In my feeling, the best method is clean the nanoparticles by centrifugation (sedimentation and re-dispersion) in water or other solvent. But take care... excess of cleaning would promote agglomeration of your nanoparticles.

I also agree with Luca.

You can remove SDS (i did it with PVP) just boiling (not really necessary but works better) with water. Keep in mind that you will have to repeat it at least 3-5 times with a quite large amount of water. EtOH could work too but I have no experience with that. There are also expensive colums to do it quantitatively, as the do it for DNA purification.

In my feeling, the best method is clean the nanoparticles by centrifugation (sedimentation and re-dispersion) in water or other solvent. But take care... excess of cleaning would promote agglomeration of your nanoparticles.

I also agree with Luca.

Thank you very much Luca and Miquel.

The product could be washed for several times by

centrifugation and redispersing in ethanol or water to remove the surfactant.

This technical note provides several purification techniques. I hope it helps.

Yes, Technical Note provided by Boon-Hong Wee is very useful and it will be useful for you. You can choose appropriate method to remove surfactant from your nanoparticles.

I understand that you can use above said methods for removing excess surfactants after synthesis. However it is difficult to remove those are chemically attached to the nanoparticles as a capping agents or so. So you should not try to remove that one which will creat problems also.

It is also possible to remove surfactant by calcination at higher temperature, but you have to see that it should not effect on your catalytic activity or changes in phase of your nanoparticle.

Also, the nanoparticles without surfactant can be obtained would be filtered through a

membrane filter (PTFE filter membrane 200 nm) using distilled water.

thank you very much... i am on to it now. The technical note by Boon-Hong Wee is also very helpful. thank you very much...

to remove surfactant from nano-particles: you can use Soxhlet extraction, or centrifugation, or dialysis, or cross-flow filtration.

By using calcination at 500 oC for 3-5 h or several washing with distillated water, SDS may be removed.

usually any surfact could be removed from nanoparticles solution simply by washing steps. In case tagged with protein etc, surfact can also be removed by dialysis in appropriate buffer.

Hey you can remove surfactant by centrifuging them and washing them with ethanol , then with methanol. You can try this for 10-11 times for 15 minutes given for each centrifugation cycle. I have tried to remove using this only. I hope it works for you also.

Thank you very much

how about calcination at high temperature like Fatemeh said.

I have tried with washing and it is working. Thank you very much. with the calcintion will the sodium be completely removed?

Actually, I am also trying to remove it now...The surfactant I used is SDS, once I got the results I will put it here.

Washing with large volumes of water by centrifuge followed by absolute ethanol worked but the nanoparticles agglomerated...

You are right, that is also my concern. So I will try calcination.

no calcination! you may sinter your product! I would recommend dialysis against water containing another stabilizer. Repeat many time and that should go.

you are right... Calcination resuted with the particles sintering

I agree with Srishty Anand

The best way, I think, is to wash your nanoparticles with some organic solvent such as ethanol or acetone etc... Moreover, you can also anneal your sample accordingly to get rid of the surfactant at elevated temperature.

do dialysis. agree with Alexandre Mantion

Suggest trying the addition of KCl - Potassium salts of SDS are remarkably insoluble. You will still need to deal with the Na+ counter-ion from NaSDS, but that may be removed/exchanged via dialysis.

Hope this helps??

The best way to remove surfactant is by washing with excess water and then acetone.

You have to watch for agglomeration. So dialysis while replacing surfactant with other small spacers that can provide electrostatic repulsion will work.

I used both ways of calcination at 500 C for several hours or reflux in ethanol for about an hour, both are ok

By washing, extraction or centrifugation

You can use as the other researches did. For example is centrifugal process, freeze dryer or using electrophoresis.

Repeated washing and centrifugation is sufficient to remove any surfactnat bound to any nanoparticles. This is well known to colloidal chemists for centuries.

Repeated washing and calcination could help to remove surfactant form the nanoparticles.

HNO3 works!

So how can we determine whether the surfactant is removed or not? I mean by which technical analysis ^^

I am late actually, consider your size also, you can use vacuum filtration system using  .22 or .45 micron filters, by this way you can get your surfactant removed concentrated NP

How do you determine or conclude that the surfactant has been removed at the end of the cleaning process?

how to pick which solvent to use for washing and centrifugation ? 

methanol is better, at the end you will have less carbon content adsorbed!!!

Give wash.. Some times with.. Disttiled water... For remove surfuctent....

Try adding acetone to your nanoparticles which readily removes surfactant from NPs. Then u can stabilize with some other surfactant depends on your application. 

Transmission electron microscopy is suitable for checking if the surfactant is removed or not ,STEM mode is better.