Academician Xu Hongxing and Researcher Chen Wen from the Institute of Precision Spectroscopy, East China Normal University, together with Dr. Hu Huatian at the Italian Institute of Technology, have achieved important progress in nanophotonics. The team proposed and demonstrated a novel detection method based on wavelength-multiplexed surface-enhanced Raman spectroscopy (WM-SERS), enabling selective probing at different spatial positions within nanocavities.

The study reveals that seemingly sealed sub‑2 nm plasmonic nanocavities form quasi‑2D open nanochannels, allowing gradual molecular infiltration and exchange from edges toward the cavity center. This discovery revises conventional understanding of nanocavity structures and provides a new experimental approach for molecular transport research under nanoconfinement. The work, titled *Spatiotemporal Raman probing of molecular transport in sub–2-nm plasmonic quasi-2D nanochannels*, was published in *Science Advances* and selected as the journal’s cover article.

Plasmonic nanocavities compress electromagnetic fields to near atomic scales, making them ideal platforms for single-molecule Raman enhancement and interfacial science. The ultrathin 1–2 nm gaps supported by ligands or self-assembled monolayers were traditionally considered highly enclosed, preventing external molecules from accessing central hotspots. Capturing dynamic molecular behavior with high spatial and temporal resolution at such extreme confinement has long been a core challenge in nanophotonics and interfacial physical chemistry.

To address this challenge, the researchers constructed an integrated plasmonic platform for transport and sensing, adopting the innovative WM-SERS strategy. By leveraging spatial distribution differences of plasmonic modes inside nanocavities, the system achieves ~20 nm spatial resolution. For the first time, the team directly observed centripetal molecular infiltration moving from edges toward the center inside a single nanocavity, confirming that nominally closed plasmonic nanocavities function as open quasi‑2D nanochannels for ordered molecular exchange.

Schematic of SERS sensing for sequential molecular exchange in NPoM plasmonic nanochannels

To validate the generality of this mechanism, the team extended experiments to microscale confined channels with an aspect ratio exceeding 1000 and a length of 5 μm. They visualized molecular infiltration and uncovered how molecules permeate, diffuse and exchange under extreme geometric confinement. These experiments further verify the broad applicability of the molecular exchange mechanism and supply valuable data for future studies of molecular behavior in complex confined nanochannels.

For practical applications, selectively etched sensing sites deliver ultra-sensitive detection down to 10 pM. Combined with digital SERS statistical analysis, the system approaches the single-molecule detection limit. Integrated microfluidic chips enable second-scale real-time monitoring of molecular transport, demonstrating strong potential for investigating dynamic chemical processes.

This research redefines the structural and functional roles of plasmonic nanocavities in nanoconfined systems. It proves nanostructures are not merely passive signal amplifiers, but controllable molecular transport channels and real-time sensing interfaces. The findings establish a robust methodological foundation for nanoreactor construction, on-chip integrated sensing and single-molecule kinetic studies.