Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) is a member of a family of polycationic PEG-grafted copolymers that have been shown to chemisorb on anionic surfaces, including various metal oxide surfaces, providing a high degree of resistance to protein adsorption. PLL-g-PEG-modified surfaces are attractive for a variety of applications including sensor chips for bioaffinity assays and blood-contacting biomedical devices. The analytical and structural properties of PLL-g-PEG adlayers on niobium oxide (Nb2O5), tantalum oxide (Ta2O5), and titanium oxide (TiO2) surfaces were investigated using reflection-absorption infrared spectroscopy (RAIRS), angle-dependent X-ray 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 electrostatically to the oxide surfaces (charged negatively at physiological pH) 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 directly 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 secondary ion fragments with strongly reduced metal (oxide) intensities pointing to an (almost) complete coverage by the densely packed PEG comblike grafts. The three different transition metal oxide surfaces with isoelectric points 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 waveguide technique. The amount of human serum adsorbed on the modified surfaces was consistently below the detection limit of the optical sensor technique used (<1-2 ng cm -2 ), and fibrinogen adsorption was reduced by 96-98% in comparison to the nonmodified (bare) oxide surfaces.
Octadecylphosphoric acid ester is shown to self-assemble on amorphous/nanocrystalline tantalum oxide (Ta2O5) layers deposited by physical vapor deposition onto glass substrates. Three complementary surfaceanalytical techniques (angle-dependent X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and atomic force microscopy in lateral force mode), showed that a 2.2 nm thick, "tailsup"-oriented adlayer is formed, which displays local near-hexagonal order, strong P-O-Ta bonding, and the presence of (-P-O-)2Ta species. A model for the binding and the structural organization of the octadecyl phosphate molecules on the tantalum oxide surface is proposed involving direct coordination of the terminal phosphate headgroup to Ta(V) cations forming a strong complexation bond, two types of bonding of the octadecyl phosphate with both monodentate and bidentate phosphate-Ta(V) coordinative interactions, and, locally, the formation of a coincidence lattice of approximately hexagonal structure defined by both the location of Ta(V) cation sites and an intermolecular spacing between the octadecyl phosphate ligands of approximately 0.5 nm. This is very similar to the self-assembled monolayer structure of long-chain alkanethiols on gold. The use of phosphoric acid ester derivatives is believed to have potential for designing specific interface architectures in sensor technology, in surface modification of oxide-passivated metallic biomaterials, and in composite metal (oxide)-polymer interfaces.
Antiwear additives, such as zinc dialkyldithiophosphate (ZnDTP), find application in many different industrial sectors. Although it is understood that certain ZnDTP concentrations need to be used to achieve an effective antiwear performance, there has been very little work published concerning the effect of temperature on the interactions of the additive and its adsorption mechanism on steel. In this article, 100Cr6 (52100) steel ball-on-disc experiments under solutions of zinc dialkyldithiophosphate (ZnDTP) in poly-a-olefin (PAO) were performed at different temperatures, ranging from 25 to 180°C. The discs were analysed after the experiments by means of small-area, imaging and angle-resolved X-ray photoelectron spectroscopy (XPS). The composition of the reaction film was found to change as a function of the applied temperature and also to vary within the film as a function of depth: Longer polyphosphate chains were found at higher temperatures as well as towards the outer part of the reaction film.
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