The use of fluorescent nanoparticles is disclosed which includes an inorganic core, a passivating layer and specific ligands having a hydrodynamic diameter of the inorganic core with the passivating layer of not more than 15 nm, preferably of not more than 10 nm, particularly preferably of not more than 5 nm, for preparing an in vivo diagnostic aid, the nanoparticles showing an emission of less than 700 nm.