Structural Comparison between Polydopamine Precipitate and Thin Coating Layers, Down to Nanometer Film Thicknesses

Abstract

Despite the large number of investigations and applications of polydopamine (PDA), an atomic-scale description of the differences between PDA as a coating layer down to nanometer layer thickness and PDA precipitate (bulk-PDA) based on clear experimental evidence is not reported yet. In this paper, a two-step approach was used to solve this shortcoming: (i) the deuteration of the dopamine’s catechol-ring to obtain deuterated PDA-based structures, i.e., bulk-PDA and PDA coating layers with different thickness deposited onto SiO2 nanospheres, and (ii) application of different solid-state MAS NMR methods (13C/15N cross-polarization and 2H solid-echo) and electronic microscopy for characterization of the prepared samples (in both deuterated and non-deuterated form). The solid-state NMR clearly demonstrates the structural equivalence of the two systems (nano-size PDA coating layer and bulk-PDA) at all length scales, down to monomeric units composing the final material. The most important result is that solid-state NMR spectroscopy gives enough sensitivity to identify surface effects when the PDA layer’s thickness is only a few nanometers.

Keywords: Polydopamine, PDA, solid-state NMR