Materials Science and Engineering C 37 (2014) 286–291
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Hydrothermal synthesis and characterization of Si and Sr co-substituted hydroxyapatite nanowires using strontium containing calcium silicate as precursors Na Zhang a,b, Dong Zhai a, Lei Chen a, Zhaoyong Zou a,b, Kaili Lin a,⁎, Jiang Chang a,⁎ a b
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 6 August 2013 Received in revised form 9 December 2013 Accepted 5 January 2014 Available online 11 January 2014 Keywords: Hydroxyapatite nanowires Silicon Strontium Co-substitution Precursor transformation Lattice constants
a b s t r a c t In the absence of any organic surfactants and solvents, the silicon (Si) and strontium (Sr) co-substituted hydroxyapatite [Ca10(PO4)6(OH)2, Si/Sr-HAp] nanowires were synthesized via hydrothermal treatment of the Sr-containing calcium silicate (Sr-CS) powders as the precursors in trisodium phosphate (Na3PO4) aqueous solution. The morphology, phase, chemical compositions, lattice constants and the degradability of the products were characterized. The Si/Sr-HAp nanowires with diameter of about 60 nm and up to 2 μm in length were obtained after hydrothermal treatment of the Sr-CS precursors. The Sr and Si substitution amount of the HAp nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors and the reaction ratio of the precursor/solution, respectively. The SiO4 tetrahedra and Sr2+ ions occupied the crystal sites of the HAp, and the lattice constants increased apparently with the increase of the substitution amount. EDS mapping also suggested the uniform distribution of Si and Sr in the synthetic nanowires. Moreover, the Si/Sr-substitution apparently improved the degradability of the HAp materials. Our study suggested that the precursor transformation method provided a facile approach to synthesize the Si/Sr co-substituted HAp nanowires with controllable substitution amount, and the synthetic Si/Sr-HAp nanowires might be used as bioactive materials for hard tissue regeneration applications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ideal hard-tissue regeneration materials should be biocompatible and bioactive, and possess excellent mechanical properties [1]. The nanowires possess excellent mechanical strength, which have been widely used as the reinforcement to fabricated biocomposites with improved mechanical properties [2,3]. Recently, the hydroxyapatite [Ca10(PO4)6(OH)2, HAp] wires/whiskers/rods have greatly aroused interests because of their excellent mechanical strength and biocompatibility, osteo-conductive properties and similarity to the inorganic component of human beings [4–6]. Previous studies revealed that the functional elements of silicon (Si) and strontium (Sr) play important roles on biological performances [7–15]. The Si element plays a critical role in the normal bone, cartilage and connective tissue growth and development [7,8]. Xynos and Honda et al. proved that the incorporation of Si into the HAp lattice enhanced the bioactivity, and stimulated the proliferation and development of osteoblasts and mesenchymal stem cells (MSCs) [9,10]. While the animal implantation studies further proved that the bone apposition, in-growth and adaptive remodelling could be apparently stimulated by the Si-substituted HAp bioceramics [11]. On the other hand, the Sr element has been shown to possess ⁎ Corresponding authors. Tel.: +86 21 52412264; fax: +86 21 52413903. E-mail addresses: [email protected] (K. Lin), [email protected] (J. Chang). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.011
dual effects of stimulating osteoblast differentiation and inhibiting osteoclast activity and bone resorption. Application of Sr might reduce the incidence of fractures in osteoporotic patients [12]. Moreover, the partial substitution of Ca by Sr can apparently improve the biological properties of Ca–P based biomaterials [1,13]. Studies have demonstrated that the Sr substitution could promote the adhesion, proliferation and bone-related gene expression of osteoblasts and MSCs [14,15]. Besides, the osteoclast differentiation was evidently inhibited by incorporation of Sr into HAp materials [15]. Therefore, it can be hypothesized that the co-substitution of HAp nanowires with Si and Sr elements (Si/Sr-HAp) will bring about synergistic effect to better stimulate the proliferation and differentiation of osteoblasts and MSCs, and to promote bone regeneration ability when comparing with the traditional Si-HAp or SrHAp materials. However, it is a challenge to synthesize the Si/Sr-HAp nanowires with controllable chemical composition of Si and Sr components. Hydrothermal process is widely used to synthesize one-dimensional HAp materials. The short rod-like HAp nanopowders with high crystallinity can be facilely obtained after hydrothermal treatment of the HAp precipitate by chemical precipitation in aqueous solutions [16,17]. The HAp nanowires and whiskers with high aspect ratio have received great interests due to their novel mechanical properties and applications. Wang [18] and Zhang [19] et al. reported the synthesis of HAp nanowires by hydrothermal-microemulsion (HM) and solvothermal-
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microemulsion (SM) methods, respectively. Recently, the hydrothermal homogeneous precipitation (HHP) method using acetamide as precipitation reagent was developed to synthesize the ultralong HAp whiskers up to 150 μm length [1,20]. However, the HM, SM and HHP methods need a large amount of environmentally unfriendly and unhealthy organic templates, solvents and additives, and the HAp product is usually in little-scale. Most recently, we developed a facile environmentally friendly strategy to hydrothermally synthesize the elementsubstituted HAp with controllable morphology (including the nanoparticles, nanowires and nano-sheets) and chemical composition using calcium silicates as precursors in the absence of any surfactants and organic solvents [21]. However, the fine control of the chemical composition, and the effect of the substitutions and the substitution amount on the crystal properties, including the lattice constants and the crystallinities, and the degradability of the products have not been well investigated. In the present study, the fine control strategy was further developed to tailor the Si-substitution amount while keeping the Sr-substitution amount on a certain level, versus, to regulate the Sr-substitution amount when keeping the Si-substitution amount on a certain value. In addition, the influences of the Si/Sr-substitutions and the substitution amount on the lattice constants and the crystallinities, and the degradability of the HAp nanowires were further investigated in detail. Herein, a facile environmentally friendly method was developed to synthesize and simultaneously control the nanowire-like structured and Si/Sr substituted HAp materials via simply hydrothermal reaction of Sr-containing calcium silicate precursors in trisodium phosphate aqueous solution, without using any surfactant, template-directed reagents and organic solvents, and the morphology, phase, chemical compositions, lattice constants and degradability of the products were characterized. 2. Materials and methods Si/Sr-HAp crystals were hydrothermally synthesized from Srcontaining calcium silicate as precursors in Na3PO4 aqueous solutions in the absence of any structure-directing reagents and organic solvents. Analytical grade reagents (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) without further purification were used in this study.
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obtained products were labeled as Si-HAp, Si/Sr5-HAp, Si/Sr10-HAp, Si/Sr20-HAp and Si/Sr20-HAp-1, respectively. In addition, the pure HAp nanoparticles were synthesized as the control sample via chemical precipitation and then hydrothermal treatment method. The solution of 0.5 M Ca(NO3)2 · 4H2O and 0.3 M (NH4)2HPO4 was first prepared, respectively. Then the pH of Ca(NO3)2 solution was adjusted above 10.8 with ammonia solution, then the (NH4)2HPO4 solution was added into the Ca(NO3)2 solution dropwise under vigorous stirring. The pH of the mixtures was controlled above 10.8 among the whole addition process by addition of 28 wt.% ammonia solution. After finishing the addition, the obtained suspension was transferred to polytetrafluoroethylene vessel for further hydrothermal treatment at 180 °C for 24 h. After the hydrothermal treatment, the products were filtered and washed as described above, and then dried at 120 °C for 24 h. The obtained pure HAp powders were labeled as HAp. The synthetic HAp and Si/Sr-HAp powders were characterized by X-ray diffraction (XRD: D/max 2550 V, Rigaku, Japan) with monochromated Cu-Kα radiation. The crystallinity degree Xc was evaluated by Eq. as follows: V 112=300 X c ¼ 1− I300 where I300 is the intensity of the (3 0 0) reflection and V112/300 is the intensity of the hollow between (1 1 2) and (3 0 0) reflections [23]. The lattice constants were calculated from the well determined positions of the intense XRD diffractions that were processed by MDI Jade 6.1 software [24]. The products were also characterized by Fourier transform infrared spectroscopy (FTIR: Nicolet Co., USA). The morphology of the synthesized powders was characterized by scanning electron microscopy (SEM: JSM-6700F, JEOL, Japan), and the chemical compositions of the powders were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES; VISTA AX, Varian Co., USA) after calcining the products at 800 °C for 3 h to eliminate the adsorbed water and contamination came from filter paper. EDS mapping analysis was also carried out to investigate the element distribution by elemental mapping (Oxford instruments). 2.3. In vitro study of the degradability of Si/Sr-HAp nanowires
The precursors were prepared by chemical precipitation method. Briefly, the calcium was partially replaced by strontium (x = 0, 0.05, 0.10 and 0.20 in molar ratio). The mixed solution with the pH about 3.85 of Ca(NO3)2 · 4H2O and Sr(NO3)2 was firstly prepared according to the expected substituted ratio. After preparation, the mixture was added dropwise to the Na2SiO3 solution to produce a white suspension under rigorous stirring. After complete addition, the white precipitate was further stirred for 24 h, followed by washing with three times of deionized water and two times of 100% ethanol. After washing, the remained liquid was removed by vacuum filtration. Then the obtained powders were dried at 120 °C for 24 h. Finally, the crystalline Srx-CS powders were obtained by calcining at 850 °C for 2 h [22].
To evaluate the effect of the Si/Sr-substitution on the degradability of the Si/Sr-HAp nanowires, the in vitro degradability of the synthetic Si/Sr-HAp nanowires was preliminarily investigated by examining the weight loss percentage of the products in Tris–HCl buffer solution, and the traditional pure HAp nano-particles were used as the control sample [25]. The 0.1 M Tris–HCl buffer solution was prepared by dissolving analytical reagent grade Tris(hydroxymethyl) amino-methane in distilled water and then was buffered at pH 7.4 at 37 °C with hydrochloric acid. The synthetic powders were soaked in the Tris–HCl at 37 °C in a shaking water bath for 4 days at a solid/liquid ratio of 1.50 mg mL−1 without refreshing the soaking medium. After soaking, the samples were centrifuged, the supernatant solution was used to evaluate the released ions by ICP-AES. Based on the fact that there was no P in Tris–HCl buffer solution, the dissolution ratio (S) of the powders was calculated by the following equation:
2.2. Hydrothermal synthesis of the Si/Sr co-substituted HAp nanowires
S ¼ Cp V =mp 100%
2.1. Synthesis of calcium silicate (CaSiO3, CS) and strontium substituted calcium silicate (Srx-Ca(1 − x)SiO3, SrxCS) powder precursors
1 g CS, 1 g Sr0.05CS, 1 g Sr0.10CS, 1 g Sr0.20CS or 1.5 g Sr0.20CS powders after calcining were mixed with 85 mL, 0.2 M trisodium phosphate aqueous solution in a polytetrafluoroethylene vessel and the vessel was sealed in a stainless steel autoclave and heated at 180 °C for 1.5 h, 3 h, 24 h and 3 days. After the hydrothermal reaction, the reaction system was cooled to room temperature at the rate of 2 °C/min and the Si/Sr substituted HAp nanowires were obtained. The products were washed and filtrated as described above, then dried at 120 °C for 24 h. The
Where Cp, V, and mp were the P concentration in Tris–HCl (mg mL−1), volume of Tris–HCl (mL) and P content (mg) of the samples soaked in Tris–HCl, respectively [25]. 3. Results and discussion The CS powders after calcining at 850 °C were chosen as the representative sample to observe the morphology of the synthetic precursors
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Fig. 1. FESEM image of the precursors (the symbol * indicated the precursor particles).
(Fig. 1). It is clear that the morphology of the precursors was particlelike shapes with diameter between 0.5 and 0.6 μm. After the hydrothermal treatment of the precursors in Na3PO4 aqueous solution at 180 °C for 24 h, the nanowire-like products with lengths up to 2 μm and diameters about 60 nm were obtained (Fig. 2B–E). As expected, the control sample was particle-like shape with 40 nm in diameter and about 200 nm in length (Fig. 2A). The synthetic Si/Sr-HAp nanowires possess three-dimensional random directions. It is suggested that the wires/ fibers with random directions will improve the mechanical properties by 20% (Krenchel factor = 0.2) [26]. In our future studies, the synthetic Si/Sr-HAp nanowires will be used as the mechanical reinforcement to fabricate the composites, and the relationship between the nanowire orientation factor and the mechanical property of the fabricated composites will be investigated systematically. The XRD patterns shown in Fig. 3 suggested that all of the products could be identified as pure HAp phase (JCPDS card: NO. 09-0432). Moreover, the similar XRD peaks indicated that the obtained products with the presence of both Si and Sr were element substituted HAp materials. The previous studies have confirmed that the element substitution will not significantly alter the XRD peaks of the HAp products [25,27,28]. In addition, the peaks of Si/Sr-HAp were apparently broader than that of
HAp, suggesting that the ion substitution resulted in the decrease the crystallinity of the HAp products [25]. Besides, the positions of the peaks of the Si/Sr-substituted HAp had a little shift from that for the pure HAp. Comparing with the pure HAp materials, the small angle XRD scanning results (Fig. 3B) clearly revealed that the corresponding peaks of the Si/Sr-HAp nanowires shifted to lower degree. In addition, the shifting degree increased with the increase of the Srsubstitution amount, indicating the increase of the lattice constants (Table 1) [29,30]. The lattice constants calculated from the XRD determination results further confirmed that the lattice constants of all the Si/Sr-HAp nanowires were larger than that of the pure HAp (Table 1). Moreover, the values increased with the increase of the element substitution amount. The increase in the lattice constants were due to the replacement of the PO4 tetrahedra and Ca2+ ions by larger SiO4 tetrahedra and Sr2+ ions, respectively [29–31]. In this study, the deviation of lattice constants suggested that the SiO4 tetrahedra and Sr2+ ions replaced and occupied the PO4 tetrahedra and Ca2+ crystal sites of the HAp, respectively [30,31]. Moreover, the crystallinity of the Si-substituted HAp and the Si/Sr co-substituted HAp were lower than that of the pure HAp, which further proved that the element substitution decreased the crystallinity. The similar phenomenon of the element substitution leading to the decrease of the crystallinity has been confirmed in previous studies [25,32]. Fig. 4 reveals the FTIR spectra of the synthesized HAp nanoparticles and Si/Sr-HAp nanowires. All spectra agreed with the reported FTIR data for HAp. The peaks at 471, 563, 604, 960, 1032 and 1092 cm− 1 were the characteristic bands for PO34 − [33]. The peaks at 3443 and 1637 cm− 1 were assigned to the bending mode of the absorbed water. The peaks around 1414 and 1466 cm−1 were attributed to carbonate ions (CO2− 3 ) in B-site, which might come from the dissolved carbon dioxide in phosphate solutions [33]. The peaks at 3568 and 631 cm−1 are the characteristic OH bands of HAp, which were weak or disappeared from the synthesized Si/Sr-HAp nanowires because of the existence of carbonate ions [33] and the replacement of PO4 tetrahedra by SiO4 tetrahedra in the HAp structure [31]. The chemical composition analysis suggested the existence of SiO4 tetrahedra. The FTIR result further confirmed that the positions of the peaks were not affected by Sr and Si substitutions. To further investigate the formation mechanism of the Si/Sr-HAp nanowires, the Sr0.10-CS was selected as the representative precursor and hydrothermally treated in Na3PO4 aqueous solution at 180 °C for
Fig. 2. SEM images of the synthesized HAp nanoparticles and Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. (A) HAp nanoparticles, (B) Si-HAp, (C) Si/Sr5-HAp, (D) Si/Sr10-HAp, (E) Si/Sr20-HAp and (F) Si/Sr20-HAp-1.
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Fig. 3. XRD patterns of the synthesized HAp nanoparticles and the Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h.
1.5 h, 3 h, 24 h and 3 days, respectively. Comparing with the morphologies of the precursor powders (Fig. 1), the short rod-like crystals with diameter around 50 nm formed on the surface of the precursor microparticles after hydrothermal treatment of the precursor in Na3PO4 aqueous solution for 1.5 h (Fig. 5A, the early stage). With the increase of the hydrothermal time to 3 h, the short rod-like crystals became longer up to 300 nm (Fig. 5B). With an increase of the hydrothermal time to 24 h, the wire-like products with lengths up to 2 μm and diameters around 60 nm were obtained (Fig. 5C). When the hydrothermal time was further increased to 3 days, the length and the diameters increased to about 2.5 μm, 70 nm, respectively (Fig. 5D). The morphology development mechanism of Si/Sr-HAp nanowires can be summarized as followings based on the SEM observations: The surface erosion of the precursors happened when they were surrounded by the phosphate solution under hydrothermal condition, accompanying with the release of the Ca2 +, Sr2 + and SiO23 − ions into the aqueous solution. With the continual ion-release, the concentration of the Ca2+/Sr2+ and PO34 reached over saturation and the hydroxyapatite nucleated on the eroded Sr-CS particle surfaces. During this process, the Sr-CS itself played both the roles of Ca2+ and substituted ion sources, and the HAp crystal nucleation sites. With the extending of the hydrothermal time, the ion released continuously and the bigger and longer HAp crystals grew at the expense of the released ions. At the same time, the Si and Sr ions released from the Sr-CS precursors incorporated into the newly formed HAp crystal lattices. Finally, the Si/Sr-substituted HAp nanowires were obtained. Moreover, the increase of the diameter and length of the Si/Sr-HAp nanowires with the increase of the hydrothermal time suggested that the size of the products could be well controlled via regulation of the hydrothermal conditions such as hydrothermal period.
The elemental analyses of the synthetic products are presented in Table 2. It is clear that the synthetic Si/Sr-HAp nanowires contained the substituted ions of Si and Sr, which came from the Sr-CS precursors. Furthermore, the amount of Sr in the obtained Si/Sr-HAp nanowires increased obviously with the increase of the Sr-substitution level in the Sr-CS precursors. The results suggested that the Sr substitution amount of the Si/Sr-HAp nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors. In addition, it is interesting to find that the co-substitution of Sr resulted in an increase of the Si substitution amount, which might be attributed to the following reason (see the data for the samples of Si-HAp, Si/Sr5-HAp, Si/Sr10-HAp and Si/Sr20-HAp shown in Table 2): the replacement of Ca2+ crystal site by bigger diameter of Sr2 + results in the crystalline swelling, which might enhance the incorporation of the SiO4 tetrahedra into the lattice of HAp crystals. Furthermore, the amount of substituted Si for the synthesized products with similar Sr substitution level could be facilely tailored by regulating the reaction ratio of the precursor/solution (see the data for the samples of Si/Sr20-HAp and Si/Sr20-HAp-1 shown in Table 2). In the present study, the amount of PO34 − ions used in the hydrothermal process is overdose. Therefore, the more precursors
Table 1 Lattice constants, 2θ value for (211) diffraction and crystallinity of the HAp nanoparticles and Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. Samples
HAp Si-HAp Si/Sr5-HAp Si/Sr10-HAp Si/Sr20-HAp Si/Sr20-HAp-1
Lattice constants a (Å)
c (Å)
9.365 9.381 9.397 9.414 9.430 9.479
6.730 6.812 6.871 6.910 6.925 6.965
2θ (°) for (2 1 1) reflection
Crystallinity (XC)
32.021 31.945 31.848 31.769 31.710 31.691
0.886 0.745 0.798 0.733 0.741 0.760
Fig. 4. The FTIR spectra of the synthesized HAp nanoparticles (A) and the nanowires of Si-HAp (B), Si/Sr5-HAp (C), Si/Sr10-HAp (D), Si/Sr20-HAp (E) and Si/Sr20-HAp-1 (F) via hydrothermal treatment of the precursors at 180 °C for 24 h.
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Fig. 5. SEM images of the products after hydrothermal treatment of Sr10-CS precursors at 180 °C for 1.5 h (A), 3 h (B), 24 (C) and 72 h (D), respectively.
Table 2 Chemical composition of the Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. Samples
Si-HAp Si/Sr5-HAp Si/Sr10-HAp Si/Sr20-HAp Si/Sr20-HAp-1
Chemical composition (wt.%) Si (wt.%)
Sr (wt.%)
Ca/P molar ratio
(Ca + Sr)/(P + Si) molar ratio
0.56 1.16 1.17 1.23 3.06
0 1.64 3.81 6.88 6.76
1.63 1.61 1.58 1.67 1.65
1.57 1.53 1.54 1.68 1.48
were used, the more SiO4 tetrahedra would be released to compete with the PO4 tetrahedra to occupy the crystal sites of HAp, resulting in higher Si incorporation level. The EDS mapping technology was further applied to investigate whether the substituted Si/Sr elements were homogeneously distributed in the obtained Si/Sr-HAp nanowires, and the Si/Sr10-HAp nanowires were selected as the representative sample (Fig. 6). The results showed that the Ca, Sr, P, Si and O elements were
uniformly distributed in the products, which might further confirm that the Si/Sr elements evenly occupied in the lattice sites of the HAp crystals. On the other hand, the Ca/P molar ratio of the obtained Si/SrHAp powders was deviated from the stoichiometric HAp (Ca/P = 1.67). Comparing with the stoichiometric HAp material, the deviation degree of Ca/P molar ratio is critically relevant to both substitution amounts of Si and Sr due to the partial replacement of the Ca and P by the Sr and Si elements, respectively. The Si-substitution amount in Si/Sr5-HAp and Si/Sr10-HAp is almost similar. However, comparing with Si/Sr5-HAp, much more Ca was replaced by Sr in the Si/Sr10HAp, which led to the decrease of the Ca/P molar ratio. As for Si/Sr20HAp nanowires, much more P was replaced by Si, which inversely increased the Ca/P molar ratio to 1.67. It is well known that all of these elements coexist in human bone and tooth minerals, and the composition may vary depending upon the sex, age and nature of the races [25,29]. The evaluation of the biological effect of the Si/Sr co-substituted HAp in vitro and in vivo to screen out the optimal amounts of Si and Sr co-substitution is critical for the biomedical applications of Si/Sr-HAp biomaterials, which will be the next step of our study.
Fig. 6. Mapping of Ca, Sr, P, Si and O distribution in the HAp nanowires.
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preparation of new bioactive materials and mechanical reinforcement materials for bone regeneration. Acknowledgments The authors gratefully acknowledge the support of the Natural Science Foundation of China (Nos.: 81171458, 81190132, 51061160499), the Science and Technology Commission of Shanghai Municipality (No.: 13NM1402102) and the Fund of the Shanghai Institute of Ceramics Chinese Academy of Sciences for Innovation of Science and Technology (No.: Y26ZC1110G). References
Fig. 7. Degradability rate of the control sample (pure HAp nanoparticles, A), and the synthetic Si/Sr-HAp nanowires: Si-HAp (B), Si/Sr5-HAp (C), Si/Sr10-HAp (D), Si/Sr20-HAp (E) and Si/Sr20-HAp-1 (F) after soaking in Tris–HCl buffer solution for 4 days.
The improvement of the degradability is usually required for wide biomedical applications of HAp materials. Fig. 7 shows the quantitative dissolutions of the Si/Sr-HAp nanowires and the traditional HAp nanoparticles after soaking in the Tris–HCl buffer solution for 4 days. The results showed that the substitution of the Si/Sr improved the degradability rate of the products, and the degradability rate increased apparently with the increase of the substitution amount of Si/Sr (Table 2 & Fig. 7). The higher degradability rate for the obtained Si/Sr-HAp nanowires was attributed to the substituted Si/Sr elements, which led to lower crystallinity [25]. 4. Conclusions In summary, the Si/Sr co-substituted HAp nanowires were successfully synthesized via hydrothermal treatment of Sr-substituted calcium silicate precursors in Na3PO4 aqueous solutions, without using any surfactant and organic solvents. The control of nanowire-like morphology and chemical compositions simultaneously was achieved, while the Sr and Si substitution amount of the nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors and the reaction ratio of the precursor/solution, respectively. During the substitution reaction, the SiO4 tetrahedra and Sr2+ ions entered into the HAp lattice and replaced part of the PO4 tetrahedra and Ca2+ ions, respectively, and the lattice constants increased apparently with the increase of the substitution amount. Moreover, the Si/Sr-substitution apparently improved the degradability of the HAp materials. Our study suggested that the Si/Sr co-substituted HAp nanowires might be suitable for
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Hydrothermal synthesis and characterization of Si and Sr co-substituted hydroxyapatite nanowires using strontium containing calcium silicate as precursors Na Zhang a,b, Dong Zhai a, Lei Chen a, Zhaoyong Zou a,b, Kaili Lin a,⁎, Jiang Chang a,⁎ a b
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 6 August 2013 Received in revised form 9 December 2013 Accepted 5 January 2014 Available online 11 January 2014 Keywords: Hydroxyapatite nanowires Silicon Strontium Co-substitution Precursor transformation Lattice constants
a b s t r a c t In the absence of any organic surfactants and solvents, the silicon (Si) and strontium (Sr) co-substituted hydroxyapatite [Ca10(PO4)6(OH)2, Si/Sr-HAp] nanowires were synthesized via hydrothermal treatment of the Sr-containing calcium silicate (Sr-CS) powders as the precursors in trisodium phosphate (Na3PO4) aqueous solution. The morphology, phase, chemical compositions, lattice constants and the degradability of the products were characterized. The Si/Sr-HAp nanowires with diameter of about 60 nm and up to 2 μm in length were obtained after hydrothermal treatment of the Sr-CS precursors. The Sr and Si substitution amount of the HAp nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors and the reaction ratio of the precursor/solution, respectively. The SiO4 tetrahedra and Sr2+ ions occupied the crystal sites of the HAp, and the lattice constants increased apparently with the increase of the substitution amount. EDS mapping also suggested the uniform distribution of Si and Sr in the synthetic nanowires. Moreover, the Si/Sr-substitution apparently improved the degradability of the HAp materials. Our study suggested that the precursor transformation method provided a facile approach to synthesize the Si/Sr co-substituted HAp nanowires with controllable substitution amount, and the synthetic Si/Sr-HAp nanowires might be used as bioactive materials for hard tissue regeneration applications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ideal hard-tissue regeneration materials should be biocompatible and bioactive, and possess excellent mechanical properties [1]. The nanowires possess excellent mechanical strength, which have been widely used as the reinforcement to fabricated biocomposites with improved mechanical properties [2,3]. Recently, the hydroxyapatite [Ca10(PO4)6(OH)2, HAp] wires/whiskers/rods have greatly aroused interests because of their excellent mechanical strength and biocompatibility, osteo-conductive properties and similarity to the inorganic component of human beings [4–6]. Previous studies revealed that the functional elements of silicon (Si) and strontium (Sr) play important roles on biological performances [7–15]. The Si element plays a critical role in the normal bone, cartilage and connective tissue growth and development [7,8]. Xynos and Honda et al. proved that the incorporation of Si into the HAp lattice enhanced the bioactivity, and stimulated the proliferation and development of osteoblasts and mesenchymal stem cells (MSCs) [9,10]. While the animal implantation studies further proved that the bone apposition, in-growth and adaptive remodelling could be apparently stimulated by the Si-substituted HAp bioceramics [11]. On the other hand, the Sr element has been shown to possess ⁎ Corresponding authors. Tel.: +86 21 52412264; fax: +86 21 52413903. E-mail addresses: [email protected] (K. Lin), [email protected] (J. Chang). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.011
dual effects of stimulating osteoblast differentiation and inhibiting osteoclast activity and bone resorption. Application of Sr might reduce the incidence of fractures in osteoporotic patients [12]. Moreover, the partial substitution of Ca by Sr can apparently improve the biological properties of Ca–P based biomaterials [1,13]. Studies have demonstrated that the Sr substitution could promote the adhesion, proliferation and bone-related gene expression of osteoblasts and MSCs [14,15]. Besides, the osteoclast differentiation was evidently inhibited by incorporation of Sr into HAp materials [15]. Therefore, it can be hypothesized that the co-substitution of HAp nanowires with Si and Sr elements (Si/Sr-HAp) will bring about synergistic effect to better stimulate the proliferation and differentiation of osteoblasts and MSCs, and to promote bone regeneration ability when comparing with the traditional Si-HAp or SrHAp materials. However, it is a challenge to synthesize the Si/Sr-HAp nanowires with controllable chemical composition of Si and Sr components. Hydrothermal process is widely used to synthesize one-dimensional HAp materials. The short rod-like HAp nanopowders with high crystallinity can be facilely obtained after hydrothermal treatment of the HAp precipitate by chemical precipitation in aqueous solutions [16,17]. The HAp nanowires and whiskers with high aspect ratio have received great interests due to their novel mechanical properties and applications. Wang [18] and Zhang [19] et al. reported the synthesis of HAp nanowires by hydrothermal-microemulsion (HM) and solvothermal-
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microemulsion (SM) methods, respectively. Recently, the hydrothermal homogeneous precipitation (HHP) method using acetamide as precipitation reagent was developed to synthesize the ultralong HAp whiskers up to 150 μm length [1,20]. However, the HM, SM and HHP methods need a large amount of environmentally unfriendly and unhealthy organic templates, solvents and additives, and the HAp product is usually in little-scale. Most recently, we developed a facile environmentally friendly strategy to hydrothermally synthesize the elementsubstituted HAp with controllable morphology (including the nanoparticles, nanowires and nano-sheets) and chemical composition using calcium silicates as precursors in the absence of any surfactants and organic solvents [21]. However, the fine control of the chemical composition, and the effect of the substitutions and the substitution amount on the crystal properties, including the lattice constants and the crystallinities, and the degradability of the products have not been well investigated. In the present study, the fine control strategy was further developed to tailor the Si-substitution amount while keeping the Sr-substitution amount on a certain level, versus, to regulate the Sr-substitution amount when keeping the Si-substitution amount on a certain value. In addition, the influences of the Si/Sr-substitutions and the substitution amount on the lattice constants and the crystallinities, and the degradability of the HAp nanowires were further investigated in detail. Herein, a facile environmentally friendly method was developed to synthesize and simultaneously control the nanowire-like structured and Si/Sr substituted HAp materials via simply hydrothermal reaction of Sr-containing calcium silicate precursors in trisodium phosphate aqueous solution, without using any surfactant, template-directed reagents and organic solvents, and the morphology, phase, chemical compositions, lattice constants and degradability of the products were characterized. 2. Materials and methods Si/Sr-HAp crystals were hydrothermally synthesized from Srcontaining calcium silicate as precursors in Na3PO4 aqueous solutions in the absence of any structure-directing reagents and organic solvents. Analytical grade reagents (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) without further purification were used in this study.
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obtained products were labeled as Si-HAp, Si/Sr5-HAp, Si/Sr10-HAp, Si/Sr20-HAp and Si/Sr20-HAp-1, respectively. In addition, the pure HAp nanoparticles were synthesized as the control sample via chemical precipitation and then hydrothermal treatment method. The solution of 0.5 M Ca(NO3)2 · 4H2O and 0.3 M (NH4)2HPO4 was first prepared, respectively. Then the pH of Ca(NO3)2 solution was adjusted above 10.8 with ammonia solution, then the (NH4)2HPO4 solution was added into the Ca(NO3)2 solution dropwise under vigorous stirring. The pH of the mixtures was controlled above 10.8 among the whole addition process by addition of 28 wt.% ammonia solution. After finishing the addition, the obtained suspension was transferred to polytetrafluoroethylene vessel for further hydrothermal treatment at 180 °C for 24 h. After the hydrothermal treatment, the products were filtered and washed as described above, and then dried at 120 °C for 24 h. The obtained pure HAp powders were labeled as HAp. The synthetic HAp and Si/Sr-HAp powders were characterized by X-ray diffraction (XRD: D/max 2550 V, Rigaku, Japan) with monochromated Cu-Kα radiation. The crystallinity degree Xc was evaluated by Eq. as follows: V 112=300 X c ¼ 1− I300 where I300 is the intensity of the (3 0 0) reflection and V112/300 is the intensity of the hollow between (1 1 2) and (3 0 0) reflections [23]. The lattice constants were calculated from the well determined positions of the intense XRD diffractions that were processed by MDI Jade 6.1 software [24]. The products were also characterized by Fourier transform infrared spectroscopy (FTIR: Nicolet Co., USA). The morphology of the synthesized powders was characterized by scanning electron microscopy (SEM: JSM-6700F, JEOL, Japan), and the chemical compositions of the powders were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES; VISTA AX, Varian Co., USA) after calcining the products at 800 °C for 3 h to eliminate the adsorbed water and contamination came from filter paper. EDS mapping analysis was also carried out to investigate the element distribution by elemental mapping (Oxford instruments). 2.3. In vitro study of the degradability of Si/Sr-HAp nanowires
The precursors were prepared by chemical precipitation method. Briefly, the calcium was partially replaced by strontium (x = 0, 0.05, 0.10 and 0.20 in molar ratio). The mixed solution with the pH about 3.85 of Ca(NO3)2 · 4H2O and Sr(NO3)2 was firstly prepared according to the expected substituted ratio. After preparation, the mixture was added dropwise to the Na2SiO3 solution to produce a white suspension under rigorous stirring. After complete addition, the white precipitate was further stirred for 24 h, followed by washing with three times of deionized water and two times of 100% ethanol. After washing, the remained liquid was removed by vacuum filtration. Then the obtained powders were dried at 120 °C for 24 h. Finally, the crystalline Srx-CS powders were obtained by calcining at 850 °C for 2 h [22].
To evaluate the effect of the Si/Sr-substitution on the degradability of the Si/Sr-HAp nanowires, the in vitro degradability of the synthetic Si/Sr-HAp nanowires was preliminarily investigated by examining the weight loss percentage of the products in Tris–HCl buffer solution, and the traditional pure HAp nano-particles were used as the control sample [25]. The 0.1 M Tris–HCl buffer solution was prepared by dissolving analytical reagent grade Tris(hydroxymethyl) amino-methane in distilled water and then was buffered at pH 7.4 at 37 °C with hydrochloric acid. The synthetic powders were soaked in the Tris–HCl at 37 °C in a shaking water bath for 4 days at a solid/liquid ratio of 1.50 mg mL−1 without refreshing the soaking medium. After soaking, the samples were centrifuged, the supernatant solution was used to evaluate the released ions by ICP-AES. Based on the fact that there was no P in Tris–HCl buffer solution, the dissolution ratio (S) of the powders was calculated by the following equation:
2.2. Hydrothermal synthesis of the Si/Sr co-substituted HAp nanowires
S ¼ Cp V =mp 100%
2.1. Synthesis of calcium silicate (CaSiO3, CS) and strontium substituted calcium silicate (Srx-Ca(1 − x)SiO3, SrxCS) powder precursors
1 g CS, 1 g Sr0.05CS, 1 g Sr0.10CS, 1 g Sr0.20CS or 1.5 g Sr0.20CS powders after calcining were mixed with 85 mL, 0.2 M trisodium phosphate aqueous solution in a polytetrafluoroethylene vessel and the vessel was sealed in a stainless steel autoclave and heated at 180 °C for 1.5 h, 3 h, 24 h and 3 days. After the hydrothermal reaction, the reaction system was cooled to room temperature at the rate of 2 °C/min and the Si/Sr substituted HAp nanowires were obtained. The products were washed and filtrated as described above, then dried at 120 °C for 24 h. The
Where Cp, V, and mp were the P concentration in Tris–HCl (mg mL−1), volume of Tris–HCl (mL) and P content (mg) of the samples soaked in Tris–HCl, respectively [25]. 3. Results and discussion The CS powders after calcining at 850 °C were chosen as the representative sample to observe the morphology of the synthetic precursors
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Fig. 1. FESEM image of the precursors (the symbol * indicated the precursor particles).
(Fig. 1). It is clear that the morphology of the precursors was particlelike shapes with diameter between 0.5 and 0.6 μm. After the hydrothermal treatment of the precursors in Na3PO4 aqueous solution at 180 °C for 24 h, the nanowire-like products with lengths up to 2 μm and diameters about 60 nm were obtained (Fig. 2B–E). As expected, the control sample was particle-like shape with 40 nm in diameter and about 200 nm in length (Fig. 2A). The synthetic Si/Sr-HAp nanowires possess three-dimensional random directions. It is suggested that the wires/ fibers with random directions will improve the mechanical properties by 20% (Krenchel factor = 0.2) [26]. In our future studies, the synthetic Si/Sr-HAp nanowires will be used as the mechanical reinforcement to fabricate the composites, and the relationship between the nanowire orientation factor and the mechanical property of the fabricated composites will be investigated systematically. The XRD patterns shown in Fig. 3 suggested that all of the products could be identified as pure HAp phase (JCPDS card: NO. 09-0432). Moreover, the similar XRD peaks indicated that the obtained products with the presence of both Si and Sr were element substituted HAp materials. The previous studies have confirmed that the element substitution will not significantly alter the XRD peaks of the HAp products [25,27,28]. In addition, the peaks of Si/Sr-HAp were apparently broader than that of
HAp, suggesting that the ion substitution resulted in the decrease the crystallinity of the HAp products [25]. Besides, the positions of the peaks of the Si/Sr-substituted HAp had a little shift from that for the pure HAp. Comparing with the pure HAp materials, the small angle XRD scanning results (Fig. 3B) clearly revealed that the corresponding peaks of the Si/Sr-HAp nanowires shifted to lower degree. In addition, the shifting degree increased with the increase of the Srsubstitution amount, indicating the increase of the lattice constants (Table 1) [29,30]. The lattice constants calculated from the XRD determination results further confirmed that the lattice constants of all the Si/Sr-HAp nanowires were larger than that of the pure HAp (Table 1). Moreover, the values increased with the increase of the element substitution amount. The increase in the lattice constants were due to the replacement of the PO4 tetrahedra and Ca2+ ions by larger SiO4 tetrahedra and Sr2+ ions, respectively [29–31]. In this study, the deviation of lattice constants suggested that the SiO4 tetrahedra and Sr2+ ions replaced and occupied the PO4 tetrahedra and Ca2+ crystal sites of the HAp, respectively [30,31]. Moreover, the crystallinity of the Si-substituted HAp and the Si/Sr co-substituted HAp were lower than that of the pure HAp, which further proved that the element substitution decreased the crystallinity. The similar phenomenon of the element substitution leading to the decrease of the crystallinity has been confirmed in previous studies [25,32]. Fig. 4 reveals the FTIR spectra of the synthesized HAp nanoparticles and Si/Sr-HAp nanowires. All spectra agreed with the reported FTIR data for HAp. The peaks at 471, 563, 604, 960, 1032 and 1092 cm− 1 were the characteristic bands for PO34 − [33]. The peaks at 3443 and 1637 cm− 1 were assigned to the bending mode of the absorbed water. The peaks around 1414 and 1466 cm−1 were attributed to carbonate ions (CO2− 3 ) in B-site, which might come from the dissolved carbon dioxide in phosphate solutions [33]. The peaks at 3568 and 631 cm−1 are the characteristic OH bands of HAp, which were weak or disappeared from the synthesized Si/Sr-HAp nanowires because of the existence of carbonate ions [33] and the replacement of PO4 tetrahedra by SiO4 tetrahedra in the HAp structure [31]. The chemical composition analysis suggested the existence of SiO4 tetrahedra. The FTIR result further confirmed that the positions of the peaks were not affected by Sr and Si substitutions. To further investigate the formation mechanism of the Si/Sr-HAp nanowires, the Sr0.10-CS was selected as the representative precursor and hydrothermally treated in Na3PO4 aqueous solution at 180 °C for
Fig. 2. SEM images of the synthesized HAp nanoparticles and Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. (A) HAp nanoparticles, (B) Si-HAp, (C) Si/Sr5-HAp, (D) Si/Sr10-HAp, (E) Si/Sr20-HAp and (F) Si/Sr20-HAp-1.
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Fig. 3. XRD patterns of the synthesized HAp nanoparticles and the Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h.
1.5 h, 3 h, 24 h and 3 days, respectively. Comparing with the morphologies of the precursor powders (Fig. 1), the short rod-like crystals with diameter around 50 nm formed on the surface of the precursor microparticles after hydrothermal treatment of the precursor in Na3PO4 aqueous solution for 1.5 h (Fig. 5A, the early stage). With the increase of the hydrothermal time to 3 h, the short rod-like crystals became longer up to 300 nm (Fig. 5B). With an increase of the hydrothermal time to 24 h, the wire-like products with lengths up to 2 μm and diameters around 60 nm were obtained (Fig. 5C). When the hydrothermal time was further increased to 3 days, the length and the diameters increased to about 2.5 μm, 70 nm, respectively (Fig. 5D). The morphology development mechanism of Si/Sr-HAp nanowires can be summarized as followings based on the SEM observations: The surface erosion of the precursors happened when they were surrounded by the phosphate solution under hydrothermal condition, accompanying with the release of the Ca2 +, Sr2 + and SiO23 − ions into the aqueous solution. With the continual ion-release, the concentration of the Ca2+/Sr2+ and PO34 reached over saturation and the hydroxyapatite nucleated on the eroded Sr-CS particle surfaces. During this process, the Sr-CS itself played both the roles of Ca2+ and substituted ion sources, and the HAp crystal nucleation sites. With the extending of the hydrothermal time, the ion released continuously and the bigger and longer HAp crystals grew at the expense of the released ions. At the same time, the Si and Sr ions released from the Sr-CS precursors incorporated into the newly formed HAp crystal lattices. Finally, the Si/Sr-substituted HAp nanowires were obtained. Moreover, the increase of the diameter and length of the Si/Sr-HAp nanowires with the increase of the hydrothermal time suggested that the size of the products could be well controlled via regulation of the hydrothermal conditions such as hydrothermal period.
The elemental analyses of the synthetic products are presented in Table 2. It is clear that the synthetic Si/Sr-HAp nanowires contained the substituted ions of Si and Sr, which came from the Sr-CS precursors. Furthermore, the amount of Sr in the obtained Si/Sr-HAp nanowires increased obviously with the increase of the Sr-substitution level in the Sr-CS precursors. The results suggested that the Sr substitution amount of the Si/Sr-HAp nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors. In addition, it is interesting to find that the co-substitution of Sr resulted in an increase of the Si substitution amount, which might be attributed to the following reason (see the data for the samples of Si-HAp, Si/Sr5-HAp, Si/Sr10-HAp and Si/Sr20-HAp shown in Table 2): the replacement of Ca2+ crystal site by bigger diameter of Sr2 + results in the crystalline swelling, which might enhance the incorporation of the SiO4 tetrahedra into the lattice of HAp crystals. Furthermore, the amount of substituted Si for the synthesized products with similar Sr substitution level could be facilely tailored by regulating the reaction ratio of the precursor/solution (see the data for the samples of Si/Sr20-HAp and Si/Sr20-HAp-1 shown in Table 2). In the present study, the amount of PO34 − ions used in the hydrothermal process is overdose. Therefore, the more precursors
Table 1 Lattice constants, 2θ value for (211) diffraction and crystallinity of the HAp nanoparticles and Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. Samples
HAp Si-HAp Si/Sr5-HAp Si/Sr10-HAp Si/Sr20-HAp Si/Sr20-HAp-1
Lattice constants a (Å)
c (Å)
9.365 9.381 9.397 9.414 9.430 9.479
6.730 6.812 6.871 6.910 6.925 6.965
2θ (°) for (2 1 1) reflection
Crystallinity (XC)
32.021 31.945 31.848 31.769 31.710 31.691
0.886 0.745 0.798 0.733 0.741 0.760
Fig. 4. The FTIR spectra of the synthesized HAp nanoparticles (A) and the nanowires of Si-HAp (B), Si/Sr5-HAp (C), Si/Sr10-HAp (D), Si/Sr20-HAp (E) and Si/Sr20-HAp-1 (F) via hydrothermal treatment of the precursors at 180 °C for 24 h.
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Fig. 5. SEM images of the products after hydrothermal treatment of Sr10-CS precursors at 180 °C for 1.5 h (A), 3 h (B), 24 (C) and 72 h (D), respectively.
Table 2 Chemical composition of the Si/Sr-HAp nanowires via hydrothermal treatment of the precursors at 180 °C for 24 h. Samples
Si-HAp Si/Sr5-HAp Si/Sr10-HAp Si/Sr20-HAp Si/Sr20-HAp-1
Chemical composition (wt.%) Si (wt.%)
Sr (wt.%)
Ca/P molar ratio
(Ca + Sr)/(P + Si) molar ratio
0.56 1.16 1.17 1.23 3.06
0 1.64 3.81 6.88 6.76
1.63 1.61 1.58 1.67 1.65
1.57 1.53 1.54 1.68 1.48
were used, the more SiO4 tetrahedra would be released to compete with the PO4 tetrahedra to occupy the crystal sites of HAp, resulting in higher Si incorporation level. The EDS mapping technology was further applied to investigate whether the substituted Si/Sr elements were homogeneously distributed in the obtained Si/Sr-HAp nanowires, and the Si/Sr10-HAp nanowires were selected as the representative sample (Fig. 6). The results showed that the Ca, Sr, P, Si and O elements were
uniformly distributed in the products, which might further confirm that the Si/Sr elements evenly occupied in the lattice sites of the HAp crystals. On the other hand, the Ca/P molar ratio of the obtained Si/SrHAp powders was deviated from the stoichiometric HAp (Ca/P = 1.67). Comparing with the stoichiometric HAp material, the deviation degree of Ca/P molar ratio is critically relevant to both substitution amounts of Si and Sr due to the partial replacement of the Ca and P by the Sr and Si elements, respectively. The Si-substitution amount in Si/Sr5-HAp and Si/Sr10-HAp is almost similar. However, comparing with Si/Sr5-HAp, much more Ca was replaced by Sr in the Si/Sr10HAp, which led to the decrease of the Ca/P molar ratio. As for Si/Sr20HAp nanowires, much more P was replaced by Si, which inversely increased the Ca/P molar ratio to 1.67. It is well known that all of these elements coexist in human bone and tooth minerals, and the composition may vary depending upon the sex, age and nature of the races [25,29]. The evaluation of the biological effect of the Si/Sr co-substituted HAp in vitro and in vivo to screen out the optimal amounts of Si and Sr co-substitution is critical for the biomedical applications of Si/Sr-HAp biomaterials, which will be the next step of our study.
Fig. 6. Mapping of Ca, Sr, P, Si and O distribution in the HAp nanowires.
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preparation of new bioactive materials and mechanical reinforcement materials for bone regeneration. Acknowledgments The authors gratefully acknowledge the support of the Natural Science Foundation of China (Nos.: 81171458, 81190132, 51061160499), the Science and Technology Commission of Shanghai Municipality (No.: 13NM1402102) and the Fund of the Shanghai Institute of Ceramics Chinese Academy of Sciences for Innovation of Science and Technology (No.: Y26ZC1110G). References
Fig. 7. Degradability rate of the control sample (pure HAp nanoparticles, A), and the synthetic Si/Sr-HAp nanowires: Si-HAp (B), Si/Sr5-HAp (C), Si/Sr10-HAp (D), Si/Sr20-HAp (E) and Si/Sr20-HAp-1 (F) after soaking in Tris–HCl buffer solution for 4 days.
The improvement of the degradability is usually required for wide biomedical applications of HAp materials. Fig. 7 shows the quantitative dissolutions of the Si/Sr-HAp nanowires and the traditional HAp nanoparticles after soaking in the Tris–HCl buffer solution for 4 days. The results showed that the substitution of the Si/Sr improved the degradability rate of the products, and the degradability rate increased apparently with the increase of the substitution amount of Si/Sr (Table 2 & Fig. 7). The higher degradability rate for the obtained Si/Sr-HAp nanowires was attributed to the substituted Si/Sr elements, which led to lower crystallinity [25]. 4. Conclusions In summary, the Si/Sr co-substituted HAp nanowires were successfully synthesized via hydrothermal treatment of Sr-substituted calcium silicate precursors in Na3PO4 aqueous solutions, without using any surfactant and organic solvents. The control of nanowire-like morphology and chemical compositions simultaneously was achieved, while the Sr and Si substitution amount of the nanowires could be well regulated by facile tailoring the Sr substitution level of the precursors and the reaction ratio of the precursor/solution, respectively. During the substitution reaction, the SiO4 tetrahedra and Sr2+ ions entered into the HAp lattice and replaced part of the PO4 tetrahedra and Ca2+ ions, respectively, and the lattice constants increased apparently with the increase of the substitution amount. Moreover, the Si/Sr-substitution apparently improved the degradability of the HAp materials. Our study suggested that the Si/Sr co-substituted HAp nanowires might be suitable for
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