Molecular conformation in oligo(ethylene glycol)-terminated self-assembled monolayers on gold and silver surfaces determines their ability to resist protein adsorption

We report data from infrared absorption (FTIR) and X-ray photoelectron spectroscopies that correlate the molecular conformation of oligo(ethylene glycol) (OEG)-terminated self-assembled alkanethiolate monolayers (SAMs) with the ability of these films to resist protein adsorption. We studied three different SAMs of alkanethiolates on both evaporated Au and Ag surfaces. The SAMs were formed from substituted 1-undecanethiols with either a hydroxyl-terminated hexa(ethylene glycol) (EG6-OH) or a methoxy-terminated tri(ethylene glycol) (EG3-OMe) end group, or a substituted 1-tridecanethiol chain with a methoxy-terminated tri(ethylene glycol) end group and a -CH2OCH3 side chain at the C-12 atom (EG[3,1]-OMe). The infrared data of EG6-OH-terminated SAMs on both Au and Ag surfaces reveal the presence of a crystalline helical OEG phase, coexisting with amorphous OEG moieties; the EG[3,1]-OMe-terminated alkanethiolates on Au and AE show a lower absolute coverage and greater disorder than the two other compounds. The molecular conformation of the methoxy-terminated tri(ethylene glycol) (EG3-OMe) is different on Au and Ag surfaces due to the different lateral densities of SAMs on these substrates: on Au we find a conformation similar to that of EG6-OH alkanethiolates, whereas on Ag the infrared spectra indicate a densely packed film with trans conformation around the C-C bonds of the glycol units. The resistance of these OEG-functionalized alkanethiolate SAMs to adsorption of fibrinogen from a buffered solution correlates with the molecular conformation of the OEG moieties. The predominantly crystalline helical and the amorphous forms of OEG on gold substrates are resistant to adsorption of proteins, while a densely packed "all-trans" form of EG3-OMe present on silver surfaces adsorbs protein. The experimental observations are compatible with the hypothesis that binding of interfacial water by the OEG moieties is important in their ability to resist protein adsorption.