Hii Bheem can u elaborate your question.

Naveen Sir, i have synthesized zinc nanoparticles by plant extract, after the solution is centrifuge for nanoparticles settle down in the form of pellet. After pellet was washed with distilled water to remove the excess biomolecules. But here pellet is not form.    

Dear Bheemanagouda,

I hope you confirmed presence of zinc nanoparticles through TEM or SEM and EDAX analysis. If you have confirmed, then you must also be awre of their size. If the size is small, you need to use ultra-centrifuge for sedimenting them. Can you please elaborate on what is the size of zinc nanoparticles and at what speed you centrifuged them? You need to be sure that the pellet (you seem to have obatined) contain nanoparticles or some insoluble complex/precipitate.

Sir,

Now only UV-Vis characterization was done, not checked the size by using SEM, TEM and AFM analysis and centrifuge speed is 3500 rpm.

Dear Bheemanagouda,

Please confirm presence of zinc nanoparticles through TEM. If the nanoparticles are small, they will not sediment down at such a low speed (i.e. 3500 rpm).

I think you may have to increase the centrifuge speed. Also please confirm the ZnNPs formation by TEM/SEM. Best of luck.

Thank you very much Saradhi and Mukherjee sir

You have to first find out what the rpm of the centrifuge available corresponds to in terms of acceleration. For a given acceleration, size and density you can calculate by Stokes law how long you have to centrifuge to separate all nanoparticles. You may succeed by simply using much longer time. Beside size additional problem might be biomaterial associated with the particles leading to lower density of particles.

Thank you very much sir