Improved photostability of silica bead impregnated with CdSe-based quantum dots prepared through proper surface silanization

The incorporation of emitting quantum dots (QDs) into silica and their sustained photostability have been the subject of research over the last two decades. This report focuses on the synthesis of luminescent silica beads by directly coating silica on the hydrophobic CdSe-based QDs by exchanging the original ligand (oleic acid) through partially hydrolyzed tetra-alkoxysilane, using a surface silanization approach. It was found that the ratio of QD/alkoxide upon silanization must be optimized to get uniform coating of silica. The thickness of silica layer was controlled judiciously by a subsequent reverse micelle method. The photostability of silica beads in water thus prepared was evaluated systematically by a newly developed, and facile method using high irradiation intensity (ca. 45 W/cm2). When the thickness of the silica layer attained a value of 5 nm, the photostability of QDs improved drastically, probably due to a change in the glass structure. We have found that the addition of small amount of water increased the degree of surface silanization, resulting in ca. 46 % enhancement of the photoluminescence (PL) degradation time of QDs. We also discuss the advantages of this direct coating method with tetra-alkoxysilanes in comparison with other organo-alkoxysilanes as primers.