Facile One-Pot Synthesis of Oriented Pure Hydroxyapatite with

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A facile one-pot synthesis of oriented pure hydroxyapatite with hierarchical architecture by topotactic phase conversion Ming Wang, Jianyong Gao, Chao Shi, Ying-chun Zhu, Yi Zeng, and Dalin Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg5013044 • Publication Date (Web): 24 Oct 2014 Downloaded from http://pubs.acs.org on October 31, 2014

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Crystal Growth & Design

A Facile One-pot Synthesis of Oriented Pure Hydroxyapatite with Hierarchical Architecture by Topotactic Conversion Ming Wanga, Jianyong Gaoc, Chao Shia, Yingchun Zhua,*, Yi Zengb, **, Dalin Wangc a Key Lab of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China c Changhai Hospital of Shanghai

KEYWORDS: hydroxyapatite; one-pot synthesis; biomineralization; topotactic transition; hierarchical architecture

ABSTRACT

The oriented hierarchical architecture of hydroxyapatite (HA) shows excellent performances in multiple functions in human hard tissues such as bone and teeth. It is a challenge to mimic the architecture of biomineralization products. This study introduces a simple method for the hierarchical architecture of pure stoichiometric HA by a one-pot hydrothermal process without adding any organic molecules. The reaction process is researched by XRD, FT-IR, SEM and TEM and the HA crystal growth mechanism is discussed. The short rod-like HA nanocrystals are orderly assembled into micron-sized platy particles with the DCPD (CaHPO4·2H2O, brushite) shape retained. The ordered HA hierarchical architectures are achieved through topotactic

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transformation due to the crystal structural similarity from DCPD through DCPA (CaHPO4, monetite) to HA. It is found for the first time that the growth of HA crystals on DCPA is accompanied with orientation relationship of HA [110] ∥ DCPA [110] and HA (112) ∥ DCPA (112) . The resemblance between the crystal structures promotes the direct phase transition. The results provide new insights into the formation mechanism of ordered HA hierarchical architectures and gives hints for designing hierarchically assembled nanoarchitecture in synthetic work.

1. Introduction Hydroxyapatite (Ca10[PO4]6[OH]2, HA) is the main inorganic component of human hard tissues such as teeth and bones1. It is commercially used as bone-substitute implants because of its excellent biocompatibility and osteogenetic ability2,3. Additionally, HA has a wide range of applications in chromatography4, pollutant removal5, and drug delivery agent6,7 based on its adsorbability for various ions and organic molecules. Natural teeth and bones are reported to possess excellent properties due to the hierarchically assembled organization of HA nanocrystals with oriented C-axis8. Therefore, investigation on the synthetic method and formation mechanism of hierarchically oriented HA is of great importance for the developing mechanical and biological properties and understanding the mechanism involved in biomineralization. In spite of the extensive researches on synthetic methods of HA with assembled nanoarchitectures, no high purity HA obtained by quick and simple method is proposed. The most commonly used method is adding various organic molecules templates or synthetic surfactants with proteins9, collagen10,11, citrate12 and CTAB13,14 etc. These reactive groups can be adsorbed on a certain crystal face of HA, thus regulating the crystal growth to mimic the function


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of organic matrix in natural hard tissues. However, organic addition inevitably causes CO32doped in HA structure. Furthermore, it is hard to thoroughly remove the residual adsorbed organics on HA surface even after repeated cleaning. While in the absence of organics additives, substantial attention is paid to the ordered arrangement of HA nanocrystals via direct phase transformation in vivo of precursor phases without changing the original morphology of precursor phases. This kind of transformation are called as solid-solid phase transition15,16, single-crystal-to-single-crystal transformation17,18 or topotactic solid-solid conversion19-23 in previous reports. It has been reported that the rapid hydrolysis of precursor phase DCPD24-26 and DCPA16,19-21,27 under alkaline condition result in HA crystals displaying oriented architecture on the original platy morphologies of DCPD and DCPA precursor. The precursor phase in HA formation is important for biomineralization which have a crucial impact on HA crystal growth28. It is generally accepted that OCP (octacalcium phosphate, Ca8H2[PO4]6·5H2O) is an important precursor phase in the formation of HA29. There has been a consensus that the similarity of crystal structure between the two substances causes the phase transition from the OCP to HA. The crystal structure relationships between OCP and HA have been researched in previous reports30-33. For the same reason, researchers have inferred that DCPA is also a precursor phase of HA formation and there possibly exists topotactic solid-solid transition from DCPA to HA, despite a lack of direct evidence. DCPA has been found to be a good choice as precursor phase to prepare highly oriented HA with hierarchical structure. Hiroaki Imai et al.16,19 reported that DCPA can be obtained from DCPD through topotactic transition by drying at 60°C, and nanotextured HA is prepared on DCPA surface via a rapid hydrolysis under alkaline condition. Jiang Chang et al. synthesized dental enamel-like HA21 and multilevel hierarchically ordered HA20 transformed directly from DCPA. However, the obtained


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HA are nonstoichiometric due to the lack of Ca2+ and Na+ doped in the crystal structure. Furthermore, the direct evidence of the transformation from precursor DCPA to HA is not given and the formation mechanisms of this kind of architectures remain unclear in previous reports. Brown29 discussed the structural resemblance among OCP, DCPA and HA and pointed out that the calcium phosphate chains of the type O3-P-O4-Ca-O3-P-O4 parallel to b-axis in DCPA is similar to those parallel to c-axis in OCP and HA. The c axis of HA is inherited from b axis of DCPA in previous reports15,18-20. In the present study, HRTEM data demonstrates the crystallographic orientation of HA[110]∥DCPA[110] and HA (112)∥DCPA(112), which is different from previous reports. In the present study, a one-pot synthesis method via quick and simple operations is proposed for stoichiometric HA with oriented hierarchical architecture. This method using Ca(OH)2 and Ca(H2PO4)2·H2O as the starting materials aims at eliminating the introduction of foreign ions in the HA structure. The crystal growth mechanism of ordered HA with hierarchical architectures is discussed in detail. Most importantly, this study illustrates the first direct observation of interface between DCPA and HA via topotactic transition, and deduces the topological crystallographic relationship between DCPA and HA. 2. Experimental Section 2.1 Synthesis The raw materials were Ca(OH)2 and Ca(H2PO4)2·H2O (MCPM) of analytical grade. The synthesis was conducted based on the following reaction: 3Ca(H2PO4)2·H2O+7Ca(OH)2→Ca10(PO4)6(OH)2+15H2O

The final products were HA and H2O, which avoided foreign ions doped in the HA crystal structure and thereby ensured the purity of HA.


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The initial reactants, including 2.333 g of Ca(OH)2 and 3.403 g of Ca(H2PO4)2·2H2O, were weighed into a 100 ml teflon pot containing 80 ml of deionized water. Because of the low solubility of Ca(OH)2 and Ca(H2PO4)2, the mixture was stirred with a glass rod to prevent reactant powders precipitating down the bottom and adhering to the wall lining of the pot. When the reactants were mixed together, the obvious phenomenon of mixture rolling over for several seconds was observed. The white precipitate at room temperature was dried and defined as 0 min product. Then, the pot was placed in a stainless-steel autoclave that was clamped to a rotatable holder in a reactor at 150°C. Precipitation powders were obtained at different time intervals (5, 15, 30, 45, 60 min, and 10 h), separated by vacuum filtration, and dried at 60°C for 12 h. 2.2 Characterization The material phases of the prepared powder samples in the 2θ range from 3° to 60° were examined on an X-ray diffractometer (XRD, Ultima IV, Rigaku, Japan) with Cu Kα radiation operating at 40 kV and 40 mA. The surface and microstructure of samples were analyzed using a field emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan). The Fourier transform infrared absorption spectra (FTIR) of samples were measured using KBr method on a spectrophotometer (FTIR-8400, Shimadzu, Japan). Structural changes in samples during the reaction were explored using a field emission transmission electron microscope (FETEM, JEM2100F, JEOL, Japan) combined with XRD analysis of selected area in FETEM images as well as high-resolution transmission electron microscopy (HRTEM) and fast Fourier transform (FFT). The FFT patterns of HRTEM images were obtained using the Gatan’s program Digital Micrograph (http://www.gatan.com). 3. Result


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Figure 1. XRD patterns of 0 min products and hydrothermal products synthesized at 150°C for 5, 15, 30, 45, 60 min and 10 h. The reaction experiences phase transition from DCPD to DCPA, and to HA. The HA grow well-crystallized with reaction time.

The XRD profiles illustrate that hydrothermal products experience phase changes during the reaction process (Fig. 1). At the initial stage of reaction between Ca(OH)2 and Ca(H2PO4)2·H2O, the precipitate at room temperature after mixing defined as 0-min product contains mainly DCPD and the residual unreacted Ca(OH)2. After 5 min hydrothermal treatment, the XRD peaks intensity of DCPD decreases and DCPA and HA peaks appear, which demonstrates that DCPD transforms to DCPA and HA. 15 min and 30 min later, the peaks intensity of DCPD decreases and that of HA increases gradually. In XRD pattern of 45 min products, the peaks of DCPD disappear and HA becomes main phase despite of small amount of DCPA.1 h later, the pure HA is obtained, indicating that the transformation from reactants to HA is completed within 1 h. However, the presence of weak and broad reflection peaks in the XRD profile of 1 h product suggests that the sample is not well crystallized at this time point. When the reaction time prolongs to 10 h, the reflection peaks become stronger and sharper, which means that the crystallinity of the product increases with crystal growth (JCPDS No. 09-0432). It is worth to mention that the amount of DCPA does not change obviously before DCPD disappears. DCPA, as dehydration product of DCPD, is presumed to be an intermediate phase in the transformation


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from DCPD to HA. These results demonstrate that the DCPD phases are formed at the initial reaction stage and then transformed to HA via DCPA intermediate with reaction time.

Figure 2. FTIR spectra of 0 min products and hydrothermal products synthesized at 150°C for 5, 15, 30, 45, 60 min and 10 h (b: brushite, DCPD; m: monetite, DCPA; h: hydroxyapatite, HA; *: identified.).

The FTIR spectra of the prepared materials are present in the wavenumber range of 400-1300 cm-1 (Fig. 2). The positions at 528 and 576 cm-1 (P-O bending mode), 795 cm-1 (P-O-H out-ofplane bending), 873 cm-1 (P-O-H stretching mode), 986 cm-1, 1006 cm-1, 1064 cm-1, 1135 cm-1 and 1215 cm-1 (P-O stretching mode) are characteristic absorption peaks of [HPO4]2- in DCPD34. The weak peak at 905 cm-1 is ascribed to P-O(H) stretching mode of [HPO4]2- in DCPA35. The peak at 860 cm-1 may be caused by residual [H2PO4]-36. The peak at 962 cm-1 and the strong board peaks around 1000 to 1200 cm-1 exhibit P-O stretching mode of [PO4]3- in HA, while the bands observed at 561 cm-1 and 602 cm-1 result from P-O bending mode of [PO4]3- in HA. The adsorption peak at 631 cm-1 is attributed to the librational mode vibration of OH- in HA structure. For 0-min product, the main adsorption bands are assigned to [HPO4]2- in DCPD. At 5 min of reaction the absorption peaks of DCPD weaken. The [PO4]3- bands in HA and weak [HPO4]2- peak in DCPA appear. At 15min and 30 min the absorption peaks of DCPD weaken further. At 45 min the absorption peaks of DCPD disappear and the peak at 873 cm-1 which is


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assigned to [HPO4]2- in DCPD right shift to 875 cm-1 which is attributed to residual [HPO4]2doped in HA structure34. Besides, with extending reaction time, the growing stronger peak at 631 cm-1 and the sharpening of [PO4]3- peaks indicates that the samples are becoming wellcrystallized with reaction time. The results obtained from FTIR spectra are in accordance with XRD results (Fig. 1).

Figure 3. (a-c) SEM images of 0 min products: the plate-like DCPD with smooth surface; (d-f) hydrothermal products synthesized at 150°C for 5 min: DCPA nanoblocks orderly stacked after dehydration of DCPD; and (h-j) 15 min: the HA unclei assembled at the surface of spindlelike particles.

The morphologies of the products at different time stage are shown in SEM images (Figs. 3-6). For the 0-min products (Figs. 3a-c), the DCPD shows plate-like shape with 1-50 µm width and 0.25 µm thickness; and the smooth surface are observed. The 5-min sample shows morphology of dehydration product of DCPD (Figs. 3d-f) which are similar with the reports of Imai19. The surface of DCPD become rough, and the grooves and pores appear on the surface. The DCPA nanoscale blocks with ca. 50 nm width are orderly stacked. For 15 min product, the macroscale observation displaying same plate-like morphology with the DCPD (Fig. 3h) is detected. The rough surface of the plates are coverd with oriented array of spindlelike particles units that are 100-300 nm in length and 50-100 nm in diameter (Fig. 3i). On the suface of the spindlelike particles are HA tiny crystal unclei.


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Figure 4. (a-e) SEM images of the 30 min product with hierarchically architecture: the HA unclei grow and assemble into spindlelike particles on original plate-like DCPD; and (f) SEM image of the 30min product after grinding operation: inside unreacted DCPD thin slices with smooth surface exposed.

In Fig.4, the 30-min products show multilevel hierarchically architecture. On the macroscale, the plate-like particles are retained (Fig. 4a-b); On the microscale, oriented array of spindlelike particles units are arranged orderly on the surface (Fig. 4c); on the nanoscale, the surface of spindlelike particles are assembled of tiny nanocrystal HA unclei with a size of ca. 10 nm (Fig. 4d). The morphology of ground product shows the irregular particles and thin slices because the grinding operation makes the inside exposed. There are still unreacted DCPD smooth thin slices inside of the macroscale plate-like particles, indicating that the reaction starts from the surface (Fig. 4f).

Figure 5. SEM images of the 45 min product (a, b) and 1h product (c, d): HA nucleus grows longer into short-rod nanoparticles.

The Fig. 5 reveals the crystal growth of HA of 45-min and 1-h products. The tiny HA unclei on spindlelike particles of 45-min product grow longer along major axis of the spindlelike


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particles compared with that of 30-min product in Fig. 4c (Fig. 5b). For 1-h product, the HA subunits grow into short rod-like particles and the shape of spindlelike units disappear. The aspect ratio of HA short rod-like nanocrystals increases further with crystal growth. The Fig. 6 shows the hierarchical morphology of final 10-h product. The micron-sized platelike particles are still observed (Fig. 6a). The grown oriented HA short rod-like nanoparticles with an average size of 20 nm in diameter and 100 nm in length are assembled into the plate-like particles in a head-to-tail way (Fig. 6b-c). The lateral view shows that HA short-rod nanoparticles are well-organized together (Fig. 6d), indicating complete transition to oriented HA inside the plate-like particles. During the reaction process, the micron-sized plate-like morphology is reserved which is inherited from DCPD. The DCPD particles are transformed to oriented HA nanoparticles on the original platy morphology.

Figure 6. SEM images of the 10h product: oriented HA short rod-like nanoparticles are assembled into the plate-like particles.

TEM analysis (Fig. 7) shows that the final HA powders synthesized for 10 h are made of short rod-like nanoparticles. Many rod-like nanoparticles are aggregated together with orientation, identical to the SEM images (Fig. 7a). The crystal structure of HA nanoparticles is demonstrated by HRTEM image and SEAD pattern (Fig. 7b and 7d). The white spots on the nanorod in Fig. 7b are damage caused by electron beam under HRTEM mode. The growth of rod-like HA


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nanoparticles along the C-axis direction [001] is confirmed. According to EDS results, the Ca/P molar radio of the final product is 1.67 which is equal to the theoretical value of ideal stoichiometric HA (Ca10[PO4]6[OH]2) (Fig. 7c).

Figure 7. TEM analysis of hydrothermal products synthesized at 150°C for 10 h (a) TEM image; (b) HRTEM image; (c) EDS spectrum; (d) SEAD pattern.

Further analysis of HRTEM image of the growing interface from DCPA to HA in 30-min products shows two sets of lattice fringe, indexed as DCPA and HA, which are separated by grain boundary (Fig. 8a). Interplanar spacing of the lattice fringes of DCPA region in Figure 8a are measured as 2.8Å and 3.4 Å, which can be indexed to (112) and (002) crystal planes of DCPA with an interplane angle of 37.2 º . HA lattice fringes lie at the edge of DCPA. Interestingly, the same lattice fringe (marked with the long white lines) runs throughout the DCPA and HA regions, indicating that the crystallographic orientation relationship of parallel lattice planes exists in the transition from DCPA structure to HA structure. The parallel lattice planes are indexed as (112) of DCPA and (112) of HA. The HA region is observed along [110] zone axis, in agreement with the observation in TEM analysis (Fig. 7); and the DCPA region is identified along [110] zone axis. The relationship of HA [110]∥DCPA [110] and HA (112)∥ DCPA (112) can obviously be concluded. Additionally, FFT patterns are obtained for different


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areas in HRTEM image (Fig. 8b-d). Fig. 8c contains two sets of grids that can be regarded as the combination of Figs. 8b and 8d. Based on the above results, the spots in Fig. 8c are distinguished and indexed. The two sets of grids share the same spot indexed as DCPA (112) and HA(112), consistent with the results in Fig. 8a.

Figure 8. HRTEM image and FFT patterns of 30-min products showing phase transformation from DCPA to HA. (a) the interface between DCPA and HA; FFT patterns of selected areas in HRTEM image (b) area I; (c) area II; and (d) area III.

4. Discussion 4.1 Transformation from DCPA to HA To investigate the transformation of DCPA to HA structure, crystal structure models are built for restoring the HRTEM image (Fig. 8) of lattice fringes in Fig. 9a with the crystallographic orientation relationship of HA [110]∥DCPA [110] and HA (112)∥DCPA (112). The crystal structural relationship between the HA and DCPA is easily obtained when we analyze a structural unit in Fig. 9a (the area besieged by black lines). The structural unit lattices are redefined with coordinate axis[001], [110] and [110] of DCPA and HA, and the topological relation between crystal lattices of DCPA and HA are shown in Fig. 9b. There are four [PO4]3-


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chains marked as 1, 2, 3 and 4 along [110]of DCPA and [110] of HA in the lattice units. The crystal lattice framed by lattice plane (110) , (001) and 2/3 (110) in DCPA structure is transformed to that framed by (110), (001) and (110) in HA structure during this topotactic conversion, with the conservation of [PO4]3-.

Figure 9. Schematic diagrams of transformation from DCPA to HA (a) crystallographic orientation relationship between DCPA and HA by HRTEM; (b) topological relation between crystals lattices of DCPA and HA

The possible mechanism of transition from DCPA to HA structure under alkaline condition is inferred. H atom in DCPA structure connects with O atom of [PO4]3- to form the [HPO4]2- unit (Fig. 9b). When DCPA is present under alkaline condition, OH- enters the DCPA structure and takes away H+ in [HPO4]2- to produce H2O and leave [PO4]3-, as shown in the equation : HPO42-+ OH-→PO43-+ H2O. This process may causes the rotation and displacement of [PO4]3- to help the transform to HA structure. In previous reports16,19-21, this process is achieved without Ca2+. By immersing DCPA in NaOH or NH4OH solutions, the HA is formed. Inevitably, the resulting HA


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is lack of Ca2+ with Na+ doped in Ca2+ location. In this study, OH- and Ca2+ may simultaneously enter the DCPA structure and form stoichiometric HA. The resemblance between crystal structures, which offers a good crystal lattice match, promotes the direct phase transition and creates favorable condition for the growth of HA on DCPA surface. Following this principle, the inorganic precursor phase DCPA can be regarded as the template of HA whose morphology is preserved despite of the transition process. 4.2 Reaction process and crystal growth mechanism Raw materials Ca(H2PO4)2 and Ca(OH)2 are seldom used to synthesize HA because of their slightly soluble property. However, this reaction system shows fast speed and high efficiency. The whole reaction process is divided into three stages shown as Equation (1-3). In Stage (I), from the XRD profile of 0 minutes in Figure 1, when the reactants are mixed together, Ca(H2PO4)2 and Ca(OH)2 react vigorously. The Ca(H2PO4)2 are exhausted in a few seconds and the main product are DCPD and excess Ca(OH)2. The previous reports have shown that in H3PO4 and Ca(OH)2 reaction system to prepare HA, DCPD is also the preferential phase26. We believe that the DCPD phase is of the most superiority in dynamics in this reaction system. The Stage (II) shows a dehydration reaction. Although DCPA is a dehydration product form DCPD, DCPA and DCPD are of different crystal structures. DCPD lose its hydration water and form DCPA via topotactic conversion19 at higher temperature37 as shown in Equation (2). In the Stage (III), because of the residual Ca2+ and OH-, the reaction is going on and results in stoichiometric HA finally because HA is the most thermodynamically stable calcium phosphate phase. DCPA transform to HA via topotactic conversion as Equation (3). In general, the reaction in this work is summed up as Equation (4) and completes in one pot. The calcium phosphates experience phase transition process in the sequence of DCPD, DCPA and HA. The pH level of the reaction system


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undergoes substantial changes during hydrothermal synthesis of HA (Fig. 10). The pH value decreases significantly within the first 1 h of reaction due to the consumption of Ca2+ and OH- in solution and finally returns to 6 which is equal to the pH of the original deionized water before reaction. This result is consistent with the corresponding XRD patterns (Fig. 1). After 1 h, there are no obvious changes in pH level of the reaction system, because the synthetic reaction is finished within 1 h while the growth of HA crystals is ongoing. In Ca(OH)2 and Ca(H2PO4)2 reaction system, the theoretically stoichiometric HA is prepared without foreign ions introduced by reactants such as Cl-, CO32-, Na+, NO3-, NH4+ doped in HA structure. Stage (I): Ca(H2PO4)2·H2O (MCPM)+Ca(OH)2→2CaHPO4·H2O (DCPD)+H2O Stage (II): CaHPO4·H2O (DCPD)→CaHPO4 (DCPA)+H2O Stage (III): 6CaHPO4 (DCPA)+4Ca(OH)2→Ca10(PO4)6(OH)2 (HA)+6H2O

(1) (2) (3)

Summing (I), (II) and (III) up: 3Ca(H2PO4)2·H2O+7Ca(OH)2→Ca10(PO4)6(OH)2+15H2O

(4)

Figure 10. Changes in pH level of the reaction system during hydrothermal synthesis.


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Figure 11. The crystal growth schematic illustration of hydrothermal process from DCPD to oriented HA.

The crystal growth process is illustrated in Fig. 11. Two steps of topotactic transitions proceed during the hydrothermal synthesis, i.e., DCPD to DCPA and DCPA to HA. The platy shaped DCPD firstly lose its hydration water and form DCPA. During this reaction, the DCPD platy single crystal are disassembled to DCPA nanoblocks with orientation arrangement by topotactic dehydration transformation, in which the crystal lattices of DCPD and DCPA show one or more crystallographically equivalent according to previous reports19. With the reaction of residual Ca(OH)2, the Ca2+ and OH- react with the DCPA and form HA. The HA crystal nucleus form at the surface of DCPA and assemble into spindlelike particles. The c axis of DCPA and HA keep a similar orientation during the transition. Although there is crystallographic relationship between crystal structures of precursor DCPA and HA, they are not the same. DCPA single crystals cleave into HA nanocrystals when DCPA transform into HA because of the slight mismatch between crystal structures of DCPA and HA. The Ca2+ and OH- may penetrate into the inner


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portion of DCPA gradually along the cracks among the HA nanocrystal boundaries. The transitions proceed from surface to inner portion gradually. Subsequently, the HA crystal nucleus keep growing to short-rod nanoparticles along c axis. Because of the certain orientation relationship in topotactic transition, the produced HA crystals are orderly aggregated and the precursor single crystal DCPD platy morphology is still preserved. 5. Conclusions This study proposes a simple method for the hierarchical architecture of pure stoichiometric hydroxyapatite (HA) by a one-pot hydrothermal process without adding any organic molecules. The obtained products are composed of short rod-like HA nanocrystals, which are orderly assembled into micron-sized plates with the DCPD shape retained. The hierarchical architectures of ordered HA nanocrystals originate from direct topotactic transformation from DCPD via DCPA based on the crystal structural similarity. HRTEM results give the direct evidence of the topotactic transformation from DCPA to HA. The crystallographic relationship between DCPA and HA is HA [110] ∥ DCPA [110] and HA (112) ∥ DCPA (112) . According to the relationship, the topological relation of DCPA and HA crystal structure are discussed. This study provides important information on ordered HA structure in hierarchical architectures and gives hints for designing hierarchically assembled nanoarchitecture in synthetic work. The proposed DCPA-HA transformation with a certain crystallographic relationship in this work implies a possible mechanism of biomineralization in nature. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Yingchun Zhu). Tel.: +86 21 52412632 Fax. : +86 21 52412632.


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**Email: [email protected] (Yi Zeng) Tel.: +86 21 52411020 Fax. : +86 21 52413903. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (No. 51072217 No. 51232007), and the Science and Technology Commission of Shanghai Municipality (No. 08JC1420700 and No. 11XD1405600).

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For Table of Contents Use Only

A crystal growth model of HA with hierarchical architecture is built according to the XRD, SEM and TEM results. The topotactic transformation from DCPA to HA is analyzed by HRTEM, and the reaction process is discussed.


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Facile One-Pot Synthesis of Oriented Pure Hydroxyapatite with

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