Controlling the pH of the solution is very important in the synthesis of Au NPs. pH=5 was recommended. Please check the following reference.

Nanoscale Research Letters, 2016,11,362.

Good luck!

I recommend Please check the following articles:

• DOI: 10.14202/vetworld.2014.72-77
• DOI: 10.3390/s18072246
• DOI: 10.4028/www.scientific.net/AMR.709.19

Hi Maheshwaran,

I recommend clean all the glassware with freshly prepared HCl: HNO3 solution (molar ratio of 3:1) and water. It will help you to avoid undesired nucleation during the synthesis. Due to contamination, the gold colloids may also aggregate.

its likely due to contamination

which gold salt have you used ?

and which acid you used to lower the pH ?

Hello Maheshwaran Velusamy

It might be contamination. Please note that the synthesis of AuNP is sensitive to impurities per my experience. I suggest cleaning your bottles with aqua regia & EtOH 3 times before you prepare.