[en] Ultrafine well-dispersed magnetite nanoparticles were directly prepared in aqueous solution using the controlled coprecipitation method. This paper discusses the preparation of novel surface silica modified colloids with a core of single-domain magnetite particles. Amino-silane was covalently coupled to the colloids and activated by glutaraldehyde. The underlying idea is to tailor the silica shell thickness and surface properties such that the colloids became stable spheres with isotropic interactions, whereas an external magnetic field produces weak dipolar attractions and consequently reversible anisotropic structures. Hepama-1, a humanized monoclonal antibody directed against liver cancer, was coated on functional magnetic nanoparticles by the cross-linker glutaraldehyde to prepare immuno-magnetic nanoparticles (IMN). A direct labeling method was adopted to radiolabel IMN with ¹⁸⁸Re. The radiolabeling efficiency was about 90% with good in vitro stability. Such nanoparticles might be useful for bio-magnetic radiotherapy. (author)