Dithiocarbamates as Capping Ligands for Water-Soluble Quantum Dots

We investigated the suitability of dithiocarbamate (DTC) species as capping ligands for colloidal CdSe-ZnS quantum dots (QDs). DTC ligands are generated by reacting carbon disulfide (CS(2)) with primary or secondary amines on appropriate precursor molecules. A biphasic exchange procedure efficiently replaces the existing hydrophobic capping ligands on the QD surface with the newly formed DTCs. The reaction conversion is conveniently monitored by UV-vis absorption spectroscopy. Due to their inherent water solubility and variety of side chain functional groups, we used several amino acids as precursors in this reaction/exchange procedure. The performance of DTC-ligands, as evaluated by the preservation of luminescence and colloidal stability, varied widely among amino precursors. For the best DTC-ligand and QD combinations, the quantum yield of the water-soluble QDs rivaled that of the original hydrophobic-capped QDs dispersed in organic solvents. The mean density of DTC-ligands per nanocrystal was estimated through a mass balance calculation which suggested nearly complete coverage of the available nanocrystal surface. The accessibility of the QD surface was evaluated by self-assembly of His-tagged dye-labeled proteins and peptides using fluorescence resonance energy transfer. DTC-capped QDs were also exposed to cell cultures to evaluate their stability and potential use for biological applications. In general, DTC-capped CdSe-ZnS QDs have many advantages over other water-soluble QD formulations and provide a flexible chemistry for controlling the QD surface functionalization. Despite previous literature reports of DTC-stabilized nanocrystals, this study is the first formal investigation of a biphasic exchange method for generating biocompatible core-shell QDs.