For a given sample, we prepare several solutions with the same number of NPs and add increasing amounts of AF488-DBCO. Upon conjugation to NDA-(EG) 2 -N 3 , the fluorescence is quenched. After around 72 h at room temperature no further changes in fluorescence intensity were observed. Although "click" reactions are known for their high reaction rates, the relatively long reaction time is due to the very low concentrations used. The latter have the advantage that the samples are immediately ready for spectrocopic characterization and minimize the cost of consumed dye. The fluorescence intensity at 518 nm was measured and plotted against the initial dye:NP ratio, see Figure -f for NPs with 0, 2, 5 and 10 targeted NDA-(EG) 2 -N 3 ligands per NP (raw PL spectra can be found in Figure ).