The generation of surfaces and interfaces that are able to withstand protein adsorption is a major challenge in the design of blood-contacting materials for both medical implants and bioaffinity sensors. Poly(ethylene glycol)-derived materials are generally considered to be particularly effective candidates for the fabrication of protein-resistant materials. Most metallic biomaterials are covered by a protective, stable oxide film; converting such oxide surfaces, which are known to strongly interact with proteins, into noninteractive surfaces requires a specific design of the surface/interface architecture. A class of copolymers based on poly(L-lysine)g-poly(ethylene glycol) (PLL-g-PEG) was found to spontaneously adsorb from aqueous solutions onto several metal oxide surfaces, such as TiO 2 , Si 0.4 Ti 0.6 O 2 , and Nb 2 O 5 , as measured by the in situ optical waveguide lightmode spectroscopy technique and by ex situ X-ray photoelectron spectroscopy. The resulting adsorbed layers are highly effective in reducing the adsorption both of blood serum and of individual proteins such as fibrinogen, which is known to play a major role in the cascade of events that lead to biomaterial-surfaceinduced blood coagulation and thrombosis. Adsorbed protein levels as low as <5 ng/cm 2 could be achieved for an optimized polymer architecture. The modified surfaces are stable to desorption under flow conditions at 37 °C and pH 7.4 in HEPES [4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid] and PBS (phosphatebuffered saline) buffers. The adsorbed layer of copolymer is thought to form a comblike structure at the surface, with positively charged primary amine groups of the PLL bound to the negatively charged metal oxide surface, while the hydrophilic and uncharged PEG side chains are exposed to the solution phase.Copolymer architecture is an important factor in the resulting protein resistance; it is discussed on the basis of packing-density considerations and the corresponding radii of gyration of the different PEG chain lengths studied. This surface functionalization technology is believed to be of value for use in both the biomaterial and biosensor areas, as the chosen macromolecules are biocompatible and the application is straightforward and cost-effective. † Part of the special issue "Gabor Somorjai Festschrift".
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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