Updated Insights Into the Mechanism of Salt‐Induced Aggregation‐Based Single‐Molecule Surface‐Enhanced Raman Spectroscopy

Since its discovery in 1997, the single molecule surface-enhanced Raman spectroscopy (SM-SERS) has attracted wide interest owing to its enormous potential in many fields. However, the commercialized applications of SM-SERS are still limited by the lack of a clear understanding of the relevant mechanism in the famous SM-SERS experiments. In this study, a salt-gradient model is proposed to deeply investigate the physical nature and update insights into the morphological, structural, and component evolution processes of Ag NPs from dispersed nanostructures to aggregation states in the salt-induced aggregation SERS strategy. A gradient interface is observed, where an ultrahigh sensitivity approaching a single molecule level, has been achieved in Ag colloidal system. An unusual dissolution of Ag, the release of Ag⁺ ions from Ag NPs, and the final precipitation of AgCl can be evidenced. Thus, except for aggregation effect, the active AgCl packaging shell on the surface of Ag NPs remarkably improves the SERS property. This work not only reveals the physics processes and nature of SM-SERS but also offers a new way to exploit the SM-SERS into practical applications by means of designing different surface states of NPs and various activation compositions to meet diverse molecule systems.