Characterization of a Chloride-Activated Surface Complex and Corresponding Enhancement Mechanism by SERS Saturation Effect

Chemical enhancement in surface-enhanced Raman scattering (SERS) may involve the presence of s charge-transfer (CT) complex either by direct binding (covalent) to the metal or by indirect binding with the assistance of an electrolyte ion. The electrolyte (e.g., NaCl, MgSO4) is very necessary for both direct binding and indirect binding. For the direct binding complex, the electrolyte functions as aggregation agents to create hot spots, while for the indirect binding complex, the electrolyte is used to not only assist the formation of the CT complex but also create hot spots by aggregating nanoparticles. Thus, it is difficult to identify the types of complexes by direct observation. In this article, the types of CT complexes can be distinguished via simply observing the change of saturation point of SERS intensity. The saturation point for indirect binding complex can easily be shifted to higher concentration by simply increasing the concentration of chloride, but the saturation point for the direct binding complex is almost unchanged. Correspondingly, the type of chemical enhancement can be studied further after the complexes are confirmed. Thus, this new method is a very simple and effective way to characterize the types of CT complexes and chemical enhancement.