MgCu2-type) structures can be stabilized by entropy alone depending on the size ratio (γ). The AlB2 is stable for 0.45 ≤ γ ≤ 0.61, NaZn13 is stable for 0.54 ≤ γ ≤ 0.625, and the Laves structures are stable for 0.76 ≤ γ ≤ 0.84, respectively.30,31,34,35 Theoretical packing densities are calculated for the binary hard-sphere model of different symmetries as a function of γ (Figure 4), in which the ranges of stability are marked with colored bands and arrows. The binary superlattice structures observed in this study,


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MgZn2 (for Au-PEG-1/C12E25/water, γ = 0.73), NaZn13 (for Au-PEG-2/C12E25/water, γ = 0.59) and AlB2 + NaZn13 (for Au-PEG-2/C12E25/water at 46/40/60, γ = 0.59) are consistent with the predictions made by the binary hard-sphere models. Among the three Laves structures, only MgZn2-type structure is observed, which can be attributed to the fact that MgZn2-type structure has the lowest free energy per particle among the Laves phases although the difference is small.31 In the Au-PEG-2/C12E25/water system (γ = 0.59), when the number ratio (Au-PEG-2:C12E25 micelle) is in the range of 1:33 ~ 1:13, only NaZn13-type structure is observed (Figure 2b). When the number ratio becomes 1:6.3, however, AlB2-type structure is formed, resulting in the coexistence of NaZn13 and AlB2-type structures. This is consistent with the simulation for the binary hard-sphere models by Eldridge et al30 which predicts that when γ = 0.58, the phase transition from the NaZn13 + fluid to the AlB2 + NaZn13 structure occurs as the number ratio changes across 1:13. All these indicate that the binary mixture of Au-PEG-1 or -2 and C12E25 micelles in water form thermodynamically stable binary superlattices, as predicted by the hardsphere model. Binary NPSLs of a rich structural diversity have been observed when mixtures of inorganic nanoparticles grafted with organic molecules are slowly evaporated.7 Although the majority of reported binary NPSLs follow the space-filling rules expected for hard spheres, many observed binary NPSL structures could not be explained by the hard sphere model. These deviations from the hard sphere model have been attributed to the softness of coronal layers of functionalized nanoparticles.36,37 It is interesting to note that the binary mixture of Au-PEG-1 or -2 and C12E25 micelles in water, both of which have molecular outer shells, follows the hard-sphere model very well. The organic coronal layers of Au-PEG-1 or -2 and C12E25 micelles are made of the same molecular units (ethylene glycol) although their rigid cores are different. Therefore, the


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interaction between Au-PEG-1 or -2 and C12E25 micelles in aqueous solution may be essentially similar to the interaction between non-ionic spherical micelles in water. The underlying micellemicelle interaction in water comes from the excluded volume effect (i.e. repulsion) to which both of the molecular chains and the hydrated water molecules contribute.38 The range of intermicellar interaction depends on the ratio of the coronal layer thickness to the core radius39 although the grafting density40 should be also considered. When the ratio is relatively small, the repulsive interaction is short-ranged and micelles behave as hard spheres, forming the fcc structure at a high micellar concentration. When the ratio is relatively large, the repulsive interaction becomes long-ranged and micelles form the bcc structure. It should be noted that the Au-PEG-1 and -2 in water form the fcc structure at a mixing ratio of 40/60 (by volume) while C12E25 micelles form the bcc structure. This indicates that Au-PEG-1 and -2 behave as hard spheres while C12E25 micelles behave as soft spheres. Therefore, to a first approximation, the mixture of Au-PEG-1 or -2 and C12E25 micelles may be considered as a mixture of hard spheres and soft spheres of different sizes. Although full understanding is yet to be made, it is conjectured that the hard sphere-like nature of Au-PEG-1 or -2 may make its mixture with C12E25 micelles to follow the hard sphere model.


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Figure 5. Structure factors of (a) Au-PEG-1/C12E25/water (31/40/60), (b) Au-PEG2/C12E25/water (15/40/60) and upon heating (left) and cooling (right). The temperature was varied in 5 °C step and the structure factors at selected temperature are shown here. The structure factors are shifted vertically for visual clarity.


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To investigate the effect of temperature on the superlattice structures, SAXS measurements were performed with increasing temperature from 25 °C to 65 °C in 5 °C step (Figure 5). Before each measurement, the samples were stabilized for 15 minutes after reaching each target temperature. In the case of Au-PEG-1/C12E25/water (31/40/60) which shows MgZn2-type structure at 25 °C, the scattering peak intensity gradually decreases with temperature and completely disappear at 65 °C, indicating a structural transition from the MgZn2-type to the isotropic phase (Figure 5a). Upon cooling down to 25 °C (in 5 °C step with 15 minutes of stabilization time at each temperature), the system returns to the MgZn2-type structure with hysteresis. In the case of Au-PEG-2/C12E25/water (15/40/60) which shows NaZn13-type structure at 25 °C, the scattering peak intensity remains almost the same up to 60 °C (Figure 5b). As the temperature is further increased to 65 °C, all the correlation peaks for the NaZn13-type structure completely disappear and the new peaks corresponding to the AlB2-type structure appear (Supporting Information, Figure S7 for peak indexing), indicating a symmetry transition from the NaZn13-type to the AlB2-type structure. Upon cooling down to 25 °C, the NaZn13-type structure re-appears with hysteresis. At 25 °C, the scattering peaks corresponding to the AlB2-type structure still remains, possibly because the stabilization time at each temperature is not sufficient for the highly viscous sample. In the case of Au-PEG-2/C12E25/water (46/40/60) (Supporting Information, Figure S8) which shows a mixed (NaZn13 + AlB2) phase at 25 °C, there is no significant change in the scattering pattern up to 60 °C. As the temperature is further increased to 65 °C, all the correlation peaks for the NaZn13-type structure completely disappear and only the correlation peaks for the AlB2-type structure remain, indicating a symmetry transition from the NaZn13 + AlB2 to the AlB2-type structure. Upon cooling down to 25 °C, the system returns to the mixed phase with hysteresis. All the results clearly indicate that the NPSLs


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formed by the spherical micelle-assisted method exhibit thermally reversible symmetry transitions. The large thermal hysteresis indicates that the relaxation time of the NPSLs is long, possibly due to the high viscosity of the systems. Eldridge et al30 and Filion et al31 calculated the Gibbs free energy for the binary hard-sphere models with different size ratios, γ, and constructed phase diagrams as a function of reduced pressure and composition, where the reduced pressure (p*) is determined by pressure (P), temperature (T), and diameter of large particle (d), p* = Pd3/kT. Our experimentally observed thermotropic symmetry transitions were compared with the phase diagrams determined by the free energy calculation for the binary hard-sphere models (for γ = 0.76 and 0.58 which are very close to γ = 0.73 in Au-PEG-1/C12E25/water and 0.59 in Au-PEG-2/C12E25/water, respectively). For these comparisons, the reduced pressures for the experimental conditions (P, d, T) investigated in this study were calculated and used. The experimentally observed thermotropic symmetry transitions were consistent with the calculated phase diagrams although there are shifts in the transition temperature. The calculated phase diagram31 for γ = 0.76 shows that the binary hard-sphere mixture (when the number ratio of large and small particles is 1:2 or higher) is predicted to exist in the MgZn2 + fccs mixed phase at ambient pressure and 25 °C, and make a transition to the fluidic phase at higher temperature. Here, the fccs stands for the fcc structure formed by small particles (in our system, C12E25 micelles are small particles which could not be observed in the SAXS measurements due to their much lower electron density compared with AuNPs). The experimentally observed phase transition of Au-PEG-1/C12E25/water (31/40/60, for which the number ratio is 1:7.5) with increasing temperature is consistent with the calculated phase diagram (from the MgZn2 to the fluidic phase) with some shift of transition temperature. The


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calculated phase diagram30 for γ = 0.58 shows that the binary hard-sphere mixture at ambient pressure and 25 °C is predicted to make transition from the NaZn13 + fluids phase to the NaZn13 + AlB2 phase as the number ratio is changed across 1:13 (which is consistent with the lyotropic phase transition observed in Au-PEG-2/C12E25/water system at 25 °C as shown in Figure 2). Here, the fluids stand for the fluidic phase of small particles. As temperature increases, both of the NaZn13 + fluids and the NaZn13 + AlB2 phases make a transition to the AlB2 + fluids and then to the fluidic phase. As shown in Figure 5b and Supporting Information Figure S8, the experimentally observed thermotropic symmetry transition of Au-PEG-2/C12E25/water (15/40/60 and 46/40/60 for which the number ratios are 1:19 and 1:6.5, respectively) is consistent with the calculated phase diagram with some shift of transition temperature. In summary, we developed a spherical micelle-assisted method to form NPSLs of different symmetries. The NPSLs fabricated by this method exhibit thermally reversible symmetry transitions, making the NPSLs excellent candidates for new thermo-responsive functional materials. The spherical micelle-assisted method is an equilibrium method as evidenced by the thermally reversible transition and its consistency with the phase diagram predicted by Gibbs free energy calculation. The equilibrium nature of this method provides a highly predictable and reliable way of designing NPSL synthesis. Considering that micellar systems are sensitive to other environmental conditions such as pressure and pH, this approach may be used to explore the responsivity of NPSLs for other stimuli, opening up new possibilities of NPSLs as materials for functional devices switchable between different states. Considering the simplicity and generality of this method, it would also be applicable to other metallic, semiconducting, and magnetic nanoparticles which are of great current interests.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental section; TEM image and size distribution of as-prepared AuNPs; TEM images of Au-PEG-1 and Au-PEG-2; UV-vis-NIR spectra of as-prepared AuNPs, AuPEG-1 and Au-PEG-2; TGA measurements of Au-PEG-1 and Au-PEG-2; SAXS intensities of C12E25/water with increasing temperature; SAXS intensities of Au-PEG1/water and Au-PEG-2/water at 25 °C; Structural information for Au-PEG-1/C12E25/water and Au-PEG-2/C12E25/water; Structure factor of Au-PEG-2/C12E25/water (15/40/60) at 65 °C; Structure factors of Au-PEG-2/C12E25/water (46/40/60) upon heating and cooling; Structure factor of Au-PEG-2/C12E25/water (46/40/60) at 65 °C (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Jae-Min Ha: 0000-0002-2430-0548 Min-Jae Lee: 0000-0002-6269-3979 Sung-Min Choi: 0000-0002-0749-2226 Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2017R1A2A1A05001425 and 2017M2A2A6A01021366) and the KUSTAR-KAIST Institute, KAIST, Korea. We acknowledge the Pohang Accelerator Laboratory for providing beam time at the beamline (4C) in this study. REFERENCES (1)

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Figure 1. Schematics of experimental design. 3.7 nm AuNPs functionalized with SH-PEG of different molecular weight 1 or 2 kDa (denoted as Au-PEG-1 or -2) are mixed with C12E25 micellar solution at the bcc phase. The mass ratio of C12E25 and water was kept constant at (40/60) and a different amount of AuPEG-1 or -2 was added to each sample. Solid core with a semi-transparent shell is also used to present micelle and functionalized AuNPs in a simplified form. The molecular structures of C12E25 and SH-PEG (1 and 2kDa) are shown in the box.


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Figure 2. SAXS measurements of Au-PEG/C12E25/water mixture at different mixing ratios. SAXS intensities of (a) Au-PEG-1/C12E25/water (mp/40/60, mp = 6, 8, 10, 15, 31 for which the number ratios, Au-PEG-1 : C12E25 micelle, are 1:40, 1:30, 1:24, 1:16, and 1:7.5, respectively) and (b) Au-PEG-2/C12E25/water (mp/40/60, mp = 9, 11, 15, 23, 46 for which the number ratios, Au-PEG-2 : C12E25 micelle, are 1:33, 1:26, 1:19, 1:13, and 1:6.3, respectively) at 25°C. The SAXS intensity of C12E25/water (40/60) is shown at the bottom of each graph for comparison. SAXS intensities are shifted vertically for visual clarity. 2-D SAXS patterns of (c) Au-PEG-1/C12E25/water (31/40/60), (d) Au-PEG-2/C12E25/water (15/40/60), and (e) AuPEG-2/C12E25/water (46/40/60) are shown on the right.


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Figure 3. Structure factors of Au-PEG/C12E25/water mixtures. Structure factors of (a) Au-PEG1/C12E25/water (31/40/60) and (b) Au-PEG-2/C12E25/water (15/40/60) are well-indexed by P63/mmc and Fm-3c space group, respectively. (c) Au-PEG-2/C12E25/water (46/40/60) shows a mixed phase which is well-indexed by Fm-3c and P6/mmm space group (for clarity, only the indexing by P6/mmm space group is shown). The measured structure factors were obtained by dividing the measured SAXS intensity by the particle form factor of AuNP. The calculated structure factors using the FullProf program33 are shown at the bottom of each figure for comparison. In the schematics for the crystalline structures (on the right), the simplified notations given in Figure 1 are used to present Au-PEG-1, -2, and C12E25 micelles. 239x259mm (300 x 300 DPI)


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Figure 4. Space-filling curves for different crystalline symmetries. The packing density of binary hard-sphere system versus size ratio is calculated for AlB2, NaZn13, and MgZn2-type symmetries, respectively. The three colored bands (with width marked by arrows) correspond to the range of stability for each symmetry predicted by computer simulations. 80x77mm (300 x 300 DPI)


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Figure 5. Structure factors of (a) Au-PEG-1/C12E25/water (31/40/60), (b) Au-PEG-2/C12E25/water (15/40/60) and upon heating (left) and cooling (right). The temperature was varied in 5 °C step and the structure factors at selected temperature are shown here. The structure factors are shifted vertically for visual clarity.


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Table of contents 80x35mm (300 x 300 DPI)


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