Probing resistance to protein adsorption of oligo(ethylene glycol)-terminated self-assembled monolayers by scanning force microscopy

Functionalized scanning force microscope (SFM) probes were used to investig ate and to mimic the interaction between fibrinogen and self-assembled mono layers (SAMs) of methoxytri(ethylene glycol) undecanethiolates -S(CH2)(11)( OCH2CH2)(3)OCH3 (EG(3)-OMe) on gold and silver surfaces. The SAMs on gold a n resistant to protein adsorption, whereas the films on silver adsorb varia ble amounts of fibrinogen. Experiments were performed with both charged and hydrophobic tips as models for local protein structures to determine the i nfluence of these parameters on the interaction with the SAMs. A striking d ifference between the two monolayers was established when the forces were m easured in an aqueous environment with hydrophobic probes. While a long-ran ge attractive hydrophobic interaction was observed for the EG3-OMe on silve r, a repulsive force was measured for EG3-OMe on gold. The strong dependenc e of the repulsive force for the EG3-OMe-gold system upon the solution ioni c strength suggests that this interaction has a significant electrostatic c ontribution. The observed differences are attributed to the distinct molecu lar :conformations of the oligo(ethylene glycol) tails on the gold-supporte d (helical) and silver-supported ("all-trans") monolayers. A comparison of the force/distance curves for the EG3-OMe SAMs with those measured under id entical conditions on end-grafted poly(ethylene glycol) (PEG 2000) on gold further emphasizes that the nature of the repulsive forces originating from the short-chain oligomers is unique and not related to a "steric repulsion " effect.