It depend on the structure of your Iron oxide (Fe2O3 hemitate, Fe3O4)

Generally, polyol-modified iron oxide nanoparticles can be stable and dispersed for a long time in aqueous solution. You said your synthesized iron oxide nanoparticles are not stalbe. Did you mean they occurred aggregation in aqueous solution or other solvent? Sometime they are not stabale due to the higher ionic strength of solution. Of courese, they also aggregate if capping agents are not enough to provide a excellent dispersion.  Another possible reason is that if the used polyols did not successfully cover the surface of iron oxide nanoparticles. As I known, the protect efficiency is not very good when you only incubate iron oxide nanoparticles and polyols directly. Most people used functional group-exposed (ex: amine group) iron oxide nanoparticles  to conjugate polyols  on the their surface. Based on this strgtegy, the polyol-modified iron oxide nanoparticles have a well-dispersion in  aqueous solution.

Thank you for your answers, yes they are unstable in water and also therefore with ionic solutions or cell culture medium. They should be well covered because the polol is the solvent, reducing agent and stabilizing agent. They are magnetite as far as I'm aware. 

Dear Dr. Hachani:

In a general way, the process of recovering of nanoparticle is not a fast procedure, sometimes are necessary  1 to 24 hours or more under a kind of stirring, being common another 5 to 15 min of sonication.  Here, the idea is develop under the nanoparticle surface, a structure similar to the double layer in particles of aqueous medium. Nowaday, there is several class of Surfactant as Anionic, cationic, Non-Ionic, Zwitterionnic, etc... etc...each one of them with tens of compounds. A priori, surfactant type oleates and ethoxylated triglycerides, in specific di- or poly-ester would give good results. Also, combination of two or more surfactants seems possible. At least, from classical oleic acid, there is a great number of references that can be consulted. In this sense, with a great list of results published. There is the class Triton (405, 305, 207, 60,...) and the classic Triton X-100 sufactant.

Kind regards.

Marcos Nobre (M. A. L. Nobre)

You may try polyol with higher alkyl chain length.

Thank you for the suggestions, i will have a look into them. 

pH, impurity ions and surface  play important role in stability of magnetic colloids. Particularly in polar medium like water unless pH is maintained and all the traces of impurity ions are removed long term stability will not be attained.