I have 1 mM AuNPs solution.

Then I mixed with buffer (ration 1:1)

Tube 1 --> Tris-HCl EDTA (TE 1X)

Tube 2 --> 10 mM Tris-acetate containing 50 mM NaCl

The color is change to purple (the parameter of aggregation).

How it becomes like that and how to prevent it??