Nano Lett. 12, 6212–6217 (2012)

Human hair is a long cylinder with a diameter of 30–100 μm, and the hair shaft — the part seen above the skin — consists of three layers. The outermost region, called the cuticle, contains many layers of cells and protects the middle layer called the cortex. The cortex, which contains lipids, keratins (structural proteins) and melanin (colour pigment), provides strength, texture and colour. The medulla, which is the innermost layer, is often absent but may be seen in thick hairs. In ancient times, lead minerals were used to dye human hair. Now, Philippe Walter and colleagues have shown that gold nanoparticles can be grown inside them.

The researchers — who are based at various institutes in France — treated white human hair with an alkaline solution of chloroauric acid (HAuCl4). The treated hair changed from pale yellow after 1 day to deep brown after 16 days. High-resolution imaging and spectroscopy showed that gold had been taken up by the cuticle and cortex, and that gold nanoparticles had formed on the surface of the hair shaft on each cuticle cell. Closer examination of the cuticle revealed that many of the nanoparticles were formed in areas with keratins, which contain cysteine-rich amino acids.

Multiple washings did not affect the colour of the hair because the gold nanoparticles were buried and stabilized inside the keratin structure, and the researchers suggest that this approach might be of use in the hair colouring industry.