Analysis of quantum dot fluorescence stability in primary blood mononuclear cells

A quantitative assessment of fluorescence signal generation and persistence in blood cells, measured at multiple points over a time course, is presented. Quantum dots (QDs) are inorganic fluorophores that are photostable and nonmetabolized and so can provide quantitative measures of cell biology over multiple cell generations. However, if the potential of these nanoparticles for long-term reporting is to be realized, an understanding of the stability of their fluorescence in living cells is essential. CdTe/ZnS and CdSe/ZnS core/shell dots with peak emission wavelengths of 705 nm and 585 nm, respectively, were loaded, via endocytosis into mononuclear cells extracted from primary blood and flow cytometry used to measure the average fluorescence intensity per cell within populations >10⁴. Time-based study showed a saturation-limited uptake of QDs with a characteristic time of 20 min and a maximum fluorescence signal that is linearly proportional to dot solution concentration. The fluorescence signal decreases after attachment and internalization within cells and is accurately described by a biexponential decay with a rapid initial decay followed by a much slower signal loss with characteristic times of 435 and 7,000 min respectively. Comparison with control samples indicates that interaction with the culture media is a major contributory factor to the initial signal decay. These results provide phenomenological descriptions of the evolving QD fluorescence within live cells with associated analytical equations that allow quantitative assessment of QD-based assays.