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.