Ring-Like Assembly of Silica Nanospheres in the ... - ACS Publications

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Ring-Like Assembly of Silica Nanospheres in the Presence of Amphiphilic Block Copolymer: Effects of Particle Size Chisato Atsumi, Shintaro Araoka, Kira Beth Landenberger, Arihiro Kanazawa, Jin Nakamura, Chikara Ohtsuki, Sadahito Aoshima, and Ayae Sugawara-Narutaki Langmuir, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Ring-Like Assembly of Silica Nanospheres in the Presence of Amphiphilic Block Copolymer: Effects of Particle Size Chisato Atsumi,† Shintaro Araoka,‡ Kira B. Landenberger,§ Arihiro Kanazawa,‡ Jin Nakamura,ǁ Chikara Ohtsuki,ǁ Sadahito Aoshima,*,‡ and Ayae Sugawara-Narutaki*,ǁ †

Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku,

Nagoya 464-8603, Japan ‡

Department of Macromolecular Science, Osaka University, Machikaneyama, Toyonaka, Osaka

560-0043, Japan §

Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510,

Japan ǁ

Department of Materials Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-

8603, Japan

KEYWORDS Self-assembly, Ring-like nanostructure, Silica nanoparticle, Amphiphilic block copolymer, Organic-inorganic hybrid, Liquid-phase process

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ABSTRACT

Block copolymer-mediated self-assembly of colloidal nanoparticles has attracted great attention for the fabrication of a wide variety of nanoparticle arrays. We have previously shown that silica nanospheres (SNSs) 15 nm in diameter assemble into ring-like nanostructures in the presence of amphiphilic block copolymers poly[(2-ethoxyethyl vinyl ether)−block−(2-methoxyethyl vinyl ether)] (EOVE−MOVE) in an aqueous phase. Here, the effects of particle size of SNSs on this polymer-mediated self-assembly are studied systematically using scanning electron microscopy to observe SNSs of seven different sizes between 13 to 42 nm. SNSs of 13, 16, 19, and 21 nm in diameter assemble into nanorings in the presence of EOVE−MOVE. In contrast, larger SNSs of 26, 34, and 42 nm aggregate heavily, form chain-like networks, and remain dispersed, respectively, instead of forming ring-like nanostructures. The assembly trend for 26−42 nmSNSs agrees with that expected from the increased colloidal stability for larger particles. Timecourse observation for the assembled morphology of 16 nm-SNSs reveals that the nanorings, once formed, assemble further into network-like structures, as if the nanorings behave as building units for higher-order assembly. This indicates that the ring-like assembly is a fast process that can proceed onto random colloidal aggregation. Detailed analysis of nanoring structures revealed that the average number of SNSs comprising one ring decreased from 5.0 to 3.1 with increasing the SNS size from 13 to 21 nm. A change in the number of ring members was also observed when the length of EOVE−MOVE varied while the size of SNSs was fixed. Dynamic light scattering measurements and atomic force microscopy confirmed the SNSs/polymer composite structures. We hypothesize that a stable composite morphology may exist that is influenced by both the size of SNSs and the polymer molecular structures.


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INTRODUCTION Self-assembly of colloidal nanoparticles (NPs) is an exciting and important research field as it promises to provide a variety of NP arrays for next-generation nanomaterials with unique optical, magnetic, electronic, and catalytic properties.1-5 Amphiphilic block copolymers that can form segregated nanostructures have been widely used as templates to assemble NPs.6 In most of these cases, NP surfaces are organically modified to control interfacial interaction with the template block copolymers.7-21 For instance, NPs have been preferentially incorporated within one of the block domains of a copolymer or localized at the interface of domains that have self-assembled with lamellar,7-10 inverse hexagonal,11 cylindrical,12,13 or vesicular structures.14,15 Increase in the volume fraction of NPs in copolymers has often led to a morphological transition of microdomain structures, indicating a synergistic self-assembly between NPs and block copolymers.16-23 Theoretical simulation approaches highlighted the importance of the interaction parameter between NP and a segment of the block copolymer in the morphological transition of NP-block copolymer coassembly systems.24,25 These interesting phenomena hint at a still unknown mechanism that could prove key in developing these systems. We previously reported synergistic self-assembly between silica nanospheres (SNSs) and amphiphilic block copolymers containing oxyethylene moieties in an aqueous phase in relatively dilute conditions (~2 wt% silica and ~2 wt% polymers).26-30 SNSs have no ligands but instead interact with the copolymers through hydrogen bonding between the silanol groups of silica and the ether oxygens of the copolymers.31 SNSs were assembled into one-dimensional (1D) nanochains,26-28 2D nanorings,29 and 3D vesicles30 depending on the polymer structure, pH, and temperature. In particular, pH critically influences the formation of ordered structures. Above the isoelectric point pH 2‒3,32 the surface charge of SNSs increases with increasing pH because of


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the deprotonation of the silanol groups. Due to the presence of the silanol groups, there is an optimum pH for the ordered assembly; SNSs remain dispersed at higher pH while they tend to randomly aggregate at lower pH, even in the presence of block copolymers. For example, SNS nanochains are formed in the presence of Pluronic® polymers consisting of poly(ethylene oxide) and poly(propylene oxide) (PEO–PPO–PEO).26-28 Well-defined unbranched chains are obtained at around pH 7.5 in the presence of PEO100-PPO65-PEO100 (Pluronic® F127) at 60 ºC. SNSs remain dispersed at above pH 8.0 while they assemble into bended chains and more random aggregates at pH 7.0 and 6.0, respectively.26 In the presence of more hydrophobic copolymers with a larger PPO content the optimum pH for 1D assembly shifts even higher.27 The balance of interactions, including electrostatic repulsions between SNSs, steric repulsions and hydrophobic attractions caused by the SNS surface-adsorbed polymers, is key to controlled SNS assembly. Ring-like assembly of NPs is of particular interest because it is relatively rare phenomenon and thus largely unexplored. Ring-like nanostructures have attracted attention because of their unique electric,33 magnetic,34 and optical properties.35-39 In general, NP rings as 2D nanostructures have been fabricated using templates, such as lithographic patterns,40 solid spheres,41 droplets42,43 or bubbles,44,45 which are prepared on solid substrates. Liquid-phase assembly of NPs into rings is an alternative approach that can yield free-standing nanorings.29,4650

One such example uses magnetic NPs that could be assembled into rings without templates

due to the flux-closure mechanism.46,47 Alternatively, ring-like structures consisting of gold and silver NPs were formed on viral protein templates in an aqueous phase.48,49 And while its mechanism is still under investigation, nanodroplet-mediated ring-like assembly of platinum NPs has also recently been reported.50 In our approach, nanorings of SNSs are obtained in the presence of a synthetic polymer, poly[(2-ethoxyethyl vinyl ether)−block−(2-methoxyethyl vinyl


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ether)] (EOVE–MOVE) at 45 ºC in the proper pH range.29 EOVE–MOVE is a thermoresponsive copolymer and therefore acts as an amphiphilic copolymer between 20 and 60°C, the range in which EOVE is hydrophobic and MOVE is hydrophilic.51-53 Nanoring formation in the liquid phase was confirmed using cryo-transmission electron microscopy.29 Interestingly, SNSs 15 nm in diameter assembled into nanorings, while larger 30 nm-SNSs organized into chain-like structures under the same experimental conditions.29 Systematic investigation of the effect of SNS size on the polymer-mediated self-assembly is expected to lead to structural control of the nano-objects as well as better understanding of the mechanism for ring-like self-assembly. Here we synthesize SNSs with seven different sizes ranging from 13 to 42 nm in diameter to investigate the effect of SNS particle size on their assembly mediated by the EOVE–MOVE block copolymer. The size-dependent self-assembly behaviors are discussed from the viewpoint of colloidal stability, kinetics, and the SNS/polymer composite structures. EXPERIMENTAL SECTION Materials. Tetraethyl orthosilicate (TEOS), L-lysine and 1M hydrochloric acid were purchased from Nacalai Tesque, Kyoto, Japan. Ethanol was purchased from Kishida Chemical, Osaka, Japan. These chemicals were used as received for the synthesis of SNSs suspensions. Synthesis of SNS suspensions. A colloidal suspension containing SNSs of 16 nm in diameter was prepared as reported elsewhere.29,54 Briefly, L-lysine (0.037 g) was dissolved in deionized water (35 mL) in a 110 mL-vial with a Teflon-coated stir bar. TEOS (2.6 g) was then added to the vial, followed by stirring at ca. 500 rpm for 24 hours in a water bath at 60 ºC. An optically transparent, 16 nm-SNS suspension (2 wt% SiO2) was obtained. A colloidal suspension of 13 nm-SNS was synthesized by adding two times the amount of L-lysine (0.074 g) while other


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conditions remained unchanged. Suspensions containing 19, 21, 26, 34, or 42 nm-SNSs were synthesized by the seed-regrowth method.55 Appropriate amounts of water, L-lysine and TEOS were added to the suspension of 16 nm-SNS to make final concentrations of each component to be same as those of the 16 nm-SNS suspension. The mixture was stirred at ca. 500 rpm for 24 hours at 60 ºC. Finally, 2 wt% SNS suspensions, each containing a different size of SNS, were obtained. The average diameters of each type of SNSs were determined from transmission electron microscopy (TEM) images (Table 1). SNS suspensions (2 wt% SiO2) were concentrated or diluted as needed. SNS suspensions were concentrated by centrifugation using a centrifugal filter unit (Amicon® Ultra-15, Merck Millipore, Germany). To dilute samples the appropriate amount of water, L-lysine and ethanol was added to the initial SNS suspension. The final concentrations of L-lysine and ethanol in the SNS dispersions were 0.1 and 6.1 wt%, respectively, for all samples tested in this paper. Polymer synthesis and analysis. A series of EOVE–MOVE block copolymers were synthesized by base-assisting living cationic polymerization according to previously reported methods.29,51-53 The polymerization degrees of EOVE and MOVE were calculated by 1H nuclear magnetic resonance spectroscopy (NMR, JEOL JNM-ECA 500 spectrometer). The numberaverage molecular weight (Mn) and the polydispersity index (Mw/Mn) (Mw: weight-average molecular weight) were determined from gel permeation chromatography curves in chloroform at 40 °C with respect to 16 polystyrene standards (Tosoh; Mn = 577 − 1.09 × 106, Mw/Mn < 1.1). The characterization of the synthesized polymers is shown in Table 2. Polymer-mediated self-assembly of SNSs. In a typical procedure, 0.10 g of EOVE−MOVE was measured out in a 10 mL-vial to which a SNS suspension (5.0 g) was added. The mixture was chilled to dissolve EOVE−MOVE. The pH of the mixture was adjusted to 7.8 using 0.1 and


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0.01 M HCl aqueous solutions at 10 ºC in the ice bath, with negligible volume change in the final sample. The mixture was left to stand in an incubator at 45 ºC for 24 hours. Characterization. Synthesized SNSs were observed using a transmission electron microscope JEOL JEM-2100plus at an accelerating voltage of 200 kV. Samples were diluted 100-fold with water, applied to a microgrid (JEOL Microgrid Cu 200 mesh), and then air-dried. Morphologies of assembled structures were observed by using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7500FA). Silicon wafers were cleaned by immersing them first in Semico Clean 56 (Furuuchi Chemical) and sonicating them with ultrasonication for 15 min and then immersing and sonicating them in ultrapure water for 15 min. They were then rinsed with ethanol and dried in air. As-prepared samples containing SNS assemblies were diluted by 50 times with ultrapure water. For dilution, the temperature of the water was set to 25 ºC to prevent undesirable dissociation of assembled structures below 20 ºC. The diluted samples (10 μL) were spin-coated onto the clean silicon wafers for 15 seconds at 3,000 rpm using a spin coater (Kyowariken, K-359SD-1SPINNER). Before FE-SEM observation, samples were treated with ultraviolet-ozone using an UV irradiator (172 nm, UER20-172VB, USHIO Inc., Japan) to remove the polymer. Finally, osmium was sputtered to 3 nm thickness onto the samples using an osmium coater (Osmium plasma coater OPC60A, Filgen, Inc., Japan). Dynamic light scattering (DLS) measurement was carried out using a ZETASIZER Nano-ZS, Marvern, Germany. All samples were filtered with 0.4 µm filters (Syringe filter NY013045, AS ONE Co., Ltd., Japan) before measurement. Atomic force microscopy was carried out using an MFP-3D OriginTM AFM, Oxford Instruments plc, U.K. by tapping mode in air at room temperature. Scans were performed with an Si cantilever (OMCL-AC240TS, Olympus, Japan) at a rate of 1.0 Hz.


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Samples were diluted by 100 times with ultrapure water, spin-coated onto a clean Si wafer, and then air-dried. RESULTS AND DISCUSSION Block copolymerization. A series of EOVE–MOVE block copolymers were synthesized using base-assisting living cationic polymerization.51-53 All copolymers obtained have narrow molecular weight distributions (Mw/Mn ≤ 1.24) (Table 2). Polymer 1 (EOVE92–MOVE270) and polymer 2 (EOVE90–MOVE260) have almost similar compositions and thus are regarded as equivalent products. For polymers 1, 3 and 4, the ratio of EOVE : MOVE is fixed to about 1:3 while the overall polymer length increases in the order of 3 < 1 < 4. Both EOVE and MOVE blocks are hydrophilic below ~20 ºC. The EOVE block becomes hydrophobic above 20 ºC, while the MOVE block becomes hydrophobic above 60 ºC, thus these copolymers are amphiphilic at 45 ºC.51-53 Effects of SNS Size on Polymer-Mediated Self-Assembly. In our previous report, well-defined nanoring structures were formed from SNSs (15 nm, 2 wt%) in the presence of either EOVE100– MOVE310 or EOVE100–MOVE194 (2 wt%) at pH 7.7~7.8 at 45 ºC.29 Here we employed similar experimental conditions to study the effect of SNS particle size on the polymer-mediated ringlike assembly. Suspensions of 2 wt% SNSs (16, 19, 21, 26, 34, or 42 nm) were incubated in the presence of polymer 1 (2 wt%) at pH 7.8, 45 ºC for 24 hours. Polymer 2 was used for the assembly of 13 nm-SNS with other experimental conditions kept constant. FE-SEM images of assembled SNSs are shown in Figure 1. It should be noted that only SNSs are present in the figure because polymers were removed before observation. Nanoparticle rings are observed for SNSs with diameters of 13, 16, 19, and 21 nm (Figure 1A–D). In clear contrast, only heavy


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aggregation is observed for 26 nm-SNSs (Figure 1E). SNSs of 34 nm assembled into chain-like structures (Figure 1F), which is consistent with a previous report.29 For 42 nm-SNSs, most are individually dispersed (Figure 1G). Ring-like nanostructures of SNSs were obtained only when the size of SNSs was equal to or smaller than 21 nm, indicating that there is an upper size threshold for SNSs to be assembled into nanorings. Above the size threshold of 21 nm, the aggregation tendency of SNSs (26‒42 nm) agrees with that which would be expected from their colloidal stability. Since the degree of aggregation is in the order of 26 nm-SNSs (heavily aggregated) > 34 nm-SNSs (moderately aggregated) > 42 nm-SNSs (dispersed) (Figure 1E–G), the colloidal stability increases in the reverse order; that is, larger SNSs are more colloidally stable. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the particle size affects both electrostatic repulsion and van der Waals attraction among colloidal particles.56 In general, the colloidal stability of charged NPs increases with increased size in NPs because of the higher energy potential barrier between particles. In the present system, the maximum potential barrier between SNSs also increases when the particle size increases (see Supporting Information). Moreover, this trend in the morphological change for the assembly of 26‒42 nm-SNSs in the presence of polymer 1 is similar to that observed for 15 nm-SNSs in the presence of a PEO‒PPO‒PEO copolymer with increasing pH; the morphology changes from an aggregated state, to a 1D chain, and finally to a dispersed state. This morphological change was also induced by the increase in their colloidal stability attributed to the increase in electrostatic repulsion at higher pH.26,27 SNSs of 13‒21 nm assembled into nanorings instead of forming large aggregates, which was not expected by simple size-dependent colloidal behavior. The assembly process of SNSs were monitored to obtain the time-course information for the ring-like assembly by observing


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assembled structures from 0.5 hours up to 6 days for three representative SNS system (16, 26 and 34 nm) (Figure 2). The 16 nm-SNSs already formed nanorings at 0.5 hours (Figure 2A). These nanorings further assembled into network-like structures after 3 days, as if the nanorings themselves behaved as building units (Figure 2C and D). This indicates that nanorings may form fast enough before random aggregation of SNSs starts, and once nanorings are formed, they behave as discrete colloidal particles, whose colloidal stability is higher than that of starting SNSs because the resulting colloidal size is larger. SNS of 26 nm already aggregated at 0.5 hours and negligible structural change was observed thereafter (Figure 2E–H). Dimers and short chains were observed for 34 nm-SNSs at 0.5 hours (Figure 2I). The chain length increased along with the chain branching over a period of 6 days (Figure 2J–L). The behaviors of 26 and 34 nmSNSs agree with their colloidal stability. The 34 nm-SNSs aggregate more slowly (Figure 2I‒L) than 26 nm-SNSs (Figure 2E‒H). The effect of particle size is further examined at various concentrations of SNSs. The assembled morphologies of SNSs as a function of SNS concentration and diameter are shown in Figure 3. Note that the polymer concentration was fixed at 2 wt% and the incubation time at 45 ºC was fixed to 24 hours. The data points outlined with a solid line correspond to the results shown in Figure 1. Nanorings are again obtained for SNSs only equal to or less than 21 nm in diameter at various concentrations examined in this study. Figure 3 also makes it easy to see how the self-assembly is influenced by the SNS concentration at a fixed particle size. It is clear that SNSs tend to aggregate when the SNS concentration is higher. The chain-like structures comprised of nanorings, which are similar to those observed for 16 nm-SNSs after 3 days (Figure 2C), were observed for 21 nm-SNSs at 2.6 wt%. The collision frequency between particles increases at higher particle concentration, which can cause a faster particle aggregation, therefore,


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the aggregation of nanorings in the case of the 21 nm-SNSs at 2.6 wt% takes less time (24 h) than for 16 nm-SNSs at 2 wt% (3 days). Morphological Control of Nanorings. Nanorings were obtained from only 13–21-nm-SNSs under the present experimental conditions. Detailed analysis of nanoring structures revealed that the size of SNSs also affected the number of SNSs comprising one ring. Here, the number of SNSs per ring were counted by randomly choosing approximately 150 nano-objects from the FESEM images (Table 3). Aggregated objects were counted as “others” for 13–19 nm-SNSs. Isolated SNSs in addition to aggregates were observed for 21 nm-SNS; for both of these cases they were counted as “others”. Based on this method, the observed average number of SNSs comprising one ring decreased from 5.0 to 3.1 when the diameter of SNSs increased from 13 to 21 nm. This inverse relationship suggests the existence of a stable size for ring-like nanostructures. As shown in Figure 4, the diameters of nanorings estimated from the SNS diameters and the corresponding average ring-members are close to 40 nm for the 13–21 nmSNSs. SNSs of equal to or larger size than 26 nm in diameter may no longer form stable nanorings because the size of the nanorings, if any, well exceeds 40 nm. For example, a theoretical nanoring consisting of three 26 nm-SNSs is calculated to be at least 56 nm in diameter. Because the ring-like assembly presented herein is a polymer-mediated phenomenon, the stable ring-size may vary according to the polymer structure. To verify this possibility, we employed polymers 3 and 4 for the assembly of 16 nm-SNSs and the results are compared to the results for polymer 1. Here, the polymer length increases in the order of 3 < 1 < 4 while the relative ratio of EOVE : MOVE is kept constant. Polymers 3 and 4 also yielded SNS nanorings after 24 hours incubation at 45 ºC (Figure 5). The average number of SNSs comprising one


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nanoring was 3.2, 4.2, and 4.6 for polymer 3, 1, and 4, respectively (Table 4). The longer polymer yielded larger nanorings with increased numbers of SNSs per ring. SNSs‒Polymer Composite Structures. We also confirmed the SNSs‒polymer composite structures that form in the liquid phase. The zeta potential and the hydrodynamic diameter were measured for the as-prepared suspension consisting of 16 nm-SNSs and polymer 1 after 24 hours at 45 ºC (Table 5). This suspension should contain SNS nanorings shown in Figure 1B. The zeta potential of nanorings formed with polymer 1 was close to that of polymer 1 rather than SNSs. In addition, the diameter of SNS nanorings in the presence of polymer 1 was determined to be 102 nm, which was larger than SNS nanorings after polymer removal (~ 40 nm in size). These results suggest that the nanorings are covered by the polymers. AFM observation was carried out for the assembled structures formed from 16 nm-SNSs and polymer 1 after 24 hours at 45 ºC (Figure 6). Since no UV-ozone treatment was performed for this sample, the polymer in addition to SNSs can be observed in this image. Nanoparticles around 100 nm in diameter were observed, which is consistent with the assembly size determined by DLS (102 nm). It should be noted that SNS nanorings, just as those in Figure 1B, can be observed after polymer removal by UV-ozone treatment for this AFM sample. It is again likely that SNSs were surrounded by the EOVE-MOVE block copolymers, allowing for the form of these composite nanostructures. CONCLUSION The particle size of silica nanospheres (SNSs) greatly influences their self-assembly behaviors mediated by the amphiphilic block copolymer poly[(2-ethoxyethyl vinyl ether)−block−(2-methoxyethyl vinyl ether)] (EOVE−MOVE). SNSs of 13, 16, 19, and 21 nm in


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diameter assembled into nanorings in the presence of an EOVE−MOVE block copolymer while larger SNSs did not form nanorings but instead aggregated heavily (26 nm-SNSs), formed chainlike networks (34 nm-SNSs), or remained dispersed (42 nm-SNSs). This assembly trend for larger SNSs is expected when considering the increased colloidal stability for larger particles. Time-dependent observations of the assembled morphology for 16 nm-SNSs showed that the nanorings formed first and then aggregated further with time, indicating that the ring-like assembly takes place faster than random colloidal aggregation. The average number of SNSs comprising a ring decreased with an increase in the diameter of SNSs from 13 to 21 nm. The number of SNSs in a ring can also be controlled by varying the polymer length of the block copolymer, EOVE−MOVE, at a fixed SNS particle size. Dynamic light scattering measurements and atomic force microscopy demonstrated that SNSs nanorings are surrounded by the EOVE−MOVE block copolymer. A stable composite morphology between SNSs and the polymers is conjectured to exist, and this morphology can be controlled by the handles of both the size of SNSs and the polymer molecular structure. The mechanistic study for the generation of ring-like nanostructures is ongoing.


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Table 1. Components for the preparation of SNS suspensions. Component Seed suspension (16 nm-SNSs) [mL] ‒ ‒ 19 14 7.8 3.3 1.6

Ultrapure water [mL] 35 35 10 15 20 25 26

L-lysine

TEOS [g]

Diameter of SNSsa [nm]

[g] 0.074 0.037 0.011 0.016 0.022 0.026 0.028

2.6 2.6 0.77 1.1 1.5 1.9 2.0

13 ± 1.7 16 ± 1.1 19 ± 1.9 21 ± 1.8 26 ± 2.0 34 ± 2.1 42 ± 2.1 a

Average ± SD.


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Table 2. Characterization of the EOVE‒MOVE block copolymers. Polymerization degreea

Polymer

EOVE

MOVE

Mnb

Mw/Mnb

1

EOVE92‒MOVE270

92 (100)

270 (300)

32,000

1.23

2

EOVE90‒MOVE260

90 (90)

260 (270)

29,800

1.19

3

EOVE70‒MOVE200

70 (75)

200 (225)

28,600

1.15

4

EOVE150‒MOVE410

150 (150)

410 (450)

35,100

1.24

Abbreviations: EOVE, (2-ethoxyethyl vinyl ether); MOVE, (2-methoxyethyl vinyl ether). a Calculated by 1H NMR spectra. The theoretical degree of polymerization based on the feed monomer composition is presented in the parentheses. b Determined by gel permeation chromatography in CHCl3, polystyrene calibration.


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Figure 1. FE-SEM images of SNS assemblies formed in the presence of polymer 1 (for 16‒42 nm SNSs) or 2 (for 13 nm SNSs) after 24 hours. The average diameters of the SNSs are (A) 13 nm, (B) 16 nm, (C) 19 nm, (D) 21 nm, (E) 26 nm, (F) 34 nm and (G) 42 nm. The insets in A‒D are the zoomed-in images of a typical ring. All samples were incubated at 2 wt% polymer and 2 wt% SNSs, pH 7.8, and 45 ºC.


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Figure 2. FE-SEM images of assembled structures of SNSs formed in the presence of polymer 1 at various timepoints.


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Figure 3. The diagram of assembled morphologies of SNSs formed after 24 hours as functions of SNS size and concentration. Plots inside the solid line correspond to the results shown in Figure 1.


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Table 3. Average number of SNSs comprising one ring. Samples were formed at 2 wt% polymer and 2 wt% SNSs, pH 7.8, 45 ºC for 24 hours. Count of nano-objects Size SNSs

of

Number of SNSs comprising one ring Others

Average number SNSs

3

4

5

6

13 nm

1

6

35

9

56 (51 %)

5.0

16 nm

10

43

20

2

74 (50 %)

4.2

19 nm

26

25

0

0

96 (65 %)

3.5

21 nm

42

6

0

0

127 (73 %)

3.1

of


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Figure 4. SNSs from 13 to 21 nm in diameter are observed to form nanorings. The observed average number of SNS per ring that were determined in Table 3 are drawn above. The resulting diameter of the nanoring is described as diameter R. Based on the size of the SNSs and the corresponding average number of SNSs comprising one ring, the calculated values for R are written below each schematic. A theoretical ring for 26 nm-SNSs is also drawn, with a corresponding minimum R of 56 nm.


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Figure 5. FE-SEM images of ring assemblies of 16 nm-SNSs mediated by (A) polymer 3, (B) polymer 1 and (C) polymer 4. All samples were obtained with 2 wt% polymer and 2 wt% SNSs and incubated at pH 7.8, 45 ºC for 24 hours. Note that the panel (B) is a part of the image of Figure 1B.


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Table 4. The average number of SNSs comprising one ring using 16 nm-SNSs and polymers 3, 1 and 4. Note that the data for polymer 1 is the same as that for 16 nm-SNS in Table 3. Count of nano-objects Polymer

Others

Average number of SNSs

0

158 (59%)

3.2

20

2

74 (50 %)

4.2

42

14

102 (50 %)

4.6

Number of SNSs comprising one ring 3

4

5

6

3

90

17

3

1

10

43

4

8

36


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Table 5. Zeta potential and hydrodynamic diameter of 16 nm-SNSs, polymer 1, and the ringlike structures formed from 16 nm-SNSs in the presence of polymer 1. Measurements were performed at 45 °C.

Zeta potential / mV

Hydrodynamic diameter / nm

16 nm-SNSsa

‒40.8

16

Polymer 1b

‒0.594

117

Ring-like structures

‒1.26

102

a

The pH of the suspension was adjusted to pH 7.8 using HCl. The composition of solvent is modified to similar to that of ring-like strutures (L-lysine 0.1 wt% and ethanol 6.1 wt%).

b


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Figure 6. AFM image of assembled structures formed from 16 nm-SNSs in the presence of polymer 1 after 24 hours at 45 ºC.


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ASSOCIATED CONTENT Supporting Information. The total potential energies for particle separation of SNSs calculated based on the DerjaguinLandau-Verwey-Overbeek (DLVO) theory is available.

AUTHOR INFORMATION Corresponding Author *E-mail [email protected] (A. S.-N.) *E-mail [email protected] (S. A.)

ACKNOWLEDGMENT A part of this work was supported by Grant-in-Aid for Challenging Exploratory Research (14K14090), JSPS, Japan. We thank Prof. Takahiro Seki, Prof. Shusaku Nagano, Prof. Mitsuo Hara and Mr. Issei Kitamura (Nagoya University) for the use of AFM and UV irradiator. A part of this work was supported in Nanotechnology Platform Program (Molecule and Material Synthesis), MEXT, Japan and conducted in High Voltage Electron Microscope Laboratory at Nagoya University.


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