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.

Recently, surface-enhanced Raman Spectroscopy (SERS) has been widely applied for rapid detection of trace targets in various fields. However, two key issues are still being explored: (1) how to form effective “SERS hotspots” for the trace qualitation with high sensitivity; (2) how to quantitatively identify trace target in complicated matrix in a highly selective and fast manner. Here, aiming at these two key points, we developed an effective and simple protocol for the rapid detection of three tropane alkaloids (TAs), typical toxicants in public security. Taking the advantage of the coadsorption with the specific adsorbed I− during the formation of hotspots by the highly concentrated halide salts, the detection sensitivity of TAs was improved up to 4 orders over the case of Cl−. Under the optimized condition, the lowest detectable concentration of these three TAs was down to the 1 μg/L level. Through the use of drink and food samples, the strong nonspecific competitive adsorption from the complicated matrix was significantly weakened by the simple dilution. The recovery was increased almost 2 orders over that of direct detection, irrespectively of the distinctly changed interferences in different samples. The proposed strategy, improving qualitative sensitivity by forming effective hotspots and quantitative analysis by dilution, not only provides a rapid detection for trace TAs in various foods in case of emergency first response but also could be easily extended to other SERS-based analysis.

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