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[reaction: see text] Oxidized bis(indolyl)methane, 1, a simple chromophore containing an acidic H-bond donor moiety and a basic H-bond acceptor moiety, can act as a selective colorimetric sensor either for F(-) in aprotic solvent or for HSO(4)(-) and weak acidic species in water-containing medium. The deprotonation/protonation of oxidized bis(indolyl)methane 1 is responsible for the dramatic color change.

Hydrogen peroxide (H2O2), as a type of reactive oxygen species (ROS), can be endogenously produced from the mitochondrial electron transport chain in aerobic respiration and plays important roles in several physiological processes. However, the design and synthesis of fluorescent probes, which can detect mitochondrial H2O2 in living cells, still remain rare. Herein, we report the preparation of a novel cationic probe 1 (Mito-H2O2), which targets the mitochondria in living cells and is sensitive to the presence of H2O2. The probe Mito-H2O2 displays desired properties such as high specificity, "Turn-On" fluorescence response with suitable sensitivity, appreciable water solubility, and rapid response time (within 5 min). The sensing mechanism was confirmed by high-resolution mass spectroscopy analysis, and the mechanism of "Turn-On" fluorescent response was also determined using a density functional theory (DFT) calculation method. Moreover, as a biocompatible molecule, the probe Mito-H2O2 has been successfully applied for the detection of the intrinsically generated intracellular H2O2 in living cells, and the fluorescence colocalization studies indicate that the probe localizes solely in the mitochondria of HeLa cells.