Spectroscopic Evaluation of the Nucleation and Growth for Microwave-Assisted CdSe/CdS/ZnS Quantum Dot Synthesis

Use of microwave radiation to speed up the synthesis of CdSe-based quantum dots (QDs) in aqueous media is of practical interest and is the focus of this study. However, such microwave methods usually lead to low quantum yield (QY) QDs as compared to the conventional organic-based synthesis. By coupling the microwave oven with a fluorometer via a fiber optic cable, the fluorescence spectra during the QD evolution were obtained. Using the composition Cd-4:Se-1:Zn-4:MPA(20) (MPA = 3-mercaptopropionic acid), several stages of QD growth including development of CdSe nuclei, followed sequentially by CdS and then ZnS deposition were noted. The temperature of the microwave reaction between 130 and 155 degrees C led to QDs with similar QYs (17-19%), with the lower temperature taking longer to reach the optimal yield. The nucleation step was carried out at temperatures varying from 0 to 100 degrees C followed by growth in the microwave oven. With increasing temperature of nucleation, there was a red-shift in emission maximum, varying from 549 to 574 nm, with comparable QY, thus providing a route for QD size control in the microwave process. There was a profound effect of light illumination on the nucleated state prior to microwave treatment. Using light illumination, QDs with a QY of 40-41% were reproducibly obtained. With such optimized QDs, both flow cytometry and confocal microscopy of QD uptake into intestinal epithelial cells at 0.8-8 nM concentration were readily observed.