Silica-coated quantum dots fluorescent spheres synthesized using a quaternary 'water-in-oil' microemulsion system

Nanoscale and microscale silica spheres embedded with multiple CdSe quantum dots (QDs, having average diameters of about 2.4 and 5.0 nm, respectively.) were synthesized by using a quaternary 'water-in-oil' microemulsion. Comparing the uncoated QDs, the quantum yields (QYs) of the silica-coated QD spheres were enhanced when the QD cores were synthesized using mercaptoacetic acid (MA) as a stabilizer, while the QYs were dramatically decreased when the cores were synthesized using citric acid (CA) as a stabilizer. The enhanced QYs could be further improved by heating the silica-coated QDs in aqueous solution. Although the QYs of the silica-coated QDs were not high, these spheres emitted bright fluorescence. The silica shells contained numerous micropores (similar to 0.58-0.91 nm), and small amounts of toxic ions (such as Cd2+) could be released from the silica spheres. However, the release rate of toxic ions from the silica spheres was significantly reduced compared with that of the uncoated QDs.