The synthetic hydroxyapatite (HAP, Ca₁₀(PO₄)₆(OH)₂) is chemically similar to the inorganic component of bone and teeth, which endows it with excellent biocompatibility, good osteoconductivity and osteoinductivity^[1,2]. HAP-based biomaterials have been intensively investigated and applied in various biomedical fields, such as bone and teeth defect repair, and drug delivery^[3,4]. Strontium (Sr) is an important trace element in human body and essential in bone metabolism by regulating bone formation and resorption^[5,6]. It was reported that Sr²⁺ ions could stimulate the differentiation of osteoblasts and inhibit the differentiation of osteoclast^[7]. Previous reports indicated that the incorporation of Sr²⁺ ions into HAP could promote bone regeneration and bone defect repair^[5,8].

Compared with other HAP nanostructured materials such as HAP nanoparticles, HAP microspheres, and 1D HAP nanorods^[2-3,9-10], ultralong hydroxyapatite nanowires (UHANWs) with lengths of several hundred micrometres have high flexibility and ultralong 1D nanostructure, showing great potential in constructing different types of multifunctional biomaterials such as the highly flexible biomedical paper and elastic porous scaffolds^{[11,12,13,14,15,16]}. Moreover, the Sr-doped UHANWs (Sr-UHANWs) can combine excellent advantages of both strontium and UHANWs, which will remarkably enhance the bioactivity and enlarge the application potentials of UHANWs- based biomaterials in various biomedical fields.

Previously, Xu et al^[17] synthesized Sr-doped HAP whiskers using acetamide as a homogeneous precipitation reagent by hydrothermal treatment. Zhang, et al^[18] hydrothermally synthesized Sr and Si co-doped HAP nanowires using Sr-containing calcium silicate as the precursor. However, to the best of our knowledge, the synthesis of Sr-doped ultralong HAP nanowires by the oleate precursor solvothermal method has not been reported in the literature.

In this study, the Sr-UHANWs with different Sr/(Sr+Ca) molar ratios have been synthesized by the one-step oleate precursor solvothermal method. The as-prepared Sr-UHANWs exhibit ultralong 1D nanostructure and high flexibility. Importantly, the preparation method developed in this work is simple, and may be applicable for the synthesis of other metal ions-doped UHANWs.

Oleic acid and SrCl₂·6H₂O were purchased from Aladdin Industrial Co. Ltd., and other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received without further purification.

The strontium-doped ultralong hydroxyapatite nanowires (Sr-UHANWs) with different Sr/(Sr+Ca) molar ratios were prepared by a facile one-step oleate precursor solvothermal method^[19,20,21]. In a typical experiment for the synthesis of Sr-UHANWs with a Sr/(Sr+Ca) molar ratio of 0.4, a mixture of deionized water (135 mL), methanol (60 mL) and oleic acid (105 mL) was prepared under vigorous mechanical stirring in an ice-water bath. Then, 150 mL of NaOH (10.500 g) aqueous solution, 120 mL of aqueous solution containing 1.998 g of CaCl₂ and 3.199 g of SrCl₂·6H₂O, and 180 mL of NaH₂PO₄·2H₂O (9.360 g) aqueous solution were separately added into the above mixture. After continuous stirring for 30 min, the resulting suspension was transferred into a Teflon-lined stainless steel autoclave (1000 mL), sealed and heated at 180 ℃ for 24 h. After cooling to room temperature, the product was washed with ethanol and deionized water 3 times, and dried, respectively. The as-prepared Sr-UHANWs with a Sr/(Sr+Ca) molar ratio of 0.4 is labeled as Sr40-UHANWs.

Similarly, the undoped UHANWs and Sr100-UHANWs were prepared under the same conditions but using 3.330 g of CaCl₂ and 7.999 g of SrCl₂·6H₂O, respectively. The Sr5-UHANWs, Sr20-UHANWs and Sr90-UHANWs were also prepared using the above method.

Scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) elemental mapping patterns of the as-prepared Sr-UHANWs with different Sr/(Sr+Ca) molar ratios were recorded with a field- emission scanning electron microscope (FEI Magellan 400, USA). The X-ray powder diffraction (XRD) patterns of Sr-UHANWs with different Sr/(Sr+Ca) molar ratios were recorded with an X-ray diffractometer (Rigaku D/max 2550 V, Cu K_α radiation, λ=0.154178 nm). Fourier transform infrared (FT-IR) spectra of Sr-UHANWs with different Sr/(Sr+Ca) molar ratios were taken using a FT-IR spectrometer (FTIR-7600, Lambda Scientific, Australia).

Fig. 1 shows the XRD patterns of the as-prepared Sr-UHANWs with different Sr/(Ca+Sr) molar ratios. The XRD pattern of the undoped UHANWs can be indexed to a single crystal phase of hydroxyapatite with a hexagonal structure (Ca₁₀(PO₄)₆(OH)₂, JCPDS 09-0432). Moreover, the XRD pattern of Sr100-UHANWs can be indexed to strontium phosphate (Sr₃(PO₄)₂, JCPDS 24-1008) and strontiumapatite (Sr₁₀(PO₄)₆(OH)₂, JCPDS 33-1348) with a hexagonal structure. The XRD patterns of Sr-UHANWs with different Sr/(Sr+Ca) molar ratios can be indexed to a hexagonal apatite crystal phase (Fig. 1(b-e)). Compared with the XRD pattern of the undoped UHANWs, all diffraction peaks of Sr-UHANWs with different Sr/(Sr+Ca) molar ratios shift to a lower diffraction angle, and the shift becomes larger as the Sr²⁺ substitution ratio increases. The diffraction peak shift indicates the crystal lattice expansion in Sr-UHANWs, which is caused by the substitution of smaller Ca²⁺ ions with larger Sr²⁺ ions^[22]. These experimental results demonstrate that the as-prepared Sr-UHANWs have a chemical compositions of Ca_10-xSr_x(PO₄)₆(OH)₂ (where 0≤x≤10).

As shown in Fig. 2(a-d), the morphology of the undoped UHANWs is similar to that of Sr40-UHANWs. Both undoped UHANWs and Sr40-UHANWs have highly flexible 1D nanostructure and ultrahigh aspect ratios. The energy-dispersive spectroscopy (EDS) elemental mapping patterns of Sr40-UHANWs (Fig. 2(e)) further confirm that the Sr, Ca, P, and O elements are homogeneously distributed in the Sr40-UHANWs, and Sr²⁺ ions have been successfully doped into the UHANWs.

As shown in Fig. 3, the effect of Sr/(Sr+Ca) molar ratios on the morphology of the as-prepared Sr-UHANWs was further investigated. Interestingly, Sr5-UHANWs, Sr20-UHANWs, Sr90-UHANWs, and Sr100-UHANWs exhibit ultralong 1D nanostructure, which is similar to that of undoped UHANWs and Sr40-UHANWs, indicating that the oleate precursor solvothermal method reported herein may be suitable for the synthesis of Sr-UHANWs with Sr/(Ca+Sr) molar ratios ranging from 0 to 100%. High-magnification SEM images of Sr-UHANWs display that the surface of Sr-UHANWs is smooth, and in many cases Sr-UHANWs can self-assemble into nanowire bundles along the longitudinal direction. SEM images in Figs. 2 and 3, display that many Sr-UHANWs can bend owing to their high flexibility and ultralong nanostructure.

According to our previously reported formation mechanism of undoped UHANWs^[12,14,19], we propose that the formation of Sr-UHANWs is an oleate precursor solvothermal process. The precursors of calcium oleate and strontium oleate are first formed in the reaction system after the addition of oleic acid, NaOH, CaCl₂, and SrCl₂·6H₂O. Then, the precursors of calcium oleate and strontium oleate transform to Sr-UHANWs after the addition of NaH₂PO₄·2H₂O under the solvothermal conditions.

The FTIR analysis was performed to characterize the Sr-UHANWs samples. As shown in Fig. 4, the broad absorption peak of all the samples at around 3440 cm^-1 derives from the adsorbed water in the samples. The Sr²⁺ substitution obviously influences the absorption peaks of hydroxyl (-OH) in Sr-UHANWs. As the Sr²⁺ substitution ratio increases, the absorption peaks of the stretching mode (3572 cm^-1) and librational mode (633 cm^-1) of -OH in Sr-UHANWs decrease in intensity and broaden, indicating that the Sr²⁺ substitution leads to the loss of -OH groups and structural disorder^[23]. The absorption peaks at around 1097, 1032, 962, 602, and 561 cm^-1 belong to the PO₄^3- group in the samples^[24]. However, the absorption peaks of the PO₄^3- group in the range of 1150-950 cm^-1 slightly shift to lower wave numbers as the Sr²⁺ substitution ratio of Sr-UHANWs increases, which also demonstrates the increase of disorder around phosphate sites caused by the Sr incorporation^[23]. The above experimental results are consistent with the XRD results, and further confirm that the Sr²⁺ ions have been successfully incorporated into the HAP crystal structure.

In summary, the strontium-doped ultralong hydroxyapatite nanowires (Sr-UHANWs) with different Sr/(Sr+Ca) molar ratios have been successfully synthesized using the one-step oleate precursor solvothermal method. The as-prepared Sr-UHANWs exhibit the ultralong 1D nanostructure and high flexibility. In addition, the EDS elemental mapping, XRD and FT-IR analyses confirm the successful incorporation of Sr²⁺ ions into the crystal structure of UHANWs. Thus, the one-step oleate precursor solvothermal method developed in this study may be suitable for the synthesis of Sr-UHANWs with Sr/(Sr+Ca) molar ratios ranging from 0 to 100%, which may broaden the biomedical applications of UHANWs such as bone and teeth defect repair.