They must be the profis. Swiss scientist are very good, polite and helpful.
Interesting Topic. Which application should the particles in solution be used for? We want to use such solutions as heat transfer fluids with adjustable properties. May be your research can be interesting.
As mentioned above, sonication is a straight way to separate, although if particles are in contact with naked surfaces it maybe not possible. You should use a 'high' power sonicator (not bath, better to use a point-probe sonicator).