@ Sina, anionic surfactant sodium dodecyl sulphate (SDS) is shown to have better steric effect against copper oxide dispersions in water or ethylene glycol even at higher particle loading.

If the nanoparticles were dried you will not be able to recover nanoparticles as single entities

Dear Sina Mahini for more answers to your question, please see this closely related RG thread entitled "How to disperse copper nanoparticles?":

https://www.researchgate.net/post/How-to-disperse-copper-nanoparticles

See this link

Conference Paper Electrochemical Preparation of Copper Nanoparticles in Choli...

Sina Mahini

There's a lot of confusion in the replies, IMHO, and also in the link that Frank T. Edelmann provided above.

If you have a Cu powder then there's little hope as Omar Gonzalez-Ortega indicates above (but we'll return to that). First, carry out a Stokes' Law calculation to see what size of Cu with its high density (9.3 g/cm3) can remain in suspension. You may be surprised. Second, measure the SSA of the material. Is it greater than 540 m2/g?

Then, we have the problem of definitions (and that's confused too) - normally agglomerates are loose and can be disrupted by sonication. What you have in a powder is solid fused chemically-bound aggregates that can't be disrupted in this manner. ISO committees understand the confusion in the literature and is starting to refer to 'clusters' to avoid possible misunderstanding and misinterpretation.

Nano Cu is best made by a bottom up process e.g. sol-gel not a top down one. OK, what else - there are no magic liquids to make a stable dispersion - the stages from a powder are wetting (if the material doesn't wet then a surfactant is needed), separation (the crucial step; sonication) to disrupt agglomerates, stabilization (if recombination is occurring after sonication. The only dispersion agent is energy, if we're pedantic. And you simply can't get enough energy in the system to separate aggregates. OK, so you're on a losing wicket starting with a powder... Some links (registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Metal colloids - their preparation, application and characterization

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W160218MetalColloids.html

Silver colloids and invisible ink

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150902SilverColloidInvisibleInk.html

Now Cu powder is one I have personally been able to use a technique called 'stress corrosion milling'. You'll see I mention it toward the end of the first presentation above (Slide 56). Here you need to know the ratio between the surface area size (from BET say) and the volume/mass size (from laser diffraction, say). In this way you can take a guess at the amount of the solid bridges in the system. Then you can calculate the strength and amount of HNO3 needed to just dissolve the bridges (they're an area of weakness). Get it wrong and you dissolve the whole system! But then you have the starter for a bottom up synthesis by reduction of the copper nitrate (usually in ammoniacal form) to copper metal in a bottom-up process...

One last point - metal powders don't usually wet well in water and a very low concentration of surfactant is employed to ensure intimate contact of continuous phase with particle avoiding a film of air.

I would like to thank everyone for providing brilliant responses, particularly Dr Alan F Rawle for his comprehensive guidance.