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 -CH 2 OCH 3 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 Ag 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.

Functionalized scanning force microscope (SFM) probes were used to investigate and to mimic the interaction between fibrinogen and self-assembled monolayers (SAMs) of methoxytri(ethylene glycol) undecanethiolates −S(CH2)11(OCH2CH2)3OCH3 (EG3-OMe) on gold and silver surfaces. The SAMs on gold are resistant to protein adsorption, whereas the films on silver adsorb variable amounts of fibrinogen. Experiments were performed with both charged and hydrophobic tips as models for local protein structures to determine the influence of these parameters on the interaction with the SAMs. A striking difference between the two monolayers was established when the forces were measured in an aqueous environment with hydrophobic probes. While a long-range attractive hydrophobic interaction was observed for the EG3-OMe on silver, a repulsive force was measured for EG3-OMe on gold. The strong dependence of the repulsive force for the EG3-OMe-gold system upon the solution ionic strength suggests that this interaction has a significant electrostatic contribution. The observed differences are attributed to the distinct molecular conformations of the oligo(ethylene glycol) tails on the gold-supported (helical) and silver-supported (“all-trans”) monolayers. A comparison of the force/distance curves for the EG3-OMe SAMs with those measured under identical 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.

Self-assembled monolayers (SAMs) of the nitro-substituted oligo(phenylene-ethynylene) (OPE) 4,4′-(diethynylphenyl)-2′-nitro-1-benzenethiolate on Au{111} were prepared, and the structures were characterized by multiple techniques, including infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. Assembly of the nitro-OPE SAM, either via acidic hydrolysis of the thioacetate derivative or from the thiol in pure solvent, produces a well-ordered SAM with a ( 3 × 3) superlattice structure and an average molecular tilt of 32-39°from the surface normal. In comparison, SAMs prepared from the unsubstituted OPE show the same lattice structure and a similar tilt of ∼33°. In contrast, when the nitro-OPE SAM is assembled by hydrolysis of the thioacetate derivative under basic conditions, extensive redox reactions arise in which oxidation of the S atoms occurs with accompanying reduction of -NO2 to -NH2, apparently via intermediates including -NH(OH), to form mixed composition SAMs typically containing ∼30% of the amino-substituted molecule. Further, the nitro-OPE SAM, regardless of the preparation method, shows significant chemical instability under storage in air and/or light exposure. Since the nitro-OPE molecule and molecules with related structures are of considerable interest for molecular electronics applications, these results indicate that extreme diligence must be used in designing conditions for the fabrication of devices utilizing these SAMs.

Low-energy electron-induced damage in hexadecanethiolate (HDT) monolayers on gold substrates has been investigated using infrared reflection−absorption spectroscopy (IRAS), angle-resolved near edge X-ray absorption fine structure spectroscopy (NEXAFS), and advancing water contact angle measurements. HDT films were exposed to electrons of energies 10−100 eV and doses between 30 and 14 000 μC/cm2. The induced damage was monitored both “in situ” by NEXAFS measurements interleaved with electron irradiations and “ex-situ” by NEXAFS, IRAS, and contact angle measurements after exposure of the irradiated samples to air. A progressive film damage was observed with increasing electron energy and dose of irradiation. This damage was found to occur during irradiation in UHV and was not induced by chemical reactions with airborne molecules during subsequent exposure of the irradiated films to air. The damage starts in the region of the terminal methyl groups of the HDT films and propagates into the bulk of the film. An analysis of the IRAS and NEXAFS data shows that the conformational and orientational order within the HDT film are most sensitive to low-energy electron irradiation. Electron-induced cleavage of C−H and C−C bonds resulting in a partial desorption of the film constituents also occurs and leads to formation of CC double bonds in the film as inferred from the appearance of a π*-resonance in the C 1s NEXAFS spectra. The obtained results are of importance for both the optimization of self-assembled-monolayers-based lithography processes and for the general understanding of irradiation-induced changes in organic films.

A detailed understanding of the interaction of proteins with artificial surfaces is essential for many applications in medicine and biochemistry. The affinity of surfaces toward proteins may, for instance, remove pharmacological proteins from media or control the adherence of pathogenic bacteria to protheses. Only a few analytical techniques now exist that can be used to study the binding process in real time, using unlabeled proteins. By investigating the adsorption kinetics of fibrinogen at differently terminated self-assembled monolayers (SAMs) of alkanethiols on thin gold films, it is demonstrated that acoustic plate-mode sensors are a promising analytical tool for studying the adsorption of proteins. In agreement with previous studies for fibrinogen, it is shown in situ that hexa(ethylene glycol)-terminated SAMs (HS(CH2)11 (OCH2CH2)6OH) exhibit very low protein adsorption and that methyl-terminated SAMs (HS(CH2)11CH3) tend to absorb large amounts of protein nonspecifically. The observed adsorption kinetics deviate from classical Langmuir behavior; these kinetics are compatible with a mechanism that involves an unfolding of fibrinogen after adsorption. Film quality is controlled by IR, XPS, and contact angle measurements.

Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH 2 ) 16 SH (NC-C16), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles in NC-C16/Ag and NC-C16/Au were estimated to be 29.5°( 5°and 42.5°( 5°from the surface normal, respectively, while the data suggest lower ordering for NC-C16/Au. The -CtN bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65°( 7°for both NC-C16/Au and NC-C16/ Ag. Comparison with previous data on CH 3 -terminated SAMs reveals that substitution of weakly interacting CH 3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by ∼7.5°a nd ∼17.5°in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile groups is assumed to underlie the structural behavior by controlling a balance between the headgroupsubstrate and interchain interactions. The near-parallel orientation of the nitrile groups to the surface in both of these SAMs can be explained on the basis of minimization of the unfavorable CN-CN dipole-dipole interactions.

We studied the adsorption of (17-aminoheptadecyl)trimethoxysilane (AHTMS; NH2(CH2)17Si(OCH3)3) onto hydroxylated Si(100) wafers, which were preexposed to methyl-terminated n-octadecyltrichlorosilane (OTS; CH3(CH2)17SiCl3) for various immersion periods, resulting in partial coverages between 42% and 88% of a complete monolayer. Preadsorption of a partial monolayer of OTS was found to drastically improve the molecular alignment of subsequently adsorbed AHTMS. This effect was observed for submonolayer OTS films exhibiting both an island or a homogeneous growth mode, as determined by atomic force microscopy. Without preadsorbed OTS, the adsorption of AHTMS from an aged solution typically resulted in films of multilayer thickness, while the use of fresh solutions of AHTMS resulted in disordered films of submonolayer coverage. We suggest that submonolayer quantities of preadsorbed OTS prevent coagulated aminosilane adsorption and enable oriented adsorption of aminosilane molecules along the domain perimeter of aligned and ordered methyl-terminated silane. In comparison to sequential adsorption, coadsorption of n-octadecyltrimethoxysilane (OTMS; CH 3(CH2)17Si(OCH3)3) with AHTMS from a mixture gave films of low quality. The OTS/AHTMS surface that was prepared by sequential adsorption was exposed to pentafluorobenzaldehyde (PFBA) to probe the reactivity and orientation of the AHTMS amino groups. Eighteen to thirty-four percent of the amine groups reacted with no or minor disturbance to the molecular order, compared to 21-44% on pure and disordered AHTMS multilayers. This indicates that the coupling reaction with PFBA in toluene involves only part of the amino groups at the interface.

No abstract