209 Polyphenol-metal Complex With Dopamine
time can still be achieved when the natural white hair time of ethanolamine soaking is 30
minutes, and the dyeing time of GA-Fe(II)+DA hair dye is 30 minutes. The dyeing time
can be adjusted according to the dyeing needs in the actual application process.
HAIR DYEING MECHANISM AND ANALYTICAL CHARACTERIZATION
To gain a deeper understanding of the self-assembly process involving GA and ferrous
chloride tetrahydrate, which undergo a coordination reaction in solution to form a
polyphenol-metal complex, and to confirm that dopamine remains independent in the
solution without participating in the coordination reaction, we prepared GA-Fe(II)+DA
solutions and analyzed their UV-Vis absorption spectra. This analysis aimed to illustrate
that GA-Fe(II) successfully reacts to complete the self-assembly process, while dopamine
does not.
The UV-Vis absorption spectrum of GA-Fe(II)+DA solution has a broad absorption
ranging from 800 to 500 nm, along with a coordination-to-metal charge transfer (LMCT)
band at 621 nm. These findings indicate the in-situ formation of GA-Fe(II) coordination
structure and the predominant presence of bi-type coordination within the polyphenol-
metal complex (Figure 6C). The shoulder peak at 230 nm–290 nm demonstrated the
intact presence of dopamine. The composite solution of GA-Fe(II)+DA and GA-Fe(II)
solution (without DA addition) were centrifuged (2,000g, 5 min) and the supernatant
was freeze-dried (SCIENTZ, SCIENTZ-12N, Freeze-dryer) to obtain FR-IT spectra for
Figure 4. (A), (B), (C) ethanolamine optimal pH screening. (D) Screening for optimal concentration of
ethanolamine. (E) The time for which ethanolamine soaks natural gray hair.

210 JOURNAL OF COSMETIC SCIENCE
solid determination. Figure 6D shows the FR-IT spectra of dopamine hydrochloride, GA,
ferrous chloride tetrahydrate, GA-Fe(II) solution, and GA-Fe(II)+DA solution. In the
FR-IT spectra of GA-Fe(II)+DA, we found that 1,611 cm−1 and 1,522 cm−1 are benzene
ring skeleton vibrational absorption peaks. Additionally, a peak at 1,286 cm−1 indicates
carbon-oxygen single bond stretching vibrations. Importantly, the strong absorption
peak between 1,100 cm−1 and 1,000 cm−1, consistent with ferrous chloride tetrahydrate,
confirms the complete formation of the GA-Fe(II) complex structure. In addition, the
spectrum of GA-Fe(II)+DA showed no bending vibration peaks of amine and stretching
vibration peaks of carbon and nitrogen bonds, which proved that DA was not involved in
the complexation in the coordination structure of GA-Fe(II), as DA was removed during
the freeze-drying process.
Although GA-Fe(II)+DA hair dye can dye commercial white hair, the dye molecules cannot
penetrate the cortical layer of hair through the protective layer of hair scales of natural white
hair without the help of external forces. This is because the maximum critical molecular
size that can be allowed to pass through the protective layer of hair scales is 1.4 nm. Figure
7A shows the van der Waals sphere model for calculating the molecular sizes of GA, DA,
and GA-Fe(II). The results show that the molecular size of GA is 0.965 nm ≤ 1.4 nm, and
when GA forms a complex with Fe(II), the molecular size increases to ≥ 1.4 nm. The larger
the molecular volume of the complex, the larger the molecular size, making it impossible
Figure 5. (A) Time screening of GA-Fe(II)+DA impregnated natural gray hair. (B) Molar ratio screening of
GA to Fe(II). (C), (D) pH screening of GA-Fe(II)+DA.