Organic-to-aqueous phase transfer of cadmium chalcogenide quantum dots using a sulfur-free ligand for enhanced photoluminescence and oxidative stability

This paper describes a procedure for transferring colloidal CdS and CdSe quantum dots (QDs) from organic solvents to water by exchanging their native hydrophobic ligands for phosphonopropionic acid (PPA) ligands, which bind to the QD surface through the phosphonate group. This method, which uses dimethylformamide as an intermediate transfer solvent, was developed in order to produce high-quality water-soluble QDs with neither a sulfur-containing ligand nor a polymer encapsulation layer, both of which have disadvantages in applications of QDs to photocatalysis and biological imaging. CdS (CdSe) QDs were transferred to water with a 43% (48%) yield using PPA. The photoluminescence (PL) quantum yield for PPA-capped CdSe QDs is larger than that for QDs capped with the analogous sulfur-containing ligand, mercaptopropionic acid (MPA), by a factor of 4 at pH 7, and by up to a factor of 100 under basic conditions. The MPA ligands within MPA-capped QDs oxidize at E_ox ∼ +1.7 V versus SCE, whereas cyclic voltammograms of PPA-capped QDs show no discerible oxidation peaks at applied potentials up to +2.5 V versus SCE. The PPA-capped QDs are chemically and colloidally stable for at least 5 days in the dark, even in the presence of O₂, and are stable when continuously illuminated for 5 days, when oxygen is excluded and a sacrificial reductant is present to capture photogenerated holes.