Centrifuge your particles at 5000 rpm for 20 minutes. Then re-dispersed nanoparticles in deionized water and again centrifuge. 

Dear Deepshikha,

If I understand the question correctly, the following two options are applicable in my experience:

1) Silver nanoparticles of 10nm or larger are centrifugally pelleted at 12-15,000rpm over 1 h-90mins, leaving silver nitrate in solution. Wash the pellet several times. For larger silver nanoparticles, try a slower speed. Ideally a soft pellet will be easier to resuspend in solution (water?).

2) Size exclusion chromatography of capped (coated) silver nanoparticles (such as using Sephadex G25 chromatography media) will easily purify the nanoparticles first, before washing off salts from the media.

Paul

It is possible to use dialysis or low speed centrifugation with membrane tubes:

http://kirschner.med.harvard.edu/files/protocols/Millipore_Centricons.pdf

Good luck!

if particles are dissolved you will not be able to separate them (solution of ions). For nanoparticle suspension do the calculation based on Stokes law. Make sure that during washing particles do not get destabilized.

Centrifuge your particles at 5000 rpm for 20 minutes. Then re-dispersed nanoparticles in deionized water and again centrifuge. 

I used 17000 g for centrifuging and washing the solution, but the supernatant was still yellow. Is that mean that certain silver nanoparticles was still in the supernatant?