Plasmonic Nanocavity-Induced Degradation Pathway of Boronic Acid Biosensing Interfaces Revealed by <i>In Situ</i> Tip-Enhanced Raman Spectroscopy.

Abstract

Boronic acid-functionalized plasmonic interfaces enable ultrasensitive molecular recognition and biosensing in plasmonic nanocavities, yet their photochemical stability under hot-carrier excitation remains poorly understood. Here, we elucidate the plasmonic nanocavity-induced degradation pathway of self-assembled monolayers (SAMs) of 4-mercaptophenylboronic acid (4-MPBA) on Au(111) using hyperspectral tip-enhanced Raman spectroscopy (TERS). <i>In situ</i> TERS measurements visualize a stepwise plasmon-driven degradation process at the solid-air interface. Plasmonic excitation initiates deboronation and intermolecular cross-linking within the monolayer, followed by progressive oxidation at the sulfur center and eventual C-S bond cleavage. Complementary electrospray ionization mass spectrometry and X-ray photoelectron spectroscopy identify the final degradation products as oxidized sulfur species, consistent with the TERS spectral signatures. Temperature-programmed desorption mass spectrometry rules out thermal heating as the primary driving force, while density functional theory calculations support plasmon-mediated molecular activation via direct excitation or hot-electron transfer. Together, these results reveal a previously unrecognized degradation pathway of 4-MPBA SAMs on Au(111) that can be directly visualized within a plasmonic nanocavity, providing molecular-level insight into plasmon-driven interfacial chemistry and guiding the design of more stable boronic acid-based plasmonic biosensors.