Gold nanoparticles make for permanent hair dyes

The process involved soaking white hair in a solution of a gold compound, turning them pale yellow and then darkening them to a deep brown.

Using an electron microscope, the scientists confirmed that the particles were forming inside the central core cortex of the hair. The colour remained even after repeated washings, the journal Nano Letters reports.

Philippe Walter, director of a lab at the Centre National de la Recherche Scientifique (CNRS) and colleagues explain that 40,000-60,000 gold nanoparticles could fit across the width of a human hair.

Gold nanoparticles have been deposited on hair for use as electrodes, and they had been used to dye wool, according to a CNRS statement.

Walter's team looked at a new use - dyeing hair, inspired by the ancient Greeks' and Romans' use of another metal, lead, to colour their hair.