Chapter 3 Synthesis and Characterisation of Calcium Phosphate Nanoparticles Abstract The in-situ prepared nanoparticles of calcium phosphate were characterized by various analytical methods such as XRD and TEM. From XRD measurements it was concluded that the particle size was around 1 nm. The TEM analysis confirmed that the particles are of size in 1 nm regimes and have narrow size distribution. Also with increase in concentration of PEO the particle size decreased. Results of this chapter have been communicated for publication in Chemistry of Materials
18 Chapter 3 3.1. Introduction In-situ deposition technique is a versatile technique for the preparation of nanoparticles of very low size. There were several reports in the literature regarding this technique and the successful application of the same [1-5]. Polyethylene oxide (PEO) mediated synthesis of nanomaterials of inorganic sulphates and phosphates were reported by Radhakrishnan and his coworkers recently [3-5]. In this technique the calcium salt is dissolved in PEO and stoichiometric amounts of sulphates or phosphates were added to the mixture. On digesting the mixture the formation of tiny particles occurs. The particles can be easily obtained from the reaction mixture because the residual components are either volatile or easily removable. So white powders of calcium phosphate were obtained by this in-situ deposition technique. Calcium phosphate nanoparticles are excellent protein carrying agents and have stable structure over a period of time [6-8]. 3.2. Nanoparticle characterization by XRD Nanoparticles of calcium phosphate were characterized by XRD as well as TEM techniques. Figure 3.1.1(a-f) corresponds to the XRD patterns of nanoparticles obtained, (a) corresponds to the calcium phosphate without PEO and (b-f) for calcium phosphate with PEO concentration from 2:1 to 32:1 respectively. In the figure, there are a number of crystalline forms reported for calcium phosphate existing in both anhydrous and hydrated states. The calcium phosphate without any polymer shows 3 types of phases. Most of the peaks in diffraction patters correspond to β calcium orthophosphate (major phase) while the minor phase contains calcium orthophosphate primary along with small amount of calcium metaphosphate hydrate respectively. With increase of PEO concentration, there occurs a large change in the diffraction patterns compared to curve A. A large number of peaks are suppressed and peaks at 2θ of 31.9 broaden with increase of PEO concentrations from 42 to 89%. Only two peaks are clearly seen. The peak at 31.9 corresponds to all the three states (Ca 3 (PO 4 ) 2, Ca 3 (H 2 PO 4 ) 2, Ca 3 (PO 3 ) 2 3H 2 O) and at 25.1 corresponds to the minor phase Ca 3 (H 2 PO 4 ) 2. It is interesting to note the extent of broadening from 42 to 89%. This shows that the small size of crystals, which could be associated due to good molecular level
Synthesis and Characterisation of Calcium Phosphate Nanoparticles 19 mixing. Thus on comparing curve a with curve b to f in figure 3.1, it is obvious that the effect of polymer plays a prominent role on the structure and growth behaviour of calcium phosphate crystals (in the curves the y axis indicates the intensity. Since CaCl 2 is first complexed with PEO, only certain crystalline phases of calcium phosphate is allowed to grow as compared to a large number of phases getting developed in normal free precipitation. From the XRD patterns, the crystallite size was calculated using Scherrer formula. d( A) = kλ 2θ cosθ (3.1) where k, the order of reflection, λ is 1.542 A θ, the diffraction angle and 2θ is the full width at half maximum (FWHM). As the concentration of the PEO increases the particle size decreases. The corresponding data for the nanoparticles is given in Table 3.1. (a) (b) (c) (d) (e) (f) Figure 3.1: XRD patterns for the calcium phosphate nanoparticles (a) Calcium Phosphate without PEO. The PEO: CaCl 2 ratio in (b) 2:1, (c) 4:1, (d) 8:1 (e) 16:1 and (f) 32:1 (in the x axis 2θ and in y axis intensity is plotted)
11 Chapter 3 (PEO: CaCl 2 ) FWHM (2θ), radians Particle size (L), nm Calcium phosphate (a).174 82.4 2:1 (b).134 1.65 4:1 (c).156 9.24 8:1 (d).162 8.96 16:1 (e).176 8.2 32:1 (f).181 7.82 Table 3.1. XRD values for the calcium phosphate nanoparticles 3.3. Nanoparticle characterization by TEM The TEM images of the particles are shown in figure 3.2 (a-c), which are taken for 2:1, 4:1 and 32: 1 respectively. The particles were made a suspension in alcoholic medium and the images were taken by putting the particles on a copper grid. The alcohol evaporated and the particles could be seen clearly in the images. The particle size distributions for each concentration were calculated using image analyzer software. (a) 1 nm (b) 1 nm (c) 1 nm Figure 3.2. TEM image of the calcium phosphate nanoparticles (PEO: CaCl 2, (a) 2:1, (b) 4:1, (c) 32:1)
Synthesis and Characterisation of Calcium Phosphate Nanoparticles 111 Particle size distribution curves for the three compositions are given in figure 3.3. Around 4 particles were taken for the analysis. It can be seen that majority of the particles have small size around 1 nm. Thus the in-situ deposition technique is very good to obtain narrow distributed nanoparticles. We can see that for each concentration there are a few particles with large size. Particle size distribution Particle size distribution Particle size distribution N o.o f p articles 45 4 35 3 25 2 15 1 5 5 1 15 2 25 Particle size (nm) No.of particles 45 4 35 3 25 2 15 1 5 5 1 15 2 25 Particle size (nm) No.of particles 6 5 4 3 2 1 5 1 15 2 Particle size (nm) (a) 2:1 (b) 4:1 (c) 32:1 Figure 3.3. Particle size distribution of the calcium phosphate nanoparticles (PEO: CaCl 2, (a) 2:1, (b) 4:1, (c) 32:1) The particle size reduction against concentration of PEO is shown in figure 3.4. The particle size reduction is attributed to the increase in the amount of oxygen atoms available from PEO matrix as we increase the concentration [5]. This enables the suppression of the formed particles and reduces the size further.
112 Chapter 3 1.4 1.2 1. Particle size (nm) 9.8 9.6 9.4 9.2 9. 8.8 8.6 5 1 15 2 25 3 35 Concentration of PEO: CaCl 2 (g/1ml) Figure 3.4. Effect of PEO concentration on particle size of calcium phosphate The particle size reduction can be mathematically expressed as a general equation, with respect to the concentration of PEO used, like y =.42x+ 1.732 (3.2) This linear equation gives a negative slope, which indicates that the concentration of PEO is directly related to the particle size reduction. The yield of the calcium phosphate was recorded as 83 and 72% for 4:1 and 16:1 ratios respectively. Calcium phosphate nanoparticles obtained with the concentration ratio 4:1 (particle size approximately 1 nm) were exclusively used for further studies in this thesis. 3.4. Conclusion Calcium phosphate nanoparticles were synthesized by the polymer induced crystallization technique. The obtained particles were characterized by XRD and TEM analysis. From XRD, using Scherrer formula the particle size was calculated and it was found that the particle size is in the order of 1 nm. TEM images also supported the above analysis. The particle size distribution was measured and it was found to be narrow. Further the size of the particles reduced with respect to increase in concentration of PEO due to the increased availability of the oxygen atoms in PEO.
Synthesis and Characterisation of Calcium Phosphate Nanoparticles 113 3.5 References 1. H Schmidt, Appl Organometal Chem 15 (21) 331 2. M Figlarz, F Fievet, JP Lagier (1985) US Patent Specification No 4, 539, 41 3. S Radhakrishnan, J Cryst Growth 141 (1994) 437 4. C Saujanya, S Radhakrishnan, J Mater Sci 33 (1998) 163 5. C Saujanya, S Radhakrishnan, Polymer 42 (21) 6723 6. BioSante Pharmaceuticals, Inc. 175 Olde Half Day Road Lincolnshire, Illinois USA 669 www.biosantepharma.com 7. HT Schmidt, BL Gray, PA Wingert, AE Ostafin, Chem Mater 16 (24) 4942 8. S Mishra, S Sonawane, A Mukherji, HC Mruthyunjaya, J Appl Polym Sci 1 (26) 419
Part I Polystyrene/calcium phosphate nanocomposites