Control of Intermolecular Interactions toward the Production of Free-Standing Interfacial Polydopamine Films

Aggregation of the polydopamine (PDA) molecular buildingblocksat the air/water interface leads to obtaining large surface nanometric-thinfilms. This mechanism follows two possible pathways, namely, covalentor non-covalent self-assembly, which result in a different degreeof structure order and, consequently, different structural properties.Control of this mechanism could be vital for applications that requiretrue self-support PDA free-standing films, for example, electrochemicalsensing or membrane technology. Here, we are considering the impactof boric acid (BA) and Cu2+ ions on the mentioned mechanismexclusively for the free-standing films from the air/water interface.We have employed and refined our own spectroscopic reflectometry methodto achieve an exceptionally high real-time control over the thicknessgrowth. It turned out that BA and Cu2+ ions significantlyimpact the film growth process. Reduction of the nanoparticles sizeand their number was examined via UV-vis spectroscopy and transmissionelectron microscopy, showing a colossal reduction in the mean diameterof nanoparticles in the case of BA and a moderate reduction in thecase of Cu2+. This modification is leading to significantenhancement of the process efficiency through moderation of the topologicalproperties of the films, as revealed by atomic force microscopy. Next,applying infrared, Raman, and X-ray photoelectron spectroscopy, wepresented small amounts of metal (B or Cu) in the final structureof PDA and simultaneously their vital role in the oxidation mechanismand cross-linking through covalent or non-covalent bonds. Therefore,we revealed the possibility of synthesizing films via the expectedself-assembly mechanism which has hitherto been out of control. Moreover,modification of mechanical properties toward exceptionally elasticfilms through the BA-assisted synthesis pathway was shown by achievingYoung's modulus value up to 24.1 & PLUSMN; 5.6 and 18.3 & PLUSMN;6.4 GPa, using nanoindentation and Brillouin light scattering, respectively.