CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2101–2107
Articles A Mild and Efficient Biomimetic Synthesis of Rodlike Hydroxyapatite Particles with a High Aspect Ratio Using Polyvinylpyrrolidone As Capping Agent Yanjie Zhang†,‡ and Jinjun Lu*,† State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China ReceiVed December 4, 2006; ReVised Manuscript ReceiVed April 3, 2008
ABSTRACT: Rodlike hydroxyapatite (HAp) nanoparticles were synthesized at 60 °C through a biomimetic pathway from Ca(NO3)2 · 4H2O, H3PO4, and NH4OH. The polymer polyvinylpyrrolidone (PVP) as a capping agent was used to regulate the nucleation and crystal growth of HAp crystal. X-ray diffraction results combined with high-resolution transmission electron microscopy indicated that single phase, nanocrystal HAp powder could be obtained in one-step and the addition of PVP facilitated the preferential growth of nanocrystal HAp along the axis. The formation mechanism of rodlike HAp crystal and the effects of PVP on the crystal nucleation and growth were discussed based on transmission electron microscopy and Fourier transform infrared spectroscopy results. Introduction Hydroxyapatite (HAp) has been widely used in orthopedic and dental implant applications because of its excellent biocompatibility and bonding with living tissues.1–4 In the natural bone and teeth, the nanometer-sized HAp crystal is embedded in the collagen matrix with an average length of 50 nm, width of 25 nm, and a thickness of only 2-5 nm.5 In fact, collagen acts as a template in the controlled biomineralization process.6 The template-directed crystallization of calcium phosphates has a great relevance to the understanding of the biomineralization process.7 In the biomineralization process of bone and teeth, the nucleation and growth of inorganic crystals occur in the presence of biological macromolecules which can interact with inorganic crystals by electrostatic and hydrogen bond effects. These effects play an important role on the morphology and grain size of the inorganic crystal. HAp powder can be synthesized by a variety of routes, such as the chemical precipitation approach,8,9 hydrothermal reaction,10–14 sol-gelsynthesis,15–17 andmechanochemicalsynthesis.18,19 Rod-shaped HAp powder is highly preferred in most of the research as motivated by imitating the microstructure of teeth. In other words, rodlike nano-HAp powder is believed to be the most favorable “building block” to construct dental material, for example, successful synthesis of the mesoporous hydroxyapatite using cationic surfactant as template through a hydro* Corresponding author. Tel: +86 931 4968198. Fax: +86 931 4968163. E-mail:
[email protected]. † Lanzhou Institute of Chemical Physics. ‡ Graduate School of the Chinese Academy of Sciences.
thermal method20 and preparation of ultrahigh-aspect-ratio hydroxyapatite nanofibers in reverse micelles under hydrothermal conditions.21 However, strict reaction conditions or complicated instrumentation in the aforesaid methods is necessary. For this reason, it is of great importance to develop inexpensive HAp synthesis methods to control the crystal morphology and grain size. Low-temperature solution techniques were expected to achieve the goal to control the morphology in a single step under mild reaction conditions, typically at a temperature less than 100 °C and pressure of 1 atm. Therefore, in this study, a mild and efficient biomimetic synthetic method has been proposed and described in detail. The word “mild” in this paper means that synthesis should be conducted at temperatures as close to the temperature of the human body as possible and at ambient pressure. Over the past decade, polyvinylpyrrolidone (PVP) has been widely used as surface-regulating polymer for synthesis of inorganic nanomaterials, especially for architecture of onedimensional (1D) nanostructure.22–27 For example, Yang et al. synthesized highly monodisperse wurtzite ZnO nanoparticles using PVP as capping molecules;22 Kim et al. reported the effect of water-soluble, nonionic polymers on the crystallization of calcium carbonate;23 Xi et al. prepared single-crystalline tellurium nanowires and nanotubes through a surfactant-assisted solvothermal method.24 PVP is introduced in this work just because of the imide (N-CdO) group in the polymer structure which also exists in collagen molecular structures. It is very difficult to study the influence of the functional group in collagen on the inorganic crystal due to the complicated structure of the
10.1021/cg060880e CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
2102 Crystal Growth & Design, Vol. 8, No. 7, 2008
Zhang and Lu
Figure 1. TEM micrographs of the synthesized HAp powders: (a) reacted at 60 °C for 5 days without PVP, (b, c) reacted at 60 °C for 2 and 5 days, respectively, in the presence of PVP, and (d) reacted at 40 °C for 5 days in the presence of PVP.
protein. Therefore, PVP can act as an appropriate “replacer” to help us understand the biomineralization process. Our research has been focusing on mild solution conditions (less than 100 °C) for the synthesis of HAp and the other calcium phosphates crystals. In the present work, rodlike HAp nanoparticles with uniform morphology have been directly synthesized by a simple method. The surface-regulating polymer PVP as a capping agent is used to regulate the nucleation and crystal growth of HAp crystal. The formation mechanism of rodlike HAp and effects of PVP on the crystal nucleation and growth have also been discussed. Experimental Procedures Reagent. The starting materials used in this work included commercially available calcium nitrate (Ca(NO3)2 · 4H2O, AR, Shanghai Chemical Reagent Factory, China), 85% phosphoric acid (H3PO4, AR,
Baiyin Chemical Reagent Factory, China), polyvinylpyrrolidone (PVP K30, Sinopharm Chemical Reagent Company, China), nitric acid (HNO3, AR, Baiyin Chemical Reagent Factory, China), and ammonium hydroxide (NH4OH, AR, Baiyin Chemical Reagent Factory, China). All chemicals were used without further purification. Synthesis of HAp. The procedure for preparing HAp powder was the following: 0.03 mol of Ca(NO3)2 · 4H2O was dissolved completely in 500 mL of deionized water and the pH was adjusted to 3 with HNO3 solution. Then 0.018 mol of H3PO4 and 0.011 or 0.14 mmol of PVP (one case in absence of PVP) were added to the above solution. The resulting solution had the Ca/P ratio of hydroxyapatite (Ca/P ) 1.67). Ammonium hydroxide was then added dropwise to the mixed solution with continuous stirring until the milky suspension appeared when pH 7 was reached. The rodlike HAp crystal would nucleate and grow from this supersaturated aqueous solution. The suspension solution was sealed in a jar and kept inside a water bath for 2-7 days at 60 °C (one case at 40 °C). The suspension was then washed several times with water and ethanol in turn and filtered by centrifuging.
Synthesis of Rodlike Hydroxyapatite Particles
Crystal Growth & Design, Vol. 8, No. 7, 2008 2103
Characterization. Phase analysis of the precipitated HAp powders were conducted using X-ray diffraction with Cu KR radiation (λ ) 1.5418 Å). The diffractometer (X’Pert PRO, Philips) was operated at 40 kV and 30 mA at a 2θ range of 20-70° employing a step size of 0.0170 and a 29.8450 s exposure. The products were also characterized by Fourier transform-infrared spectroscopy (FTIR, IFS 66V/S FTIR, Bruker) in the range of 4000 cm-1 to 400 cm-1. The morphology and grain sizes of HAp powders were observed by transmission electron microscopy (TEM, H-600, Hitachi) and high-resolution transmission electron microscopy (HRTEM, JEOL 2010). Zeta potentials were performed on the precipitated HAp particles using a Malvern Nano ZS ZEN3600 instrument. Measurements were averaged over 30 runs using deionized water at pH ) 7. Each sample was measured for three times.
Results and Discussion The morphologies of HAp crystals respectively prepared with and without PVP are shown in Figure 1. Figure 1a is the TEM micrograph of the synthesized HAp powders without PVP. It is clear that the product consists of two kinds of particles. The typical one is rodlike, which is at least 100 nm in length and has an aspect ratio of 5 or more, and is not prevailing in quantity in Figure 1a. Instead, numerous immature particles (the length is less than 50 nm, and the diameter is larger than that of the typical rodlike particles) are found. Although the immature particles have a tendency to preferentially grow along one direction they are not classified as rodlike particles in this paper. Figure 1b,c shows the HAp crystals synthesized at 60 °C for 2 and 5 days in the presence of PVP, respectively. Although some immature particles could still be observed in Figure 1b, the number of them is far less than that in Figure 1a. Meanwhile, the typical size of the rodlike particles in Figure 1b is found to be larger than that in Figure 1a. It should be noted that the immature particles can hardly be seen in Figure 1c. Except for some rodlike particles of about 100 nm in length, the typical size of the rodlike particles in Figure 1c is 10-20 nm in diameter and 250-300 nm in length. Hence, high aspect ratios (20-30) can be obtained at 60 °C for 5 days, see Figure 1c. Compared to the as-synthesized HAp powders without PVP in Figure 1a, the particles in Figure 1c exhibited a rodlike morphology with a higher aspect ratio, which indicated that PVP greatly facilitated the formation of rodlike HAp crystal with a high aspect ratio. The influence of the reaction temperature has also been included in this paper. HAp crystals obtained at 60 °C (Figure 1b,c) have a better aspect ratio than that obtained at 40 °C (Figure 1d). At 40 °C, which is close to the temperature of the human body, a large number of HAp particles possess rodlike shape with a length of less than 100 nm (Figure 1d). It could be deduced that the particles tend to preferentially grow along the direction at a higher temperature.8 At 60 °C, a long duration of reaction resulted in a better aspect ratio of rodlike HAp crystal, as shown in Figure 1b,c. The XRD pattern of HAp powders synthesized at 60 °C for 5 days with PVP are shown in Figure 2. It can be observed that the as-precipitated HAp powder is single phase and highly crystallized without any heat treatment. The relative intensity of the (002) diffraction peak (88%) in Figure 2 is much higher than that (36%) in standard XRD pattern of HAp (JCPDS card no. 09-432). That is, this mild biomimetic synthetic method is in favor of the HAp crystal growth along the c-axis. The results are coincident to the observation of the HRTEM, shown in Figure 3. The HRTEM images indicate a typical apatite crystalline structure. In Figure 3, the lattice fringes are clearly visible with a spacing of 0.32 nm, which is in agreement with the spacing of the (002) plane of HAp.
Figure 2. XRD patterns of HAp powder synthesized at 60 °C for 5 days with PVP. All peaks correspond to hydroxyapatite based on the standard XRD pattern card of HAp (JCPDS card no. 09-432).
Figure 3. High-resolution TEM (HRTEM) image of rodlike HAp powder synthesized at 60 °C for 5 days in the presence of PVP.
On the basis of the aforementioned results, it is feasible to use the employed method for preparation of rodlike HAp particles with a high aspect ratio. Meanwhile, the reaction condition is mild and efficient since the reaction occurs at low temperature (60 °C) and ambient pressure. To accurately understand the growth process of rodlike crystal, the characterization of the sample reaction for 1 h and 24 h at 60 °C was conducted. The as-precipitated powders showed the needle-like and rodlike morphologies respectively for 1 h and 24 h, observed from the TEM images (Figure 4a,b). A representative XRD pattern in Figure 5 shows that HAp phase for 1 h and 24 h respectively at 60 °C is indeed well-crystalline (i.e., sharp and intense diffraction peaks). And it is also found that there is a single phase of HAp in the product without the diffraction of PVP. The interaction between HAp crystal and PVP was elucidated with FTIR spectrum. In the HAp crystal structure, hydroxyls (O-H group) are filled in the channels formed by triangles in the c direction. The frequencies of stretching band and bending band are influenced by the ion which will bond with the O-H group. When the O-H group interacted with O of another hydroxyl in HAp crystal structure, the hydroxyl bond (OHsO) absorption was observed at 3571 cm-1 stretching band (νOH-O) and 630 cm-1 bending band (δOH-
2104 Crystal Growth & Design, Vol. 8, No. 7, 2008
Zhang and Lu
Figure 5. XRD patterns of HAp powder synthesized at 60 °C for 1 h and 24 h, respectively, in the presence of PVP. All peaks correspond to hydroxyapatite based on the standard XRD pattern card of HAp (JCPDS card no. 09-432).
Figure 6. FTIR spectra of HAp powder obtained at 60 °C for (a) 1 h, (b) 24 h, and (c) 5 days respectively.
Figure 4. TEM micrographs of the synthesized HAp powders at 60 °C for (a) 1 h and (b) 24 h, respectively.
respectively. It was reported that νOH-F were shifted to 3540 cm-1 and the 630 cm-1 liberational mode (δOH-F) disappeared when O-H group formed bond with F in crystal structure.28,29 Figure 6 shows the FTIR spectrum of the HAp powders obtained at 60 °C for 1 h, 24 h, and 5 days respectively. In particular, a slight blue-shift of hydroxyl group mode from 3568 cm-1 to 3571 cm-1 might originate from the disappearance of a weak hydrogen bond of CdO in PVP to O-H at the surface of HAp crystal. In Figure 6a, the weak band at 2925 cm-1 is attributed to the stretching vibration of C-H in PVP molecular structure and the peak at 1653 cm-1 is due to the amide I band. The absorption band at 3568 cm-1 (Figure 6a) was nearly identical O),
with those of hydroxyapatite-gelatin nanocomposite30 and hydroxyapatite-chitosan composite.31 The stretching vibration of the hydroxyl group band of the sample reacting for 24 h has shifted to 3570 cm-1 (Figure 6b), which means the absence of PVP at the surface of HAp crystal. Figure 6c shows a typical FTIR spectrum of rodlike HAp crystal. The stretching vibration of the hydroxyl group band of HAp is assigned to 3571 cm-1 and the 634 cm-1 originating from libration bands of OHgroup, while the broadband at 3427 cm-1 is attributed to the absorbed water. The bands at 1096, 1032, 962, 603, and 564 cm-1 are attributed to PO43- ions (ν1s962 cm-1, ν3s1032 cm-1 and 1096 cm-1, ν4s564 cm-1 and 603 cm-1).32,33 It is demonstrated from the results of FTIR spectrum that no impurity exists in the as-precipitated HAp powder. The fingerprint region of FTIR spectra was shown in Figure 7. A progressive enhancement of the amplitude of the out-of-plane bending band of O-H group at 634 cm-1 was noted with increasing the duration of reaction, which was good agreement with the progressive enhancement of the stretching vibration band at 3571 cm-1 in Figure 6. The sharp peak at 3571 cm-1 and 634 cm-1 indicated that along with the duration of reaction the hydrogen bonding effect between PVP and O-H at surface of HAP finally
Synthesis of Rodlike Hydroxyapatite Particles
Crystal Growth & Design, Vol. 8, No. 7, 2008 2105
acts as a surface-regulating polymer to result in the rodlike morphology of the HAp crystal.5,7 The interaction between inorganic crystal and organic matrix is mainly by hydrogen bond. PVP has a polyvinyl skeleton with polar group, shown in Scheme 1a. The role of PVP lies in the combination of two effects: (1) spatial effect, and (2) electrostatic and hydrogen bond effects. The Ostwald ripening and oriented attachment mechanisms have been extensively used to understand the crystal growth in the solution.34–39 Ostwald ripening usually refers to the solution process in which “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones”.34 Oriented attachment growth is related to the direct selforganization of two particles into a single crystal by sharing a common crystallographic orientation, despite the presence of strong surface-bound ligands.35,36 On the basis of the experimental results, the formation mechanism of rodlike HAp crystal has been proposed, as shown in Scheme 1. The whole formation process includes nucleate, surface-regulating, growth of needle-like crystal, oriented attachment and Ostwald ripening. In the first process, HAp crystal will nucleate from the precursor solution (Stage I). Taking into account the HAp lattice constants (a ) 9.422 Å and c ) 6.883 Å) and the hexagonal symmetry with the space group P63/m, its unit cell will be arranged along the c-axis. The growth habit of HAp along the c-axis can be found from Figure 1a without PVP, which has also been proved by the previous work.5 For a crystal in equilibrium with its surroundings, the surface energy must be minimal for a given volume.40 When HAp nucleated from the solution, the growth habit emerged simultaneously. PVP in the solution will greatly prompt this growth habit. And the capping agent PVP was favorable for the formation of rodlike HAp crystal mainly at the earlier stage of crystal growth (Stage II). The N-CdO group in PVP is easily attached to the surface of HAp crystal and slows down the growth speed of the crystal faces (Scheme 1b). Because the O-H groups are abundantly located in the surface of HAp crystal, the hydrogen band will formed between PVP and HAp, which has been proved by FTIR results (Figure 6a). In a low concentration of PVP solution which is adopted in this work, the PVP molecular
Figure 7. Finger-print region in FTIR spectra of HAp powder obtained at 60 °C for (a) 1 h, (b) 24 h, and (c) 5 days, respectively.
changed to the interaction between hydroxyls in HAp crystal structure. Therefore, it can be concluded from these results that the effect of PVP on HAp crystal (forming hydrogen bond at earlier stage of reaction) worked on the crystal morphology but not on the phase composition of product. Table 1 shows the zeta potentials of the HAp particles synthesized under different reaction conditions. It can be seen that the zeta potential for sample 1 (HAp particle without PVP) is 0.25, which indicates the minimum repulsion force and particles tend to agglomerate together. The zeta potential for sample 2 increases to 4.66 when PVP is introduced in the raw material solution. According to Table 1, the zeta potentials show a significant difference for the particles synthesized in presence of PVP at 60 °C for 1 h, 24 h, and 5 days, respectively. The increased zeta potential of 6.10 for sample 3 demonstrates the interaction between PVP and HAp occurs mainly at earlier stage of reaction, which is clearly in agreement with FTIR results (Figure 6). As for the role of PVP, it is supposed that it has similar effect as the collagen matrix in the biomineralization process does. It
Table 1. Zeta Potentials for the Synthesized HAp Powders sample
1
2
3
4
reaction condition zeta potential/mV
without PVP for 5 days 0.25 ( 0.08
with PVP for 5 days 4.66 ( 0.16
with PVP for 1 h 6.10 ( 0.09
with PVP for 24 h 4.63 ( 0.25
Scheme 1. Formation Mechanism of Rodlike HAp Crystal
2106 Crystal Growth & Design, Vol. 8, No. 7, 2008
Zhang and Lu
process, PVP can be desorbed into water because of the unstable surface adsorption. When needle-like HAp crystals were selforganized by oriented attachment (Scheme 1c), the absorbed species irreversibly removed from the surface of HAp crystal during the bundle-like crystal coalescence based on FTIR spectrum (Figure 6). Along with desorption of PVP, the crystal growth is corporately controlled by Ostwald ripening and oriented attachment process. In the presence of PVP, the formation of the needle-like HAp nanocrystallites facilitates aggregation along the c axis and form rodlike structure via oriented attachment. While in the absence of PVP, it is difficult to form the rodlike structure with a high aspect ratio from the immature particles, although Ostwald ripening also occurs in the solution. A typical oriented attachment process is shown in Figure 8. The morphology transformation of synthetic HAp crystal has been studied in the condition of a high concentration of PVP (0.14 mmol) and long duration (7 days). Oriented attachment process would be accelerated under this reaction condition. Because of the oriented attachment and Ostwald ripening process, some of the small size HAp crystals grow to the large rodlike HAp (Figures 8a and 8b). Because of the effect of the strong van der Waals attraction, the HAp rodlike structure tends to aggregate together and form side-by-side rodlike structure along the c axis.41 In Figure 8b, the formation of the bundled rodlike crystal is shown in the area marked A. The sharp edges of the bundled crystals will dissolve due to the higher solubility and the bundled structure will coalesce into the larger rodlike crystals in the end, shown in Figure 8b, the area marked B. As a result of the crystallization process of HAp controlled by the surface-regulating (PVP) and the oriented attachment process, the synthesized HAp crystals are uniform nanorods with a high aspect ratio. Conclusions High aspect ratio rodlike nano-HAp powder has been prepared using a mild and efficient biomimetic synthetic method. In the crystallization process, the oriented attachment and the effect of PVP were vital for the regulation of the nucleation and crystal growth of rodlike HAp crystal. On the basis of the spatial effect and hydrogen bond effects, the capping agent (PVP) prompts the fast formation of rodlike HAp particles with a high aspect ratio. The formation behavior mechanism is introduced to explain the great tendency of HAp crystal to preferentially grow along the direction. The reaction temperature and duration of reaction are also critical toward obtaining controlled morphology and grain size of HAp crystals.
Figure 8. TEM images of the sample synthesized in the condition of 0.14 mM PVP and 7 days, (a) 20 K magnification and (b) 100 K magnification.
shows a linear structure. Under the capping effect of PVP, HAp crystals preferentially grew along the long chain of PVP and finally needle-like morphology (Figure 4a) is obtained after reacting for 1 h at 60 °C (Stage III). In this stage, the capping agent (PVP) on the needle-like HAp crystal surface blocks the Ostwald ripening growth. Thus, needle-like HAp crystal will grow via oriented attachment (Stage IV). The shift in stretching mode of the hydrogen bond from 3568 to 3571 cm-1, as well as the progressive enhancement of OH stretching mode and liberational mode respectively at 3571 and 634 cm-1 indicates the disappearance of the interaction between HAp and PVP and the formation of rodlike HAp crystal. During the growing
Acknowledgment. The authors acknowledge the financial support from the National Natural Science Foundation of China (50572107) and “West Light Program of 2003” of the Chinese Academy of Sciences.
References (1) Afshar, A.; Ghorbani, M; Ehsani, N.; Saeri; M, R.; Sorrell, C. C. Mater. Des. 2003, 24, 197–202. (2) Riman, R. E.; Suchaned, W. L.; Byrappa, K.; Chen, C. W.; Shuk, P. Solid State Ionics 2002, 151, 393–402. (3) Chen, J. D.; Wang, Y. J.; Wei, K.; Zhang, S. H.; Shi, X. T. Biomaterials 2007, 28, 2275–2280. (4) Mobasherpour, I.; Heshajin, M. S.; Kazemzadeh, A.; Zakeri, M. J. Alloys Compd. 2007, 430, 330–333. (5) Vallet-Regi, M.; Gonzalez-Calbet, J. M. Prog. Solid State Chem. 2004, 21, 1–31. (6) Kikuchi, M.; Ikoma, T.; Itoh, S.; Matsumoto, H. N.; Koyama, Y.; Takakuda, K.; Shinomiya, K.; Tanaka, J. Compos. Sci. Technol. 2004, 64, 819–825.
Synthesis of Rodlike Hydroxyapatite Particles (7) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. AdV. Mater. 2003, 15, 313–316. (8) Kumar, R.; Prakash, K. H.; Cheang, P.; Khor, K. A. Langmuir 2004, 20 (13), 5196–5200. (9) Cai, S.; Yu, X.; Xiao, Z.; Xu, G.; Lv, H.; Yao, K. Ceram. Int. 2007, 33, 193–196. (10) Kim, H. W.; Koh, Y. H.; Li, L. H.; Leec, S.; Kim, H. E. Biomaterials 2004, 25, 2533–2538. (11) Liu, H. S.; Chin, T. S.; Lai, L. S.; Chiu, S. Y.; Chung, K. H.; Changb, C. S.; Lui, M. T. Ceram. Int. 1997, 23, 19–25. (12) Ioku, K.; Yamauchi, S.; Fujimori, H.; Goto, S.; Yoshimura, M. Solid State Ionics 2002, 151, 147–150. (13) Zhang, X.; Vecchio, K. S. J. Cryst. Growth 2007, 308, 133–140. (14) Sun, Y.; Guo, G.; Tao, D.; Wang, Z. J. Phys. Chem. Solids 2007, 68, 373–377. (15) Liu, D. M.; Yang, Q. Z.; Troczynski, T. Biomaterials 2002, 23, 691– 698. (16) Kim, T. S.; Kumta, P. N. Mater. Sci. Eng. B 2004, 111, 232–236. (17) Han, Y. C.; Li, S. P.; Wang, X. Y.; Chen, X. M. Mater. Res. Bull. 2004, 39, 25–32. (18) Yeong, K. C. B.; Wang, J.; Ng, S. C. Biomaterials 2001, 22, 2705– 2712. (19) Mochales, C.; Briak-Benabdeslam, H. E.; Ginebra, M. P.; Terol, A.; Planell, J. A.; Boudeville, P. Biomaterials 2004, 25, 1151–1158. (20) Yao, J.; Tjandra, W.; Chen, Y. Z.; Tam, K. C.; Ma, J.; Soh, B. J. Mater. 2003, 13, 3053–3057. (21) Cao, M. H.; Wang, Y. H.; Guo, C. X.; Qi, Y. J.; Hu, C. W. Langmuir 2004, 20, 4784–4786. (22) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G. K. L. Chem. Mater. 2000, 12, 2268–2274. (23) Kim, I. W.; Robertson, R. E.; Zand, R. Cryst. Growth Des. 2005, 5, 513–522. (24) Xi, B.; Xiong, S.; Fan, H.; Wang, X.; Qian, Y. Cryst. Growth Des. 2007, 7, 1185–1191.
Crystal Growth & Design, Vol. 8, No. 7, 2008 2107 (25) Rodriguez, A. T.; Chen, M.; Chen, Z.; Brinker, J.; Fan, H. J. Am. Chem. Soc. 2006, 128, 9276–9277. (26) Deng, Z.; Tang, F.; Chen, D.; Meng, X.; Cao, L.; Zou, B. J. Phys. Chem. B 2006, 110, 18225–18230. (27) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. ReV. 2007, 107, 2228–2269. (28) Kim, H. W.; Noh, Y. J.; Koh, Y. H.; Kim, H. E.; Kim, H. M. Biomaterials 2002, 23, 4113–4121. (29) Rendon-Angeles, J. C.; Yanagisawa, K.; Ishizawa, N.; Oishi, S. Chem. Mater. 2000, 12, 2143–2150. (30) Chang, M. C.; Ko, C. C.; Douglas, W. H. Biomaterials 2003, 24, 2853– 2862. (31) Murugan, R.; Ramakrishna, S. Biomaterials 2004, 25, 3829–3835. (32) Slosarczyk, A.; Pasziewicz, Z.; Paluszkiewicz, C. J. Mol. Struct. 2005, 744-747657-661.. (33) Rapacz-Kmita, A.; Paluszkiewicz, C.; Slosarczyk, A.; Pasziewicz, Z. J. Mol. Struct. 2005, 744-747653-656.. (34) Zeng, H. C. J. Mater. Chem. 2006, 16, 649–662. (35) Zhang, J.; Wang, Y.; Zheng, S.; Huang, F.; Chen, D.; Lan, Y.; Ren, G.; Lin, Z.; Wang, C. J. Phys. Chem. B 2007, 111, 1449–1454. (36) Zhang, J.; Lin, Z.; Lan, Y.; Ren, G.; Chen, D.; Huang, F.; Hong, M. J. Am. Chem. Soc. 2006, 128, 12981–12987. (37) Li, Z.; Xu, F.; Sun, X.; Zhang, W. Cryst. Growth Des. 2008, 8, 805– 807. (38) Reeja-Jayan, B.; Rosa, E. D.; Sepulveda-Guzman, S.; Rodriguez, R. A.; Yacaman, M. J. J. Phys. Chem. C 2008, 112, 240–246. (39) Cao, X.; Lan, X.; Guo, Y.; Zhao, C. Cryst. Growth Des. 2008, 8, 575–580. (40) Zhu, W.; Wu, P. Chem. Phys. Lett. 2004, 396, 38–42. (41) Chen, H. F.; Tang, Z.; Liu, J.; Sun, K.; Chang, S. R.; Peters, M. C.; Mansfield, J. F.; Czajka-Jakubowska, A.; Clarkson, B. H. AdV. Mater. 2006, 18, 1846–1851.
CG060880E
