Quenching of CdSe-ZnS core-shell quantum dot luminescence by water-soluble thiolated ligands

Water-soluble thiolated molecules are among the most commonly used ligands to render quantum dots (QDs) water-soluble and biocompatible. These ligands maintain a relatively small total QD size, which makes them useful for many biological applications, but often cause a reduction of the quantum yield. The resulting quantum yields vary significantly between different reports and a complete understanding of the mechanism underlying the quenching of luminescence is lacking. We have studied the effect of ligand-exchange reaction time with 11-mercaptoundecanoic acid (MUA) and find an exponential decrease of quantum yield with time. We have also investigated the quenching effect of various commonly used thiolated ligands. A strong dependence on size and charge of the quenching molecule was observed; the Stern-Volmer plots were nonlinear and represent multiple quenching pathways. By comparison with nonthiolated analogues, the relative contributions of the thiol group, carboxyl group, and alcohol group to the quenching behavior were assessed. Furthermore, luminescence lifetime analysis revealed that quenching is static rather than diffusive in origin, indicating that it only arises from ligands coordinated to the QD surface. Our results will be helpful for QD chemists and biophysicists to select the optimal ligands for their particular application.