Single-Molecule Fluorescence Detection of Effective Adsorption Sites at the Metal Oxide–Solution Interface

Nanoscale mapping of adsorption sites for molecules or ions at solid–liquid interfaces has not been explored in detail because of the difficulty in probing both stochastic adsorption/desorption events and heterogeneous surface structures. We report here the application of single-molecule-based super-resolution fluorescence microscopy using a catechol-modified boron–dipyrromethene dye (CA-BODIPY), which serves as a fluorescent reporter, to identify the locations of effective adsorption sites on metal oxide surfaces. Upon adsorption on a TiO₂ nanoparticle, individual CA-BODIPY molecules exhibited detectable fluorescence because of the formation of chelating complexes between the catechol moiety and the surface Ti sites. Interestingly, a significant effect of the crystal face on the adsorption preference for CA-BODIPY was found in the case of anatase TiO₂ microcrystals in neutral water: {101} > {001} ≈ {100}. In an aprotic solvent such as acetonitrile, however, the opposite crystal face effect was observed; this implies a significant contribution of solvent molecules to the adsorption of organic compounds on specific surfaces. From the quantitative analysis of the formation rate of fluorescent complexes per unit area, it was found that nanometer-sized TiO₂ crystals have superior adsorptivity over micrometer-sized TiO₂ crystals and an atomically flat TiO₂ surface. This observation is consistent with the higher density of surface defects on the nanoparticles. Furthermore, it was revealed that CA-BODIPY molecules are preferentially adsorbed on the top branches of α-Fe₂O₃ micropines, where a high density of exposed Fe cations is expected. Our methodology and findings yield new insights into the mechanisms underlying the synthesis and (photo)­catalytic activity of metal oxide particles with different sizes and shapes