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

## TL;DR

Researchers discovered that sealed plasmonic nanogaps can act as channels for molecular transport, enabling ultra-sensitive real-time sensing at the nanoscale.

## Contribution

The study reveals that ligand-capped nanoparticle-on-mirror structures form accessible nanochannels for molecular transport over micrometer-length scales.

## Key findings

- Ligand-capped NPoM-type nanogaps form quasi-2D nanochannels with extreme aspect ratios.
- Wavelength-multiplexed Raman spectroscopy resolves centripetal infiltration pathways with ~20 nm spatial resolution.
- The platform enables in situ, real-time molecular sensing at ~10−11 molar sensitivity.

## Abstract

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.

Seemingly sealed plasmonic nanogaps act as accessible channels for real-time, ultrasensitive molecular sensing.