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We applied neutron reflectivity measurements to hydrophilic and hydrophobic self-assembled monolayers (SAMs) to probe water density at these interfaces. The measurements were motivated by a previous theoretical study which reported a reduced water density at a hydrophobic surface (Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570) and by our own computer simulations on the hydration force and water density between two methoxy tri(ethylene glycol) terminated undecylthiolate SAMs adsorbed on gold substrates used to study protein adsorption. These simulations predicted that the surfaces are slightly hydrophobic and are characterized by a reduced water density at the interface (Pertsin, A. J.; Hayashi, T.; Grunze, M. J. Phys. Chem. B 2002, 106, 12274). In disagreement with the marginal reduction in water density derived in the simulations, the neutron reflectivity measurements reported here indicate an unexpectedly extended (~4 nm) water layer at the SAM surface with a noticeably reduced density (85−90% of the density of bulk water). The reproducibility of the experimental results with the methoxy tri(ethylene glycol) terminated undecylthiolate SAMs was confirmed with four different samples and by one measurement using a contrast-matched D2O/H2O water mixture. We also used neutron reflectivity measurements to study the water density at the water/SAM interface of hydroxy hexa(ethylene glycol) and hydroxy tri(ethylene glycol) terminated undecylthiolate SAMs. Except for one of the hydroxy hexa(ethylene glycol) samples studied, the results are consistent with the presence of bulk water in direct contact with the interface. We also discuss possible artifacts and problems in the analysis. Our results on nonfunctionalized hydrophobic octadecanethiolate and hydrophilic hydroxy-terminated undecylthiolate SAMs give physically unreasonable and nonconclusive models for the water interface on these surfaces, respectively. The best fit of the data for the hydrophobic surface gives an unreasonably low water density, possibly due to the presence of air inclusions in the film and/or adsorbed air “nanobubbles”. The results obtained on the hydrophilic hydroxy-terminated surface can be fitted equally well with a model assuming an interface water density that is higher or lower than that of bulk water, demonstrating the ambiguities associated with describing the organic/liquid interface with a box model for the Q range accessible in our experiment.

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.

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 self-assembled 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.

Acoustic plate mode (APM) sensors have been used to detect immunochemical reactions in on-line measurements. In order to determine the exact mechanisms of interaction involved and to enhance reproducibility, both metallized and non-metallized devices have been investigated. The metallization is found to reduce total frequency shifts by 40–50%, an effect which must be attributed to either acoustoelectric interaction with the protein layer or different mass sensitivities for the different surface boundary conditions. In spite of the lower frequency shifts, metallized sensors may be advantageous for detecting biochemical reactions in ionic solutions because spurious signals are found to be effectively eliminated.

Acoustic plate mode (APM) devices have been used as detectors in liquid environments. One of the primary detection mechanisms in such devices is mass loading. In the present work, the potential of APMs as mass sensitive biosensors is evaluated using five APM devices on ZX–LiNbO3 with frequencies of operation ranging from 13 to 42 MHz. Mass sensitivity is measured as a function of frequency and is compared to data predicted by perturbation theory. Discussions are given relative to typical antigen binding capacities of derivatized crystal surfaces.

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 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.