Colloidal Stability of Aqueous Suspensions of Polymer-Coated Iron Oxide Nanorods: Implications for Biomedical Applications

International audienceIron oxide nanorods are considered to be very promising platforms for biomedical applications, such as magnetic hyperthermia, magnetic resonance imaging or immunoassays based on magneto-optical effects. However, their efficient colloidal stabilization is challenging , and colloidal aggregation could lead to the total loss of their performance. This work is focused on synthesis and colloidal stabilization of iron oxide nanorods of an average length and diameter L×d=31×6 nm, synthetized by hydrolysis of iron (III) salt followed by reduction of the obtained akaganeite to iron oxide in a microwave reactor. Synthesized nanorods exhibited a weak ferrimagnetic behavior with remnant magnetization Mr ∼3 emu/g and saturation magnetization Ms ∼13 emu/g. The nanorods were dispersed in water after adsorption on their surface of three different polymers-linear bisphosphonate-polyethylene glycol (PEG) molecules (denoted OPT), polymethacrylate backbone / PEG side chains comb polymer (denoted PCP) (both with PEG brushes extended towards the solvent and having molecular weight Mw ∼3000 g/mol) and polyacrylic sodium salt (PAA, M w ∼15000 g/mol). Experiments and theoretical evaluation of the interaction potential shows that increasing polymer grafting density on the nanorod surface as well as decreasing concentration of non-adsorbed polymer improve the nanorod colloidal stability. The best stability is obtained on an optimal range of weight ratio of the added polymer to the nanorods between 0.5 and 1.6 mg/mg. Higher grafting density reached with OPT polymer with a bisphosphonate terminal group (2-4 nm-2) allows much better stability than using multiple adsorption with PCP (0.2-0.4 nm-2) or PAA. Even though the nanorods are still subject to some aggregation (effective hydrodynamic diameter ∼60 nm, as compared to their TEM size L×d=31×6 nm), significant progress towards understanding their colloidal stability was achieved