Hybrid nanoparticles with nanosphere mean diameter of 2-9 nm, comprising 90 wt% or more of Ln 2O 3, where Ln represents a rare earth, optionally doped with a rare earth, an actinide or a mixture in which at least 50% of the metallic ions are rare earth ions, the nanosphere is coated largely by functionalized polysiloxane, its mean thickness is 0.5-10 nm, preferably 2-10 nm, and a biological ligand is covalently bond-grafted to the polysiloxane coating, are new. An independent claim is included for the corresponding preparation of hybrid nanoparticles, which comprises: (a) preparing a colloidal suspension of nanospheres； (b) coating the nanosphere with organoalcoxysilane and a polysiloxane agent； (c) attaching a biological ligand to the nanosphere by bonding to a reactive group present on the coating； (d) separating and drying the hybrid nanoparticles obtained.
