Improved method for the preparation of carboxylic acid and amine terminated self-assembled monolayers of alkanethiolates

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.     

单壁碳纳米管在金表面的图形化组装

利用湿法化学组装技术在金表面得到了图形化的单壁碳纳米管阵列.在混酸氧化条件下,初合成的交缠状态的单壁纳米管被截短成带有羧基等功能化末端的短管.这些功能化的短管在缩合剂DCC的作用下与氨基/甲基图形化表面进行缩合反应时,纳米管将选择性地结合到氨基区域从而形成规则的纳米管阵列.     

Experimental study of the interplay between long-range electron transfer and redox probe permeation at self-assembled monolayers: evidence for potential-induced ion gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.     

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.     

Improving the surface biocompatibility with the use of mixed zwitterionic self-assembled monolayers prepared by a proper solvent

In this study, the mixed self-assembled monolayers (SAMs) containing the mixture of long-chain alkanethiol, SH(CH(2))(11)NH(2) and SH(CH(2))(10)SO(3)H, was prepared as a model surface to examine the interaction between the biological environment and artificial surface. The 10% (v/v) NH(4)OH ethanolic solution and DMSO were chosen as the solvents for the preparation of these mixed SAMs and the "solvent effect" was discussed. X-ray photoelectron spectroscopy (XPS) has indicated that -SO(3)H/-NH(2) mixed SAMs formed from 10% (v/v) NH(4)OH ethanolic solution were surface "-SO(3)H poor", while a nearly equivalent amount of surface -SO(3)H functionality was presented on the mixed SAMs formed from DMSO. This has resulted from the different solvation capability between solvent molecules and the alkanethiol. Such solvent effects were also reflected in various surface properties such as surface wettability and surface zeta potential. The mixed SAMs formed from DMSO were more surface hydrophilic and less negatively surface charged than from 10% (v/v) NH(4)OH ethanolic solution. In addition, these mixed SAMs formed from DMSO exhibited the least amount of protein adsorbed as well as a better platelet compatibility than its counterpart from 10% (v/v) NH(4)OH ethanolic solution. These findings indicated that choosing a proper solvent for mixed zwitterionic SAM can greatly affect its surface properties and biocompatibility, such as to form a surface with near neutrality for reducing protein adsorption and subsequent platelet adhesion and activation.     

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.     

Adsorption and wetting characterization of hydrophobic SBA-15 silicas

This work describes adsorption and wetting characterization of hydrophobic ordered mesoporous silicas (OMSs) with the SBA-15 motif. Three synthetic approaches to prepare hydrophobic SBA-15 silicas were explored: grafting with (1) covalently-attached monolayers (CAMs) of C(n)H(2)(n+1)Si(CH(3))(2)N(CH(3))(2), (2) self-assembled monolayers (SAMs) of C(n)H(2)(n+1)Si(OEt)(3), and (3) direct ("one-pot") co-condensation of TEOS with C(n)H(2)(n+1)Si(OEt)(3) in presence of P123 (n=1-18). The materials prepared were characterized by nitrogen adsorption, TEM, and chemical analysis. The surface properties of the materials were assessed by water contact angles (CAs) and by BET C constants. The results showed that, while loadings of the alkyl groups (%C) were comparable, the surface properties and pore ordering of the materials prepared through different methods were quite different. The best quality hydrophobic surfaces were prepared for SBA-15 grafted with CAMs of alkylsilanes. For these materials, the water CAs were above ～120°/100° (adv/rec) and BET C constants were in the range of ～15-25, indicating uniform low-energy surfaces of closely packed alkyl groups on external and internal surfaces of the pores respectively. Moreover, surfaces grafted with the long-chained (C(12)-C(18)) silanes showed super-hydrophobic behavior (CAs～150-180°) and extremely low adhesion for water. The pore uniformity of parental SBA-15 was largely preserved and the pore volume and pore diameter were consistent with the formation of a single layer of alkylsilyl groups inside the pores. Post-synthesis grafting of SBA-15 with SAMs worked not as well as CAMs: the surfaces prepared demonstrated lower water CAs and higher BET C constants, thereby indicating a small amount of accessible polar groups (Si-OH) related to packing constrains for SAMs supported on highly curved surfaces of mesopores. The co-condensation method produced substantially more disordered materials and less hydrophobic surfaces than any of the grafting methods. The surfaces of these materials showed low water CAs and high BET C constants (～100-200) thereby demonstrating a non-uniform surface coverage and presence of unmodified silica. It is concluded that CAMs chemistry is the most efficient approach in preparation of the functionalized OMS materials with uniform surfaces and pores.     

Self-assembled monolayers of CH3COS- terminated surfactant-encapsulated polyoxometalate complexes

Self-assembled monolayers (SAMs) on gold surfaces based on three kinds of acetylthio-surfactant-encapsulated polyoxometalate clusters (thio-SECs) terminated with multiple CH(3)COS- groups, (NC(26)H(55)S(CO)CH(3))(6)H(2)[Co(H(2)O)CoW(11)O(39)], (NC(26)H(55)S(CO)CH(3))(13)H(3)[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)], and (NC(26)H(55)S(CO)CH(3))(13)[Fe(4)(H(2)O)(2)(P(2)W(15)O(56))(2)]Br, have been prepared, which is representative of a general methodology to fabricate polyoxometalate-based SAMs. Thio-SECs self-assembled into monolayers on gold surfaces through the hydrolysis of CH(3)COS- groups and the subsequent formation of S-Au bonds, which was confirmed by grazing angle infrared spectroscopy, X-ray photoelectron spectroscopy, and ellipsometric and scanning tunneling microscopy (STM) measurements. Furthermore, the SAMs of the thio-SECs possess closely packed structures, and the local short-range order is clearly observed in the magnified STM image. We have also investigated the electrochemical behavior of SAMs of thio-SECs by cyclic voltammetry in detail, and the redox potential of the original polyoxometalates has been well retained. The electrochemical signals of the SAMs are very weak because of the small moiety of thio-SECs that are electrochemically accessible in the cyclic voltammetry experiments. The polyoxometalate-modified electrodes that we prepared are not only highly ordered in the local short range but also stable in electrochemical cycling because of the multiple S-Au bonds of thio-SECs on the gold substrates that aid in the construction of functional materials such as electrochemical and electrocatalytic devices.     

Surface-enhanced infrared ellipsometry of self-assembled undecanethiol and dodecanethiol monolayers on disordered gold nanoisland substrates

Infrared spectroscopic ellipsometry (IRSE) is a powerful optical probe of various chemical and physical properties of molecules adsorbed onto solid surfaces. In particular, IRSE can be useful for detecting adsorption-induced changes in the IR spectra of self-assembled monolayers (SAMs), and unlike traditional IR absorption spectroscopies, IRSE provides useful information about the phase of the reflected radiation from the SAMs. However, in the standard IRSE experimental geometry using flat substrates for SAMs, the detectable signal containing these phase data is considerably weaker than the corresponding absorbance data. In our present work, we demonstrate that enhancing the local optical fields at the sample surface through the use of a disordered Au nanoisland substrate can substantially increase both these absorbance and phase signals. We also demonstrate how this surface-enhanced infrared spectroscopic ellipsometry (SEIRSE) can be utilized for straightforward analysis of absorption peak widths, as well as to obtain information about the orientation of the terminal methyl on adsorbed SAMs. As model SAMs for this study, we use undecanethiol (UDT) containing 10 CH(2) units and a terminal CH(3) group, as well as dodecanethiol (DDT) containing 11 CH(2) units with its terminal CH(3) at a different orientation than UDT. We show that surface-enhanced IRSE is sensitive to subtle vibrational signatures of the differently oriented terminal methyls of these two homologous alkanethiol SAMs.     

Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.     

Soft-UV photolithography using self-assembled monolayers

We report thiol-on-gold self-assembled monolayers (SAMs) that can be photodeprotected using soft UV irradiation (lambda = 365 nm) to yield CO(2)H functionalized surfaces complementing those reported previously, which yielded NH(2) functionalized surfaces. The photolysis of these SAMs were monitored using a combination of surface sensitive techniques. In the SAM environment the photodeprotection yields are lower than those obtained for equivalent reactions in dilute solution. The protected carboxylic acids SAMs are shown to have a low yield approximately 50% due to competing photoreduction reactions of the nitro group. The results from infrared studies show that, as the photolysis progresses, the long chain protected residues reorganize and shield the functional COOH groups, thereby reducing the hydrophilic character of the surface.     

Effect of formation temperature and roughness on surface potential of octadecyltrichlorosilane self-assembled monolayer on silicon surfaces

Kelvin probe force microscopy (KPFM) and atomic force microscopy (AFM) are employed to probe the surface potential and topography of octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3] self-assembled monolayers (SAMs) on oxidized Si(100) and polycrystalline silicon surfaces as a function of deposition temperature and substrate roughness with particular attention paid to the monitoring of SAM adsorption on highly rough surfaces. In these studies, it is found that the surface potential magnitude of the adsorbed layer is larger for monolayers formed in the liquid-condensed (LC) phase than for those formed in the liquid-expanded (LE) phase. Experiments on individual islands in the LC phase show that surface potential and monolayer thickness increase with increasing island size; islands larger than about 1.5 microm reach maximum potential and height values of 48+/-4 mV and 2.7+/-0.1 nm, with respect to the underlying oxidized surface. It is also shown that KPFM is suitable for the study of monolayer adsorption on polycrystalline surfaces, for which preexisting surface texture makes the use of traditional scanning probe techniques for molecular recognition difficult. In these scenarios it is shown that OTS growth occurs preferentially along grain boundaries in fingerlike patterns having a molecular arrangement comparable to that of LC phase islands on atomically smooth silicon. These findings indicate that surface potential measurements provide a highly accurate, local means of probing monolayer morphology on rough surfaces encountered in many applications.     

Designing highly specific biosensing surfaces using aptamer monolayers on gold

To build highly specific surfaces using aptamer affinity reagents, the effects of linker and coadsorbents were investigated for maximizing target binding and specificity for aptamer-based self-assembled monolayers (SAMs) supported on gold. An aptamer that binds the protein thrombin was utilized as a model system to compare different mixed monolayer systems toward maximizing binding and selectivity to the immobilized aptamer. Important factors used to optimize binding characteristics of thrombin to the aptamer-based monolayer films include changes in design elements of the linker and different coadsorbent thiols. Binding events measured by surface plasmon resonance (SPR) and ellipsometry showed that the binding performance of the aptamer SAMs depends principally on the linker and to a lesser extent on the coadsorbent. SAMs formed with HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer exhibited a 4-fold increase in binding capacity versus SAMs made using HS-(CH2)6-TTTTT-aptamer. Furthermore, SAMs made using HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer showed nearly complete specificity for thrombin versus bovine serum albumin (BSA, less than 2% bound), while a SAM incorporating a random DNA fragment (HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-RANDOM) showed little binding of thrombin. Irrespective of the aptamer-linker system, use of HS-(CH2)11(OCH2CH2)3OH, referred to as EG(3), as a coadsorbent enhanced binding of thrombin by approximately 2.5-fold compared to that of HS-(CH2)6-OH (mercaptohexanol, MCH).     

Substrate effects in poly(ethylene glycol) self-assembled monolayers on granular and flame-annealed gold

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH3O(CH2CH2O)17NHCO(CH2)2SH (PEG(17)) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG(17) SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted "soft" and "hard" due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG(17), the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG(17) SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG(17) SAM.     

Mixed self-assembled monolayers (SAMs) consisting of methoxy-tri(ethylene glycol)-terminated and alkyl-terminated dimethylchlorosilanes control the non-specific adsorption of proteins at oxidic surfaces

Monolayers from the newly synthesized compound methoxy-tri(ethylene glycol)-undecenyldimethylchlorosilane (CH3O(CH2CH2O)3(CH2)11Si(CH3)2Cl, MeO(EG)3C11DMS) and dodecyldimethylchlorosilane (DDMS), both pure and mixed, were prepared by self-assembly from organic solution in the presence of an organic base. The films obtained were characterized by advancing and receding contact angle measurements and ellipsometry to confirm the formation of self-assembled monolayers (SAMs). The resulting data on the covalently attached dimethylsilanes were compared to known oligo(ethylene glycol) (OEG)-terminated SAM systems based on terminal alkenes, thiolates or trihydrolyzable silanes. The composition of the mixed SAMs was found to depend directly and linearly on the composition of the silanization solution. Enhanced protein repellent properties were found for the SAMs using a variety of proteins, including the Ras Binding Domain (RBD), a protein with high relevance for cancer diagnostics. Roughly a RBD protein monolayer amount was adsorbed to silicon oxide surfaces silanized with DDMS or non-silanized silicon wafers, and in contrast, no RBD was adsorbed to surfaces silanized with MeO(EG)3C11DMS or to mixed monolayers consisting of DDMS and MeO(EG)3C11DMS if the content of OEG-silane overcame a critical content of X(EG) approximately 0.9.     

Protein adsorption on oligo(ethylene glycol)-terminated alkanethiolate self-assembled monolayers: The molecular basis for nonfouling behavior

A study of protein resistance of oligo(ethylene glycol) (OEG), HS(CH2)11(OCH2CH2)nOH (n = 2, 4, and 6), self-assembled monolayers (SAMs) on Au(111) surfaces is presented here. Hydroxyl-terminated OEG-SAMs are chosen to avoid the hydrophobic effect observed with methyl-terminated OEG-SAMs, particularly at high packing densities. The structure of the OEG-SAM surfaces is controlled by adjusting the assembly solvent. These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Protein adsorption on these surfaces was investigated by surface plasmon resonance (SPR). OEG-SAMs assembled from mixed ethanol and water solutions show higher packing density on gold than those from pure ethanol solution. For EG2OH- and EG4OH-SAMs, proteins (i.e., fibrinogen and lysozyme) adsorb more on the densely packed SAMs prepared from mixed ethanol and water solutions, while EG6OH-SAMs generally resist protein adsorption regardless of the assembly solvent used.     

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).     

In-situ characterization of self-assembled monolayers of water-soluble oligo(ethylene oxide) compounds

In-situ spectroscopic ellipsometry (SE) was utilized to examine the formation of the self-assembled monolayers (SAMs) of the water-soluble oligo(ethylene oxide) [OEO] disulfide [S(CH(2)CH(2)O)(6)CH(3)](2) {[S(EO)(6)](2)} and two analogous thiols - HS(CH(2)CH(2)O)(6)CH(3) {(EO)(6)} and HS(CH(2))(3)O(CH(2)CH(2)O)(5)CH(3) {C(3)(EO)(5)} - on Au from aqueous solutions. Kinetic data for all compounds follow simple Langmuirian models with the disulfide reaching a self-limiting final state (d=1.2nm) more rapidly than the full coverage final states of the thiol analogs (d=2.0nm). The in-situ ellipsometric thicknesses of all compounds were found to be nearly identical to earlier ex-situ ellipsometric measurements suggesting similar surface coverages and structural models in air and under water. Exposure to bovine serum albumin (BSA) shows the self-limiting (d=1.2nm) [S(EO)(6)](2) SAMs to be the most highly protein resistant surfaces relative to bare Au and completely-formed SAMs of the two analogous thiols and octadecanethiol (ODT). When challenged with up to near physiological levels of BSA (2.5mg/mL), protein adsorption on the final state [S(EO)(6)](2) SAM was only 3% of that which adsorbed to the bare Au and ODT SAMs.     

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.     

分子动力学模拟多巴在自组装膜上的黏附性

为了更好地理解贻贝在表面的黏附机理,实现水下胶黏,采用分子动力学方法研究了多巴在自组装膜上的黏附性:采用伞形取样和加权柱状图分析方法计算了多巴在不同自组装膜表面的黏附自由能,使用拉伸分子动力学模拟研究了多巴在不同自组装膜表面上黏附后的脱附力.结果表明,多巴在带负电的羧基自组装膜上的黏附能比在带正电的氨基自组装膜上的大,多巴更容易黏附到带负电表面;多巴在带电表面的黏附能比未带电表面的黏附能更强,表明在带电表面黏附更稳定.进一步分析了多巴在不同表面的取向分布,发现多巴与不同表面相互作用的方式不同:与疏水表面主要通过苯环相互作用;与亲水表面主要通过羟基相互作用;与负电表面主要通过氨基相互作用;与正电表面主要通过羧基相互作用.通过模拟比较了多巴在不同自组装膜上的脱附力,发现多巴在带电表面的脱附力比在未带电表面的大,与黏附能的趋势一致.对比4种非带电表面的脱附力,发现多巴在疏水性甲基自组装膜表面的脱附力最大,黏附更稳定,随着表面疏水性的增加,脱附力增大,黏附稳定性增强.本工作可为研发新型水下胶黏剂提供理论指导.