Depending upon what size range you are interested in, there are many ways to do it.

Best method would be start from tetrachloroauric acid. After that you can.

1. Dissolve it into epoxy or ploymer; cure it and then degrade it using deep UV radiation.

2. Reduce it using various agents (hydrazine, sodium borohydride, formaldeghyde, glucose).

3. Disintegrate in an autoclave.

4. And use alcohol-nitric acid combustion route if proper safety precautions can be arranged.

I hope it helps.

Thank you so much for your fruitful reply.Can you just suggest me if there is any green synthesis method using microbes.

The protocols for synthesis of TiO2 nano particles with the help Titamiun(IV) Isoproxide

The Turkevich method (citrate reduction) - slides 41/42 in the first url below - is the common approach for Au nanoparticles but almost any other reducing agent and appropriate precursor can be used (HAuCl4.xH2O is common). Recipe on slide 18 for Au colloids in the second url below.  For general overviews see:

February 18th, 2016 Metal colloids - their preparation, application and characterization

https://www.brainshark.com/malvern/vu?pi=529368070&b=1&tx=120486&c1=22

September 2nd, 2015 Silver colloids and invisible ink

https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22

For more entertainment see:

July 30th, 2013 Optical properties of gold

https://www.brainshark.com/malvern/vu?pi=883587233&text=M021507

@Manmohan Kamboj,I am already acquainted with the TiO2 method and infact my co-workers are working on that.I want some exact protocols for green synthesis of gold nano and silver nano particles.

@Alan F Rawle sir..This is so nice of you that you made me understand every minute details of the whole process.Thank you so much for your humble reply.I am going through it.