Real-Time Single-Molecule Imaging of the Spatial and Temporal Distribution of Reactive Oxygen Species with Fluorescent Probes:  Applications to TiO₂ Photocatalysts

Abstract

The single-molecule detection of airborne reactive oxygen species (ROS), such as singlet oxygen (1O2) and hydroxyl radical (•OH), diffused from the photoirradiated TiO2 surface, was successfully demonstrated using single-molecule fluorescence spectroscopy. Airborne single 1O2 and •OH molecules were selectively detected by the fluorescent probes, terrylenediimide (TDI) and 3‘-(p-hydroxyphenyl) fluorescein (HPF), respectively. Generation of the airborne 1O2 and •OH 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, 1O2 and •OH were detected from both the pure and N-doped TiO2 samples, while 1O2 was exclusively detected only from the N-doped TiO2 upon visible excitation. Furthermore, the spatial and temporal distribution of the airborne •OH molecules diffused from the photoirradiated TiO2 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 TiO2 photocatalysts, air, and oxidizing substrates.