Surface-Selective Molecular Binding and Replacement Selectivity in Plasmonic Nanocavities

Molecular self-assembled monolayers (SAMs) have a strong role in nanoscience and nanotechnology, being exploited for sensing, molecular electronics, catalysis, spin transport, and more. Typically, thiol binding to coinage metals produces well-ordered, robust molecular layers. While their replacement by substituting thiols has been studied on planar surfaces, little is yet known when the SAMs are confined at the nanoscale. Here, using strong plasmonic confinement, we optically track how thiol SAMs are replaced in nanocavities and show that an unexpected mechanism is introduced when nanoparticles are placed on top of the SAM. Using a range of model molecules demonstrates that replacement thiols preferentially attach to the nanoparticle and are rotated into the nanogap. These dynamics can be selectively prevented using dithiols that fix facet metal atoms in place. This mechanism offers a promising route for spatially selective chemical control, enabling asymmetric molecular architectures and postdeposition functionalization of plasmonic nanostructures.