Click chemistry is an appealing approach for the covalent coupling of fragile biomolecules such as oligonucleotides, peptides or protein templates to the surface of fluorescent quantum dots. However, click chemistry requires the use of copper cations as catalysts, which irreversibly quench the photoluminescence of quantum dots. On p. 2247 I. Texier et al. demonstrate by means of energy-filtered transmission electron microscopy (EFTEM) that the copper ions are incorporated into the ZnS shell of polyethyleneglycol-capped CdSeTe/ZnS core/shell quantum dots, shedding new light on the quenching mechanism. The image shows the EFTEM cadmium, copper and zinc chemical maps of 7 nm×13 nm CdSeTe/ZnS quantum dots as well as their superposition.