Stable water-soluble quantum dots capped by poly(ethylene glycol) modified dithiocarbamate

A new bifunctional ligand (IA-PEG-DTC), poly(ethylene glycol) (PEG) appended dithiocarbamate derived from iminodiacetic acid, has been designed and synthesized. The obtained IA-PEG-DTC ligand with the dithiocarbamate group strongly bonding to the surface atom of quantum dots (QDs), the PEG segment rendering QDs water-soluble, and the free carboxylic group available for further bioconjugation was used successfully to transfer initial oil-soluble QDs into water-soluble ones. The resulting IA-PEG-DTC capped water-soluble QDs have relatively small hydrodynamic size, and importantly, retain high fluorescence quantum yield of the original hydrophobic QDs. In addition, the obtained water-soluble QDs exhibit excellent colloidal stability under conditions of wide pH range (2–13), long-term UV-irradiation, the boiling state, and high salt concentrations. These results indicate that the water-soluble QDs coated by the bifunctional IA-PEG-DTC ligand have potential for applications in cellular imaging and biosensor.