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 (CS2) 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-ligands 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 abalance calculation which suggested nearly complete covereage of the available nanocrystal surfact. The accessibility of the QD surface was evaluatged by self assembly of Hls-tagged dye-labelled 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 appliactions. In general DTC-capped CdSe-ZnS CDs have many advantages over other water soluble QD formulations and provide a flexible chemistry for controlling the QD surfact functionalization. Despite previous literature reports of DTC stabilized nanocrystals, this is the first formal investigation of a biphasic exchange method for generating biocompatible-shell QDs.