Preparation of Anisotropic Silica Nanoparticles via Controlled

Nov 11, 2010 - ASNPs are prepared via controlled self-assembly of spherical silica seeds (22 nm) in alcohol−water mixed media, followed by their in ...
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Preparation of Anisotropic Silica Nanoparticles via Controlled Assembly of Presynthesized Spherical Seeds Junzheng Wang, Ayae Sugawara, Atsushi Shimojima, and Tatsuya Okubo* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received September 6, 2010. Revised Manuscript Received October 22, 2010 A facile solution process for the preparation of anisotropic silica nanoparticles (ASNPs) is presented. ASNPs are prepared via controlled self-assembly of spherical silica seeds (22 nm) in alcohol-water mixed media, followed by their in situ fixation and overgrowth with tetraethoxysilane (TEOS). Ethanol and L-arginine (Arg) are used to modify the dielectric constant and ionic strength of the reaction media, by which seed assembly is controlled through the adjustment of electrostatic interaction. Ethanol and Arg also serve as a cosolvent and a catalyst for hydrolysis and condensation of TEOS, respectively, which enables us to produce ASNPs in a simple one-pot process. In addition to ASNPs with wormlike structures, different kinds of NPs (bimodal spherical NPs, monodisperse spherical NPs, and spherical aggregates) have also been obtained by changing the concentrations of ethanol and Arg. The length, thickness, or both of ASNPs are controlled systematically by varying the concentrations of Arg, seed NPs, and TEOS. Other alcoholic cosolvents, such as methanol, 1-propanol, 2-propanol, and t-butanol, are also effective to give ASNPs when the dielectric constant of the alcohol-water mixed media is properly adjusted, showing the versatility of the present method.

1. Introduction There has been growing interest in the synthesis of anisotropic colloidal particles as a new class of building blocks toward mimicking molecular assembly as well as for many potential applications.1 Anisotropic particles made of organic polymers or inorganics are among the critically important particles in the field of materials and colloid science. Anisotropic organic polymer particles with a variety of shapes have been prepared by many sophisticated methods including deformation in a polymeric matrix,2 flow-lithography,3 seeded emulsion polymerization,4 polymer protrusions,5 and salting out-quenching-fusing technique.6 In contrast, most of anisotropic inorganic particles have been synthesized by exploiting their intrinsic crystallographic anisotropy.7-10 The selective adsorption of surfactants to a particular crystal facet affects the rate of crystal growth, leading to the formation of rod-,8 disk-,9 *To whom correspondence should be addressed. Tel: þ81-3-5841-7348. Fax: þ81-3-5800-3806. E-mail: [email protected].

(1) (a) Yang, S.-M.; Kim, S-.H.; Lim, J.-M.; Yi, G.-R. H. J. Mater. Chem. 2008, 18, 2177–2190. (b) Zerrouki, D.; Baudry, J.; Pine, D.; Chaikin, P.; Bibette, J. Nature 2008, 455, 380–382. (2) Champion, J. A.; Katare, Y. K.; Mitragotri, S. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 11901–11904. (3) Dendukuri, D.; Pregibon, D. C.; Collins, J.; Hatton, T. A.; Doyle, P. S. Nat. Mater. 2006, 5, 365–369. (4) (a) Mock, E. B.; Bruyn, H. D.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037–4043. (b) Kim, J. W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374–14377. (c) Kraft, D. J.; Vlug, W. S.; van Kats, C. M.; van Blaaderen, A.; Imhof, A.; Kegel, W. K. J. Am. Chem. Soc. 2009, 131, 1182–1186. (d) Park, J. G.; Forster, J. D.; Dufresne, E. R. Langmuir 2009, 25, 8903–8906. (5) Yu, H. K.; Mao, Z. W.; Wang, D. Y. J. Am. Chem. Soc. 2009, 131, 6366– 6367. (6) Yake, A. M.; Panella, R. A.; Snyder, C. E.; Velegol, D. Langmuir 2006, 22, 9135–9141. (7) (a) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (b) Costi, R.; Saunders, A. E.; Banin, U. Angew. Chem., Int. Ed. 2010, 49, 4878–4897. (8) (a) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957–1962. (b) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633–3640. (c) Chen, Y. H.; Hung, H. H.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 9114–9121. (9) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874–12880. (10) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C. Nat. Mater. 2003, 2, 382–385.

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or branching-shaped crystals.8c,10 However, such an intrinsic anisotropy and rigid nature of inorganic materials limits the variety of particle shapes compared with those of flexible polymer materials. The fixation of anisotropically assembled colloidal clusters is one of the alternative ways to obtain anisotropic particles.11 This method is applicable for any kind of particles including polymer and inorganic particles. Anisotropic particles with intricate shapes can be obtained if anisotropically assembled colloidal clusters that are temporarily formed in the suspension are fixed in situ. For example, Johnson et al. prepared anisotropic silica particles such as dumbbells by destabilizing seed particles (over 200 nm in size), followed by silica shell coating.11a Seed particles were destabilized to form clusters through either shear-induced aggregation in high ionic strength or depletion-induced aggregation driven by surfactant micelles. Xia and coworkers also obtained silica dumbbells in relatively high yield (50%) using spherical silica (280 nm in size) as seeds by controlling the density of charges on the seed surface and the dielectric constant of the medium.11b Gold nanoparticles with chain-like structures were also fixed by silica shells; otherwise, they aggregated with time and finally precipitated.11e To take advantage of the above in situ fixing approach, it is important to control the assembly of particles12 prior to the fixation process. One-dimensional assembly has been one of the challenging targets because of its highly anisotropic nature. Some examples of 1D assembly of metal and semiconductor particles in liquid phases have been reported. Most of them are based on inherent anisotropy of magnetic13 or electric dipoles,14 crystalface specific heterogeneity,15 and nonuniform distribution of ligands on the surfaces.16,17 In some cases, external forces such (11) (a) Johnson, P. M.; van Kats, C. M.; van Blaaderen, A. Langmuir 2005, 21, 11510–11517. (b) Ibisate, M.; Zou, Z. Q.; Xia, Y. N. Adv. Funct. Mater. 2006, 16, 1627–1632. (c) Cho, Y.-S.; Yi, G.-R.; Lim, J.-M.; Kim, S.-H.; Manoharan, V. N.; Pine, D. J.; Yang, S.-M. J. Am. Chem. Soc. 2005, 127, 15968–15975. (d) Li, F.; Stein, A. J. Am. Chem. Soc. 2009, 131, 9920–9921. (e) Cho, E. C.; Choi, S.-W.; Camergo, P. H. C.; Xia, Y. Langmuir 2010, 26, 10005–10012. (12) (a) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. ACS Nano 2010, 4, 3591–3605. (b) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Small 2009, 14, 1600–1630.

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as magnetic18 and gravity fields19 were applied. Templating methods using linear biomacromolecules20 and carbon nanotubes21 were straightforward ways to achieve linear particle assembly. Polymer-assisted 1D assembly of gold NPs was also reported.22 1D assembly of silica particles has attracted particular attention from both fundamental and practical viewpoints. The flocs of silica with chain-like structures were theoretically well described by Iler23 and Thomas et al.24 The formation of elongated-shaped silica sol was experimentally observed by Watanabe et al. through the fine control of several parameters (ionic strength, pH value, and heating temperature).25 The elongated-shaped silica sol displays an excellent property as a coating material, and thus it has widespread use in industrial fields.25 Polystyrene-grafted silica NPs were assembled linearly in the corresponding polymer matrix.26 Our group found out that silica NPs assembled into straight chainlike structures in the presence of an amphiphilic block copolymer.27 Chain-like silica NPs were also obtained with the aid of thermoresponsive polymers.28 Great progress has been made to date; however, the systematic control on the self-assembly of silica sol as well as fine-tuning of the morphology of the resultant silica NPs are still to be explored. Here we report a facile approach to obtain anisotropic silica NPs via 1D assembly of seed NPs in the absence of organic polymers, followed by their in situ fixation and growth with the aid of additional silicate species. Silica nanospheres of 22 nm in size are used as seeds, and tetraethoxysilane (TEOS) is used for the fixation and further growth of the preassembled seeds. It is well known that placing hydrophilic colloidal particles in a solution of (13) (a) Gao, J. H.; Zhang, B.; Zhang, X. X.; Xu, B. Angew. Chem., Int. Ed. 2006, 45, 1220–1223. (b) Salgueiri~no-Maceira, V.; Correa-Duarte, M. A.; Hucht, A.; Farle, M. J. Magn. Magn. Mater. 2006, 303, 163–166. (c) Guo, L.; Liang, F.; Wen, X. G.; Yang, S. H.; He, L.; Zheng, W. Z.; Chen, C. P.; Zhong, Q. P. Adv. Funct. Mater. 2007, 17, 425– 430. (d) Zhang, F.; Wang, C. C. J. Phys. Chem. C 2008, 112, 15151–15156. (e) Huang, J.; Chen, W. M.; Zhao, W.; Li, Y. Q.; Li, X. G.; Chen, C. P. J. Phys. Chem. C 2009, 113, 12067–12071. (f) Korth, B. D.; Keng, P.; Shim, I.; Bowles, S. E.; Tang, C. B.; Kowalewski, T.; Nebesny, K. W.; Pyun, J. J. Am. Chem. Soc. 2006, 128, 6562–6563. (14) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (15) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751–754. (16) (a) Zhang, H.; Fung, K.-H.; Hartmann, J.; Chan, C. T.; Wang, D. J. Phys. Chem. C 2008, 112, 16830–16839. (b) Sethi, M.; Knecht, M. R. ACS Appl. Mater. Interfaces 2009, 1, 1270–1278. (17) (a) DeVries, G. A.; Brunnbauer, M.; Hu, Y.; Jackson, A. M.; Long, B.; Neltner, B. T.; Uzun, O.; Wunsch, B. H.; Stellacci, F. Science 2007, 315, 358–361. (b) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. Adv. Mater. 2005, 17, 2553–2559. (c) Sardar, R.; Shumaker-Parry, J. S. Nano Lett. 2008, 8, 731–736. (d) Hussain, I.; Brust, M.; Barauskas, J.; Cooper, A. I. Langmuir 2009, 25, 1934–1939. (18) (a) Singh, H.; Laibinis, P. E.; Hatton, T. A. Langmuir 2005, 21, 11500– 11509. (b) Wang, H.; Chen, Q. W.; Sun, L. X.; Qi, H. P.; Yang, X.; Zhou, S.; Xiong, J. Langmuir 2009, 25, 7135–7139. (c) Zhang, Y.; Sun, L.; Fu, Y.; Huang, Z. C.; Bai, X. J.; Zhai, Y.; Du, J.; Zhai, H. R. J. Phys. Chem. C 2009, 113, 8152–8157. (d) Cheng, G.; Romero, D.; Fraser, G. T.; Hight Walker, A. R. Langmuir 2005, 21, 12055–12059. (19) Roca, M.; Pandya, N. H.; Nath, S.; Haes, A. J. Langmuir 2010, 26, 2035– 2041. (20) (a) Hong, M.; Wu, L. L.; Tian, L. F.; Zhu, J. Chem.;Eur. J. 2009, 15, 5935– 5941. (b) Sharma, N.; Top, A.; Kiick, K. L.; Pochan, D. J. Angew. Chem., Int. Ed. 2009, 48, 7078–7082. (c) Harnack, O.; Ford, W. E.; Yasuda, A.; Wessels, J. M. Nano Lett. 2002, 2, 919–923. (d) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272–277. (e) Leunissen, M. E.; Dreyfus, R.; Cheong, F. C.; Grier, D. G.; Sha, R. J.; Seeman, N. C.; Chaikin, P. M. Nat. Mater. 2009, 8, 590–595. (f) Ofir, Y.; Samanta, B.; Rotello, V. M. Chem. Soc. Rev. 2008, 37, 1814–1825. (21) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182–193. (22) (a) Polavarapu, L.; Xu, Q. H. Langmuir 2008, 24, 10608–10611. (b) Kang, Y.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800–13801. (23) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979; p 225. (24) Thomas, I. L.; McCorkle, K. H. J. Colloid Interface Sci. 1971, 36, 110–118. (25) Watanabe, Y.; Ando, M.; Tanimoto, K.; Kagamimiya, T.; Kawashima, M. European Patent EP 0335195 A2, 1989. (26) Akcora, P.; Liu, H. J.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; Ganesan, V.; Ilavsky, J.; Thiyagarajan, P.; Colby, R. H.; Douglas, J. F. Nat. Mater. 2009, 8, 354–359. (27) Fukao, M.; Sugawara, A.; Shimojima, A.; Fan, W.; Arunagirinathan, M. A.; Tsapatsis, M.; Okubo, T. J. Am. Chem. Soc. 2009, 131, 16344–16345. (28) Babayan, D.; Chassenieux, C.; Lafuma, F.; Ventelon, L.; Hernandez, J. Langmuir 2010, 26, 2279–2287.

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low dielectric constant, high ionic strength, or both results in aggregation of the particles due to suppression of electrostatic repulsion.16 Our present approach is to use water-miscible organic solvent ethanol and basic amino acid L-arginine (Arg) to lower the dielectric constant and to increase the ionic strength of the suspension, respectively. The ethanol and Arg have additional roles in this system because they also serve as a cosolvent and a catalyst for hydrolysis and condensation of TEOS, respectively. We have systematically examined the effects of ethanol and Arg on the morphologies of the resultant NPs. Four distinct morphological types of NPs including wormlike anisotropic silica NPs (ASNPs) have emerged according to the concentration of ethanol and Arg. The shape of ASNPs has been further tuned by adjusting the concentration of seeds or TEOS.

2. Experimental Section 2.1. Materials. TEOS and L-alanine were purchased from Tokyo Chemical Industry and used as received. Methanol, ethanol, 1-propanol, 2-propanol, t-butanol, L-arginine, and L-glutamic acid were obtained from Wako Pure Chemical Industries and were used without further purification. Milli-Q water (18.2 MΩ cm-1) was used for all experiments. Silicon wafers (SEH) were cleaned up with Semico Clean 56 (Furuuchi Chemical) in an ultrasonic bath for 15 min and then with water for 20 min at room temperature, followed by rinsing with ethanol and air drying. 2.2. Synthesis of Spherical Silica Seeds. Spherical silica seeds were synthesized by the method developed by us.29 In a typical synthesis, Arg (54.6 mg) was dissolved in water (41.4 g) in a 110 mL glass vial at room temperature. TEOS (3.13 g) was added to the Arg-water solution. The reaction was carried out at 60 °C in a water bath under magnetic stirring for 24 h. The stirring rate was maintained at ca. 500 rpm to obtain TEOS-in-water emulsion system using a 2 cm long Teflon-coated magnetic stirring bar. Spherical silica seeds having an average diameter of 22 nm were synthesized with excellent reproducibility under these reaction conditions. 2.3. Preparation of Silica NPs with Different Morphologies. Silica nanostructures were prepared by a seed-assembly and growth approach. In brief, an appropriate portion of the 2 wt % silica seed suspension (0.4-4.4 g) was added to a ethanol-water solution (ethanol 23-83 wt %) containing Arg (0.05 to 0.4 wt %). Subsequently, the required amount of TEOS (0.05 to 0.9 g) was added to the system. The total weight of ethanol and water was kept constant (17.2 g) when changing ethanol-water weight composition in each experiment. The final concentrations of silica seeds and TEOS are in the range of 0.045 to 0.50 and 0.29-5.0 wt %, respectively. The reaction was conducted in a constant-temperature water bath while maintaining constant stirring. The typical composition for the preparation of ASNPs was as follows: 1.2 g of the 2 wt % silica seed suspension, 51.8 mg of Arg, 12.8 g of ethanol, 3.20 g of water, and 0.5 g of TEOS. After the addition of TEOS, the reaction was allowed to proceed at 60 °C for 12 h under constant stirring at ca. 500 rpm. In some experiments, other types of water-miscible organic solvents and amino acids were used in place of ethanol and Arg, respectively. Here for simplicity, Arg (0.12 wt %) and ethanol (ca. 6 wt %) in seed suspension and ethanol generated by hydrolysis of TEOS during growth process were not taken into account for the calculation of final composition of reaction media. 2.4. Characterization. Scanning electron microscopy (SEM) images were obtained using Hitachi S-900 at an accelerating voltage of 6 kV. Samples were prepared in the following manner: one droplet (3 μL) of dispersion was spread onto a cleaned silicon wafer using the spin-coating method. The rotational speed was set (29) (a) Yokoi, T.; Sakamoto, Y.; Terasaki, O.; Kubota, Y.; Okubo, T.; Tatsumi, T. J. Am. Chem. Soc. 2006, 128, 13664–13665. (b) Yokoi, T.; Wakabayashi, J.; Otsuka, Y.; Fan, W.; Iwama, M.; Watanabe, R.; Aramaki, K.; Shimojima, A.; Tatsumi, T.; Okubo, T. Chem. Mater. 2009, 21, 3719–3729.

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Figure 1. Morphology diagram of silica NPs at different ethanol and Arg concentrations. (I) Square symbols represent bimodal spherical silica NPs, (II) triangle symbols show monodisperse spherical silica NPs, (III) diamond symbols indicate anisotropic silica NPs, and (IV) circle symbols show spherical aggregates. at 2500 rpm, and the time of spinning was 20 s. Prior to imaging, the samples were coated with a Pt layer for 20 s in an argon atmosphere using an ion sputter system, Hitachi E-1030. The average sizes of silica particles are calculated from the diameters of 100 independent particles. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2000EXII microscope at an accelerating voltage of 200 kV. Dynamic light scattering (DLS) and zeta potential measurements of colloidal suspensions were performed with a Malvern Zetasizer Nano ZS90 instrument at 25 °C, and the data were analyzed by Dispersion Technology Software (version 5.10). The hydrodynamic diameter of particles was calculated by cumulant method. The zeta potential of seed NPs was determined from the measured electrophoretic mobility using the Smoluchowski approximation.

3. Results and Discussion 3.1. Morphology Diagram. The effects of the concentrations of Arg and ethanol have been systematically examined by observing the resultant silica NPs with SEM. It is revealed that as the Arg or ethanol concentrations increase, four distinct morphological types of silica NPs are obtained: bimodal spherical silica NPs, monodisperse spherical silica NPs, ASNPs, and spherical aggregates (Figure 1). The individual morphologies formed in each region of the diagram are discussed in the following. Here ethanol, Arg, and TEOS concentrations were as follows: 2383 wt % of ethanol, 0.05-0.4 wt % of Arg, 0.14 wt % of silica seeds, and 2.8 wt % of TEOS. 3.1.1. Bimodal Spherical Silica NPs (Figure 1I). At lower ethanol concentrations between 23 and 65 wt %, spherical silica NPs with bimodal size distributions are obtained in the presence of an appropriate amount of Arg (Figure 1). The representative SEM images of the resultant silica NPs are shown in Figure 2A-C. Note that densely packed NPs on Si substrate are not the result of NPs aggregation in suspension but the result of sample preparation by spin coating. The suspension is optically transparent, showing good colloidal stability of the bimodal NPs Langmuir 2010, 26(23), 18491–18498

(Figure 2D). The sizes of the bimodal NPs are analyzed from the SEM images. Bimodal NPs with average sizes of 9 and 28 nm are obtained at 45 wt % ethanol and 0.1 wt % Arg (Figure 2A). Considering that the size of the seed NPs is 22 nm, it is suggested that both new particle formation and growth of the seeds occur simultaneously by hydrolysis and condensation of TEOS. The sizes of both the bigger and smaller NPs increase with increasing ethanol concentration under constant Arg concentration (0.1 wt %). For example, the average sizes of bimodal silica NPs increase to 12 and 34 nm at 65 wt % ethanol (Figure 2C). A similar tendency is observed at a constant Arg concentration of 0.3 wt %. (See Figure S1 in the Supporting Information). In contrast, the concentration of Arg (0.1 to 0.3 wt %) has a negligible effect on particle size distributions under constant ethanol concentration (e.g., 55 wt % ethanol, data not shown). It should be noted that TEOS is not completely soluble in the reaction media at ethanol concentrations below 39 wt %, and thus the reaction system is two-phase. The reaction mixture becomes homogeneous at higher ethanol concentrations (>39 wt %). However, no obvious morphological difference is recognized between silica NPs formed in two-phase (Figure S1A of the Supporting Information) and one-phase (Figure S1B of the Supporting Information) reaction systems. Both systems tend to induce secondary seed formation as long as the ethanol concentration is kept at the lower range (23-65 wt %). 3.1.2. Monodisperse Spherical Silica NPs (Figure 1II). Monodisperse spherical silica NPs with excellent colloidal stability are formed at higher ethanol concentrations of 62-74 wt % (Figure 1). Uniform-sized silica NPs with diameter of ∼38 nm are obtained under all experimental conditions that give monodisperse NPs. The representative SEM images are shown in Figure 3 and Figure S2 of the Supporting Information. These results suggest that the TEOS added is evenly consumed for the growth of the seed NPs without forming new particles. Therefore, the concentration of Arg again does not exert effects on the final particle size. Interestingly, however, bimodal spherical silica NPs are observed at ethanol concentration of 62 (or 65 wt %) and 0.1 wt % Arg. It is unclear at present why the bimodal spherical silica NPs start to emerge at lower Arg concentration. Harten et al. reported the preparation of monodisperse silica spheres through a regrowth approach using silica NPs and TEOS as seeds and growth precursor, respectively.30 They succeeded in seeded regrowth without forming new particles by using either (1) water as reaction medium (two-phase reaction system) with a small amount of Arg or (2) ethanol-water mixed medium containing ammonia (St€ober method31). Very recently, Watanabe et al. found that mono disperse silica spheres could be prepared by the seed regrowth method using ethanol-water mixed medium containing Arg.32 Our results obtained here are in good agreement with previous works that are described above. 3.1.3. Anisotropic Silica Nanoparticles (Figure 1III). Anisotropic silica nanoparticles (ASNPs) are formed at a narrow range of ethanol concentration around 74 wt % and 0.1 to 0.3 wt % Arg (Figure 1). Unlike the results at lower ethanol concentrations (23-65 wt %), the morphology of silica NPs is drastically affected by the concentration of Arg (Figure 4). For example, at the constant ethanol concentration of 74 wt %, the silica NP morphology evolves from spherical, dumbbell-like to wormlike with increasing Arg concentration (Figure 4A-D). In other (30) Hartlen, K. D.; Athanasopoulos, A. P. T.; Kitaev, V. Langmuir 2008, 24, 1714–1720. (31) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (32) Watanabe, R.; Yokoi, T.; Kobayashi, E.; Otsuka, Y.; Shimojima, A.; Okubo, T.; Tatsumi, T. J. Colloid Interface Sci. in press.

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Figure 2. (A-C) SEM images of bimodal silica NPs prepared at different ethanol concentrations (A) 45, (B) 55, and (C) 65 wt %, whereas Arg concentration is kept at 0.1 wt %. The upper and lower insets in parts A and C show the high-magnification SEM images and the size distribution histograms of bimodal silica NPs on the corresponding SEM images, respectively. (D) Photograph of a suspension of bimodal silica NPs obtained at 45 wt % ethanol and 0.1 wt % Arg.

Figure 3. SEM image of monodisperse silica NPs prepared at 70 wt % ethanol, 0.15 wt % Arg. The inset shows the size distribution histogram of monodisperse silica NPs. 18494 DOI: 10.1021/la103564p

words, the chain length increases progressively along with the increase in Arg concentration. Additionally, these ASNPs are stable in suspension, and no precipitation has been observed for at least several months. 3.1.4. Spherical Aggregates (Figure 1IV). When the ethanol concentration reaches ∼80 wt %, the suspension becomes turbid in several hours after the addition of TEOS, and precipitation occurs at the end of the reaction. Highly aggregated particles are observed by SEM, as shown in Figure S3 of the Supporting Information. 3.1.5. Summary of the Morphology Evolution. As discussed above, four different morphological types of silica NPs have been produced simply by changing the ethanol and Arg concentrations (Figure 1). At lower ethanol concentrations (23-65 wt %), bimodal spherical silica NPs are formed (Figure 2 and Figure S1 of the Supporting Information). Monodisperse spherical silica NPs are generated with slightly higher concentrations of ethanol (62-74 wt %) and comparable Arg concentration (Figure 3 and Figure S2 of the Supporting Information). When ethanol concentration increases to ∼74%, ASNPs start to appear (Figure 4). With higher ethanol concentration (80 wt %), silica seeds aggregate with each other, finally leading to the formation of spherical aggregates (Figure S3 of the Supporting Information). It seems that ASNPs can only be obtained under relatively limited reaction conditions where anisotropic assembly of seed NPs is attained. Hereafter, we focus on the formation of ASNPs because the Langmuir 2010, 26(23), 18491–18498

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Figure 4. SEM images of ASNPs formed at 74 wt % ethanol and varied concentrations of Arg: (A) 0.1, (B) 0.2, (C) 0.25, and (D) 0.3 wt %. The insets show the histograms of the number of silica seeds that constitute each ASNP. Scheme 1. Seed-Assembly and Growth Process

preparation of such intricate silica nanostructures with the aid of Arg is currently not well studied. The formation process, effects of ethanol and Arg, as well as morphological control of ASNPs is discussed in the following section. 3.2. Formation Process of ASNPs. The formation process of ASNPs can be summarized in schematic illustration as shown in Scheme 1. The growth process of ASNPs should contain two stages: (1) 1D assembly of silica seeds under the optimized ethanol and Arg concentrations and (2) fixation of the assembled structure by hydrolysis and condensation of TEOS. These stages can proceed simultaneously. To verify the presence of the assembly stage 1, we have analyzed the assembled structure of seed NPs formed before the addition of TEOS by using DLS and SEM. Silica seeds were added to a growth medium (ethanol 74 wt %, Arg 0.3 wt %), followed by aging at 60 °C. To monitor the assembly process, we aged seed NPs for varied aging time of 1, 3, 6, and 18 h without adding TEOS. DLS shows that the average particle size of the suspension increases with time (Figure 5A), Langmuir 2010, 26(23), 18491–18498

indicating the seed assembly in the suspension. It should be noted that the particle sizes determined here are qualitative and not an accurate reflection of the actual size, shape, or both. The morphologies of the assembled seeds have been observed by SEM. Wormlike 1D structures are observed, as shown in Figure 5B (a and b), which correspond to the samples after 1 and 18 h of aging, respectively. It is suggested that silica seeds selfassemble into wormlike 1D structure at a relatively high speed (several hours), whereas silica NPs assemble into straight chainlike structures in the presence of an amphiphilic block copolymer at a low speed (several days).27 ASNPs are generated by the addition of 1.1 wt % TEOS (Figure 5B, c and d) from the corresponding seed suspensions. It is hard to recognize from the SEM images whether longer ASNPs are obtained with longer aging time, whereas DLS indicates the progress of the seed assembly with time. Considering the fact that ASNPs with high aspect ratio are generated even when TEOS is added just after seed introduction to the reaction media (Figure 4D), DOI: 10.1021/la103564p

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Figure 6. SEM images of silica NPs formed in alcohol-water mixed media (approximate dielectric constant 36.4) in the presence of 0.3 wt % Arg. The alcohol contents are as follows: (a) 93 wt % methanol, (b) 63 wt % of 1-propanol, (c) 65 wt % of 2-propanol, and (d) 50 wt % of t-butanol.

Figure 5. (A) Time-dependent increase in the average particle size of seed suspension determined by DLS. (B) SEM images of (a,b) silica seeds aged at 74 wt % ethanol and 0.3 wt % Arg, 60 °C for (a) 1 and (b) 18 h; (c,d) corresponding ASNPs formed by the addition of 1.1 wt % TEOS to assembled seeds (a,b).

it is likely that the assembly and growth of seed NPs proceed simultaneously. 3.3. Role of Ethanol and Arg. 3.3.1. Effects of Ethanol and Other Alcohols. As discussed in the Section 3.1, the concentration of ethanol drastically affects the morphology of silica NPs. At the constant Arg concentration, for example, 0.3 wt %, ASNPs are formed at very narrow ethanol concentration range (74-76 wt %), whereas seed NPs remain dispersed and randomly aggregate at the lower and higher ethanol concentrations, respectively. 1D assembly is likely accomplished under intermediate ethanol concentration, where attraction and repulsion between particles are well-balanced. The original seed NPs used in this study are colloidally stable because of their large negative charge in the presence of Arg.29b The zeta potential of as-made seed NPs is -51 mV at pH 9.2. It is well known that the addition of organic solvents to water results in the decrease in dielectric constant of the medium, which is effective to decrease electrostatic repulsion between charged particles.33 The relative dielectric constants of ethanol and water are (33) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, 1992.

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24.3 and 80.4 at room temperature, respectively. At 74 wt % ethanol concentration, where ASNPs are successfully obtained, the dielectric constant of the ethanol-water mixed media is calculated to be 36.4. (See Figure S4 in the Supporting Information for the calculation.) It is expected that the use of other water-miscible organic solvents is also effective for the formation of ASNPs if the electrostatic repulsion between seed NPs is adequately controlled. Comparative experiments have been carried out by replacing ethanol with other alcohols such as methanol, 1-propanol, 2-propanol, and t-butanol. As shown in Figure 6, ASNPs can be prepared in these alcohol-water mixtures having approximately the same dielectric constant (36.4) in the presence of 0.3 wt % of Arg. Moreover, the same tendency such that monodisperse, bimodal NPs, or both are formed with decreasing the concentration of alcohol is also observed (Figure S5 in the Supporting Information). 3.3.2. Effects of Arg and Other Amino Acids. The length of ASNPs increases with the increase in Arg concentration (Figure 4A-D) at constant ethanol concentration (74 wt %). This result indicates that Arg promotes the assembly of seed NPs in the suspension. Here the amount of Arg is varied from 0.05 to 0.4 wt %, which corresponds to approximately (1.4  104)-fold to (1.1  105)-fold excess with respect to the number of seed NPs. Although it is not easy to evaluate the charge state of Arg in ethanol-water mixed solvent, Arg may exist as cationic or zwitterionic state under weakly basic conditions. Supposing that Arg can bind to seed NPs through electrostatic interaction with a surface density of 0.5 nm-2, as in the case of L-lysine,29b unbound Arg molecules are present in the solution. Therefore, the higher concentration of Arg leads to higher ionic strength of the medium, which in turn weakens the electrostatic repulsion between the seed NPs. We have tested whether ASNPs can be produced by using other types of amino acids having different charge characteristics. Glutamic acid (Glu) and alanine (Ala) have been used for comparison, which are negatively charged and zwitterionic when Langmuir 2010, 26(23), 18491–18498

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Figure 7. SEM images of ASNPs formed from varied concentrations of seed NPs: (A) 0.045, (B) 0.20, (C) 0.36, and (D) 0.50 wt % of silica seeds. The insets show the histograms of the number of silica seeds that constitute each ASNP.

dissolved in water, respectively. Seed-assembly and growth technique has been performed in an ethanol-water mixed medium (ethanol 74 wt %, single phase) with Glu or Ala concentration ranges similar to the case of Arg. It is found that the type of amino acids affects the morphology of resultant NPs dramatically. For example, bimodal silica NPs are obtained in the presence of 0.25 wt % of Glu (Figure S6A of the Supporting Information) and monodisperse NPs are formed with 0.15 wt % of Ala (Figure S6B of the Supporting Information). The pH value of the growth media changes by the addition of different types of amino acids, which alters the surface electronic potential of silica NPs as well as the rate of hydrolysis and condensation of TEOS. This may lead to the different NPs morphology. The origin of linear assembly is still unclear. Wang et al. reported linear assembly of gold nanoparticles by utilizing selective end-on attachment, where charge repulsion is bigger attaching to the sides than attaching to the ends of a nanochain.34 We suppose that a similar mechanism might exist in our system. More recently, Sethi et al. reported that citrate-stabilized gold NPs assemble into 1D chainlike structures in the presence of Arg.16b They suppose that Arg was partially replaced by the citrate stabilizer to form segregated patchy network owing to its zwitterionic nature of Arg. As a result, an electronic dipole arises on the NPs surface, which triggers 1D assembly. Such a surface segregation model might also be applicable in the present system. Further studies are (34) Zhang, H.; Wang, D. Angew. Chem., Int. Ed. 2008, 47, 3984–3987.

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required to elucidate the mechanism of 1D self-assembly of the silica seeds. 3.4. Morphological Control of ASNPs. 3.4.1. Effect of the Concentration of Seeds. The length-to-diameter ratio of ASNPs changes dramatically depending on the seed concentration. The concentration of silica seeds has been varied from 0.045 to 0.50 wt % while keeping other parameters constant: 74 wt % ethanol, 0.3 wt % Arg, and 2.8 wt % TEOS. The resultant NPs are typically found as monomer, dimer, or trimer at lower seed concentrations (Figure 7A). ASNPs with high aspect ratio start to form with the increase in seed concentration (Figure 7B-D). These results can be explained by the faster 1D assembly of seed NPs at higher concentrations because the collision frequency between NPs increases with the increase in the number density of them. The decrease in the diameter of ASNPs with an increase in the seed concentration is reasonable provided that TEOS is evenly consumed for the growth of the seeds. 3.4.2. Effect of the Concentration of TEOS. Only the diameter of the ASNPs is changed by varying the concentration of TEOS. The concentration of TEOS added was changed from 0.29 to 4.96 wt %, whereas the other parameters were fixed as follows: 0.14 wt % silica seeds, 74 wt % ethanol, and 0.3 wt % Arg. Figure 8A shows the photographs of the colloidal suspensions with different concentration of TEOS. The turbidity of the suspension increases with increasing dosage of TEOS. All samples are colloidally stable over several weeks. Figure 8B illustrates diameters of the ASNPs as a function of the concentration of TEOS DOI: 10.1021/la103564p

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Figure 8. (A) Photograph of the silica sols prepared with different concentrations of TEOS: (a) 0.29, (b) 0.58, (c) 1.1, (d) 1.7, (e) 2.3, (f) 3.4, (g) 4.0, and (h) 5.0 wt %. (B) Average diameter of the ASNPs as a function of TEOS concentration calculated from SEM images (C). (C) SEM images of ASNPs corresponding to the samples in part A. Insets show the TEM images.

determined from the SEM images (Figure 8C). The diameters change from 23 to 64 nm just by increasing the concentration of TEOS. Meanwhile, the chain length remains almost unchanged.

as long as silica is coated on their surface. This route may provide a powerful means of preparing intricate anisotropic particles with high aspect ratio.

4. Conclusions ASNPs are prepared in ethanol-water mixed media containing L-arginine (Arg) through a facile and efficient seed-assembly and growth method. The key of this method is the anisotropic 1D self-assembly of silica seeds and subsequent in situ structural fixation by the hydrolysis and condensation of TEOS. A systematic investigation on the concentration of ethanol has revealed that ASNPs are formed at relatively limited ethanol concentration range. Four distinct morphology types of silica NPs, bimodal spherical NPs, monodisperse spherical NPs, ASNPs, and spherical aggregates, emerge as the concentration of ethanol increases. The use of other alcohols is also effective if the dielectric constant of the mixed media is properly adjusted. The thickness and length of ASNPs can be finely tuned simply by changing the concentrations of Arg, seed NPs, and TEOS. This method can, in principle, be applied to other types of nanoparticles (e.g., metal oxide NPs)

Acknowledgment. We thank Prof. Yukio Yamaguchi (The University of Tokyo) for DLS and zeta potential measurements. J.W. is grateful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for the Monbukagakusho Scholarship. A. Sugawara acknowledges Grant-in-Aid for Young Scientists (B) (21750213) from MEXT. A part of this work was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by MEXT. Global COE Program (GMSI), in the University of Tokyo, supported by MEXT is also acknowledged.

18498 DOI: 10.1021/la103564p

Supporting Information Available: SEM images of silica NPs under various synthetic conditions and the calculations for dielectric constants of organic solvent-water mixed media. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(23), 18491–18498