Hello, 

If your QDs are surface derivatized with carboxyl groups, their solubility should be enhanced at high pH. (at least a couple of units above the pKa of the carboxylic acid bound to the surface of the nanoparticles.  They may take a while to dissolve if they come from a powder form. This is because they may be aggregated into lumps bound by hydrogen bridges formes among carboxyl groups of particles in near contact.

Try dissolving them at the highest pH that is safe. Use a strong base like 1 mM of NaOH. If they disolve then lower the pH slowly until they precipitate out of solution.

Thanks Jose, I have, in fact, tried dissolving them at a pH = 8, but I got a lot of aggregates and the solution wasn't stable over > 48 h. I'll tried an even higher pH, but I fear that this will not be enough and I need to add some salts to screen the electrostatic interactions. Strangely enough, I couldn't find a clean protocole so far.