A Hidden Silver Oxide Layer Rewrites Molecular Fingerprinting in Surface-Enhanced Raman Spectroscopy

Gold (Au) and silver (Ag) are considered as interchangeable plasmonic components based on the implicit assumption that freshly prepared Ag nanomaterials remain metallic. However, we show, using in-situ surfaceaccessible SERS, that a sub-nanometer AgOx layer forms ubiquitously and instantaneously on colloidal Ag nanoparticles, and that this layer fundamentally alters our picture of how molecules interact with silver surfaces. Unlike the welldocumented long-term oxidation that slowly degrades Ag over weeks, this layer is an intrinsic property of a wide range of freshly prepared Ag nanomaterials and persists even in biological environments. This establishes the oxidationmodulated-adsorption (OMA) effect as a tunable parameter and rational basis for material selection in plasmonic technologies. We show that OMA can be quantified via the standard deviation of relative enhancement (𝜎𝑅𝐸) in SERS. Using SERS sensing as a representative application, we demonstrate that OMA causes the same molecule to yield distinct spectral fingerprints on Ag versus Au, and that harnessing OMA improves detection sensitivity by 500-fold, pushes clinically relevant concentrations into reach, and restores machine-learning classification from random chance to perfection. More generally, recognizing the OMA effect not only rewrites the underlying logic for designing plasmonic materials but also implies that the surface chemistry-related mechanisms in previously reported plasmonic technologies should be carefully re-examined.