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Hydroxyapatite (HA) nano powders have been synthesized using surfactant templating approach. Cationic surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB), was used as template to regulate the nucleation and crystal growth. Yield of final product was examined by varying the surfactant concentration. The synthesized powders were characterized using x-ray diffraction, Fourier transform spectrograph and scanning electron microscope. The diffraction data revealed the characteristic peaks of HA, where a hexagonal structure can be deduced. Calculation of the lattice parameters indicate that the HA synthesized in the present work had a larger value of c as compared to the standard values shown in the JPCDs data. In addition, the results indicate that the concentration of surfactant in the solution had a significant effect on morphology and crystallite sizes of the powder particles.
Hydroxyapatite (HA) has been widely studied as an important biocompatible materials because of its chemicals similarity to natural Ca-phosphate mineral present in biological hard tissues [1-4].Owing to its bioactive, biodegradable and osteoconductive properties [5-6], HA have been widely used in many medical applications e.g. HA can be used as a material for metallic implant coating or for bone cavity fillings .
In recent years, preparation, characterization and applications of the nanosized calcium phosphates have received increasing attention from many researchers around the world [8-9]. New developments on the production of nanosized HA particles have led to many new applications for example, nanosized HA particles can retard the multiplication of cancer cells and can be used as an efficient drug delivery agents [10-11].
Calcium phosphate nanoparticles can be fabricated by employing a variety of chemical based processing routes such as wet chemical route, sol-gel technique, solid state reactions at elevated temperature, biosynthesis route, chemical precipitation, hydrothermal reaction, microwave heating and emulsion processing route [10-13]. Recently surfactant based systems have proved to be very efficient templates for producing HA nanoparticles with controlled particle size and shape [14-15]. Although this control over the structure seems to be a big challenge, but controlled nucleation and crystal growth process mediated by macromolecular control and cell organization would finally result in uniform products. In the presence of extraneous additives, the role of crystal surface is more analogous to the conventional view of host-guest systems. The surface layers of the crystal can incorporate soluble additives provided that there is a degree of complementarity in charge and size between the guest ions and the interstices in the structure of the crystal boundary layers. Several macromolecules such as stearic acid, monosacchrides and related molecules have been explored with desired control on the morphology. Cetyl trimethyl ammonium bromide (CTAB), as one of the macromolecules, is also widely used to achieve the control of the morphology in many aqueous synthetic methods [16-18].
In this article, we report the synthesis of HA nanoparticles using CTAB micelles as nucleation points for the crystal growth. The influence of surfactant concentration on the precipitated phases, yield and morphology of the product has been studied/ analyzed.
Materials and Methods
The flow chart for the preparation of HA is illustrated in Fig. 1. The materials used in this work included calcium chloride (CaCl2), di-potassium hydrogen phosphate (K2HPO4), NaOH and Cetyl trimethyl ammonium bromide (CTAB). All chemicals were of analytical grade from Merck and aqueous solutions were made by dissolving them in doubly distilled de-ionized water.
Solutions of calcium chloride and di-potassium hydrogen phosphate were prepared by dissolving the required amounts in 60ml and 100ml of de-ionized water respectively. The surfactant solutions, with concentrations ranging from 0.01M to 0.06M, were prepared by dissolving the required amounts of CTAB in 100ml of de-ionized water.
Micellization and co-operative interaction
Formation of HA precursors in surfactant solution
Aqueous soln.of CTAB
Aqueous soln. of
Reflux at 120°C
Addition of CaCl2
Drying and Calcination
wash with de-ionized waterFig 1: Flow chart procedure for HA synthesis
Table 1: Sample ID and experimental parameters
Sr. Sample Surfactant conc. Yield Crystallite size Lattice parameter Lattice parameter
No. ID (M) (gm) (nm) a (A°) c (A°)
1 a 0.01 2.0 3.27 9.424 10.8175
2 b 0.02 2.3 3.82 9.4314 11.7439
3 c 0.04 2.4 3.83 9.4716 12.074
4 d 0.05 2.7 5.77 9.8905 13.687 5 e 0.06 2.9 5.67 11.997 13.626 6 f 0.4The reaction parameters of different batches are given in Table 1. Both the surfactant and phosphate solutions were added into 100ml of de-ionized water and the pH of the resulting solution was adjusted at 12 using NaOH. The above solution was kept for two hours to ensure that the co-operative interaction and self assembly process has been completed .
Subsequently the CaCl2 was slowly added drop-wise to the solution mixture, yielding a milky suspension, which was refluxed at 120°C for 24 hours. The precipitates were than filtered off and washed with de-ionized water. A gel like paste was produced which was then dried in an oven at 100°C for 24 hrs. The powder was then calcined in a furnace at 550°C for 5 hrs to yield HA powder.
Fourier transform infrared (FTIR) spectroscopic measurements were taken on Perkin-Elmer FTIR (model Midac 1000 series) spectrophotometer. Infrared spectra were recorded in the region of 500cm-1 to 4500cm-1 with a resolution of 16 cm-1.
The X-ray diffraction measurements were taken in a Bruker D8 Discover powder diffractometer using CuKα radiation at wavelength of 1.548 A°. The instrument was operated at 40kV with current of 40mA. The specimens were scanned between 2θ range of 10° to 70° at scanning speed of 4 degree/min.
Results and Discussion
All the HA powders obtained after calcination were of white color. The precipitates were observed to have agglomerated into large size aggregates. The yield of the product obtained for different samples is shown in Table 1 and it is clear that with the increasing concentration of surfactant the yield of HA powder increases.
Fig. 2 shows the FTIR spectra of the calcined samples. These spectra show the characteristic bands of HA along with some additional bands that are ascribed to impurity ions (CO32-, HPO42-) and some associated water. Bands at 1413 cm-1, 1415 cm-1 to 1464cm-1 indicate CO32- ions. The band at 874 cm-1 is attributed to arise due to the presence of HPO42-.Phosphate absorption bands also appear at 1034cm-1, 962cm-1, and at 605cm-1. The broad band from 3300cm-1 to 3700cm-1 is due to stretching mode of hydrogen bonded water molecules and at 1630cm-1 to 1635cm-1 derives from bending mode of water molecules. The sharpness of bands at 569cm-1 and 574 cm-1 indicate a well crystallized HA.
Fig. 2. FTIR Spectra of the synthesized samples
The XRD patterns of the powders synthesized with various CTAB concentrations in the post micellar region are shown in Fig. 3.
The diffraction patterns revealed characteristic peaks of HA, where the filled squares correspond to the standard characteristic peaks for HA. a high consistency between the data of our prepared samples and that obtained from standard JPCD International Centre for diffraction (JPCD card no. 024-0033) is observed. The results indicated that the products prepared consisted of pure phases. As is shown in Fig. 3, the effect of CTAB concentrations on the aggregate phase is hard to ascertain so the crystallite sizes and lattice parameters were calculated to visualize the behavior of the surfactant
Fig. 3. XRD Spectra of the synthesized samples
The crystallite size of the synthesized HA is calculated by the Scherrer's formula  as follows:
where t is the crystallite size of the synthesized HA, λ is wavelength of Cu Kα radiation (1.54 A°), β is full width at half maximum intensity (FWHM) and θ is the Bragg's angle. By examining the peak width obtained from distinct families of crystal phases the apparent crystallite size in a particular direction can be determined.
Fig.4. shows the crystallite sizes of the synthesized HA with varying CTAB concentrations. As the CTAB concentration increases it is observed that the crystallite size also increases. Studies indicate that the increasing surfactant concentration in the post micellar region has the same effect as that of increasing the hydrophobic character resulting in an increase in the aggregation number and size of the micelles . Acting as templates, these micellar structures help in the epitaxial growth of the product .
The lattice parameters of the synthesized powders are shown in fig.5. All the as-synthesized HA with different CTAB concentration show an increase in the values of both a and c as compared with standard values (a=9.418 A°and c = 6.884 A°). However the increase along c-axis is more compared along a-axis, which shows greater epitaxial growth of HA along c-axis.
Fig. 4. Plot between surfactant concentration, powder yield and resulting crystallite size.
As for the effect of CTAB, it was thought to be able to act as template resulting in the epitaxial growth of the product. a process called molecule recognition could have taken place at the organic/inorganic interface .
In an aqueous system, CTAB would ionize completely and result in a cation with tetra- hedral structure. At the same time, phosphate anion is also tetrahedral structure. It has been proposed in earlier studies that the charge and structure complementarity endows anisotropic CTAB with the capability to incorporate into the phosphate anion and control the crystallization process . In the present case, it is assumed that after nucleation of HA in CTAB micelles, the CTAB head groups were preferentially adsorbed on surface planes with phosphate anion on the surface. Intensive charge interaction on the surface planes resulted in the HA crystals having a larger c- value compared with standard value of JPCD's card. From this it is certain that the surfactant binds to certain faces of crystals and to certain ions so as to change the lattice parameters.
Fig. 5 reveals the SEM images of the prepared samples done with JEOL SEM (JSM - 5910) equipped with a field emission gun.
Sem images show the typical morphology of the particles which are clearly below 100nm in
Fig. 5. SEM micrographs of HA powders
produced using various surfactant
size. At lower concentrations of CTAB, the particles have a shape closer to, but not exactly, spherical with very little elongation in one direction. but at 0.4M CTAB concentration (0.4M) a significant change in the morphology of the product is being observed and particles have been transformeb into elongated cylinders.
Nano powders of HA have been successfully prepared using surfactant self assembling templates approach.
XRD patterns reveal the phase purity of the product. Also the patterns indicated hexagonal structure.
The calculations of lattice parameters indicate an increase in values of a and c with increasing surfactant concentration, but this increase in c-value is more as compared to a due to epitaxial growth of HA along an axis.
The yield of the product and crystallite size was observed to increase with the increasing surfactant concentration.
Surfactant concentration has a profound effect on the morphology and size of the product.