Surface and Tip-Enhanced Raman Spectroscopy at the Plasmonic Hot Spot of a Coordination Complex-Conjugated Gold Nanoparticle Dimer

We report a synthetic method to generate surface enhanced Raman scattering (SERS) active gold nanodimer, by employing rationally designed co-ordination complex of europium (Eu) metal with 4-mercaptobenzoic acid (4-MBA) and heterocyclic phenanthroline ligands. The reaction between the Raman active 4-MBA ligand of the Eu-complex and citrate-stabilized Au nanoparticles (cit-Au NPs) led to selective dimerization in considerable yield and precise control over hot-spot engineering to maximize SERS. Additionally, scanning probe microscopy (SPM)-correlated tip-induced Raman enhancement using He–Ne laser focusing on the plasmonically coupled Au nanodimers helped obtain intense Raman signal at the spatial resolution of single dimer. The average SERS amplification was measured to be on the order of 10⁹, whereas that in the presence of tip-enhancement was found to be 10⁴. This extraordinary Raman activity of an inorganic complex-coupled gold nanodimer therefore augurs well for applying chemical synthetic strategies in designing plasmonically active assembled nanomaterials for applications in photovoltaics, photocatalysis, biosensing, and single molecule spectroscopy.