硫辛酸在金电极上自组装成膜动力学研究

自组装单层膜 (ＳＡＭ)应用广泛[1～ 3] 。但对于硫辛酸 (ＴＡ)自组装成膜动力学过程的研究尚未见报道。本研究利用石英晶体微天平 (ＱＣＭ)对ＴＡ自组装成膜过程进行在线监测 ,研究成膜动力学特征 ,并探讨浓度、温度对成膜过程的影响 ,以及在不同ｐＨ值下 ,ＳＡＭ的稳定性 ,推算了不同温度下的速率常数及成膜的活化能。1　实验部分1 1　基本原理通过监测成膜过程中石英晶体微天平 (ＱＣＭ)的振荡频率的变化 ,据Ｓａｕｅｒｂｒｅｙ方程 ,可推知在石英谐振器 (ＱＣＭ)的金电极表面 ,ＴＡ的吸附质量Γ(Γ =Δｍ Ａ ,ｇ ｃｍ2 )与ＱＣＭ的频率…     

Surface-enhanced Raman scattering study of the kinetics of self-assembly of carboxylate-terminated n-alkanethiols on silver

Adsorption of 11-mercaptoundecanoic acid (MUA) on silver from methanol and aqueous solutions was monitored in situ by surface-enhanced Raman scattering (SRES) spectroscopy. While adsorption of MUA from methanol is a one-step formation of a thiol-bound monolayer, SERS spectra reveal that monolayer formation from aqueous solution involves interactions of both carboxylate and thiol groups of MUA with the silver surface. Several Raman scattering bands, including the ν(C-S), ν(s)(COO(-)), and ν(C-C), were used to investigate the evolution of the structure of adsorbed MUA on silver surfaces. The time-dependent profiles of these bands for assembly from aqueous solution indicate a multistep process, which is initiated by the binding of both carboxylate and thiol groups to silver, producing a mixture of gauche and trans conformations. In a subsequent step, the COO-Ag interactions are displaced by stronger S-Ag bonds, leading to ordering of the resulting monolayer with formation of a complete SAM with all-trans conformations. The results also showed that the adsorption process depended strongly on the solution pH and surface potential of the metal. These factors can significantly affect the participation and displacement of -COO(-) during self-assembly of MUA from aqueous solution.     

Supported lipid bilayers lifted from the substrate by layer-by-layer polyion cushions on self-assembled monolayers

The formation of lipid bilayers, lifted from the solid substrate by layer-by-layer polyion cushions, on self-assembled monolayers (SAMs) on gold was investigated by surface plasmon resonance (SPR) and fluorescence recovery after photobleaching (FRAP). The polyions poly(diallyldimethylammonium chloride) (PDDA) and polystyrene sulfonate (PSS) sodium salt were used for the layer-by-layer polyion macromolecular assembly. The cushion was formed by electrostatic interaction of PDDA/PSS/PDDA layers with a negatively charged surface of an SAM of 11-mercaptoundecanoic acid (MUA) on gold. The lipid bilayer membranes were deposited by vesicle fusion with different compositions of SOPS (an anionic lipid, 1-stearoyl-2-oleoyl-phosphatidylserine) and POPC (a zwitterionic lipid, 1-palmitoyl-2-oleoylphosphatidylcholine). In the case of pure SOPS and for lipid mixtures with a POPC composition up to 25%, single bilayers were deposited. FRAP experiments showed that single bilayers supported on PDDA/PSS/PDDA/MUA were mobile at room temperature, with lateral coefficients of approximately (1.2–2.1)×10⁻⁹ cm²/s. The kinetics of the addition of the ion-channel-forming peptide protegrin-1 to the supported bilayers was detected by SPR. A two-step interaction was observed, similar to the association behavior of protegrin-1 with bilayers supported on PDDA/MUA. The results are similar to that of supported lipid bilayers without a layer-by-layer cushion. The model membrane system in this work is a potential biosensor for mimicking the natural activities of biomolecules and is a possible tool to investigate the fundamental properties of biomembranes.     

Modulation of direct electron transfer of cytochrome c by use of a molecularly imprinted thin film

We describe the preparation of a molecularly imprinted polymer film (MIP) on top of a self-assembled monolayer (SAM) of mercaptoundecanoic acid (MUA) on gold, where the template cytochrome c (cyt c) participates in direct electron transfer (DET) with the underlying electrode. To enable DET, a non-conductive polymer film is electrodeposited from an aqueous solution of scopoletin and cyt c on to the surface of a gold electrode previously modified with MUA. The electroactive surface concentration of cyt c was 0.5 pmol cm^?2. In the absence of the MUA layer, no cyt c DET was observed and the pseudo-peroxidatic activity of the scopoletin-entrapped protein, assessed via oxidation of Ampliflu red in the presence of hydrogen peroxide, was only 30 % of that for the MIP on MUA. This result indicates that electrostatic adsorption of cyt c by the MUA–SAM substantially increases the surface concentration of cyt c during the electrodeposition step, and is a prerequisite for the productive orientation required for DET. After template removal by treatment with sulfuric acid, rebinding of cyt c to the MUA–MIP-modified electrode occurred with an affinity constant of 100,000 mol^?1 L, a value three times higher than that determined by use of fluorescence titration for the interaction between scopoletin and cyt c in solution. The DET of cyt c in the presence of myoglobin, lysozyme, and bovine serum albumin (BSA) reveals that the MIP layer suppresses the effect of competing proteins.     

Influence of defects on the electrical characteristics of mercury-drop junctions: self-assembled monolayers of n-alkanethiolates on rough and smooth silver

This paper compares the structural and electrical characteristics of self-assembled monolayers (SAMs) of n-alkanethiolates, SCn (n = 10, 12, 14), on two types of silver substrates: one used as-deposited (AS-DEP) by an electron-beam evaporator, and one prepared using the method of template-stripping. Atomic force microscopy showed that the template-stripped (TS) silver surfaces were smoother and had larger grains than the AS-DEP surfaces, and reflectance-absorbance infrared spectroscopy showed that SAMs formed on TS substrates were more crystalline than SAMs formed on AS-DEP substrates. The range of current densities, J (A/cm2), measured through mercury-drop junctions incorporating a given SAM on AS-DEP silver was, on average, several orders of magnitude larger than the range of J measured through the same SAM on TS silver, and the AS-DEP junctions failed, on average, 3.5 times more often within five current density-voltage (J-V) scans than did TS junctions (depending on the length of the alkyl chains of the molecules in the SAM). The apparent log-normal distribution of J through the TS junctions suggests that, in these cases, it is the variability in the effective thickness of the insulating layer (the distance the electron travels between electrodes) that results in the uncertainty in J. The parameter describing the decay of current density with the thickness of the insulating layer, beta, was either 0.57 A-1 at V = +0.5 V (calculated using the log-mean of the distribution of values of J) or 0.64 A-1 (calculated using the peak of the distribution of values of J) for the TS junctions; the latter is probably the more accurate. The mechanisms of failure of the junctions, and the degree and sources of uncertainty in current density, are discussed with respect to a variety of defects that occur within Hg-drop junctions incorporating SAMs on silver.     

Weak interactions between deposited metal overlayers and organic functional groups of self-assembled monolayers

The purpose of our research is to study the reactions, interactions or penetration between vacuum-deposited metals (M) and the organic functional end groups (OFGs) of self-assembled monolayers (SAMs) under controlled conditions. Metal/SAM systems are models for understanding bonding at M/organic interfaces and the concomitant adhesion between the different materials. In broad terms, the M/OFGs form interacting interfaces (e.g., Cr/COOH or Cu/COOH) in which the deposit resides on top of the OFGs or weakly interacting interfaces through which the overlayer penetrates and resides at the SAM/gold interface. We present a review of XPS results from weakly interacting systems (e.g., Cu/OH, Cu/CN, Ag/CH₃, Ag/COOH) and discuss in more depth the time-temperature dependence of the disappearance of the metal from the M/SAM interface following deposition. In this work, XPS and ISS were used to characterize octadecanethiol (ODT, HS(CH₂)₁₇CH₃), mercaptoundecanoic acid (MUA, HS(CH₂)₁₀COOH), and mercaptohexadecanoic acid (MHA, HS(CH₂)₁₅COOH) SAMs before and after depositing up to 1.0 nm Ag or Cu at ca. 10⁻⁷ torr. The SAMs were prepared by self-assembly onto gold films on <100> silicon substrates in an ethanolic thiol solution. XPS spectra indicate that no strong interaction occurs between the deposited Ag and the COOH organic functional group (OFG) of MUA or MHA, although a stronger interaction is evident for MHA, and a unidentate is formed for Cu on mercaptoundecanol (MUO). The Ag interaction with ODT is weak. ISS compositional depth profiles (CDPs) for Ag on MHA and MUA and ODT are compared over a temperature range of 113 to 293 K. The ISS results indicate that Ag remains on the surface of MUA for up to 1 h after deposition, whereas Ag penetrates ODT in less than 5 min at 295 K. The time for Ag to penetrate into MHA is several times longer than for MUA, depending on the SAM temperature. The time dependence of the slower Ag penetration through MUA and MHA is compared with that for ODT at temperatures below 295 K. Although Ag/OFGs are expected to have relatively weak interactions, the Ag/COOH system was anticipated to be more interactive than was found, so rapid penetration of Ag through the COOH SAM is an unexpected result.     

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.     

Immobilization of Protein A on SAMs for the elaboration of immunosensors

Binary mixtures of 11-mercaptoundecanoic acid (MUA) and other thiols of various lengths and terminal functions were chemisorbed on gold-coated surfaces via S–Au bonds to form mixed self-assembled monolayers (SAMs). Several values of the mole fraction of MUA in the thiol mixtures were tested and the structure and composition of the resulted thin films were characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The results made it clear that co-adsorption of MUA with thiols of similar chain length led to well-ordered monolayers whereas the co-adsorption of MUA with shorter thiols yielded less crystalline-like thin films, but with more reactive carboxylic acid terminal groups. This criterion appeared decisive for efficient covalent binding of Staphylococcus aureus Protein A (PrA), a protein that displays high affinity for the constant fragment (Fc) of antibodies of the IgG type from various mammal species. The ability of immobilized Protein A to recognize and bind a model IgG appeared to be optimal for the mixed SAM of MUA and the short-chain, ω-hydroxythiol 6-mercaptohexanol in the proportion 1–3.     

Effect of amino,hydroxyl, and carboxyl terminal groups of alkyl chains of self-assembled monolayers on the adsorption pattern of gold nanoparticles

The adsorption pattern of gold nanoparticles (AuNPs) on functionalized self-assembled monolayers (SAMs) produced on a Au(111) surface was characterized. The Au(111) was modified with 11-amino-1-undecanethiol hydrochloride (AUT), 11-mercapto-1-undecanol (MUT), or 11-mercaptoundecanoic acid (MUA) at an elevated temperature and pressure. The AuNPs aggregated on the AUT-SAM surface, whereas they were well dispersed on the MUT-SAM surface and localized on the MUA-SAM surface. The results suggest that interactions between AuNPs differ according to the degree of peeling of citrate-layer-capped AuNPs. The degree of peeling, which is related to both the surface randomness of the SAMs and the functional characteristics of the terminal group of each SAM, was discussed on the basis of scanning tunneling microscopy observations, X-ray photoelectron spectroscopic analyses, and contact angle measurements. Our study shows that AuNP patterns can be controlled by changing the terminal group of the alkyl thiol SAM on a Au(111) surface.     

pH-Induced changes in adsorbed cytochrome c. voltammetric and surface-enhanced resonance Raman characterization performed simultaneously at chemically modified silver electrodes

The influence of pH on the redox properties of cytochrome c (cyt c) adsorbed on roughened silver electrodes chemically modified with a self-assembled monolayer (SAM) of 11-mercapto-1-undecanoic acid (MUA) was studied with voltammetric techniques in combination with surface-enhanced resonance Raman scattering (SERRS). The experiments were performed simultaneously on the same electrode sample in a homemade spectroelectrochemical cell suitable for such applications. At pH 7.0 cyt c was found in its native state; at higher pH values (ranging from 8.0 to 9.0) the redox properties of the adsorbed protein varied considerably, featuring a redox behavior which does not resemble the one reported for the alkaline transition. Our results instead indicate the presence of an electrochemically inactive 6cLS species immobilized on MUA at pH 9.0. The pH-induced conformational changes observed for cyt c immobilized on the SAM of MUA were found to be repeatable and chemically reversible, meaning that the recovery of the electrochemical signal due to the native protein occurred instantaneously (on the second time scale) when the electrode was switched back to pH 7.0. The pH-induced changes observed were attributed to a conformational change involving a heme reorientation with respect to the electrode surface.     

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.     

Formation and dissolution processes of the 6-thioguanine (6TG) self-assembled monolayer. A kinetic study

This is a report on the kinetics of the destruction and formation processes of the 6-thioguanine self-assembled monolayer (6TG SAM) on a mercury electrode from acid solutions by chronoamperometry. The destruction of the 6TG SAM that has been previously formed under open circuit potential conditions is carried out by stepping the potential from an initial value where the chemisorbed layer is stable up to potentials where the molecules are no longer chemisorbed. The destruction of the SAM has been described by a model that involves three types of contributions: (i) a Langmuir-type adsorption process, (ii) a 2D nucleation mechanism followed by a growth controlled by surface diffusion, and (iii) a 2D nucleation mechanism followed by a growth at a constant rate. The nonlinear fit of the experimental transients by using this procedure allows the quantitative determination of the individual contributions to the overall process. The kinetics of the formation process is studied under electrochemical conditions. The chronoamperometric experiment allows us to monitor the early stages of 6TG SAM formation. The implications of the physisorbed state at low potentials in the type of monolayer formation and destruction processes as well as the influence of temperature are also discussed.     

Labelling and binding of poly-(L-lysine) to functionalised gold surfaces. Combined FT-IRRAS and XPS characterisation

We compare herein the interfacial reactivity of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA), 1-undecanethiol (UDT) and 11-mercaptoundecanol (MUD) on gold surfaces towards aqueous solutions of poly-(L-lysine) (PL). Liquid-phase labelling of PL with the alkyne dicobalt hexacarbonyl cluster 1 combined with analysis of the substrates by Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS) revealed that irreversible binding of PL occurred in all cases. However, the mechanism of binding involved differed markedly from one monolayer to the other. The main mode of interaction of PL to MUA SAM was of electrostatic nature between the terminal carboxylate of MUA and the ammonium groups of PL. For a similar number of bound thiolate molecules, the UDT adsorbed layer was found less continuous than the MUA one, allowing a higher fraction of PL to directly bind to the gold surface. As for MUD, very little thiolate molecules were adsorbed, leaving bare gold surface areas for non specific adsorption of PL.     

Surface stress, kinetics, and structure of alkanethiol self-assembled monolayers

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 +/- 0.6 N/m results when a c(4x2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 +/- 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.     

Directed growth of poly(isobenzofuran) films by chemical vapor deposition on patterned self-assembled monolayers as templates

This paper describes a method to direct the formation of microstructures of poly(isobenzofuran) (PIBF) by chemical vapor deposition (CVD) on chemically patterned, reactive, self-assembled monolayers (SAMs) prepared on gold substrates. We examined the growth dependence of PIBF by deposition onto several different SAMs each presenting different surface functional groups, including a carboxylic acid, a phenol, an alcohol, an amine, and a methyl group. Interferometry, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and optical microscopy were used to characterize the PIBF films grown on the various SAMs. Based on the kinetic and the spectroscopic analyses, we suggest that the growth of PIBF is surface-dependent and may follow a cationic polymerization mechanism. Using the cationic polymerization mechanism of PIBF growth, we also prepared patterned SAMs of 11-mercapto-1-undecanol (MUO) or 11-mercaptoundecanoic acid (MUA) by microcontact printing (microCP) on gold substrates as templates, to direct the growth of the PIBF. The directed growth and the formation of microstructures of PIBF with lateral dimensions of 6 microm were investigated using atomic force microscopy (AFM). The average thickness of the microstructures of PIBF films grown on the MUO and the MUA patterns were 400 +/- 40 nm and 490 +/- 40 nm, respectively. SAMs patterned with carboxylic acid salts (Cu2+, Fe2+, or Ag+) derived from MUA led to increases in the average thickness of the microstructures of PIBF by 10%, 12%, or 27%, respectively, relative to that of control templates. The growth dependence of PIBF on the various carboxylic acid salts was also investigated using experimental observations of the growth kinetics and XPS analyses of the relative amount of metal ions present on the template surfaces.     

Labelling and binding of poly-( -lysine) to functionalised gold surfaces. Combined FT-IRRAS and XPS characterisation

We compare herein the interfacial reactivity of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA), 1-undecanethiol (UDT) and 11-mercaptoundecanol (MUD) on gold surfaces towards aqueous solutions of poly-( -lysine) (PL). Liquid-phase labelling of PL with the alkyne dicobalt hexacarbonyl cluster 1 combined with analysis of the substrates by Fourier transform infrared reflection–absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS) revealed that irreversible binding of PL occurred in all cases. However, the mechanism of binding involved differed markedly from one monolayer to the other. The main mode of interaction of PL to MUA SAM was of electrostatic nature between the terminal carboxylate of MUA and the ammonium groups of PL. For a similar number of bound thiolate molecules, the UDT adsorbed layer was found less continuous than the MUA one, allowing a higher fraction of PL to directly bind to the gold surface. As for MUD, very little thiolate molecules were adsorbed, leaving bare gold surface areas for non specific adsorption of PL.     

偶氮苯硫醇衍生物自组装成膜过程考察

The formation kinetics of the self-assembled monolayer (SAM) of an azobenzenealkanethiol, denoted as AzoC2SH on surface was studied using electrochemical techniques, and the monolayers prepared at different assembly times and concentration were also investigated. A two-step adsorption kinetics has been confirmed: the fast adsorption process and the following long-term reorganization. The equilibrium constant(K)for the adsorption and the interaction factor between adsorbate-adsorbate molecules (δ) were evaluated based on the Frumkin isotherm and determined to be (3. 17±0. 13)× 106 mol•L(-1) and -0.34±0.04, respectively. The Gibbs free energy (G)of SAM was determined to be -(37. 07±0. 13)kJ•mol(-1).     

"Sliding kinetics" of single-walled carbon nanotubes on self-assembled monolayer patterns: beyond random adsorption

We present the experimental results and theoretical model describing new adsorption kinetics of single-walled carbon nanotubes (swCNTs) onto self-assembled monolayer (SAM) including their sliding motion. The adsorption behavior of swCNTs on large-size SAM patterns is similar to the Langmuir isotherm, while that on nanoscale patterns shows a significant deviation which can be explained by the sliding motion of adsorbed nanotubes. The "sliding chamber" experiment confirms that swCNTs can align along the SAM patterns by sliding motion right above the SAM surfaces. This result provides new scientific insights regarding the adsorption kinetics of one-dimensional nanostructures, and, from a practical point of view, it can be an important guideline to design SAM patterns to assemble carbon nanotubes and nanowires into desired device structures.