Figure 2.

Higher temperature enhances polydopamine deposition and hair darkening. (A, B) Photographs of hair samples before (1) and after dyeing with polydopamine using 3% NH₄OH (2), 0.05 N NaOH (3), 10 mM CuSO₄/15 mM H₂O₂ (4), Tris buffer pH 8.5 (5), as well as color of the resulting polydopamine solutions. Dopamine polymerizations were carried out for 2 h at (A) room temperature and (B) 37–40 °C. (C) Optical microscopy images of the hair samples dyed at 37–40 °C are also reported. (D) UV–Vis spectra of solutions obtained after 2 h dopamine polymerization using 3% NH₄OH and 0.05 N NaOH at either room temperature or at 37–40 °C. In every case, hair samples were washed five times with water and three times with a 10% shampoo solution prior to imaging/analysis (see Methods for protocols and for hair sample descriptions from alternative vendors).