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Synthesis of Hollow Silica Spheres with Mesostructured Shell Using Cationic-Anionic-Neutral Block Copolymer Ternary Surfactants Yi-Qi Yeh,† Bi-Chang Chen,‡ Hong-Ping Lin,*,† and Chih-Yuan Tang§ Department of Chemistry, National Cheng Kung UniVersity, Tainan, Taiwan 701, Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106, and Department of Zoology, National Taiwan UniVersity, Taipei, Taiwan 106 ReceiVed August 4, 2005. In Final Form: NoVember 3, 2005 Hollow silica spheres with mesostructured shells (HSSMS) were prepared with a vesicle template of cetyltrimethylammonium bromide-sodium dodecyl sulfate-Pluronic P123 (C16TMAB-SDS-EO20PO70EO20) at a SDS/C16TMAB ratio of 0.6-0.8 following a fast silicification in dilute silicate solution at pH ≈ 5.0. The mesostructure of the shell is disordered, and the mesopore size is about 5.5-7.5 nm. Moreover, the direction and length of the nanochannels of the shell change with the SDS/C16TMAB ratios. A bi-template model, in which the C16TMA+-DS- form the stable bilayer vesicle structure and the P123 copolymers anchored on C16TMA+-DS- vesicle act as the template for the mesoporous silica, was proposed to explain the formation of the HSSMS. This bi-template model can be applied extensively to prepare the HSSMS with different diameters and pore sizes by using other CnTMAX-SDS-EOnPOmEOn ternary-surfactant mixtures.
Recently, the synthesis of useful porous materials with hollow interiors has attracted much attention in the chemistry and material community because of their potential applications in encapsulation, controlled drug release, confined-space catalysis, separation, and as solid nanotemplates.1,2 Hollow spheres of various diameters and wall thicknesses have been synthesized using polymer beads or emulsions as interior templates.3-10 However, the removal of the expensive polymer bead is uneconomic and fine-tuning of the compositions of surfactant, oil and water, and mechanical strength (e.g., stirring rate) to prepare a metastable emulsion is not simple. Up to now, developing a convenient synthetic method using a highly stable template to obtain hollow silica spheres with mesoporous shells remains a desirable target. Based on the chemistry of surfactants, various shapes and morphologies of surfactant micelles, vesicles, and liquid crystal phases can be found in mixed surfactant components.11-13 Usually, the thermodynamically stable and biocompatible PEOcoated vesicles are widely used in the applications of gene therapy * To whom correspondence should be addressed. E-mail: hplin@ mail.ncku.edu.tw. † National Cheng Kung University. ‡ Department of Chemistry, National Taiwan University. § Department of Zoology, National Taiwan University. (1) (a) Mdischer, B.; Won, Y.-Y.; Ege, D. S.; Lee, J. C.-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (b) Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (2) (a) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1998, 27, 113. (b) Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389. (3) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740. (4) Bourlinos, B.; Karakassides, M. A.; Petridis, D. Chem. Commun. 2001, 1518. (5) Zhu, G.; Qui, S.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723. (6) Li, W.; Coppens, M.-O. Chem. Mater. 2005, 17, 2241. (7) Sun, Q.; Kooyman, P. J.; Grossmann, G.; Bomans, P. H. H.; Frederik, P. M.; Magusin, P. C. M. M.; Beelen, T. P. M.; Santen, R. A. van S.; Sommerdijk, N. A. J. M. AdV. Mater. 2003, 15, 1097. (8) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F.; Science 1996, 273, 768. (9) Wang, B.; Shan, W.; Zhang, Y.; Xia, J.; Yang, W.; Gao, Z.; Tang, Y. AdV. Mater. 2005, 17 578. (10) Li, W.; Sha, X.; Dong, W.; Wang, Z. Chem. Comm. 2002, 2434. (11) Holland, P. M.; Rubingh, D. N. Mixed Surfactant Systems; American Chemical Society: Washington, DC, 1992. (12) Yin, H.; Zhou, Z.; Huang, J.; Zheng, R. Zhang, Y. Angew. Chem. Int. Ed. 2003, 42, 2188. (13) (a) Shioi, A.; Hatton, T. A. Langmuir 2002, 18, 7341. (b) Xia, Y.; Goldmints, I.; Johnson, P. W. T.; Hatton, A.; Bose, A. Langmuir 2002, 18, 3822.
or drug release.14,15 In addition to acting as nanocarriers, highly stable PEO-coated vesicles could be considered newly novel templates for preparing porous silica vesicles because the surface PEO segments can aggregate with the silica species via hydrogenbonding interactions.16 Consequently, we used a mixture of catanionic surfactant (i.e., a mixture of cationic and anionic surfactants) and Pluronic EOn-POm-EOn polymeric surfactant to generate thermodynamically stable neutral surfactant-coating vesicles as an organic template. After fast silicification in a dilute silicate solution at neutral pH value, hollow silica spheres with mesoporous shells were conveniently obtained. Hollow silica spheres with mesostructured shells (denoted as HSSMS) were synthesized using a ternary-surfactant mixture as the template and dilute sodium silicate solution at neutral pH value as the silica source. Typically, 26.5 g of 5.66 wt % C16TMAB (cetyltrimethylammonium bromide, Acroˆs) aqueous solution was combined with 26.0 g of 3.66 or 2.77 wt % SDS (sodium dodecyl sulfate, Acroˆs) water solution, and then 50.7 g of 1.38 wt % Pluronic P123 (EO20PO70EO20, Aldrich) surfactant was added to form a ternary-surfactant mixture solution at 4045 °C. After stirring for 2-3 h, the homogeneous ternarysurfactant solution was poured into a neutralized sodium silicate solution at pH ≈ 5.0. That dilute silicate solution was prepared via the following sequences: fast acidification of a mixture of 5.50 g of sodium silicate and 300.0 g of water with 10.0 g of 1.20 M H2SO4 aqueous solution, fine-tuning the pH-value to around 5.0, and then aging for 3-5 min. A white precipitate was formed within seconds. The gel-solution was hydrothermally treated under a static condition at 100 °C for 24 h. Filtration, washing, and drying gave the as-synthesized mesoporous silica product. The organic template was removed after calcination at 560 °C in air for 6 h. Figure 1, panels A and B, shows the representative TEM images of the HSSMS prepared with C16TMAB-SDS-P123-silicate composition at the SDS/C16TMAB molar ratios (denoted as S) of 0.8 and 0.6. One can clearly see that both silica samples have hollow spherical morphologies. The diameter of the hollow silica (14) Fo¨rster, S.; Plantenberg, T. Angew. Chem. Int. Ed. 2002, 41, 688. (15) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323. (16) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979.
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Figure 1. TEM images of the hollow silica spheres with mesostructured shell prepared with P123-C16TMAB-SDS ternary surfactants as template at different SDS/C16TMAB ratios (S). (A) S ) 0.8; (B) S ) 0.6; (C) High-magnification TEM image of sample A; (D) highmagnification TEM image of sample B.
spheres is not uniform and distributed within 100-500 nm. Higher-magnification TEM images confirm that the shell of the HSSMS consists of the disordered nanochannels with a mesopore size of about 7.0 nm. Moreover, the mesostructures of the HSSMS shell prepared at varied S values are different (Figure 1 C, D). At S ) 0.8, the channel is rodlike and its direction is nearly parallel to the shell. Although the S value is 0.6, the channel length becomes shorter (i.e., less than 20 nm) and the channel direction is perpendicular to the shell. Using low-magnification SEM observations (see the Supporting Information, Figure S1), we found the homogeneity of the HSSMS to be very high (>90%), and the mesoporous silica particulates were seldom found. From the analysis of N2 adsorption-desorption isotherms, one can obviously see that both samples possess a two-step shaped adsorption isotherm (Figure 2A). A steep adsorption at relative pressure (P/P0) around 0.60-0.75 is ascribed to the mesopore of the shell. The other less steep but strong adsorption at P/P0 close to 1.0 is a result of the huge hollow volume of the HSSMS. This novel adsorption isotherm of a combination of type IV and type II is consistent with TEM observations. By analyzing the adsorption isotherm, the HSSMS samples possess a high BET surface area of about 350-450 m2/g and pore size of 5.5-7.5 nm (calculated by BJH method). Given that the shell pore size of about 7.0 nm is close to that of the P123-templated SBA-15 silica, it is reasonable to suppose that the mesopores are templated with the P123 copolymers rather than with the C16TMAB, SDS, or C16TMAB-SDS catanionic surfactant (see the Supporting Information, Figure S2).17,18 Consequently, the novel morphology of the mesoporous silica hollow spheres can be efficiently prepared (17) (a) Chen, B. C.; Lin, H. P.; Chao, M. C.; Mou, C. Y.; Tang, C. Y. AdV. Mater. 2004, 16, 1657. (b). Lin, H. P.; Mou, C. Y. Acc. Chem. Res. 2002, 35, 927. (18) Hubert, D. H.; Jung, W. M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. AdV. Mater. 2000, 12, 1286.
with ternary-surfactant mixtures and a highly dilute sodium silicate solution under ambient conditions. When analyzing XRD patterns of the as-synthesized HSSMS prepared at S ) 0.6 and 0.8 (Figure 2B, curves I and III), one can find a broad peak at 2θ ≈ 0.7° in each sample that is due to the disordered mesostructure of the HSSMS’s shell templated by P123 copolymers. Besides, there exists a broad peak of lowintensity at 2θ ≈ 2.0° (indicated by arrows). That peak was supposed to be the lamellar mesostructure of the C16TMA+DS- mixed surfactants and thus disappeared after calcination (Figure 2B, curves II and IV). On the basis of these aforementioned results, we proposed a bi-template model to explain the formation of the HSSMS with the ternary-surfactant mixture. It is wellknown that the mixture of cationic alkyltrimethylammonium and anionic alkyl sulfate surfactant has a strong tendency to form a bilayer or vesicle structure at the appropriate cationic/ anionic ratio via the strong electrostatic interactions.11 Moreover, it is generally accepted that neutral block copolymers with the correct hydrophilic/hydrophobic balance may absorb onto the C16TMA+-DS- bilayer via the PPO blocks and hence reduce the bending elasticity of the C16TMA+-DS- bilayer.19,20 With the anchoring of Pluronic 123 on the C16TMA+-DS-, the protruding PEO chains possessing the steric-stabilization effect can prevent the mutual aggregation between the vesicles. In addition, the C16TMA+-DS--Pluronic 123 can aggregate with silica species via the hydrogen-bonding interaction at near neutral pH.4 Therefore, after fast silicification at pH ≈ 5.0, the distinct mesoporous hollow silica spheres with the P123-templated nanochannels were formed. Figure 2C is the schematic diagram of the bi-templating configuration of the HSSMS, where the C16TMA+-DS- catanionic surfactant forms the stable vesicle (19) Bergstrand, N.; Edwards, K. J. Colloid Interface Sci. 2004, 276, 400. (20) Kostarelos, T.; Tadros, F.; Luckham, P. F. Langmuir 1999, 15, 369.
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Figure 2. (A) N2 adsorption-desorption isotherms of the calcined HSSMS of a different S value. (I) S ) 0.8; (II) S ) 0.6, (curve II is vertically shifted by 100 cm3/g). (B) XRD patterns (λ ) 0.1326 nm) of the as-synthesized HSSMS. (C) A schematic diagram of the bi-template model for the HSSMS. (D) The TGA plots of the as-synthesized silicas with different surfactants.
structure and the P123 copolymers anchored on the C16TMA+DS- vesicle act as the template of mesoporous silica shell. Changing the surface charge density on the C16TMA+-DS- bilayer might lead to different anchoring models of the anchored P123 copolymer and alter the channel direction and length. This interesting but complicated phenomenon requires further study. To prove the inclusion of the P123 into the C16TMA+-DS- bilayer, the TGA curve of the as-synthesized HSSMS at S ) 0.8 is intermediate between the TGA-curves of the C16TMA+-DS-made and P123-made silicas (Figure 2D). This average in combustion temperature occurs commonly in homogeneous organic or polymer blending.21 According to the bi-template model, the C16TMA+-DS- vesicle acts as the morphology template of the HSSMS. The stable C16TMA+-DS- vesicles can form within a narrow range of surfactant mixing ratios.11-13 When the S value is outside the range of 0.6-0.8, we found the yield of the HSSMS decreased at S ) 0.9 and the spherical morphology was distorted at S ) 0.5 (see the Supporting Information, Figure S3). In practice, the bi-template model can be extensively applied to other surfactants with similar hydrophobic properties and charges. The physical properties of the HSSMS samples prepared with CnTMAX-SDS-Pluronic copolymer ternary surfactants were listed in Table 1.Using other CnTMAX surfactants with n ) 12, 14, and 18 to replace the C16TMAB, also could obtain the HSSMS products of different diameters. However, the mesopore (21) Stevens, M. P. Polymer Chemistry; Oxford University Press: New York, 1999.
Table 1. Physical Properties of the Hollow Silica Spheres with Mesostructured Shell Prepared with the CnTMAX-SDS-Pluronic Surfactant Ternary-Surfactant Template CnTMAX-SDS-Pluronic surfactant templatea
BET surface area/ m2g-1
pore sizeb/ nm
diameter range of HSSMS/nm
C18TMACl/SDS/P123 C16TMAB/SDS/P123 C14TMAB/SDS/P123 C12TMAB/SDS/P123 C16TMAB/SDS/P103 C16TMAB/SDS/PE64
389 350 383 393 397 438
5.4 5.5 5.3 5.5 4.3 3.2
100-350 100-500 80-500 100-400 100-800 200-800
a
The SDS/CnTMAX molar ratio is 0.6. b Calculated by BJH method.
size of the HSSMS is not affected by the chain length of the CnTMAX surfactants. This result is in agreement with the bitemplate model we proposed. To change the pore size of the mesoporous shell, other neutral tri-block copolymers (such as P103 (EO17PO85EO17) and PE64 (EO13PO30EO13)) were used. With the suitable chemical compositions, the HSSMS with different mesopore sizes were readily prepared. In contrast, HSSMS could not be obtained using either PEO homopolymers or PEO-rich Pluronics such as F127 (EO106PO70EO106), since such highly hydrophilic polymers are only weakly incorporated within the catanionic bilayer. Other experimental factors (such as temperature, pH value, S value, reactant concentration, and preparation procedure) affecting the formation of the vesicles will be further investigated to find the optimal condition and compositions for the preparation of HSSMS in the desired size.
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Unlike the nonporous silica shell of the silica vesicles templated with double-tail cationic surfactant, gemini surfactant, or catanionic surfactant, the mesoporous hollow silica spheres would make the hollow interior more accessible.18,22,23 In conclusion, the ternary-surfactant template could provide a new method to prepare mesoporous silica hollow spheres. The HSSMS of mesoporous and permeable shell, large pore size, high adsorption capacity, and different nanochannel’s directions may be useful as the solid nanocapsules for drug delivery and DNA therapy applications. In addition, the synthetic compositions (i.e., highly dilute sodium silicate solution and surfactant) and mild conditions (i.e., T ) 40-45 °C and pH ≈ 5.0) are close to the silicification environment for diatom formation. This
ternary-surfactant vesicle-templating method could provide a route to mimic the lipid vesicle-templating silicification in the diatoms.24
(22) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069. (23) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267.
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Acknowledgment. Authors thank Dr. Hwo-Shuenn Sheu for helps in taking X-ray patterns (17A, NSRRC, Shing-Chu, Taiwan). This work was financially supported by the National Science Council, Taiwan (NSC93-2113-M-006-003 and NSC932323-B-006-009). Supporting Information Available: SEM micrographs of the hollow silica spheres with mesostructured shells (Figure S1). TEM images of the silica sample (Figure S2). TEM images of hollow silica spheres (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. (24) Noll, F.; Sumper, M.; Hampp, N. Nano Lett. 2002, 2, 91.
