Key Role of Direct Adsorption on SERS Sensitivity: Synergistic Effect among Target, Aggregating Agent, and Surface with Au or Ag Colloid as SERS Substrate.

It is widely accepted that the sensitivity of surface-enhanced Raman spectroscopy (SERS) is mainly manipulated by the electromagnetic enhancement mechanism (EM). Herein, we determined that the direct adsorption of the target on the SERS active surface is vital as well, through the systematic investigation of the SERS behavior of three positively charged molecules on negatively charged gold (Au) or silver nanoparticles (Ag NPs). Facilitated by the synergistic effect among the molecule, the surface, and the specific adsorbed halide ions (Cl-, Br-, and I-), high SERS sensitivity for trace target was realized, which was mainly from the directly adsorbed molecules. Noteworthy, little contribution from the nondirectly adsorbed molecules was discernible, although the EM enhancement was at the same level for these two surface species dwelling within a distance significantly less than 1 nm from the surface. Further, the related strategy for trace detection sheds light on how to realize sensitive SERS detection of new targets.