Encapsulation of Multiple QDs into SiO2 Beads by Reflux without Degrading Initial Photoluminescence Properties

Lipophilic quantum dots (QDs, CdSe/ZnS) were encapsulated into size-controlled SiO2 beads by reflux. Slow silanization of QDs by adjusting the amount of tri-n-octylphosphine oxide (TOPO) and tetraethyl orthosilicate (TEOS) in toluene solution is the key to maintaining the initial photoluminescence efficiency. The silanized QDs formed assemblies when transferred into the aqueous phase by a condensation reaction. Beads were created after further deposition of SiO2 monomers on the assemblies. The speed of the transfer into the aqueous phase was controlled by adjusting the amount of TOPO on the surface of the QDs. Reflux was used to accelerate this transfer speed so as to prevent linear-type assemblies and degradation of the photoluminescence properties. Under optimal preparation conditions, SiO2 beads containing multiple QDs (20 +/- 2.5 per bead) were formed with a narrow size distribution (44 +/- 4 nm in diameter). Their photoluminescence efficiency was 29%, whereas the initial value in toluene was 30%. This method is applicable to the preparation of SiO2 beads encapsulating other lipophilic QDs for biological usage.