Competition between Molecular Adsorption and Diffusion: Dramatic Consequences for SERS in Colloidal Solutions

This study highlights a crucial but often overlooked consideration during sample preparation involving surface-adsorbing species: the competition between analyte adsorption and analyte diffusion/mixing strongly affects the distribution of analytes throughout the sample. In cases of fast analyte adsorption, we argue that the use of large-dilution factors, a common approach for sample preparation in surface-enhanced Raman spectroscopy (SERS), may result in an extreme nonuniformity of the surface coverage. This has a direct effect on the aggregation state of the colloidal solution and therefore on the overall SERS signal. Explicitly, we show that the average SERS signal obtained from typical dyes in colloidal solutions can be drastically different for two seemingly equivalent samples, differing only in the method by which the dye molecules were diluted. We, in addition, discuss the implications of such nonuniformity on the statistics of SERS intensities in the context of single-molecule detection. These results vividly highlight the importance of the dilution step in any experiments involving surface-adsorbing species and position SERS as an ideal tool to evidence such effects. In such cases, a simple half-half dilution procedure should be adopted as the standard method to mitigate these effects.