An Analysis of Commercial Oxidative Hair Dyes Using Raman Spectroscopy

Here, we aim to further the potential use of surface-enhanced Raman spectroscopy (SERS) in analyzing hair dyes for use in forensic investigations. Current methods include light microscopy and thin-layer chromatography (TLC). However, these techniques can be destructive to the hair samples, do not have high discriminatory power, and have had problems with giving inconclusive results. Raman spectroscopy, on the other hand, is a fast, simple technique with a high discriminatory power. Previous work has shown that SERS can be used for in-situ analyses that is not destructive to the sample. This previous work also demonstrated that SERS could definitively differentiate between semi-permanent and permanent dyes and dyes of the same color and permanency but different brand. In this study, we chose to examine 10 different color couplers that are common precursors in the production of permanent hair dyes. These couplers were then divided into four different groups based on the color dye they are used in. Both experimental SERS and theoretical Raman spectra were collected for 3-aminophenol and 5-amino-o-cresol. These results showed that 1) silver surfaces are better than gold for these type of molecules, 2) SERS is acting mainly as a fluorescence quencher with these molecules, thus, 3) the computational B3LYP/6-311G method and basis set are a good model for these molecules. With this in mind, theoretical Raman spectra were created for each of the color couplers and were compared with the others of the same group. Majority of the color couplers showed excellent differentiation and could be distinguished from the other molecule(s) in the group. The only problems arose with the amino-o-cresols in the pink-red category where their spectra were nearly the exact same. Overall, though, this study showed that using the known spectrum of each of these precursor molecules can assist forensic analysts in determining which color coupler was used in the hair dye on the sample