Template-Based Synthesis of Nanoporous Hydroxyapatite

Hydroxyapatite (HAp) particles, a potential starting material for bone substitutes, with nanopores were synthesized in the presence of cetyltrimethylammonium bromide (CTAB) and P123 as cationic and nonionic surfactants as the structuring units. Effect of nonionic surfactant concentration on surface areas is also investigated. Based on N2 adsorption-desorption isotherms investigation, surface area increased up to 50 m2/g by using P123 and 147 m2/g by using CTAB as porosity agent. Pore structure remained even after the removal of surfactant and calcinations at 400◦C.


Introduction
Hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 (denoted as HA), is a form of bioceramics material [1].It is chemically similar to bones and hard tissues found in humans.It is biocompatible and can be rapidly integrated into the human body.Owing to its bioactive property, HA is widely used in medicine and dentistry as a material for metallic implant coatings, or for bone cavity fillings [2,3].In addition, HA can be used as drug delivery agent, such as for the delivery of antitumor agents and antibodies in the treatment of osteomyelitis: a bone infection that is often treated by excision of necrotic tissue and irrigation of the wound [4].Potential use of HA as a protein delivery agent was also examined [5].
Porosity in a biomaterial implant allows for the possibility of growth of natural bone.In recent years, HA has also been used as an acidic catalyst for Friedel-Crafts alkylation and Knoevenagel reactions [6].
The synthesis of mesoporous materials can be achieved using a supramolecular templating technique.In 1992 [7,8] a group of Mobil scientists successfully demonstrated that such a templating technique could be used to prepare siliconbased mesoporous materials.The cooperative self-assembling of inorganic silicates and cationic surfactants produces a mesophase structure, and the removal of the organic component produces well-organized porous structures.Surfactant-templated mesoporous materials have some specific features such as high surface areas and high adsorption capacities including the uniformity and periodicity of tunable mesopores [9][10][11], which are widely applicable to adsorbents and catalytic supports [12][13][14][15][16][17][18].
In this study, we attempt to achieve the direct crystallization of mesoporous hydroxyapatite by chemical precipitation method using CTAB and P123 as structure-directing agents.
We succeeded in synthesizing a mesoporous hydroxyapatite with high surface area (140 m 2 /g).HAp powders were synthesized using the micelle as a template system which was used as the surfactant.The effect of the surfactant on the BET specific average surface area of the particle was studied by  varying the P123 surfactant concentration as 0.01, 0.02, and 0.03 M. CTAB and P123 as surfactant were used to synthesize nanoporous hydroxyapatite powders of this study.The chemical structures of P123 and CTAB are shown in Figure 1.

Experimental
In a successful synthesis run to yield only single-phase (Ca 10 (PO 4 ) 6 (OH) 2 , HA), 5.91 gr diammonium hydrogen phosphate ((NH 4 ) 2 HPO 4 ) and 5 gr CTAB were dissolved in 70 mL deionized water at room temperature with stirring for 1 hr pH that was adjusted to 10 using sodium hydroxide.8.21 gr of CaCl 2 was weighed into 75 mL of de-ionized water.Subsequently, the CaCl 2 solution was added dropwise to the solution mixture, yielding a milky suspension, which was refluxed for 24 h.
The precipitate was filtered and washed several times with distilled water to remove contaminated ions and surfactant.It was dried at 100 • C for 24 h.Calcination of powders was carried out at 400 • C for 4 h to yield a white powder.
Nanoporous hydroxyapatite powders are also synthesized by using P123 as structure directing agent.Different amount of surfactant P123 was first dissolved in 50 gr of deionized water with overnight stirring at room temperature.Calcium hydroxide Ca(OH) 2 and phosphoric acid (H 3 PO 4 85%) were used as calcium and phosphorus sources, respectively.
Aqueous solutions of calcium and phosphorus precursors were prepared by dissolving 2.22 gr of Ca(OH) 2 and 1.764 gr of H 3 PO 4 into 10 mL of deionized water.The calcium and phosphorus precursor solutions were added into the surfactant solution which was refluxed for 24 h.
The precipitate was filtered and washed several times with distilled water to remove contaminated ions and

Results and Discussion
Figure 2 shows the WAXRD patterns of the as-dried hydroxyapatite.WAXRD patterns of the nanoparticles consist of narrow peaks with d spacing consistent with well-ordered crystalline hydroxyapatite.It indicates that the sample consisted of a single phase of crystalline hydroxyapatite with a hexagonal structure (JCPDS 09-0432).
The XRD pattern demonstrates that pure and wellcrystalline HA is the only phase present.
The peaks within small angle range (2 theta 1-10 • ) provide information on the arrangement presented by porous structure [26].
For LAXRD (Figure 3), the pattern of the as-dried samples shows a single intense diffraction peak at 2θ value of 1.46, 1.33 • for samples synthesized in the presence of 3, 15, and 30 wt% P123, respectively.The single peak suggested the presence of disordered mesoporous structure inside the sample, such as worm-like mesopores [26,27].It is believed that d spacing in small angle range is corresponded to the distance between the neighboring pore centers, which can be regarded as the pore diameter while neglecting the wall thickness.
The FTIR spectra of nanoporous hydroxyapatite synthesized using P123 and CTAB as template are shown in characteristic for a typical HAP FTIR spectrum.The bands at 2922 and 2877 cm −1 were ascribed to CH 2 and CH groups, indicating the existence of surfactant chains within the HAP particles.The additional bands at 1684 and 1362 cm −1 were attributed to urea.The absorption band at 3200 cm −1 is due to adsorbed water.
Adsorption-desorption isotherms and pore size distributions of mesoporous hydroxyapatite calcined at 400 • C are shown in Figure 5. Sample synthesized in the presence of CTAB exhibits a mesoporous materials type IV curve with a hysteresis loop.The BET surface area is calculated to be 147 m 2 /g.The distribution of pore diameters was plotted according to the BJH nitrogen desorption.Figure 5 (insets) shows the distribution curves.Sharp peak appears at 46.5 A • which means that pore diameters around 4.6 nm are in the majority.
Figures 6(a), 6(b), and 6(c) show the BET surface area in meter square per gram and the effect of various surfactant concentrations during synthesis on the final BET surface area of HAp particles.It was observed that a 30 wt% P123 concentration gave the highest surface area in the final HAp powders, 50 m 2 /g.This resulting HAp precipitate interacts with the micelles to form a gel-like mass by electrostatic interaction.This leads to the formation of an organized structure, where the HAp precipitate is templated by micelles and solidified to form a matrix.During heat treatment, the micelle template decomposes into gases, such as CO 2 and HO, and leaves pores in the product.This type of mechanism is known in the silica-based inorganic system.
The surface area of the final powders is controlled by the particle size as well as the pore size and shape, which are determined by the micelle structure during powder preparation.In general, the micelle geometry is controlled by the concentration of the surfactant in solution and by the processing conditions.At different concentrations, the surfactant molecules aggregate into different forms, which influence the pore structure and surface area.
The thermal properties were studied by TGA in the temperature ranging from 25 to 800 • C under nitrogen  atmosphere, in order to choose precisely the calcinations temperature (Figure 5).The TGA curves of nanoporous hydroxyapatite synthesized by using CTAB (Figure 7) and synthesized by using P123 (Figure 8) show continuing weight losses from room temperature to 200 • C and also a sharp weight loss between 350 and 450 • C, approximating 1%

Conclusions
We have demonstrated the synthesis of phase-pure nanoporous hydroxyapatite inorganic using the P123 and CTAB micelle systems, where the nonionic surfactant concentration was found to have an influence on nanoparticle surface area.At the 30 wt% P123 surfactant concentration, HAp particles showed the highest surface area 50 m 2 /g.By using cationic surfactant CTAB, surface area increased to 147 m 2 /g.

Figure 8 :
Figure 8: Thermal analysis for nanoporous hydroxyapatite using P123 as template.