Spatiotemporal Raman probing of molecular transport in sub-2-nm plasmonic quasi-2D nanochannels.

Capturing molecular dynamics in nanoconfined channels with high spatiotemporal resolution is a key challenge in nanoscience, crucial for advancing catalysis, energy conversion, and molecular sensing. Bottom-up ultrathin plasmonic nanogaps, such as nanoparticle-on-mirror (NPoM) structures, are ideal for ultrasensitive probing due to their extreme light confinement, but their perceived sealed geometry has cast doubt on the existence of accessible transport pathways. Here, counterintuitively, we demonstrate that ubiquitous ligand-capped NPoM-type nanogaps can form a natural quasi-two-dimensional nanochannel, supporting molecular exchange and infiltration over unprecedented length scales (≳5 micrometers) with an extreme aspect ratio (>103). Using wavelength-multiplexed Raman spectroscopy, we resolve the underlying centripetal infiltration pathway with a spatial resolving power of ~20 nanometers. This redefines the NPoM architecture as a sensitive and hotspot-accessible platform, enabling in situ, real-time, reusable monitoring of analyte with ~10-11 molar. This work establishes a versatile platform for advancing super-resolved in situ molecular sensing, nanoscale physicochemical studies, and on-chip nanophotofluidics.