Still Dyeing After 2,000 Years

Nanotechnology may seem like the latest fad in beauty products, but a new report suggests that people have been using nanomaterials to improve upon nature for at least 2,000 years. According to researchers in France, an ancient hair-coloring concoction turns tresses black via the formation of lead sulfide nanoparticles within the hair shaft (Nano Lett., DOI: 10.1021/nl061493u).

"During the second century C.E., Claudius Galen, the most famous doctor in the Roman Empire, described exactly how to use a mixture of lead oxide and slaked lime [Ca(OH)₂] to dye hair black," explains Philippe Walter, a senior scientist at the Paris-based CNRS Research & Restoration Center of Museums in France, who spearheaded the research. Applied as a paste, the lead-based formula strips sulfur from amino acids in the hair's keratin proteins and forms darkly colored PbS within the hair.

Similar formulas for dyeing hair and wool were recorded during the medieval period and the Renaissance and by 18th-century chemists in France. The coloring product Grecian Formula still uses this lead-based chemistry to gradually darken gray hair.

Working in collaboration with scientists at L'Orèal, Walter and colleagues re-created Galen's recipe and studied progressively darkening hair with modern analytical techniques. They found that the PbS forms quantum-dot-like crystals about 5 nm across. "What is particularly surprising in this reaction is that, despite the structural complexity of hair and its relative chemical inertness, metal sulfide nanoparticles easily crystallize and get organized inside this biomaterial," Walter notes.

In contrast to modern nanotechnology, Walter adds, the dyeing process uses basic chemistry techniques and inexpensive natural materials.