X-ray photoelectron spectroscopy and radiometry studies of biotin-derivatized poly(L-lysine)-grafted-poly(ethylene glycol) monolayers on metal oxides

This paper describes the first detailed analytical characterization of the surface properties of a new class of biomolecular interfaces based on derivatized poly(L-lysine)-grafted-polyethylene glycol) (PLL-g-PEG) copolymers. Such copolymers spontaneously adsorb to negatively charged surfaces under physiological pH and efficiently repel nonspecific protein adsorption while providing PEG-tethered functional/active sites for specific biomolecular recognition. As a model system, we synthesized biotin-derivatized (PLL-g-PEG) copolymers, PLL-g-[(PEG)(1-x)(PEG-biotin)(x)], where x varies from 0 to 1. The copolymers were adsorbed on titanium dioxide substrates. Surface characteristics and biorecognition properties were investigated using X-ray photoelectron spectroscopy and radiometry. We show that the monolayer formed is similar to 20-25 Angstrom in thickness. It is organized with its PLL backbone located within the first 10 Angstrom on the substrate and with the PEG side chains located above the PLL. The resulting biotin surface concentration depends linearly on the biotin concentration in the bulk copolymer. This aspect implies that the surface concentration of functional groups can be adjusted by adapting their concentration within the bulk copolymer. The PLL-g-PEG(-biotin) monolayers are efficient in repelling nonspecific protein adsorption but can specifically bind streptavidin (SA). Within the biotin range considered, the SA surface concentration increases linearly with the biotin surface concentration of the monolayer.