In Vitro Tracking of Human Umbilical Vein Endothelial Cells Using Ultra-Sensitive Quantum Dot-Embedded Silica Nanoparticles

The nanoscale spatiotemporal resolution of single-particle tracking (SPT) renders it a powerful method for exploring single-molecule dynamics in living cells or tissues, despite the disadvantages of using traditional organic fluorescence probes, such as the weak fluorescent signal against the strong cellular autofluorescence background coupled with a fast-photobleaching rate. Quantum dots (QDs), which enable tracking targets in multiple colors, have been proposed as an alternative to traditional organic fluorescence dyes; however, they are not ideally suitable for applying SPT due to their hydrophobicity, cytotoxicity, and blinking problems. This study reports an improved SPT method using silica-coated QD-embedded silica nanoparticles (QD(2)), which represent brighter fluorescence and are less toxic than single QDs. After treatment of QD(2) in 10 μg/mL, the label was retained for 96 h with 83.76% of labeling efficiency, without impaired cell function such as angiogenesis. The improved stability of QD(2) facilitates the visualization of in situ endothelial vessel formation without real-time staining. Cells retain QD(2) fluorescence signal for 15 days at 4 °C without significant photobleaching, indicating that QD(2) has overcome the limitations of SPT enabling long-term intracellular tracking. These results proved that QD(2) could be used for SPT as a substitute for traditional organic fluorophores or single quantum dots, with its photostability, biocompatibility, and superior brightness.