Real-time single-molecule imaging of the spatial and temporal distribution of reactive oxygen species with fluorescent probes:: Applications to TiO2 photocatalysts

The single-molecule detection of airborne reactive ox gen species (ROS), such as singlet oxygen (O-1(2)) and hydroxyl radical ((OH)-O-center dot), diffused from the photoirradiated TiO2 surface, was successfully demonstrated using single-molecule fluorescence spectroscopy. Airborne single O-1(2), and (OH)-O-center dot molecules were selectively detected by the fluorescent probes, terrylenediimide (TDI) and 3'-(p-hydroxyphenyl) fluorescein (HPF), respectively. Generation of ' the airborne O-1(2), and (OH)-O-center dot from the TiO2 surface has been investigated under various conditions, such as the excitation wavelengths (UV or visible) and the types of TiO2 (pure or nitrogen (N)-doped). Upon UV excitation, O-1(2) and (OH)-O-center dot were detected from both the pure and N-doped TiO, samples, while O-1(2) was exclusively detected only from the N-doped TiO2 upon visible excitation. Furthermore, the spatial and temporal distribution of the airborne (OH)-O-center dot molecules diffused from the photoirradiated TiO2 surface was investigated by the real-time single-molecule imaging technique. The bimolecular reaction rate constant between (OH)-O-center dot and HPF, which obeys pseudo first-order kinetics, can be regarded as a steady-state concentration of the airborne (OH)-O-center dot molecules. Additionally, the anomalous diffusion of (OH)-O-center dot molecules through an air-water interface (on glass surface, roughly 50 mu m per 1 s) was directly observed and interpreted in terms of the superdiffusive model. Our finding in this single-molecule study provides new insights into the generation, diffusion, and reaction processes of the airborne ROS at the solid-air interfaces including TiO2 photocatalysts, air, and oxidizing substrates.