Plasmonic catalysts have the potential to accelerate
and control
chemical reactions with light by exploiting localized surface plasmon
resonances. However, the mechanisms governing plasmonic catalysis
are not simple to decouple. Several plasmon-derived phenomena, such
as electromagnetic field enhancements, temperature, or the generation
of charge carriers, can affect the reactivity of the system. These
effects are convoluted with the inherent (nonplasmonic) catalytic
properties of the metal surface. Disentangling these coexisting effects
is challenging but is the key to rationally controlling reaction pathways
and enhancing reaction rates. This study utilizes super-resolution
fluorescence microscopy to examine the mechanisms of plasmonic catalysis
at the single-particle level. The reduction reaction of resazurin
to resorufin in the presence of Au nanorods coated with a porous silica
shell is investigated in situ. This allows the determination
of reaction rates with a single-molecule sensitivity and subparticle
resolution. By variation of the irradiation wavelength, it is possible
to examine two different regimes: photoexcitation of the reactant
molecules and photoexcitation of the nanoparticle’s plasmon
resonance. In addition, the measured spatial distribution of reactivity
allows differentiation between superficial and far-field effects.
Our results indicate that the reduction of resazurin can occur through
more than one reaction pathway, being most efficient when the reactant
is photoexcited and is in contact with the Au surface. In addition,
it was found that the spatial distribution of enhancements varies,
depending on the underlying mechanism. These findings contribute to
the fundamental understanding of plasmonic catalysis and the rational
design of future plasmonic nanocatalysts.