Poly(l-lysine) grafted with poly(ethylene glycol) (PLL-g-PEG), a polycationic copolymer that is positively charged at neutral pH, spontaneously adsorbs from aqueous solution onto negatively charged surfaces, resulting in the formation of stable polymeric monolayers and rendering the surfaces protein-resistant to a degree related to the PEG surface density. A set of PLL-g-PEG polymers with different architectures was synthesized. The grafting ratio, g, of the polymer, defined as the ratio of the number of lysine monomers to the number of PEG side chains, was systematically varied between 2 and 23, and PEG molecular weights of 1, 2, and 5 kDa were used. The polymers were adsorbed onto niobium oxide-coated substrates, leading to highly different but well-controlled PEG surface densities with maximal values of 0.9, 0.5, and 0.3 chains/nm2 for the three PEG molecular weights, respectively. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) was used in conjunction with the in situ optical waveguide lightmode spectroscopy (OWLS) technique to investigate the interface architecture. While ToF-SIMS provided surface-analytical data on the polymeric adlayer, OWLS allowed the quantitative determination of the adsorbed polymer mass. Extremely good correlations were established between the ToF-SIMS data (obtained in UHV) and the in situ OWLS results. The amount of serum adsorbed, determined quantitatively by OWLS, was found to depend systematically on the surface coverage in terms of the ethylene glycol (EG) density, controlled by both PEG molecular weight and grafting ratio, g. Serum adsorption dropped gradually from 590 ng/cm2 on bare Nb2O5 to <2 ng/cm2 (=detection limit of the OWLS technique) for EG densities ≥ 20 nm-2. The PLL-g-PEG technology shows itself to be an efficient, cost-effective, and robust tool for the immobilization of PEG chains onto metal oxide surfaces. The precise control over PEG surface density across a wide range allows for the production of tailored surfaces with controlled degrees of bio-interactiveness. Such surfaces are expected to have a substantial potential for applications in biomedical and bioanalytical devices.

A novel biosensor interface exploiting the spontaneous surface assembly of a polycationic, PEG-grafted, biotinylated copolymer was developed and tested on optical waveguide chips in a model immunoassay based on sequential immobilization of (strept)avidin and biotinylated goat antirabbit immunoglobulin (RRIgG-biotin) as a capture molecule to sense the rabbit immunoglobulin (RIgG) target molecule. Optical waveguide lightmode spectroscopy with niobium oxide waveguiding layers was used to monitor quantitatively and in situ the spontaneous adsorption of the (biotinylated) copolymer onto the waveguide surface, the resistance of the resulting adlayer to nonspecific protein adsorption, and the mass uptakes in each step of the model immunoassay. Poly(L-lysine)-g-poly(ethylene oxide) (PLL-g-PEG) is a polycationic copolymer that adsorbs spontaneously from aqueous solutions onto negatively charged surfaces via electrostatic interactions. It forms monolayers with densely packed PEG chains. PLL-g-PEG graft copolymers carrying terminal biotin groups on 0, 20, 30, or 50% of the PEG chains were synthesized and assembled onto the surface of niobium oxide (negatively charged at neutral pH). The surface concentration of biotin was tailored by adjusting the biotin grafting ratio in the polymeric molecule or by assembling mixed [PLLg-PEG/PEGbiotin + PLL-g-PEG] adlayers from the corresponding mixed solutions. These biotinylated surfaces are shown to be highly resistant to nonspecific adsorption from serum while still allowing for the specific surface binding of the linkage proteins: streptavidin, avidin, or neutral avidin. The amount of immobilized linkage protein is shown to be closely related to the biotin surface concentration. The subsequent adsorption behavior of RRIgG-biotin and RIgG, however, depends in a more complex manner on each individual surface modification step and is discussed in the light of specific and nonspecific interactions, as well as of orientational and steric repulsion effects within the adlayers. In terms of the sensing signalto-background ratio, the [PLL-g-PEG/PEGbiotin//NeutrAvidin//RRIgG-biotin] architecture demonstrated particularly promising performance as an interface architecture for bioaffinity sensing of proteins.

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In this work, we have explored the application of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) as an additive to improve the lubricating properties of water for metal-oxide-based tribo-systems. The adsorption behavior of the polymer onto both silicon oxide and iron oxide has been characterized by optical waveguide lightmode spectroscopy (OWLS). Several tribological approaches, including ultra-thin-film interferometry, the mini traction machine (MTM), and pin-on-disk tribometry, have been employed to characterize the frictional properties of the oxide tribo-systems in various contact regimes. The polymer appears to form a protective layer on the tribological interface in aqueous buffer solution and improves both the load-carrying and boundarylayer-lubrication properties of water.

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.

The dynamics of cross-polarization from the central transition of a quadrupolar nucleus (27Al or 23Na) to a spin-1/2 nucleus (29Si) during magic-angle spinning and using low-radio-frequency field strengths are analyzed for the mineral low albite. Under these conditions additional complications in the spin-lock behavior of the quadrupolar nucleus and in the cross-polarization process were found experimentally and are examined in detail. A step-by-step procedure for optimizing cross-polarization from the central transition of a quadrupolar nucleus to a spin-1/2 nucleus is described. Significant enhancement of 29Si NMR sensitivity and several applications are demonstrated.

Besides surface chemistry, the surface roughness on the micrometer scale is known to dominate the wetting behavior and the biocompatiblity properties of solid-state materials. The significance of topographic features with nanometer size, however, has yet to be demonstrated. Our approach is based on well-defined Ge nanopyramids naturally grown on Si(001) using ultrahigh vacuum chemical vapor deposition, where the nanopyramid density can be precisely controlled by the growth conditions. Since the geometry of the nanopyramids, often termed dome clusters, is known, the surface roughness can be characterized by the Wenzel ratio with previously unattainable precision. Dynamic contact-angle measurements and adsorption of γ-globulin as a function of that ratio demonstrate the strong correlation between surface nanoarchitecture, on one hand, and wetting behavior and biocompatibility, on the other hand. Related x-ray photoelectron spectroscopy measurements reveal that potential changes of surface composition can be definitely excluded.