Dear Aiman,

you can dissolve your nanoparticles same way as macroparticles at high pH and in acids, in acetic acid as well. I doubt that you really want to do this. Acetic acid does not oxidize especially not III valent iron oxide. Iron oxides are only very sparingly soluble in ethanol.

To disperse your nanoparticles you need dispersants/stabilizers. Anionic surfactants and anionic polyelectrolytes like CMC should work.

Your nanoparticles maybe highly agglomerated initially. Then you need ultrasonication or alternatively high shear mixing.

I presume you need to suspend the particles as in a ferrofluid. The rate of settlement of the particles depends on their size, and agglomeration. To resolve the problem of agglomeration of fine Fe3O4 particles we used Oleic acid as a surfactant. Particles of typical size below ~10 nm were suspended in several carriers such as Diester and Toluene.   

The first thing you should do is to identify the ligands on your NP surface. Polar ligands work with water and alcohols; aliphatic ligands will need a similar solvent like toluene or hexane. Polar ligands in nonpolar solvents and vice versa will almost always lead to aggregation or settling.

I agree with the answers above. Anyway, I add that for the use of surfactant the surfactant type (cataionic, anionic, nonionic) and the solvent are important. Surfactant concentration should be slightly higher than the CMC (critical micellar concentration), but avoid high fraction of micelles. Furthermore, to be sure of a good surfactant adosption on your surface you should consider the solvent-surfactant and surfactant-nanoparticles interactions. I would add that sedimentation is not an exclusive effect of aggregation, it could be cause by a high density difference between the NP and the dispersion matrix. In this case, you should consider using solvent of suitable density.

I agree with Titus, you should not mix the term of dissolution with the dispersion. Using acetic (or any other) acid you would partially or even completely dissolve your NPs. To disperse your NPs, you should select some mild surfactant or prepare adequate dispersant..