Viability and metabolic activity of human neuroblastoma (SH-SY5Y) cells treated with NAC-modified and unmodified cadmium telluride QDs. a. Quantum dot toxicity differs depending on their surface modifications by NAC. Flow cytometry light scatter dot plots reveal two distinct cell populations corresponding to viable cells (R1), and cells in various stages of apoptotic death (R2). FSC, forward scatter (proportional to cell size); SSC, side scatter (proportional to cell complexity or granularity). b. Cell death in neuroblastomas after 24 hours of QD treatments. Graph shows percentage of dead cells (gated on R2) for each treatment: Ctrl = cells under serum-deprivation with no drug or QD added; QD = cysteamine-QDs; NAC-conj QD = NAC-conjugated QDs; NAC-cap QD = NAC-capped QDs; NAC (2 mM); NAC + QD (2 mM NAC + 5 μg/mL cysteamine-QD). All QDs were added at 5 μg/mL. Mean values and standard deviations from three independent experiments (N = 9) are shown. (*p < 0.05; **p < 0.01). c. Mitochondrial metabolic activity was assessed using MTT and its conversion to formazan was measured at 595 nm. All values are expressed relative to cells without any drug or QD addition (Ctrl) taken as 100%. Note significant decrease with QD treatments and full recovery in the presence of 2 mM NAC. Mean values and standard deviations from quadruplicate measurements in two independent experiments (N = 8) are shown. (**p < 0.01).