Solubilization and bioconjugation of QDs and their application in cell imaging

Quantum dot (QD) solubilization, conjugation with biomolecules, column purification, and labeling of human HepG2 cells with transferrin-QD (Tf-QD) conjugates are reported in detail in this article. Water-soluble QDs (WQDs) were obtained using sodium thiolycolate to replace the surface ligand tri-n-octylphosphine oxide (TOPO) on the surface of oil-soluble QDs, and Tf-QD conjugates were produced by coupling Tf to WQD. The resulting Tf-QDs were characterized by UV and luminescence spectrophotometry and purified by Sephadex column. The results indicate that Tf has been conjugated to QD successfully. Based on transferrin/transferrin-receptor-mediated delivery system, the Tf-QD conjugates were used to label human HepG2 cells. After a short incubation, the QDs were mainly localized to the membrane of cells. After 12-h incubation, QDs appear mainly in the cytoplasm portion. However, QDs were not found in the nucleus of the cells. Furthermore, the fluorescence intensity of QDs in the cells reduces gradually over time, and fluorescence cannot be observed after 10 days. However, the growth of the labeled cells was not markedly affected by the toxicity of QDs, and they are alive for 10 days. These results can be used for further application of QDs in bioscience.