Silica encapsulation of highly luminescent hydrophobic quantum dots by two-step microemulsion method

A method has been developed to incorporate multiple hydrophobic CdSe/CdxZn1−xS QDs into SiO2 beads by a two-step microemulsion process and retain the high photoluminescence (PL) efficiencies of the QDs. In step 1, the QDs were silanized by using partially hydrolyzed tetraethyl orthosilicate (TEOS) via replacing oleic acid (OA) on the surface of QDs. In step 2, these silanized QDs were transferred into aqueous phase in the reverse micelle solution by adding a small amount of water and stirring for a few hours. Then these silanized QDs assembled and subsequently encapsulated in a SiO2 shell by a reverse micelle synthesis. The initial silanization plays an important role for the QDs to be coated within a homogeneous SiO2 shell and retain a high PL efficiency in water. Single QDs were coated into silica beads by this two-step microemulsion process without water addition before ammonia and TEOS addition and pre-stirring for a certain time. The number of QDs incorporated into the silica beads can be controlled by adjusting the stirring time. More QDs can be incorporated into silica spheres by partially hydrolyzed 3-mercaptopropyl trimethoxysiliane (MPS) or 3-aminopropyltrimethoxysilane (APS) functionalization at step 1. An improvement in the PL efficiency (up to 35%) of silica spheres was obtained after APS functionalization of QDs. The silica coating shows a bit blueshift for the PL peak wavelength of the QDs. The full width at half maximum (FWHM) of PL spectra was decreased 4 nm after silica coating of the QDs.