Carboxylic Acid-terminated Self-assembled Monolayers Research Articles

Growth Processes and Control of Two-Dimensional Structure of Carboxylic Acid-Terminated Self-Assembled Monolayers on Au(111)

The growth processes and solvent effects on the two-dimensional structure of self-assembled monolayers (SAMs) formed by 6-mercaptohexanoic acid (MHA) on Au(111) were examined by scanning tunneling microscopy (STM) and contact angle (CA) measurements. The STM study revealed for the first time that the striped phases of 6-mercaptohexanoic acid (MHA) on Au(111) are preferentially formed near the step edges of gold terraces in the initial stage of SAM growth. In addition, it was found that MHA SAMs prepared from a polar aprotic solvent form a more well-ordered structure and a better hydrophilic surface than those prepared from a polar protic solvent. From this study, we clearly demonstrated that the properties of solvents are very important factors in controlling the two-dimensional structure of carboxylic acid-terminated SAMs.

Cu adsorption on carboxylic acid-terminated self-assembled monolayers: a high-resolution X-ray photoelectron spectroscopy study

The interaction between vacuum-deposited Cu and a carboxylic acid-terminated self-assembled monolayer (SAM) on Au(111) has been investigated using high-resolution X-ray photoelectron spectroscopy (XPS). Upon adsorption of 3-mercaptopropionic acid from the gas phase on Au(111), C 1s, O 1s, and S 2p line shapes and binding energies are consistent with the formation of a thiolate surface intermediate with an intact carboxylic acid function. Deposition of increasing amounts of Cu results in preferential modification of the hydroxyl group O 1s peak indicating unidentate complexation. Consistent with selective bonding, Cu induces changes in the carboxylic acid C 1s region with no evidence of reaction with the methylene or thiolate units. The Cu 2p core level is comparable with bulk, suggesting cluster growth and weak Cu-SAM interactions. The reactivity is nevertheless sufficient to prevent metal penetration to the Au substrate surface.

Grafting Nitrilotriacetic Groups onto Carboxylic Acid-Terminated Self-Assembled Monolayers on Gold Surfaces for Immobilization of Histidine-Tagged Proteins

In this paper, we report a common intermediate method to present nitrilotriacetic acid (NTA) groups on gold surfaces for immobilizing His-tagged proteins onto the surfaces, and a full characterization of self-assembled monolayers (SAMs) terminating in carboxylic acids [HS(CH2)15COOH (C15-COOH), HS(CH2)11(OCH2CH2)3OCH2COOH (EG3-COOH), and HS(CH2)11(OCH2CH2)5OCH2COOH (EG5-COOH)] and coupling reactions of an NTA-containing primary amine [(1S)-N-(5-amino-1-carboxypentyl)iminodiacetic acid; NTA−NH2] with the carboxylic acid on surfaces. The lateral packing densities of the COOH-terminated SAMs were calculated to be 4.32 (for C15-COOH), 3.49 (for EG3-COOH), and 2.65 (for EG5-COOH) molecules/nm2. The packing densities were decreased by incorporating a relatively flexible ethylene glycol (EG) group into the backbone of alkanethiols and increasing the number of the EG groups in the backbone of alkanethiols. The NTA group was then attached by coupling NTA−NH2 with the COOH group on the surfaces, followed by a Ni(II...

Enzyme-amplified electrochemical detection of DNA using electrocatalysis of ferrocenyl-tethered dendrimer.

We have developed a sandwich-type enzyme-linked DNA sensor as a new electrochemical method to detect DNA hybridization. A partially ferrocenyl-tethered poly(amidoamine) dendrimer (Fc-D) was used as an electrocatalyst to enhance the electronic signals of DNA detection as well as a building block to immobilize capture probes. Fc-D was immobilized on a carboxylic acid-terminated self-assembled monolayer (SAM) by covalent coupling of unreacted amine in Fc-D to the acid. Thiolated capture probe was attached to the remaining amine groups of Fc-D on the SAM via a bifunctional linker. The target DNA was hybridized with the capture probe, and an extension in the DNA of the target was then hybridized with a biotinylated detection probe. Avidin-conjugated alkaline phosphatase was bound to the detection probe and allowed to generate the electroactive label, p-aminophenol, from p-aminophenyl phosphate enzymatically. p-Aminophenol diffuses into the Fc-D layer and is then electrocatalytically oxidized by the electronic mediation of the immobilized Fc-D, which leads to a great enhancement in signal. Consequently, the amount of hybridized target can be estimated using the intensity of electrocatalytic current. This DNA sensor exhibits a detection limit of 20 fmol. Our method was also successfully applied to the sequence-selective discrimination between perfectly matched and single-base mismatched target oligonucleotides.

Use of Atomic Force Microscopy for the Study of Surface Acid−Base Properties of Carboxylic Acid-Terminated Self-Assembled Monolayers

Atomic force microscopy was used to measure the forces between a silica probe and a carboxylic acid-terminated self-assembled monolayer (SAM) on a gold substrate in the presence of KCl electrolyte solutions of different pH. Silica−silica interaction force measurements were conducted at different pH values to determine the silica probe surface electrostatic potentials under these conditions. The interaction between two silica surfaces is repulsive and can be accurately predicted (except at short distances) by the Derjaguin−Landau−Verwey−Overbeek theory. The interaction between silica and clean gold surfaces exhibits an attractive interaction at neutral pH. The interaction between a silica probe and carboxylic acid-terminated SAM-covered gold substrate was a strong function of the pH value of the electrolyte. The surface electrostatic potentials of the surface-confined monolayers of carboxylic acid were obtained by theoretical fits of the force data to solutions of the complete nonlinear Poisson−Boltzmann e...