Controlled Silver Nanoparticle Formation in Hair Fibers Dyed with Reseda luteola L.: A Study on Additive-Dependent Penetration and Aggregation.

Applying physico-analytical methods to whole hair fibers enables investigation of hair dye performance. Light microscopy, SEM imaging and EDX mapping of intact hair fibers, as well as TEM imaging of microtome cuts, provided insights into the distribution, size, shape and growth patterns of the dyeing species and particles, thus demonstrating the correlation between silver nanoparticles (AgNPs) and dye impression. Yak hair fibers were treated with a polyphenol-containing Reseda luteola L. extract (RE), which had been acidified using either hydrochloric acid (HCl) or citric acid (CA), and subsequently exposed to silver nitrate (AgNO₃), resulting in the formation of quasi-spherical silver nanoparticles (AgNPs) that were depicted several microns deep inside the hair fiber, regardless of the additive used. The particles appeared to aggregate preferentially in sulfur-rich domains within the hair fiber, probably due to the affinity of silver ions on the NP's surface towards sulfur. The additives significantly affected the size and aggregation behavior of the particles. Using HCl, larger, aggregated particles were formed, whereas the application of CA yielded smaller, more uniform particles and a higher penetration depth. Despite different particle sizes, the dye outcome was comparable. In strands treated with HCl, washing brought the particles deeper into the hair cortex and resulted in further aggregation. Thus, HCl promoted the formation of larger particles whereas CA yielded more uniformly sized particles. These findings open a new route for metal nanoparticle-based hair dyes with excellent wash fastness.