The colloidal stability of a fluid will depend first, on the dimensions of the particles, which should be sufficiently small so that precipitation due to gravitation forces can be avoided, and second on the charge and surface chemistry, which give rise to both, steric and coulombic repulsions. Additional restrictions for biomedical applications strongly depend on whether these particles are going to be used for in vivo or in vitro applications.
Dimercaptosuccinic acid (DMSA, ligand attached to the surface of iron oxide after ligand exchange with oleic acid) is a small molecule that probably does not provide enough repulsive forces for larger particles than 50 nm. It has demonstrated to be effective for iron oxide nanoparticles with smaller size providing negative surface charges at physiological pH and allowing a wide range of pharmaceutical applications, especially for targeted drug delivery, biomedical imaging, biosensing, hyperthermia, or nano-thermometry, improving either the efficiency of the therapy, or the detection limit of the technique [Magnetic nanoparticles coated with dimercaptosuccinic acid: development, characterization, and application in biomedicine].
For magnetic nanoparticles above 30 nm, polymers such as poly-(maleic anhydride alt-1-octadecene) or PEG are more suitable. It must be highlighted the importance of the amphiphilic polymers, whose alkyl intercalate with the chains of the surfactants bound at the nanoparticle hydrophobic shells, at the surface, i.e. to oleic acid, and the anhydride rings hydrolyze and ensure colloidal stability by means of negative electrostatic repulsions. Inorganic coating such as silica are also very interesting, in particular for anisometric particles [How shape and internal structure affect the magnetic properties of anisometric magnetite nanoparticles].