Doubling Photoluminescence of Red-Emitting Quantum Dots by Direct Transfer from Hexane into Water Without Adding Extra Reagents

The demand for converting hydrophobic nanoparticles (NPs) into water-soluble NPs, specifically red-emitting quantum dots (QDs) synthesized in organic solvents, has led to extensive research. However, the current methods for transferring these NPs face challenges related to their efficiency and a decrease in their photoluminescence quantum yield (PLQY). In this study, a remarkably facile and efficient technique was utilized to transfer oleic acid (OA)-coated CdSe/CdS core-shell QDs into water without the need for additional chemicals or agents. This process results in an unexpected increase of over 100% in PLQY upon transfer into basic water (pH 8). The transfer method is highly reproducible and can be extended to other oleic acid-based particles like magnetic NPs. The process is a straightforward method using ultrasonic assistance completed within only 4 hours at room temperature, involving the rearrangement of OA ligands at the QD surface, leading to the formation of a 'mixed' monolayer. This innovative configuration exposes approximately half of the carboxylic acid moieties to the QD surface and directs the remaining towards the aqueous medium. Density-functional-theory (DFT) calculations corroborate this rearrangement, indicating its favorable energetic state in aqueous media. The behavior of the transferred QDs was investigated and compared to that of the original QDs in hexane through absorption spectroscopy, fluorescence spectroscopy, and transmission electron microscopy. Regarding DFT calculations, Quantum ESPRESSO was utilized and CdS surfaces were generated to mimic core-shell quantum dot surfaces. This discovery can open promising avenues for employing NPs especially conventional QDs synthesized in organic solvents for applications in aqueous media. The ease of transferring QDs into a water environment will also facilitate a variety of assembly methods that rely on aqueous surface chemistry, and applications in the field of biomedical imaging.