The invention concerns hybrid nanoparticles containing: a nanosphere, of mean diameter included in the range from 2 to 9 nm, of which at least 90% by weight consists of Ln2O3 where Ln represents a rare earth, optionally doped with a rare earth or an actinide, or a mixture of rare earths, or a rare earth and actinide mixture, in which at least 50% of the metal ions are rare earth ions, a coating around the nanosphere chiefly consisting of functionalized polysiloxane, having a mean thickness included in the range from 0.5 to 10 nm, preferably greater than 2 nm and no more than 10 nm, and at least one biological ligand grafted by covalent bonding to the polysiloxane coating and their method of preparation.