Computer Simulation of Water-Mediated Adhesion Between Phospholipid Bilayer and Solid Support Functionalized with Self-Assembled Monolayers

We report the deposition of DNA nanostructures on self-assembled monolayers (SAMs), focusing on the stability of DNA nanostructures on both hydrophilic and hydrophobic SAMs. Our study reveals distinct outcomes based on the nature of the SAMs. DNA nanostructures maintain structural integrity on hydrophilic SAMs, whereas they experience deformation on the most hydrophobic SAMs. Interestingly, the stability of DNA nanostructures is also sensitive to postdeposition washing procedures. The observations shed light on the intricate interplay between the wettability of SAMs and the structural stability of the DNA nanostructures. An empirical trend emerged where increased hydrophobicity is associated with a more severe deformation of DNA nanostructures. This deformation is hypothesized to arise from disrupted hydrogen bonding within DNA nanostructures and is exacerbated by interfacial tension during the drying process. Our study also highlights the potential role of π-π stacking interactions between the DNA bases and the SAMs in stabilizing the DNA nanostructures. Our work expands the type of substrates that can be used for applications of DNA nanotechnology and highlights the need for a comprehensive understanding of the interactions between DNA nanostructures with different surfaces.

A method for the spectroscopic characterization of interfacial fluid molecular structure near solid substrates is reported. The thickness and interfacial molecular structure of residual ultrathin D20 films remaining after forced dewetting on alkanethiolate self-assembled monolayers (SAMs) of 11 1-mercaptoundecanoic acid (11-MUA), 11-mercaptoundecanol (11-MUD), and undecanethiol (UDT) on Ag are investigated using ellipsometry and surface Raman spectroscopy. The residual film thickness left after withdrawal is greater on hydrophilic SAMs than on hydrophobic SAMs. This behavior is rationalized on the basis of differing degrees of fluid slip within the interfacial region due to different interfacial molecular structure. The v(O-D) regions of surface Raman spectra clearly indicate unique interfacial molecular properties within these films that differ from bulk D20. Although the residual films are created by shear forces and Marangoni flow at the three-phase line during the forced dewetting process, the nature of the films sampled optically must also be considered from the standpoint of thin film stability after dewetting. Thus, the resulting D20 films exist in vastly different morphologies depending on the nature of the water-SAM interactions. Residual D20 is proposed to exist as small nanodroplets on UDT surfaces due tospontaneous rupture of the film after dewetting. In contrast, on 11-MUD and 11-MUA surfaces, these films exist in a metastable state that retains their conformal nature on the underlying modified surface. Analysis of the peak intensity ratios of the so-called "ice-like" to "liquid-like" v(O-D) modes suggests more ice-like D20 character near 11-MUD surfaces, but more liquid-like character near 11-MUA and UDT surfaces. The creation of residual ultrathin films by forced dewetting is thus demonstrated to be a powerful method for characterizing interfacial molecular structure of fluids near a solid substrate under ambient conditions of temperature and pressure.

The hydration forces operating between two parallel methoxy tri(ethylene glycol)-terminated alkanethiol self-assembled monolayers (SAMs) immersed in water are simulated using the grand canonical Monte Carlo technique, the TIP4P model for water, and a quantum chemistry-based force field for the SAM−water interactions. Two structural modifications of the SAMs are studied, one assembled on the Ag(111) and the other on the Au(111) substrate. Both the Ag- and Au-supported SAMs show typical hydrophobic behavior: the water-mediated interaction between the SAMs is attractive, and the water density level between the SAMs is noticeably reduced. In addition, at small separations, the SAMs induce capillary evaporation of the confined water. The Ag- and Au-supported SAMs show substantial differences in their interfacial behavior, which can be attributed to the difference in the areal density between the SAMs. The simulation results are discussed in the context of the experimentally observed differences in protein ads...

Method of oriented and activity-preserved immobilization of biologically active proteins based on concepts of active-site masking and kinetic control was previously reported. We extended our study and found that the surface hydrophobicity had a noticeable effect on the activity of active-site protected immobilized (PIM) Taq DNA polymerase on mixed self-assembled monolayer (SAM) manufactured on Au surface. Hydrophobic SAM was created by using 12-mercaptododecanoic acid and 1-heptanethiol. The resulting Taq DNA polymerase activity was measured by performing PCR amplification and compared with previously reported values acquired with hydrophilic SAM. The maximum activity of immobilized Taq DNA polymerase was achieved at 17.5% of 12-mercaptododecanoic acid and within 90 min of reaction time which are higher than those acquired with hydrophilic SAM; maximum activity at 5% of 12-mercaptododecanoic acid and at 10 min. In order to be applicable to commercial level, immobilized enzyme need to be stable, reusable and storable. PIM Taq DNA polymerase was stably attached to the surface and could be used for as many as 10 PCR runs which were comparable to solution-phase enzyme. Even after 56 days of storage at 4 °C, immobilized enzyme pertained 70% of the initial PIM enzyme activity. These data suggest that PIM Taq DNA polymerase can be used for various commercial applications.

A rapid surface modification technique for the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films using microwave heating in <10 min is reported. In this regard, SAMs of two model alkanethiols, 11-mercaptoundecanoic acid (11-MUDA, to generate a hydrophilic surface) and undecanethiol (UDET, a hydrophobic surface), were successfully formed on gold thin films using selective microwave heating in (1) a semicontinuous fashion and (2) a continuous fashion at room temperature (24 h, control experiment, no microwave heating). The formation of SAMs of 11-MUDA and UDET was confirmed by contact angle measurements, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The contact angles for water on SAMs formed by the selective microwave heating and conventional room temperature incubation technique (24 h) were measured to be similar for 11-MUDA and UDET. FT-IR spectroscopy results confirmed that the internal structures of SAMs prepared using both microwave heating and room temperature were similar. XPS results revealed that the organic and sulfate contaminants found on bare gold thin films were replaced by SAMs after the surface modification process had been conducted using both microwave heating and room temperature.

Department of Chemistry, Chungbuk National University, Cheongju 361-763, KoreaReceived September 21, 2013, Accepted September 27, 2013Molecular dynamics simulations were performed to study the structure and stability of a nanoscale self-assembled monolayer (SAM) of alkanethiol on a gold (111) surface. The tilt angle and orientational order ofthe alkyl chains in the SAM island were examined by systematically varying the size of the island. The chainlength dependence of the SAM island was examined by considering alkanethiols containing 12, 16, 20, and 24carbon atoms. The minimum diameter of SAM islands made from 1-tetracosanethiol, 1-ecosanethiol, 1-hexadecanethiol and 1-dodecanethiol were 2.29, 1.9, 4.7 and 4.76 nm, respectively. These set the ultimateresolution that can be patterned by soft nanolithography. As the length of alkanethiol increases, the SAMislands became more ordered in both orientation and conformation of the alkyl chains. Key Words : Self-assembled monolayer, Alkanethiol, Chain length, NanolithographyIntroductionThe self-assembled monolayers (SAMs) of alkanethiolhave numerous applications as coating materials.

Silver nanocubes are self-assembled into unique structures via hydrophobic interactions. The structures could be predesigned by selectively functionalizing the faces of each nanocube with hydrophobic and hydrophilic self-assembled monolayers (SAMs). The cubic lattice shown in this SEM image was formed at an air-water interface from Ag nanocubes whose entire surface had been derivatized with a hydrophobic SAM.

To understand protein adsorption on a surface is very important in bio-related domains of technology and application such as biomaterials, implant biocompatibility, and biosensor technology. (Gray 2004) Self-assembled monolayers (SAMs) are excellent model surfaces for biology and biochemistry because they are stable, highly ordered, easy to prepare, and provide a wide range of organic functionality. (Love et al. 2005) Previous experimental studies (Ostuni et al. 2003; Prime & G.M. Whitesides 1991) have indicated that hydrophobic interactions are the major mechanism for protein adsorption on surfaces, and the dehydration of both the protein and hydrophobic SAMs provides an entropic driving force for protein adsorption. (Ostuni et al. 2003) Nonetheless, there are still many questions needed to be clarified and difficult to be investigated in experiments. Molecular dynamics (MD) simulation is a powerful tool that is able to investigate the atomic details of a molecular system. This paper summarizes the investigations of proteins adsorption on alkanethiol SAMs by means of MD simulations in order to expand the limited information that the experiments can provide. (Hung et al.2006; Hung et al.2010; Hung et al. 2011)

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.

Barrier height of self-assembled monolayers (SAMs) on Au(111) was measured using scanning tunneling microscopy (STM) with a vertical modulation. We obtained nanometer scale STM images of alkanethiol SAMs, and binary component SAM films fabricated by an implantation technique. Local barrier height on the alkanethiol SAM at the depressions and the domain boundaries is larger than that on the domains where the molecular lattice is visible, probably due to the molecular density or orientation. A molecular resolution contrast was obtained at a conjugated molecule embedded in the alkanethiol SAM, reflecting the difference in the molecular species. We found that the tip-sample interaction force affects the barrier height measurements, especially when the tunneling gap is small, e.g., on nonanethiol SAM at the tunneling resistance smaller than 50 GΩ.

Self-assembled monolayers of alkanethiols on gold modulate electrophysiological parameters and cellular morphology of cultured neurons

Alkanethiol Self-Assembled Monolayers (SAMs), typically deposited on Au substrates, have been commonly utilized to immobilize receptor biomolecules in the construction of a variety of biosensors, such as the detection of glucose oxidase by the immobilization of calixarenes thiols 1. The appeal of using SAMs derives from their ease of fabrication and component flexibility, allowing for more than one alkanethiol to be deposited in a single monolayer. This results in the formation of SAMs in which each thiol component adds a different property or role, thus introducing versatility to the construction and operation of resulting sensors 2.In our recent work, alkanethiol SAMs, composed of three different thiol molecules, were constructed for use as an electrochemical Toll-Like Receptor 4 (TLR4)-based sensor that can detect Gram-negative bacteria. TLRs are a family of immunoproteins in the innate immune system, which are able to detect a broad category of pathogens, rather than specific targets, as is the case with the adaptive system (e.g., antibodies). In our published work 3, SAMs consisted of a carboxylic acid terminated thiol that allowed for the conversion to a nitrilotriacetic (NTA) group. This moiety could then chelate a Ni2+ cation, followed by attachment of the TLR4 immunoprotein via its His-tag. The second thiol in the SAM contained a terminal ferrocene (Fc) group, where the Fc group was bound to the C11 chain via an amide group. Here, the Fc group serves as a mediator for charge transfer between ferrocyanide in solution and the Au substrate, resulting in relatively high current signals, thus making the sensor potentially field deployable. The third thiol in these SAMs was a hydroxy-terminated thiol, added as a diluent between the immobilized TLR4 receptors. SAMs were prepared by co-deposition on sputter Au on glass.Electrochemical testing involved cyclic voltammetry (CV) studies in pH 7 buffer solution in order to determine the amount of Fc-thiol present, as well as CV and electrochemical impedance spectroscopy (EIS) studies in ferrocyanide solutions to examine its redox-mediation response and to obtain a measure of the charge transfer rate. In the presence of lysed bacteria, the immobilized TLR4 dimerizes, which blocks electrochemical mediation between ferrocyanide and the surface bound Fc. This results in an increase in RCT which is our biosensor signal. However, a key problem with these sensors is that their RCT increased with electrochemical testing in the absence of lysed bacteria, producing signal drift. Thus, in the present work, the reasons for sensor signal drift and methods to overcome it were explored.It was found in recent work that the drift of the full TLR4 sensor was correlated with the loss in the Fc redox signal, although it was not clear if underlying SAM was at the root of the problem. If so, then it was necessary to determine if the loss in the Fc redox activity was caused by the physical loss of the Fc thiol from the Au surface or deactivation of the Fc surface complex and/or its electrochemistry. To determine which of these options was responsible for signal drift, the relative % v/v fraction of the three thiols was varied, and in later work, based on chemical considerations, the Fc molecule containing an amide group was replaced with a C11 chain bonded directly to the Fc. It was found that this latter strategy led to a very stable SAM, showing negligible drift in the Fc CV charges and in the RCT values in ferrocyanide solutions, with or without the TLR4 receptor attached on the outer surface of the SAM.This presentation will focus on presenting the electrochemistry of the various binary and ternary thiol SAMs investigated in this work and examining the Fc CV peak charges, potential, and kinetics, as well as the double layer capacitance, to better understand how and why the Fc redox response was stabilized. This is important because this correlates directly with the stability of the TLR4/Gram-negative bacteria sensor, as will be demonstrated for a TLR2 biosensor tested against Gram-positive bacteria cell wall ligands.

One key route for controlling reaction selectivity in heterogeneous catalysis is to prepare catalysts that exhibit only specific types of sites required for desired product formation. Here we show that alkanethiolate self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation and hydrodeoxygenation products during the reaction of furfural on supported palladium catalysts. Vibrational spectroscopic studies demonstrate that the selectivity improvement is achieved by controlling the availability of specific sites for the hydrogenation of furfural on supported palladium catalysts through the selection of an appropriate alkanethiolate. Increasing self-assembled monolayer density by controlling the steric bulk of the organic tail ligand restricts adsorption on terrace sites and dramatically increases selectivity to desired products furfuryl alcohol and methylfuran. This technique of active-site selection simultaneously serves both to enhance selectivity and provide insight into the reaction mechanism.

The effect of X-ray and low-energy (50 eV) electron irradiation on short-chain alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) was studied by synchrotron-based high resolution X-ray...

We present an improved method to prepare carboxylic acid (COOH) and amine (NH2) terminated self-assembled monolayers (SAMs) of alkanethiolates. In this method, a small amount of CF3COOH (for COOH-SAM) or N(CH2CH3)3 (for NH2-SAM) is added into the ethanolic solution of alkanethiols during SAM formation. The freshly formed COOH- and NH2-SAMs are then rinsed with an ethanolic solution of NH4OH or CH3COOH, respectively. Both SAMs prepared with the improved method show better quality in terms of surface chemical composition, roughness, and wettability as measured by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle, respectively. The formation of better SAMs can be attributed to the disruption of interplane hydrogen bonds.