Switching on SERS from SERS-inactive substrates through simple electrodeposition guided by defective molecular layers

Raman scattering is a powerful method for molecular identification but lacks sufficient intensity. This limitation is overcome by surface-enhanced Raman scattering (SERS), which relies on plasmonic surfaces to enable electromagnetic enhancement of Raman scattering-originally discovered serendipitously in an electrochemical (EC) cell. Electrochemistry can also induce charge transfer (CT) resonance, offering chemical enhancement. Recently, a new way of applying electrochemistry to SERS has been developed, in which secondary plasmonic components are electrochemically deposited onto "already-SERS-active nanostructured surfaces" while CT resonance conditions are formed simultaneously. Despite its superior analytical performance, the underlying enhancement mechanisms of this unconventional EC-SERS remain unclear, limiting its practical applications. Here, we elucidate the key factors governing this unconventional EC-SERS, enabling its application even with "SERS-inactive flat metal surfaces". High-resolution electron microscopy reveals that surface pre-modification with a defective molecular monolayer is critical for the formation of plasmonic nanostructures. We also clarify the role of CT resonance by tuning the chemical conditions and performing theoretical calculations. We believe that these insights not only remove the need for sophisticated preparation of SERS substrates, expanding practical access to SERS, but also open a way to further improve the enhancement of SERS from molecular, surface, theoretical, and electrochemical perspectives, reinvigorating SERS research.