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 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 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. It was found that nano-sized PDA coating layers and bulk-PDA are structurally equivalent at all length scales, down to monomeric units composing the final material. Ss-NMR proved sensitive to identify the distinctive contributions of PDA film surfaces in the 13C and 15N ss-NMR spectra for film thicknesses of 5 nm and less. They consist of amplified line intensities for the aliphatic carbons and NH3+, suggesting the larger number of monomeric units carrying these moieties that exist at the film surfaces compared to the inner oligomer layers. The relevance of these findings for better explaining PDA adhesion is discussed.