Onset of Intense Surface Enhanced Raman Scattering and Aggregation in the Au@Ag System

Abstract

Gold core/silver shell (Au@Ag) nanoparticles of ~37 ± 5 nm diameter generate intense SERS (λEX=785 nm) responses in solution when they interact with the SERS labels rhodamine 6G (R6G), 4-mercaptopyridine (MPY), and 4-mercaptobenzoic acid (MBA). Herein the relationship between SERS intensity, aggregation, and adsorption phenomenon isobserved by titrating Au@Ag with the above labels. As the labels adsorb to the Au@Ag, they drive aggregation as evidenced by the creation of NIR extinction peaks, and the magnitude of this NIR extinction (measured at 830 nm) correlates very closely to magnitude of the intense SERS signals. The label MBA is an exception since it does not trigger aggregation nor does it result in intense SERS; rather intense SERS is recovered only after MBA coated Au@Ag is aggregated with KCl. An “inner filter” model is introduced and applied to compensate for solution extinction when the exciting laser radiation is significantly attenuated. This model permits a summary of the SERS responses in the form of plots of SERS intensity versus the aggregate absorption at 830 nm, which shows the excellent correlation between intense SERS and LSPR bands extinction.