Modulating and probing the dynamic intermolecular interactions in plasmonic molecule-pair junctions

Reversible intermolecular interactions play critical roles in nature. However, it is still challenging to monitor the dynamic intermolecular interactions at single-molecule level in aqueous solution. Here, we studied the dynamic changes of intermolecular interactions at the carboxyl/carboxyl interfaces between a pair of molecules trapped in a plasmonic nanocavity formed between a gold nanoparticle (GNP) and a gold nanoelectrode (GNE). The development of intermolecular interactions, including the appearance of hydrogen bonds (h-bonds), during and after single GNP collision events on the GNE, were monitored by time-resolved surface-enhanced Raman spectroscopy at tens of millisecond time resolution. Spectral fingerprints of carboxyl group corresponding to non-specific intermoleular interactions and h-bonds are identified. Furthermore, we demonstrated that the strength of intermolecular interaction could be mechanically modulated by changing the applied bias at the GNE, which resulted in small and controllable changes in the nanogap distance. Different from non-specific intermolecular interactions, the intermolecular h-bonds can only be formed stochastically and are more senstive to the gap distance modulation. This report demonstrates a new approach to modulate and probe intermolecular interactions within nanogaps.