Four strategies for water transfer of oil-soluble near-infrared-emitting PbS quantum dots

The successful transfer of oil-soluble quantum dots (QDs) into water is critical for many of their bioapplications. In this paper, the impacts of four various strategies (i.e., via micelles, nanohydrogels, amphiphilic polymers and water-soluble thiol small molecules) on the phase transfer of oil-soluble oleic acid-capped NIR-emitting PbS QDs into water were evaluated systematically. It was found that the process of water transfer and the optical property of the resulting water-soluble QDs highly hinge on the type of the phase transfer agents used due to their different interactions with QD surface. Among all these phase transfer agents, SOC micelles and glutathione (thiol) molecules are more favorable for retaining the optical property of the initial oil-soluble PbS QDs. As a result, the obtained water-soluble QDs show strong NIR fluorescence (PL QY > 30% in water). However, in the case of nanohydrogel and amphiphilic polymers, the corresponding water-soluble ones display relatively weak fluorescence emission. These results suggest fully that "correct" phase transfer agents should be selected in order to obtain high-quality water-soluble PbS QDs. The possible reasons for this obvious difference were further analyzed and revealed. Besides, the preliminary results obtained also indicate that the NIR-emitting PbS QDs will be a potential probe in the in vivo biomedical imaging of small animals.