Synthesis of Monodisperse Mesoporous Silica Hollow Microcapsules

Jul 28, 2010 - Monodisperse polystyrene (PS) particles were used as templates for ..... spheres by treating layered mesoporous silica with a basic sol...
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Synthesis of Monodisperse Mesoporous Silica Hollow Microcapsules and Their Release of Loaded Materials Noritaka Kato,* Tomohiro Ishii, and Shouzou Koumoto Department of Electronics and Bioinformatics, School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan Received June 16, 2010. Revised Manuscript Received July 17, 2010 Monodisperse mesoporous silica hollow capsules (MSHCs) with sizes ranging from 570 to 75 nm were synthesized using the sol-gel method combined with the template-assisted method. Monodisperse polystyrene (PS) particles were used as templates for the hollow structure of the MSHCs, and cylindrical micelles of cationic surfactant were used to create mesopores across the shell of the MSHC. To obtain MSHCs with a degree of polydispersity in diameter comparable to that of the PS templates and spherical in shape with uniform shell thicknesses, the conditions for the synthesis were systematically examined. It was found that the ranges of the reaction conditions to obtain such MSHCs had to be narrow because (1) the colloidal stability of the particles must be maintained before and after the sol-gel reaction and (2) the rate of the silica formation during the reaction must be regulated to attain sufficient shell thickness to retain the hollow structure and to achieve smooth surfaces. The sustained release of dye molecules loaded in the MSHCs was confirmed, indicating that our MSHC is a candidate for use as a drug carrier in drug delivery systems or as a container for microreactors.

Introduction Mesoporous silica (MS) particles1,2 are of great interest for applications as drug carriers in drug delivery systems (DDSs)3-7 and microcontainers for catalysts in microreactors.8-11 Cylindrical micelles of surfactants are utilized as templates for mesoporous structures, and the structures of the MS particles, such as pore periodicity, pore size, particle size, and particle shape, can be controlled by the species of surfactant4,12-16 and molar ratio of the silica precursor to surfactants17 in the reaction solution. The sol-gel reaction of silica precursors (hydrolysis and condensation of alkoxysilanes) to form silica/micelle complexes can be *To whom correspondence should be addressed: e-mail [email protected]. ac.jp, Fax þ81-44-934-7292.

(1) Monnier, A.; Sch€uth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299–1303. (2) Ciesla, U.; Sch€uth, F. Microporous Mesoporous Mater. 1999, 27, 131–149. (3) Slowing, I. I.; Vivero-Escoto, J. L.; Wu, C.-W.; Lin, V. S.-Y. Adv. Drug Delivery Rev. 2008, 60, 1278–1288. (4) Vallet-Regı´ , M.; Balas, F.; Arcos, D. Angew. Chem., Int. Ed. 2007, 46, 7548– 7558. (5) Sousa, A.; Souza, K. C.; Sousa, E. M. B. Acta Biomater. 2008, 4, 671–679. (6) Muoz, B.; Rmila, A.; Prez-Pariente, J.; Daz, I.; Vallet-Reg, M. Chem. Mater. 2003, 15, 500–503. (7) Lu, J.; Choi, E.; Tamanoi, F.; Zink, J. I. Small 2008, 4, 421–426. (8) Sun, J.; Bao, X. Chem.-Eur. J. 2008, 14, 7478–7488. (9) Yang, Q.; Liu, J.; Zhang, L.; Li, C. J. Mater. Chem. 2009, 19, 1945–1955. (10) Lee, C.-H.; Lin, T.-S.; Mou, C.-Y. Nano Today 2009, 4, 165–179. (11) Ispas, C.; Sokolov, I.; Andreescu, S. Anal. Bioanal. Chem. 2009, 393, 543– 554. (12) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147–1160. (13) Romero, A. A.; Alba, M. D.; Zhou, W.; Klinowski, J. J. Phys. Chem. B 1997, 101, 5294–5300. (14) Zhu, G.; Qiu, S.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723– 7724. (15) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865–867. (16) Ogura, T.; Sakai, K.; Sakai, H.; Abe, M. J. Phys. Chem. C 2008, 112, 12184– 12187. (17) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317–2326. (18) Lind, A.; du Fresne von Hohenesche, C.; Sma˚tt, J.-H.; Linden, M.; Unger, K. K. Microporous Mesoporous Mater. 2003, 66, 219–227.

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performed under a wide range of conditions, from high temperature and high pressure using an autoclave to an ambient condition.18,19 After the reaction, the micelle templates are removed by pyrolysis2-7,9,10,12-14,16-19 or dissolution3,4,7,10,15 of the micelle templates. Microcapsules prepared by the colloid-templating method20-24 also have potential applications in catalysis and DDS. The colloid particles as core templates are coated by either polymer or inorganic materials via electrostatic interaction, and the core particle is subsequently removed, resulting in microcapsules. Using such microcapsules, model drugs can be loaded and the release of the loaded materials can be controlled by external stimulation. Light25-27 and ultrasound28,29 irradiations to the capsules trigger rupture of the capsules via thermal shocks induced by the absorption of light by the capsules and the shear force of the sound wave, respectively. A pH change to a medium that can alter the permeability of the polyelectrolyte shell is also applied in controlled release.30 To incorporate the advantages of both mesoporous particles and microcapsules into drug carriers and microcontainers for catalysis, the synthesis of mesoporous silica hollow capsules (MSHC) has been proposed. A combination of two templates, (19) Kumar, D.; Schumacher, K.; du Fresne von Hohenesche, C.; Gr€un, M.; Unger, K. K. Colloids Surf., A 2001, 187-188, 109–116. (20) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111–1114. (21) Caruso, F. Aust. J. Chem. 2001, 54, 349–353. (22) De Geest, B. G.; Sanders, N. N.; Sukhorukov, G. B.; Demeester, J.; De Smedt, S. C. Chem. Soc. Rev. 2007, 36, 636–649. (23) Tong, W.; Gao, C. J. Mater. Chem. 2008, 18, 3799–3812. (24) Jewell, C. M.; Lynn, D. M. Adv. Drug Delivery Rev. 2008, 60, 979–999. (25) Radt, B.; Smith, T. A.; Caruso, F. Adv. Mater. 2004, 16, 2184–2189. (26) Skirtach, A. G.; Javier, A. M.; Kreft, O.; K€ohler, K.; Alberola, A. P.; M€ohwald, H.; Parak, W. J.; Sukhorukov, G. B. Angew. Chem., Int. Ed. 2006, 45, 4612–4617. (27) Katagiri, K.; Koumoto, K.; Iseya, S.; Sakai, M.; Matsuda, A.; Caruso, F. Chem. Mater. 2009, 21, 195–197. (28) Shchukin, D. G.; Gorin, D. A.; M€ohwald, H. Langmuir 2006, 22, 7400– 7404. (29) De Geest, B. G.; Skirtach, A. G.; Mamedov, A. A.; Antipov, A. A.; Kotov, N. A.; De Smedt, S. C.; Sukhorukov, G. B. Small 2007, 3, 804–808. (30) Chung, M.-H.; Park, C.; Chun, B. C.; Chung, Y.-C. Colloids Surf., B 2004, 34, 179–184.

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i.e., surfactant micelles as mesopore structures1,2 and microparticles as hollow structures,20-24 yields MSHCs. During the gelation of silica, silica/micelle complexes are formed on the core template, and both templates are subsequently removed by either calcinations or dissolutions, resulting in the MSHC. The MSHCs have a large volume of the inner chamber, similar to microcapsules, and its shell has mesoporous channels for loading and release of materials. Better mechanical and chemical stabilities compared with polymer capsules and liposomes are expected. Moreover, the shell of the MSHC is made of silica, whose surface can be easily modified by various chemicals such as poly(ethylene glycol) in DDS applications.31 Even if the surface is not modified, the MSHC is expected to be biocompatible,4 as bare MS particles exhibit low toxicity toward immunocompetent cells.32 Therefore, the MSHCs have great potential for biological and medical applications. To date, various cores have been used as templates to obtain MSHCs: emulsions,33-40 gas bubbles,41-44 vesicles,45-47 inorganic particles,48-52 and latexes.53-58 Specifically, monodisperse latexes can be used to obtain monodisperse MSHCs.53-58 The loading and release of drugs using MSHCs has been demonstrated.36,38,44,48-52 By using MSHCs, sustained release was observed, and the release behavior was able to be controlled by the pH value36,49 and temperature49 of the media and the shell thickness50 of the MSHC. However, a large polydispersity in size, aggregation, and fusion of MSHCs, which must be avoided to achieve uniform properties as microcarriers and microreactors, can be seen in the literature. It is (31) He, X.; Nie, H.; Wang, K.; Tan, W.; Wu, X.; Zhang, P. Anal. Chem. 2008, 80, 9597–9603. (32) Witasp, E.; Kupferschmidt, N.; Bengtsson, L.; Hultenby, K.; Smedman, C.; Paulie, S.; Garcia-Bennett, A. E.; Fadeel, B. Toxicol. Appl. Pharmacol. 2009, 239, 306–319. (33) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Sch€uth, F. Science 1996, 273, 768–771. (34) Li, W.; Sha, X.; Dong, W.; Wang, Z. Chem. Commun. 2002, 2434–2435. (35) Li, Y.; Shi, J.; Hua, Z.; Chen, H.; Ruan, M.; Yan, D. Nano Lett. 2003, 3, 609–612. (36) Zhu, Y.; Shi, J. Microporous Mesoporous Mater. 2007, 103, 243–249. (37) Sun, Q.; Kooyman, P. J.; Grossmann, J. G.; Bomans, P. H. H.; Frederik, P. M.; Magusin, P. C. M. M.; Beelen, T. P. M.; van Santen, R. A.; Sommerdijk, N. A. J. M. Adv. Mater. 2003, 15, 1097–1100. (38) Botterhuis, N. E.; Sun, Q.; Magusin, P. C. M. M.; van Santen, R. A.; Sommerdijk, N. A. J. M. Chem.-Eur. J. 2006, 12, 1448–1456. (39) Zhao, Y.; Zhang, J.; Li, W.; Zhang, C.; Han, B. Chem. Commun. 2009, 2365–2367. (40) Teng, Z.; Han, Y.; Li, J.; Yan, F.; Yang, W. Microporous Mesoporous Mater. 2010, 127, 67–72. (41) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 2028–2029. (42) Rana, R. K.; Mastai, Y.; Gedanken, A. Adv. Mater. 2002, 14, 1414–1418. (43) Wu, Z.; Zhang, M.; Yu, K.; Zhang, S.; Xie, Y. Chem.-Eur. J. 2008, 14, 5346–5352. (44) Wang, J.-G.; Li, F.; Zhou, H.-J.; Sun, P.-C.; Ding, D.-T.; Chen, T.-H. Chem. Mater. 2009, 21, 612–620. (45) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1302–1305. (46) Yeh, Y.-Q.; Chen, B.-C.; Lin, H.-P.; Tang, C.-Y. Langmuir 2006, 22, 6–9. (47) Yu, M.; Wang, H.; Zhou, X.; Yuan, P.; Yu, C. J. Am. Chem. Soc. 2007, 129, 14576–14577. (48) Chen, J.-F.; Ding, H.-M.; Wang, J.-X.; Shao, L. Biomaterials 2004, 25, 723– 727. (49) Wen, L.-X.; Li, Z.-Z.; Zou, H.-K.; Liu, A.-Q.; Chen, J.-F. Pest Manage. Sci. 2005, 61, 583–590. (50) Li, Z.-Z.; Xu, S.-A.; Wen, L.-X.; Liu, F.; Liu, A.-Q.; Wang, Q.; Sun, H.-Y.; Yu, W.; Chen, J.-F. J. Controlled Release 2006, 111, 81–88. (51) Zhou, J.; Wu, W.; Caruntu, D.; Yu, M. H.; Martin, A.; Chen, J. F.; O’Connor, C. J.; Zhou, W. L. J. Phys. Chem. C 2007, 111, 17473–17477. (52) Liu, Y.; Miyoshi, H.; Nakamura, M. Colloids Surf., B 2007, 58, 180–187. (53) Zhu, G.; Qiu, S.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723– 7724. (54) Tan, B.; Rankin, S. E. Langmuir 2005, 21, 8180–8187. (55) Sadasivan, S.; Sukhorukov, G. B. J. Colloid Interface Sci. 2006, 304, 437– 441. (56) Wu, X.; Tian, Y.; Cui, Y.; Wei, L.; Wang, Q.; Chen, Y. J. Phys. Chem. C 2007, 111, 9704–9708. (57) Yang, M.; Wang, G.; Yang, Z. Mater. Chem. Phys. 2008, 111, 5–8. (58) Blas, H.; Save, M.; Pasetto, P.; Boissiere, C.; Sanchez, C.; Charleux, B. Langmuir 2008, 24, 13132–13137.

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known that the enhanced permeability and retention effect59 in tumor vasculature depends on the particle size, and it was recently reported that the organ distribution60,61 of nanoparticles after intravenous administration depends on the particle size. In microreactor applications, a wide distribution of the capsule size cannot provide uniform reaction conditions among the capsules. Therefore, monodispersity of the capsules is crucial for particular applications. Although there have been efforts to obtain monodisperse MSHCs, the degree of polydispersity of the obtained capsules has not been evaluated, and only the polydispersity factor of the particles before calcination for the removal of templates has been investigated.58 In the present work, not only the polydispersity of the particles before the removal of the templates but also that of the resultant MSHCs were investigated by light scattering methods. Electron microscopy was also used to observe the size, the shell thickness, and the surface morphology of the MSHCs. We focused mainly on the volume ratio of water to ethanol and the concentration of the surfactant in the sol-gel reaction solution. These parameters were systematically varied to find the conditions to produce MSHCs with a degree of polydispersity the same as that of the core particles used and with shells having uniform thickness and a smooth surface. The synthesis procedure for MSHCs was divided into three main stages: before the sol-gel reaction, after the sol-gel reaction (before calcination), and after calcination. The conditions where the particles or the capsules were well dispersed were experimentally evaluated at each stage along with physicochemical considerations such as a colloidal stability and a sol-gel reaction behavior. The release characteristics of dye molecules loaded into the obtained MSHCs was also observed.

Experimental Details Materials. Aqueous dispersions of polystyrene (PS) particles (10% w/v) with average diameters of 330 and 153 nm (standard deviation: 7 nm) were purchased from Microparticles GmbH, and PS particles with an average diameter of 50 nm (10% w/w, standard deviation: 7 nm) were purchased from Magsphere Inc. There were sulfate groups on the surface of the PS particles, and their surface charges were negative. The average diameters of the particles and their coefficient of variation (CV) were remeasured by dynamic light scattering (DLS) using our system (see below for an explanation of the DLS apparatus and analysis). The results are summarized in Table 1. The difference between the official and remeasured values is due to the difference in the evaluation method. These remeasured CV values were compared with those of the MSHCs measured by our DLS system. Hereafter, these core particles are termed PS360, PS168, and PS57. The dispersions were diluted into 0.25% w/v by pure water. The chemicals for the MSHC synthesis; tetraethoxysilane (TEOS, >95%), cetyltrimethylammonium bromide (CTAB, 98%), aqueous ammonia (25% w/w), and ethanol (EtOH, >99.5%), were obtained from Wako Pure Chemical Industry Co., Ltd., Japan. Calcein, used as a model drug, was obtained from Dojindo Molecular Technologies Inc. Other chemicals for the load and release experiments; NaCl (>99.5%) and NaOH (>97%), were purchased from Wako Pure Chemical Industry Co., Ltd. An aqueous solution of calcein at 12.5 mM with 0.625 M NaCl and an aqueous solution of NaCl at 0.5 M were prepared. The solutions were adjusted to pH 7 by NaOH. All chemicals were used as received. Pure water was prepared using a Millipore system (Direct-Q System). (59) Torchilin, V. In Nanoparticles as Drug Carriers; Torchilin, V. P., Ed.; Imperial College Press: London, 2006; Chapter 1. (60) De Jong, W. H.; Hagens, W. I.; Krystek, P.; Burger, M. C.; Sips, A. J. A. M.; Geertsma, R. E. Biomaterials 2008, 29, 1912–1919. (61) Sonavane, G.; Tomoda, K.; Makino, K. Colloids Surf., B 2008, 66, 274–280.

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Kato et al. Table 1. Particle Properties Remeasured by Dynamic Light Scattering official values

remeasured values

average diameter (nm)

standard deviation (nm)

coefficient of variation (%)

average diameter (nm)

standard deviation (nm)

coefficient of variation (%)

abbreviation

330 153 50

7 7 7

2.1 4.6 14

360.4 168.0 57.4

102.0 44.0 21.8

23.8 26.2 38.0

PS360 PS168 PS57

Figure 1. Synthesis flowchart.

Synthesis of Mesoporous Silica Hollow Capsules. The procedure was divided in three main processes, as shown in Figure 1. Process I was the preparation of the CTAB solution that contained the PS particles and aqueous ammonia (PS þ CTAB-NH3 dispersion). Process II corresponded to the sol-gel reaction to form the silica/micelle complex on the surface of the PS particles by adding TEOS into the PS þ CTAB-NH3 dispersion. Process III consisted of calcination of the coated PS particles to remove the templates and the redispersion of the capsules in aqueous medium. To optimize the reaction conditions, we focused on two parameters: the CTAB concentration (CCTAB) and the volume fraction of EtOH (VFEtOH) in the disperse medium of the PS þ CTABNH3 dispersion. In process I, x mol of CTAB was dissolved in a mixture of y mL of H2O, z mL of EtOH, 0.1 mL of aqueous ammonia, and 1 mL of PS dispersion (0.25% w/v). We fixed y þ z at 15 mL. By taking account of the volume of the added aqueous ammonia and PS dispersion, the total volume of the disperse medium in the PS þ CTAB-NH3 dispersion was ∼16.1 mL. Thus, the CCTAB was represented by x/(16.1  10-3) M and the VFEtOH by z/16.1. Note the VFEtOH values did not take into account the shrinkage after mixing EtOH and water. After these chemicals were put into a conical flask, the PS þ CTAB-NH3 dispersion was stirred by a magnetic stirrer (250 rpm) for 10 min at 30 °C. In process II, the sol-gel reaction was carried out. 50 μL of TEOS was slowly added into the PS þ CTAB-NH3 dispersion with stirring at 250 rpm, and the mixture was continuously stirred at 30 °C for 2 h to allow the hydrolysis and condensation of TEOS and the formation of the silica/micelle complex. After the reaction, the PS particles coated by the silica/micelle complex were separated from the excess chemicals and byproducts (silica particles). The resultant suspension was centrifuged at an appropriate centrifugal force, where only the coated PS particles were settled and the supernatant was removed. Subsequently, pure water was added, and the sediment was redispersed. This separation procedure was repeated twice, and an aqueous dispersion 14336 DOI: 10.1021/la1024636

of the PS particles coated by the silica/micelle complex was obtained. In process III, the disperse medium of the coated PS particle dispersion was exchanged from pure water to EtOH. The dispersion of the coated PS particles in EtOH was centrifuged, and the sediment was placed in a crucible and dried at RT. After the evaporation of EtOH, the coated PS particles were calcined at 550 °C for 6 h using a muffle furnace. The furnace was heated at a rate of 1 °C/min, and after calcination, it was cooled naturally. During calcination, the PS and CTAB micelles were thermally decomposed and evaporated, and the further gelation of silica was promoted. The calcined sample was redispersed in 1 mL of pure water using sonication, and the monodisperse MSHCs were separated from aggregated and fused capsules by centrifugation. Finally, 1 mL of an aqueous dispersion of the monodisperse MSHCs was obtained. Light Scattering Methods. The dispersity of the particles was checked at three different stages throughout the synthesis procedure shown in Figure 1: the dispersity of the PS particles in the PS þ CTAB-NH3 dispersion at the end of process I, that of the PS particles coated by the silica/micelle complex at the end of process II, and that of the MSHCs at the end of process III. The PS þ CTAB-NH3 dispersion was investigated by two different methods. Under conditions of high CCTAB and large VFEtOH, we merely evaluated whether or not the PS particles were aggregated. The intensity of the scattered light from the dispersion increases when the particles are aggregated. Hereafter, this method is termed simple static light scattering (SLS). The reason for employing the SLS method instead of commonly used dynamic light scattering (DLS) method is because of the difficulty in obtaining the values of refractive index and viscosity at high CCTAB and large VFEtOH. A He-Ne laser beam (633 nm) was irradiated into the dispersion in a glass cylinder perpendicular to the cylinder axis. The intensity of light scattered at an angle of ca. 14° from the beam axis was observed (see Supporting Information, Figure S1, for the optical setup). As the PS particles did not aggregate, irrespective of the VFEtOH value at CCTAB = 0, the scattered light intensity at CCTAB = 0 was set as the baseline; i.e., when the scattered light intensity at a certain value of CCTAB was sufficiently high above that at CCTAB = 0, we judged aggregation of the PS particles occurred. Under conditions of low CCTAB and small VFEtOH, the PS þ CTAB-NH3 dispersion was investigated by the DLS method using particle size analyzer (ELSZ, Otsuka Electronics Co., Ltd.). The light source was a semiconductor laser (660 nm), and the selfcorrelation function was accumulated 70 times. For simplicity, the values of refractive index and viscosity of water/EtOH mixture62 without CTAB were used to analyze the average diameter and its coefficient of variation (CV). The average diameter and the CV of the PS particles coated by the silica/micelle complex and the MSHCs were also evaluated by the DLS method using the same particle size analyzer. Because the disperse medium of all samples were pure water in these cases, the refractive index (1.333) and viscosity (0.8904 cP) of pure water at 25 °C were used for the analysis. The distribution of scattered intensity versus particle diameter was obtained by the Marquardt method.63 (62) Kagaku-Binran Ksio-Hen, 5th ed.; Chemical Society of Japan: Tokyo, 2004; pp II-49 and II-641. (63) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431–441.

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Electron Microscopes. A sample was prepared by placing 1 μL of the as-prepared dispersion of the capsules at the center of the Cu grid covered by collodion film followed by evaporation of the disperse medium at room temperature. A transmission electron microscope (TEM, JEM-1200EXII, JEOL) was used to observe the size, shape, and shell thickness of the MSHC and was operated at an acceleration voltage of 80 kV. A field emission scanning electron microscope (FESEM, S-5200, Hitachi Hightechnologies Corp.), operated at an acceleration voltage of 5 kV, was used to observe the surface topology of the MSHC. The sample for the FESEM observation was the same as that for the TEM observation. The FESEM was also used for evaluation of the number density of the capsules in the obtained dispersion. X-ray Diffraction. The periodicity of the mesopores in the shell of the MSHC was evaluated by the X-ray diffraction (XRD) method. The powder diffraction pattern was obtained by a θ-2θ diffractometer (GeigerFlex4037, Rigaku), and the X-ray wavelength was 0.154 nm (Cu KR). The scan rate of the 2θ axis was 0.25°/s, and the sampling width was 0.01°. Load and Release of Dye Molecules. To obtain a sufficient amount of MSHC, the reaction volume shown in Figure 1 was scaled up 5 times. The obtained dispersion of MSHC in pure water was concentrated to 1 mL, and 4 mL of aqueous solution of 12.5 mM calcein with 0.625 M NaCl was added to the concentrated dispersion, resulting in ∼10 mM calcein and 0.5 M NaCl in the dispersion. The dispersion was then incubated for 15 h to load calcein into the MSHCs. After the 15 h incubation, the dispersion was centrifuged, the supernatant was removed, and 6 mL of 0.5 M NaCl aqueous solution was added to remove excess calcein in the disperse medium. This procedure was repeated three times, and the incubation for release was then begun. The dispersion of the calcein loaded MSHC was incubated for seven different durations (3, 9, 27, 81, 243, 729, and 1458 min), and after each duration, 0.8 mL of the dispersion was taken and centrifuged. The visible absorption spectrum of the supernatant was then measured to analyze the amount of released calcein, and the temporal release profile was observed. All processes were carried out at ambient temperature, and a miniature fiber-optic spectrometer (StellarNet) equipped with a tungsten-krypton lamp was used to measure the visible absorption spectra.

Figure 2. CCTAB dependence of the scattered light intensity of the PS þ CTAB-NH3 dispersion measured by the SLS method. The VFEtOH values were (a) 0.16, (b) 0.31, and (c) 0.93, and PS360 particles were used.

Results and Discussion Colloidal Stability at the End of Process I. To prevent the formation of fused and aggregated MSHCs, the sol-gel reaction has to be performed under conditions where the template PS particles exhibit good colloidal stability and are well dispersed. To find such conditions, the colloidal stability of the PS particles (PS360) in the PS þ CTAB-NH3 dispersion was investigated as functions of VFEtOH and CCTAB by the SLS and DLS methods. In the case of the SLS method, a standard sample was prepared by adding 1 mL of the PS360 dispersion (0.25% w/v) to 15.1 mL of pure water, and its scattered intensity was observed to be 50 in arbitrary units. The scattered light intensity of the PS þ CTABNH3 dispersion was compared with the value of 50 to judge whether or not the PS particles were dispersed. When the CCTAB value was 0 mM, it was found that the PS360 particles were not aggregated at either VFEtOH value. Therefore, we observed the scattered light intensity at given VFEtOH values as a function of CCTAB. For example, Figure 2 shows the CCTAB dependence of the scattered light intensity at VFEtOH values of 0.16, 0.31, and 0.93. When VFEtOH = 0.16, the value of the intensity did not exceed 60, even at CCTAB = 430 mM (Figure 2a). When the VFEtOH = 0.31, the intensity increased above CCTAB = 130 mM and the value reached around 80 (Figure 2b). When the VFEtOH value was increased at 0.93, an increase in the intensity was observed at CCTAB = 8.5 mM (Figure 2c). These results indicate that as the VFEtOH value increased, the minimum value of CCTAB Langmuir 2010, 26(17), 14334–14344

Figure 3. CCTAB dependence of the average particle size obtained by the DLS method. The VFEtOH values of the PS þ CTAB-NH3 dispersion was 0.31, and PS360 particles were used. The dashed line indicates the average particle size of PS360 at CCTAB = 0.

where the aggregation of the PS particles occurred decreased. Because the intensity value increased gradually as the CCTAB value increased (Figure 2b,c), we concluded that the PS360 particles were aggregated when the observed intensity value exceeded 60 rather than 50. This criterion was also applied to the samples at other VFEtOH values. At low CCTAB values, we used the DLS method to investigate the dispersity of the PS360 particles more precisely. For example, Figure 3 shows the CCTAB dependence of the average particle size at VFEtOH = 0.31. At low CCTAB values, the aggregated and dispersion behaviors of the PS360 were rather complicated. Below CCTAB = 0.01 mM, the PS360 dispersed well, and as the CCTAB increased, the PS360 particles showed aggregation around CCTAB = 0.1 mM. A further increase in the CCTAB value (>0.3 mM) resulted in a well-dispersed state of PS360 up to 30 mM, DOI: 10.1021/la1024636

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and at 100 mM, the PS360 particles aggregated again. The aggregation at CCTAB = 100 mM observed by the DLS method (Figure 3) corresponds well to that at CCTAB = 130 mM observed by the SLS method (Figure 2b). In the case of the DLS method, we judged that the PS360 particles were aggregated when the average particles size was sufficiently larger than that observed at CCTAB = 0 mM, and this criterion was also applied to the samples at other VFEtOH values. All results obtained by the SLS and DSL measurements are summarized in the CCTAB-VFEtOH diagram shown in Figure 4. The crosses and open circles indicate the aggregated and welldispersed states, respectively. There are a few disagreements between the results obtained by the SLS and DSL measurements at VFEtOH = 0.55, but the overall trends are coincident. The diagram can be divided into three regions: the region surrounded by the boundary III and II (area 1), that below the boundary I (area 2), and that except areas 1 and 2 (area 3). Areas 1 and 2 are the conditions where the PS360 particles were in a well-dispersed state, and area 3 corresponds to the conditions where the PS360 particles aggregated. The colloidal stability of the PS360 in PS þ CTAB-NH3 dispersion is determined by the interaction between the positively ionized CTAB and the negatively charged PS particles.64 CTAB adsorbs on the PS particles by electrostatic interaction. At sufficiently low CCTAB, CTAB cannot cover the whole surface of the PS particles, resulting in a dispersed state of the PS particles, and this situation corresponds to area 2. Once the CCTAB reaches a sufficient concentration where a CTAB monolayer covers the entire surface of the PS particle, the surface of PS particles becomes neutral and hydrophobic, resulting

in the aggregated state by hydrophobic interaction.64 This situation corresponds to area 3. The minimum CCTAB values at each VFEtOH to induce this type of aggregation correspond to the values on boundary I in Figure 4, and the CCTAB values on boundary I were below the critical micelle concentration (cmc) of the CTAB.65 This minimum CCTAB value tends to increase as the VFEtOH increases because the solubility of CTAB increases as the VFEtOH increases. When the CCTAB value was increased above boundary II, area 1, which indicates the well-dispersed state, appears below VFEtOH = 0.5, and the minimum CCTAB values at each VFEtOH to redisperse the hydrophobized PS particles correspond to the values on boundary II in Figure 4. In area 1, a bilayer of CTAB covers the PS particle, and the surface of the covered PS particle is positively charged and becomes hydrophilic, resulting in a welldispersed state.64 Further increase in CCTAB results in the aggregated state again and the boundary between the dispersed and aggregated states corresponds to boundary III in Figure 4. The CCTAB values on boundary III up to VFEtOH = 0.5 is above the cmc of CTAB.65 The mechanism of this aggregation can be ascribed to the depletion effect from free CTAB micelles and/or the screening effect on the electrostatic repulsion from the free ionized CTAB molecules.64,66 At lower VFEtOH on boundary III, the depletion flocculation should be the main driving force for the aggregation, and as the VFEtOH increases, the screening effect from the ionized CTAB and its counterion may increase together with the depletion effect. Similar to the decrease in the critical coagulation concentration of an ionic surfactant with a decrease in the surfactant chain length (increase in the solubility of surfactant) observed in the literature,66 an increase in VFEtOH increases the solubility of CTAB and the CCTAB value on boundary III decreases as VFEtOH increases, and boundary III intersects with boundary II around VFEtOH = 0.5. To obtain well-dispersed MSHCs, we carried out the reaction under the conditions in area 1 because it was expected that the PS templates covered by the bilayers of CTAB have positively charged surface to interact with the negatively charged silica. In early works, it was found that low VFEtOH conditions did not provide any capsules and high CCTAB conditions only provided solid particles of silica/micelle complex. Therefore, the proper condition was sought precisely in the area surrounded by the dashed line indicated in area 1 (see Figure 4). Particle Size Distribution at the End of Process II. The conditions on CCTAB and VFEtOH in the area surrounded by the dashed line shown in Figure 4 were precisely examined in process II. After process I, we added TEOS to the PS þ CTAB-NH3 dispersion and allowed the sol-gel reaction to occur (see Figure 1). During the reaction, the silica sols and the CTAB molecules were incorporated into the silica/micelle complex, and the nucleation and growth of the complexes apparently occurred in the dispersed medium of the PS þ CTAB-NH3 dispersion as well as on the surface of the PS particles surrounded by the CTAB bilayer. As both the CTAB micelles and the surface of the CTAB-coated PS particles have positive charges, the negatively charged silica sols would adhere between them. This process may trigger the nucleation and growth of the complexes in the latter case. When the complexes appeared in the dispersed medium, solid silica/ micelle particles (byproducts) were formed. Therefore, after the sol-gel reaction, the byproducts had to be separated from the PS particles coated by the silica/micelle complex. Under the present

(64) Furusawa, K.; Sato, A.; Shirai, J.; Nashima, T. J. Colloid Interface Sci. 2002, 253, 273–278.

(65) Li, W.; Han, Y.-C.; Zhang, J.-L.; Wang, B.-G. Colloid J. 2005, 67, 159–169. (66) Kline, S. R.; Kaler, E. W. Langmuir 1996, 12, 2402–2407.

Figure 4. CCTAB-VFEtOH diagram of the PS þ CTAB-NH3 dispersion. Open circles and crosses indicate the conditions where the PS360 particles were in a well-dispersed state and an aggregated state, respectively. The small open circles and crosses were obtained by the SLS method, and the large ones were obtained by the DLS method. Boundaries I, II, and III indicate the estimated boundaries between the dispersed and aggregated states of the PS360 particles. The conditions in the area surrounded by the dashed line were examined in process II.

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Figure 5. Particle size distribution of the supernatant (a) and that of the sediment (b) of the dispersion after the sol-gel reaction. The reaction was carried out at CCTAB = 5.5 mM and VFEtOH = 0.31 using the PS360 particles, and the centrifugation was carried out at 5000g for 5 min.

CCTAB and VFEtOH conditions, the byproduct size (50-350 nm in diameter, depending on the conditions) was smaller than that of the silica/micelle complex coated PS particles, and fortunately, separation was able to be made by centrifugation; i.e., the density of the byproducts seemed to be less than that of the coated PS particles. For example, the particle size distributions of the supernatant and sediment of the dispersion after the sol-gel reaction made at CCTAB = 5.5 mM and VFEtOH = 0.31 are shown in parts a and b of Figure 5, respectively. Although the distributions overlapped, the separation was sufficient, and this was confirmed by a TEM image where almost no byproducts were found with the MSHC (see next subsection). Because not only the byproducts but also the excess CTAB and TEOS had to be separated from the silica/micelle complex coated PS particles before calcination, the separation procedure in process II was essential. Besides the formation of the byproducts, the aggregation of the PS particles coated by the silica/micelle complex was found under certain conditions. Therefore, the size distribution of the dispersion of the coated PS particles was investigated after the separation of the byproducts and the excess materials for the sol-gel reaction, and the conditions where almost no aggregation occurred were explored. Figure 5b shows that there were no aggregated particles under the condition of VFEtOH = 0.31 and CCTAB = 5.5 mM. In addition, the size distributions of the coated PS particles obtained under two different conditions (VFEtOH=0.37 and 0.44 at CCTAB = 5.5 mM) are given in Figure 6. At VFEtOH = 0.37, two separated distributions of the coated PS particles were observed (Figure 6a). One distribution with a higher peak has a similar width to that of the bare PS particles, indicating that this distribution corresponds to that of the coated PS particles in a well-dispersed state. The other distribution from 6 to 10 μm in diameter shows the existence of aggregated particles. At VFEtOH = 0.44, almost all particles were aggregated (Figure 6b). Thus, the frequency of particle aggregation depended on the conditions; i.e., as the VFEtOH value increased, the fraction of the aggregated particles increased. We must avoid conditions that result in large amounts of aggregated particles to obtain monodisperse MSHCs. To judge whether or not the condition for the reaction was proper, the criterion at the end of process II was given as follows: if the condition gave a fraction of Langmuir 2010, 26(17), 14334–14344

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Figure 6. Size distribution of the PS360 particles coated by the silica/micelle complex. The disperse medium was pure water. The data were obtained by the DLS method. (a) PS particles coated by the silica/micelle complex were prepared at a condition of CCTAB = 5.5 mM and VFEtOH = 0.37. (b) PS particles coated by the silica/ micelle complex were prepared under a condition of CCTAB = 5.5 mM and VFEtOH = 0.44. Narrower and wider bars indicate the distribution of the bare PS360 particles and that of the coated PS360 particles, respectively.

the distribution of scattered light intensity above 1.5 μm in diameter less than 30%, we accepted the condition, and vice versa.67 According to this criterion, the condition of CCTAB = 5.5 mM and VFEtOH = 0.37 was accepted (Figure 6a, the distribution of scattered light intensity above 1.5 μm was 18%), and that of CCTAB = 5.5 mM and VFEtOH = 0.44 was not (Figure 6b, the distribution of scattered light intensity above 1.5 μm was 99%). Other conditions in the area surrounded by the dashed line in Figure 4 were examined, and the results are summarized in Figure 7. The conditions marked by open circles met the criterion, and those marked by crosses did not. The conditions at low CCTAB and large VFEtOH resulted in particle aggregations (see Figure 7) because these conditions were close to boundary II in Figure 4. However, the reason for the aggregation obtained at high CCTAB conditions in Figure 7 was not clear. Thus, at the conditions marked by the open circles in Figure 7, the synthesis procedure was carried forward to process III. DLS and TEM Observations of the Capsules at the End of Process III. At the conditions marked by open circles in Figure 7, the synthesis procedure (Figure 1) from process I to III was carried out. Although under these conditions the PS particles coated by the silica/micelle complex were in a well-dispersed state, fused capsules were found by the DLS measurement after calcination and redispersion in pure water. The fusion of the capsules during the calcination could not be prevented by changing the CCTAB and VFEtOH conditions. Therefore, the fused capsules had to be separated from the dispersed MSHCs. For example, the results of the DLS measurements on the MSHC dispersion obtained under the conditions CCTAB = 5.5 mM and VFEtOH = 0.31 before and after the removal of the fused MSHCs are shown in parts a and b of Figure 8, respectively. The distribution from 6 to (67) Here, we used the distribution of scattered light intensity instead of the particle number distribution. Note that, for larger particles, the ratio of the distribution of scattered light intensity will be greater than that of the particle number distribution because the larger particles scatter light more intensely than the smaller ones even both have the same particle number.

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Figure 7. CCTAB-VFEtOH diagram of the dispersion of the PS360 particles coated by the silica/micelle complex obtained at the end of process II. Open circles correspond to the conditions that gave the coated PS360 particles in a well-dispersed state, and the crosses correspond to the conditions that gave large amount of aggregated particles. The value on the right-hand side of each open circle and cross is the coefficient of variation (CV) in the particle diameter and the fraction of the distribution of the scattered light intensity above 1.5 μm in diameter, respectively.

Figure 8. Size distribution of the MSHCs in pure water before (a) and after (b) the removal of the fused MSHCs. The data were obtained by the DLS method. Narrower and wider bars indicate the distributions of the bare PS360 particles and the MSHCs, respectively. The reaction was carried out using PS360 particles at a condition of CCTAB = 5.5 mM and VFEtOH = 0.31.

10 μm in diameter corresponds to that of fused MSHCs. Separation was achieved by centrifugation, and Figure 8b clearly shows that the isolation of the monodisperse MSHCs was successful. The resultant size distribution of the MSHCs was similar to that of the bare PS particles, indicating that the degree of polydispersity was similar to that of the template PS particles. For all conditions marked by open circles in Figure 7, the TEM and DLS observations were made after the removal of fused MSHCs. For example, TEM images and the size distributions of the MSHCs synthesized at CCTAB = 5.5 mM are shown in Figure 9. As shown in Figure 9a, b, at VFEtOH = 0.19 and 0.25, the shells of the MSHCs were too thin to maintain their shape and collapsed capsules were found, and due to the thin shell, the scattered light intensity was too low to obtain a size distribution by the DLS method. At VFEtOH = 0.31 and 0.37, the shells of the MSHCs were thick enough to maintain 14340 DOI: 10.1021/la1024636

Figure 9. VFEtOH-dependent morphology of the MSHCs prepared at CCTAB = 5.5 mM using the PS360 template. TEM images of the capsules prepared at (a) VFEtOH = 0.19 and (b) VFEtOH = 0.25. TEM images and size distributions of those prepared at (c, d) VFEtOH = 0.31 and (e, f) 0.37. (g) Dependence of shell thicknesses and diameters on VFEtOH. The thicknesses and diameters were evaluated from the TEM images.

their shape, and the synthesis of uniform shaped and sized MSHCs was successful (Figure 9c,e). Their size distributions were similar to that of the bare PS particles used as templates, indicating that the obtained MSHCs were monodisperse (Figure 9d,f). The shell thickness depended on VFEtOH; as VFEtOH increased, the thickness increased (Figure 9g). In contrast, the inner diameter was almost constant with VFEtOH because we used the same template PS360 particles (Figure 9g). Dependence of the MSHC morphology on CCTAB is also indicated. For example, the TEM images and the size distributions of the MSHCs synthesized at VFEtOH = 0.31 are shown in Figure 10. At high CCTAB (21 mM), the byproducts (small mesoporous silica particles) were fused on the capsules surface (Figure 10a), and aggregated capsules were formed (Figure 10b). At CCTAB = 16 mM, the capsules fused with byproducts and broken capsules were found (Figure 10c), but aggregated capsules were not detectable (Figure 10d). At CCTAB = 11 mM, the resultant capsules exhibited uniform shapes and there were almost no fused capsules (Figure 10e,f), and at lower CCTAB (7.9 and 3.1 mM), Langmuir 2010, 26(17), 14334–14344

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Figure 11. CCTAB-VFEtOH diagram obtained at the end of process III. The template for the hollow structure was the PS360 particle. Open circles correspond to conditions where the monodisperse MSHCs with uniform shell thickness and shape were obtained; crosses correspond to conditions where broken or fused capsules or those with lumpy surfaces were obtained.

Figure 10. CCTAB-dependent morphology of the MSHCs prepared at VFEtOH = 0.31 using the PS360 template. TEM images and size distributions of those prepared at (a, b) CCTAB = 21 mM, (c, d) 16 mM, (e, f) 11 mM, (g, h) 7.9 mM, and (i,j) 3.1 mM. (k) Dependence of shell thicknesses and diameters on CCTAB. The thicknesses and diameters were evaluated from the TEM images. See Figure 9c,d for the result at CCTAB = 5.5 mM.

the capsules also exhibited uniform shapes and no aggregated capsules were detected (Figure 10g-j). The shell thickness also depended on CCTAB. As CCTAB decreased, the thickness increased (Figure 10k), but the dependence was not as strong as that observed in Figure 9g. Langmuir 2010, 26(17), 14334–14344

To select the conditions that produced monodisperse MSHCs from those marked by open circles in Figure 7, the criterion at the end of process III was given as follows: if the condition resulted in TEM images that showed monodisperse capsules with uniform shell thickness and no broken or fused capsules and if the fraction of the distribution of scattered light intensity above 1.2 μm in diameter was less than 15%,67 we accepted the condition, and vice versa. The results of the judgments according to this criterion are summarized in Figure 11. The conditions marked by the open circles in area iv in Figure 11 met the criterion, and those marked by crosses did not. The conditions in area i resulted in no capsule formation because there was almost no formation of the silica/ micelle complex on the template surface. The conditions in area ii resulted in broken capsules because the shell thickness was not thick enough to maintain their shape (see Figure 9a,b). The conditions in area iii resulted in fused capsules and/or the capsules with lumpy surfaces (see Figure 10a-d). Thus, our systematic investigation on the reaction conditions of CCTAB and VFEtOH disclosed that the conditions in area iv in Figure 11 provide the monodisperse MSHCs with uniform shape and shell thickness, and their average diameters, shell thicknesses, and CV values in diameter are summarized in Table 2. As shown in Figures 9g and 10k, the shell thickness depends on VFEtOH and CCTAB; i.e., under the conditions shown in Figure 11, as VFEtOH increases, the thickness increases, and as CCTAB increases, the thickness decreases. The former dependence can be ascribed to the dependence of the rate of hydrolysis of silicon alkoxides on VFEtOH in the sol-gel reaction solution.68 As the fraction of alcohol in water increases up to 60%, the rate of hydrolysis decreases. The higher rate of hydrolysis at small VFEtOH < 0.3 resulted in a higher frequency of the nucleation and faster growth of silica, not only on the surface of the PS template but also in the liquid phase. Also, TEOS is consumed on the formation of byproducts, resulting in thin or no shell formation under the conditions in Figure 11, areas i and ii. In contrast, under the lower rate of hydrolysis around VFEtOH = 0.3, the surface of the PS template acts as a main field of silica nucleation, resulting in a thicker shell. The mechanism for the CCTAB dependence can be ascribed by the number density of free CTAB micelles in the reaction solution. (68) Wright, J. D.; Sommerdijk, N. A. J. M. Sol-Gel Materials: Chemistry and Applications; Gordon and Breach Science Publishers: Amsterdam, 2001; Chapter 3.

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Table 2. Average Diameter, Standard Deviation (SD) in Diameter, Coefficient of Variation (CV) in Diameter, and Shell Thickness of MSHCsa CCTAB (mM)

VFEtOH

diameterb (nm)

SDb (nm)

CVb (%)

shell thicknessb (nm)

diameterc (nm)

SDc (nm)

CVc (%)

11 0.31 411 16 4 57 425.1 227.7 53.6 7.9 0.31 458 12 3 86 458.2 192.3 42.0 7.9 0.37 568 15 3 140 686.9 248.5 36.2 5.5 0.31 457 8 2 90 484.6 132.7 27.4 5.5 0.37 570 16 3 142 767.1 254.9 33.2 d d N/A N/Ad 3.1 0.25 377 19 5 52 N/A 3.1 0.31 464 8 2 93 513.4 210.8 41.1 a The template for the hollow structure was the PS360 particle. b Obtained from TEM image. c Obtained by the DLS method. d Because of thin shell thickness sufficient scattered light was not obtained for the DLS method.

A large number of CTAB micelles in the solution are formed at higher CCTAB. These micelles also provide the field of silica nucleation, and a larger amount of TEOS is consumed by the formation of byproducts, resulting in thinner shells at higher CCTAB, as shown in Figure 10k. Shell Morphology and Periodicity of Mesopores. FESEM observations were performed to confirm that there were no macroscopic holes, defects, and cracks in the shells of the MSHCs. A typical FESEM image of the MSHCs prepared under the condition of CCTAB = 5.5 mM and VFEtOH = 0.31 is shown in Figure 12a. Such macroscopic holes cannot be observed on the MSHCs prepared under the conditions in area iv shown in Figure 11. Periodic structures similar to the mesoporous silica particles have been observed on the MSHC shell,33,35-37,40,42,44-46,53-55,57 and the mesopores align perpendicular to the shell surface.58 Figure 12b indicates the X-ray diffraction (XRD) pattern of the MSHCs prepared under the condition of CCTAB = 11 mM and VFEtOH = 0.31 and that of CCTAB = 3.1 mM and VFEtOH = 0.31. The obtained diffraction peaks were very broad for both, indicating that periodicity of the mesopores was highly disordered. Because of a lack of long-range order, periodic mesopores were not clearly observed in the TEM image. Thus, the diffraction peaks in Figure 12b indicate the correlation length of the pore to pore distance was ca. 3.4 nm. For control, we synthesized hollow capsules without CTAB, and the XRD pattern of the capsules was observed (see control in Figure 12b). Because the XRD pattern showed no diffraction in the control, the broad peak observed on the MSHCs originated in the mesopore structure formed by the CTAB micelle templates. Note that most of the hollow capsules synthesized without CTAB were broken and their shell thicknesses were not uniform (see Supporting Information for the synthesis procedure). MSHCs Made by the PS168 and PS57 Templates. To synthesize smaller MSHCs than those obtained using the PS360 template, smaller templates (PS168 and PS57 particles, Table 1) were used. The syntheses were performed under the conditions around area iv shown in Figure 11, and monodisperse MSHCs were also obtained under the condition of CCTAB = 3.1 mM and VFEtOH = 0.25 for both smaller templates (see Figure 13). The average outer diameter and shell thickness obtained from the TEM images were 231 and 55 nm, respectively, for the MSHCs prepared using the PS168 template and were 73 and 21 nm, respectively, for those prepared using the PS57 template (see Figure 13a,c). The CV values in diameter of the MSHCs prepared using the PS168 and PS57 templates are comparable with those values of the bare PS168 and PS57 templates, respectively (see Figure 13b,d and Table 1), indicating that our conditions for MSHC synthesis are applicable for a wide range of template sizes from submicrometer to nanometer scale. In the case of the MSHCs obtained using the PS168 template, a striped pattern at the outer edge of the shell and a dotted pattern in the hollow 14342 DOI: 10.1021/la1024636

Figure 12. (a) FESEM image of the MSHCs prepared under the condition of CCTAB = 5.5 mM and VFEtOH = 0.31 using the PS360 template. The corresponding TEM image can be found in Figure 9c. (b) X-ray diffraction patterns obtained from the MSHCs prepared under the condition of CCTAB = 11 mM and VFEtOH = 0.31 and that of CCTAB = 3.1 mM and VFEtOH = 0.31. For control, capsules synthesized without CTAB (CCTAB = 0 mM) were also observed. Note that the capsules obtained without CTAB were aggregated and/or broken (see Supporting Information, Figure S2).

core region, which could be observed when the mesopores were ordered perpendicular to the shell surface,58 were observed in the shell (see inset of Figure 13a). To obtain such periodic patterns for other templates, we are exploring which parameter in the synthesis procedure affects the periodicity of the mesopore structure. Load and Release Properties. The MSHCs synthesized using the PS360 template were used to observe the load and release properties of the capsules. The condition of the synthesis was CCTAB = 5.5 mM and VFEtOH = 0.31, and the average diameter and shell thickness were 457 and 90 nm, respectively (see Table 2 and Figure 9c,d). Calcein was loaded into the MSHCs by incubating the MSHCs in a 10 mM calcein aqueous solution with 0.5 M NaCl. After removal of excess calcein by centrifugation, the calcein-loaded MSHCs were redispersed in a 0.5 M NaCl aqueous solution, and the temporal evaluation of the calcein absorption peak at 495 nm in the disperse medium was observed. As shown in Figure 14, the temporal release profile exhibits good reproducibility and sustained release was observed. After 1458 min, almost all calcein was released and the absorbance at 1458 min Langmuir 2010, 26(17), 14334–14344

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Figure 14. Temporal calcein release profile of MSHCs synthesized using the PS360 template.

suggests that our MSHCs had an encapsulation volume of 2.9  10-14 mL per capsule as expected for a hollow capsule with the mesoporous shell, and the sustained release observed in Figure 14 coincides with the fact that no macroscopic holes were observed in the shells by FESEM (see Figure 12a).

Conclusions

Figure 13. TEM images and size distributions of the MSHCs prepared under the condition of CCTAB = 3.1 mM and VFEtOH = 0.25 using the PS168 and PS57 templates. (a,b) The case using the PS168 template; (c,d) the case using the PS57 template.

corresponded to a concentration of 5.7 μM. To estimate the concentration of calcein in the MSHCs, the number density of the MSHC in the dispersion was estimated by counting the MSHCs using FESEM to be 3.0  1010 mL-1. If the mesopores in the shell were assumed to pack hexagonally with a lattice constant of d(100) = 3.4 nm and the diameter of the pores was assumed to be 2.3 nm, as observed in the literature,58 the total vacancy (mesopore and hollow structures) per capsule was calculated to be 2.9  10-14 mL. Therefore, the calcein concentration in the MSHCs was estimated to be 6.6 mM. This estimated value is lower than the concentration of the calcein solution (10 mM) used for loading, indicating that the calcein leaked from the capsule during the removal of excess calcein after the incubation for loading. This leakage was reasonable because there were no gatekeepers3 in our MSHCs. Thus, the 6.6 mM of calcein in the MSHC reasonably Langmuir 2010, 26(17), 14334–14344

We found the best reaction conditions to obtain monodisperse MSHCs. Under a fixed volume of the sol-gel reaction solution (y þ z = 15 mL in Figure 1), the concentration of CTAB (CCTAB) and the volume fraction of EtOH (VFEtOH) were systematically varied, and the conditions that provided monodisperse MSHCs were found to be around CCTAB = 5 mM and VFEtOH = 0.3. The reaction conditions for the monodisperse MSHCs were located in the small region shown in Figure 11, area iv, because good colloidal stability of the template particles has to be maintained before and after the sol-gel reaction and the formation of spherical capsules with a uniform shell thickness has to be achieved after the removal of the templates. Using three different-sized template particles for the hollow structure, MSHCs with outer diameters from 570 to 75 nm were obtained, and the shell thicknesses were from 140 to 20 nm. The degree of polydispersity for the diameter of the obtained MSHCs was observed by the DLS method and confirmed to be comparable to that of the bare template particles. TEM and FESEM observations also showed uniform shape and size of MSHCs. The load and release properties of our MSHCs confirmed that they have a hollow structure with a mesoporous shell, and there were no macroscopic holes in the shell. Thus, our MSHCs could be used as drug carriers in DDS or microcontainers for catalysis. Moreover, such high-quality capsules could be used further as templates or building blocks to fabricate functional hierarchic structures.69-72 If we define the yield of the MSHC as the number of obtained MSHCs divided by the number of the template particles used for the synthesis, the MSHC yield was estimated to be ca. 30%. The dominant factor that decreases the yield is the fusion of the MSHCs induced by calcination for the removal of the templates. To improve the yield of the MSHCs, refinements of the calcination conditions and environment must be achieved. Acknowledgment. We are grateful to Prof. H. Yoshimura of Meiji University for his support in the TEM and FESEM observations. We also acknowledge Prof. M. Notomi of Meiji (69) Ji, Q.; Miyahara, M.; Hill, J. P.; Acharya, S.; Vinu, A.; Yoon, S. B.; Yu, J.-S.; Sakamoto, K.; Ariga, K. J. Am. Chem. Soc. 2008, 130, 2376–2377. (70) Ji, Q.; Yoon, S. B.; Hill, J. P.; Vinu, A.; Yu, J.-S.; Ariga, K. J. Am. Chem. Soc. 2009, 131, 4220–4221. (71) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109. (72) Ariga, K.; Ji, Q.; Hill, J. P.; Vinu, A. Soft Matter 2009, 5, 3562–3571.

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University for his support in the XRD measurements. This work was partially supported by a grant of the Strategic Research Foundation Grant-aided Project for Private Universities from the Ministry of Education, Culture, Sport, Science, and Technology, Japan (S0901027).

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Supporting Information Available: Experimental setup for the simple static light scattering method (section 1); synthesis procedure for the sample without CTAB for control in Figure 12b (section 2). This material is available free of charge via the Internet at http://pubs.acs.org.

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