Ratiometric assay for hydrolytic enzyme quantification

The invention refers to a nanoparticle comprising (i) a core comprising a first population of quantum dots (QDs) embedded in silica, (ii) a shell comprising a second population of QDs embedded in silica, (iii) at least one biomolecule selected from a peptide, a nucleic acid, a carbohydrate or a lipid which comprises a cleavage site that is susceptible of being cleaved by a hydrolytic enzyme, said biomolecule being bound to the surface of the shell through a moiety, and (iv) a photoluminescent label for each biomolecule, wherein the label is bound to the part of the biomolecule which detaches from the nanoparticle after cleavage of said biomolecule by a hydrolytic enzyme, wherein the first and second QD populations have different maximum photoluminescence emission wavelengths, and only the second QD population is susceptible of producing Forster resonance energy transfer (FRET) with the photoluminescent label.