This protocol may help you.

http://www.sciencedirect.com/science/article/pii/S0167577X05001539

Hydroxyapatite nano-particle can be otained by the controling the parameters of the precipitation process.

Well You can synthesize HAp nanofiber. Please refer to my paper 

http://www.sciencedirect.com/science/article/pii/S0167577X14019491

They are stable for 6 months. Incase you specifically need nanoparticles try to change capping agent BSA would be good for this. 

I hope this workout for you. 

I recommend you this simple and well establish methodology:

https://www.researchgate.net/publication/230875618_Ion_exchange_behaviour_of_silver-doped_apatite_micro-and_nanoparticles_as_antibacterial_biomaterial

If you need more information contact me directly  

Article Ion exchange behaviour of silver-doped apatite micro-and nan...

Thank you so much for your answers and articles

Problem concern is when ever I am dealing with particle size analysis by using DLS, size is not coming into nano. We require size of particles between 50-100 nm. 

Well sir DLS is not correct way to measure size because it show hydrodynamic range 

you should perform SEM or TEM analysis for exact size measurement. 

And it may be possible that morphology of your particle is different that's why DLS is showing you size in higher range. 

My suggestion is to perform SEM analysis. 

All the Best 

Thank You Akshay S Bhadwal for your corresponding reply. From SEM or TEM we will get to know the exact size of nanoparticles.

I have gone through some literature in which people are measuring particle size by using DLS and they are getting size properly.