Protein resistance of oligoether self-assembled monolayers (SAMs) on gold and silver surfaces has been investigated systematically to elucidate structural factors that determine whether a SAM will be able to resist protein adsorption. Oligo(ethylene glycol) (OEG)-, oligo(propylene glycol)-, and oligo(trimethylene glycol)-terminated alkanethiols with different chain lengths and alkyl termination were synthesized as monolayer constituents. The packing density and chemical composition of the SAMs were examined by XPS spectroscopy; the terminal hydrophilicity was characterized by contact angle measurements. IRRAS spectroscopy gave information about the chain conformation of specific monolayers; the amount of adsorbed protein as compared to alkanethiol monolayers was determined by ellipsometry. We found several factors that in combination or by themselves suppress the protein resistance of oligoether monolayers. Monolayers with a hydrophobic interior, such as those containing oligo(propylene glycol), show no protein resistance. The lateral compression of oligo(ethylene glycol) monolayers on silver generates more highly ordered monolayers and may cause decreased protein resistance, but does not necessarily lead to an all-trans chain conformation of the OEG moieties. Water contact angles higher than 70 degrees on gold or 65 degrees on silver reduce full protein resistance. We conclude that both internal and terminal hydrophilicity favor the protein resistance of an oligoether monolayer. It is suggested that the penetration of water molecules in the interior of the SAM is a necessary prerequisite for protein resistance. We discuss and summarize the various factors which are critical for the functionality of "inert" organic films.

Self-assembled monolayers of methyl(1-mercaptoundec-11-yl)tri(ethylene glycol) (CH3O(C2H4O)3C11H22SH, EG3-OMe) adsorbed on gold were investigated by IR−vis sum frequency generation in the range of the C−H stretching vibrations. Comparison of the monolayers in ambient atmosphere, in contact with water, and in contact with carbon tetrachloride revealed that the film structure is strongly disturbed by the interaction of the liquid with the monolayer. The ordered structure found in air undergoes an amorphization upon exposure to the solvents. The experiments demonstrate that in situ analysis of the film structure is mandatory.

We report the synthesis, film formation, protein resistance, and specific antigen binding capability of a carboxy-functionalized poly(ethylene glycol) alkanethiol [HOOC-CH2-(OCH2-CH2)n-O-(CH2)11-SH, n ) 22-45]. Despite its polymeric character, the molecule is found to form a densely packed self-assembled monolayer on polycrystalline gold, if adsorbed from dimethylformamide solution. Due to its chain length distribution, the carboxy tailgroups are expected to be partially buried within the film and, thus, do not affect the protein repulsive characteristics of the ethylene glycol moieties when exposed to fibrinogen and immunoglobulin G (IgG). However, if activated by N-hydroxysuccinimide and N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride, antibodies can be covalently coupled to the monolayer. While resistance to nonspecific fibrinogen and IgG adsorption is maintained for this biologically active layer, it exhibits high specific antigen binding capacity. The performance of this highly selective surface is compared to that of antibody films prepared by standard aminosilane chemistry.

The grafting of alkanethiol-terminated poly(ethylene glycol) [HS(CH2)11(OCH2CH2)n-OCH3; n ) 34-56, MW ≈ 2224 Da] onto polycrystalline gold from dilute solutions was investigated by ellipsometry, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and in situ second harmonic generation. After immersion of a gold-coated Si wafer into a 50 µM dimethylformamide solution, the thickness of the grafted layer increases in a first rapid step up to ∼20 Å. After about 10 min, the thickness rises notably again and reaches saturation after ∼2 h at ∼120 Å. The kinetics of film formation clearly deviate from Langmuir kinetics, which is normally observed for the self-assembly of nonfunctionalized alkanethiols. Our observation can be explained by a conformational transition of the grafted poly(ethylene glycol) chains from amorphous coils to a brush morphology, predominantly consisting of helices with an orientation perpendicular to the surface. The second harmonic generation experiments show that the coverage at saturation of adsorption corresponds to ∼90% that of self-assembled monolayers of alkanethiols, indicating a densely packed film.

Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) has been used to study the protein resistance of poly-and oligo(ethylene glycol) (PEG and OEG) terminated alkanethiolate selfassembled monolayers (SAMs) on Au and Ag in the temperature range from 0 to 85 °C. These experiments extend previous room-temperature studies by Harder et al. 1 who related the protein adsorption characteristics of OEG-SAMs to the lateral density and corresponding molecular conformation of the ethylene glycol (EG) moieties in the film. In addition to the short oligomer OEG-SAMs, we investigated PEG-derivatized alkanethiolate monolayers with an average chain length of 45 EG units and a mean molecular mass of 2000 g/mol (PEG2000). We observe that films, which are protein resistant at room temperature, maintain their protein repulsive characteristics up to 85 °C but may adsorb significant amounts of protein if the temperature is lowered.