Dithiocarbamates as Capping Ligands for Water-Soluble Quantum Dots

• Published in: ACS applied materials & interfaces 2010-11, Vol.2 (11), p.3384-3395
• Main Authors: Zhang, Yanjie, Schnoes, Allison M, Clapp, Aaron R
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Ligands; Molecular Structure; Quantum Dots; Solubility; Thiocarbamates - chemistry; Water - chemistry
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/am100996g

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-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.