The adsorption of fibrinogen (Fb) and bovine serum albumin onto polycrystalline Au coated with HS(CH2)3O(CH2CH2O)5CH3 was determined by surface plasmon resonance from bare Au (0% coverage) to the complete ( approximately 100% coverage) self-assembled monolayer (SAM). Both proteins exhibit similar adsorption curves with common onset ( approximately 60% coverage) and range ( approximately 60% to 80% coverage) of minimal protein adsorption. Reflection-absorption infrared spectroscopic data show that widespread order is not present in the films over this range of coverage, indicating loosely packed, bound oligomers that are uniformly distributed and fully screen the underlying substrate. On the basis of our data, we propose a mechanism of protein rejection by oligo(ethylene oxide) (OEO)-modified surfaces in terms of changes in free energy (DeltaGsystem; system = protein + surface) due to oligomer conformational constriction over an area greater than the contact area. Minimal protein adsorption corresponds to the maximum DeltaGsystem for a given compression. This controlled study of protein adsorption provides insights into the molecular level understanding of protein adsorption unavailable from previous polymer and comparative SAM studies.

The structure and ordering conditions of the self-assembled monolayers (SAMs) of HS(EO)xR, where R ) CH3, EO ) CH2CH2O, and x ) 3-6, on polycrystalline gold (Au) were determined by reflectionabsorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy (EIS). For x ) 5 and 6, RAIRS and SE data show that the 1-thiaoligo(ethylene oxide) [TOEO] segments adopt the highly ordered helical structure of the folded-chain crystal polymorph of crystalline poly(ethylene oxide) and are oriented normal to the substrate. RAIRS and SE data obtained as a function of immersion time show that x ) 6 attains the highly ordered, all helical conformation faster than x ) 5. For x ) 3 and 4, RAIRS and SE data indicate largely disordered structures, with some all-trans conformation but little evidence of the helical conformation. EIS measurements, in an aqueous environment, indicate a significant presence of water in all SAMs with x < 6 while x ) 6 remains essentially free of water and/or ions for extensive periods (2 h). These data suggest that a minimum of five ethylene oxide units are necessary for TOEO SAMs to adopt the helical conformation and that SAMs with x < 6 are penetrated by water or ionic species in an aqueous environment, regardless of the order of the TOEO segment in air.

The structural order and ordering conditions of the self-assembled monolayers (SAMs) of HSCH2CH2CH2O(EO)xCH3, where EO = CH2CH2O and x = 3-9, on polycrystalline gold (Au) were determined by reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy. For x = 5-7, RAIRS and SE data show that the oligo(ethylene oxide) [OEO] segments adopt the near single phase, 7/2 helical conformation of the folded-chain crystal polymorph of crystalline poly(ethylene oxide), oriented normal to the substrate. These SAMs exhibit OEO segment structure and orientation identical to that found in a previous isostructural series [HS(CH2CH2O)6-8C18H37 SAMs. Vanderah, D. J., et al. Langmuir 2003, 19, 3752] and are anisotropic films for surface science metrology where structure is constant and thickness increases in 0.30 nm increments. In addition, this is the first example of OEO SAMs to attain this highly ordered, helical conformation where the (EO)x segment is separated from the Au-sulfur headgroup by a polymethylene chain. For x = 4, 8, and 9, the SAMs are largely helical but show evidence of nonhelical conformations and establish the upper and lower limits of the isostructural set. For x = 3, the SAMs are largely disordered containing some all-trans conformation. SAM order as a function of immersion time from 100% water and 95% ethanol indicates that the HSCH2CH2CH2O(EO)5-7CH3 SAMs order faster and under a wider range of conditions than omega-alkyl 1-thiaolio(ethylene oxide) [HS(EO)xCH3] SAMs, reported earlier (Vanderah, D. J., et al. Langmuir 2002, 18, 4674 and Vanderah, D. J., et al. Langmuir 2003, 19, 2612).

Self-assembled monolayers (SAMs) of a series of linear thiols containing a 1-thiaoligo(ethylene oxide) [TOEO] moiety, i.e., HS(CH2CH2O)xCH3, where x = 3 - 6, were prepared on polycrystalline gold (Au) and characterized by reflection absorption infrared spectroscopy (RAIRS) and spectroscopic ellipsometry (SE). For x = 5 and 6, the RAIRS data show that the TOEO segment, oriented normal to the substrate, adopts the highly ordered 7/2 helical structure of the folded-chain crystal polymorph of poly(ethylene oxide). For x = 3 and 4, the RAIRS and SE data indicate disordered, “amorphous” SAMs with essentially no evidence of the helical conformation in the TOEO segment. These data suggest that, for SAMs with TOEO segments, a minimum of five ethylene oxide units is required to adopt a helical conformation.