Surface-Enhanced Raman Spectroscopy: Investigations at the Nanorod Edges and Dimer Junctions

Raman signal enhancement of two analyte molecules, containing bipyridine and phenyl moieties, were investigated by linking them (i) onto the edges of Au nanorods using monothiol derivatives and (ii) at the junctions of two Au nanorods using dithiol derivatives. Edges of Au nanorods are regions of high electric field, and specific interaction of the thiol molecules on the {111} planes at the edges resulted in an enhanced Raman signal. When two Au nanorods are brought together in a linear fashion through dithiol linkages, their longitudinal plasmon oscillations couple each other, creating regions of enhanced electric field (hot spots) at the junctions. Interestingly, dimerization leads to a spontaneous enhancement in the intensity of Raman signals (enhancement factor of ∼1.4 × 10⁵) due to the localization of molecules at the junctions of Au nanorod dimers