Polypeptoid-coated
surfaces and many surface-grafted hydrophilic
polymer brushes have been proven efficient in antifouling-the prevention
of nonspecific biomolecular adsorption and cell attachment. Protein
adsorption, in particular, is known to mediate subsequent cell-surface
interactions. However, the detailed antifouling 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
antifouling coating or adsorbed proteins. In this study, interfacial
assembly of polypeptoid brushes with different chain lengths has 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 (IEPs).
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 antifouling effect against FIB, which was corroborated by atomic
force microscopy imaging. Moreover, NR enabled characterization of
the
in situ
hydrated layer structures of the polypeptoids
together with proteins adsorbed under selected conditions. While adsorption
on bare SiO
2
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 antifouling behavior of polypeptoids.