Characterization of molecular redox states on silica surfaces using shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) with various shell thicknesses

Understanding the molecular redox state is crucial for investigating chemical activities involving electron exchange, particularly in optical electrochemistry. Methyl viologen (MV) is commonly employed as a redox mediator and electron acceptor, exhibiting three distinct redox states (MV⁰, MV⁺, and MV²⁺), each characterized by a unique molecular structure and Raman spectrum. Utilizing surface-enhanced Raman spectroscopy (SERS), we explore the discrete molecular redox states of MV on shell-isolated nanoparticles (SHINs), which are gold nanoparticles (AuNPs) coated with silica shells of varying thicknesses, ranging from 1 to 10 nm. Our study, employing 532 nm excitation, reveals that all three redox forms of MV are sporadically observed on the metallic surfaces of AuNPs. However, the radical cation (MV⁺) state is predominantly detected on the silica surfaces of the SHINs, irrespective of the shell thickness. This consistency across different shell thicknesses suggests that electromagnetic (EM) effect predominantly contributes to the Raman enhancement in shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), rather than enhancement via electron transfer. If electron transfer were induced by laser excitation, varying redox species would likely appear dependent on shell thickness. Given the absence of external perturbation such as applied potential or reducing agents, we believe our findings can provide a crucial reference for future studies using MV as a redox state-sensing probe. Furthermore, our results demonstrate the efficacy of SHINs as a robust nano-sensing platform that efficiently prevents direct contact with the metallic surface and unwanted reactions.