Gap mode induced photo-oxidation of p-methyl thiophenol and relating phenomena

Abstract

Surface plasmon excitation at a nanogap between adjacent metal nanostructures generates extremely enhanced electric field, which enables us to detect and characterize individual molecules by surface enhanced Raman scattering (SERS). However, versatile nanogap fabrication methods have not been established, making difficult to efficiently exploit a gap mode plasmon in practical applications such as biosensing, biomedical, and environmental analyses. Here, we describe reproducible nanogap assemblies using adjacent metal nanoparticles (MNPs) and MNPs/adsorbates/metal substrates based on the Derjaguin − Landau − Verway − Overbeek theory and irreversible optical trapping of MNPs on metal substrates. The assembled nanogaps between adjacent MNPs, here, M is Ag or Au, enabled us to characterize adsorbates such as solvent shared ion pairs of hydrated metal ions, and three distinct adsorbed states of adenine depending on pH in aqueous solutions. Also we found plasmon-induced oxidation of p-alkyl thiophenol (p-AlkTP) at the nanogap on AgNP/p-AlkTP/Ag films. This oxidation has a specific reactivity at the p-position in the p-AlkTP phenyl ring, while providing intermediate species under nitrogen atmosphere. This specific oxidation is not driven by a thermal process but by a hot carrier transfer process. Thus, the nanogap fabrication methods using adjacent metal nanostructures we developed are promising to elucidate static and dynamical nature of target molecules using SERS spectroscopy.