Photophysics of dopamine-modified quantum dots and effects on biological systems.

Semiconductor quantum dots (QDs) have been widely used for fluorescent labelling. However, their ability to transfer electrons and holes to biomolecules leads to spectral changes and effects on living systems that have yet to be exploited. Here we report the first cell-based biosensor based on electron transfer between a small molecule (the neurotransmitter dopamine) and CdSe/ZnS QDs. QD-dopamine conjugates label living cells in a redox-sensitive pattern: under reducing conditions, fluorescence is only seen in the cell periphery and lysosomes. As the cell becomes more oxidizing, QD labelling appears in the perinuclear region, including in or on mitochondria. With the most-oxidizing cellular conditions, QD labelling throughout the cell is seen. Phototoxicity results from the creation of singlet oxygen, and can be reduced with antioxidants. This work suggests methods for the creation of phototoxic drugs and for redox-specific fluorescent labelling that are generalizable to any QD conjugated to an electron donor.