Polyphenolization: Hair Browning by Phenolic Adhesion, Ambient Oxygen, and Transition Metals

Similar to how astringency and spiciness persist on the tongue due to their strong anchoring to mucosal proteins, polyphenols demonstrate nonwashable properties. In this study, we investigated how polyphenol coating and subsequent oxidation, activated by ambient oxygen and transition metals, provide a convenient hair dyeing solution, such as apple browning. A synthetic polyphenol, dimeric 1,2,4-benzenetriol (diBTO), immediately coats to hair keratin, forming a nonwashable layer that resists detergent washing. The coated diBTO gradually oxidizes, exhibiting a brown color on the gray hair. Cu(II) ions serve as coordinating linkers between diBTO ligands, increasing coating stability and enhancing the brown color. Cysteine on the hair surface acts as an additional ligand, facilitating the rapid coating of polyphenols. This study reduces the long exposure time required for professional hair dyeing and demonstrates that gray hair care can be effectively achieved through polyphenol browning in daily life, offering a practical solution for maintaining hair color with regular shampooing.