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Crystallisation of hydroxyapatite nanocrystals under magnetic field
Materials Letters 60 (2006) 761 – 765 www.elsevier.com/locate/matlet
Crystallisation of hydroxyapatite nanocrystals under magnetic field N. Meenakshi Sundaram a , E.K. Girija a , M. Ashok b , T.K. Anee c , R. Vani a , R.V. Suganthi a , Y. Yokogawa d , S. Narayana Kalkura a,⁎ a
Crystal Growth Centre, Anna University, Chennai-600 025, India School of Physics, Seoul National University, Seoul, South Korea c Department of Physics, St. Joseph's College, Irinjalakuda, Kerala, India Bioceramics group, Ceramic Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya, Japan b
d
Received 14 February 2005; accepted 3 October 2005 Available online 14 November 2005
Abstract Crystallisation of hydroxyapatite (HAp) was studied in the presence of low magnetic field. The magnetic field favoured the formation of hydroxyapatite. The number of spherulites in the Lisesegang rings increased in the presence of magnetic field and that might be due to the acceleration in the diffusion of the reactants. The major plane of brushite crystals, which were formed along with HAp were found to orient perpendicular to the direction of the applied magnetic field. The crystals were also further characterised by XRD, SEM, FT-IR and DSC analyses. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Brushite; Crystallisation; Single diffusion; Magnetic field
1. Introduction Hydroxyapatite (HAp) {Ca10(PO4)6(OH)2} is one of the important biominerals of interest in the broad group of calcium phosphate based bioceramics and the main inorganic constituent of bone and teeth. HAp is mainly used as bone and dental replacement material [1]. Morphological control of HAp crystals is a stringent requirement in many applications. One of the reasons why HAp has been proposed as a potential material for filler is that, it absorbs proteins with high efficiency. The major advantage of hydroxyapatite phase of calcium phosphate materials is that the nano size (10–100 nm) hydroxyapatite crystalline arrays along the collagen fibers inside the bone is able to bond directly with the hard tissue replacement implants and provides perfect lattice matching between the fracture bone surfaces [2]. The strength and other mechanical properties of bone depend upon orientation of the hydroxyapatite nanocrystals and collagen fibers [3]. HAp has also been used as the ⁎ Corresponding author. Tel.: +91 44 22352774, +91 44 22203577; fax: +91 44 22352870. E-mail addresses: [email protected] (N. Meenakshi Sundaram), [email protected] (S. Narayana Kalkura). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.034
fillers for column chromatography. HAp crystals were prepared by various methods such as solution [4], sol–gel [5], high temperature [6] and solid-state reaction [7]. Recently, crystallisation of hydroxyapatite at physiological temperature was carried out [8]. An acicular crystal of hydroxyapatite-silica composite of size 100 nm was synthesised by diffusion of calcium ions through an alkaline silica gel matrix [9]. There are only few reports on the influence of magnetic field on the calcium phosphate crystallisation process [10–12]. There are reports at 0.08 to 0.3 Tesla magnetic field, the dia and paramagnetic organic polymers [13] and inorganic materials [10] have significant effect on the nucleation process. It is very clear that 0.1 Tesla can influence the nucleation of dia, paramagnetic organic and inorganic materials. In the case of oriented crystalline hydroxyapatite, bone grows directly on the surface of the implant resulting in an intimate bond, probably due to the direct chemical attachment for normal bone healing (calcification) [14]. Bioactive behaviour are depends on a and b-axis or caxis of HAp crystal [11]. Thus, the crystal orientation of HAp has been very attractive in the biomaterial field. But on the other hand, when supersaturation of calcium apatite exceeds the solubility limit in the biological fluid, the unwanted nucleation and crystallisation occurs, which acts as a nucleation centre for
762
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765 Table 2 Number, size and morphology of the crystals formed in the gel medium Field strength (T)
Number of spherulites (HAP phase)
Average radius of spherulites (mm)
Number of brushite crystals
Size of the brushite crystals (mm2)
0 0.1
120 180
1.0 1.4
20 12
4 × 1.5 6 × 2.0
·
sodium meta silicate (Na2SiO3 9H2O) of specific gravity 1.03 and Na2HPO4 (0.6 M) was taken in 1 : 1 ratio adjusted to the pH 7.4. About 5 ml of this solution was allowed to gel in a test tube at the physiological temperature of 37 ± 0.3 °C in an incubator over a period of 3 days. After gelation, about 5 ml of CaCl2 2H2O was carefully layered over the gel as a supernatant solution without damaging the gel. The pH of the supernatant solution was found to be 6.6 after the crystallisation was completed. A steady and homogeneous magnetic field was applied by using a pair of permanent Nd2Fe14B bar magnets of size 1 × 4 × 6 cm3 separated by 1.2 cm. The field strength is 0.1 Tesla in between the magnets and is homogeneous all over the gap which was measured by Gauss meter. The schematic diagram of the magnetic field set up is shown in the Fig. 1. The samples were harvested and thoroughly washed with distilled water, dried and kept in a dessicator. Shimadzu UVVisible spectrophotometer UV-1601, Japan was used in the estimation of concentration of phosphate ions. Rigaku X-ray
·
Fig. 1. Schematic diagram of magnetic field set up.
the cholesterol deposition, which leads to the deposition of a mixture of cholesterol and hydroxyapatite on the inner walls of the blood arteries [15]. The epidemiological study on the magnetic field exposure and cardiovascular disease mortality is still under investigation [16]. Gel acts as an ideal medium for studying the crystallisation of biomolecules in vitro [17]. This method provides a convenient technique to assess the effect of various factors such as pH, temperature, additives and external forces in altering the crystallisation parameters like nucleation time, crystal morphology and growth rate of the crystals. Also the main reason for using the gel medium in this study is that it holds the growing crystals in its own orientation. To the best of our knowledge, there is no systematic investigation on the effect of magnetic field on the calcium phosphate crystallisation in a controlled diffusion medium at the physiological temperature. In this report, the synthesis of hydroxyapatite crystals in silica gel medium by single diffusion technique at physiological temperature and the effect of low magnetic field on the periodic precipitation of HAp and brushite are reported. 2. Experimental
·
The analytical grade CaCl2 2H2O and Na2HPO4 (E-Merck) were used as reagents. The mixture of the aqueous solution of Table 1 Number, size and morphology of crystals grown at the gel-solution interface in 0.1 T (brushite phase) Field strength (T)
Number of crystals
Average size (mm2)
Morphology
0 0.1
225–300 150–170
0.5 × 1.2 0.8 × 1.4
Thin plates Thin plates
Fig. 2. XRD of (a) hydroxyapatite and (b) brushite.
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
detector was used in the diffraction measurement. Varian make spectrometer was used for the FT-IR analysis. DSC measurements were done using Perkin Elmer DSC 7 model in a dynamic atmosphere. 3. Results Immediately after the addition of the supernatant solution in control and magnetic field exposed test tubes, a dense white precipitate was formed at the interface and it continued to grow in the solution and inside the gel medium to a thickness of about 0.5 cm. Just below the continuous precipitate, periodic white discs (Liesegang rings) of micro crystals were observed inside the gel. Nearly, 10 discs of about 0.5 mm thickness were seen in the gel medium
763
after two weeks. The space between successive Liesegang rings was found to increase towards the bottom of the tube from the interface. The rapid mixing of the reactants was avoided due to the presence of gel, which slow down diffusion of the reactants and the rate of crystallisation. The thick precipitate formed at the gel-solution interface contained many brushite crystals. The number, average size and the morphology of the brushite crystals formed at the gel-solution interface in control and in 0.1 T magnetic field are presented in Table 1. The results indicate that in the presence of magnetic field, the number of brushite crystals formed at the gel-solution interface reduces and the average size of the crystals increases. Inside the gel medium, the Liesegang rings contained predominantly spherulites of HAp along with the crystals of brushite. The number
Fig. 3. (a) Scanning electron micrograph of spherulite of hydroxyapatite, Further magnification at (b) control and (c) 0.1 T magnetic field, (d) SEM micrograph of brushite crystal. (e) Single diffusion crystal growth of calcium phosphates. Magnetic field applied perpendicular to the plane of this paper.
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N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
Fig. 4. FT-IR spectrum of hydroxyapatite.
and average size of spherulites and brushite crystals formed inside the gel medium are given in Table 2.
4. Characterisation 4.1. Calcium Phosphorus ratio The Ca / P ratio of the sample was determined by titration and UV-Visible spectroscopic analysis. Calcium concentration was measured by titrating the sample solution with 0.2 N EDTA solution. The Ca / P ratio of the sample was found to be 1.67. Though Ca / P ratio ranging from 1.61 to 1.71 was accepted for HAp, a good stoichiometric HAp was expected to give Ca / P ratio of 1.67. Hence present sample could be considered to be of good stoichiometric ratio and free of contamination. The HAp and brushite samples synthesized in magnetic field yield the same stoichiometric range as in the control. 4.2. Powder X-ray diffraction (XRD) The platy micro-crystals in the Liesegang ring were identified as HAp from XRD analysis. The XRD results were in good agreement with the ICDD standard (9-432) values for HAp (Fig. 2). The density was found to be 3.14 (± 0.01) g/cm3 from the XRD data. Also the platy crystals formed inside the gel medium confirmed to be the brushite phase and the XRD pattern is shown in Fig. 2. Table 3 FT-IR Assignments of functional groups Vibrational frequency (cm− 1)
Assignments
570 600 963 1119 1086 1032 1651 3492
O–P–O bending O–P–O bending P–O symmetric stretching bonds P–O asymmetric stretching P–O asymmetric stretching P–O asymmetric stretching O–H in-plane bending O–H stretching
Fig. 5. DSC spectrum of hydroxyapatite.
4.3. Scanning electron microscopy Spherulites of hydroxyapatite were observed under SEM (Fig. 3a), which revealed that spherulites made of nano crystalline platy crystals of HAp in control having the size ∼10 × 2 nm as shown in the Fig. 3b. The array of nano crystalline hydroxyapatite needles under 0.1 T magnetic field as shown in Fig. 3c. 4.4. FT-IR analysis FT-IR spectrum of the HAp sample is shown in the Fig. 4. The stretching mode of the OH− group appeared at 3500 cm− 1. O–H in-plane bending is found at 1651 cm− 1. This peak confirms the presence of OH in the sample. The bands, which correspond to O–P–O bending, occurred at 570 cm− 1 and 600 cm− 1 and PO stretching was observed at 1033 cm− 1, 1085 cm− 1 and 960 cm− 1. FT-IR assignments are given in Table 3. FT-IR spectrum confirms that the HAp crystals are free from silica inclusion. There was no difference in FT-IR of HAp synthesised at control and magnetic field. 4.5. Differential scanning calorimetric (DSC) analysis The DSC analysis of HAp was carried out between 50–500 °C with a heating rate of 10 °C per minute. The DSC trace obtained is shown in Fig. 5. The endothermic weight loss below 100 °C is clearly seen in DSC, indicating the loss of water. The dehydro-condensation of OH group produces weight loss at 255 and 400 °C, which represents at least two different types of arrangement of OH groups in the crystal lattice. 5. Discussion The above results indicate that the external magnetic field inhibited the formation of brushite. Furthermore, static magnetic fields cause orientation of brushite crystals with respect to the external field. The major plane of the platy brushite crystals was oriented perpendicular to the direction of the applied
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
magnetic field. The magnetic field influences the crystal formation is due to the increase in the free energy of the nuclei [18]. During the nucleation process, the magnetic anisotropy of the nucleated clusters becoming larger than the individual molecule that leads to the orientation of nucleated clusters and it enhances the ordering of nano size crystals in the mother solution with respect to the magnetic field. Sørensen and Madsen [10] reports that 0.08 to 0.3 Tesla give the significant effect on the nucleation of calcium phosphates in gel medium. It is evident that the magnetic field increases the overall rate of nucleation. In the presence of magnetic field, the number of Liesegang rings increased. It means, the magnetic field accelerates the diffusion and rate of reaction. 6. Conclusion HAp was crystallized by single diffusion technique in the presence of magnetic field. In presence of magnetic field, the number of brushite crystals reduced and the size of the crystals increased at the gel solution interface. Inside the gel medium, Liesegang's rings consisted of spherulites of hydroxyapatite. In the presence of magnetic field, formation of hydroxyapatite phase was favoured. The major plane of the platy brushite crystals was oriented perpendicular to the direction of the applied magnetic field. The number of spherulites in the Lisesegang rings increased in the presence of magnetic field and that might be due to the acceleration in the diffusion of the reactants along the gel medium. The spherulites formed were confirmed to be stochiometric hydroxyapatite by titration and UV-visible spectrometry. The crys-
765
tals were also further characterised by XRD, SEM, FT-IR and DSC analyses. Acknowledgment This work was supported by University Grants Commission, New Delhi. References [1] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94. [2] J.B. Park, Biomater. Sci. and Engg., Plenum Press, New York, 1987, p. 243. [3] W.J. Landis, Bone 16 (1995) 533. [4] P.W. Brown, N. Hocker, H. Susan, J. Am. Ceram. Soc. 74 (1991) 1848. [5] T.K. Anee, M. Palanichamy, M. Ashok, N.M. Sundaram, S.N. Kalkura, Mater. Lett. 58 (2004) 478. [6] K. Teraoka, A. Ito, K. Onuma, T. Tateishi, S. Tsutsumi, J. Mater. Res. 14 (1999) 2655. [7] J.C. Elliott, R.A. Young, Nature 214 (1967) 904. [8] M. Ashok, N.M. Sundaram, S.N. Kalkura, Mater. Lett. 57 (2003) 2066. [9] A.I. Villacampa, J. Ma, R. Garcia, J. Cryst. Growth 211 (2000) 111. [10] J.S. Sørensen, H.E.L. Madsen, J. Cryst. Growth 216 (2000) 399. [11] S. Asai, K. Sassa, M. Tahashi, Sci. Technol. Adv. Mater. 4 (2003) 455. [12] H.E.L. Madsen, J. Cryst. Growth 152 (1995) 94. [13] A.P. Chiriac, C.I. Simionescu, Prog. Polym. Sci. 25 (2000) 219. [14] Y. Yokogawa, F. Nagata, M. Toriyama, Chem. Lett. (1999) 527. [15] M. Epple, Z. Kardiol. 90 (2001) 64. [16] D.A. Savitz, D. Liao, A. Sastre, R.C. Kleckner, R. Kavet, Am. J. Epidemiol. 149 (1999) 135. [17] G.R. Sivakumar, S.N. Kalkura, P. Ramasamy, Mater. Chem. Phys. 57 (1999) 238. [18] N. Meenakshi Sundaram, M. Ashok, S. Narayana Kalkura, Acta Crystallogr., D Biol. Crystallogr. 58 (2002) 1711.
Crystallisation of hydroxyapatite nanocrystals under magnetic field N. Meenakshi Sundaram a , E.K. Girija a , M. Ashok b , T.K. Anee c , R. Vani a , R.V. Suganthi a , Y. Yokogawa d , S. Narayana Kalkura a,⁎ a
Crystal Growth Centre, Anna University, Chennai-600 025, India School of Physics, Seoul National University, Seoul, South Korea c Department of Physics, St. Joseph's College, Irinjalakuda, Kerala, India Bioceramics group, Ceramic Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya, Japan b
d
Received 14 February 2005; accepted 3 October 2005 Available online 14 November 2005
Abstract Crystallisation of hydroxyapatite (HAp) was studied in the presence of low magnetic field. The magnetic field favoured the formation of hydroxyapatite. The number of spherulites in the Lisesegang rings increased in the presence of magnetic field and that might be due to the acceleration in the diffusion of the reactants. The major plane of brushite crystals, which were formed along with HAp were found to orient perpendicular to the direction of the applied magnetic field. The crystals were also further characterised by XRD, SEM, FT-IR and DSC analyses. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Brushite; Crystallisation; Single diffusion; Magnetic field
1. Introduction Hydroxyapatite (HAp) {Ca10(PO4)6(OH)2} is one of the important biominerals of interest in the broad group of calcium phosphate based bioceramics and the main inorganic constituent of bone and teeth. HAp is mainly used as bone and dental replacement material [1]. Morphological control of HAp crystals is a stringent requirement in many applications. One of the reasons why HAp has been proposed as a potential material for filler is that, it absorbs proteins with high efficiency. The major advantage of hydroxyapatite phase of calcium phosphate materials is that the nano size (10–100 nm) hydroxyapatite crystalline arrays along the collagen fibers inside the bone is able to bond directly with the hard tissue replacement implants and provides perfect lattice matching between the fracture bone surfaces [2]. The strength and other mechanical properties of bone depend upon orientation of the hydroxyapatite nanocrystals and collagen fibers [3]. HAp has also been used as the ⁎ Corresponding author. Tel.: +91 44 22352774, +91 44 22203577; fax: +91 44 22352870. E-mail addresses: [email protected] (N. Meenakshi Sundaram), [email protected] (S. Narayana Kalkura). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.034
fillers for column chromatography. HAp crystals were prepared by various methods such as solution [4], sol–gel [5], high temperature [6] and solid-state reaction [7]. Recently, crystallisation of hydroxyapatite at physiological temperature was carried out [8]. An acicular crystal of hydroxyapatite-silica composite of size 100 nm was synthesised by diffusion of calcium ions through an alkaline silica gel matrix [9]. There are only few reports on the influence of magnetic field on the calcium phosphate crystallisation process [10–12]. There are reports at 0.08 to 0.3 Tesla magnetic field, the dia and paramagnetic organic polymers [13] and inorganic materials [10] have significant effect on the nucleation process. It is very clear that 0.1 Tesla can influence the nucleation of dia, paramagnetic organic and inorganic materials. In the case of oriented crystalline hydroxyapatite, bone grows directly on the surface of the implant resulting in an intimate bond, probably due to the direct chemical attachment for normal bone healing (calcification) [14]. Bioactive behaviour are depends on a and b-axis or caxis of HAp crystal [11]. Thus, the crystal orientation of HAp has been very attractive in the biomaterial field. But on the other hand, when supersaturation of calcium apatite exceeds the solubility limit in the biological fluid, the unwanted nucleation and crystallisation occurs, which acts as a nucleation centre for
762
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765 Table 2 Number, size and morphology of the crystals formed in the gel medium Field strength (T)
Number of spherulites (HAP phase)
Average radius of spherulites (mm)
Number of brushite crystals
Size of the brushite crystals (mm2)
0 0.1
120 180
1.0 1.4
20 12
4 × 1.5 6 × 2.0
·
sodium meta silicate (Na2SiO3 9H2O) of specific gravity 1.03 and Na2HPO4 (0.6 M) was taken in 1 : 1 ratio adjusted to the pH 7.4. About 5 ml of this solution was allowed to gel in a test tube at the physiological temperature of 37 ± 0.3 °C in an incubator over a period of 3 days. After gelation, about 5 ml of CaCl2 2H2O was carefully layered over the gel as a supernatant solution without damaging the gel. The pH of the supernatant solution was found to be 6.6 after the crystallisation was completed. A steady and homogeneous magnetic field was applied by using a pair of permanent Nd2Fe14B bar magnets of size 1 × 4 × 6 cm3 separated by 1.2 cm. The field strength is 0.1 Tesla in between the magnets and is homogeneous all over the gap which was measured by Gauss meter. The schematic diagram of the magnetic field set up is shown in the Fig. 1. The samples were harvested and thoroughly washed with distilled water, dried and kept in a dessicator. Shimadzu UVVisible spectrophotometer UV-1601, Japan was used in the estimation of concentration of phosphate ions. Rigaku X-ray
·
Fig. 1. Schematic diagram of magnetic field set up.
the cholesterol deposition, which leads to the deposition of a mixture of cholesterol and hydroxyapatite on the inner walls of the blood arteries [15]. The epidemiological study on the magnetic field exposure and cardiovascular disease mortality is still under investigation [16]. Gel acts as an ideal medium for studying the crystallisation of biomolecules in vitro [17]. This method provides a convenient technique to assess the effect of various factors such as pH, temperature, additives and external forces in altering the crystallisation parameters like nucleation time, crystal morphology and growth rate of the crystals. Also the main reason for using the gel medium in this study is that it holds the growing crystals in its own orientation. To the best of our knowledge, there is no systematic investigation on the effect of magnetic field on the calcium phosphate crystallisation in a controlled diffusion medium at the physiological temperature. In this report, the synthesis of hydroxyapatite crystals in silica gel medium by single diffusion technique at physiological temperature and the effect of low magnetic field on the periodic precipitation of HAp and brushite are reported. 2. Experimental
·
The analytical grade CaCl2 2H2O and Na2HPO4 (E-Merck) were used as reagents. The mixture of the aqueous solution of Table 1 Number, size and morphology of crystals grown at the gel-solution interface in 0.1 T (brushite phase) Field strength (T)
Number of crystals
Average size (mm2)
Morphology
0 0.1
225–300 150–170
0.5 × 1.2 0.8 × 1.4
Thin plates Thin plates
Fig. 2. XRD of (a) hydroxyapatite and (b) brushite.
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
detector was used in the diffraction measurement. Varian make spectrometer was used for the FT-IR analysis. DSC measurements were done using Perkin Elmer DSC 7 model in a dynamic atmosphere. 3. Results Immediately after the addition of the supernatant solution in control and magnetic field exposed test tubes, a dense white precipitate was formed at the interface and it continued to grow in the solution and inside the gel medium to a thickness of about 0.5 cm. Just below the continuous precipitate, periodic white discs (Liesegang rings) of micro crystals were observed inside the gel. Nearly, 10 discs of about 0.5 mm thickness were seen in the gel medium
763
after two weeks. The space between successive Liesegang rings was found to increase towards the bottom of the tube from the interface. The rapid mixing of the reactants was avoided due to the presence of gel, which slow down diffusion of the reactants and the rate of crystallisation. The thick precipitate formed at the gel-solution interface contained many brushite crystals. The number, average size and the morphology of the brushite crystals formed at the gel-solution interface in control and in 0.1 T magnetic field are presented in Table 1. The results indicate that in the presence of magnetic field, the number of brushite crystals formed at the gel-solution interface reduces and the average size of the crystals increases. Inside the gel medium, the Liesegang rings contained predominantly spherulites of HAp along with the crystals of brushite. The number
Fig. 3. (a) Scanning electron micrograph of spherulite of hydroxyapatite, Further magnification at (b) control and (c) 0.1 T magnetic field, (d) SEM micrograph of brushite crystal. (e) Single diffusion crystal growth of calcium phosphates. Magnetic field applied perpendicular to the plane of this paper.
764
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
Fig. 4. FT-IR spectrum of hydroxyapatite.
and average size of spherulites and brushite crystals formed inside the gel medium are given in Table 2.
4. Characterisation 4.1. Calcium Phosphorus ratio The Ca / P ratio of the sample was determined by titration and UV-Visible spectroscopic analysis. Calcium concentration was measured by titrating the sample solution with 0.2 N EDTA solution. The Ca / P ratio of the sample was found to be 1.67. Though Ca / P ratio ranging from 1.61 to 1.71 was accepted for HAp, a good stoichiometric HAp was expected to give Ca / P ratio of 1.67. Hence present sample could be considered to be of good stoichiometric ratio and free of contamination. The HAp and brushite samples synthesized in magnetic field yield the same stoichiometric range as in the control. 4.2. Powder X-ray diffraction (XRD) The platy micro-crystals in the Liesegang ring were identified as HAp from XRD analysis. The XRD results were in good agreement with the ICDD standard (9-432) values for HAp (Fig. 2). The density was found to be 3.14 (± 0.01) g/cm3 from the XRD data. Also the platy crystals formed inside the gel medium confirmed to be the brushite phase and the XRD pattern is shown in Fig. 2. Table 3 FT-IR Assignments of functional groups Vibrational frequency (cm− 1)
Assignments
570 600 963 1119 1086 1032 1651 3492
O–P–O bending O–P–O bending P–O symmetric stretching bonds P–O asymmetric stretching P–O asymmetric stretching P–O asymmetric stretching O–H in-plane bending O–H stretching
Fig. 5. DSC spectrum of hydroxyapatite.
4.3. Scanning electron microscopy Spherulites of hydroxyapatite were observed under SEM (Fig. 3a), which revealed that spherulites made of nano crystalline platy crystals of HAp in control having the size ∼10 × 2 nm as shown in the Fig. 3b. The array of nano crystalline hydroxyapatite needles under 0.1 T magnetic field as shown in Fig. 3c. 4.4. FT-IR analysis FT-IR spectrum of the HAp sample is shown in the Fig. 4. The stretching mode of the OH− group appeared at 3500 cm− 1. O–H in-plane bending is found at 1651 cm− 1. This peak confirms the presence of OH in the sample. The bands, which correspond to O–P–O bending, occurred at 570 cm− 1 and 600 cm− 1 and PO stretching was observed at 1033 cm− 1, 1085 cm− 1 and 960 cm− 1. FT-IR assignments are given in Table 3. FT-IR spectrum confirms that the HAp crystals are free from silica inclusion. There was no difference in FT-IR of HAp synthesised at control and magnetic field. 4.5. Differential scanning calorimetric (DSC) analysis The DSC analysis of HAp was carried out between 50–500 °C with a heating rate of 10 °C per minute. The DSC trace obtained is shown in Fig. 5. The endothermic weight loss below 100 °C is clearly seen in DSC, indicating the loss of water. The dehydro-condensation of OH group produces weight loss at 255 and 400 °C, which represents at least two different types of arrangement of OH groups in the crystal lattice. 5. Discussion The above results indicate that the external magnetic field inhibited the formation of brushite. Furthermore, static magnetic fields cause orientation of brushite crystals with respect to the external field. The major plane of the platy brushite crystals was oriented perpendicular to the direction of the applied
N. Meenakshi Sundaram et al. / Materials Letters 60 (2006) 761–765
magnetic field. The magnetic field influences the crystal formation is due to the increase in the free energy of the nuclei [18]. During the nucleation process, the magnetic anisotropy of the nucleated clusters becoming larger than the individual molecule that leads to the orientation of nucleated clusters and it enhances the ordering of nano size crystals in the mother solution with respect to the magnetic field. Sørensen and Madsen [10] reports that 0.08 to 0.3 Tesla give the significant effect on the nucleation of calcium phosphates in gel medium. It is evident that the magnetic field increases the overall rate of nucleation. In the presence of magnetic field, the number of Liesegang rings increased. It means, the magnetic field accelerates the diffusion and rate of reaction. 6. Conclusion HAp was crystallized by single diffusion technique in the presence of magnetic field. In presence of magnetic field, the number of brushite crystals reduced and the size of the crystals increased at the gel solution interface. Inside the gel medium, Liesegang's rings consisted of spherulites of hydroxyapatite. In the presence of magnetic field, formation of hydroxyapatite phase was favoured. The major plane of the platy brushite crystals was oriented perpendicular to the direction of the applied magnetic field. The number of spherulites in the Lisesegang rings increased in the presence of magnetic field and that might be due to the acceleration in the diffusion of the reactants along the gel medium. The spherulites formed were confirmed to be stochiometric hydroxyapatite by titration and UV-visible spectrometry. The crys-
765
tals were also further characterised by XRD, SEM, FT-IR and DSC analyses. Acknowledgment This work was supported by University Grants Commission, New Delhi. References [1] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94. [2] J.B. Park, Biomater. Sci. and Engg., Plenum Press, New York, 1987, p. 243. [3] W.J. Landis, Bone 16 (1995) 533. [4] P.W. Brown, N. Hocker, H. Susan, J. Am. Ceram. Soc. 74 (1991) 1848. [5] T.K. Anee, M. Palanichamy, M. Ashok, N.M. Sundaram, S.N. Kalkura, Mater. Lett. 58 (2004) 478. [6] K. Teraoka, A. Ito, K. Onuma, T. Tateishi, S. Tsutsumi, J. Mater. Res. 14 (1999) 2655. [7] J.C. Elliott, R.A. Young, Nature 214 (1967) 904. [8] M. Ashok, N.M. Sundaram, S.N. Kalkura, Mater. Lett. 57 (2003) 2066. [9] A.I. Villacampa, J. Ma, R. Garcia, J. Cryst. Growth 211 (2000) 111. [10] J.S. Sørensen, H.E.L. Madsen, J. Cryst. Growth 216 (2000) 399. [11] S. Asai, K. Sassa, M. Tahashi, Sci. Technol. Adv. Mater. 4 (2003) 455. [12] H.E.L. Madsen, J. Cryst. Growth 152 (1995) 94. [13] A.P. Chiriac, C.I. Simionescu, Prog. Polym. Sci. 25 (2000) 219. [14] Y. Yokogawa, F. Nagata, M. Toriyama, Chem. Lett. (1999) 527. [15] M. Epple, Z. Kardiol. 90 (2001) 64. [16] D.A. Savitz, D. Liao, A. Sastre, R.C. Kleckner, R. Kavet, Am. J. Epidemiol. 149 (1999) 135. [17] G.R. Sivakumar, S.N. Kalkura, P. Ramasamy, Mater. Chem. Phys. 57 (1999) 238. [18] N. Meenakshi Sundaram, M. Ashok, S. Narayana Kalkura, Acta Crystallogr., D Biol. Crystallogr. 58 (2002) 1711.