Nanoscale Disassembly and Free Radical Reorganization of Polydopamine in Ionic Liquids

Despite the growing scientific and technological relevance of polydopamine (PDA), a eumelanin-like adhesive material widely used for surface functionalization and coating, knowledge of its structural and physicochemical properties, including in particular the origin of paramagnetic behavior, is still far from being complete. Herein, we disclose the unique ability of ionic liquids (ILs) to disassemble PDA, either as a suspension or as a thin film, up to the nanoscale, and to establish specific interactions with the free radical centers exposed by deaggregation of potential investigative value. Immersion of PDA-coated glasses into four different ILs ([C₁C₁im][(CH₃O)HPO₂], [C₁C₁im][(CH₃O)CH₃PO₂], [C₁C₁im][(CH₃O)₂PO₂], [N₁₈₈₈][C_18:1]) at room temperature caused the fast and virtually complete removal of the coating as determined by UV-visible spectroscopy and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) analysis of the colored supernatants from PDA suspensions in ILs revealed the presence of nanostructures not exceeding 50 nm in diameter. Electron paramagnetic resonance (EPR) analysis indicated profound IL-dependent modifications in signal intensity, line-width, and g-factor values of PDA. These differences were interpreted in terms of a partial conversion of C-centered radicals into O-centered semiquinone-type components following destacking and interaction with the anion component in ILs. The discovery of ILs as a powerful tool to disassemble PDA under mild conditions provides a new entry both to detailed investigations of this biopolymer on the nanoscale and to mild removal of coatings from functionalized surfaces, greatly expanding the scope of PDA-based surface functionalization strategies.