Size tuning of MAA capped CdSe and CdSe/CdS quantum dots and their stability in different pH environments

The present work reports synthesis of mercaptoacetic acid capped CdSe nanoparticles soluble in water at different temperatures and with different precursor ratios. This enabled to tune the particle size of QDs from 2.7 to 5.8 nm. The particles consist of nanocrystals; with mixed phase, hexagonal wurtzite as well as sphalerite cubic and are luminescent with quantum yield 10%. The quantum yield up to 20% has been obtained by growing a shell of CdS over the CdSe. HR-TEM images, XRD patterns and the photoluminescence excitation spectra shows epitaxial growth of CdS shell over CdSe and with average size 3.2 ± 1.2 and 4.7 ± 1.2 nm for CdSe and CdSe/CdS quantum dots respectively. FT-IR spectrum and the negative zeta potential value together confirms the attachment of mercaptoacetic acid to the QD surface, where the carboxylic acid group is facing towards solvent and provides stability due to electrostatic hindrance. Further, the QDs are checked for their stability and the luminescence in environments of different pH (4-11 pH). Both CdSe and CdSe/CdS agglomerate with total elimination of fluorescence for 4 pH medium, and no shift in the fluorescence emission peak observed for the 6-9 pH, therefore QDs can be applicable as the fluorescence tags in this specific range of pH.