Highly bright water-soluble silica coated quantum dots with excellent stability

Silica coating via a Stöber method is an effective route to render luminescent quantum dots (QDs) with great biocompatibility, low toxicity and water-solubility for bioapplications. However, the bottleneck in this route is the access of highly luminescent, colloidally stable QD dispersion in alcoholic solution. Herein, we report a facile route based on the Stöber method for the synthesis of isolated silica coated QDs (QD@SiO₂) with high emission efficiencies, tunable small size (less than 30 nm) and excellent stability. Prior to silica coating, the initial oil-soluble QDs were made dispersible in alcohol-water media by replacing the native hydrophobic ligands with adenosine 5'-monophosphate (AMP). Then, 3-mercaptopropyl-trimethoxysilane (MPS) was introduced to serve as silane nucleation primers. Finally, a silica shell with controllable thickness was obtained on the QD surface by hydrolysis/condensation of tetraethyl orthosilicate (TEOS). Remarkably, the resultant QD@SiO₂ had nearly the same high luminescent efficiency (50-65%) as that of initial oil-soluble QDs and exhibited excellent long-term photo and colloidal stability in harsh environments (pH range of 3-13, saturated NaCl solution and thermal treatment at 100 °C). It was demonstrated that the cytotoxicity of the resultant QD@SiO₂ was significantly diminished. Moreover, the QD@SiO₂ conjugated with folic acid exhibits high specific binding toward receptor-positive Hela cells over receptor-negative A549 cells, indicating the potential of our obtained QD@SiO₂ as robust biomarkers in cells due to their chemical processibility and low cytotoxicity.