Cost-Effective Strategy for Surface Modification via Complexation of Disassembled Polydopamine with Fe(III) Ions

Mussel-inspired polydopamine (PDA) deposition provides a prominent approach for constructing functional coatings, which has received much research interest over the past decade. However, large PDA aggregates often formed and precipitated from the solution during the deposition process, significantly lowering the utilization efficiency of dopamine for surface modification. It is of both fundamental and practical importance to "reactivate" and reuse the precipitated aggregates to achieve higher usage efficiency of PDA in surface modifications. In this work, we report a facile, substrate-independent, and cost-effective coating strategy, by disassembling the precipitated PDA aggregates, to achieve the coating deposition through the complexation of disassembled polydopamine (d-PDA) species with Fe(III) ions on various substrates. Adsorption tests determined by a quartz crystal microbalance with dissipation (QCM-D) monitoring technique indicated that the pH of the solution and the ratio of d-PDA to Fe(III) significantly influence the deposition behavior of d-PDA/Fe(III). Force measurements using a surface force apparatus demonstrated that the coordination interaction between d-PDA and Fe(III) was the major force leading to the formation of coatings. The deposited d-PDA/Fe(III) coatings featured controllable nanoscale thickness, uniform surface morphologies, and light color. Furthermore, the d-PDA/Fe(III) coating could act as an intermediate layer in the preparation of hydrophobic polyurethane sponge for highly efficient oil/water separation. This work provides a useful strategy to realize the reusability of PDA aggregates for versatile surface functionalization, with implications for the fundamental understanding of the formation mechanism in the metal-phenolic complexation systems and development of new coating approaches in various engineering applications.