Materials Science and Engineering C 45 (2014) 29–36
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Hydroxyapatite nanocrystals: Simple preparation, characterization and formation mechanism Fatemeh Mohandes a, Masoud Salavati-Niasari a,b,⁎, Mohammadhossein Fathi c,d, Zeinab Fereshteh c a
Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P. O. Box. 87317-51167, Islamic Republic of Iran Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box 87317-51167, Islamic Republic of Iran Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Islamic Republic of Iran d Dental Materials Research Center, Isfahan University of Medical Sciences, Isfahan, Islamic Republic of Iran b c
a r t i c l e
i n f o
Article history: Received 1 May 2014 Received in revised form 6 August 2014 Accepted 29 August 2014 Available online 6 September 2014 Keywords: Bioceramic Hydroxyapatite Nanocrystal Precipitation
a b s t r a c t Crystalline hydroxyapatite (HAP) nanoparticles and nanorods have been successfully synthesized via a simple precipitation method. To control the shape and particle size of HAP nanocrystals, coordination ligands derived from 2-hydroxy-1-naphthaldehyde were first prepared, characterized by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopies, and finally applied in the synthesis process of HAP. On the other hand, the HAP nanocrystals were also characterized by several techniques including powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). According to the FE-SEM and TEM micrographs, it was found that the morphology and crystallinity of the HAP powders depended on the coordination mode of the ligands. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The use of specially designed ceramics for the repair of damaged parts of the body have improved the quality of life. Excellent biocompatibility, the ability to promote cellular functions, and osteoconductivity are the remarkable properties of bioceramics [1,2]. Bioceramics can be divided into two large groups: bioinert and bioactive ceramics [3]. The bioinert ceramics have almost no influence on the surrounding living tissues like ZrO2 and Al2O3. In contrast, the bioactive ceramics like calcium phosphates are able to bond with living tissues. Hydroxyapatite (Ca10(PO4)6(OH)2; Ca/P (molar ratio) = 1.67) is one of the most attractive calcium phosphates due to its clinical applications. Since HAP has excellent biocompatibility and surface active properties with living tissues, it has become one of the most important materials for artificial bone [4,5]. For bone regeneration, different morphologies of HAP including globular-like and plate-like crystals [6], particle-like nanocrystals [7], one-dimensional nanostructures such as nanorods [8] and porous microspheres [9] have been applied. So far, numerous morphology and size-controlled synthesis methods based on a precipitation approach by using macromolecules have been
⁎ Corresponding author at: Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P. O. Box 87317-51167, Islamic Republic of Iran. Tel.: +98 31 55912383; fax: +98 31 55552935. E-mail address: [email protected] (M. Salavati-Niasari).
http://dx.doi.org/10.1016/j.msec.2014.08.058 0928-4931/© 2014 Elsevier B.V. All rights reserved.
introduced for the synthesis of HAP nanocrystals. Wang et al. synthesized HAP nanorods at 200 °C for 8 h in the presence of citric acid, sodium dodecyl sulfate, and sodium dodecylbenzene sulfonate as organic modifiers [10]. Sadat-Shojai and his co-workers introduced a chemical precipitation method followed by a hydrothermal method for large-scale production of HAP at 200 °C after heating for 60 h [11]. Herein, we report a novel precipitation route with the aid of Schiff base compounds to control particle size and morphology of HAP nanocrystals. In the present method, the starting reagents were heated at 120 °C for 5 h to fabricate pure monophasic HAP nanostructures. By comparing the available methods to prepare HAP crystals with the present method, it is found that this precipitation method has several advantages such as short reaction time and low reaction temperature. Schiff base compounds, containing an imine or azomethine group (\RC_N\), were discovered by Hugo Schiff [12]. The electrophilic carbon atoms of aldehydes and ketones can be targets of nucleophilic attack by amines. The result of this reaction is a compound in which the C_O double bond is replaced by a C_N double bond. In this synthesis procedure, the coordination ligands derived from 2-hydroxy-1naphthaldehyde and different diamines were prepared, characterized, and then used to produce HAP nanocrystals with different morphologies. The products were analyzed by techniques such as Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (1H-NMR), powder X-ray diffraction (XRD), fieldemission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The obtained results showed that pure monophasic HAP nanocrystals were produced by this method.
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2. Materials and methods 2.1. Materials All the chemicals with analytical grade were used as received. 2Hydroxy-1-naphthaldehyde, ethylenediamine, o-phenylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,6-diaminohexane, 1,8diamino-3,6-dioxaoctane, triethyltetramine, calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), diammonium hydrogen phosphate ((NH4)2HPO4), sodium hydroxide (NaOH) methanol and chloroform were purchased from Merck Co. 2.2. Synthesis of coordination ligands According to Scheme 1, the compounds 2a, 2b, 3a, 4a, 6a, 8a and 8b were prepared by reaction between 2 mol of 2-hydroxy-1naphthaldehyde and 1 mol of the corresponding diamine in 50 mL of methanol. The mixture was refluxed and heated at 70 °C for 3 h. After evaporating the solvent, crystallization was induced by adding petrol ether. Afterward, the crystals obtained were filtered on a Buchner funnel. In Table 1, the corresponding diamine and yield of each compound have been illustrated.
Table 1 Experimental results of the as-synthesized coordination ligands. Ligand no.
Corresponding diamine
Yield (%)
2a 2b 3a 4a 6a 8a 8b
Ethylenediamine o-Phenylenediamine 1,3-Diaminopropane 1,4-Diaminobutane 1,6-Diaminohexane 1,8-Diamino-3,6-dioxaoctane Triethyltetramine
85.0 83.5 83.0 87.2 92.5 83.0 82.5
dropwise into the ammonium hydrogen phosphate solution to adjust the pH of the solution to 12. At this time, the ammonium hydrogen phosphate solution was added slowly at a rate of 5 mL/min into the calcium solution. During the addition of the ammonium hydrogen phosphate solution into the calcium solution, a yellow-colored suspension including white colloids was obtained. A round-bottom flask loaded
2.3. Synthesis of HAP nanocrystals Generally, HAP nanocrystals were prepared by a reaction between a calcium–ligand solution and an ammonium hydrogen phosphate solution. In the all experiments, the molar ratios of Ca/ligand and Ca/P were 1.0 and 1.67, respectively. Details of the synthesis process are described. At first, 10 mmol of coordination ligand was dissolved in 40 mL of methanol at 40 °C under a constant stirring of 1000 rpm. When the ligand was completely dissolved in methanol, 10 mmol of Ca(NO3) 2·4H2O was added into the solution. After that, additional stirring was performed for 30 min, and finally a yellow clear solution was obtained. To prepare the ammonium hydrogen phosphate solution, 6 mmol of (NH4)2HPO4 was dissolved in 40 mL of distilled water under magnetic stirring. A sodium hydroxide (NaOH) solution (0.1 M) was added
Scheme 1. Chemical structures of the as-synthesized coordination ligands.
Fig. 1. 1H-NMR spectra of the as-synthesized ligands: (a) 2b; (b) 3a; (c) 4a; (d) 6a; (e) 8a; and (f) 8b.
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with the resulting mixture was kept in an oil bath. The reaction mixture was heated at 120 °C for 5 h. After cooling the reaction mixture to room temperature, the final precipitates were collected by centrifugation at 4500 rpm for 5 min, and washed three times with distilled water, methanol, and chloroform, sequentially. The white precipitates obtained were dried at 50 °C for 5 h. 2.4. Sample characterization Fourier transform infrared (FT-IR) spectra were obtained on a Shimadzu IR-460 spectrometer in KBr pellets in the range of 400–4000 cm−1. Proton nuclear magnetic resonance (1H-NMR) spectra were obtained on a BRUKER (400 MHz) spectrometer. Proton chemical shifts are reported in ppm relative to an internal standard of Me4Si. Powder X-ray diffraction (XRD) patterns were collected from a diffractometer of Philips Company with X'Pert Pro monochromatized Cu Kα radiation (λ = 1.54 Å, operated on 35 mA and 40 kV current). The diffraction angles (2θ) were scanned from 10○ to 80○. SEM micrographs were taken by using a field-emission scanning electron microscope (HITACHI S4160, Japan). Transmission electron images (TEM) were recorded on a JEM-2100 with an accelerating voltage of 200 kV equipped with a high resolution CCD Camera. To prepare the samples for TEM and SEM analyses, the grids were first covered by a Au film, and then a small amount of sample was placed onto the grids covered by Au. For TEM analysis, a small amount of sample was first dispersed in 20 mL of ethanol by sonication for 20 min, and then the grid was treated with the solution. For SEM analysis, the sputter coating was the process of applying an ultra-thin coating of Au onto the samples. 3. Results and discussion 3.1. 1H-NMR and FT-IR studies of coordination ligands
Fig. 2. FT-IR spectra of the as-synthesized ligands: (a) 2a; (b) 2b; (c) 3a; (d) 4a; (e) 6a; (f) 8a; and (g) 8b.
Chemical structures of the coordination ligands shown in Scheme 1 were studied by NMR. The 1H-NMR spectra of these compounds are seen in Fig. 1. Since the compound 2a was insoluble in DMSO-d6, CDCl3, CO(CD3)2 and their mixtures, we could not study its chemical structure by NMR spectroscopy. Details of the NMR spectra for the compounds 2b, 3a, 4a, 6a, 8a and 8b are as follows: 1H-NMR data of the compound 2b (Fig. 1a, CDCl3, δ, ppm): 7.184 (d, 2H aromatic), 7.356 (t, 2H aromatic), 7.420 (s, 4H aromatic), 7.522 (t, 2H aromatic), 7.736 (d, 2H aromatic), 7.823 (d, 2H aromatic), 8.142 (d, 2H aromatic), 9.467 (s, 2H, 2 × HC = N), and 15.100 (s, 2H, 2 × OH); compound 3a (Fig. 1b, CDCl3, δ, ppm): 2.204 (s, 2H, CH2), 3.748 (s, 4H, 2 × CH2), 6.911 (d, 2H aromatic), 7.008 (d, 2H aromatic), 7.457 (t, 2H aromatic), 7.569 (t, 2H aromatic), 7.635 (d, 2H aromatic), 7.979 (s, 2H aromatic), 8.437 (d, 2H, 2 × HC = N), and 13.823 (s, 2H, 2 × OH); compound 4a (Fig. 1c, CDCl3, δ, ppm): 1.938 (s, 4H, 2 × CH2), 3.713 (s, 4H, 2 × CH2), 6.946 (d, 2H aromatic), 7.250 (t, 2H aromatic), 7.435 (t, 2H aromatic), 7.626 (d, 2H aromatic), 7.702 (d, 2H aromatic), 7.877 (d, 2H aromatic), 8.793 (d, 2H, 2 × HC = N), and 14.653 (s, 2H, 2 × OH); compound 6a (Fig. 1d, CDCl3, δ, ppm): 1.512 (s, 4H, 2 × CH2), 1.775 (s, 4H, 2 × CH2), 3.561 (s, 4H, 2 × CH2), 6.780 (d, 2H aromatic), 6.930 (d, 2H aromatic),
Table 2 Experimental results of the as-prepared HAP nanocrystals. HAP samples
Coordination ligand
Morphology
Particle size (nm)
Yield (%)
HAP1 HAP2 HAP3 HAP4 HAP5 HAP6 HAP7
2a 2b 3a 4a 6a 8a 8b
Sponge-like structures composed of nanoparticles Particle-like Rod-like Rod-like Rod-like Rod-like Scale-like
80–110 20–25 (150–200) (200–250) (200–300) (120–200) (100–120)
98.0 97.5 97.5 97.3 98.5 97.0 97.5
× × × × ×
(40–55) (50–55) (50–70) (40–45) (50–70)
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Fig. 4. FE-SEM images of HAP nanocrystals: (a) HAP1 and (b) HAP2.
Fig. 3. FT-IR spectra of HAP nanocrystals: (a) HAP1; (b) HAP2; (c) HAP3; (d) HAP4; (e) HAP5; (f) HAP6; and (g) HAP7.
2.594 (t, 4H, 2 × CH2), 3.742 (d, 4H, 2 × CH2), 6.683 (d, 2H aromatic), 7.170 (t, 2H aromatic), 7.416 (t, 2H aromatic), 7.614 (d, 2H aromatic), 7.702 (d, 2H aromatic), 8.024 (d, 2H aromatic), 9.049 (d, 2H, 2 × HC = N), and 13.793 (s, 2H, 2 × OH). The 1H-NMR results prove the chemical structures of the coordination ligands presented in Scheme 1. Fig. 2a–g shows the FT-IR spectra of the compounds 2a, 2b, 3a, 4a, 6a, 8a and 8b, respectively. All of these spectra depict a strong peak corresponding to the v(C_N), which proves the formation of the Schiff base compounds [13]. The stretching vibrations of the C_C bond in aromatic rings are seen in the range of 1470–1540 cm−1. The broad adsorption peaks in the range of 3400\3600 cm−1 can be attributed to the stretching vibration of OH. Although we could not investigate the chemical structure of the compound 2a by NMR spectroscopy due to its insolubility in the NMR solvents, the observation of the v(C_N) at 1643 cm−1 in the FT-IR spectrum of the compound 2a (Fig. 2a) proved its chemical structure. 3.2. FT-IR, FE-SEM, TEM and XRD studies of HAP nanocrystals
7.427 (t, 2H aromatic), 7.568 (quartet, 4H aromatic), 7.798 (d, 2H aromatic), 8.453 (d, 2H, 2 × HC = N), and 13.499 (s, 2H, 2 × OH); compound 8a (Fig. 1e, CDCl3, δ, ppm): 3.639 (d, 8H, 4 × CH2), 3.697 (t, 4H, 2 × CH2), 6.901 (d, 2H aromatic), 7.212 (t, 2H aromatic), 7.396 (t, 2H aromatic), 7.586 (d, 2H aromatic), 7.662 (d, 2H aromatic), 7.804 (d, 2H aromatic), 8.675 (s, 2H, 2 × HC = N), and 14.298 (s, 2H, 2× OH); and compound 8b (Fig. 1f, DMSO-d6, δ, ppm): 2.510 (s, 4H, 2 × CH2),
Table 2 gives a list of the preparation conditions that we have tested to synthesize HAP nanocrystals with the aid of different coordination ligands. As shown in the FT-IR spectra of the samples HAP1–7 (Fig. 3), all the characteristic peaks of HAP are observed. The weak adsorption band located at around 472 cm− 1 is corresponding to the symmetric stretching vibration (v2) of the PO3− ions, while the bands at 4
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Fig. 6. FE-SEM images of HAP nanocrystals: (a) HAP6 and (b) HAP7.
Fig. 5. FE-SEM images of HAP nanocrystals: (a) HAP3; (b) HAP4; and (c) HAP5.
around 557–568 cm− 1 are attributed to the bending vibrations (v4) −1 of the PO3− are de4 ions. The bands centered around 1030–1045 cm rived from the asymmetric stretching vibration (v3) of the PO3− 4 ions. The frequency of the symmetric stretching vibration (v1) of the PO3− 4 ions appeared at lower wavenumbers rather than the asymmetric stretching vibration. Moreover, the stretching vibration of the OH− ions in the HAP lattice is seen at 3570 cm−1 [14,15]. The broad adsorption band at 3100–3500 cm−1 (stretching vibration) and the weak band
at 1630–1637 cm−1 (bending vibration) are attributed to the crystal water and surface adsorbed water. In addition, the presence of the −1 v(CO2− are observed in the 3 ) vibrations at around 1383–1458 cm FT-IR spectra of the products [16]. The carbonate ions might come from the atmosphere carbon dioxide which combined into the crystal structure during the dissolving and stirring processes [17]. This evidence may improve the biological activity of the carbonated HAP, which is essential for bonding with natural bone [18]. The CO2− ions 3 can be introduced into the structure of hydroxyapatite for the substitution of the PO3− ions (type B). On the other hand, type A represents 4 the substitution of OH– ions by CO2− ions [19,20]. By considering 3 the IR bands attributed to the v(CO2− 3 ) vibrations at around 1383–1458 cm −1, it is found that the B-type carbonated hydroxyapatite has been formed during the synthesis route. Based on the results of energy dispersive spectrometry, the Ca/P ratio in the final products is higher than the stoichiometric ratio of Ca/P in HAP (1.67) due to the re2− placement of some PO3− 4 ions by CO3 ions [21]. FE-SEM images of the samples HAP1–7 were studied to find the effect of different coordination ligands on the morphology of the products. As shown in Fig. 4a, sponge-like structures composed of nanoparticles are produced by compound 2a. By using compound 2b instead of compound 2a, homogeneous nanoparticles are formed because of the high steric hindrance effect of compound 2b (Fig. 4b). In Fig. 5a–c, FE-SEM images of the samples HAP3–5 synthesized with the aid of compounds 3a, 4a and 6b are observed. By comparing the presented images in Fig. 5a–c, it can be concluded that
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one-dimensional (1-D) nanocrystals of HAP including nanorods are produced by increasing the CH2 groups between two \C_N\ bands. In addition, the dimensions of the nanorods increased by increasing the CH2 groups between the \C_N\ groups in the used ligands. As seen in Fig. 6a and b, rod-like and scale-like nanocrystals of HAP are prepared by using compounds 8a and 8b, respectively. Particle sizes of the samples HAP1–7 were illustrated in Table 2. The morphological feature of the sample HAP2 is given in the TEM images of Fig. 7. The TEM images show that morphology of the sample HAP2 is particle-like nanostructures with particle sizes in the range of 15–20 nm.\20 nm. The crystalline structures of the as-synthesized HAP nanocrystals were confirmed by XRD. Fig. 8 shows the XRD pattern of the sample HAP2. All the diffraction peaks can be indexed to the hexagonal phase hydroxyapatite with the space group of P63/m and cell constants a = b = 9.4180 Å and c = 6.8840 Å (JCPDS: 09-0432). In this pattern, no XRD peaks can be observed for the presence of other calcium phosphate phases in the products. The XRD patterns of the samples HAP4 and HAP5 are shown in Fig. 9a and b, respectively. The broader peaks with low intensities can be attributed to the hexagonal phase hydroxyapatite with the space group of P63/m and cell constants a = b = 9.4240 Å, and c = 6.8790 Å (JCPDS: 74-0565). By comparing the XRD patterns in Figs. 8 and 9, it is clear that the crystallinity degree of the sample HAP2 is more than that of the samples HAP4 and HAP5 due to the sharp peaks in Fig. 8. The relationship between lattice distance (λ = 2dsinθ) and lattice parameters (a = b, and c) of the hexagonal structure is expressed as follows [22]: 2 2 2 2 2 2 1=d ¼ ð4=3Þ h þ hk þ k =a þ l =c
Fig. 7. (a, b) TEM images of HAP nanoparticles (sample HAP2).
ð1Þ
where h, k, and l as reflection planes (Miller index) and d as lattice distance are known. The lattice parameters of the samples HAP2, HAP4 and HAP5 were also calculated. According to Table 3, it is concluded that the c-axis of the unit cells in the samples HAP4 and HAP5 is longer than that of the sample HAP2. This finding exhibits the crystal growth direction of the nanorods along the c-axis. Moreover, the decrease and increase of a-axis and c-axis values for the carbonated hydroxyapatite can be expected due to the substitution of the tetragonal PO3− 4 ions with the trigonal planar CO2− 3 ions [23,24]. The XRD results coincide with the FT-IR
Fig. 8. XRD pattern of HAP nanoparticles (sample HAP2).
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Fig. 9. XRD patterns of HAP nanorods: (a) sample HAP4 and (b) sample HAP5.
analyses. The volume of the hexagonal unit cell for the samples was calculated by Eq. (2) and seen in Table 3 [25]. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 ¼ 3=2a2 c V A˚
ð2Þ
3.3. Possible formation and crystal growth mechanism of HAP nanocrystals The possible formation and crystal growth mechanism of HAP nanocrystals can be assumed as follows. At first, the in situ formation of the [Ca–ligand]2+ complexes may take place by a reaction between Ca(NO3)2·4H2O and the coordination ligand in methanol. Then, the nucleation of HAP can be induced by adding the (NH4)2HPO4 solution into the [Ca–ligand]2+ solution at a pH of 12. The HAP nuclei capped by the coordination ligands grow during the thermal treatment process at 150 °C. At last, pure HAP nanocrystals are obtained after washing by distilled water, methanol and chloroform, sequentially.
Table 3 Lattice parameters and volume of the hexagonal unit cell for HAP nanocrystals. HAP samples
XRD pattern
a = b (Å)
c (Å)
V (Å3)
HAP2 09-0432 HAP4 HAP5 74-0565
Fig. 8 – Fig. 9a Fig. 9b –
9.437 9.418 9.013 9.139 9.424
6.890 6.884 6.960 6.984 6.879
531.36 528.80 489.64 505.16 529.09
On the basis of the FE-SEM images, it is concluded that the shape of the HAP nanocrystals is dependent on the coordination mode of the assynthesized coordination ligands and stability of their complexes. Here, two main coordination modes may be assumed to grow the 0-D and 1-D HAP nanocrystals. In the first coordination mode, the ligand is introduced as a tetradentate chelating ligand to form [CaN2O2]2+ complexes. In the second one, the ligand can play as a bidentate chelating ligand to form [CaN2]2+, [CaO2]2+ and [CaNO]2+ complexes. In the tetradentate unit which can be assumed for the compounds 2a and 2b, the ligand can wrap itself around the Ca2+ ion with two nitrogen and two oxygen atoms to form six-membered chelate rings. In fact, in this coordination mode, all aspects of HAP nuclei are capped by the ligands, and the 0-D HAP nanocrystals obtained. In the bidentate units suggested for the compounds 3a, 4a, 6a, 8a and 8b, the Ca2+ ion can interact with the complexing agents through nitrogen and oxygen atoms to form fivemembered and six-membered chelate rings. When the ligand 8a is used as a complexing agent, two oxygen atoms can participate to design five-membered chelate rings. The proposed formation mechanism of HAP nanocrystals was illustrated in Scheme 2. Although the use of coordination ligands to control the shape and particle size of various nanocrystals has been developed [26–30], this work is the first successful attempt in HAP synthesis by using these compounds. 4. Conclusions In summary, a novel and simple precipitation route has been developed to prepare HAP nanoparticles and nanorods. The coordination ligands derived from 2-hydroxy-1-naphthaldehyde were applied to control shapes and particle sizes of HAP nanocrystals. Based on the
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Scheme 2. Possible coordination modes of the as-synthesized ligands to form 1-D and 0-D HAP nanocrystals.
coordination modes proposed for the as-synthesized ligands, it was found that the formation of tetradentate and bidentate units led to the fabrication of 0-D (nanoparticles) and 1-D (nanorods) HAP nanocrystals. Acknowledgments The authors are grateful to the council of the University of Kashan for providing financial support to undertake this work by Grant No. 159271/163. References [1] L.L. Hench, Bioceramics, J. Am. Ceram. Soc. 81 (1998) 1705–1728. [2] T. Yamamuro, L.L. Hench, J. Wilson, Handbook of bioactive ceramics, Calcium Phosphate and Hydroxyapatite Ceramics, vol. 2, CRC Press, Boca Raton, FL, 1990. 3. [3] M. Vallet-Regi, J.M. Gonzalez-Calbet, Calcium phosphates as substitution of bone tissues, Prog. Solid State Chem. 32 (2004) 1–31. [4] S.I. Roohani-Esfahani, S. Nouri-Khorasani, Z.F. Lu, M.H. Fathi, M. Razavi, R.C. Appleyard, H. Zreiqat, Modification of porous calcium phosphate surfaces with different geometries of bioactive glass nanoparticles, Mater. Sci. Eng. C 32 (2012) 830–839. [5] M.H. Fathi, A. Hanifi, V. Mortazavi, Preparation and bioactivity evaluation of bonelike hydroxyapatite nanopowder, J. Mater. Process. Technol. 202 (2008) 536–542. [6] M.M. Zogbi Jr., E. Saito, H. Zanin, F.R. Marciano, A.O. Lobo, Hydrothermal– electrochemical synthesis of nano-hydroxyapatite crystals on superhydrophilic vertically aligned carbon nanotubes, Mater. Lett. 132 (2014) 70–74. [7] T. Liu, X. Ding, X. Yang, Z. Gou, J. Chen, X. Feng, Effect of carbonization treatment on the morphology and microstructure of mesoporous bioactive glass/nanocrystalline hydroxyapatite nanocomposite, J. Non-Cryst. Solids 389 (2014) 104–112. [8] Z. Fan, J. Wang, Z. Wang, Y. Li, H. Ran, L. Niu, P. Gong, B. Liu, S. Yang, One-pot synthesis of graphene/hydroxyapatite nanorod composite for tissue engineering, Carbon 66 (2014) 407–416. [9] K. Lin, P. Liu, L. Wei, Z. Zou, W. Zhang, Y. Qian, Y. Shen, J. Chang, Strontium substituted hydroxyapatite porous microspheres: surfactant-free hydrothermal synthesis, enhanced biological response and sustained drug release, Chem. Eng. J. 222 (2013) 49–59. [10] A. Wang, D. Liu, H. Yin, H. Wu, Y. Wada, M. Ren, T. Jiang, X. Cheng, Y. Xu, Sizecontrolled synthesis of hydroxyapatite nanorods by chemical precipitation in the presence of organic modifiers, Mater. Sci. Eng. C 27 (2007) 865–869. [11] M. Sadat-Shojai, M.-T. Khorasani, A. Jamshidi, Hydrothermal processing of hydroxyapatite nanoparticles—a Taguchi experimental design approach, J. Cryst. Growth 361 (2012) 73–84. [12] R.H. Holm, G.W. Everett Jr., A. Chakravorty, Metal complexes of Schiff bases and βketoamines, Prog. Inorg. Chem. 7 (1989) 83–214.
[13] M. Salavati-Niasari, M. Bazarganipour, Synthesis, characterization and alcohol oxidation properties of multi-wall carbon nanotubes functionalized with a cobalt(II) Schiff base complex, Transit. Met. Chem. 34 (2009) 605–612. [14] J.S. Cho, S.-H. Rhee, Formation mechanism of nano-sized hydroxyapatite powders through spray pyrolysis of a calcium phosphate solution containing polyethylene, J. Eur. Ceram. Soc. 33 (2013) 233–241. [15] V.K. Mishra, S.K. Srivastava, B.P. Asthana, D.J. Kumar, Structural and spectroscopic studies of hydroxyapatite nanorods formed via microwave-assisted synthesis route, J. Am. Ceram. Soc. 95 (2012) 2709–2715. [16] P. Layrolle, A. Ito, T. Tateishi, Sol–gel synthesis of amorphous calcium phosphate and sintering into microporous hydroxyapatite bioceramics, J. Am. Ceram. Soc. 81 (1998) 1421–1428. [17] Y.J. Wang, J.D. Chen, K. Wei, S.H. Zhang, X.D. Wang, Surfactant-assisted synthesis of hydroxyapatite particles, Mater. Lett. 60 (2006) 3227–3231. [18] P. Li, C. Ohtsuki, T. Kokubo, The role of hydrated silica, titania, and alumina in inducing apatite on implants, J. Biomed. Mater. Res. 28 (1994) 7–15. [19] M.E. Fleet, Infrared spectra of carbonate apatites: v2-region bands, Biomaterials 30 (2009) 1473–1481. [20] M.E. Fleet, X. Liu, Location of type B carbonate ion in type A–B carbonate apatite synthesized at high pressure, J. Solid State Chem. 177 (2004) 3174–3182. [21] F. Mohandes, M. Salavati-Niasari, Particle size and shape modification of hydroxyapatite nanostructures synthesized via a complexing agent-assisted route, Mater. Sci. Eng. C 40 (2014) 288–298. [22] W.J. Shih, Y.F. Chen, M.C. Wang, M.H. Hon, Crystal growth and morphology of the nano-sized hydroxyapatite powders synthesized from CaHPO4·2H2O and CaCO3 by hydrolysis method, J. Cryst. Growth 270 (2004) 211–218. [23] R.Z. Legeros, O.R. Trautz, J.P. Legeros, E. Klein, W.P. Shirra, Apatite crystallites: effects of carbonate on morphology, Science 155 (1967) 1409–1411. [24] R.Z. Legeros, Apatites in biological systems, Prog. Cryst. Growth Charact. 41 (1981) 1–45. [25] P. Minh, H. Sebei, A. Nzihou, P. Sharrock, Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution, Chem. Eng. J. 198–199 (2012) 180–190. [26] S. Ahmadian-Fard-Fini, M. Salavati-Niasari, F. Mohandes, Synthesis and characterization of PbTe micro/nanostructures through hydrothermal method by using a novel capping agent, Bull. Mater. Sci. 37 (2014) 1–7. [27] S. Ahmadian-Fard-Fini, M. Salavati-Niasari, F. Mohandes, Sonochemical and hydrothermal synthesis of PbTe nanocrystals with the aid of a novel capping agent, Mater. Res. Bull. 48 (2013) 4332–4338. [28] M. Salavati-Niasari, B. Shoshtari-Yeganeh, F. Mohandes, Schiff-base assisted synthesis of lead selenide nanocrystals, Mater. Res. Bull. 48 (2013) 1745–1752. [29] F. Tavakoli, M. Salavati-Niasari, F. Mohandes, Sonochemical synthesis and characterization of lead iodide hydroxide micro/nanocrystals, Ultrason. Sonochem. 21 (2014) 234–241. [30] S. Ahmadian-Fard-Fini, M. Salavati-Niasari, A. Monfared, F. Mohandes, PbTe nanocrystals: microwave-assisted synthesis by using lead Schiff-base precursor, characterization and formation mechanism, C. R. Chim. 16 (2013) 778–788.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Hydroxyapatite nanocrystals: Simple preparation, characterization and formation mechanism Fatemeh Mohandes a, Masoud Salavati-Niasari a,b,⁎, Mohammadhossein Fathi c,d, Zeinab Fereshteh c a
Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P. O. Box. 87317-51167, Islamic Republic of Iran Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box 87317-51167, Islamic Republic of Iran Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Islamic Republic of Iran d Dental Materials Research Center, Isfahan University of Medical Sciences, Isfahan, Islamic Republic of Iran b c
a r t i c l e
i n f o
Article history: Received 1 May 2014 Received in revised form 6 August 2014 Accepted 29 August 2014 Available online 6 September 2014 Keywords: Bioceramic Hydroxyapatite Nanocrystal Precipitation
a b s t r a c t Crystalline hydroxyapatite (HAP) nanoparticles and nanorods have been successfully synthesized via a simple precipitation method. To control the shape and particle size of HAP nanocrystals, coordination ligands derived from 2-hydroxy-1-naphthaldehyde were first prepared, characterized by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopies, and finally applied in the synthesis process of HAP. On the other hand, the HAP nanocrystals were also characterized by several techniques including powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). According to the FE-SEM and TEM micrographs, it was found that the morphology and crystallinity of the HAP powders depended on the coordination mode of the ligands. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The use of specially designed ceramics for the repair of damaged parts of the body have improved the quality of life. Excellent biocompatibility, the ability to promote cellular functions, and osteoconductivity are the remarkable properties of bioceramics [1,2]. Bioceramics can be divided into two large groups: bioinert and bioactive ceramics [3]. The bioinert ceramics have almost no influence on the surrounding living tissues like ZrO2 and Al2O3. In contrast, the bioactive ceramics like calcium phosphates are able to bond with living tissues. Hydroxyapatite (Ca10(PO4)6(OH)2; Ca/P (molar ratio) = 1.67) is one of the most attractive calcium phosphates due to its clinical applications. Since HAP has excellent biocompatibility and surface active properties with living tissues, it has become one of the most important materials for artificial bone [4,5]. For bone regeneration, different morphologies of HAP including globular-like and plate-like crystals [6], particle-like nanocrystals [7], one-dimensional nanostructures such as nanorods [8] and porous microspheres [9] have been applied. So far, numerous morphology and size-controlled synthesis methods based on a precipitation approach by using macromolecules have been
⁎ Corresponding author at: Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P. O. Box 87317-51167, Islamic Republic of Iran. Tel.: +98 31 55912383; fax: +98 31 55552935. E-mail address: [email protected] (M. Salavati-Niasari).
http://dx.doi.org/10.1016/j.msec.2014.08.058 0928-4931/© 2014 Elsevier B.V. All rights reserved.
introduced for the synthesis of HAP nanocrystals. Wang et al. synthesized HAP nanorods at 200 °C for 8 h in the presence of citric acid, sodium dodecyl sulfate, and sodium dodecylbenzene sulfonate as organic modifiers [10]. Sadat-Shojai and his co-workers introduced a chemical precipitation method followed by a hydrothermal method for large-scale production of HAP at 200 °C after heating for 60 h [11]. Herein, we report a novel precipitation route with the aid of Schiff base compounds to control particle size and morphology of HAP nanocrystals. In the present method, the starting reagents were heated at 120 °C for 5 h to fabricate pure monophasic HAP nanostructures. By comparing the available methods to prepare HAP crystals with the present method, it is found that this precipitation method has several advantages such as short reaction time and low reaction temperature. Schiff base compounds, containing an imine or azomethine group (\RC_N\), were discovered by Hugo Schiff [12]. The electrophilic carbon atoms of aldehydes and ketones can be targets of nucleophilic attack by amines. The result of this reaction is a compound in which the C_O double bond is replaced by a C_N double bond. In this synthesis procedure, the coordination ligands derived from 2-hydroxy-1naphthaldehyde and different diamines were prepared, characterized, and then used to produce HAP nanocrystals with different morphologies. The products were analyzed by techniques such as Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (1H-NMR), powder X-ray diffraction (XRD), fieldemission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The obtained results showed that pure monophasic HAP nanocrystals were produced by this method.
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2. Materials and methods 2.1. Materials All the chemicals with analytical grade were used as received. 2Hydroxy-1-naphthaldehyde, ethylenediamine, o-phenylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,6-diaminohexane, 1,8diamino-3,6-dioxaoctane, triethyltetramine, calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), diammonium hydrogen phosphate ((NH4)2HPO4), sodium hydroxide (NaOH) methanol and chloroform were purchased from Merck Co. 2.2. Synthesis of coordination ligands According to Scheme 1, the compounds 2a, 2b, 3a, 4a, 6a, 8a and 8b were prepared by reaction between 2 mol of 2-hydroxy-1naphthaldehyde and 1 mol of the corresponding diamine in 50 mL of methanol. The mixture was refluxed and heated at 70 °C for 3 h. After evaporating the solvent, crystallization was induced by adding petrol ether. Afterward, the crystals obtained were filtered on a Buchner funnel. In Table 1, the corresponding diamine and yield of each compound have been illustrated.
Table 1 Experimental results of the as-synthesized coordination ligands. Ligand no.
Corresponding diamine
Yield (%)
2a 2b 3a 4a 6a 8a 8b
Ethylenediamine o-Phenylenediamine 1,3-Diaminopropane 1,4-Diaminobutane 1,6-Diaminohexane 1,8-Diamino-3,6-dioxaoctane Triethyltetramine
85.0 83.5 83.0 87.2 92.5 83.0 82.5
dropwise into the ammonium hydrogen phosphate solution to adjust the pH of the solution to 12. At this time, the ammonium hydrogen phosphate solution was added slowly at a rate of 5 mL/min into the calcium solution. During the addition of the ammonium hydrogen phosphate solution into the calcium solution, a yellow-colored suspension including white colloids was obtained. A round-bottom flask loaded
2.3. Synthesis of HAP nanocrystals Generally, HAP nanocrystals were prepared by a reaction between a calcium–ligand solution and an ammonium hydrogen phosphate solution. In the all experiments, the molar ratios of Ca/ligand and Ca/P were 1.0 and 1.67, respectively. Details of the synthesis process are described. At first, 10 mmol of coordination ligand was dissolved in 40 mL of methanol at 40 °C under a constant stirring of 1000 rpm. When the ligand was completely dissolved in methanol, 10 mmol of Ca(NO3) 2·4H2O was added into the solution. After that, additional stirring was performed for 30 min, and finally a yellow clear solution was obtained. To prepare the ammonium hydrogen phosphate solution, 6 mmol of (NH4)2HPO4 was dissolved in 40 mL of distilled water under magnetic stirring. A sodium hydroxide (NaOH) solution (0.1 M) was added
Scheme 1. Chemical structures of the as-synthesized coordination ligands.
Fig. 1. 1H-NMR spectra of the as-synthesized ligands: (a) 2b; (b) 3a; (c) 4a; (d) 6a; (e) 8a; and (f) 8b.
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with the resulting mixture was kept in an oil bath. The reaction mixture was heated at 120 °C for 5 h. After cooling the reaction mixture to room temperature, the final precipitates were collected by centrifugation at 4500 rpm for 5 min, and washed three times with distilled water, methanol, and chloroform, sequentially. The white precipitates obtained were dried at 50 °C for 5 h. 2.4. Sample characterization Fourier transform infrared (FT-IR) spectra were obtained on a Shimadzu IR-460 spectrometer in KBr pellets in the range of 400–4000 cm−1. Proton nuclear magnetic resonance (1H-NMR) spectra were obtained on a BRUKER (400 MHz) spectrometer. Proton chemical shifts are reported in ppm relative to an internal standard of Me4Si. Powder X-ray diffraction (XRD) patterns were collected from a diffractometer of Philips Company with X'Pert Pro monochromatized Cu Kα radiation (λ = 1.54 Å, operated on 35 mA and 40 kV current). The diffraction angles (2θ) were scanned from 10○ to 80○. SEM micrographs were taken by using a field-emission scanning electron microscope (HITACHI S4160, Japan). Transmission electron images (TEM) were recorded on a JEM-2100 with an accelerating voltage of 200 kV equipped with a high resolution CCD Camera. To prepare the samples for TEM and SEM analyses, the grids were first covered by a Au film, and then a small amount of sample was placed onto the grids covered by Au. For TEM analysis, a small amount of sample was first dispersed in 20 mL of ethanol by sonication for 20 min, and then the grid was treated with the solution. For SEM analysis, the sputter coating was the process of applying an ultra-thin coating of Au onto the samples. 3. Results and discussion 3.1. 1H-NMR and FT-IR studies of coordination ligands
Fig. 2. FT-IR spectra of the as-synthesized ligands: (a) 2a; (b) 2b; (c) 3a; (d) 4a; (e) 6a; (f) 8a; and (g) 8b.
Chemical structures of the coordination ligands shown in Scheme 1 were studied by NMR. The 1H-NMR spectra of these compounds are seen in Fig. 1. Since the compound 2a was insoluble in DMSO-d6, CDCl3, CO(CD3)2 and their mixtures, we could not study its chemical structure by NMR spectroscopy. Details of the NMR spectra for the compounds 2b, 3a, 4a, 6a, 8a and 8b are as follows: 1H-NMR data of the compound 2b (Fig. 1a, CDCl3, δ, ppm): 7.184 (d, 2H aromatic), 7.356 (t, 2H aromatic), 7.420 (s, 4H aromatic), 7.522 (t, 2H aromatic), 7.736 (d, 2H aromatic), 7.823 (d, 2H aromatic), 8.142 (d, 2H aromatic), 9.467 (s, 2H, 2 × HC = N), and 15.100 (s, 2H, 2 × OH); compound 3a (Fig. 1b, CDCl3, δ, ppm): 2.204 (s, 2H, CH2), 3.748 (s, 4H, 2 × CH2), 6.911 (d, 2H aromatic), 7.008 (d, 2H aromatic), 7.457 (t, 2H aromatic), 7.569 (t, 2H aromatic), 7.635 (d, 2H aromatic), 7.979 (s, 2H aromatic), 8.437 (d, 2H, 2 × HC = N), and 13.823 (s, 2H, 2 × OH); compound 4a (Fig. 1c, CDCl3, δ, ppm): 1.938 (s, 4H, 2 × CH2), 3.713 (s, 4H, 2 × CH2), 6.946 (d, 2H aromatic), 7.250 (t, 2H aromatic), 7.435 (t, 2H aromatic), 7.626 (d, 2H aromatic), 7.702 (d, 2H aromatic), 7.877 (d, 2H aromatic), 8.793 (d, 2H, 2 × HC = N), and 14.653 (s, 2H, 2 × OH); compound 6a (Fig. 1d, CDCl3, δ, ppm): 1.512 (s, 4H, 2 × CH2), 1.775 (s, 4H, 2 × CH2), 3.561 (s, 4H, 2 × CH2), 6.780 (d, 2H aromatic), 6.930 (d, 2H aromatic),
Table 2 Experimental results of the as-prepared HAP nanocrystals. HAP samples
Coordination ligand
Morphology
Particle size (nm)
Yield (%)
HAP1 HAP2 HAP3 HAP4 HAP5 HAP6 HAP7
2a 2b 3a 4a 6a 8a 8b
Sponge-like structures composed of nanoparticles Particle-like Rod-like Rod-like Rod-like Rod-like Scale-like
80–110 20–25 (150–200) (200–250) (200–300) (120–200) (100–120)
98.0 97.5 97.5 97.3 98.5 97.0 97.5
× × × × ×
(40–55) (50–55) (50–70) (40–45) (50–70)
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Fig. 4. FE-SEM images of HAP nanocrystals: (a) HAP1 and (b) HAP2.
Fig. 3. FT-IR spectra of HAP nanocrystals: (a) HAP1; (b) HAP2; (c) HAP3; (d) HAP4; (e) HAP5; (f) HAP6; and (g) HAP7.
2.594 (t, 4H, 2 × CH2), 3.742 (d, 4H, 2 × CH2), 6.683 (d, 2H aromatic), 7.170 (t, 2H aromatic), 7.416 (t, 2H aromatic), 7.614 (d, 2H aromatic), 7.702 (d, 2H aromatic), 8.024 (d, 2H aromatic), 9.049 (d, 2H, 2 × HC = N), and 13.793 (s, 2H, 2 × OH). The 1H-NMR results prove the chemical structures of the coordination ligands presented in Scheme 1. Fig. 2a–g shows the FT-IR spectra of the compounds 2a, 2b, 3a, 4a, 6a, 8a and 8b, respectively. All of these spectra depict a strong peak corresponding to the v(C_N), which proves the formation of the Schiff base compounds [13]. The stretching vibrations of the C_C bond in aromatic rings are seen in the range of 1470–1540 cm−1. The broad adsorption peaks in the range of 3400\3600 cm−1 can be attributed to the stretching vibration of OH. Although we could not investigate the chemical structure of the compound 2a by NMR spectroscopy due to its insolubility in the NMR solvents, the observation of the v(C_N) at 1643 cm−1 in the FT-IR spectrum of the compound 2a (Fig. 2a) proved its chemical structure. 3.2. FT-IR, FE-SEM, TEM and XRD studies of HAP nanocrystals
7.427 (t, 2H aromatic), 7.568 (quartet, 4H aromatic), 7.798 (d, 2H aromatic), 8.453 (d, 2H, 2 × HC = N), and 13.499 (s, 2H, 2 × OH); compound 8a (Fig. 1e, CDCl3, δ, ppm): 3.639 (d, 8H, 4 × CH2), 3.697 (t, 4H, 2 × CH2), 6.901 (d, 2H aromatic), 7.212 (t, 2H aromatic), 7.396 (t, 2H aromatic), 7.586 (d, 2H aromatic), 7.662 (d, 2H aromatic), 7.804 (d, 2H aromatic), 8.675 (s, 2H, 2 × HC = N), and 14.298 (s, 2H, 2× OH); and compound 8b (Fig. 1f, DMSO-d6, δ, ppm): 2.510 (s, 4H, 2 × CH2),
Table 2 gives a list of the preparation conditions that we have tested to synthesize HAP nanocrystals with the aid of different coordination ligands. As shown in the FT-IR spectra of the samples HAP1–7 (Fig. 3), all the characteristic peaks of HAP are observed. The weak adsorption band located at around 472 cm− 1 is corresponding to the symmetric stretching vibration (v2) of the PO3− ions, while the bands at 4
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Fig. 6. FE-SEM images of HAP nanocrystals: (a) HAP6 and (b) HAP7.
Fig. 5. FE-SEM images of HAP nanocrystals: (a) HAP3; (b) HAP4; and (c) HAP5.
around 557–568 cm− 1 are attributed to the bending vibrations (v4) −1 of the PO3− are de4 ions. The bands centered around 1030–1045 cm rived from the asymmetric stretching vibration (v3) of the PO3− 4 ions. The frequency of the symmetric stretching vibration (v1) of the PO3− 4 ions appeared at lower wavenumbers rather than the asymmetric stretching vibration. Moreover, the stretching vibration of the OH− ions in the HAP lattice is seen at 3570 cm−1 [14,15]. The broad adsorption band at 3100–3500 cm−1 (stretching vibration) and the weak band
at 1630–1637 cm−1 (bending vibration) are attributed to the crystal water and surface adsorbed water. In addition, the presence of the −1 v(CO2− are observed in the 3 ) vibrations at around 1383–1458 cm FT-IR spectra of the products [16]. The carbonate ions might come from the atmosphere carbon dioxide which combined into the crystal structure during the dissolving and stirring processes [17]. This evidence may improve the biological activity of the carbonated HAP, which is essential for bonding with natural bone [18]. The CO2− ions 3 can be introduced into the structure of hydroxyapatite for the substitution of the PO3− ions (type B). On the other hand, type A represents 4 the substitution of OH– ions by CO2− ions [19,20]. By considering 3 the IR bands attributed to the v(CO2− 3 ) vibrations at around 1383–1458 cm −1, it is found that the B-type carbonated hydroxyapatite has been formed during the synthesis route. Based on the results of energy dispersive spectrometry, the Ca/P ratio in the final products is higher than the stoichiometric ratio of Ca/P in HAP (1.67) due to the re2− placement of some PO3− 4 ions by CO3 ions [21]. FE-SEM images of the samples HAP1–7 were studied to find the effect of different coordination ligands on the morphology of the products. As shown in Fig. 4a, sponge-like structures composed of nanoparticles are produced by compound 2a. By using compound 2b instead of compound 2a, homogeneous nanoparticles are formed because of the high steric hindrance effect of compound 2b (Fig. 4b). In Fig. 5a–c, FE-SEM images of the samples HAP3–5 synthesized with the aid of compounds 3a, 4a and 6b are observed. By comparing the presented images in Fig. 5a–c, it can be concluded that
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one-dimensional (1-D) nanocrystals of HAP including nanorods are produced by increasing the CH2 groups between two \C_N\ bands. In addition, the dimensions of the nanorods increased by increasing the CH2 groups between the \C_N\ groups in the used ligands. As seen in Fig. 6a and b, rod-like and scale-like nanocrystals of HAP are prepared by using compounds 8a and 8b, respectively. Particle sizes of the samples HAP1–7 were illustrated in Table 2. The morphological feature of the sample HAP2 is given in the TEM images of Fig. 7. The TEM images show that morphology of the sample HAP2 is particle-like nanostructures with particle sizes in the range of 15–20 nm.\20 nm. The crystalline structures of the as-synthesized HAP nanocrystals were confirmed by XRD. Fig. 8 shows the XRD pattern of the sample HAP2. All the diffraction peaks can be indexed to the hexagonal phase hydroxyapatite with the space group of P63/m and cell constants a = b = 9.4180 Å and c = 6.8840 Å (JCPDS: 09-0432). In this pattern, no XRD peaks can be observed for the presence of other calcium phosphate phases in the products. The XRD patterns of the samples HAP4 and HAP5 are shown in Fig. 9a and b, respectively. The broader peaks with low intensities can be attributed to the hexagonal phase hydroxyapatite with the space group of P63/m and cell constants a = b = 9.4240 Å, and c = 6.8790 Å (JCPDS: 74-0565). By comparing the XRD patterns in Figs. 8 and 9, it is clear that the crystallinity degree of the sample HAP2 is more than that of the samples HAP4 and HAP5 due to the sharp peaks in Fig. 8. The relationship between lattice distance (λ = 2dsinθ) and lattice parameters (a = b, and c) of the hexagonal structure is expressed as follows [22]: 2 2 2 2 2 2 1=d ¼ ð4=3Þ h þ hk þ k =a þ l =c
Fig. 7. (a, b) TEM images of HAP nanoparticles (sample HAP2).
ð1Þ
where h, k, and l as reflection planes (Miller index) and d as lattice distance are known. The lattice parameters of the samples HAP2, HAP4 and HAP5 were also calculated. According to Table 3, it is concluded that the c-axis of the unit cells in the samples HAP4 and HAP5 is longer than that of the sample HAP2. This finding exhibits the crystal growth direction of the nanorods along the c-axis. Moreover, the decrease and increase of a-axis and c-axis values for the carbonated hydroxyapatite can be expected due to the substitution of the tetragonal PO3− 4 ions with the trigonal planar CO2− 3 ions [23,24]. The XRD results coincide with the FT-IR
Fig. 8. XRD pattern of HAP nanoparticles (sample HAP2).
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Fig. 9. XRD patterns of HAP nanorods: (a) sample HAP4 and (b) sample HAP5.
analyses. The volume of the hexagonal unit cell for the samples was calculated by Eq. (2) and seen in Table 3 [25]. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 ¼ 3=2a2 c V A˚
ð2Þ
3.3. Possible formation and crystal growth mechanism of HAP nanocrystals The possible formation and crystal growth mechanism of HAP nanocrystals can be assumed as follows. At first, the in situ formation of the [Ca–ligand]2+ complexes may take place by a reaction between Ca(NO3)2·4H2O and the coordination ligand in methanol. Then, the nucleation of HAP can be induced by adding the (NH4)2HPO4 solution into the [Ca–ligand]2+ solution at a pH of 12. The HAP nuclei capped by the coordination ligands grow during the thermal treatment process at 150 °C. At last, pure HAP nanocrystals are obtained after washing by distilled water, methanol and chloroform, sequentially.
Table 3 Lattice parameters and volume of the hexagonal unit cell for HAP nanocrystals. HAP samples
XRD pattern
a = b (Å)
c (Å)
V (Å3)
HAP2 09-0432 HAP4 HAP5 74-0565
Fig. 8 – Fig. 9a Fig. 9b –
9.437 9.418 9.013 9.139 9.424
6.890 6.884 6.960 6.984 6.879
531.36 528.80 489.64 505.16 529.09
On the basis of the FE-SEM images, it is concluded that the shape of the HAP nanocrystals is dependent on the coordination mode of the assynthesized coordination ligands and stability of their complexes. Here, two main coordination modes may be assumed to grow the 0-D and 1-D HAP nanocrystals. In the first coordination mode, the ligand is introduced as a tetradentate chelating ligand to form [CaN2O2]2+ complexes. In the second one, the ligand can play as a bidentate chelating ligand to form [CaN2]2+, [CaO2]2+ and [CaNO]2+ complexes. In the tetradentate unit which can be assumed for the compounds 2a and 2b, the ligand can wrap itself around the Ca2+ ion with two nitrogen and two oxygen atoms to form six-membered chelate rings. In fact, in this coordination mode, all aspects of HAP nuclei are capped by the ligands, and the 0-D HAP nanocrystals obtained. In the bidentate units suggested for the compounds 3a, 4a, 6a, 8a and 8b, the Ca2+ ion can interact with the complexing agents through nitrogen and oxygen atoms to form fivemembered and six-membered chelate rings. When the ligand 8a is used as a complexing agent, two oxygen atoms can participate to design five-membered chelate rings. The proposed formation mechanism of HAP nanocrystals was illustrated in Scheme 2. Although the use of coordination ligands to control the shape and particle size of various nanocrystals has been developed [26–30], this work is the first successful attempt in HAP synthesis by using these compounds. 4. Conclusions In summary, a novel and simple precipitation route has been developed to prepare HAP nanoparticles and nanorods. The coordination ligands derived from 2-hydroxy-1-naphthaldehyde were applied to control shapes and particle sizes of HAP nanocrystals. Based on the
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Scheme 2. Possible coordination modes of the as-synthesized ligands to form 1-D and 0-D HAP nanocrystals.
coordination modes proposed for the as-synthesized ligands, it was found that the formation of tetradentate and bidentate units led to the fabrication of 0-D (nanoparticles) and 1-D (nanorods) HAP nanocrystals. Acknowledgments The authors are grateful to the council of the University of Kashan for providing financial support to undertake this work by Grant No. 159271/163. References [1] L.L. Hench, Bioceramics, J. Am. Ceram. Soc. 81 (1998) 1705–1728. [2] T. Yamamuro, L.L. Hench, J. Wilson, Handbook of bioactive ceramics, Calcium Phosphate and Hydroxyapatite Ceramics, vol. 2, CRC Press, Boca Raton, FL, 1990. 3. [3] M. Vallet-Regi, J.M. Gonzalez-Calbet, Calcium phosphates as substitution of bone tissues, Prog. Solid State Chem. 32 (2004) 1–31. [4] S.I. Roohani-Esfahani, S. Nouri-Khorasani, Z.F. Lu, M.H. Fathi, M. Razavi, R.C. Appleyard, H. Zreiqat, Modification of porous calcium phosphate surfaces with different geometries of bioactive glass nanoparticles, Mater. Sci. Eng. C 32 (2012) 830–839. [5] M.H. Fathi, A. Hanifi, V. Mortazavi, Preparation and bioactivity evaluation of bonelike hydroxyapatite nanopowder, J. Mater. Process. Technol. 202 (2008) 536–542. [6] M.M. Zogbi Jr., E. Saito, H. Zanin, F.R. Marciano, A.O. Lobo, Hydrothermal– electrochemical synthesis of nano-hydroxyapatite crystals on superhydrophilic vertically aligned carbon nanotubes, Mater. Lett. 132 (2014) 70–74. [7] T. Liu, X. Ding, X. Yang, Z. Gou, J. Chen, X. Feng, Effect of carbonization treatment on the morphology and microstructure of mesoporous bioactive glass/nanocrystalline hydroxyapatite nanocomposite, J. Non-Cryst. Solids 389 (2014) 104–112. [8] Z. Fan, J. Wang, Z. Wang, Y. Li, H. Ran, L. Niu, P. Gong, B. Liu, S. Yang, One-pot synthesis of graphene/hydroxyapatite nanorod composite for tissue engineering, Carbon 66 (2014) 407–416. [9] K. Lin, P. Liu, L. Wei, Z. Zou, W. Zhang, Y. Qian, Y. Shen, J. Chang, Strontium substituted hydroxyapatite porous microspheres: surfactant-free hydrothermal synthesis, enhanced biological response and sustained drug release, Chem. Eng. J. 222 (2013) 49–59. [10] A. Wang, D. Liu, H. Yin, H. Wu, Y. Wada, M. Ren, T. Jiang, X. Cheng, Y. Xu, Sizecontrolled synthesis of hydroxyapatite nanorods by chemical precipitation in the presence of organic modifiers, Mater. Sci. Eng. C 27 (2007) 865–869. [11] M. Sadat-Shojai, M.-T. Khorasani, A. Jamshidi, Hydrothermal processing of hydroxyapatite nanoparticles—a Taguchi experimental design approach, J. Cryst. Growth 361 (2012) 73–84. [12] R.H. Holm, G.W. Everett Jr., A. Chakravorty, Metal complexes of Schiff bases and βketoamines, Prog. Inorg. Chem. 7 (1989) 83–214.
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