Forming highly fluorescent near-infrared emitting PbS quantum dots in water using glutathione as surface-modifying molecule

We present a new facile procedure for transferring oil-soluble oleic acid-capped NIR-emitting PbS quantum dots (QDs) into water, using hydrophilic thiol ligands as the surface-modifying agents of the primary capping molecules (oleic acid). The influence of exchange of the primary capping molecules with five different types of thiol molecules is investigated. The results show that highly fluorescent water-soluble PbS QDs are obtained using glutathione as a surface-modifying agent (photoluminescence quantum yield (PL QY), >30%); significantly less fluorescent water-soluble QDs were obtained using l-cysteine (PL QY, ~5%); with other three thiol molecules, PbS QDs lose almost completely their fluorescence in aqueous solution. This striking difference among the five thiol molecules may be attributed to the difference in the molecular structure. Next, we explored systematically the conditions of QD water solubilization, storage stability, photostability and cytotoxicity and tested further the resulting water-soluble PbS QDs for the imaging of living animals. The preliminary results from these studies illustrate that our synthesis procedure is very facile and that the as-prepared water-soluble PbS QDs are stable and low-cytotoxic and will be an important potential probe in the imaging of living animals due to free carboxyl and amino groups on the external surface of the QDs.