Multifunctional Hair Dyeing with Curcumin via Thiol-Michael Click Reaction

Plant-based colorants in hair dyeing are popular due to their lower toxicity for health and the environment compared to conventional chemically synthesized dyes. However, the application of plant colorants in hair dyeing is limited due to their low adsorption and poor fastness. In this study, we aimed to enhance the hair dyeing performance of a plant-based colorant (curcumin) by prelinking thiol groups (referred to as “click dyeing”) and investigated the dyeing effect and restorative function. The results demonstrated that click dyeing significantly improved curcumin’s coloring ability and color fastness, with the retention ratio of color reaching 80% after seven shampoo washes. Additionally, click dyeing enhanced the mechanical properties of the bleached hair and reduced keratin loss. Fluorescence labeling experiments confirmed that curcumin reacted with the introduced thiol groups through a thiol-Michael click reaction. After click dyeing, the mechanical properties of the hair fibers were repaired due to the formation of chemical bonds during the click reaction, resulting in a 9.81% increase in tensile strength compared to bleached hair before the hair dyeing process. The mechanism behind this enhancement lies in the restoration of the keratin conformation by click dyeing, bringing various properties back to their original state. Furthermore, the study verified curcumin’s ability to resist photoaging through xenon arc lamp radiation and electron spin resonance (ESR) spectroscopy.