Quantum Dots in Ancient Dyed Hair

Inorganic hair dyes obtained from lead compounds for black color have been used in as early as the Greco-Roman period, a few recipes describing several methods to dye hair and wool. These ancient techniques, remarkable, have been used up to today, where similar formula is still being implemented: a mixture of lead oxide (PbO) and slaked lime (Ca(OH)₂), with a small amount of water to form a paste, which is applied on the hair. Successive applications of the paste on gray or light hair give rise to a black color. It is already known that the blackening of hair is due to the precipitation of galena (PbS) crystals by a chemical reaction. The lead in the paste precipitates with sulfur from the hair keratins. A recent letter (Nano Lett., DOI: 10.1021/nl061493u), however, told us a more 'modern' implication about this ancient technique: the microscopic consequence of these practices is the formation of nanocrystals of PbS. With a size of about 5 nm, their appearance is quite similar to PbS quantum dots synthesized by the latest development of nanotechnology, in contrast to which, the dyeing process is characterized by a simple chemical reaction that have been developed for more than 2,000 years(!), based on low-cost natural materials. Further investigation into the dyeing process via X-ray diffraction, scanning and transmission electrion microscopy gives more surprising result that, despite the structural complexity of hair and its relative chemical inertness, metal sulfide nanoparticles easily crystallize and get organized inside this biomaterial.