Fig. 6. Fluorescence response of GCS-2 to 50 μM Zn²⁺ on live HeLa cell surfaces followed by addition of 50 μM Cu²⁺. (a) Images of 1 min prior to Zn²⁺ addition, 3 min after the addition of Zn²⁺, and 1 and 3 min after the addition of Cu²⁺. Scale bar: 10 μm. (b) Plot of the average response of 13 cells vs. time after 50 μM Zn²⁺ addition. All fluorescence intensity values were subtracted by background signals and normalized by the fluorescence before Zn²⁺ addition. The time of 50 μM Cu²⁺ addition is shown. The error bars correspond to the standard error of the mean value of 13 individual cells.