Laccase-mediated dopamine polymerization: a bioinspired and sustainable coating technology

Polydopamine (PDA) has emerged as a widely used coating for various materials due to its unique properties such as robustness, biocompatibility, and antioxidant and photothermal activities. PDA coatings are usually prepared through autoxidation of dopamine under alkaline conditions, but this approach is generally time-consuming and requires high concentrations of the starting monomer. To overcome these drawbacks, enzymatic catalysis has emerged as a novel approach, allowing also to work in a wider range of pH values. In this work, the laccase-assisted dopamine polymerization and PDA film formation have been systematically investigated and optimized by varying reaction parameters, including dopamine and laccase concentrations as well as stirring speed and deposition time. The best results have been obtained at a low dopamine concentration (2.5 mM) in the presence of 0.25 mg/mL of laccase at pH 5.5, and under high stirring (1250 rpm). Under these conditions, laccase-assisted PDA film deposition resulted in significantly improved coating formation than that obtained through traditional autoxidation at pH 8.5 or sodium periodate-induced oxidation at pH 5.5 by approximately 5- and 1.6-fold, respectively. Insights into the chemical structure and morphological features of the PDA coatings were obtained by liquid chromatography–ultraviolet detection–mass spectrometry (LC-UV-MS), atomic force microscopy (AFM), kelvin-probe force microscopy (KPFM) and attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) spectroscopy analysis. In addition, water contact angle (WCA) measurements were performed. These results provide a significant step forward for the obtainment of organic coatings under mild and sustainable conditions opening new possibilities for future applications in PDA-based functional materials.