You can get more information from"https://www.ttu.ee/public/m/Mehaanikateaduskond/Instituudid/Materjalitehnika_instituut/MTX9100/Lecture11_Synthesis.pdf".

BW

straight forward approach keep them in suspension without lyophilization

I agree with @Titus Sobisch, lyophilization often yields agglomerates that are hard to break down. But if you're determined to work with the ones you've already lyophilized, I suggest trying dispersing them using probe sonication, at low concentrations (wouldn't go higher than 1mg/ml). This usually gives much better results than bath sonicator. Watch out for the overheating, use low amplitudes and consider sonicating on ice.

If you don't have access to the probe sonicator, you can also try gentle mincing the particles with the mortar before bath sonication. But this might also damage the particles if they're too soft, so you have to be careful. If you suspect that your particles might break, I would avoid that.

Aleksandra Benko, thank you so much for your detailed response. In fact, the particles that were dispersed with ultrasonic processor showed better dispersion (with lower PDI), and we'll analyse by SEM if the nanoparticles mantain their shape. The correct way for dispersing using the probe would be before or after nanoparticle functionalization?

In most cases, presence of functional groups facilitates dispersion. But this will depend on the type of particles, solvent and type of functional groups. For example, if you oxidize particles that have a large potency to agglomerate via Van der Waals attraction, this generally would lead to better dispersion in polar liquids. So I'd say it really depends on what you mean by functionalization.