Delivering octadecylphosphonic acid self-assembled monolayers on a Si wafer and other oxide surfaces

We describe a simple experimental approach for delivering self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) on many oxide surfaces using a nonpolar medium with a dielectric constant around 4 (e.g., trichloroethylene). This approach readily results in the formation of full-coverage OPA SAMs on a wide variety of oxide surfaces including cleaved mica, Si wafer, quartz, and aluminum. Especially, the availability of delivering full-coverage OPA SAM on a Si wafer is unique, as no OPA SAMs at all could be formed on a Si wafer when using a polar OPA solution. The reason a nonpolar solvent is superior lies in the very fact that the hydrophilic OPA headgroup tends to escape from the nonpolar solution and is thus enriched at the medium-air interface. It is these OPA headgroups seeking a hydrophilic surface that make possible the well-controlled OPA monolayer on an oxide surface.     

Properties of a water layer on hydrophilic and hydrophobic self-assembled monolayer surfaces: A molecular dynamics study

The microscopic behaviors of a water layer on different hydrophilic and hydrophobic surfaces of well ordered self-assembled monolayers (SAMs) are studied by molecular dynamics simulations. The SAMs consist of 18-carbon alkyl chains bound to a silicon(111) substrate, and the characteristic of its surface is tuned from hydrophobic to hydrophilic by using different terminal functional groups ( CH 3 , COOH). In the simulation, the properties of water membranes adjacent to the surfaces of SAMs were reported by comparing pure water in mobility, structure, and orientational ordering of water molecules. The results suggest that the mobility of water molecules adjacent to hydrophilic surface becomes weaker and the molecules have a better ordering. The distribution of hydrogen bonds indicates that the number of water-water hydrogen bonds per water molecule tends to be lower. However, the mobility of water molecules and distribution of hydrogen bonds of a water membrane in hydropho- bic system are nearly the same as those in pure water system. In addition, hydrogen bonds are mainly formed between the hydroxyl of the COOH group and water molecules in a hydrophilic system, which is helpful in understanding the structure of interfacial water.     

Mechanical stabilization effect of water on a membrane-like system

The penetration resistance of a prototypical model-membrane system (HS-(CH2)11-OH self-assembled monolayer (SAM) on Au(111)) to the tip of an atomic force microscope (AFM) is investigated in the presence of different solvents. The compressibility (i.e., height vs tip load) of the HS-(CH2)11-OH SAM is studied differentially, with respect to a reference structure. The reference consists of hydrophobic alkylthiol molecules (HS-(CH2)17-CH3) embedded as nanosized patches into the hydrophilic SAM by nanografting, an AFM-assisted nanolithography technique. We find that the penetration resistance of the hydrophilic SAM depends on the nature of the solvent and is much higher in the presence of water than in 2-butanol. In contrast, no solvent-dependent effect is observed in the case of hydrophobic SAMs. We argue that the mechanical resistance of the hydroxyl-terminated SAM is a consequence of the structural order of the solvent-SAM interface, as suggested by our molecular dynamics simulations. The simulations show that in the presence of 2-butanol the polar head groups of the HS-(CH2)11-OH SAM, which bind only weakly to the solvent molecules, try to bind to each other, disrupting the local order at the interface. On the contrary, in the presence of water the polar head groups bind preferentially to the solvent that, in turn, mediates the release of the surface strain, leading to a more ordered interface. We suggest that the mechanical stabilization effect induced by water may be responsible for the stability of even more complex, real membrane systems.     

Fabrication of flexible gold films with periodic sub-micrometer roughness and their wettability control by modification of SAM

Flexible honeycomb gold films supported by polymer sheets are fabricated by using polystyrene particle monolayers. The surfaces of the flexible gold films are covered with self-assembled monolayers (SAMs) of hydrophobic or hydrophilic thiol compounds, and the wettability of the modified surface is evaluated by measurements of the contact angles of water droplets. The contact angle of the film covered with hydrophobic SAM is ca. 150 degrees, which is greater than the value of 112 degrees for a flat gold surface, while the values for hydrophilic SAM are below 10 degrees.     

Understanding the nonfouling mechanism of surfaces through molecular simulations of sugar-based self-assembled monolayers

This paper presents a molecular simulation study of the interactions of a protein (lysozyme) with self-assembled monolayers (SAMs) of mannitol and sorbitol terminated alkanethiols in the presence of explicit water molecules and ions. The all-atom simulations were performed to calculate the force generated on the protein as a function of its distance above the SAM surfaces. The structural and dynamic properties of water molecules both above the SAM surfaces and around the SAM head groups were analyzed to provide a better understanding of the nonfouling behavior of the sugar-based SAM surfaces. Results from this work suggest that both mannitol and sorbitol SAMs generate a tightly bound, structured water layer around the SAM chains. This hydration layer creates a repulsive force on the protein when it approaches the surface, resulting in a nonfouling surface despite the presence of hydrogen-bond donor groups. This work demonstrates the importance of strong surface-water interactions for surface resistance to nonspecific protein adsorption.     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

Computer simulation of adhesion between hydrophilic and hydrophobic self-assembled monolayers in water

The grand canonical Monte Carlo technique and atomistic force fields are used to calculate the force-distance relations and free energies of adhesion between carboxyl and methyl terminated alkanethiolate self-assembled monolayers (SAMs) in water. Both symmetric and asymmetric confinements are considered, as formed by like and unlike SAMs, respectively. As the confinement is increased, water confined by the hydrophobic methyl terminated SAMs experiences capillary evaporation. As a consequence, the adhesion energy is determined by the direct interaction between bare SAMs. In the asymmetric system, an incomplete capillary evaporation is observed, with the number of water molecules dropped by more than an order of magnitude. The remaining water molecules are all adsorbed on the hydrophilic SAM, while the hydrophobic SAM is separated from the rest of the system by a thin vapor layer. The calculated free energies of adhesion are in acceptable agreement with experiment.     

Influence of nanobubbles on the adsorption of nanoparticles

A quartz crystal microbalance was used to study the influence of nanobubbles on the adsorption of polystyrene nanoparticles onto surfaces coated with gold, or coated with dodecanethiol or mercaptoundecanoic acid self-assembled monolayers (SAMs). Adsorption of the nanoparticles onto the surface causes the resonant frequency of the quartz crystal to decrease. We found that particles were adsorbed onto the gold-coated quartz crystal in air-rich water, but not in degassed water. This finding supports the long-standing hypothesis that nanobubbles play a key role in the long-range attractive force between hydrophobic surfaces in aqueous solutions. When the experiments were conducted using quartz crystals coated with a hydrophobic dodecanethiol SAM, the nanoparticles were adsorbed onto the surface even in degassed water due to the short-range hydrophobic interactions between the nanoparticles and the dodecanethiol molecules. In contrast, the nanoparticles were adsorbed to a lesser degree onto the hydrophilic mercaptoundecanoic acid-coated crystals due to electrostatic repulsive forces.     

Water hydrogen bond analysis on hydrophilic and hydrophobic biomolecule sites

Elastic and quasielastic neutron scattering experiments have been used to investigate the hydrogen bonding network dynamics of hydration water on hydrophilic and hydrophobic sites. To this end the evolution of hydration water dynamics of a prototypical hydrophobic amino acid with polar backbone, N-acetyl-leucine-methylamide (NALMA), and hydrophilic amino acid, N-acetyl-glycine-methylamide (NAGMA), has been investigated as a function of the molecular ratio water : peptide. The results suggest that the dynamical contribution of the intrinsic and low hydration molecules of water is characteristic of pure librational/rotational movement. The water molecule remains attached to the hydrophilic site with only the possibility of hindered rotations that eventually break the bond with the peptide and reform it immediately after. A gradual evolution from librational motions to hindered rotations is observed as a function of temperature. When the hydration increases, we observe (together with the hindered rotations of hydrogen bonds) a slow diffusion of water molecules on the surface of the peptides.     

A tetrafluorophenyl activated ester self-assembled monolayer for the immobilization of amine-modified oligonucleotides

A tetrafluorophenyl (TFP) ester-terminated self-assembled monolayer (SAM) for the fabrication of DNA arrays on gold surfaces is described. Activated ester SAMs are desirable for biomolecule array fabrication because they readily react with amine-containing molecules to form a stable amide linkage. N-Hydroxysuccinimide (NHS) ester SAMs are commonly used for this purpose but are subject to a competing hydrolysis side reaction, limiting their effectiveness under basic conditions. TFP was evaluated here as an alternative activated ester leaving group with a potentially greater stability under basic conditions. It is shown that TFP SAMs are much more stable to basic pH than their NHS analogs and are also more hydrophobic, which is an advantage in the fabrication of high-density spotted arrays. DNA arrays prepared on TFP SAMs at pH 10 have a 5-fold greater surface density of DNA molecules, reduced fluorescence background, and smaller spot radii than those prepared on NHS SAM analogs.     

Odd-even variations in the wettability of n-alkanethiolate monolayers on gold by water and hexadecane: a molecular dynamics simulation study

Molecular dynamics (MD) simulations were performed to investigate odd-even chain length dependencies in the wetting properties of self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)n-1SH] on gold by water and hexadecane. Experimentally, the contact angle of hexadecane on the SAMs depends on whether n is odd or even, while contact angles for water show no odd-even dependence. Our MD simulations of this system included a microscopic droplet of either 256 water molecules or 60 hexadecane molecules localized on an n-alkanethiolate SAM on gold with either an even or odd chain length. Contact angles calculated for these nanoscopic droplets were consistent with experimentally observed macroscopic trends in wettability, namely, that hexadecane is sensitive to structural differences between odd- and even-chained SAMs while water is not. Structural properties for the SAMs (including features such as chain tilt, chain twist, and terminal methyl group tilt) were calculated during the MD simulations and used to generate IR spectra of these films that compared favorably with experimental spectra. MD simulations of SAMs in contact with slabs of water and hexadecane revealed that the effects of these solvents on the structure of the SAM was restricted to the chain terminus and had no effect on the inner structure of the SAM. The density profiles for water and hexadecane on the SAMs were different in that water displayed a significant depletion in its density at the liquid/SAM interface from its bulk value, while no such depletion occurred for hexadecane. This difference in contact may explain the lack of an odd-even variation in the wetting characteristics of water on these surfaces, because the water molecules are positioned further away from the surface and, therefore, are not sensitive to the structural differences in the average orientations for the terminal methyl groups in odd- and even-chained SAMs. In contrast, the differences in the wetting properties of hexadecane on the odd- and even-chained SAMs may reflect the closer proximity of these molecules to the SAM surface and a resulting greater sensitivity to the differences in the terminal methyl group orientations in the SAMs. SAM-solvent interaction energies were calculated during the MD simulations, yielding interaction energies that differed on the even- and odd-chained surfaces by approximately 10% for hexadecane and negligibly for water, in accord with estimates using experimental wetting results.     

Structure and dynamics of hydrogen bonds in the interface of a C12E6 spherical micelle in water solution: a MD study at various temperatures

The temperature dehydration of a C(12)E(6) spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C(12)E(6) surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol(-1). On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, tau approximately 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, tau approximately 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm(2), close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.     

Role of surface heterogeneity and molecular interactions in the charge-transfer process through self-assembled thiolate monolayers on Au(111)

A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n = 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negative-charged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate- terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.     

Contact angles of surface nanobubbles on mixed self-assembled monolayers with systematically varied macroscopic wettability by atomic force microscopy

The dependence of the properties of so-called "surface nanobubbles" at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (θ(macro)) of the SAMs spanned the range between 107° ± 1° and 15° ± 3°. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles θ(nano) < 86° were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of θ(nano) calculated from these data were found to change from 167° ± 3° to 33° ± 58°, when θ(macro) decreased from 107° ± 1° to 37° ± 3°. While the values for θ(nano) significantly exceeded those of θ(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.     

SURFACE ENTHALPY AND ENTROPY AND THE PHYSICO-CHEMICAL NATURE OF HYDROPHOBIC AND HYDROPHILIC INTERACTIONS

The attractive Interactions between typically hydrophobic molecules such as hexane or CCl₄, and the repulsive Interactions between extremely hydrophilic molecules such as poly(ethylene oxide) (PEO), when immersed in water, as well as the interactions between these molecules and water, have been examined from a surface thermodynamic viewpoint, taking the changes in surface free energy into account, as a function of temperature. It was found that attractive hydrophobic Interactions are not, as vas generally believed up to now, invariably entropic. Hydrophobic Interactions can be mainly enthalpic or mainly entropic, or more or less equal mixtures of both, depending on each individual case; however, all hydrophobic interactions are polar (in the sense of Lewis acid-base) in nature. Repulsive hydrophilic interactions are enthalpic, and also polar in nature. The interaction between hydrophobic solutes and water is mainly enthalpic, and is apolar in nature.     

Role of hydration force in the self-assembly of collagens and amyloid steric zipper filaments

In protein self-assembly, types of surfaces determine the force between them. Yet the extent to which the surrounding water contributes to this force remains as a fundamental question. Here we study three self-assembling filament systems that respectively have hydrated (collagen), dry nonpolar, and dry polar (amyloid) interfaces. Using molecular dynamics simulations, we calculate and compare local hydration maps and hydration forces. We find that the primary hydration shells are formed all over the surface, regardless of the types of the underlying amino acids. The weakly oscillating hydration force arises from coalescence and depletion of hydration shells as two filaments approach, whereas local water diffusion, orientation, or hydrogen-bonding events have no direct effect. Hydration forces between hydrated, polar, and nonpolar interfaces differ in the amplitude and phase of the oscillation relative to the equilibrium surface separation. Therefore, water-mediated interactions between these protein surfaces, ranging in character from "hydrophobic" to "hydrophilic", have a common molecular origin based on the robustly formed hydration shells, which is likely applicable to a broad range of biomolecular assemblies whose interfacial geometry is similar in length scale to those of the present study.     

THE ROLE OF INTERFACIAL INTERACTIONS IN THE PARTICLE/BUBBLE ATTACHMENT

The Van Oss surface thermodynamic theory of polar and apolar interfacial interactions was extended to the interaction between mineral surfaces and bubbles across liquid media. The acid base (polar) interfacial interactions are supposed to be responsible for the hydration repulsion between a hydrophilic mineral and a bubble as well as for the hydrophobic attraction between a hydrophobic mineral and the bubble.     

Investigation into pH-responsive self-assembled monolayers of acylated anthranilate-terminated alkanethiol on a gold surface

This article describes the preparation of pH-responsive self-assembled monolayers (SAMs) of acylated anthranilate-terminated alkanethiol. These monolayers are formed by chemisorption of the alkanethiol molecules onto a gold surface, resulting in different wetting properties of the surfaces depending upon the pH. By using various characterization techniques (e.g., infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis), we have found that the changes in the wetting properties originate from the different surface structures of the monolayers in different pH environments. From surface energy analysis, we found that the disperse components of the surface energy on such SAMs predominate after treatment with pH 1 water, whereas the polar components of the surface energy on such SAMs predominate after treatment with pH 13 water. It is greatly anticipated that this line of research will provide new insight into the mechanism behind pH-responsive properties, facilitating the design and synthesis of new surface-active molecules for the fabrication of pH-responsive functional surfaces.     

State and structure of water in vicinity of hydrophobic surfaces

Criterial values of the specific heat of water wetting, surface pressure, and contact angle classifying surfaces into hydrophilic and hydrophobic are proposed based on the analysis of own and published data. The most characteristic properties of hydrophobic surfaces, i.e., large surface area per water molecule in the conventional adsorption monolayer and the absence of continuous two-layer water film on the adsorbent surface at vapor pressure close to saturation, are discussed using nonporous carbon-based materials as example. The presence of residual hydrophilic groups that act as sites of the clusterization of polar molecules on the surface of graphitized carbon black is confirmed by gas chromatography and the concentration of these sites is calculated. The amount of water molecules in the surface cluster is determined at different stages of adsorption. Procedures for preparing organically modified layered silicates and silica as basic objects of the study of the interaction between water molecules and hydrophobic surfaces are considered. It is proven that the boundary water layer in the vicinity of hydrophobic surface consists of a thin (∼0.5 nm) depletion layer with a density of 0.4 g/cm³ and a considerable amount (25–30%) of water molecules with free OH groups and thicker (∼35 nm) layer, which is characterized by a more ordered network of hydrogen bonds compared to liquid water. Data obtained by X-ray scattering and neutron and reflection methods, and sum-frequency vibrational spectroscopy are compared with the results of calorimetric study of the interaction between water and hydrophobic surface, as well as with the data of molecular-statistical calculations of the state of water molecules in the surface layer.     

Effect of surface wettability on the adhesion of proteins

Besides significantly broadening the scope of available data on adhesion of proteins on solid substrates, we demonstrate for the first time that all seven proteins (tested here) behave similarly with respect to adhesion exhibiting a step increase in adhesion as wettability of the solid substrate decreases. Also, quantitative measures of like-protein-protein and like-self-assembled-monolayer (SAM)-SAM adhesive energies are provided. New correlations, not previously reported, suggest that the helix and random content (as measures of secondary structure) normalized by the molecular weight of a protein are significant for predicting protein adhesion and are likely related to protein stability at interfaces. Atomic force microscopy (AFM) was used to directly measure the normalized adhesion or pull-off forces between a set of seven globular proteins and a series of eight well-defined model surfaces (SAMs), between like-SAM-immobilized surfaces and between like-protein-immobilized surfaces in phosphate buffer solution (pH 7.4). Normalized force-distance curves between SAMs (alkanethiolates deposited on gold terminated with functional uncharged groups -CH3, -OPh, -CF3, -CN, -OCH3, -OH, -CONH2, and -EG3OH) covalently attached to an AFM cantilever tip modified with a sphere and covalently immobilized proteins (ribonuclease A, lysozyme, bovine serum albumin, immunoglobulin, gamma-globulins, pyruvate kinase, and fibrinogen) clearly illustrate the differences in adhesion between these surfaces and proteins. The adhesion of proteins with uncharged SAMs showed a general "step" dependence on the wettability of the surface as determined by the water contact angle under cyclooctane (thetaco). Thus, for SAMs with thetaco < approximately 66 degrees, (-OH, -CONH2, and -EG3OH), weak adhesion was observed (>-4 +/- 1 mN/m), while for approximately 66 < thetaco < approximately 104 degrees, (-CH3, -OPh, -CF3, -CN, -OCH3), strong adhesion was observed (< or =8 +/- 3 mN/m) that increases (more negative) with the molecular weight of the protein. Large proteins (170-340 kDa), in contrast to small proteins (14 kDa), exhibit characteristic stepwise decompression curves extending to large separation distances (hundreds of nanometers). With respect to like-SAM surfaces, there exists a very strong adhesive (attractive) interaction between the apolar SAM surfaces and weak interactive energy between the polar SAM surfaces. Because the polar surfaces can form hydrogen bonds with water molecules and the apolar surfaces cannot, these measurements provide a quantitative measure of the so-called mean hydrophobic interaction (approximately -206 +/- 8 mN/m) in phosphate-buffered saline at 296 +/- 1 K. Regarding protein-protein interactions, small globular proteins (lysozyme and ribonuclease A) have the least self-adhesion force, indicating robust conformation of the proteins on the surface. Intermediate to large proteins (BSA and pyruvate kinase-tetramer) show measurable adhesion and suggest unfolding (mechanical denaturation) during retraction of the protein-covered substrate from the protein-covered AFM tip. Fibrinogen shows the greatest adhesion of 20.4 +/- 2 mN/m. Unexpectedly, immunoglobulin G (IgG) and gamma-globulins exhibited very little adhesion for intermediate size proteins. However, using a new composite index, n (the product of the percent helix plus random content times relative molecular weight as a fraction of the largest protein in the set, Fib), to correlate the normalized adhesion force, IgG and gamma-globulins do not behave abnormally as a result of their relatively low helix and random (or high sheet) content.