Not sure if it will work for your research, but you can try cross flow filtration (which works for nanotubes, at least), AC dielectrophoresis, or gel diffusion (although this technique is used mainly for analysis or separation of small amount of material).

Thanks Oleg. I think better to try it. 

By ultrafiltration AKA dialysis.

We usually separate  solid particles having very little density difference with the coal liquids by assisted settling methods  including thermal coagulation, flocculation, agglomeration, etc.You can try these methods. Nanoparticles are usually a form of associate colloids. The associate colloidal particle can be grown by micellea formation. Try it.

Thanks you Sir, I will try all these methods. This can be a good project. Dr. Iva, i read the paper. It is very interesting to try.

It depend of the kind of NP you have.

Do you want to purify it from the reactants?

Is it the first purification step?

Do you want to separate the particles by size?

Did you perform a post-modification onto NPs?

Which is the media where NPs are soluble/dispersed?

Every method have pro and cons ... would you tell us more details about your problem? to give you better ideas

Thanks Mr. Julio for detailed questionnaire. 

Suppose they are in first step, synthesised from plant extract. I prepared them by using silver or gold. Now i need to centrifuge them. Can i separate without using centrifuge?

The problem is plant extract has hundreds of compounds and only a few will take part towards NPs. 

@ Said Nadeem Had your problem solved? i am facing the same problem

Hi dear Said, depending on the nature and properties of the nanoparticles, methods such as Electrophoretic methods, ultrafiltration AKA dialysis, chromatography and etc. can be used.

I think the following articles can be useful.

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Article On-line enhancement and separation of nanoparticles using ca...

Article Purification of Molecularly Bridged Metal Nanoparticle Array...

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