Fe³⁺-Mediated Interfacial Polyphenol Coatings on Hair Fibers Using an Aronia melanocarpa Extract

Aronia melanocarpa contains abundant phenolic compounds, including procyanidins, flavonols, flavan-3-ols, and phenolic acids, but its pronounced astringency limits its broader use in food applications. Catechol and other oxygen-donor motifs in plant phenolics are known to associate with Fe³⁺ and can support metal-phenolic assembly. In this study, ultrasound-assisted cellulase-pectinase extraction was used to recover an A. melanocarpa polyphenol extract, which was subsequently combined with Fe³⁺ to construct a plant-derived hair dye coating on keratin fibers. Under optimized extraction conditions, the total phenolic yield reached 82.315 mg gallic acid equivalents (GAE)/g dry fruit. A limited targeted LC-MS/MS panel was used to evaluate 12 selected non-anthocyanin phenolic analytes, including procyanidin dimers, a flavan-3-ol, flavonols, and phenolic acids; the contribution of the pH-sensitive anthocyanin fraction was not resolved. Among the tested metal ions, Fe³⁺ produced the strongest chromogenic response. The optimized L-cysteine pretreatment-Fe³⁺ mordanting-polyphenol dyeing sequence produced a ΔE* of 55.42. Adsorption experiments empirically described the uptake behavior of the polyphenol extract, whereas complementary surface, spectroscopic, elemental, diffraction, thermal, and wettability analyses were consistent with the presence of a relatively uniform Fe-containing polyphenol coating at the hair fiber interface. Neither the adsorption-model fits nor the individual characterization techniques uniquely resolved the underlying molecular mechanism. Preliminary HET-CAM, single-exposure dermal, and acute eye irritation assessments showed no obvious acute irritation under the tested conditions. An exploratory image-based analysis examined whether standardized hair tress photographs could approximate a predefined colorimetric score. Because only 30 independent original samples were available and no external validation was performed, this analysis was treated solely as a proof of concept. This laboratory proof-of-concept extends established Fe³⁺-polyphenol assembly chemistry to a compositionally complex A. melanocarpa extract for hair fiber coloration. Further work is required to clarify anthocyanin behavior, simplify the sequential protocol, and benchmark its performance against representative commercial hair dyes.