Controllable synthesis of fluorapatite nanocrystals with various morphologies: Effects of pH value and chelating reagent

Journal of Alloys and Compounds 485 (2009) 396–401

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Controllable synthesis of fluorapatite nanocrystals with various morphologies: Effects of pH value and chelating reagent Min Chen ∗ , Deli Jiang, Di Li, Jianjun Zhu, Guofang Li, Jimin Xie School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

a r t i c l e

i n f o

Article history: Received 21 October 2008 Received in revised form 22 May 2009 Accepted 24 May 2009 Available online 30 May 2009 Keywords: Nanostructured materials Inorganic materials Crystal growth X-ray diffraction Scanning and transmission electron microscopy

a b s t r a c t Fluorapatite (FHAp) nanocrystals with various morphologies were synthesized through a hydrothermal process in the presence of three different chelating reagents: citric acid (CA), ethylenediamine tetraacetate disodium salt (Na2 EDTA), and CA/Na2 EDTA mixed chelating reagent. The morphology of FHAp nanocrystal is highly dependent on the pH value and nature of chelating reagent. In the case of CA, two main morphologies of FHAp nanocrystals were observed as pH value was varied from 3.6 to 10.0; while onedimensional (1D) FHAp nanocrystals with hexagonal structure were always produced in the presence of Na2 EDTA under different pH values. When the two chelating reagents mentioned above were simultaneously used, tuning the pH value resulted in not only a morphology change but also a structural transformation due to the synergistic effect of the chelating reagents. A possible mechanism for the formation of FHAp nanocrystals with various morphologies was discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Recently, apatites [Ca10 (PO4 )6 (OH, F, Cl)2 ], the principle inorganic composition of the bone and teeth, have been attracting great interest in a large spectrum of applications, such as biomedical uses [1], drug delivery carriers [2], catalysts carriers [3], and adsorbents [4]. Among the family of apatites, fluorapatite (FHAp) is considered as an alternative material as biomaterial due to its low solubility and good biocompatibility with comparison to hydroxyapatite (HAp) [5]. The performance of HAp or FHAp in these applications is influenced by the nanoscale morphology and crystallinity [6–7]. For example, fibrous and needle-like morphologies with a high specific surface area are advantageous for adsorption and ion exchange. Moreover, the chemical and biological properties are known to depend on the crystal faces of FHAp. Therefore, techniques for the preparation of FHAp with a controlled structure would be highly important for the development in biologically applicable inorganic materials. There are several well-established methods available for preparation of FHAp, such as using surfactant [8], glutamic acid [9], biological proteins [10], chelating reagents [11], etc. to control the nucleation and growth of crystals under varying synthetic conditions. Among them, chelating reagent assisted controlled

∗ Corresponding author. Tel.: +86 11 88791708; fax: +86 11 88791800. E-mail addresses: [email protected] (M. Chen), [email protected] (D. Jiang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.121

fabrication of FHAp is of particular interest because of the simplicity. Ethylenediaminetetraacetic acid (EDTA) was often used in the morphology-controlled fabrication of nanocrystals with various shapes and sizes owing to its appropriate chelating and capping effect, which influence the growth rate of different facets distinguishingly [12]. Seoa and Lee synthesized HAp one-dimensional (1D) whisker through dissolution–reprecipitation process using EDTA [13]. Zhu et al. synthesized hexagonal HAp using EDTA under hydrothermal condition [14]. Liu et al. prepared HAp nanorods, bowknot-like nanostructures and flower-like architectures under microwave irradiation with the help of EDTA [15]. However, limited studies have been reported for the controlled synthesis of FHAp nanocrystals with various morphologies by using chelating reagent as a shape modifier. It is still a significant challenge to fabricate FHAp nanocrystals with various shapes and sizes by a simple method. As a chelating reagent, citric acid (CA) is an important biological ligand for metal ions and can form stable complexes with Ca2+ , Zn2+ , and Ag+ ions, etc. [16]. It can also serve as shape controller and stabilizer in the synthesis of HAp, calcite, Fe2 O3 , Ni(OH)2 , ZnO, etc. [17,18]. However, controlled fabrication of inorganic crystals with various nanostructures utilizing CA/Na2 EDTA mixed chelating reagents has not been reported yet. In this study, we report a first example for the controllable synthesis of FHAp nanocrystals in the presence of three different chelating reagents: CA, Na2 EDTA and CA/Na2 EDTA mixed chelating reagent. The morphology of FHAp products, such as flower-like nanostructures, nanorod aggregates, dumbbell-like nanostructures, hexagonal prism-like nanorods, whiskers, and nanowires can be easily controlled by sim-

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ply altering the pH value of initiative reaction solution. The effects of pH value and chelating reagent on the growth of FHAp are systematically investigated. The possible formation mechanisms for the hydrothermally synthesized FHAp nanocrystals with various morphologies are presented. 2. Experimental section All chemicals are analytical-grade reagents and used without further purification. A typical procedure for the preparation of FHAp nanocrystals under pH 5.2 in the presence of CA and Na2 EDTA is given below. 0.0025 mol of Na2 EDTA and 0.0025 mol of CA were dissolved into 80 mL of distilled water in a 250 mL three neck flask, then the flask was kept in a water bath at a temperature of 40 ◦ C. Under continuous stirring, 0.0025 mol of Ca(NO3 )2 , 0.0015 mol of (NH4 )2 HPO4 and 0.0004 mol NaF were added into the flask, sequentially. Then the pH value was adjusted to 5.2 using NaOH (1 M). After being stirred for 20 min, the mixture was transferred into a 100 mL Teflon-lined stainless autoclave and kept at 150 ◦ C for 8 h. After the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and ethanol in sequence, and then dried in a vacuum oven at 50 ◦ C for 8 h. Other samples were prepared by the similar procedure, except for different chelating reagents and pH conditions. The pH of the mixture was adjusted to a specific value in the same manner by using NaOH solution (1 M). The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD) using D8 Advance X-ray diffraction (Bruker axs company, Germany) equipped with Cu KR radiation (␭) 1.5406 (Å), employing a scanning rate of 0.02◦ s−1 in the 2Â range from 20◦ to 70◦ . The morphologies of the products were observed using field emission scanning electron micrographs (FESEM) and transmission electron microscopy (TEM). The FESEM images were taken with a field emission scanning electron microscope (Hitachi S-4800 II, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). TEM was recorded on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV.

3. Results and discussion 3.1. XRD patterns of the products The composition and phase purity of the products were investigated using XRD. XRD patterns of the samples prepared by using mixed chelating reagents with hydrothermal treatment at 150 ◦ C for 8 h under various pH conditions of (a) pH 3.6, (b) pH 5.2, (c) pH 7.1 and (d) pH 10.0 are shown in Fig. 1. The diffraction peaks of the four samples can be indexed as a pure hexagonal structure, which coincides well with the literature values (JCPDS No. 15–0876). No evidence of impurities can be found in the XRD patterns. The peaks shown in the XRD patterns of the prepared samples are sharp and intense, indicating that good crystallinity of the samples can be obtained at a relatively low hydrothermal treatment temperature (150 ◦ C). The XRD patterns also indicate that there is a moderate

Fig. 1. The XRD patterns of FHAp products obtained using mixed chelating reagents with hydrothermal treatment at 150 ◦ C for 8 h at different pH values: (a) pH 3.6, (b) pH 5.2, (c) pH 7.1 and (d) pH 10.0. The ratios of relative intensity (RRT) based on (0 0 2), (2 1 1) and (3 0 0) peaks for four samples are shown here.

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difference from each other in the relative intensity (RRT) based on (0 0 2 ), (2 1 1 ) and (3 0 0 ) peaks for four samples (Fig. 1), indicating the possibility of different preferential orientation growth of FHAp crystals under different pH conditions. 3.2. Morphologies of the products and effects of pH value and chelating reagent To investigate the effect of pH value on the morphology of FHAp products prepared using CA/Na2 EDTA mixed chelating reagents, a series of contrast experiments were conducted by using single Na2 EDTA or CA as chelating reagent. In this work, the molar ratios of CA/Ca2+ and Na2 EDTA/Ca2+ are both fixed at 1:1. Fig. 2 shows a series of typical FESEM and TEM images of FHAp products obtained by hydrothermal treatment at 150 ◦ C for 8 h under different pH values in the presence of different chelating reagents. One can see that FHAp nanocrystals with various novel morphologies including flower-like nanostructures, hexagonal prism-like nanorods, hexagonal nanorods with a thin part, and nanorod aggregates were generated. 3.2.1. CA as chelating reagent CA is a minority bone component. It plays an important role in crystal habit growth inhibition and acts as a mineralization inhibitor in vivo [19–21]. Fig. 2a1–d1 are FESEM images of FHAp nanocrystals prepared under different pH values using CA as chelating reagent. We found that CA favors the growth of FHAp nanorods aggregates under low pH value (below 6.1). At pH 3.6, the FHAp product is composed of uniform 3D nanorods aggregates, which are composed of numerous well-aligned nanorods (Fig. 2a1). From the inset, it can be clearly found that the nanorods with a diameter of about 50 nm were parallel and self-assembled to construct the superarchitecture. Under pH of 5.2, the sample is composed of dumbbell-like crystals as well as some particles with irregular shape (Fig. 2b1). Note that the ends of the dumbbell-like crystal are still composed of nanorods, which is similar with the structure of nanorods aggregates (Fig. 2a1). When pH was increased up to 6.1, however, the morphology and size of the FHAp nanocrystals changed remarkably. Fig. 2c1 and d1 show the FESEM and TEM (inset) images of FHAp products prepared in the presence of CA at pH 6.1 and 10.0, respectively. The FHAp products obtained under the two different pH values are both made up of a large number of short nanorods with a mean length of about 50 nm. It reveals that pH value has little influence on the shape and size of FHAp nanocrystal when it is above 6.1. 3.2.2. Na2 EDTA as chelating reagent When Na2 EDTA was used as single chelating reagent, the morphologies of the FHAp products are different from those produced by using CA as chelating reagent under the corresponding pH values. In the case of pH 3.6, the sample consists of novel flower-like crystals with diameters of around 3 ␮m (Fig. 2a2). It can be clearly found that the flower-like crystal has a perfect hexagonal screw caplike center part and five nail-like clusters laterally initiating from the center part. The FHAp product obtained under pH 5.2 in the presence of Na2 EDTA consists of hexagonal nanorods with lengths ranging from 2–5 ␮m and diameters of about 500 nm (Fig. 2b2). Note that some of the nanorods have a thin middle part (marked with the white arrow), revealing that the nanorods trend to crack. The nanorods with the sharp ends (marked with the white ring in Fig. 2b2) confirmed this speculation. As shown in Fig. 2c2, the similar sample can be also obtained under pH of 6.1 in the presence of Na2 EDTA. However, this kind of nanorods was not produced at pH 10.0, whereas the nanowires with lengths of up to 20 ␮m and diameter of around 200 nm as well as a few of hexagonal microrods were generated under that condition (Fig. 2d2). It is worthwhile to men-

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Fig. 2. FESEM and TEM images of as-prepared FHAp products obtained by hydrothermal treatment at 150 ◦ C for 8 h at different pH values in the presence of different additives: (a1, b1, c1 and d1) in the presence of CA; (a2, b2, c2 and d2) in the presence of Na2 EDTA and (a3, b3, c3 and d3) in the presence of mixed CA/Na2 EDTA chelating reagents system. (a, b, c and d) FESEM images of as-prepared FHAp products obtained at pH 3.6, 5.2, 6.1 and 10.0, respectively. The insets in Fig. c1 and Fig. d1 are the TEM images of the corresponding products.

tion that, almost all the products obtained at pH 5.2 and above are composed of 1D FHAp structures (except for the sample obtained in the presence of CA), suggesting that the presence of Na2 EDTA favor the formation of FHAp 1D nanostructure. 3.2.3. CA and Na2 EDTA as chelating reagent We mainly studied the effect of solution pH value on the morphology of FAHp nanocrystals in the presence of two different chelating reagents. The experiment conditions and the corresponding morphologies and dimensions of the samples were summarized in Table 1. When a mixture of Na2 EDTA and CA was added into the solution, FHAp nanocrystals morphology changed dramatically compared to the morphology of the samples prepared in single chelating system. Fig. 2a3–2d3 present FESEM images of FHAp samples prepared under pH 3.6, 5.2, 6.1 and 10.0, respectively. At pH 3.6, novel flower-like crystal with shuttle-shaped petals was observed. When pH value was increased up to 5.2, the FHAp sample is made up of 1D hexagonal nanorods with about 2 ␮m in length and 300 nm in diameter. It is noteworthy that the nanorods are in well-defined hexagonal prism-like morphology instead of nanorods with a thin middle part. We believed that the formation of such well-defined nanorods may be due to the combinative effect of two chelating reagents. Continuously increasing pH value, the FHAp nanocrystals

with different morphologies were formed. As shown in Fig. 2c3, the FHAp sample obtained at pH 6.1 consists of nanorods aggregates, which are composed of many well aligned nanorods with diameters of about 200 nm. As pH value was increased up to 10.1, the sample is composed of a wealth of branch-like nanorods with lengths of ranging from 5 to 10 ␮m and diameters of about 300 nm (Fig. 2d3). Noticeably, from the inset in Fig. 2d3 we found that the cross-section of the branch-like nanorods is pentagonal rather than hexagonal. It implied that the combination of these two chelating reagents induce not only morphology change but also structural transformation. Table 1 Summary of the experiment conditions and the corresponding morphologies and dimensions of the samples. Sample

pH

Morphology

Length (␮m)

S1 S2 S3 S4 S5 S6 S7 S8

3.6 4.5 4.9 5.2 6.1 7.1 8.3 10.0

Flower-like crystals Nanoprisms Hexagonal nanorods Hexagonal nanorods Nanorods aggregates Nanorods Whiskers Branch-like nanorods

3 1 2 2 3 5 10 20

± ± ± ± ± ± ± ±

0.3 (petal) 0.2 0.5 0.3 1 1 2 3

Diameter (␮m) 0.6 0.3 0.2 0.2 0.2 0.2 0.2 0.3

± ± ± ± ± ± ± ±

0.1 (petal) 0.1 0.05 0.05 0.08 0.8 0.1 0.1

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Fig. 3. (a–c) FESEM images with different magnifications of as-prepared FHAp products obtained in the mixed chelating reagents solution under pH 4.5; (d–f) FESEM and TEM images of as-prepared FHAp products obtained in the mixed chelating reagents solution under pH 4.9.

To confirm the structural transformation, we further investigated the influence of pH value on the morphology of FHAp nanocrystals under other pH conditions. Fig. 3a shows FESEM image of FHAp product obtained under pH 4.5, which is composed of rodlike FHAp crystals with uniform morphology and size. From the magnified FESEM image (Fig. 3b), we observed that the 1D nanostructures give the novel prism-like morphology. These prism-like nanocrystals with two strange ends have the uniform size of about 300 nm in diameter and 1 ␮m in length. Surprisingly, a typical magnified image (Fig. 3c) displays that such hexagonal prism-like FHAp crystals have two tower-like ends. The formation of such interesting ends may be due to the complex effect of the two different chelating reagents on the growth of FHAp crystals. When pH was 4.9, the resultant product was composed of hexagonal nanorods with high symmetry, monodispersity, and well-defined crystallographic facets (Fig. 3d–f), which had an average length of 2 ␮m and a mean diameter of about 200 nm. The surfaces of the hexagonal nanorods are extremely smooth without obvious defects. With comparison to the nanorods prepared at pH 5.2 (Fig. 2b3), we conclude that the pH value 4.9 is more favorable for the synthesis of perfect hexagonal FHAp nanorods. When pH value was continuously increased up to 7.1 and above, the product consists of a large quantity of randomly distributed nanorods with lengths up to 5 ␮m and diameters of about 200 nm, as shown in Fig. 4a. Note that these nanorods have irregular hexagonal structure (Fig. 4b). At pH 8.3, the morphology of FHAp product was basically maintained. As shown in Fig. 4c and an enlarged FESEM image in Fig. 4d, FHAp whiskers in irregular morphology with lengths up to 10 ␮m and mean diameters of about

200 nm were obtained. Based on the above experimental results, we concluded that the combination of CA and Na2 EDTA enable the structural transformation of FHAp nanocrystals, possibly due to the synergistic effect of two chelating reagents. 3.3. Possible growth mechanism There are two important key factors for determining the final shapes of FHAp nanocrystals. The first one is an intrinsic factor, that is, the crystallographic phase of the nucleated seeds. In general, for the materials of hexagonal structure, the anisotropic growth along the c-axis is available to form the 1D nanostructure. Our observations of crystal morphology indicate that many crystals take 1D hexagonal shape, suggesting that FAHp nucleated seeds prefer to preferential grow into hexagonal nanocrystals along the [0 0 0 1] direction. The other key factor is external factors including pH value and chelating reagent, which may render the FHAp nanocrystals anisotropic growth along crystallographically reactive directions possible. How pH value and chelating reagent affect the growth of FHAp particles into various shapes? To address this question, we compared the samples obtained under different pH values and in the presence of different chelating reagents and demonstrated a possible growth mechanism for the formation of FHAp nanocrystals with various morphologies, as shown in Fig. 5. Firstly, we discussed the effect of pH values on the shape of FHAp nanocrystals prepared in the presence of single chelating reagent, CA or Na2 EDTA. Zhang et al. reported a morphologically controllable synthesis of FHAp

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Fig. 4. FESEM images of as-prepared FHAp products obtained in the mixed chelating reagents solution under different pH value of (a–b) 7.1 and (c–d) 8.3.

nanostructures in the presence of glutamic acid by hydrothermal process [9]. They assumed that the interaction between calcium ions and carboxyl groups of glutamate ions during the formation of FHAp crystals would be one of the main factors which affected the morphological development. Similarly, CA has a hydroxyl and three carboxyl groups in its molecule and is ionized to the H2 (C6 H5 O7 ), H(C6 H5 O7 )2− (Hcit2− ) and (C6 H5 O7 )3− (cit3− ) by loss of hydrogen ions in the solution [22]. When citrate ions are introduced in the precipitating medium, they mobilize calcium ions as calciumcitrate complexes, most probably as Cacit− and CaHcit species [23]. At relatively low pH value, for example pH 3.6, citrate is completely deprotonated and has three negatively charged sites. Since the amount of calcium-citrate complexes in solution with low pH value is less than that in high pH value solution, more Ca2+ ions were released from the calcium-citrate complexes under hydrothermal condition, which produced a large quantity of nucleates of a large

size. These large nucleates will aggregate together and adopt a spherical structure in order to keep their surface energy low, finally resulting in the formation of nanorod aggregates. However, when pH was above certain value, for example 6.4, Cacit− was the main calcium-citrate complex. When these more stable Cacit− decomposed under high temperature conditions, less FHAp nucleates with smaller size were formed. In this case, only a single short nanorod forms from each nucleate, as the samples shown in Fig. 2c1and d1. Similar with CA, the chelating ability of Na2 EDTA to Ca2+ also depends on the pH value of solution. Na2 EDTA can exist in seven forms: H6 Y2+ , H5 Y+ , H4 Y, H3 Y− , H2 Y2− , HY3− , and Y4− [10]. We considered that the stability of calcium-Na2 EDTA complex enhanced with the increase of pH value. Since the anion Y4− is the ligand species with the strongest complex ability, the larger the fraction of Y4− , the more stable the complex is. So, relatively low pH value favors the formation of 3D nanocrystals aggregates in order to

Fig. 5. Schematic illustration of a possible growth mechanism for the formation of FHAp nanocrystals with various morphologies under different experimental conditions.

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reduce the surface energy because there are more nuclei formed in the solution, whereas at high pH values, for example at pH 6.1 and 10.1, 1D FHAp nanorods were produced (see Fig. 2c and d). Murphy had claimed that the preferential adsorption of molecules and ions in solution onto different crystal facets directs the growth of particles into various shapes by controlling the growth rates along different  crystal axes [24]. In the present cases, ¯ since the rectangular 1010 facets are rich in calcium ions or positive charge as compared to the hexagonally arranged {0 0 0 1} facets [25]; Thus, the citrate and  Na2EDTA molecule or ions may adsorb ¯ preferentially onto the 1010 facets of the growing FHAp seeds, contributing to electrostatic inhibition of hexagonal facet-to-facet particle aggregation, which leads to preferential growth of FHAp crystal along the [0 0 0 1] direction, as shown in Fig. 5. However, we found that when CA was used as a growth modifier, short FHAp nanorods tend to form in high pH value solution, where more cit3− ions exist. It implied that citrate inhibits FHAp nanocrystals axial growth along the [0 0 0 1] direction. In contrary, the formation of long FHAp nanorods at high pH value in the presence of Na2 EDTA indicated that Na2 EDTA inhibits equatorial growth of FHAp crystals (Fig. 5). When Na2 EDTA and CA were introduced simultaneously into the reaction system, species with multiple coordination sites were formed under different pH value, making the reaction between chelating reagent and Ca2+ more complex. Under pH 3.6, because the prominent chelating ligands are Hcit2− , H6 Y2+ , H5 Y+ , H4 Y, H3 Y− and H2 Y2− which can yield relative unstable complexes, so at the nucleating stage more Ca2+ ions were released from these unstable complexes under hydrothermal condition. In order to keep surface energy low, usually, 3D clusters of critical size would be formed, which could act as nuclei for FHAp crystals and develop into crystallites under hydrothermal conditions. Note that CA can induce the formation of 1D nanorods aggregates (Fig. 2a1) and Na2 EDTA can result in flowerlike crystal with hexagonal petals (Fig. 2a2). These nuclei would aggregate together and adopt the 3D flowerlike morphology, so the novel flower-like crystals with shuttle-shaped hexagonal petals were obtained (Fig. 2a3) in the two chelating reagents system owing to the synergistic effect between Na2 EDTA and CA. We have already found that citrate inhibits axial growth along the [0 0 0 1] direction, while Na2 EDTA favors axial growth along the [0 0 0 1] direction. Therefore, we considered that the synergistic effect between Na2 EDTA and CA is competitive. This competitive effect may result in 1D anisotropic crystal growth and enable this mixed chelating reagent system more advantageous in morphologically controlled growth of FHAp nanocrystals. For example, with comparison to the samples synthesized assisted by single chelating reagent (Na2 EDTA or CA), 1D FHAp nanorods with different shapes were produced in the presence of Na2 EDTA and CA from pH 5.2 to 10.0, although 1D FHAp nanorods were indeed formed in the presence of Na2 EDTA. Interestingly, a crystal structural transformation from hexagonal nanorods to irregular hexagonal nanorods and then to pentagonal nanorods takes place when pH value is increased from 4.9 to 10.0. Furthermore, the formation of the well-defined FHAp hexagonal prism-like nanorods synthesized in the presence of Na2 EDTA and CA at pH 4.9 further suggests the synergistic effect between Na2 EDTA and CA play a crucial role in the controlled synthesis of nanocrystals with reg-

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ular shapes and perfect facets. It should be stated that the above explanation about the formation of the various nanostructures is somewhat conjectural and phenomenological. Intensive study is necessary to substantially explore the formation mechanism of the nanocrystals with various shapes reported herein. 4. Conclusions In summary, we have demonstrated a facile hydrothermal route to the morphology-controlled synthesis of FHAp nanostructures in the presence of chelating reagents. The pH value and chelating reagents both play a key role in the growth habit and final morphology of FHAp crystals. By comparing the morphology and size of the samples synthesized using different chelating reagents, it was found that the usage of CA/Na2 EDTA mixed chelating reagent can result in not only a morphology change but also a structural transformation due to the synergistic effect of the two chelating reagents. This synthetic strategy may open new routes to the chelating reagent-assisted synthesis of inorganic nanostructures with various morphologies. Acknowledgments We gratefully ackownledge the financial supports of the National Natural Science Foundation of China (No. 30772117), Industrial High Technology Foundation of Jiangsu Province (No. BG2007025) and Research Foundation for Talented Scholars of Jiangsu University (No. 08JDG052). References [1] S. Kannan, A.F. Lemos, J.M.F. Ferreira, Chem. Mater. 18 (2006) 2181–2186. [2] V.S. Komleva, S.M. Barinova, E.V. Koplikb, Biomaterials 23 (2002) 3449–3454. [3] K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 124 (2002) 11572–11573. [4] T. Kawasaki, J. Chromatogr. 544 (1999) 147–184. [5] H.B. Qu, M. Wei, Acta Biomaterialia 2 (2006) 113–119. [6] J. Vandiver, D. Dean, N. Patel, W. Bonfield, C. Ortiz, Biomaterials 26 (2005) 271–283. [7] B. Viswanath, N. Ravishankar, Biomaterials 29 (2008) 4855–4863. [8] J.J.J.M. Donners, R.J.M. Nolte, N.A.J.M. Sommerdijk, Adv. Mater. 15 (2003) 313–316. [9] G. Zhang, Q.S. Zhu, Y. Wang, Chem. Mater. 17 (2005) 5824–5830. [10] S. Busch, Angew. Chem. Int. Ed. 43 (2004) 1428–1431. [11] H.F. Chen, K. Sun, Z.Y. Tang, V.L. Robert, F.M. John, C.J. Agata, B.H. Clarkson, Cryst. Growth Des. 6 (2006) 1504–1508. [12] D.E. Zhang, X.J. Zhang, X.M. Ni, J.M. Song, H.G. Zheng, Cryst. Growth Des. 7 (2007) 2117–2119. [13] D.S. Seoa, J.K. Lee, J. Crystal Growth 310 (2008) 2162–2167. [14] R.H. Zhu, R.B. Yu, J.X. Yao, D. Wang, J.J. Ke, J. Alloys Compd. 457 (2008) 555–559. [15] J. Liu, K. Li, H. Wang, M. Zhu, H. Xu, H. Yan, Nanotechnology 16 (2005) 82–87. [16] J.A. Parkinson, H.Z. Sun, P. Sadler, J. Chem. Commun. 8 (1998) 881–882. [17] P.G. Simon, S. Steve, Chem. Mater. 19 (2007) 4016–4022. [18] J.B. Wu, H. Zhang, N. Du, X.Y. Ma, D.R. Yang, J. Phys. Chem. B 110 (2006) 11196–11198. [19] F. Dickens, Biochem. J. 35 (1941) 1011–1023. [20] J. Christoffersen, M.R. Christoffersen, J. Arends, Croat. Chem. Acta 56 (1983) 769–777. [21] M. Johnsson, C.F. Richardson, J.D. Sallis, G.H. Nancollas, Calcif. Tissue Int. 49 (1991) 134–137. [22] P.C. Hidber, T.J. Graule, L. Gauckler, J. Am. Ceram. Soc. 79 (1996) 1857–1867. [23] A.L. Macipe, J.G. Morales, R.R. Clemente, Adv. Mater. 10 (1998) 49–53. [24] C.J. Murphy, Science 298 (2002) 2139–2141. [25] K. Kandori, A. Fudo, T. Ishikawa, Colloids Surf. B Biointerfaces 24 (2002) 145–153.