Sample self‐absorption in surface‐enhanced Raman spectroscopy (SERS): influence of the resonance, dilution and depth of the measurements

In this paper, we demonstrate that sample self‐absorption can strongly attenuate the surface‐enhanced Raman spectroscopy (SERS) signal when resonant conditions are used. Below a monolayer coverage of the SERS substrate (silver nanoparticles), the signal is weakly attenuated, and a linear relationship to the concentration is maintained, as suggested by the fact that the scattered signal is proportional to the concentration. For concentrations of analytes superior to a monolayer coverage, the intensity reaches a maximal plateau value, and the increasing amount of molecules in solution acts as filters. As a result, it strongly attenuates the intensity of the SERS scattered main peaks and of the incident excitation light. These molecules contribute to the absorption but not to the Raman scattering. At this point, the attenuation of the light follows an exponential relationship to the concentration, and a decrease in intensity is observed for very concentrated solutions. A practical example is given with the analysis of an orange dye, having a strong absorption in the visible (~400–500 nm). We show that the effect of self‐absorption is even more pronounced when depth‐related measurements are made. We also demonstrate that exciting the sample off‐resonance eliminates the self‐absorption and that a linear relationship of the Raman intensities to the concentration is respected. We finally compare the surface‐enhanced with normal Raman measures. Copyright © 2017 John Wiley & Sons, Ltd.