Paint it black

I don’t generally tend to post my article for Nature’s nanozone here, as they are a bit too techie. But this was just such a cute story…

Nanotechnology is older than we thought. The Egyptians were using it four millennia ago to darken their graying locks.

Artisans were making semiconductor quantum dots more than four thousand years ago, a team in France has claimed. Needless to say, the motivation was far removed from that today, when these nanoparticles are of interest for making light-emitting devices and as components of photonic circuits and memories. It seems that the ancient Egyptians and Greeks were instead making nanocrystals to dye their hair black.

Philippe Walter of the Centre for Research and Restoration of the Museums of France in Paris and his colleagues have investigated an ancient recipe for blackening hair using lead compounds. They find that the procedure described in historical sources produces nanoparticles of black lead sulphide (PbS), which are formed deep within the protein-rich matrix of hair [1].

That the chemical technologies of long ago sometimes involved surprisingly sophisticated processes and products is well known [2]. The synthesis of nanoparticles has, for example, been identified in metallic, lustrous glazes used by potters in the Middle Ages [3]. Such practices are remarkable given that ancient craftspeople generally had no real knowledge of chemical principles and had only crude means of transforming natural materials, such as heating, at their disposal.

The nanocrystal hair dye is particularly striking. Walter and colleagues say that these particles, with a size of about 5 nm, are “quite similar to PbS quantum dots synthesized by recent materials science techniques.” Moreover, the method alters the appearance of hair permanently, because of the deep penetration of the nanoparticles, yet without affecting its mechanical properties.

That makes the process an attractive dyeing procedure even today, despite the potential toxicity of lead-based compounds. Walter and colleagues point out that some modern hair darkeners indeed contain lead acetate, which forms lead sulphide in situ on hair fibres. In any event, safety concerns do not seem to have troubled people in ancient times, perhaps because of their short life expectancy – as well as using lead to dye hair, the Egyptians used lead carbonate as a skin whitener, and toxic antimony sulphide for eye shadow (kohl).

The recipe for making the lead-based hair dye is simple. Lead oxide is mixed with slaked lime (calcium hydroxide, which is strongly alkaline) and water to make a paste, which is then rubbed into the hair. A reaction between the leads ions and sulphur from hair keratins (proteins) produces lead sulphide. These proteins have a high sulphur content: they are strongly crosslinked by disulphide bonds formed from cysteine amino acids, which gives hair its resilience and springiness (such bonds are broken in hair-straightening treatments). The researchers found that the alkali seems to be essential for releasing sulphur from cysteine to form PbS.

The French team dyed blond human hairs black by applying this treatment for three days. They then looked at the distribution of lead within cross-sections of the hairs using X-ray fluorescence spectroscopy, and saw that it was present throughout. X-ray diffraction from treated hairs showed evidence of lead sulphide crystals, which electron microscopy revealed as nanoparticles about 4.8 nm across.

The nanoparticles decorate fibrillar aggregates of proteins within the cortex of hair strands – the inner region, beneath the cuticle of the hair surface. High-resolution microscopy revealed that these particles are highly organized: they seem to be attached to individual microfibrils, which are about 7 nm in diameter and are formed from alpha-helical proteins. Thus the distribution of particles echoes the supramolecular arrangement of the microfibrils, being placed in rows about 8-10 nm apart and aligned with the long axis of the hair strands. So the ancient recipe provides a means not only of making nanocrystals but of organizing them in a roughly regular fashion at the nanoscale – one of the major objectives of modern synthetic methods.

The discovery throws a slightly ironic light on the debate today about the use of nanoparticles in cosmetics [4]. Quite properly, critics point out that the toxicological behaviour of such particles is not yet well understood. It now seems this is a much older issue than anyone suspected.

References
1. Walter, P. et al. Early use of PbS nanotechnology for an ancient hair dyeing formula. Nano Lett. 6, 2215-2219 (2006) [article here]
2. Ball, P. Where is there wisdom to be found in ancient materials technologies? MRS Bull. March 2005, 149-151.
3. Pérez-Arantegui et al. Luster pottery from the thirteenth century to the sixteenth century: a nanostructured thin metallic film. J. Am. Ceram. Soc. 84, 442 (2001) [article here]
4. ‘Nanoscience and nanotechnologies: opportunities and uncertainties.’ Report by the Royal Society/Royal Academy of Engineering (2004). [Available here]