During the past decade vibrational sum frequency generation as a method to study interfaces has matured and can now be applied more routinely to systems of increasing complexity. The article provides a brief overview of technical aspects of infrared–visible sum frequency generation, compares this nonlinear technique with its linear analogs, and highlights the latest applications.

Adsorption of docosanethiol (CH3(CH2)21SH) onto polycrystalline gold from ethanol solution was investigated by nonlinear vibrational spectroscopy. The formation of the self-assembled monolayer (SAM) was monitored by recording of the temporal evolution of the C−H stretching modes. Three different steps with significantly different time scales were identified. The fastest step in the adsorption process is related to the Au−S bond formation. Progressing with a rate constant of 2500−3000 L mol-1 s-1, it is characterized by hydrocarbon chains with a high fraction of gauche kinks. The second step proceeds 3−4 times more slowly and comprises the straightening of the alkane chains. During the first two steps a pronounced band around 2813 cm-1 appears, which is interpreted as mode softening of the C−H vibrations due to the interaction with the substrate. The time scale of the final step is a factor of 35−70 slower than the rate of chain stretching and encompasses the reorientation of the terminal methyl end groups. The methylene group adjacent to the methyl group exhibits the same temporal behavior and thus behaves oppositely to the other methylene groups. The SFG data imply that the last few percent of the thiol molecules adsorbing produce the complete monolayer by inducing the transition from a high number of gauche defects to an all-trans conformation. The sequence of changes in the vibrational spectra suggests an ordering and annealing of the film from the interface to the film surface.

We present a method for increasing the lateral chain density in self-assembled monolayers (SAMs) of alkanethiols on polycrystalline gold. This method relies on exposure of the alkanethiolate monolayers to mercury vapor and subsequent reimmersion into the thiol solution. Mercury adsorption on the gold surface induces a structural rearrangement in the alkanethiolate monolayers, as indicated by changes in dichroism in the Fourier transform infrared (FTIR) spectra, in line shape in the sum frequency generation (SFG) spectra, and in the macroscopic wetting behavior of the monolayers. X-ray photoelectron spectroscopy (XPS) data show that saturation of the thiolate samples with mercury occurs after 20−30 min of exposure to air saturated with mercury vapor. For 100 nm evaporated gold films a mercury bulk concentration of 14−16 atom % was determined by energy-dispersive X-ray analysis (EDX). Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) data indicate that after mercury adsorption the monolayers consist of gold thiolate and mercury thiolate molecules. From the FTIR and XPS data we conclude that the mercury-exposed SAMs exhibit an inhomogeneous structure with differently tilted domains. As determined from the IR experiments, the average tilt angle of the alkyl chains in hexadecanethiolate monolayers decreases by ∼16° by mercury adsorption and by an additional ∼3° after reimmersion into the thiol solution. The corresponding changes obtained from near edge X-ray absorption fine structure (NEXAFS) spectra are ∼9° and ∼3°, respectively.

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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.

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Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.