Do not know how you established "colloidal brown" or that the nanoparticles are "aggregated" or that you are not getting AgNPs. Please read regular, available literature first and then do not sonicate.

You should get Golden yellow colloidal solution for Ag nanoparticles. To prevent it from agglomeration you need to use a suitable stabilizing or capping agent in requisite proportion. Ag NP' s colloidal solution will never get brown or red but it may get grey or black on agglomeration.

When silver ions are reduced by plant extracts, a change in color is possible. Silver ions are first dehydrated, then various colored complexes with the substances of the extract can form. Plant extract substances can act as capping agents. Try to disperse the ultrasound as you suggest. Change the color with a change in the degree of hydration during ion reduction, see the article.

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Dear Drishti Garg,

As we know that plant extract is actually the combination of different biomolecules which act as both reducing and stabilizing agents. These silver NPs always get reduced by these biomolecules. After reaction (Silver nitrate+ Plant extract) the color of the solution get golden yellow or even sometime black, which is an indication for silver nanoparticles. Even sometime people add reducing agents, which is not as much beneficial as through plant extract which is safer and green approach.