The dynamic properties of PDA-laccase films at the air-water interface

Abstract

Polydopamine (PDA) films at the air - water interface can be easily obtained in slightly alkaline dopamine solutions using 02 dissolved in water as an oxidant but their insufficient stability limits their applications. In this work PDA films at the water surface were obtained by polymerization of dopamine hydrochloride (DA) by the enzyme laccase in a slightly acidic environment at a constant concentration of enzyme and DA concentrations in the range from 0.1 to 10 g/l. The dynamic surface elasticity, effective surface tension, film thickness and refractive index were measured as function of time to characterize the main steps of film formation. The obtained results show that laccase accelerates the polymerization process, decreases the required DA concentration for film formation, and insight from Brewster angle microscopy shows that laccase increases the resistance of the films to mechanical deformation. The dynamic surface elasticity of the obtained films is about seven times higher than the highest values for either pure PDA or pure laccase films. Laccase is inferred not only to provide crosslinking of the polymer but is also incorporated in the film resulting in its higher stability. The kinetic dependences of the surface properties allow the main steps of film formation to be distinguished: nucleation of polymer domains in a rapidly-formed protein film, coexistence of the protein and polymer domains, and their transformation into a highly dense composite film with encapsulated protein. Insight from atomic force microscopy images of films transferred to mica show that PDA fills in the gaps of spherical laccase aggregates when the DA concentration is increased and the film becomes less rough. The inclusion of laccase in PDA films significantly increases their stability.