Dear Tushar,

If you have used metallic silver nanoparticles (not silver oxide), I assume that you have purchased commercially (Because in chemical synthesis, we get only AgO NPs). If that's the case, AgNPs appear yellow and on centrifugation, the Ag would have got oxidized to AgO NPs and thus appear dark.

You can confirm this with the XRD peaks of the particles before and after centrifugation.

All the Best.

Abdul Azeez Nazeer Thanks for your reply. Actually I have prepared AgNPs in my lab using AgNo3, NaBH4 and Sodium citrate. After, centrifuge the peak and color both changed.

Yes. Even in that case, your particles got oxidized. Ag turns dark with oxidation.

Abdul Azeez Nazeer Thank You. Another query is what does it mean when they say the Synthesized AgNPs were centrifuged and washed thrice. What kind of washing the researchers are talking about ? Because once you centrifuge you get the Separated NPs from water. In one protocol they have mentioned that after centrifuge the NPs were washed thrice. Do you know what washing are they talking about and with what ?

Tushar Ramesh Pavase The washing is with ethanol. It is washed more than once to remove the reducing agents (basic salts), unreacted precursors. Centrifugation is basically to rip of the solutes from the solution. Hence for the better yield and purity, centrifugation and washing are performed.