Red era: Dyeing yak hair fibres red with in situ generated silver nanoparticles using silver nitrate and a flavonoid‐containing plant extract

Abstract Plants and their extracts have been used for dyeing fabrics, skin and hair for thousands of years and have been experiencing a revival in the last years due to increasing health risks arising from oxidative hair dye precursors. We developed a novel two‐step method to dye keratin fibres based on in situ formed silver nanoparticles including a flavonoid‐containing plant extract from Reseda luteola L. and silver nitrate. Yak belly and human hair strands were treated with the aqueous plant extract, adjusted to pH 3 using hydrochloric acid (HCl) or citric acid (CA) or combined with sodium chloride (NaCl). In a second step, the keratin fibres were treated with a silver nitrate solution facing subsequent ultraviolet‐visible (UV‐visible) irradiation. The proposed method resulted in an intense and long‐lasting red dye result, with over 85% colour retention after 24 washes. Microscope images of microtome cuts revealed an increase in penetration depth of the red colour of more than 10% after washing compared to unwashed strands, demonstrating the dye's longevity on and in the hair fibre. No mechanical damage of the hair fibre resulting from dyeing was depicted. The dye did not change the chemical composition of the hair surface as confirmed by X‐ray photoelectron spectroscopy and Fourier‐transform infrared spectroscopy. UV‐visible spectra recorded during red dye formation in aqueous solution showed a peak at 450 nm being characteristic for surface plasmon resonance of silver nanoparticles. Transmission electron microscopy (TEM) images of dried nanoparticles isolated from solution were of a quasi‐spherical shape with different sizes, ranging from 1 to 38 nm.