Dependence of the eGFP (a) and tagRFP (b) fluorescence emission signal as a function of the energy dose at several excitation intensities. In case of the eGFP molecules, the higher the intensity, the faster was the bleaching of the signal in fixed HeLa cells. (c) Long-term illumination of living HeLa cells loaded with

to monitor the production of ROS species in the 473GFP and (d) 532RFP channels. No increase of the fluorescence due to the illumination-induced oxidation of the

molecules was observed during 45 minutes. Few seconds of strong illumination by a Hg-lamp (

for 5 seconds) were sufficient to generate ROS and increase the fluorescence signal. The intensity of the fluorescence signals is given in arbitrary units [a.u.].