Raman and SERS characterization of solvent dyes: An example of shoe polish analysis

Surface-Enhanced Raman Spectroscopy (SERS) has significantly contributed to the analysis and identification of dyes over the years. SERS takes advantage of the high molecular absorption of chromophores molecules in the visible to further enhance their signal by resonance mechanisms. The main categories of dyes have all been successfully described by this technique, with the exception of non-water-solubles dyes such as solvent dyes. Due to their non-polarity, solvent dyes do not mix well with aqueous silver colloids nor provide the necessary adsorption to the SERS substrate. We show that by varying the proportions of solvent to water, we can easily and rapidly obtain very intense spectra from solvent soluble dyes, which were otherwise not possible to measure using the conventional silver colloids. We demonstrate this procedure with a preliminary collection of 13 solvent dyes which were characterized by standard Raman and SERS at 488, 633 and 785 nm. Depending on their molecular absorptions these dyes display resonant conditions with either standard Raman, SERS or both. The second part of the paper focuses on the identification of solvent dyes in shoe polish. The use of shoe polish has already been described in a forensic context, but the identification of colorants has never been possible due to the high overlapping absorption of the waxes used in the composition. We demonstrate here that sensitive results can be obtained from amounts of shoe polish as small as a few micrograms. Positive identification of solvent dyes in four different colored shoe polish is presented. (C) 2016 Elsevier Ltd. All rights reserved.