Prior bleaching of virgin or colored hair has minimal impact on SERS-based hair dye identification

In many criminal investigations, scalp hair is commonly used as physical evidence that can be used to identify the suspect. People commonly dye their hair, which requires prebleaching of virgin and previously colored hair. Surface-enhanced raman spectroscopy (SERS) is a minimally destructive, fast, and reliable technique that can detect and identify dyes present on hair. SERS is based on the enhancement provided by Raman scattering from metal nanostructures that can be drop-casted on a small fraction of hair. In this pilot study, we investigate the extent to which prebleaching of virgin and previously colored hair from one subject can alter the accuracy of SERS-based identification of permanent and semipermanent dyes. We found that prebleaching of virgin hair does not affect spectroscopic signatures of dyes applied on such hair afterwards. Prebleaching of colored hair results in only small changes in the intensity of the SERS spectra of dyes applied on it afterwards. However, these small changes do not alter the accuracy of SERS-based analysis of hair dyes. Furthermore, if prebleached, the underlying colorants are not dominant enough to affect SERS spectra of the dyes applied after bleaching, except in cases where the new dye gives weak Raman signals. These findings help to understand the robustness of SERS in the detection and identification of dyes present on hair, highlight the potential for the use of SERS in forensic identification of bleached hair and, consequently, advance the forensic analysis of hair.