Detection of adsorption sites at the gap of a hetero-metal nano-dimer at the single molecule level

In situ surface-enhanced Raman scattering (SERS) measurements were carried out using Au homo, Ag homo and Au–Ag hetero-dimer structures with controlled gap distances. Controlled adsorption of 2,2′-bipyridine and 4,4′-bipyridine molecules onto the surface of metals in an aqueous solution resulted in characteristic SERS peak shifts reflecting switching of the adsorption site at the gap of the Au–Ag hetero-dimer. Changes in the relative intensities and the wavenumbers of the SERS bands provided information on the adsorption environment of the target molecule at the single molecule level.

► We succeeded in preparing a SERS active Au–Ag hetero nanodimer structure.
► The orientation of target molecules at the gaps of the hetero-dimer were evaluated qualitatively.
► The switching of the adsorption site between different metals was observed as the change in the SERS spectra.
► Novel feature of the SERS spectra suggests the formation of a single molecular bridge at the gap.