Two-Dimensional Ag Nanoparticle Tetramer Array for Surface-Enhanced Raman Scattering Measurements

A two-dimensional Ag nanoparticle tetramer array which served as a hotspot matrix for surface-enhanced Raman scattering detection of rhodamine 6G (R6G) molecules down to a concentration as low as 10^–15 M was successfully fabricated by electrochemical deposition on an anodized aluminum substrate. The high detection sensitivity was attributed to both the electromagetic enhancement at the dense Ag nanoparticle tetramer hotspot matrix and chemical enhancement on the corrugated substrate. A single molecule dynamic adsorption behavior was experimentally sensed by the abrupt changes of the charateristic peak intensity and line shape in the spectroscopy when the R6G concentration was lowered to 10^–15 M. Time-evolved spectroscopies revealed the adsorption behavior of either the single molecule in the nanogaps of 2–5 nm or multiple molecules in the nanogaps of 5–9 nm between the Ag nanoparticles.