The nano- hydroxyapatite (n-HA) has been synthesized using a wet
chemical method, which is also known as precipitation method. The
wet chemical precipitation, combined with the thermal treatment at
high temperatures results in the formation of crystalline phases. The
process involves drop-wise addition of aqueous solution of one reagent
to the aqueous solution of another reagent, under continuous stirring.
The product obtained after precipitation and centrifugation, is then
sintered at high temperatures. The detailed experimental protocol
used for n-HA synthesis is as follows: first the calcium hydroxide
[Ca(OH)2] solution (0.5 M) and orthophosphoric acid (H3PO4) solution
(0.3 M) was prepared in deionized (DI) water, separately. Then, the
H3PO4 solution was added drop-wise to 100 mL solution of Ca(OH)2,
till the pH of solution reached 12.5. In this synthesis, maintaining the
pH was crucial step as the pH and temperature play important role in
deciding morphology and crystallinity of the product [30,31]. Manoj et
al. [32] studied the effect of processing parameters like temperature,
pH and stirring time on HA formation. At pH 7.0 and 9.0, a mixture of
calcium phosphate phases was observed and the rate of HA formation
was very poor. At pH 11.5, no calcium phosphate phases were present
and the reaction showed fast particle growth. At alkaline conditions,
the instable and amorphous CaP phases transform into a stable HA according
to the following reaction