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Morphosynthesis of Vesicular Mesostructured Calcium Phosphate under Electron Irradiation Z. Y. Yuan,† J. Q. Liu, and L. M. Peng Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China B. L. Su* Laboratoire de Chimie des Mate´ riaux Inorganiques, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Received October 8, 2001. In Final Form: December 10, 2001
Introduction The discovery of M41S periodic mesoporous silicates in 1992 based on the use of surfactant supramolecular templates had an immediate strong impact on the area of mesostructured inorganic materials.1,2 A variety of related synthesis strategies have been developed, and a great diversity of mesostructured materials in terms of both composition and structure has been achieved.3-6 An extension to the synthesis of mesostructured transition metal oxides, metal phosphates, metal sulfides, and metals has aroused much interest as well.7-18 Among the many mesostructured phosphate materials such as aluminophosphates,19-21 oxovanadium phosphates,22 zirconium * Corresponding author. Fax: +32-81-725414. E-mail:
[email protected]. † Present address: Laboratoire de Chimie des Mate ´ riaux Inorganiques, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Sayari, A. Chem. Mater. 1996, 8, 1840. (4) Corma, A. Chem. Rev. 1997, 97, 2373. (5) Biz, S.; Occelli, M. I. Catal. Rev. 1998, 40, 329. (6) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (7) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149. (8) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (9) Ciesla, U.; Fro¨ba, M.; Stucky, G. D.; Schu¨th, F. Chem. Mater. 1999, 11, 227. (10) Mamak, M.; Coombs, N.; Ozin, G. A. Adv. Mater. 2000, 12, 198. (11) Sokolov, I.; Jiang, T.; Ozin, G. A. Adv. Mater. 1998, 10, 942. (12) Li, J.; Delmotte, L.; Kessler, H. Chem. Commun. 1996, 1023. (13) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. (14) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (15) Wong, M. S.; Ying, J. Y. Chem. Mater. 1998, 10, 2067. (16) Tian, Z. R.; Wang, J. Y.; Duan, N. G.; Krishnan, V. V.; Suib, S. L. Science 1997, 276, 926. (17) Blin, J. L.; Flamand, R.; Su, B. L. Int. J. Inorg. Mater. 2001, 3, 959. (18) Attard, G. S.; Barlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. Science 1997, 278, 838. (19) Khimyak, Y. Z.; Klinowski, J. Phys. Chem. Chem. Phys. 2000, 2, 5275. (20) Kimura, T.; Sugahara, Y.; Kuroda, K. Microporous Mesoporous Mater. 1998, 22, 115. (21) Tiemann, M.; Froba, M.; Rapp, G.; Funari, S. S. Stud. Surf. Sci. Catal. 2000, 129, 559. (22) Haskouri, J. E.; Roca, M.; Cabrera, S.; Alamo, J.; Beltran-Porter, A.; Beltran-Porter, D.; Marcos, M. D.; Amoros, P. Chem. Mater. 1999, 11, 1446.
Figure 1. Powder X-ray diffraction pattern of synthesized mesolamellar calcium phosphate.
phosphates,23 and titanium phosphates,24 little attention has been paid to mesostructured calcium phosphate although calcium phosphate is involved in the biomineralization of most vertebrates.25 It is well-known that assemblies of both charged and uncharged amphiphilic molecules can interact with inorganic precursors to form complex architectures, in which morphology and structure can be controlled over two length scales and spatial dimensions.26-30 The morphosynthesis of silica and calciferous materials displaying open skeletal frameworks has been reported by Mann and co-workers.31,32 The voids created by the amorphous silica, calcium phosphate, or carbonate skeletons are of the order of micrometers and thus begin to approach the structures with void dimensions and compositions desirable for bioceramic applications.33,34 We report here the synthesis of mesostructured calcium phosphate materials for the potential application in biomimic fabrication of stable bioceramic composite materials. It is significant that the formation of vesicular mesolameller calcium phosphate under electron-beam irradiation can be directly monitored by transmission electron microscopy (TEM). (23) Jime´nez-Jime´nez, J.; Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Jones, D. J.; Rozie`re, J. Adv. Mater. 1998, 10, 812. (24) Jones, D. J.; Aptel, G.; Brandhorst, M.; Jacquin, M.; Jime´nezJime´nez, J.; Jime´nez-Lo´pez, A.; Maireles-Torres, P.; Piwonski, I.; Rodrı´guez-Castello´n, E.; Zajac, J.; Rozie`re, J. J. Mater. Chem. 2000, 10, 1957. (25) Suchanek, W.; Yoshimura, M. J. Mater. Res. 1998, 13, 94. (26) Chomski, E.; Khushalani, D.; MacLachlan, M.; Ozin, G. A. Curr. Opin. Colloid Interface Sci. 1998, 3, 181. (27) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (28) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (29) Yang, S. M.; Yang, H.; Coombs, N.; Sokolov, I.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 52. (30) Yuan, Z.; Zhou, W. Chem. Phys. Lett. 2001, 333, 427. (31) Sims, S. D.; Walsh, D.; Mann, S. Adv. Mater. 1998, 10, 151. (32) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11, 324. (33) Walsh, D.; Mann, S. Chem. Mater. 1996, 8, 1944. (34) An Introduction to Bioceramics; Hench, L. L., Wilson, J., Eds.; Advanced Series in Ceramics Vol. 1.; World Scientific: Singapore, 1993; Vol. 1.
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Figure 2. Transmission electron micrographs of the vesicle formation: (a) initial smaller vesicles; (b) two joining adjacent vesicles; (c) further larger vesicles; (d) character of hollow vesicles with a thick “shell”.
Experimental Section Our process started with a homogeneous aqueous solution of amphiphilic hexadecylamine (C16H33NH2), into which hydrochloric acid solution was added to give an acidic environment. Na2HPO4 and CaCl2 were then consecutively introduced under stirring to the above mixture, giving a final reactant molar ratio of hexadecylamine/Na2HPO4/CaCl2 of 1/1/1. The pH value of the final mixture was 5. Then, the resulting gel was loaded into Teflon-lined stainless steel autoclaves and heated at 100 °C for 96 h. The products were filtered, washed with copious amounts of deionized water and ethanol, and dried in ambient air. X-ray powder diffraction (XRD) experiments were performed on a Rigaku D/max 2500 diffractometer in θ/2θ-geometry with Cu KR radiation at a scanning rate of 0.01°/min. Transmission electron microscopy was carried out on a JEOL JEM-2010 microscope operating at 200 kV. The specimens for TEM examination were prepared by dispersing the powdered samples in alcohol by ultrasonic treatment and dropping them onto a holey carbon film supported on a copper grid. Energy-dispersive X-ray (EDX, element > B) analysis was also carried out.
Results and Discussion The powder X-ray diffraction pattern of the solid product is shown in Figure 1, which reveals a mesolamellar phase with a primary d001 spacing of 3.14 nm. However, no evident striped pattern consistent with the lamellar structure was found during TEM observation. This lamellar mesostructured calcium phosphate material is unstable under the electron-beam irradiation. The surface structure features can be changed to vesicular particle morphology with curved patterns at submicrometer scales under electron-beam irradiation (Figure 2). Figure 2a shows a low-magnification TEM micrograph of the mesolamellar calcium phosphate after electron-beam irradiation at an initial period, which reveals the formation of many small vesicles with the size of several nanometers. Some vesicles with the size of more than 10 nm can also be found in Figure 2a, which are a mixture of these smaller vesicles. With further electron irradiation, two neighboring vesicles can adhere, join, and merge with each other to become one bigger vesicle of 20-40 nm (Figure 2b). These vesicular particles can continue to grow to achieve larger sizes of more than 50 nm (Figure 2c). Figure 2d shows a
part of a very large vesicle with the size of ∼150 nm, in which the vesicular particle is hollow with an approximately 10 nm thick “shell”. EDX measurement was used to characterize the chemical composition of the vesicles, and it revealed the presence of Ca, P, C, N, and O in the shell of the vesicles. However, it is difficult to estimate the accurate composition of vesicular calcium phosphate on the basis of the EDX profile, since the content of element H is immeasurable in the present experiment. It is interesting to note that the hollow vesicles made of mesolamellar calcium phosphate described for the first time in this paper were produced under electron-beam irradiation, which is distinct from the formation processes of mesoporous silica “shell mimics” grown in solution,35-37 mesoporous silica “hollow spheres” grown at the interface between oil and water,38 and lamellar silica or aluminophosphate with a vesicular structure,39,40 though the acidcatalyzed synthesis of highly ordered mesostructured materials in the solution system has proven to be an effective route for the generation of hierarchical ordering with various topological genuses.26-29,35-38,41 Surfactants have been used in the formation of calcium phosphate materials with interconnected microskeletal architectures by bicontinuous microemulsions,33 but organic surfactants are difficult to be inserted in the crystallized solids.42 Only Ozin et al. reported a mesolamellar phase of calcium dodecyl phosphate obtained from the reaction of calcium and potassium dodecyl phosphate,43 which could be (35) Lin, H.-P.; Mou, C.-Y. Science 1996, 273, 765. (36) Ozin, G. A.; Yang, H.; Sokolov, I.; Coombs, N. Adv. Mater. 1997, 9, 662. (37) Lin, H.-P.; Cheng, Y.-R.; Mou, C.-Y. Chem. Mater. 1998, 10, 3772. (38) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. (39) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47. (40) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (41) Yang, S. M.; Sokolov, I.; Coombs, N.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 1427. (42) Sarda, S.; Heughebaert, M.; Lebugle, A. Chem. Mater. 1999, 11, 2722. (43) Ozin, G. A.; Varaksa, N.; Coombs, N.; Davies, J. E.; Perovic D. D.; Ziliox, M. J. Mater. Chem. 1997, 7, 1601.
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Figure 3. Proposed formation mechanism of vesicular calcium phosphate.
cocrystallized with the calcium phosphate mineral phase43 or hydroxyapatite44 to create a chemical composite for biomedical application in the field of bone replacement. However, no interesting morphology for mesolamellar calcium dodecyl phosphate was found. The hexadecylamine surfactant was protonated to be C16H33NH3+ in the initial mixture with an acidic environment, which produced a lamellar (C16H33NH3+)2(HPO42-) phase with HPO42- from sodium hydrogen phosphate.39 The packing requirements of the small amine surfactant headgroup favor a planar bilayer structure of surfactants and water.39 Mineralized calcium(II) species encounter (C16H33NH3+)2(HPO42-) to induce an organic/inorganic hybrid bilayer structure of calcium phosphate lamellae with no surface patterning, which has been supported by the XRD data (Figure 1). The layered structure can be stabilized by the electrostatic and entropic undulation repulsion force between the membrane layers and the condensation of inorganic phosphates.35 Because of electron bombardment in high vacuum during TEM observation, curvature and new assembly of ionic surfactant of the bilayers would take place, leading to vesicle formation. (44) Soten, I.; Ozin, G. A. J. Mater. Chem. 1999, 9, 703.
Notes
Such bilayer-to-vesicle transformations are well documented;39,40 however, most cases took place directly in the mixed gel system of either aqueous or nonaqueous solutions. Actually the temperature of the particle surface may be increased quickly with the energy from instant electron-irradiation. This may cause the restructuring of the calcium phosphate mesophase. The morphogenesis of the organic-inorganic complex mesolamellar structure will occur in the process of C16H33NH3+ micelle vesiculation. Momentary high energy from electron-beam irradiation may cause the rupture of organic/inorganic hybrid bilayers and vesiculation. Minimization of their surface free energy through a balance of adhesion and elastic forces could also lead to vesicle distortion and to the enlargement of elliptical vesicles. Meanwhile with the supply of further energy from electron-beam irradiation for a longer time, vesicles of mesolamellar structure could grow through coalescence between neighboring vesicles and followed reassembly. Figure 3 illustrates our proposed formation mechanism of visicular mesostructured calcium phosphate material. Two adjacent vesicles would join and emerge to give a larger one via self-assembly by polymerization of respective inorganic species and aggregation of ionic surfactant species, forming the spherical calcium phosphate morphology. Or these small vesicles may directly reassemble with the contiguous bilayer patches to become bigger ones. Those aggregated ionic surfactant species from the merged visicles could assemble again into two or more bilayers of visicle structure, resulting in the enlargement of the bilayer vesicle. Of course, further work to develop and consummate this proposed model for morphogenesis of hollow vesicles based on the theorization and experimentation should be processed. Therefore, self-organizing transformation synthesis involving a new kind of formation mechanism has been described that produces novel hollow vesicle morphologies made up of mesostructured lamellar calcium phosphate. Electron-beam irradiation induces the curvature of a surfactant-inorganic layers system, leading to vesiculation of a hybrid of the surfactant and calcium phosphate. The formation of vesicular calcium phosphate would provide insight into morphogenesis of mineralized dimensional forms in biology and nanotechnology and ideas for new opportunities in materials science.41 The present work could supply some valuable information on the understanding of the formation mechanism of mesostructure and morphology. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (NSFC) and Belgium Federal Government PAI-IUAP-4/ 10 project. LA011518Z
