Fig. 2.

PEG-dopa₄-Fe³⁺crosslinking. (A) Dopa-modified polyethylene glycol (PEG-dopa₄, 10 kDa PEG core). (B) Relative fractions of mono- (green), bis- (blue), and tris-catechol-Fe³⁺ (red) complexes in solutions of PEG-dopa₄ with FeCl₃ at a dopa∶Fe molar ratio of 3∶1 as a function of pH. (C) Physical state and color of PEG-dopa₄ gels in mono- (green/blue), bis- (purple), and tris-catechol-Fe³⁺ (red) complexation (dopa∶Fe 3∶1). Insets highlight color of bis- and tris-catechol-Fe³⁺ cross-linked gels. (D) Resonance Raman spectroscopy of the same samples as in (C). Spectra of PEG-dopa₄ without (dashed black) and with (dashed green) FeCl₃ (unadjusted pH ∼3–4) are shown for comparison. Additionally, the spectrum from the native mussel thread cuticle is included (dashed red). For each spectra N = 3.