Abstract: Luminescent and/or electroactive nanoparticles suitable for MRI (Magnetic Resonance Imaging) and/or PET (Positron Emission Tomography) are prepared by mixing luminescent or electroactive compounds and ethyl oxide/propyl oxide block-copolymers in an organic solvent, which is thereafter evaporated in order to obtain a residue; and by hydrolyzing-condensating tetraalkoxysilanes in an aqueous solution in the presence of the residue; the obtained nanoparticles show no or a negligible release of the luminescent or electroactive compounds and are useful for bio-analytic and bio-medic applications.