Parallel Plate Capacitor Model at the Nanoscale for Stable and Gigantic SERS Activity of the...

Selective use of ingredients out of a specific natural product (e.g., fruit, leaf, flower, or honey extract) or their mixture (e.g., bacteria, viruses, fungi, plants, etc.) by smart manipulation of precursors and reaction conditions to synthesize nanoparticles can provide us a low-cost, environmentally friendly route for their industrial-scale production. The presence of more than one active ligand (sourced natural product extract) on the surface not only makes them the most stable (electrostatically) and monodispersed (controlled kinetics) but also devoid of any external ligand-assisted aggregation. This empowered us to modify the surface of the nanoparticles in a monolayered fashion or to couple between nanoparticles through a ligand-assisted chemical coupling pathway to avoid their aggregation and hence to keep their nanoscale property intact. A metal-to-ligand charge transfer (MLCT) trajectory combined with electromagnetic field-induced coherent capacitive coupling between two nanoparticles was introduced to explain the gigantic Raman enhancement observed from these nanoparticles. As a model system, we have synthesized the nanoparticles from rose extract as the active ligand ingredient source for 2-phenyl ethanol, linalool, citronellol, nerol, geraniol, pyrogallol (C₆H₃(OH)₃), and quercetin (3,3',4',5,7-pentahydroxyflavone) and the surface of the synthesized nanoparticles has been modified by 4-mercaptobenzoic acid (4-MBA) acting as a Raman tag. The obtained structural and spectroscopic data correlate well between our numerical and density functional theory (DFT)-based calculations to justify their gigantic SERS activity, which may lead us to propose an unexplored coherent capacitive coupling-based Raman enhancement mechanism.