Stable aqueous ZnS quantum dots obtained using (3-mercaptopropyl)trimethoxysilane as a capping molecule

We have examined the synthesis and stability of ZnS quantum dots (QDs) using an all-aqueous route at pH = 12 with (3-mercaptopropyl)trimethoxysilane (MPS) as the capping molecule. The MPS-capped ZnS QDs obtained were well dispersed with a particle size around 5 nm and a cubic zinc blende crystalline structure. The QDs exhibited optimal photoluminescence (PL) emission when the MPS:Zn:S ratio was between 1/4:2:1 and 1/2:2:1. Compared with the earlier obtained ZnS QDs capped with 3-mercaptopropionic acid (MPA), the MPS-capped ZnS QDs exhibited a similar, high quantum yield, 42% and 25% for MPS:Zn:S 1/2:2:1 and 1/4:2:1, respectively, but much better photostability. With the MPS:Zn:S ratio of 1/4:2:1, we showed that at room temperature and under the normal laboratory lighting conditions, the MPS-capped QDs were able to maintain their PL intensity for more than 50 days without degradation. We further showed that the MPS-capped QDs were stable not only in their synthesis solution but also in deionized (DI) water and in phosphate buffer saline (PBS) solution. The QDs with 1/2:2:1 were able to stay at 50 degrees C for more than 20 h without degrading the PL intensity. They were also stable under continuous UV exposure for 3 h. With the high quantum yield and significantly improved photostability, the MPS-capped ZnS QDs could be good imaging tools for many biological applications.