CSI: Hair Dye

Hair is commonly found at crime scenes and DNA analysis can certainly help identify suspects. But DNA analysis of hair is limited to samples with intact bulbs and the technique is time consuming, not to mention that there are always hundreds of thousands of samples waiting to be processed...

Looking for alternative methods, researchers from Northwestern University, Illinois, USA, believe that surface-enhanced Raman spectroscopy (SERS) could be used, at the very least, to help pin down suspects that use artificial dyes. SERS can be used directly at crime scenes using portable units to detect hair that have been dyed, as well as exactly what brand was used (1). Gold nanoparticles adsorbed on hair amplify Raman signal from dyes, which are present on the hair surface. Dyes with different chemical structures yield different SERS spectra, which can be considered as ‘dye fingerprints’. Since SERS is non-destructive, samples could still be subject to DNA analyses in the lab.

“The technique is so precise that we can even identify distinct brands of dye that were used to color hair. Moreover, only a few microns of hair is required for this analysis,” says Dmitry Kurouski, lead author on the paper. “It was previously shown that SERS can detect chemical analytes down to the single molecule level. On average, people who artificially dye their hair apply the dye every two month and we expect that artificial dyes can be detected with SERS on hair that was colored more than four weeks prior to the analysis.”

Besides the actual dye, commercial colorants contain numerous ingredients, such as lauryl, cetearyl, myristyl, and stearyl alcohols, fragrance, sodium sulfide, ammonia, and detergents. SERS can selectively see the dye rather than pick up signals from all the additional components. At the moment, the difficulties with DNA analysis lead many forensic investigators to use a microscopic to visually compare hair from crime scenes to known samples, which isn’t always conclusive. Liquid chromatography and mass spectrometry can also be used to detect warfare agents and numerous drugs in hair, but they destroy the sample in doing so.

Richard Van Duyne, the discoverer of SERS and co-author on the paper, adds, “Next, we will explore uncharted territories of potential applications of SERS, as well as study the fundamental principles of the technique. Almost all things that surround us contain dyes. SERS can also be used to detect dyes in food products, plastics, metal and wood paints. For example, SERS analysis of car paints could potentially help in police investigations of car incidents and crashes.”