Abstract
The adsorption properties and structure of polyamine-ended poly(ethylene glycol) (PEG) derivatives on a flat gold surface were studied by means of surface plasmon resonance (SPR) and X-ray photoelectron spectroscopy (XPS) using PEG(5k)-block-poly[2-(N,N-dimethylamino)ethyl methacrylate](7.5k) [PEG-b-PAMA(5k/7.5k)] and pentaethylenhexamine-ended PEG(5k) [N6-PEG(5k)], which had 48 and 6 amino groups at the omega-end, respectively. The SPR analysis showed that the amount of PEG-b-PAMA(5k/7.5k) adsorbed onto the gold surface was not affected by the change in pH, and the desorption of this copolymer from the surface was not observed upon the addition of a solution at high salt concentration. The angle-resolved XPS (ARXPS) analysis revealed the structure of the PEG-b-PAMA polymer layer constructed on the gold surface: the PAMA segments were concentrated and located at the interface between the PEG layer and the gold surface. On the other hand, in the case of the PEG-graft-PAMA copolymer (PAMA-g-PEG)-modified gold surface, both the PAMA and the PEG segments homogeneously migrated to all regions of the constructed copolymer layer. The adsorbed amounts of N6-PEG(5k) under different pH conditions were constant and 2-3 times higher than those caused by the adsorption of single amino group-terminated PEG(5k) [PEG-NH(2)(5k)] and hydroxyl group-terminated PEG(5k) [PEG-OH(5k)]. The N6-PEG(5k)-modified gold surface showed a higher nonfouling property toward the adsorption of bovine serum albumin compared with the bare and the N6-modified gold surface. These results indicate that polyamine-ended PEGs were strongly immobilized onto the gold surface by polyamine anchors, even though electrostatic interaction between the polyamine and the gold substrate was not the dominant factor in this adsorption event. Furthermore, the formation of an almost complete phase-separated PEG/polyamine layer on the gold surface by polyamine-ended PEGs was strongly suggested.
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