Hydrazine will be better then sodium borohydride

Citric acid could be tried for pH 3 and boric acid or tris could be used for pH 9. You need to be cautious with several things: the buffer concentration is critical, the speciation of gold ions will change, you need to provide time for proper gold speciation, the buffer molecules could associate with the gold ions, the reduction potential of gold ions will change, etc. I am not familiar with PVP as reducing agent but I assume it also adsorbs on the surface of gold nanoparticles to impart steric stability. If so the effect of buffer ions on the ionic strength or particle stability (by charge) will not be notorious.

Thanks for your reply sir. I already tried using tris of pH 9, I couldn't get any nanoparticles. It seems Tris itself is a reducing agent.

Citric acid can also reduce as well as stabilize AuNPs ( but if Dr. Omar Gonzalez-Ortega said then you should try and if it works then please share with us).

Speaking about the pH, i would suggest the phosphate buffer for acidic and carbonate-bicarbonate for basic. i am not sure about this but this can be tried.

if it is not necessary to synthesize at a particular pH, then pH can be adjusted after synthesis also.

Regards!

Best of luck.

Are you running the synthesis at high temperature?

@Omar Gonzalez, no sir reaction is being carried out at room temperature conditions.

Then you can use citric acid to buffer pH. High temp is required for citric acid to reduce gold ions.

Thank you sir, @Dr Omar Gonzalez-Ortega

Should I make separate buffer and use it for dilution of reactants (i.e using buffer solutions as solvent rather than water ) or should I make reactants solution buffer themselves, i.e adding required amount of buffer chemicals in the reactants solutions itself?

For proper gold speciation, the buffer must be added to the gold solution. Also PVP must be buffered.

Hydrazine will be better then sodium borohydride