CdSe/ZnS core/shell quantum dots for bio-application

Quantum dots, with functional group on surfaces, can be attached to bio-molecules to form quantum-dots/bio-molecule complex. With their excellent photo-electronic properties, quantum dots are now widely used to label the bio-molecules and highlight a cell's nucleus and microtubule fibers. They may also be used to control and operate the bio-molecules. We investigate the synthesis of the CdSc/ZnS core-shell quantum dots. To make the quantum dots water-soluble, the TOPO, TOPSe, and HAD ligands on the surface of CdSe/ZnS QDs were replaced by mercaptoundecanoic acid (MUA). The MUA-coated quantum dots are dispersible in water. For the use in biology, it is required that the quantum dots, when surrounded by a bio-environment, have high quantum yield and long fluorescence lifetime. Our study indicates that quantum yield of the core-shell quantum dots exceeds 60% and fluorescence lifetime of the electrons in the excited state is around 20 nano seconds. The CdSe/ZnS core/shell structure is very stable. No significant fluorescence decay is observed with a prolong excitation by a 365 rim ultro-violet light source of 30 mW. Here the function of the ZnS is two-fold: as both the barrier and the passivation layer for the CdSe quantum dots. In addition, the S-2 bonds are bio-active, which form stable bonding to bio-molecules.