Platelet compatibility improvement by proper choice of acidic terminal functionality for mixed-charge self-assembled monolayers

In this study two different series of mixed-charge self-assembled monolayers (SAMs) prepared with -N(+)(CH(3))(3)-terminated alkanethiol and strong dissociated monovalent -SO(3)H acid-terminated or weaker dissociated divalent -PO(3)H(2) acid-terminated alkanethiol in pure ethanol were characterized. The influence of the acidity of the anionic functionality in the mixed-charge SAMs on the surface characteristics and platelet compatibility was investigated. X-ray photoelectron spectroscopy indicated that a nearly equivalent amount of countercharged terminal groups was noted on the surface of -SO(3)H/-N(+)(CH(3))(3) mixed SAMs, while "-N(+)(CH(3))(3) thiol poor" phenomena were found on -PO(3)H(2)/-N(+)(CH(3))(3) mixed SAMs instead. This was caused by the distinct differences in solvation capability between the acidic anionic functional groups and solvent molecules and/or the interactions among the terminal ends of the thiols. This acidity difference also affected other interfacial properties and the platelet compatibility. The mixed SAMs formed from the mixture of -SO(3)H- and -N(+)(CH(3))(3)-terminated thiols showed higher surface hydrophilicity and exhibited the least amount of platelets adhered, but these two mixed SAMs were all fairly negatively surface charged. The structure of the hydration layer near the surfaces was likely affected by the acidity of the anionic functionality, and this would cause such a distinct behavior in platelet compatibility. It was concluded that the hydrophilic surfaces with nearly equal amounts of surface positively and negatively charged components could exhibit better platelet compatibility. This work demonstrated that the nature of the acidic terminal ends of alkanethiol is also a key factor for preparing mixed-charge SAMs with good platelet compatibility.     

Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(1 1 1)

Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.     

Orientation modulation of a synthetic polypeptide in self-assembled monolayers: a TOF-SIMS study

Structure and orientation of molecules are key properties of functionalized surfaces. Using time-of-flight secondary ion mass spectrometry (TOF-SIMS), here we investigate how to modulate these parameters upon the immobilization process varying the conditions of self-assembly. The molecule of interest, a template-assembled synthetic protein (TASP), consists of a central peptide ring with orthogonally arranged residues. Thioalkane chains allow the directed self-assembly of the molecule on a gold surface; four serine residues on the opposite side of the ring can be used as anchoring sites for various functional sensing molecules. The TASP conformation and its orientation in self-assembled monolayers (SAMs) play a central role for the accessibility of these serine residues. To study the influence of the self-assembly conditions, two series of samples were prepared. Pure TASP monolayers of different surface densities are compared to mixed TASP/alkanethiol monolayers prepared by sequential adsorption varying sequence and particular incubation times as well as by coadsorption modifying incubation times and TASP/alkanethiol mass ratios. Switching the TASP orientation from a state where the molecules are lying flat on the surface to an upright orientation turned out to be possible by inserting the TASP into a preformed alkanethiol monolayer of an appropriate surface density. This study demonstrates that TOF-SIMS is an excellent tool not only to investigate the surface composition, but also the molecular structure of functionalized surfaces.     

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 of thiol self-assembled monolayers commensurate with the GaAs (001) surface

Observed properties of thiol self-assembled monolayers (SAMs) on GaAs (001) surfaces can be explained by the presence of surface reconstructions, but their exact form is generally unknown. We propose a new approach to modeling the SAM-surface interface based on using alkanethiol dense packing structures as a starting point and adjusting the surface reconstruction to accommodate them. Obtained in such a way, model SAMs adsorb along the trenches in the [110] direction and exhibit a 19 degrees tilt and +/- 45 degrees twist angles, in agreement with available experimental data. The molecules of the SAM bind to both Ga and As, and cover only 50% of the available surface sites. The requirements for the SAM formation process to achieve the proposed structures are discussed.     

分子自组装单层膜的表面浸润性研究现状和展望

立足于分子自组装单层膜的制备及结构, 讨论了分子自组装单层膜的头基基团与基底的作用机理、 主链与环境的温度依赖关系, 特别是其端基基团的化学性质及构象对表面浸润行为的影响. 重点讨论了分子自组装单层膜的端甲基基团对表面能的贡献、 极性端基基团与水分子之间的相互作用以及自组装单层膜表面的分子尺寸粗糙度对表面浸润的影响. 最后, 基于理论和实验基础对以上问题提出新的认知与看法, 并对未来该领域发展的机遇与挑战进行了展望.     

Thermo-kinetics study of laser-induced desorption of self-assembled monolayers from gold: case of laser micropatterning

Laser-induced desorption of self-assembled monolayers (SAMs) from gold surfaces within context of the direct laser patterning methodology was investigated through combining results of a heat diffusion thermal model with desorption kinetics of alkanethiol SAMs. It was found that contrast plots of experimental scanning electron microscopy (SEM) images, which are correlated to surface coverage of SAMs desorbed after laser irradiation, agreed with the theoretically predicted surface composition of SAMs. The surface composition of SAM was then interpreted in terms of the wetting property of the resulting surface. The effect of incident laser beam power and size on the final spatial coverage of SAMs on the surface and feature sizes was investigated both experimentally and by modeling. Theoretical modeling and experimental evidence showed that the resulting feature sizes are wider when the surface is heated by a laser of higher power. Increasing the laser beam size results in broadening of feature sizes. Considering the correlation of the theoretical and experimental results, we concluded that the feature sizes are controllable in a predictable way (using the presented thermal-kinetics model) through varying laser beam power and beam size.     

Mixed alkanethiol monolayers on gold surfaces: Wetting and stability studies

Studies of wetting and stability of mixed monolayers containing hydrophobie and hydrophilic components are discussed. We are reporting the observation of an apparent concentration-driven transition in the cosine of the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible (true or rounded) surface phase transition, resulting in the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. The variation of the surface free-energy, both polar and dispersive parts, has been determined as a function of surface OH-concentration. The surface free-energy of the 100% OH surface is close to that found for water, as might be expected for a surface coated with several monolayers of water. Zisman plots obtained for several of the surfaces using polar and nonpolar liquids give γ_c values which follow the observed dispersive contribution to the total surface free energy, and thus do not present a good approximation to the surface free energy (i.e., γ_c < γ_sv).Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH₃ groups at their air-monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (%OH). The effect of different storage environments and temperature on the changing contact angles are discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize over time, depending on the chain length, by surface reorganization and the adsorption of contaminants.     

Dynamic and collective electrochemical responses of tetrathiafulvalene derivative self-assembled monolayers

Electroactive tetrathiafulvalene (TTF)-containing alkanethiol self-assembled monolayers (SAMs) were designed and synthesized to elucidate the relationship between electrochemical responses and film structures. Two TTF derivative molecules having one alkanethiol chain (1) and two alkanethiol chains (2) were utilized to modulate the molecular packing arrangements in the SAMs, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR). SPR measurements in various contacting media demonstrated loose packing of SAM 1 and close packing of SAM 2 due to the different space fillings of the molecules. Two successive one-electron redox waves were observed for both SAMs by cyclic voltammetry. The peak widths of the redox waves were strongly dependent on the oxidation states of the TTF moieties, the packing arrangement of the SAMs, and the contacting medium. We found that TTF-based SAMs exhibited collective electrochemical responses induced by dynamic structural changes, depending on the degree of freedom for the component molecules in the SAMs. These results imply that the molecular design, taking into account the electrochemical responses, extends the available range of molecular-based functionalities in TTF-based SAMs.     

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

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.     

金表面不同链长烷基硫醇自组装单分子膜表面电势的变化规律

从Helmholtz模型出发,对生长在金表面不同链长烷基硫醇自组装单分子膜(SAM)表面电势的变化规律进行了理论研究.利用量子化学软件Gaussian03和MOPAC,讨论了分子偶极矩、相对介电常数以及分子的倾斜角对SAM表面电势的影响.研究表明,不同链长烷基硫醇SAM中分子的倾斜角随烷基链长度的规律性变化是引起SAM表面电势变化的主要原因.从SAM形成机制出发,对金表面不同链长烷基硫醇SAM表面电势的变化规律及其成因提出了新的解释.     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

Molecular-level approach to inhibit degradations of alkanethiol self-assembled monolayers in aqueous media

A molecular-level approach is developed to prevent or inhibit the degradation processes of alkanethiol self-assembled monolayers (SAMs). Previous studies revealed two degradation pathways: direct desorption and oxidation-desorption. By use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in situ and time-dependent imaging reveals and confirms that degradations of alkanethiol SAMs on gold mainly initiate at defect sites, such as domain boundaries and vacancy islands, and then propagate into the ordered domains. Our approach targets at attaching small molecules with preferred adhesion to the defects. The best candidates are aqueous media containing a small amount of amphiphilic surfactant molecules, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). High-resolution studies demonstrate that DMSO and DMF molecules attach to SAM surfaces and more favorably at defect sites, forming relatively stable adsorbates. This attachment increases the activation energy sufficiently to inhibit both degradation pathways. The robustness of this approach has been investigated as a function of surfactant concentration, solution temperature, and the stirring condition. Molecular-level mechanisms and energetics for degradation inhibition of SAMs are also discussed in detail.     

Gas-phase ion-mobility characterization of SAM-functionalized Au nanoparticles

We present results of a systematic examination of functionalized gold nanoparticles (Au-NPs) by electrospray-differential mobility analysis (ES-DMA). Commercially available, citrate-stabilized Au colloid solutions (10-60 nm) were sized using ES-DMA, from which changes in particle size of less than 0.3 nm were readily discerned. It was found that the formation of salt particles and the coating of Au-NPs by salt during the electrospray process can interfere with the mobility analysis, which required the development of sample preparation and data correction protocols to extract correct values for the Au-NP size. Formation of self-assembled monolayers (SAMs) of alkanethiol molecules on the Au-NP surface was detected from a change in particle mobility, which could be modeled to extract the surface packing density of SAMs. A gas-phase temperature-programmed desorption (TPD) kinetic study of SAMs on Au-NPs found the data to be consistent with a second-order Arrhenius-based rate law, yielding an Arrhenius factor of 1.0 x 10 (11) s (-1) and an activation energy approximately 105 kJ/mol. For the size range of SAM-modified Au-NP we considered, the effect of surface curvature on the energetics of binding of carboxylic acid terminated SAMs is evidently negligible, with binding energies determined by TPD agreeing with those reported for the same SAMs on planar surfaces. This study suggests that the ES-DMA can be added to the tool set of characterization methods used to study the structure and properties of coated nanoparticles.     

Tunable resistive m-dPEG acid patterns on polyelectrolyte multilayers at physiological conditions: template for directed deposition of biomacromolecules

This paper describes a new class of salt-responsive poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) on top of polyelectrolyte multilayer (PEMs) films. PEM surfaces with poly(diallyldimethylammonium chloride) as the topmost layer are chemically patterned by microcontact printing (muCP) oligomeric PEG molecules with an activated carboxylic acid terminal group (m-dPEG acid). The resistive m-d-poly(ethylene glycol) (m-dPEG) acid molecules on the PEMs films were subsequently removed from the PEM surface with salt treatment, thus converting the nonadhesive surfaces into adhesive surfaces. The resistive PEG patterns facilitate the directed deposition of various macromolecules such as polymers, dyes, colloidal particles, proteins, liposomes, and nucleic acids. Further, these PEG patterns act as a universal resist for different types of cells (e.g., primary cells, cell lines), thus permitting more flexibility in attaching a wide variety of cells to material surfaces. The patterned films were characterized by optical microscopy and atomic force microscopy (AFM). The PEG patterns were removed from the PEM surface at certain salt conditions without affecting the PEM films underneath the SAMs. Removal of the PEG SAMs and the stability of the PEM films underneath it were characterized with ellipsometry and optical microscopy. Such salt- and pH-responsive surfaces could lead to significant advances in the fields of tissue engineering, targeted drug delivery, materials science, and biology.     

1-Dodecanethiol self-assembled monolayers on cobalt

Cobalt and its alloys are used in a broad range of application fields. However, the use of this metal is especially limited by its strongly oxidizable nature. The use of alkanethiol self-assembled monolayers (SAMs) is a very efficient way to protect against such oxidation and/or to inhibit corrosion. This surface modification method has been particularly applied to oxidizable metals such as copper or nickel, yet the modification of cobalt surfaces by alkanethiol SAMs received limited attention up to now. In this work, we study the influence of parameters by which to control the self-assembly process of 1-dodecanethiol monolayers on cobalt: nature of the surface pretreatment, solvent, immersion time, and concentration. Each of these parameters has been optimized to obtain a densely packed and stable monolayer able to efficiently prevent the reoxidation of the modified cobalt substrates. The obtained monolayers were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy, and contact angle measurements. The stability of the optimized 1-dodecanethiol monolayer upon air exposure for 28 days has been confirmed by XPS.     

Determination of self-exchange rate of alkanethiolates in self-assembled monolayers on gold using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In this paper, we describe a new method for determining the exchange rates of alkanethiolates in self-assembled monolayers (SAMs) on gold using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to analyze the compositions of the alkanethiolate in SAMs rapidly and directly. In particular, to investigate the self-exchange of alkanethiols, we prepared a deuterated alkanethiol that has the same molecular properties as the non-deuterated alkanethiol but a different molecular weight. SAMs consisting of deuterated alkanethiolates were immersed in a solution of the non-deuterated alkanethiol, and the influences of the immersion time, temperature, concentration, and solvent on the self-exchange rates were investigated. Furthermore, we assessed the exchange rates among alkanethiols with different carbon chain lengths and different size of ethylene glycol units. In addition, we performed molecular dynamics simulations using a model SAM system in order to understand the molecular mechanism of the exchange process.     

Electroactive self-assembled monolayers of unique geometric structures by using rigid norbornylogous bridges

Herein, we describe the synthesis of straight (S) and L-shaped (L) norbornylogous bridges (NBs) with an anthraquinone moiety at the distal end as the redox-active head group and two thiol feet at the proximal end, by which the molecules assemble on gold surfaces. The NB molecules were shown to form self-assembled monolayers (SAMs) with a well-behaved surface redox process. The SAMs were characterized by using in situ IR spectroscopy, cyclic voltammetry, scanning tunnelling microscopy and electrochemical impedance spectroscopy. The surface selection rules associated with the IR band intensities allowed the estimation of the position of the anthraquinone moiety with respect to the surface and the tilt of the bridge with respect to the surface normal, both in pure and diluted monolayers. It is shown that the S- and L-NBs hold the plane of the anthraquinone moiety close to the surface normal or the surface tangent, respectively. Neither NB molecule changes its orientation if spaced by diluents on the surface. The difference in the structure of the S- and L-NB SAMs provides a suitable framework for the investigation of factors that govern electron transfer of anthraquinone moieties across self-assembled monolayers with limited structural ambiguity, as compared with the commonly used structurally flexible alkanethiol monolayers.     

Superhydrophobic effects of self-assembled monolayers on micropatterned surfaces: 3-D arrays mimicking the lotus leaf

The contact angle of water has been measured on a series of self-assembled monolayers (SAM) on thermally evaporated and sputter coated silver surfaces. It is found that micropatterning the surface using nanosphere lithography leads to large increases in the contact angle and generates superhydrophobic surfaces with contact angles >150 degrees. The type of functional groups on the SAMs, the metal island size, and the metal island thickness all contribute to the measured contact angle. The maximum contact angle found was 161 degrees for a fluorinated alkanethiol on 80 nm thick silver islands.     

Electrochemically deposited palladium as a substrate for self-assembled monolayers

The vast majority of reports of self-assembled monolayers (SAMs) on metals focus on the use of gold. However, other metals, such as palladium, platinum, and silver offer advantages over gold as a substrate. In this work, palladium is electrochemically deposited from PdCl2 solutions on glassy carbon electrodes to form a substrate for alkanethiol SAMs. The conditions for deposition are optimized with respect to the electrolyte, pH, and electrochemical parameters. The palladium surfaces have been characterized by scanning electron microscopy (SEM) and the surface roughness has been estimated by chronocoulometry. SAMs of alkane thiols have been formed on the palladium surfaces, and their ability to suppress a Faradaic process is used as an indication for palladium coverage on the glassy carbon. The morphology of the Pd deposit as characterized by SEM and the blocking behavior of the SAM formed on deposited Pd delivers a consistent picture of the Pd surface. It has been clearly demonstrated that, via selection of experimental conditions for the electrochemical deposition, the morphology of the palladium surface and its ability to support SAMs can be controlled. The work will be applied to create a mixed monolayer of metals, which can subsequently be used to create a mixed SAM of a biocomponent and an alkanethiol for biosensing applications.