Fig. 5. (a) A ratiometric sensing scheme. (b) Time dependent fluorescence changes of the probe (10 μM) in the presence of H₂O₂ (200 μM) in PBS (10 mM, pH 7.4), showing a gradual decrease of the NIR emission at 712 nm (excitation at 702 nm) with the concomitant increase of the far-red emission at 655 nm (excitation at 481 nm). (c) Ratiometric imaging (I_Red/I_NIR) of H₂O₂ in A549 cells using the probe: the cells were incubated with either the probe alone (10 μM; incubation for 30 min) or with the probe and exogenously added H₂O₂ (50 and 200 μM; incubation for 30 min). The ratio images (I_Red/I_NIR) were constructed from the pixel-to-pixel intensity ratio of the images collected in the red channel (500–650 nm, λ_ex = 488 nm) to those collected in the NIR channel (650–800 nm, λ_ex = 633 nm). Scale bar: 100 μm.