Effect of Poly (Sodium 4-Styrene Sulfonate) on the Morphology of Hydroxyapatite Particles

Abstract

Nanorods hydroxyapatite, (HAP) _{Ca10(PO4 )6(OH)2} is successfully prepared by water in oil microemulsion using, _CaCL2 and _H3PO4 (water phase), poly(sodium 4-styrene sulfonate) (PSSS) as template and cyclohexane as oil phase. The nano-structure of the product was studied by means of X-ray diffraction (XRD), Fourier transmission infrared spectrometer (FT-IR), scanning electron microscopy (SEM), and atomic force microscope (AFM). With this system, we could synthesize nano-particles of hydroxyapatite with high crystallinity and least agglomeration.

Recommended by Anastasios Keramidas

Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran

Received 16 March 2009; Revised 20 June 2009; Accepted 30 June 2009

1. Introduction

Hydroxyapatite (HAP) has been widely studied as an important biocompatible material because of its chemical similarity to the natural calcium phosphate mineral present in a biological hard tissue [1-4]. HAP also finds applications in fields of industrial or technological interests as catalyst in chromatography or gas sensor [5], waterpurification, fertilizers production, and drug carrier [6]. Properties of HAP, including bioactivity, biocompatibility, solubility, sinterability, castability, fracture toughness. and absorption can be tailored over wide ranges by controlling the particle composition, size, and morphology [7-9].

The morphology of calcium phosphate nanoparticles made by traditional methods as chemical coprecipitation [10], sol-gel [11], spray-pyrolysis [12], hydrothermal synthesis [13], emulsion processing [13], mechano-chemical method [14], and autocombustion methods [15] are needle-like, sheet-like, or spherical which are not more than 300 nm in length.

Microemulsions are thermodynamically stable dispersions of oil and water stabilized by a surfactant and, in many cases, also a cosurfactant. The microemulsions can be of the droplet type, either with spherical oil droplets dispersed in a continuous medium of water (oil in water microemulsions, O/W) or with spherical water droplets dispersed in continuous medium of oil (water in oil microemulsions, W/O) [16].

In our work, we investigated the morphology of nanohydroxyapatite particles formed in the presence of PSSS as a crystal modifier using microemulsion method. The only phase in product as prepared was hydroxyapatite and it was well crystallized.

2. Experimental

HAP nanopowders were synthesized using the micelle as a template system where poly(sodium 4-styrene solfonate) (_CH2 CH (_C6H4SO3 Na ), Aldrich) was used as the template. Calcium chloride (_CaCl2 , Merck) and phosphoric acid (_H3PO4 85%, Merck) were used as calcium and phosphorus sources, respectively. Cyclohexane (Merck) was used as oil phase. For preparing reverse micelle system 10% volume of aqueous solution of 0.02 M polymer was added to cyclohexane, after that aqueous solution of 5 M _CaCl2 and aqueous solution of 3 M _H3PO4 were added slowly to the reverse micelle, respectively. The mixture was stirring in all steps. Then the pH of microemulsion was adjusted at 12 by adding aqueous solution of NaOH. The final milky suspension was kept for 12 hours at room temperature. The obtained precipitate was then filtered off and washed several times with deionized water. A gel-like paste was produced which was then dried at 15^0° C for 3 hours and calcined at 65^0° C for 1 hour.

The morphologies of the as-prepared HAP were observed by a scanning electron microscopy (SEM) (Cambridge-S365) equipped with energy-disperse X-ray spectroscopy and AFM (nanoscope 2). The powder X-ray diffractometer using Cu Kα (siemens D500) and Fourier transform infrared (FTIR) spectroscopy (shimadzu, KBr pellet technique) was used to identify the quality and composition of hydroxyapatite.

3. Results and Discussion

The wide angle (2θ > 1^0° ) X-ray diffraction patterns of the obtained sample is shown in Figure 1. The diffraction peaks correspond to the standard characteristic peaks of hexagonal HAP. There is a high consistency between the data from our sample and that from the standard database, with lattice dimensions of a = b = 0.9414 nm, c = 0.6879 nm (space group p₆₃ /m, JCPDS no. 09-0432). No other impurity was observed in the XRD pattern, indicating the chief inorganic phase of the sample is HAP crystal.

Figure 1: XRD pattern of HAP nanorods.

[figure omitted; refer to PDF]

Figure 2 shows the FT-IR spectra of the sample. The peak at 3420 c^m-1 is attributed to the _ν2 bending mode of adsorbed water [17]. The stretching vibration band of O^H- is observed at 3569 c^m-1 [18]. Tow adsorptionbands at 561 and 601 c^m-1 are ascribed to the _ν4 bending mode of P^O43- [19]. The characteristic band at 1024 and 1091 c^m-1 are related to the stretching vibration of P^O43- . The band at 951 c^m-1 is assigned to _ν1 stretching mode of P^O43- . The typical splitting peaks at 567 and 603 c^m-1 derived from the _ν4 phosphate mode [19]. The FT-IR results indicate that no PSSS molecule is incorporated in the HAP.

Figure 2: FT-IR spectra of synthesized hydroxyapatite.

[figure omitted; refer to PDF]

Figure 3 shows the SEM image of the HAP nanoparticles. It reveals that the overall morphology of the obtained powders at mentioned situation is rod-like. This suggested that the presence of PSSS had greatly influenced the morphology of the product due to a strong interaction between the sulfate groups of PSSS and the ^Ca2+ ions in the solution and on the surface of HAP particles [20-22].

Figure 3: Scanning electron microscopy for synthesized HAP.

[figure omitted; refer to PDF]

Figure 4 shows the AFMimage of nanoparticles of hydroxyaopatite. AFM image confirmed that the particles of synthesized sample are rod shape. AFM imageshows the resulted rod shapes HAP have an average width and length about 30 and 200 nm, these sizes are adherent with SEM results which measured the width and length of the shown particle about 40 and 177 nm and also these dimensions evidence that the particles are rod shape.

Figure 4: AFM image of hydroxyapatite nanoparticles.

[figure omitted; refer to PDF]

We propose here a mechanism for the formation of HAP nanoparticles in the compositions containing the anionic polymer. Earlier studies on PSSS demonstrated that the sulfate groups are able to interact with calcium ions present in an aqueous solution. Surfactant molecules in micelles or emulsion droplet interact with ^Ca2+ ions to form zwitterions structures [20-22]. These numerous calcium-rich domains lead to the fast formation of HAP particles upon contact with phosphate ions in the aqueous phase. The reaction between _H3PO4 and _CaCl2 in the micelles is deemed to be rapid because of the localized ^Ca2+ concentration effect. In addition, the positional stabilization of ^Ca2+ ions within each zwitterions structure as a result of the electrostatic interaction effect by PSSS molecules favors the formation of ordered HAP crystals.

4. Conclusion

Drop-wise addition of 0.6 M _H3PO4 solution into 1.0 M _CaCl2 solution resulted in a fast precipitation of HAP particles. Using PSSS as a nucleation and growth controlling agent, well-crystallized hydroxyapatite nanoparticles were precipitated via a microemulsion route. During the mixing of PSSS with calcium precursor, it formed rod-like micelles which control the morphology and crystallization of nano-hydroxyapatite. X-ray diffraction pattern and FTIR spectrum of the resulted precipitates confirmed the formation of high purity and well-crystallized HAP. The SEM and AFM investigations showed that the obtained HAP nanorods have an average width and length of about 30 and 200 nm, respectively.

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