Colloidal stabilisation of barium hexaferrite nanoplatelets in different media

Barium ferrite, BaFe12O19, is a hexagonal ferrite, which is distinguished from other ferrites by a large magnetocrystalline anisotropy and high intrinsic coercivity. Recently there has been an increasing interest for biomedical applications of hexaferrite nanoparticles, for instance in hyperthermia1,2. For such application, a material has to be colloidally stable in a given media. Stabilizing molecules that can provide steric repulsive forces, electrostatic repulsive forces or both can provide for long-term stability. Different molecules have been used in water-based ferrofluids, including small molecules (citric acid - CA), polymers (polyethyleneglycol), polysaccharides (dextran), polypeptides, and surfactants (cetyl trimethylammonium bromide). In our research, we aim to modify the nanoplatelets surface to ensure their colloidal stability and biocompatibility. Here we studied barium hexaferrite nanoplatelets substituted with Sc3+, which were synthesized using hydrothermal synthesis3 . The nanoplatelets were coated with silica. Namely, silica coating provides reactive –OH surface groups, that can be used for further functionalization. CA was adsorbed on the hydrothermally synthesised nanoplatelets. In a subsequent reaction, particles were coated with silica using a modified Stöber process4 . The silica coated nanoplatelets were further grafted with dextran that was previously reacted with (3-Glycidyloxypropyl)trimethoxysilane (GLYMO-dextran). The efficiency of the colloidal stabilisation was tested in different media such as deionised water (dH2O), phosphate buffer (PBS) and fetal bovine serum (FBS). The colloidal stability of the nanoplatelets in dH2O was achieved after the adsorption of CA. The nanoplatelets remained stable in dH2O for long periods of time and through all the following coating procedures. However, a long-term colloidal stability in PBS was only achieved after grafting nanoplatelets with GLYMO-dextran, which provided steric repulsive forces. When nanoplatelets grafted with GLYMO-dextran were introduced into FBS the suspension remained stable for up to 5 days. This represents a substantial progress in the colloidal stabilization of highly magnetic nanoplatelets and a step further to their possible application in biomedicine. Acknowledgements: The authors acknowledge financial support of the Slovenian Research Agency through the project PR-060806.