Interest in using colloidal quantum dots (QDs) as fluorescent biological probes has driven the exploration of surface chemistries that can stabilize highly fluorescent QDs in aqueous environments while maintaining small hydrodynamic size. Much of this research has focused on describing the exchange of hydrophobic native ligands used to control synthesis at high temperatures with hydrophilic ligands that enable stable aqueous QD dispersions. Numerous examples exist of QDs stabilized by monodentate, polydentate, and polymeric ligands, but relatively few studies have directly examined relative binding strengths and the density of surface coordination in these systems. In contrast, systematic studies of ligand exchange have favored nonpolar solvents. As such there is yet a need to provide a deeper understanding of surface modifications in polar, protic environments. Here, we have studied the surface modification of zinc blende, phosphonate-capped CdSe/ZnS and CdSe/CdS core/shell QDs with thiolate ligands. We aim to shed light on how small molecule thiols bind to QDs with different surface compositions. By exchanging the native hydrophobic ligands with a labile, small molecule phosphonate intermediate, we are able to quantify and compare the installation of other monodentate anionic ligands in a subsequent ligand exchange step. This work should aid in interpretation of ligand exchanges in water with bio-relevant binders towards producing versatile and stable QDs for biological applications.

Competitive binding of monodentate ligands on aqueous core/shell quantum dots

ACS Fall 2021

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