Bioinspired Approach to Silica Nanoparticle Synthesis Using Amine

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A Bioinspired Approach to Silica Nanoparticle Synthesis Using AmineContaining Block Copoly(vinyl ethers): Realizing Controlled Anisotropy Ayae Sugawara-Narutaki, Sachio Tsuboike, Yukari Oda, Atsushi Shimojima, Kira Beth Landenberger, Tatsuya Okubo, and Sadahito Aoshima Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01493 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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A Bioinspired Approach to Silica Nanoparticle Synthesis Using Amine-Containing Block Copoly(vinyl ethers): Realizing Controlled Anisotropy Ayae Sugawara-Narutaki*,†,ǁ, Sachio Tsuboike,‡,ǁ Yukari Oda,§,¶ Atsushi Shimojima,⁑,ǂ Kira B. Landenberger,§,| Tatsuya Okubo,‡ and Sadahito Aoshima*,§ †

Department of Materials Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Chemical System Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Department of Macromolecular Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan ⁑ Department of Applied Chemistry, Waseda University, Okubo-3, Shinjuku-ku, Tokyo 169-8555, Japan ǂ Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, Nishiwaseda-2, Shinjukuku, Tokyo 169-0051, Japan | Department of Polymer Chemistry, Kyoto University, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡

ABSTRACT: Core-shell polymer-silica hybrid nanoparticles smaller than 50 nm in diameter were formed in the presence of micelles of poly(2-aminoethyl vinyl ether-block-isobutyl vinyl ether) (poly(AEVEm-b-IBVEn)) through the hydrolysis and polycondensation of alkoxysilane in aqueous solution at a mild pH and temperature. The size of the nanoparticles as well as the number and size of the core parts were effectively controlled by varying the molecular weight of the copolymers. The polymers could be removed by calcination to give hollow silica nanoparticles with Brunauer-Emmett-Teller (BET) surface areas of more than 500 m2 g-1. Among these, silica nanoparticles formed with poly(AEVE115-b-IBVE40) displayed an anisotropy of single openings in the shell. The use of an alternative copolymer, poly(AEVE-b-2-naphthoxyethyl vinyl ether) (poly(AEVE113-b-βNpOVE40)) yielded core-shell nanoparticles with less pronounced anisotropy. These results showed that the degree of anisotropy could be controlled by the rigidity of micelles; the micelle of poly(AEVE115-b-IBVE40) was more deformable during silica deposition than that of poly(AEVE113-b-βNpOVE40) in which aromatic interactions were possible. This bioinspired, environmentally friendly approach will enable large scale production of anisotropic silica nanomaterials, opening up applications in the field of nanomedicine, optical materials, and self-assembly.

INTRODUCTION Silica biominerals, such as diatom cell walls and sponge spicules exhibit incredibly intricate nanostructures that are produced at mild temperatures and pH.1-3 These natural systems provide an attractive challenge for materials scientists to elucidate and replicate the biosilicification process. In natural systems, various macromolecules produced by the organisms are believed to assist in the formation of these complex silica structures. In diatoms, for instance, the biosilica contains long-chain polyamines, with species-specific molecular structures, that are thought to play a decisive role in biosilicification.4-7 When these naturally occurring polyamines are used in vitro with solutions of silicic acid, they can facilitate the precipitation of silica nanospheres.4-6 Inspired by this polyamine-mediated biosilicification, the effects of synthetic polyamines on silica formation under ambient conditions have been studied intensively.8-14 For example, it was found that poly(allylamine) affords spherical particles of various sizes.8 Crystalline aggregates of poly(ethyleneimine) induce the formation of a variety of nanostructured silica with dendritic, cylindrical, and sheet-like shapes.9 In the case of poly(L-lysine), where the polypeptide chains pack into a hexagonal lattice as the silicification proceeds, hexagonal silica platelets were produced.10 Additionally, a series of tailored poly(ethyleneimine)s and poly(propyleneimine)s were synthesized as

analogs of polyamines in diatoms, and the relationship between the molecular structure and silica-precipitating ability was systematically investigated.11,12 It was determined that the amount of precipitated silica increased as the number of protonated amine groups as well as the hydrophobicity of the molecule increased.12 It is generally believed that the positive charges of the amino groups play a significant role in the condensation of silicic acid, while the self-assembly structure of the polyamine derived from the hydrophobic interactions result in the templation for silica formation.12-14 Since the self-assembled structures can serve as templates for silica formation, the use of amine-functionalized, amphiphilic block copolymers, with well understood self-assembly structures, represents a more strategic way to achieve control over the resulting morphology of silica nanomaterials. For example, aggregates of poly(L-cysteine-block-L-lysine) yielded micronsized particles or packed columns of silica.15 Crystalline aggregates of comb-like polyethyleneimine on polystyrene backbone produced silica nanoribbons.16 Hollow silica nanospheres of 100‒200 nm in diameter were formed in the presence of vesicles of poly(-caprolactone-block-2-aminoethyl methacrylate),17 poly(L-lysine-block-L-phenylalanine),18 while those ca.30 nm in diameter were formed with micelles of triblock copolymer, poly(styrene-block-2-vinyl pyridine-block-ethylene

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oxide).19 Micelles of poly{2-(diisopropylamino)ethyl methacrylate-block-2-(dimethylamino)ethyl methacrylate} resulted in core-shell hybrid copolymer-silica nanoparticles.20 Such hollow or core-shell structures are of particular interest in a variety of practical applications including anti-reflective coatings, drug delivery systems, and catalyst supports. Among hollow silica nanoparticles, special attention has been given to the preparation of anisotropic, single-hole nanoparticles (i.e. nanocups) because they may find many applications such as carriers for nanomedicine materials,21 templates for optical materials,22 and as anisotropic building units for self-assembly.23 Although there have been no bio-inspired approaches, several methods for the fabrication of nanocups were reported. Lim et al. prepared silica nanocapsules (ca. 300 nm in diameter) with a single hole by utilizing a sol-gel reaction around a surfactant-stabilized oil-water droplet; the single hole was generated during the reaction by the partial rupture of the silica wall due to the evaporation of the organic solvent.24 Fu et al. synthesized single-hole hollow silica microspheres (136–310 nm in diameter) by the coprecipitation of poly(styrene-co-methyl methacrylate) and TEOS, followed by a phase separation.25 Li et al. synthesized single-hole mesoporous nanocages of organosilica with uniform particle size (100–240 nm) by using SiO2 nanoparticles as seeds.26 Only a few reports demonstrated the preparation of single-hole nanoparticles smaller than 100 nm in diameter. Lo et al. obtained buckled bowl-like organosilica nanoparticles (50–60 nm in diameter) using 3-mercaptopropyl trimethoxysilane as a silane source in an aqueous solution containing poly(vinyl alcohol).27 The buckled nanoparticles formed as a result of drying. Ma et al. reported the synthesis of sub-10 nm silica nanoparticles bearing half pores by using micelles of hexadecyltrimethylammonium hydroxide as templates.28 The half-pore shape was formed through strict control of the kinetics of hydrolysis and condensation of tetramethyl orthosilicate and subsequent quenching of the particle growth by addition of polyethylene glycol-silane. However, flexible control over the pore anisotropy as well as various features of the core-shell structure (i.e. size, number, and location of the cores) still remains a significant challenge in nanoscale synthesis.

Figure 1. Chemical structures of block copolymers used in this study.

EXPERIMENTAL SECTION Materials. 2-Vinyloxyethyl phthalamide (PIVE) was prepared by the reaction of 2-chloroethyl vinyl ether (TCI; 97.0%) with potassium phthalimide and recrystallized from methanol twice, then ethyl acetate (Wako; >99.5%), and dried for 3 h prior to use.29 2-Phenoxyethyl vinyl ether (PhOVE) was prepared by the reaction of 2-chloroethyl vinyl ether with phenol (Nacalai Tesque; 99%) and purified by double distillation over calcium hydride under reduced pressure. 2-Naphthoxyethyl vinyl ether (NpOVE, Canon Inc., >99.5%) was recrystallized from methanol, then dry hexane and vacuum dried for 3 hours prior to use. 1,4-Dioxane (Wako, >99.5%) was distilled over calcium hydride and then lithium aluminum hydride. 1-(Isobutoxy)ethyl acetate [IBEA; CH3CH(OiBu)OCOCH3] was prepared from IBVE and acetic acid as was previously reported.30 Commercial Et1.5AlCl1.5 (Nippon Aluminum Alkyls; 1.0 M solution in toluene) was used without further purification. Toluene (Wako, 99.5%) and dichloromethane (Nacalai Tesque; 99%) were dried using solvent purification columns (Glass Contour). All materials except for PIVE, NpOVE, toluene, and dichloromethane were stored in brown ampoules under dry nitrogen.

Precisely synthesized block copolymers have a high potential to produce controlled silica nanostructures. We have developed a method for achieving precision synthesis of amine-functionalized block and random copolymers of vinyl ethers of various molecular weights.29 Since primary amine groups can be specifically incorporated at various positions and lengths, these polymers are ideally suited for designing various unique coreshell polymer-silica hybrid structures using a bioinspired approach. Here we report the unique and well-controlled synthesis of both single hole and multi-core, core-shell silica nanoparticles less than 50 nm in diameter, with a narrow size distribution, using designed copolymers containing primary amine groups as templates. By varying the molecular weight of the amine-containing copolymers poly(2-aminoethyl vinyl ether-block-isobutyl vinyl ether) (poly(AEVEm-b-IBVEn)) (Figure 1), the number of cores as well as their size and position can be controlled. Hollow silica nanoparticles with high surface area are produced after calcination to remove the polymers. Furthermore, by varying the hydrophobic comonomers employed, the degree of anisotropy of the core position and thus the size of the openings can be controlled.

Preparation of polymers bearing amino groups. Polymerization: A series of poly(PIVE-b-IBVE)s were synthesized according to a previously reported method.29 Similar methods were used to synthesize both poly(PIVE-b-PhOVE) and poly(PIVE-b-NpOVE).31 All polymerizations were carried out under a dry nitrogen atmosphere using a glass tube equipped with a three-way stopcock. The glassware was baked at >300 C for 10 minutes under a dry nitrogen flow to remove any residual water. A typical example for the polymerization of a block copolymer containing PIVE is as follows: NpOVE was dried for three hours in a baked glass tube equipped with a three-way stopcock prior to use and then dissolved in toluene (in the case of IBVE and PhOVE, these monomers were stored in ampoules and then added directly to toluene). 1,4-Dioxane and IBEA were added and the mixture and the solution was cooled to 0 C. Prechilled Et1.5AlCl1.5 solution in toluene was added to initiate the polymerization. When polymerization of

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NpOVE neared 90%, a prechilled solution of PIVE, vacuum dried for 3 hours and dissolved in dichloromethane, was added to the reaction mixture. The polymerization was quenched with a prechilled methanol solution containing 0.1wt% ammonia. The reaction mixture was diluted with dichloromethane and washed three times with hydrochloric acid and then multiple times with water to remove the initiator residues. The volatiles were removed under reduced pressure and the remaining solid was dried for 6 hours.

2100plus operated at 200 kV. Samples of nanoparticles for TEM were prepared by casting the dispersions onto carboncoated copper grids. Dynamic light scattering (DLS) and zeta potential measurements were performed with a Malvern Zetasizer Nano ZS90 instrument at 25 C. The zeta potentials were calculated from the electrophoretic mobility using the Helmholtz-Smoluchowski equation. N1–5 were recovered by centrifugation and dried at 60 C, and thermogravimetric and differential thermal analysis (TG-DTA), and nitrogen adsorption measurement were performed. TG-DTA were performed on a Thermo plus (Rigaku) with a heating rate of 5 K/min under the O2 10% and He 90% flow. Polymer contents were calculated assuming that the weight loss over 100 C was derived from polymer combustion. Nitrogen adsorption-desorption measurements were performed at 77 K using Autosorb-iQ2-MR (Quantachrom Instruments) for the samples after TG-DTA. Before measurements, samples are degassed at 150 C for 6 hours. The Brunauer-Emmett-Teller (BET) specific surface areas of samples were calculated from adsorption data at a relative pressure range of 0.03–0.13. The microporous surface areas were calculated from t-plot using relative pressure range of 0.3–0.5.

Deprotection Reaction: The phthalimide protecting groups were removed by treating the polymers with hydrazine as previously reported.29 The obtained polymers were purified by dialysis against distilled water for at least 2 days and then MilliQ water for a day. Characterization of polymers bearing amino groups. All of polymers were characterized as reported previously.30 The number-average molecular weight (Mn), and the polydispersity index (Mw/Mn) (Mw: weight-average molecular weight) of the polymers were determined indirectly by 1H NMR and gel permeation chromatography (GPC) measurements using the protected precursor polymers. The deprotection reaction quantitatively yielded amino-containing polymers as confirmed by 1H NMR analysis.

Evaluation of particle size and anisotropy. The histograms of particle size, core size, the number of core parts, and the degree of anisotropy were made by analyzing over 200 particles in TEM images. The lengths of long axes of the particles and cores were measured using ImageJ software. The centroid of the particle and core was defined as the intersection of the long axis and the short axis of the particle and the core, respectively.

Preparation of silica/polymer nanoparticles. Polymers (0.01 wt%) were each dissolved in distilled water to obtain polymer solution (15 g). Tetraethyl orthosilicate (TEOS, 0.021 g) was then added to the solution. They were stirred at room temperature for 4 days at 500 rpm.

RESULTS AND DISCUSSION

Characterization. Field-emission scanning electron microscopy (FE-SEM) images were obtained with a Hitachi S-900 at accelerating voltage of 6 kV. Samples for SEM observation were prepared by casting of the dispersions onto silicon substrates followed by removal of polymers by UV ozone treatment using USHIO eximer lamp, and sputter deposition of Pt were carried out for 15 s in argon atmosphere with a Hitachi E-1030 ion sputter. Transmission electron microscopy (TEM) observations were carried out on JEOL JEM 2000EXII or JEM-

Synthesis and characterization of amine-containing polymers. A series of precursor copolymers of poly(2-vinyloxyethyl phthalimide-b-isobutyl vinyl ether) (poly(PIVE-b-IBVE)) with varying compositions were synthesized, purified and then transformed through a deprotection reaction to yield the corresponding poly(AEVE-b-IBVE) copolymers.29 The characterization of these polymers are summarized in Table 1.

Table. 1 Characterization of the polymers used in this study.

polymer sample 1

poly(AEVE115-b-IBVE40)

protected precursor polymer degree of Mnb polymerizationa IBVE(1–3, 5) PIVE PhOVE(6) βNpOVE(7) 115 (120) 40 (40) 15,600

Mw/Mnb

1.11

hydrodynamic diameterc [nm]

zeta potentiald [mV]

48±3

+32±4

2


61 (60)

20 (20)

11,600

1.17

43±1

+28±7

3


30 (30)

10 (10)

5,400

1.19

24±1

+26±4

4

poly(AEVE100)

100 (100)

0 (0)

11,200

1.12

n.d.

n.d.

5

poly(AEVE105-r-IBVE40)

105 (120)

40 (40)

11,700

1.23

n.d.

n.d.

6

poly(AEVE121-b-PhOVE40)

121 (120)

40 (40)

16,900

1.17

65±1

+48±4

7

poly(AEVE113-b-βNpOVE40)

113 (150)

40 (40)

9,000

1.32

53±0.4

+47±2

a

Calculated by 1H NMR spectra. The theoretical degree of polymerization based on the feed monomer composition is presented in the parentheses. bDetermined by GPC in CHCl3, polystyrene calibration. cDetermined by DLS. c,dThe values represent the mean ± SD of three independent measurements.

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Block copolymer 1 has the highest molecular weight among 1‒ 3. The sizes of both the AEVE and IBVE blocks of polymer 2 and 3 are approximately half and quarter of that of polymer 1, respectively. The AEVE homopolymer, 4, random copolymer, 5, were also synthesized for comparison. Two additional block copolymers, poly(AEVE-b-2-phenoxyethyl vinyl ether) (poly(AEVE-b-PhOVE)) (6), and poly(AEVE-b-2naphthoxyethyl vinyl ether) (poly(AEVE-b-βNpOVE)) (7) were designed and synthesized to test the role of hydrophobic comonomers. The precursor polymers all have narrow molecular weight distributions (Mw/Mn = 1.11‒1.32) and complete deprotection was confirmed by 1H NMR analysis. The critical (intermolecular) aggregation concentration of poly(AEVE28-bIBVE10), almost equivalent polymer to 3, was 36 μg/mL (0.0036 wt%) in an aqueous solution, which was determined by static light scattering measurement in our previous study.32 Self-assembled structures of the polymers. The polymers were dissolved at 0.01 wt% in deionized water at room temperature to give a clear suspension, with a resulting pH of 6.0, irrespective of the polymer employed. The hydrodynamic diameters determined from DLS measurements and zeta potentials are listed in Table 1. The block copolymers poly(AEVE-bIBVE) (1–3) form aggregates below 50 nm in diameter, with narrow polydispersity indexes not exceeding 0.4. The size of the aggregates tends to decrease with decreasing molecularweight of the polymers. The zeta potentials of the aggregates are positive at pH 6.0, indicating that there are protonated amino groups at the surface. It is expected that block copolymers 1–3 will self-assemble to form micelles with amino groups located at the corona. The size of the AEVE homopolymer 4 or the random copolymer 5 in solution could not be estimated by DLS because of the low scattering intensity. The block copolymers with PhOVE (6) and βNpOVE (7) blocks also form aggregates the sizes of which are slightly larger than those of 1–3. Figure 2. Samples N1 (a-c), N2 (d-f), N3 (g-i), N4 (j-l), and N5 (m-o) were characterized using TEM (a,d,g,j,m) and SEM (b,e,h,k,n). The particle size distributions determined from TEM images (c,f,i,l,o) are also shown. Insets in the TEM images show the magnified images of the corresponding samples. Scale bars are 20 nm.

Synthesis of the silica/polymer hybrid nanoparticles. TEOS (0.04 wt% as SiO2) was added to polymers 1–5 (0.01 wt%) suspension and allowed to react for 4 days at room temperature. The suspensions remained transparent and the pH of the suspensions remained unchanged (pH 6.0) even after 4 days, which is likely due to how dilute the synthesis system was. The TEM and SEM images of the samples, and the particle size histograms determined from the TEM images are shown in Figure 2. Since polymers are not visible via TEM without staining, the black contrasted parts in the TEM images represent only the silica. Likewise, SEM images show only the silica structures, since the polymers were removed prior to SEM measurements by UV/ozone treatment.

darkened portion is observed at the surface of some particles, which indicates the presence of openings in these nanoparticles. Considering the various angles of the nanoparticles in the prepared sample, it can be concluded that almost all of N1 has single openings in the shell. In the case of nanoparticles synthesized using polymers with lower molecular weights, N2 and N3, core-shell structures were also formed, but, as can be ascertained from the TEM images (Figure 2d and g), most of the particles have multi-core structures. In further contrast to the higher molecular weight block copolymer, open pores are not observed from the SEM images for N2 and N3 (Figure 2e and h). Finally, as the molecular weight of the block copolymers decreases, the size of the silica nanoparticles also decreases (Figure 2c, f and i), which agrees with the trend in the change of the micelle sizes for the polymers (Table 1).

Silica nanoparticles under 50 nm in diameter are successfully obtained under mild conditions in the presence of polymers 1– 5. The nanoparticles obtained using these polymers will hereafter be designated N1–5, with the number corresponding to the templating polymer. While N1 exhibits a clear core-shell structure with a narrow size distribution (Figure 2a), these nanoparticles also have a unique structural feature: namely, most of these particles have only one core with a distinct, single opening to the outside of the nanosphere. Thus, these nanoparticles display a natural anisotropy. From the SEM image (Figure 2b) a

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This intriguing difference in the resulting structures of the silica nanoparticles based upon the molecular weights of the polymers employed prompted us to further examine this relationship. Histograms of both the core size and the number of cores per particle are shown in Figure 3a and b, respectively. As the molecular weight decreases, the size of the core(s) becomes smaller, and the number of cores per particle is greater.

more effective templation ability of longer polymers. The micropores are likely derived from molecular templating of amine-containing AEVE segments. The stronger electrostatic interactions between AEVE segments and silicate species are expected for longer polymers because of the higher zeta potential (Table 1). A stronger interaction is also suggested by the higher content of polymers (N1 > N2 > N3) in the nanoparticles (Table 2). Table 2. Polymer content of N1–5 and BET surface area (SBET) and microporous surface area (Smicro) of N1–5 after polymer removal. sample N1 N2 N3 N4 N5

Figure 3. Histograms of (a) core sizes and (b) number of cores in each nanoparticle for N1, N2, and N3.

polymer content [%] 20 18 17 12 14

SBET [m2 g-1] 560 585 630 449 567

Smicro [m2 g-1] 323 316 202 214 228

Considered collectively, the above results indicate that the polymers serve as templates to produce the silica/polymer hybrid nanoparticle and effectively allow for the creation of meso/micropores after their removal. The relation of the polymer structures to the resultant silica nanoparticles is summarized in Figure 4. While the amino groups provide a source of positive charge that allows for the deposition of silica, the hydrophobic portions account for mesopores. For the multi-core structures of N2 and N3 we postulate that some polymer micelles aggregate before or during silica deposition, resulting in these unique multi-core structures. The structures of the silica nanoparticles, therefore, are dependent on the structure of the polymer employed and change as the polymer structure changes. In this way unique silica nanoparticle structures, such as single-holes and multi-core nanospheres, can be selectively formed.

Formation of silica nanoparticles is also induced by the aminecontaining homopolymer, 4, as well as the random copolymer, 5, which is of comparable composition to 1. Neither core-shell structures nor holes are observed for N4 (Figure 2j and k) while very small voids are present in N5 (Figure 2m). In the absence of any polymers, indistinct particles with diameters ca. 10 nm and their aggregates are observed (Figure S1). This fact further highlights the role of polymers in controlling particle morphologies. Characterization of the silica/polymer hybrid nanoparticles. After synthesis, silica samples were recovered via centrifugation. TG-DTA as well as N2-adsorption-desorption studies were employed to determine the polymer contents, BET surface areas, and microporous surface areas (Table 2). Since the TG-DTA data (Figure S2) shows a decrease in mass, presumably due to polymer combustion, all samples are thought to contain polymer. It is important to note that when removing this polymer, through either TG-DTA analysis or calcination, the nanoparticle structure is perfectly retained, as confirmed by the TEM and SEM images of N1–3 (Figure S3) and the SEM images of N4 and N5 (Figure S4). The N2-adsorption-desorption isotherm of N1–5 (Figure S5) shows a type IV isotherm and the BET surface areas are higher than that of non-hollow silica nanoparticles of the same or smaller particle size synthesized using a comparable method (average particle diameter of 22 nm and BET surface of 168 m2 g-1).33 Notably, the microporous surface areas of N1–5, as calculated using t-plots, are also high. When considering the nanoparticles formed with block copolymers, the BET surface area was found to increase in the order of N1 < N2 < N3. This can be explained by the decrease in particle size (Figure 2c, f, and i) in the order of N1 > N2 > N3. The contribution of internal pores of these nanoparticles might not be significant because there is inverse relationships between the size and number of cores in each nanoparticle (Figure 3). On the other hand, the microporous surface area decreases in the order of N1 ~ N2 > N3. The possible explanation for this is the

Figure 4. Scheme of the relationships between polymer structures and the resulting silica nanoparticles.

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Formation process for N1. Of the silica nanoparticles formed using these amine-containing polymers, N1 is of particular interest due to the unique anisotropic shape of this nanomaterial. To better understand the formation of the anisotropic silica in this study, the growth process was monitored using microscopic observation. TEM and SEM images of the intermediate structures of N1 formed after 1 to 3 days were acquired (Figure 5). On the first day, crescent moon-like structures are frequently observed in the TEM image (Figure 5a). The corresponding SEM image reveals the presence of concave discs (Figure 5b). On day 2, the morphology is bowl-like (i.e. hollow hemispherical) in the TEM image (Figure 5c). Single-hole nanoparticles are already recognized in the SEM image after day 2 (Figure 5d and f). On day 3, the shape of the nanoparticles is more spherical (Figure 5e) than that on day 2 (Figure 5c). In other words, if the bright contrasted parts in the nanoparticles obtained after 2 days are regarded as cores in TEM images, the size of the core is largest on day 2 and grew successively smaller till day 4. After this, the nanoparticle size and shape does not change, indicating that silica formation is complete after 4 days under this synthesis condition. From these images, we infer that the silica nucleates at only one part of the polymer micelle at the corona, and continues to deposit in the corona to yield single-hole structures. Time-course measurement of the zeta potentials gives further insights into the formation process (Figure S6). The zeta potential of the template micelle (polymer 1) is +32±4 mV (Table 1). After the reaction with TEOS, the zeta potential decreases until day 2 where it levels off at around +27 mV. No further decrease in the zeta potential was detected even at day 6. This result indicates that amino groups of the polymer 1 are still exposed on the final silica nanoparticles. It should be noted that the diameter of final silica nanoparticles, N1 (34±3 nm, determined by TEM, Figure 2(c)), is smaller than that of the initial polymer micelles (48±3 nm, determined by DLS, Table 1). Taken together, there is a possibility that hydrophobic TEOS was initially encapsulated in the core of the polymer micelle, thereby facilitating the hydrolysis and condensation of TEOS at the interface between the hydrophobic core and the amine-containing corona, leaving outermost corona unsilicified. In addition, it is expected based on the geometry of the micelles that the density of AEVE polymer chains (i.e. amino groups) is higher around the core than at the corona-water interface, a situation which would further facilitate deposition of silica around the core.

Figure 5. TEM (a,c,e) and SEM (b,d,f) images of the intermediate structures of N1 at 1 day (a,b), 2 days (c,d), and 3 days (e,f). Insets in the TEM images show the magnified images of the corresponding samples.

arrows in the TEM image (Figure 6b). Based on this finding together with the study about the formation process, we hypothesize that the relative rigidity of the micelle core influences the anisotropy of the core. During silica aggregation, it is possible that IBVE, which is a relatively flexible monomer and only has van der Waals interactions, in the micelle core in 1 is pushed in one direction as the silica grows, leaving a hollow portion open to the outside.

Effect of concentration of polymer and TEOS. By increasing the concentration of TEOS added, core-shell silica nanoparticles with closed hole can also be prepared. The presence/absence of open holes can be controlled over various concentrations of polymer and TEOS as shown in the morphology diagram (Figure 6a). For example, when the concentration of TEOS is doubled (0.08 wt% as SiO2) while the concentration of 1 is kept at 0.01 wt%, core-shell nanoparticles without open holes are obtained (Figure 6b and c). Black contrasted parts that correspond to holes are not observed from SEM images. Smaller nanoparticles with diameters ca. 10 nm are also generated in addition to core-shell particles probably because of the homogeneous nucleation of silica. Interestingly, the closed-hole nanoparticles has natural anisotropy: the position of holes are off-centered as observed by many particles indicated by black

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18.8±8.7 %. Owing to this decreased anisotropy, smaller openings (~ 5 nm) in the shell than those of N1 (~ 13 nm, Figure 3a) were observed for N7 (Figure S7). It is expected that the micelle core for 7 is the most rigid due to the strong ring-ring interactions between naphthyl groups. Thus, the micelles of 7 are harder to deform during silica aggregation. Careful selection of the hydrophobic comonomer allows for control over the anisotropy and the size of pore openings of core-shell nanoparticles.

Figure 6. (a) Morphology diagram of the silica nanoparticles formed in the presence of polymer 1 with varied TEOS and the polymer concentrations. ○ and ● represents nanoparticles with open- and closed-hole structures, respectively. (b) TEM and (c) SEM image of silica nanoparticles synthesized from TEOS (0.08 wt% as SiO2) and 1 (0.01 wt%).

Effect of comonomer. The above results gave us an idea; the anisotropy of the core part could be controlled by changing the rigidity of the hydrophobic core of the polymer micelle. To verify this possibility, two additional block copolymers, poly(AEVE-b-PhOVE)(6) and poly(AEVE-b-βNpOVE)(7) were employed for silica synthesis (Figure 7). These amphiphilic block copolymers are similar to 1 in block lengths. However, instead of IBVE, which is a relatively flexible monomer and has only van der Waals interactions, 6 and 7 both incorporate more rigid hydrophobic monomers that are capable of aromatic interactions as well. Like 1, 6 and 7 also produce anisotropic nanoparticles (N6 and N7) that have single off-centered cores. We estimated the degree of anisotropy (DA) for these particles using equation (1): DA %

2𝑑 𝐿

100

Figure 7. (a,b) TEM images of the silica nanoparticles synthesized with 6 (a) and 7 (b), respectively. (c) The degree of anisotropy of N1, N6, and N7.

CONCLUSIONS Core-shell silica nanoparticles, 50 nm in diameter or less, with unique structures including single-hole and multi-core nanospheres, were formed using amphiphilic block copolymers containing amino groups. Both the size and number of cores were changed systematically by varying the chain length of the copolymer additives. When a random copolymer was employed, even though it had a less well-defined self-assembly structure in water, a multi-hole structure with ill-defined voids was still obtained. Use of a homopolymer, with no notable self-assembly in water, produced solid silica nanospheres, confirming both that the amino-group containing polymer assists in silica formation and that the hydrophobic portions of copolymers are key to achieving various silica morphologies. Of particular interest is a formation of a unique, single-hole nanosphere that showed consistent anisotropy when the longest block copolymer were

1

where dcore-shell is the distance between the centroid of the core and the shell, and L is the long axis of the shell. According to equation (1), the DA is 0% if the core is located at the center of nanoparticle, while the DA is 100 % when the centroid of the core is located on the outer circumference of a nanoparticle (Figure 7). N1 and N6 show similar distribution of the DA; the average±SD of DA is 32.7±15.0 % and 30.5±15.0 %, respectively. In contrast, the DA of N7 is significantly smaller with a narrower distribution such that the average±SD of the DA is

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employed. By changing the identity of the hydrophobic comonomer but not the polymer structure, control over the degree of anisotropy could be achieved. By applying the principles developed in this bioinspired approach, which enables fine-tune control over silica nanostructures, many more novel nanomaterials have the potential to be developed.

7.

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ASSOCIATED CONTENT

9.

Supporting Information. Figure S1–S7. This material is available free of charge via the Internet at http://pubs.acs.org.

10.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

11.

Author Contributions ‖

A.S-N. and S.T. contributed equally to this article.

12.

Present Addresses ¶Department

of Applied Chemistry, Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0385, Japan

13.

Notes 14.

The authors declare no competing financial interest.

ACKNOWLEDGMENT

15.

We thank Prof. Yukio Yamaguchi (The University of Tokyo) for DLS and zeta potential measurements. We also thank Dr. Junzheng Wang (The University of Tokyo) and Dr. Yu Shinke (Osaka University) for help in FE-SEM observation and polymer synthesis, respectively. This work was supported by a Grant-in-Aid for Scientific Research on the Innovative Areas: “Fusion Materials” (Area no. 2206) from MEXT and by JSPS KAKENHI Grant Numbers 17H03068 and 16K14090, Japan. A part of this work was supported in Nanotechnology Platform Program (Molecule and Material Synthesis), MEXT, Japan and conducted in Center for Nano Lithography & Analysis, The University of Tokyo and the High Voltage Electron Microscope Laboratory, Nagoya University.

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18. 19.

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