you can try turkewich method to get desired Au nanoparticles .

Possible, you have too much residue sodium chloride in the solution; NaCl is effective coagulant for nanogold. Try the test assay by using the stabilizing polymer for preventing of agglomeration. PVP (polyvinylpyrrolidone) is the best, but polysaccharides (as soluble starch) are approved too. Heat to boiling an aqueous solution of citrate (and polymer), transfer glass to a magnetic stirrer and quickly add the gold salt solution in a small amount of water. If you'll obtain red wine - the next step sould be purification of precursors.

Yes i will also suggest you to follow the turkevich method or seed mediated method. And in the prepararation of Gold Nano particles you should carefully controll the Parameters like Concentration of HAuCl4,Temperature, RPM, Equipment cleanliness and also use some references such "One-step seeded growth of Au nanoparticles with widely tunable sizes†

Chuanbo Gao, John Vuong, Qiao Zhang, Yiding Liu and Yadong Yin". Thanks

Thank you for all of your answers. I found them really helpful

Actually, I tried again using a larger volume (of course, a less concentration of gold precursor) to start with and finally obtained the red wine color. In order to achieve such result, I tried repelling the residual acid vapor during the aqua regia dissolution and reconstituted the similar amount of DI water. The resulted solution (without excess salt), formed red wine AuNPs perfectly.

I just wanna ask further about the role of starch as the stabilizer. Is the bond between AuNPs - starch so stable that I have to use another reagent to break them apart? I need the free AuNPs for the next step of my experiment, so this is definitely my concern.

Thank you so much in advance.

In fact, Au-glucan bond is weak and can be replaced easy. For example, the most of biomolecules contain thiol groups, Au-S bond is very strong (>125 kJ/mol) and practically irreversible. From the other side, it will be difficult to remove polysaccharide as high-weight molecule (while small molecules can be dialyzed). It depends on your project and permissible contaminations. In some processes, starch will not interfere - see an enclosed example of such paper.

Article Starch-supported gold nanoparticles and their use in 4-nitro...

Dear Dr.Scherbakov.

Thank you for your information. I will conduct some more experiments and discuss whenever possible.

Best regards,

i obtained different time red wine while i controle pH at 10

prepare the following solutions

1- %1 wt sodium citrate in Distilled water

2-1%wt NaBH4 in the above solution

3-20mg fo HAuCl4 in water and add 1ml solution NO.1stirrer for 20min.

4-add 1ml of Solution No.2 to Solution NO.3 and stirred for 30 min