Structural comparison between polydopamine precipitate and thin coating layers, down to nanometer film thicknesses

• Published in: Applied surface science 2024-03, Vol.649, p.159190, Article 159190
• Main Authors: Filip, Xenia, Simion, Andrea, Grosu, Ioana-Georgeta, Raluca Gherman, Ana-Maria, Lar, Claudia, Filip, Claudiu
• Format: Article
• Language: English
• Subjects: PDA; Polydopamine; Solid-state NMR
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2023.159190

[Display omitted] •2H/13C/15N solid-state NMR for bulk PDA and PDA coating layer deposited onto SiO2 NPs was performed.•Ss-NMR spectroscopy proves sensitive to separate the contribution of surface and interface for layer’s thickness of only a few nanometers.•Rigid packing of oligomers in PDA, regardless the nature of the sample, bulk, or coating layer was evidenced by 2H MAS NMR.•Structural equivalence between bulk PDA and PDA coating layer with at least 10 nm thickness was demonstrated. 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.