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 oxygen species (ROS), such as singlet oxygen ( 1 O 2 ) and hydroxyl radical ( • OH), diffused from the photoirradiated TiO 2 surface, was successfully demonstrated using single-molecule fluorescence spectroscopy. Airborne single 1 O 2 and • OH molecules were selectively detected by the fluorescent probes, terrylenediimide (TDI) and 3‘-( p -hydroxyphenyl) fluorescein (HPF), respectively. Generation of the airborne 1 O 2 and • OH from the TiO 2 surface has been investigated under various conditions, such as the excitation wavelengths (UV or visible) and the types of TiO 2 (pure or nitrogen (N)-doped). Upon UV excitation, 1 O 2 and • OH were detected from both the pure and N-doped TiO 2 samples, while 1 O 2 was exclusively detected only from the N-doped TiO 2 upon visible excitation. Furthermore, the spatial and temporal distribution of the airborne • OH molecules diffused from the photoirradiated TiO 2 surface was investigated by the real-time single-molecule imaging technique. The bimolecular reaction rate constant between • OH and HPF, which obeys pseudo first-order kinetics, can be regarded as a steady-state concentration of the airborne • OH molecules. Additionally, the anomalous diffusion of • OH molecules through an air−water interface (on glass surface, roughly 50 μm 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 TiO 2 photocatalysts, air, and oxidizing substrates.