Paper NS+BI-ThA11
Controlled Surface Modification of Ultra-stable Superparamagnetic Iron Oxide Nanoparticles

Thursday, October 21, 2010, 5:20 pm, Room La Cienega

Biocompatibility, magnetic properties and ease of synthesis renders iron oxide nanoparticles (NPs) attractive for many especially biomedical applications such as magnetic resonance (MR) contrast agents, triggered drug release and cell separation. Good NP stability under physiologic conditions and controlled surface chemistry are key to successful application not only in the biomedical field but also for assembly into various materials.

NPs with close control over the interfacial chemistry and good stability at high salt concentrations and elevated temperatures can only be achieved if dispersants are irreversibly bound to the NP surface. The dispersant binding affinity is determined by its anchor group. Low molecular weight dispersants which consist of one high affinity anchor covalently linked to poly(ethylene glycol) (PEG) spacers have been proven well suited to sterically stabilize Fe₃O₄ NPs. However, we found that electronegatively substituted catechols such as nitrocatechols vastly outperform the well-known and often used catechol anchors such as DOPA and dopamine. Because of the optimized binding affinity of nitrocatechols, PEG-nitrocatechol coated Fe₃O₄ NPs remained stable under physiologic conditions up to 90 °C whereas e.g. PEG-dopamine stabilized Fe₃O₄ NPs started to agglomerate below body temperature.^[1] Further investigations showed that the optimal binding affinity of nitrocatechols to Fe₃O₄ is closely related to a redox reaction between Fe²⁺ located at the Fe₃O₄ NP surface and nitrocatechols, which leads to electron delocalization in the adsorbed catechol ring, and a close to covalent bond of nitrocatechols to Fe₃O₄ surfaces. Irreversible binding of PEG-nitrocatechols to Fe₃O₄ NPs allowed us to closely control and investigate the influence of dispersant layer thickness by varying the nitrocatechol-PEG molecular weight. Furthermore, NPs could easily be functionalized by co-adsorbing differently end-functionalized dispersants on the Fe₃O₄ NP surface.^[2]

In summary, nitrocatechols have a close to optimal binding affinity to Fe₃O₄ surfaces. This optimized binding affinity not only leads to ultra-stable PEG-nitrocatechol coated superparamagnetic Fe₃O₄ NPs but also allows for close control over the hydrodynamic diameter and interfacial chemistry, factors which crucially determine NP performance especially in biomedical applications.

[1] E. Amstad, T. Gillich, I. Bilecka, M. Textor, E. Reimhult, Nano Letters 2009, 9, 4042.

[2] E. Amstad, S. Zurcher, A. Mashaghi, J. Y. Wong, M. Textor, E. Reimhult, Small 2009, 5, 1334.