Interfacial Assembly Inspired by Marine Mussels and Anti-fouling Effects of Polypeptoids: A Neutron Reflection Study.

Polypeptoid coated surfaces and many surface-grafted hydrophilic polymer brushes have been proven efficient in anti-fouling-the prevention of non-specific biomolecular adsorption and cell attachment. Protein adsorption, in particular, is known to mediate subsequent cell-surface interactions. However, the detailed anti-fouling mechanism of polypeptoid and other polymer brush coatings at the molecular level is not well understood. Moreover, most adsorption studies focus only on measuring a single adsorbed mass value, and few techniques are capable of characterizing the hydrated in situ layer structure of either the anti-fouling coating or adsorbing proteins. In this study, interfacial assembly of polypeptoid brushes with different chain lengths have been investigated in situ using neutron reflection (NR). Consistent with past simulation results, NR revealed a common two-step structure for grafted polypeptoids consisting of a dense inner region that included a mussel adhesive-inspired oligopeptide for grafting polypeptoid chains, and a highly hydrated upper region with very low polymer density (molecular brush). Protein adsorption was studied with human serum albumin (HSA) and fibrinogen (FIB), two common serum proteins of different sizes but similar isoelectric points (IEP). In contrast to controls, we observed higher resistance by grafted polypeptoid against adsorption of the larger FIB, especially for longer chain lengths. Changing the pH to close to the IEPs of the proteins, which generally promotes adsorption, also did not significantly affect the anti-fouling effect against FIB, results that were corroborated by AFM imaging. Moreover, NR characterization enabled characterization of the in situ hydrated layer structures of the polypeptoids together with proteins adsorbed under certain conditions. While adsorption on bare SiO2 controls resulted in surface-induced protein denaturation, this was not observed on polypeptoids. Our current results therefore highlight the detailed in situ view that NR may provide for characterizing protein adsorption on polymer brushes as well as the excellent anti-fouling behaviour of polypeptoids.