Abstract
The tyrosinase-catalyzed oxidation of tyramine, leading to the deposition of pseudo-polydopamine (ψ-PDA) thin films, is disclosed herein as a superior technology for surface functionalization and coating at a neutral pH and at a low substrate concentration, compared to the standard autoxidative PDA coating protocols. Smooth ψ-PDA thin films of variable thickness up to 87 nm were obtained from 1 mM tyramine by varying tyrosinase concentrations (5–100 U/mL). Compared to the PDA films obtained by the similar enzymatic oxidation of 1 mM dopamine with tyrosinase (T-PDA), ψ-PDA displayed slower deposition kinetics, lower water contact angles in the range of 11°–28°, denoting higher hydrophilicity but similar UV-vis absorption profiles, as well as electrochemical properties and antioxidant activity. MALDI-MS analysis indicated for ψ-PDA a well defined pattern of peaks compatible with dopamine tetrameric structures degraded to a variable extent. The exposure to a tyramine solution of tyrosinase-loaded alginate spheres, or films deposited on glass or polyethylene, resulted in a rapid gel-confined ψ-PDA formation with no leakage or darkening of the solution, allowing the complete recovery and re-utilization of the unreacted tyramine. In contrast, an abundant PDA precipitation outside the gel was observed with dopamine under the same conditions. The ψ-PDA deposition by tyrosinase-catalyzed tyramine oxidation is thus proposed as a controllable and low-waste technology for selective surface functionalization and coating or for clean eumelanin particle production.
Highlights
The multifunctional and universal wet surface dip-coating technology with highly adhesive polydopamine (PDA) films has dominated the scene of materials science over the past decade, since the first report of the classical autoxidation protocol from 10 mM dopamine at pH 8.5
Since the main goal of this study was the development of a clean protocol for the generation of an adhesive functionalizing polymer with no waste of unreacted precursor or the massive precipitation of black polymer, in a final set of experiments, tyrosinase was incorporated into alginate gel, both spheres and films on glass, and the enzyme-containing gels were exposed to 1 mM tyramine, and for comparison, dopamine, in a buffer at pH 6.8
Dipped into tyramine solution; (B) tyrosinase (0.1%) alginate hydrogel beads dipped in 1 mM tyramine tyramine or dopamine
Summary
The multifunctional and universal wet surface dip-coating technology with highly adhesive polydopamine (PDA) films has dominated the scene of materials science over the past decade, since the first report of the classical autoxidation protocol from 10 mM dopamine at pH 8.5.
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