The characteristic Ag(core)Au(shell) nanoparticles as SERS substrates in detecting dopamine molecules at various pH ranges

Ag(core)Au(shell) nanoparticles (NPs) are a promising surface-enhanced Raman scattering (SERS) substrate, which can offer a high enhancement factor through the combined effect of the high SERS activity of the Ag core and the biocompatibility of the Au shell. In this study, Ag(core)Au(shell) NPs were examined as SERS substrates for the sensitive detection of dopamine (DA) molecules in an aqueous solution. The SERS activity of the Ag(core)Au(shell) NPs was strongly dependent on the pH of the solution. When the pH of the solution was acidic (pH <5) or basic (pH >9), the Ag(core)Au(shell) NPs exhibited negligible SERS activity toward the DA molecules, due to the weakened interactions (or repulsive forces) between the DA molecules and the core–shell NPs. On the other hand, the Ag(core)Au(shell) NPs exhibited a high SERS activity in the intermediate pH ranges (pH 7–9), due to the molecular bridging effect of DA molecules, which allows probe molecules to be located at the interstitial junctions (so-called hot spots) between the core–shell NPs. The results of this study highlight the importance of probe-induced clustering of core–shell NPs in the SERS measurements at physiological pH.