Effects of concentration and temperature on the formation process of decanethiol self-assembled monolayer on Au(111) followed by electrochemical reductive desorption

Adsorption and desorption of self-assembled L-cysteine monolayers on nanoporous gold monitored by in situ resistometry

Surface modifications of nanoporous metals have become a highly attractive research field as they exhibit great potential for various applications, especially in biotechnology. Using self-assembled monolayers is one of the most promising approaches to modify a gold surface. However, only few techniques are capable of characterizing the formation of these monolayers on porous substrates. Here, we present a method to in situ monitor the adsorption and desorption of self-assembled monolayers on nanoporous gold by resistometry, using cysteine as example. During the adsorption an overall relative change in resistance of 18% is detected, which occurs in three distinct stages. First, the cysteine molecules are adsorbed on the outer surface. In the second stage, they are adsorbed on the internal surfaces and in the last stage the reordering accompanied by additional adsorption takes place. The successful binding of cysteine on the Au surface was confirmed by cyclic voltammetry, which showed a significant decrease of the double-layer capacitance. Also, the electrochemically controlled desorption of cysteine was monitored by concomitant in situ resistometry. From the desorption peak related to the (111) surface of the structure, which is associated with a resistance change of 4.8%, an initial surface coverage of 0.48 monolayers of cysteine could be estimated.

Atrazine-based self-assembled monolayers and their interaction with anti-atrazine antibody: building of an immunosensor

As a part of our objective to build an immunosensor for the detection of the pesticide atrazine (ATZ) in environmental samples, we studied the self-assembling process of the disulfide derivative of the pesticide atrazine on a gold substrate. Atrazine-based self-assembled monolayers were characterized by ellipsometry, scanning tunneling microscopy, polarization-modulation infrared reflection-absorption spectroscopy (PM IRRAS), X-ray photoelectron spectroscopy and quartz crystal microbalance (QCM) measurements. Two different time constants for the adsorption process were observed, depending on the experimental method used. The QCM data reflect adsorption kinetics of the original disulfide compound, whereas ellipsometry and ex situ PM IRRAS refer to the formation of thiolate (ATZS) monolayers. In situ QCM data demonstrated the suitability of such monolayers for the detection of atrazine in aqueous samples. Exposure of the ATZS sensing surface to an anti-atrazine antibody (anti-ATZ IgG) resulted in complete coverage of the surface by antibody, whereas approximately half of the antibody molecules were displaced from the QCM sensor surface by further addition of atrazine into the solution.

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.

Fundamental calculations on the surface area determination of supported gold nanoparticles by alkanethiol adsorption

Surface area determination is a crucial step for the characterization of the activity of noble metal catalysts. Not only the development of useful determination methods but foremost the understanding of surface properties and their conversion into mathematical expressions are essential to obtain reliable results. A selective method to gain access to the specific surface area of gold on oxidic supports is the chemisorption of alkanethiol from suspensions. Therefore, the concentration of a 1-dodecanthiol solution before and after immersion of supported gold catalysts was determined by gas chromatography. To convert the concentration information into a specific surface area, the surface coverage, the surface atom concentration, the interatomic Au-Au distance, and the particle morphology were considered. Further calculations afforded the determination of a mean particle diameter. A good agreement was found between gold particle sizes obtained from transmission electron microscopy and thiol adsorption. The given mathematical expressions are highly valuable for a broad range of chemisorption methods and noble metal catalysts.

Reorientation of thiols during 2D self-assembly: interplay between steric and energetic factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.