Size control synthesis of hydroxyapatite plates and their application in

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Size control synthesis of hydroxyapatite plates and their application in the preparation of highly oriented films Naohiro Horiuchi, Kotaro Shibata, Hironori Saito, Yuki Iwabuchi, Norio Wada, Kosuke Nozaki, Kazuaki Hashimoto, Yumi Tanaka, Akiko Nagai, and Kimihiro Yamashita Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00480 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Crystal Growth & Design

Cover Page Title: Size control synthesis of hydroxyapatite plates and their application in the preparation of highly oriented films Authors: Naohiro Horiuchi*†, Kotaro Shibata†, Hironori Saito‡, Yuki Iwabuchi§, Norio Wada†, Kosuke Nozaki†, Kazuaki Hashimoto‡, Yumi Tanaka§, Akiko Nagai†, and Kimihiro Yamashita† Affiliations: †Department of Inorganic Biomaterials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Kanda-Surugadai, Chiyoda, Tokyo 101-0062, Japan ‡Department of life and Environmental Science, Chiba Institute of Technology, 2-17-1 Tudanuma, Narashino, Chiba 275-0016, Japan §Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan *Corresponding author: Naohiro Horiuchi TEL +81-3-5280-8014 / FAX +81-3-5280-8015 / E-mail: [email protected] Abstract: Hydroxyapatite (HAp) plates were successfully synthesized using dodecanedioic acid (DDDA). Following the preparation of an ethanolic solution of DDDA, solutions of calcium and phosphate were added in succession to produce a white slurry upon mixing. Subsequent processing of this slurry under hydrothermal conditions provided HAp plates of various sizes. Indeed, the plate size could be changed by varying the quantity of DDDA employed in the synthesis, with larger particles being obtained in the presence of larger quantities of DDDA. It was confirmed that a calcium-DDDA complex (Ca-DDD, calcium dodecanedioate) was formed upon mixing of the DDDA and calcium solutions. The observed morphological control could potentially be accounted for by the formation of Ca-DDD. More specifically, during crystal growth, Ca-DDD plays the role of a nucleation substrate (template), a pH buffer, and a calcium reservoir. Finally, we demonstrated that the obtained HAp plates could be employed to prepare a HAp film with high crystallographic orientation. The most important illustration in the paper: Figure 2.

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Size control synthesis of hydroxyapatite plates and their application in the preparation of highly oriented films Naohiro Horiuchi*†, Kotaro Shibata†, Hironori Saito‡, Yuki Iwabuchi§, Norio Wada†, Kosuke Nozaki†, Kazuaki Hashimoto‡, Yumi Tanaka§, Akiko Nagai†, and Kimihiro Yamashita† †Department of Inorganic Biomaterials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Kanda-Surugadai, Chiyoda, Tokyo 101-0062, Japan ‡Department of life and Environmental Science, Chiba Institute of Technology, 2-17-1 Tudanuma, Narashino, Chiba 275-0016, Japan §Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan *Corresponding author: [email protected]

ABSTRACT Hydroxyapatite (HAp) plates were successfully synthesized using dodecanedioic acid (DDDA). Following the preparation of an ethanolic solution of DDDA, solutions of calcium and phosphate were added in succession to produce a white slurry upon mixing. Subsequent processing of this slurry under hydrothermal conditions provided HAp plates of various sizes. Indeed, the plate size could be changed by varying the quantity of DDDA employed in the synthesis, with larger particles being obtained in the presence of larger quantities of DDDA. It was confirmed that a calcium-DDDA complex (Ca-DDD, calcium dodecanedioate) was formed upon mixing of the DDDA and calcium solutions. The observed morphological control could potentially be accounted for by the formation of Ca-DDD. More specifically, during crystal growth, Ca-DDD plays the role of a nucleation substrate, a pH buffer, and a calcium reservoir. Finally, we demonstrated that the obtained HAp plates could be employed to prepare a HAp film with high crystallographic orientation.


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INTRODUCTION Hydroxyapatite [Ca10(PO4)6(OH)2, HAp] has been extensively employed in the field of biomedical engineering because of its excellent biocompatibility. In addition, its structural and compositional similarities to mineral components of bone have attracted attention from researchers in biology and materials science. Furthermore, the use of HAp is not limited to the medical field, with applications being reported in environmental science and catalysis,1 a proton conductor,2,3 an electret,4 and an adsorbent for organic matter and heavy metals.5,6 For various applications, the morphology should be optimized to achieve the best performance. As such, significant efforts have been made to control of the size and shape of HAp particles.7,8 In biominerals such as bones and teeth, a hierarchical structure composed of organic and inorganic parts components is present, where the inorganic component has a crystallographic structure similar to that of HAp. More specifically, bone has a hierarchical self-assembled structure constructed from a collagen matrix and nanoscale HAp plates of several nanometers thickness and tens of nanometers in length and width.9,10 The artificial synthesis of HAp plates could therefore help prepare bone mimics for use as orthopedic devices. For example, Zhuang et al. successfully demonstrated that the zeta potentials of HAp particles depend on the exposed crystal faces and that cell adhesion behavior is also influenced by the surfaces.11,12 The properties of HAp are also expected to be favorable in the context of energy devices. For example, the proton conductivity of HAp2,3 allows it to be employed in solid electrolytes, while the piezoelectric,13–15 flexoelectric,16 and electret4 nature of HAp render it suitable for application in vibration energy harvesting devices. As



these properties depends on the anisotropic nature of the crystals, specific oriented structures are required to optimize the performance of HAp in the above applications. One potential route toward such materials is a self-assembly using nano-sized plates (nanosheets) to yield oriented film or architectures.17–19 Although the shape and size of HAp fibers can be successfully controlled,20 such control has not yet been achieved in the synthesis of HAp plates.7,8 One potential route to HAp plates involves the use of two-dimensionally grown calcium phosphate. More specifically, following the facile preparation of dicalcium phosphate dihydrate (DCPD) and octacalcium phosphate (OCP) in plate form, these materials are transformed into plate-shaped HAp by thermal treatments under alkaline conditions.21–23 In addition, organic molecules that can behave as structure-directed templates also enable the synthesis of plate-shaped HAp.22,24,25 Other examples from the literature include the synthesis of nanowires and nanosheets using calcium silicate precursors,26 the direct preparation of HAp plates under hydrothermal conditions,25 and the synthesis of nanometric HAp sheets by controlling the supersaturation in a solution system based on simulated body fluid (SBF).27 However, despite these research efforts, no quick and simple method for the preparation of HAp plates has been established, with size control being a particular challenge. Thus, we herein report the preparation of HAp plates using a hydrothermal method based on the dicarboxylic acid dodecanedioic acid (DDDA, HO2C−(CH2)10−CO2H). Initially, a calcium dodecanedioate (Ca-DDD) complex will be prepared for use as a precursor, from which the HAp plates will be synthesized under hydrothermal conditions. This method is inspired by a study by Lu


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et al.,28 who synthesized ultra-long HAp fibers using a calcium-oleic acid complex. We also aim to determine the effect of DDDA quantity on the sizes of the obtained plates, in addition to applying the synthesized HAp plates to prepare oriented architectures.

EXPERIMENTAL SECTION Calcium chloride (CaCl2, 95%), sodium hydroxide (NaOH, 97%), dodecanedioic acid (DDDA, 99%), and ethanol (99.5%) were purchased from Wako Pure Chemical Industries. Sodium dihydrogen phosphate dihydrate (NaH2PO4ꞏ2H2O, 99%) was purchased from Kanto Chemical Co., Inc. Oleic acid (technical grade, 90%), which was used for surface modification in a subsequent coating process, was purchased from Sigma-Aldrich. Cyclohexane (99.5%) was purchased from Tokyo Chemical Industry. A typical synthetic procedure is described as follows. An ethanolic solution of DDDA was prepared by mixing various quantities of DDDA (1, 2, 5 or 10 mmol, which is the variable parameter in this experiment) with ethanol (10 g). A 0.1 M aqueous solution of CaCl2 (10 mL) was then added to the DDDA solution, followed by the addition of a 1.2 M NaOH solution to adjust the pH to ~6 to give a slurry containing a white precipitate (i.e., Ca-DDD). After stirring for a further 30 min, a 0.2 M aqueous solution of NaH2PO4 (5 mL) was added dropwise. The resulting suspension was then transferred into a 50 mL polytetrafluoroethylene-lined stainless-steel pressure vessel (DAB-2, Berghof, Germany), and heated at 150 °C for 18 h. After this time, the precipitate was separated from the suspension by centrifugation then washed with ethanol (×2) and purified water (×1). The



resulting precipitate was dried at 60 °C overnight and the as-synthesized powder was characterized as described below. Coatings were then fabricated on glass substrates using the precipitate (i.e., synthesized HAp plates). To obtain a highly oriented film, the plates were assembled at the air-water interface.29 These plates were then modified with oleic acid by adding the HAp plates (0.1 g) synthesized using 2 mmol DDDA to a 0.15 M solution of oleic acid prepared in cyclohexane (50 mL) and dispersing the mixture by ultrasonication over 60 min. The HAp plates were recovered by centrifugation and washed once with ethanol. The HAp plates were then dispersed again in ethanol (50 mL), and the resulting suspension was added slowly onto the surface of water in a beaker. The HAp plate suspension then spread over the surface of water due to ultrasonication for several minutes. The spread HAp plates were transferred onto a glass substrate (10 mm diameter) by dip-coating. Subsequently, the coated substrate was dried on a hot plate at 60 °C. The transfer and drying processes were repeated 4 times. The morphologies of the obtained HAp plates were then observed by either scanning electron microscopy (SEM; S-3400NX; Hitachi) or by field emission scanning electron microscopy (FESEM; S-4500; Hitachi). The surface morphology was also observed by atomic force microscopy (AFM; SPM-9600, Shimadzu). The crystal structures were identified by powder X-ray diꞏraction (XRD) (D8 Advance; Bruker AXS GmbH) with nickel-ﬁltered Cu Kα radiation at a voltage of 40 kV and a current of 40 mA. Fourier transform infrared (FTIR) absorption spectroscopy was carried out using the KBr method (FTIR4700; Jasco Corp.). Transmission electron microscopy (TEM, H-7100,


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Hitachi) was employed for morphological observations and high-resolution TEM (HRTEM, JEM-2800, JEOL) gave information regarding the crystal structure.

RESULTS AND DISCUSSION Following preparation of the HAp plates using different quantities of DDDA, the FTIR spectra were recorded, as shown in Figure 1a. For simplicity, products prepared using 1, 2, 5 and 10 mmol DDDA are hereafter referred to as DDDA-01, DDDA-02, DDDA-05 and DDDA-10, respectively. All spectra exhibited absorption peaks at 3570 and 630 cm−1, which could be attributed to the presence of OH− ions. In addition, the sharp signal at 3570 cm−1 represented the characteristic O-H stretching vibration of HAp. The signals at 1120–960 and 600–560 cm−1 were attributed to the PO43− ions. Furthermore, the spectra of DDDA-05 and DDDA-10 exhibited small peaks at 2950 and 2920 cm−1, which originated from the DDDA C-H stretching vibration. The XRD patterns (Figure 1b) also confirmed that HAp was obtained through the hydrothermal route employed herein, as all diffraction peaks in all patterns could be assigned to HAp (ICDD 09-0432). These results suggest that HAp was precipitated in the crystalline phase in all cases, with no secondary crystalline phases being observed at any DDDA loadings. Notably, the intensity of the 300 diffraction peak increased with the increasing quantity of DDDA employed, which is likely due to changes in the crystal morphology, as confirmed by SEM observations.



Figure 1. (a) FTIR spectra and (b) XRD patterns of the products obtained using different quantities of DDDA.

The FESEM and TEM images of the HAp synthesized using different DDDA loadings are shown in Figures 2a–2d and in Figures 2e and 2f, respectively. As shown in these images, the obtained products exhibit a plate-shaped morphology under all DDDA loadings examined herein. Indeed, the morphology of DDDA-10 (Figures 2d and 2f) is similar to that of plate-shaped HAp as reported by Viswanath and Ravishankar.25 However, it should be noted that the plate size varied with the DDDA loadings, with larger plates (i.e., micrometer scale) being obtained in the presence of


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greater quantities of DDDA (Figure 2g). In addition, laminated structures consisting of thin plates can be seen in the products synthesized using 5 and 10 mmol DDDA (Figures 2c, 2d, and 2f). The plate shape was also confirmed by the AFM image of DDDA-02 (Figure 2h). As shown in Figures 2a–2d, the HAp using 1 mmol DDDA was slightly agglomerated, and the HAp using 5 and 10 mmol DDDA exhibited laminated structures. Thus, to determine the thickness, the HAp using 2 mmol DDDA was selected for AFM observations as the plate morphology of this sample was easiest to trace. The height profile in Figure 2h presents a thickness of 15–30 nm, indicating the thickness was much smaller than the length of the plates. The HAp plates were then characterized by HRTEM, which indicated that the plate is electron transparent and so must be extremely thin (Figure 3a). In addition, Figure 3c shows the lattice patterns obtained from a magnified image of the highlighted area of the fast Fourier transform (FFT) pattern of Figure 3b, which indicates lattice parameters of a = 0.94 nm and c = 0.68 nm and confirms that the longitudinal direction of the plate is the c-axis.

Figure 2. FESEM images of HAp synthesized using different amounts of DDDA: (a) 1 mmol; (b)



2 mmol; (c) 5 mmol; and (d) 10 mmol. TEM images of the HAp synthesized using (d) 1 mmol and (f) 10 mmol DDDA. (g) Width of the HAp plate as a function of DDDA loading. (h) AFM image of HAp plates synthesized using 2 mmol DDDA.

Figure 3. (a)–(c) HRTEM images of HAp synthesized using 10 mmol DDDA (DDDA-10). (d) FFT pattern obtained from image (b). (c) A magnified image of the highlighted area of (b).

As shown above, the plate-shaped HAp products were successfully prepared, and the sizes of these plates were controlled by varying the quantity of DDDA employed during synthesis. To determine a potential mechanism for plate formation, we investigated the intermediate materials obtained prior to the addition of phosphate. Indeed, the formation of a complex between DDDA and Ca2+ ions was confirmed by IR absorption measurements. Figure 4a shows the IR spectrum of the white precipitate prepared using 10 mmol DDDA, which is denoted by Ca-DDD. Spectra were recorded and analyzed after drying the slurry containing the white precipitate at 60 °C overnight.


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The spectrum of the raw DDDA is denoted as “DDDA.” In this spectrum, the peak at 1698 cm−1 was assigned to the C=O stretching vibration of the carboxylic acid group, while that at 1467 cm−1 was attributed to the CH bending vibration.30 In addition, the peak at 1430 cm−1 was assigned to the C-OH bending vibration. Furthermore, the peaks at 1418 and 1410 cm−1 in the spectra of Ca-DDD and DDDA, respectively, were attributed to symmetric COO− stretching. Upon comparison of the two spectra, it was apparent that an additional three peaks were present in the spectrum of Ca-DDD. These peaks were therefore attributed to the asymmetric COO− stretching vibration,30–32 with the peak at 1625 cm−1 being associated with pseudobridging carboxylate groups.32 In addition, Lu and Miller reported that the presence of two peaks at 1578 and 1543 cm−1 implies the formation of a three-dimensional calcium–carboxylate complex which has unidentate and bidentate carboxylates within its structure.30 These results suggest that the obtained intermediate material (i.e., the white precipitate) is the calcium carboxylate salt, which is formed through the chelation of a single Ca2+ ion to two carboxylate (COO−) groups. Furthermore, Figure 4b shows the XRD patterns of the slurries obtained using different quantities of DDDA. Peaks that could be attributed to d spacing values in the ratios 1:1/2:1/4 were observed, where the d values correspond to the diffraction indices of 001, 002, and 004. This result indicates that the intermediate material has a layered structure. In addition, the d value determined using the various peak positions was 1.79 nm, which is comparable to the value calculated from the sum of the ionic radius of a calcium ion and the length of a DDDA molecule (i.e., dcal = 1.86 nm):33 ݀ୡୟ୪ = 11݀େିେ sin 55° + 2݀େି୓ + 2‫ݎ‬େୟమశ

(1)



where dC-C = 0.154 nm and dC-O = 0.136 nm are the C-C and C-O bond lengths, respectively, and rCa2+ = 0.100 nm is the ionic radius of the Ca2+ ion. This result suggests that the layered structure of

the precipitate consists of a complex between Ca2+ and DDDA (i.e., Ca-DDD). Moreover, the peak at 21.7° can be attributed to an in-plane ordered layered structure, with this peak also being found in the XRD pattern of DDDA.34 The spacing was calculated to be dh00 = 0.41 nm, which is comparable to that of calcium stearate (0.438 nm).35 An increase in the peak intensities attributed to the layered structure upon increasing the DDDA loading suggests that the layered Ca-DDD structure grows in the presence of higher concentrations of DDDA, as a large excess of DDDA (i.e., 1 mmol Ca2+ and 10 mmol DDDA) promotes growth of the calcium–carboxylate complex. The reason the Ca-DDD grew larger as the DDDA quantity increased is unknown because the mechanism of Ca-DDD formation remains unclear. However, precipitations of complexes between calcium and monocarboxylic acids with long alkyl chains could help to discuss the mechanism. Pereira et al. investigated the precipitation of long-chain calcium carboxylates using saturated fatty acids. Their experiments also required excess carboxylic acid for larger precipitates. They reported that the precipitation and growth process consists of several distinct stages.33 In the first stage, calcium ions associate with monocarboxylic acids one by one, thus providing small units of complexes. In the second stage, these units aggregate, and some larger complexes serve as nuclei for further growth. The third stage is the growth of crystalline solids. In our experiment, a larger excess of DDDA would form higher supersaturation in the precipitation, which provides finer crystallites. These finer crystallites promote the crystal growth stage because finer crystallites have a larger surface energy,


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which is the driving force for grain growth. We then observed the obtained Ca-DDD complex by SEM, and the morphology of the precipitate obtained using 10 mmol DDDA is shown in Figure 4c. Interestingly, the shape and size of these particles are comparable to those of the obtained HAp particles. Indeed, a strong correlation was observed between the size of the resulting HAp plates and the growth of Ca-DDD, with larger HAp plates being produced from larger Ca-DDD particles. This result implies that Ca-DDD plays an important role in the size-controlled crystal growth of HAp plates, for example, as a templates or structure-directing agents.

Figure 4. (a) FTIR spectra of DDDA and Ca-DDD. (b) XRD patterns of Ca-DDD prepared using different quantities of DDDA. (c)SEM image of Ca-DDD synthesized using 10 mmol DDDA.



A possible mechanism for the synthesis of the HAp plates using DDDA is outlined schematically in Figure 5. It should be noted here that Sato et al. investigated HAp crystal growth on aligned carboxyl groups using films prepared by the Langmuir–Blodgett (LB) method. Their study revealed that the HAp plates grow with the (100) plane, and that their c-axes are roughly parallel to the interface in SBF.36 As described above, Ca-DDD has a layered structure and is considered to have aligned carboxyl groups, which is similar to the structure of the LB film. In addition, the Ca2+ concentration at the initial stage of synthesis was maintained at a low level (as was the case for SBF in the LB method), as the formation of Ca-DDD suppressed the Ca2+ concentration. Crystal growth on Ca-DDD would therefore be expected to be similar to that on the LB film, as shown in Figure 5a. This figure illustrates the growth of HAp on the Ca-DDD intermediate. The (100) plane of the growing HAp crystal is parallel to the surface of the Ca-DDD. Furthermore, DDDA was also expected to behave as a pH buffer. The acid dissociation constant (pKa) of DDDA was assumed to be ~6 based on the value of sebacic acid (i.e., HO2C−(CH2)8−CO2H, pKa = 4.59, 5.59).37 This would be expected to maintain the pH of the hydrothermal reaction at ~6, which is the optimal value predicted by Viswanath and Ravishankar for the growth of HAp plates under hydrothermal conditions.25 The growth processes are illustrated in Figures 5b and 5c. Upon the addition of phosphate ions to the Ca-DDD suspension, the HAp crystals nucleate heterogeneously on Ca-DDD, and the phosphate ion promotes the release of calcium ions from Ca-DDD into solution. These calcium ions can then be used in the continuous growth of HAp, i.e., Ca-DDD acts as a Ca-reservoir. Furthermore, the


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dependence of the crystal size on the quantity of DDDA employed can be explained by considering that a large excess of DDDA provides large Ca-DDD particles. While the Ca-DDD particles become larger, the total volume of the Ca-DDD does not change because the quantity of calcium used in the synthesis is constant. Therefore, the total surface area of the Ca-DDD particles become smaller as the particles grow larger. The decrease in the surface area implies a decrease in nucleation sites, which leads larger crystals. In addition, these larger Ca-DDD particles will serve as a template for the crystal growth (see Figure 5b). The excess DDDA also functions as an adhesive, which bonds the plates to each other. This process produces the aggregates and laminated structures formed by thin plate stacking, which are shown in Figures 2c, 2d, and 2f. In contrast, when low quantities of DDDA provide smaller Ca-DDD particles, the number of nucleation sites become larger. Thus, these abundant nucleation sites provide smaller crystals (Figure 5c).



Figure 5. Schematic illustrations of the plausible mechanism of HAp plate formation using DDDA.

As shown in Figure 6, we successfully prepared crystallographically-oriented HAp film. The XRD pattern obtained from the film exhibited a peak corresponding to the 300 diffraction, while no peaks assigned to the 002 and 211 diffractions were observed (Figure 6a). In the case where no crystallographic orientation is present, the most intense peak would be expected to be that attributed to the 211 diffraction. However, as shown in Figure 6a, no such peak was observed, thereby


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suggesting that the film has a strong crystallographic orientation. This high orientation was also confirmed by the SEM image (Figure 6b), where HAp plates aligned in parallel were observed. The crystallographic orientation indicates that the film mainly exposes the surfaces parallel to (100) plane. It is known that the exposed crystal faces change the zeta potentials, and additionally behavior of living cells is affected by the exposed surfaces.11,12 The films prepared in this study could be utilized in such biological experiments. In addition, as shown in Figure 6c, the film was optically transparent due to its thin nature. This is particularly advantageous when employing the film for cell culture.

Figure 6. (a) XRD pattern and (b) top-view SEM image of the HAp film prepared on a glass substrate. (c) Photographic image of the HAp film on the glass substrate.

CONCLUSIONS We herein described the successful use of a long-chain dicarboxylic acid (i.e., dodecanedioic acid, DDDA) in the synthesis of hydroxyapatite (HAp) plates of different sizes via a facile hydrothermal treatment method. We found that upon increasing the quantity of DDDA employed in the synthesis,



the HAp particle size also increased, thereby indicating the controllable nature of this preparative route. Interestingly, we found that calcium dodecanedioate (Ca-DDD) formed prior to the addition of a phosphate solution exhibited a layered structure with a rectangular shape. During this process, Ca-DDD was found to act as a nucleation substrate (template), a pH buffer, and a calcium reservoir, thereby allowing us to present a plausible mechanism to explain the formation of these HAp plates. Furthermore, using the synthesized HAp plates, a HAp film with crystallographic orientation was successfully prepared on a glass substrate. This result implies that the prepared HAp plates can be employed to prepare higher-ordered structures. As such, the synthetic approach presented herein, based on the use of a metal-dicarboxylic acid complex as a precursor, constitutes a novel strategy for controlling the morphologies of crystal growth. Moreover, the preparation of HAp plates of various sizes could be considered important for the preparation of bone mimics as orthopedic devices.

ACKNOWLEDGEMENT This work was supported in part by JSPS KAKENHI (grant numbers JP16H05531 and JP17K17691) and the Murata Science Foundation. We thank Dr. K. Miyazawa for his assistance with the HRTEM observations. The TEM and FESEM observations were supported by the Research Center for Medical and Dental Sciences at Tokyo Medical and Dental University.

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For Table of Contents Use Only

Size control synthesis of hydroxyapatite plates and their application in the preparation of highly oriented films

Naohiro Horiuchi, Kotaro Shibata, Hironori Saito, Yuki Iwabuchi, Norio Wada, Kosuke Nozaki, Kazuaki Hashimoto, Yumi Tanaka, Akiko Nagai, and Kimihiro Yamashita

Table of Contents image:

Synopsis: Hydroxyapatite (HAp) plates were synthesized using dodecanedioic acid (DDDA). The plate sizes were controlled by varying the quantity of DDDA employed. We also demonstrated that the synthesized plates could be employed to prepare HAp films with high crystallographic orientations.


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DDDA-05

×5 DDDA-02

×5

DDDA-01

Intensity (arb. unit)

DDDA-10

DDDA-05

DDDA-02

H 2O

DDDA-01

H 2O

ICDD 09-0432

2θ, CuKα (°)


213

222

×5

001 210

100

211 300

DDDA-10

×5

Transmittance (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


310

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Figure 2. FESEM images of HAp synthesized using different amount of DDDA: (a) 1 mmol; (b) 2 mmol; (c) 5 mmol; and (d) 10 mmol. TEM images of the HAp synthesized using (d) 1 mmol and (f) 10 mmol DDDA. (g) Width of the HAp plate as a function of DDDA loading. (h) AFM image of HAp plates synthesized using 2 mmol DDDA. 160x71mm (300 x 300 DPI)


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(a)–(c) HRTEM images of HAp synthesized using 10 mmol DDDA (DDDA-10). (d) FFT pattern obtained from image (b). (c) A magnified image of the highlighted area of (b). 71x71mm (300 x 300 DPI)



Figure 4. (a) FTIR spectra of DDDA and Ca-DDD. (b) XRD patterns of Ca-DDD prepared using different quantities of DDDA. (c)SEM image of Ca-DDD synthesized using 10 mmol DDDA. 79x99mm (300 x 300 DPI)


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(100)

p

HA

HA

p

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ca-DDD

(b) DDDA 10 mmol

(c) DDDA 1 mmol


300

(a)

Glass ICDD 09-0432

211

Coated

002

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (arb. unit)


2θ, CuKα (°)


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Hydroxyapatite (HAp) plates were synthesized using dodecanedioic acid (DDDA). The plate sizes were controlled by variation in the quantity of DDDA employed. We also demonstrated that the synthesized plates could be employed in the preparation of HAp films with high crystallographic orientations. 88x33mm (300 x 300 DPI)


Size control synthesis of hydroxyapatite plates and their application in

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