Poly(L-lysine)-g-poly(ethylene glycol) layers on metal oxide surfaces: Surface-analytical characterization and resistance to serum and fibrinogen adsorption

Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) is a member of a family of polycationic PE G-grafted copolymers that have been shown to chemisorb o n anionic surfaces, including various metal oxide surfaces, providing a hig h degree of resistance to protein adsorption. PLL-g-PEG-modified surfaces a re attractive for a variety of applications including sensor chips for bioa ffinity assays and blood-contacting biomedical devices. The analytical and structural properties of PLL-g-PEG adlayers on niobium oxide (Nb2O5), tanta lum oxide (Ta2O5), and titanium oxide (TiO2) surfaces were investigated usi ng reflection-absorption infrared spectroscopy (RAIRS), angle-dependent X-r ay photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The combined analytical information provides clear evidence for an architecture with the cationic poly(L-lysine) attached ele ctrostatically to the oxide surfaces (charged negatively at physiological p H) and the poly(ethylene oxide) side chains extending out from the surface. The relative intensities of the vibrational modes in the RAIRS spectra and the angle-dependent XPS data point to the PLL backbone being located direc tly at and parallel to the oxide/polymer interface, whereas the PEG chains are preferentially oriented in the direction perpendicular to the surface. Both positive and negative ToF-SIMS spectra are dominated by PEG-related se condary ion fragments with strongly reduced metal (oxide) intensities point ing to an (almost) complete coverage by the densely packed PEG comblike gra fts. The three different transition metal oxide surfaces with isoelectric p oints well below 7 were found to behave very similarly, both in respect to the kinetics of the polymer adlayer adsorption and properties as well as in terms of protein resistance of the PLL-g-PEG-modified surface. Adsorption of serum and fibrinogen was evaluated using the OWLS optical planar wavegui de technique. The amount of human serum adsorbed on the modified surfaces w as consistently below the detection limit of the optical sensor technique u sed(<1-2 ng cm(-2)), and fibrinogen adsorption was reduced by 96-98% in com parison to the nonmodified (bare) oxide surfaces.