Have you thought about 2D Gel electrophoresis ? But the first question is What do you want to be removed : Nanoparticles or Electrolytes ?

Best regards

Ultracentrifugation?

http://static.thermoscientific.com/images/D20943~.pdf

http://www.nature.com/ncomms/journal/v2/n6/full/ncomms1338.html

More precise i want to have a electrolyte free dispersion of "unprotected" metal nanoparticle. Which should be more stable against aggregation.....

by unprotected do you mean there will be no ligand protecting the surface? This will cause aggregation and will not be stable... when metal nanoparticles are not protected the collisions they experience upon each other in solution can cause fusing. You also need a ligand on the surface to maintain solubility. Otherwise they are just TINY rocks suspended in solution until gravity eventually pulls them out.

if you just want to get rid of the electrolyte anything above 10nm you can centrifuge out of solution no problem. like Giovanna said. Anything smaller than that will require a high concentration of salt to precipitate out and then you can centrifuge that out. To ensure electrolyte free particles I would sonicate or vortex the pellet in 70% ethanol and this will dissolve electrolytes while keeping the NPs out of solution. Then you just centrifuge again.

electrophoresis will not work because you need electrolytes to do it...

if you don't have a centrifuge then you can simple do dialysis.

Nanoparticle stability depends mainly on their surface charge and their magnetic properties. If the nanoparticles are naturally charged (either positive or negative) under the desired conditions then removal of electrolytes will increase their stability due to Debye length increases, unless thermal energy is enough to cause collisions and bring them close enough to coagulate. If the natural nanoparticle surface charge is close to zero then the presence or absence of electrolytes will make no difference to their stability. If the particles are magnetic then keeping them unaggregated is even harder and you need to protect the particle with a large enough steric barrier to avoid magnetic attractive forces, which can be very long range.

If you still want to remove electrolytes; dialysis, ultrafiltration and centrifugation are the main ways of doing it.

I would suggest membrane filtration, I used down to 10 nm membrane pore size, maybe there are smaller ones as well.

@Franca Jones: no they are "non" magnetic and the zeta potential is about -50 mV. So i think the surface is more like MxOy than M (also they are stable for one/two weeks with 2 - 4% of weight, depends on the "rate/time" of dialysis) but centrifugation cause irreversible aggregation! @ Bernhard Wessling can you please tell me a supplier?

you may want to check with Merck (Millipore) or Sartorius

Another question: how did you measure the zeta potential?

Re membrane filtration: we had only used it for analytical purposes, if you plan to use for preparative purposes, you may need to evaluate how to handle it, also the question which memebranes are useful requires some studies.

zeta potential with dynamic light scattering, also the particle size and you are right I`m looking for a preparative method or better a high effort procedure.

Preferably low cost!

And for sure, i had to dilute for zeta potential measure

I know that "zeta potential" is a very popular term in colloidal science, all kinds of measurement devices and calculation methods are offered. But does the DLVO theory behind it really describe the properties and behaviour of colloidal systems? Colloids are usually sketched as particles statistically evenly distributed in the liquid, but is that really the case?

I found it is not, and we should not be surprised to find dissipative structures in colloidal dispersions, as dispersions are non-equilibrium systems which form dissipative structures:

https://www.researchgate.net/publication/245535674_Dissipative_Structures_in_Colloidal_Systems?ev=prf_pub

https://www.researchgate.net/publication/233979584_Dispersion_hypothesis_and_non-equilibrium_thermodynamics_key_elements_for_a_material_science_of_conductive_polymers._A_key_to_understanding_polymer_blends_or_other_multiphase_polymer_systems?ev=prf_pub (look especially in chapter 4., p 134 ff)

Is this compatible with DLVO theory and the theory of zeta potential?

(I know it does not answer your question, Ralph, about how to collect the nanoparticles, but with your "zeta potential" comment, you triggered this question which I already have since long time.)

Article Dissipative Structures in Colloidal Systems

Article Dispersion hypothesis and non-equilibrium thermodynamics: ke...

It´s worth to think about!