Thiol-Enhanced Interfacial and Internal Deposition of Metal-Polyphenol Networks for Permanent Hair Dyeing

Metal polyphenolic networks (MPNs) are becoming more and more attractive for nontoxic hair dyeing, but their coloring effect is not satisfactory because of the limited interfacial deposition and the absence of internal deposition. Moreover, there is a lack of understanding of the driving factors of the interfacial deposition of MPNs on hair. Herein, we develop a simple yet efficient strategy that transforms disulfide bonds of the hair into thiol groups by thioglycolic acid (TGA) to highly enhance the coloring effect of MPNs at a low temperature. The highly reactive thiol groups react with Fe²⁺ to form Fe-S bonds, greatly facilitating the interfacial and internal deposition of MPNs and thus resulting in rapid coloration and high darkness. Moreover, the TGA-assisted MPNs-dyed (TGA/MPN) hair shows high resistance to washing with a shampoo. Further, it is found that the connection of thiol groups to the MPNs endows the TGA/MPN hair with similar mechanical and structural properties to the natural white hair and even enables simultaneous hair dyeing and perming. This study offers a novel universal strategy for hair dyeing and permeation with MPNs.