Ordered molecular assemblies of substituted bis(phthalocyaninato) rare earth complexes on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Substituted bis(phthalocyaninato) rare earth complexes ML2 (M = Y and Ce; L = [Pc(OC8H17)8]2, where Pc = phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm(-3) HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 x 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 x 5 radical3)rect (theta = 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.     

Surface bonding and dynamical behavior of the CH3SH molecule on Au(111)

The chemisorption of the undissociated CH3SH molecule on the Au(111) surface has been studied at 5 K using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The molecule was found to adsorb on atop Au sites on the defect-free surface. CH3SH undergoes hindered rotation about the Au-S bond on the defect-free surface which is seen in STM as a time-averaged 6-fold pattern. The pattern suggests that the potential minima directions occur for the rotating molecule at the six hollow sites surrounding the atop adsorption site. The barrier for rotation, obtained by DFT calculations, is approximately 0.1 kcal.mol(-1). At low coverages, preferential adsorption occurs at defect sites in the surface, namely, the herringbone "elbows" and random atomic step sites. Molecules adsorbed on these sites do not exhibit rotational freedom.     

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Organized Monolayers of Biological Macromolecules on Au(111) Surfaces

Single-crystal electrochemistry and scanning tunneling microscopy directly in aqueous electrolyte solution (in situ STM) are established in physical electrochemistry but new in studies of adsorption and interfacial electrochemistry of biological macromolecules. These high-resolution techniques have now been applied comprehensively to proteins and other biomolecules in recent studies, discussed in this report. Focus is on three systems. The first one is a pair of amino acids, cysteine and cystine. These are strongly adsorbed via thiolate and disulfide, respectively, with identical reductive desorption and in situ STM patterns. Long-range lateral order can be imaged to molecular resolution. The amino acids are also reference molecules for the blue single-copper protein Pseudomonas aeruginosa azurin. This protein assembles in two well-defined orientations. One applies on bare Au(111) to which the protein is linked via its surface disulfide group. This orients the copper center away from the electrode surface. The other mode is by hydrophobic interactions with variable-length alkanethiols self-assembled on Au(111). In this mode the copper center is directed towards the surface. Adsorption and long-range electron tunneling in both modes have been characterized in detail using different electrochemical and spectroscopic techniques, as well as STM. Other data show that penta-(A–T) oligonucleotide adsorbed via a covalently bound thiol linker also displays reductive desorption and in situ STM to molecular resolution. The three systems thus appear to open new perspectives for broader use of high-resolution electrochemical techniques in comprehensive investigations of large biological molecules.     

Thiol with an unusual adsorption-desorption behavior: 6-mercaptopurine on Au(111)

A multitechnique study of 6-mercaptopurine (6MP) adsorption on Au(111) is presented. The molecule adsorbs on Au(111), originating short-range ordered domains and irregular nanosized aggregates with a total surface coverage by chemisorbed species smaller than those found for alkanethiol SAMs, as derived from scanning tunneling microscopy (STM) and electrochemical results. X-ray photoelectron spectroscopy (XPS) results show the presence of a thiolate bond, whereas density functional theory (DFT) data indicate strong chemisorption via a S-Au bond and additional binding to the surface via a N-Au bond. From DFT data, the positive charge on the Au topmost surface atoms is markedly smaller than that found for Au atoms in alkanethiolate SAMs. The adsorption of 6MP originates Au atom removal from step edges but no vacancy island formation at (111) terraces. The small coverage of Au islands after 6MP desorption strongly suggests the presence of only a small population of Au adatom-thiolate complexes. We propose that the absence of the Au-S interface reconstruction results from the lack of significant repulsive forces acting at the Au surface atoms.     

Revisiting the S-Au(111) interaction: static or dynamic?

The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry.     

Direct imaging of hexaamine-ruthenium(III) in domain boundaries in monolayers of single-stranded DNA

We describe adsorption and identification of the binding sites of [Ru(NH3)6]3+ (RuHex) molecules in a closely packed monolayer of a 13-base ss-DNA on Au(111) electrodes by electrochemical in situ scanning tunneling microscopy (STM), cyclic voltammetry and interfacial capacitance data. In situ STM at single-molecule resolution shows that RuHex adsorbs only at the domain borders and near defects. Together with the electrochemical data that show a negative redox potential shift for RuHex adsorbed to DNA strands, this strongly suggests that RuHex binds only to the exposed phosphate groups in the DNA backbone.     

金电极表面植物原生质体的固定化

近年来 ,基于聚电解质同携带相反电荷的基团之间的静电吸引作用组装超薄复合膜的研究受到广泛的关注 [1～ 7] .但是 ,利用聚电解质来固定生物活细胞的研究尚不多见 .Wen等 [8]曾在水溶液中利用聚电解质复合物构成的微囊来固定人体肿瘤活细胞 .本文将该技术应用于生物活细胞在电极表面的固定化 .利用吸附在金电极上的荷正电的聚二烯丙基二甲基氯化铵 [Poly( diallyldimethylammoniumchloride) ,PDADMAC]将携带负电荷的大麦细胞固定在金电极表面 ,运用扫描隧道显微镜 ( STM)、石英晶体微天平 ( QCM)及共焦激光扫描荧光显微镜对该过程进…     

Adsorption and in situ scanning tunneling microscopy of cysteine on Au(111): Structure, energy, and tunneling contrasts

The amino acid L-cysteine (Cys) adsorbs in highly ordered (3 square root of 3 x 6) R30 degrees lattices on Au(111) electrodes from 50 mM ammonium acetate, pH 4.6. We provide new high-resolution in situ scanning tunneling microscopy (STM) data for the L-Cys adlayer. The data substantiate previous data with higher resolution, now at the submolecular level, where each L-Cys molecule shows a bilobed feature. The high image resolution has warranted a quantum chemical computational effort. The present work offers a density functional study of the geometry optimized adsorption of four L-Cys forms-the molecule, the anion, the neutral radical, and its zwitterion adsorbed a-top-at the bridge and at the threefold hollow site of a planar Au(111) Au12 cluster. This model is crude but enables the inclusion of other effects, particularly the tungsten tip represented as a single or small cluster of W-atoms, and the solvation of the L-Cys surface cluster. The computational data are recast as constant current-height profiles as the most common in situ STM mode. The computations show that the approximately neutral radical, with the carboxyl group pointing toward and the amino group pointing away from the surface, gives the most stable adsorption, with little difference between the a-top and threefold sites. Attractive dipolar interactions screened by a dielectric medium stabilize around a cluster size of six L-Cys entities, as observed experimentally. The computed STM images are different for the different L-Cys forms. Both lateral and vertical dimensions of the radical accord with the observed dimensions, while those of the molecule and anion are significantly more extended. A-top L-Cys radical adsorption further gives a bilobed height profile resembling the observed images, with comparable contributions from sulfur and the amino group. L-Cys radical a-top adsorption therefore emerges as the best representation of L-Cys adsorption on Au(111).     

Adsorption of N-Decyl-N,N,N-trimethylammonium triflate (DeTATf), a cationic surfactant, on the Au(111) electrode surface

The adsorption behavior of the cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) on the Au(111) electrode surface was characterized using cyclic voltammetry, differential capacity, and chronocoulometry. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DeTATf has a multistate character. At low bulk DeTATf concentrations, the adsorption state is consistent with the formation of an adsorbed film of nearly flat molecules. At higher concentrations this film may represent a three-dimensional aggregated state. At negative potentials and charge densities close to 0 microC cm-2, the data suggest the formation of a film of tilted molecules oriented with the hydrocarbon tail toward the metal surface and the polar head toward the solution. A surprising result of this study is that DeTATf displays adsorption characteristics of a zwitterionic rather than a cationic surfactant. This behavior indicates that the adsorbed species is an ion pair.     

Thiol- and disulfide-modified oligonucleotide monolayer structures on polycrystalline and single-crystal Au(111) surfaces

We provide a comprehensive study of single- (ss) and double-strand (ds) oligonucleotides with either 25 or 10 bases or base pairs (bp) immobilized on polycrystalline and single-crystal Au(111) surfaces. The study is based on X-ray photoelectron spectroscopy, cyclic and differential pulse voltammetry, interfacial capacitance data, and electrochemical scanning tunnelling microscopy (in situ STM). The sequences used were the 25-bp sequence from the BRCA1 gene (25-mer), while the 10-bp oligonucleotides contained solely linear adenine and thymine sequences. The oligonucleotides were modified by the dimethoxytrityl group (DMT) via a disulfide group [DMT-S-S-ss25-mer and DMT-S-S-ds(AT)₁₀], a pure disulfide group (A₁₀-S-S-T₁₀), or a thiol group [HS-ss25-mer and HS-ds-(AT)₁₀], all via a hexamethylene linker. The overall pattern suggests strategies for controlled adsorption of DNA-based molecules and recognition of complementary strands or other molecules.     

Assembled monolayers of Mo3S4(4+) clusters on well-defined surfaces

A class of inorganic monolayers formed by assembling the molybdenum-sulfur cluster, Mo3S4(4+), onto a well-defined Au(111) surface is presented. The monolayers have been comprehensively characterized by electrochemistry, X-ray photoelectron spectroscopy (XPS), and in situ scanning tunneling microscopy (in situ STM). The voltammetric data show strong reductive and oxidative desorption signals from Au-S interactions, supported by the presence of both S and Mo signals in XPS. In situ STM shows many small pits in the dense adlayers uniformly spread over the surface, which is a typical feature of self-assembled monolayers (SAMs) of alkanethiols. The density of the pits is ca. 23 (+/-5)% and is significantly higher than for straight-chain alkanethiol SAMs with a single -SH group. The pit shapes are irregular, suggesting multiple Au-S interactions from Mo3S4(4+). High resolution images disclose bright round spots of ca. 8 A diameter representing individual molecules in the SAM. This is the first example of in situ monolayer formation by a metal-chalcogenide cluster directly anchored onto the gold surface through core ligands and offers a simple way to prepare a new class of functionalized inorganic monolayers.     

In situ STM imaging and direct electrochemistry of Pyrococcus furiosus ferredoxin assembled on thiolate-modified Au111 surfaces

We have addressed here electron transfer (ET) of Pyrococcus furiosus ferredoxin (PfFd, 7.5 kDa) in both homogeneous solution using edge plane graphite (EPG) electrodes and in the adsorbed state by electrochemistry on surface-modified single-crystal Au111 electrodes, This has been supported by surface microscopic structures of PfFd monolayers, as revealed by scanning tunneling microscopy under potential control (in situ STM). Direct ET between PfFd in phosphate buffer solution, pH 7.9, and EPG electrodes is observed in the presence of promoters. Neomycin gives rise to a pair of redox peaks with a formal potential of ca -430 mV (vs SCE), corresponding to [3Fe-4S]1+/0. The presence of an additional promoter, which can be propionic acid, alanine, or cysteine, induces a second pair of redox peaks at approximately -900 mV (vs SCE) arising from [3Fe-4S]0/1-. A robust neomycin-PfFd complex was detected by mass spectrometry. The results clearly favor an ET mechanism in which the promoting effect of small organic molecules is through formation of promoter-protein complexes. The interaction of PfFd with small organic molecules in homogeneous solution offers clues to confine the protein on the electrode surface modified by the same functional group monolayer and to address diffusionless direct electrochemistry, as well as surface microstructures of the protein monolayer. PfFd molecules were found to assemble on either mercaptopropionic acid (MPA) or cysteine-modified Au111 surfaces in stable monolayers or submonolayers. Highly ordered (2 radical 3 x 5)R30 degrees cluster structures with six MPA molecules in each cluster were found by in situ STM. Individual PfFd molecules on the MPA layer are well resolved by in situ STM. Under Ar protection reversible cyclic voltammograms were obtained on PfFd-MPA/Au111 and PfFd-cysteine/Au111 electrodes with redox potentials of -220 and -201 mV (vs SCE), respectively, corresponding to the [Fe3S4]1+/0 couple. These values are shifted positively by 200 mV relative to homogeneous solution due to interactions between the promoting layers and the protein molecules. Possible mechanisms for such interactions and their ET patterns are discussed.     

碱基腺嘌呤与胸腺嘧啶在Au(111)电极上的共吸附

用循环伏安法(CV)和电化学扫描隧道显微镜(ECSTM)在HClO4溶液中研究了配对碱基腺嘌呤(Adenine,A)与胸腺嘧啶(Thymine,T)在Au(111)电极上的共吸附行为.CV曲线表明,A和T的电化学共吸附行为更接近于A的电化学吸附行为.高分辨STM图像显示,在物理吸附区域碱基A和T分子之间通过氢键作用形成一种不同于单组分的网络结构.根据STM图像提出一个可能的模型,并给出了在Au(111)电极上共吸附时A和T分子之间可能的氢键作用方式.     

Adsorption and electrochemical activity: an in situ electrochemical scanning tunneling microscopy study of electrode reactions and potential-induced adsorption of porphyrins

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H(2)SO(4) solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V(SCE). The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.     

Monolayers of a de novo designed 4-alpha-helix bundle carboprotein and partial structures on Au(111)-surfaces

Mapping of structure and function of proteins adsorbed on solid surfaces is important in many contexts. Electrochemical techniques based on single-crystal metal surfaces and in situ scanning probe microscopies (SPM) have recently opened new perspectives for mapping at the single-molecule level. De novo design of model proteins has evolved in parallel and holds promise for test and control of protein folding and for new tailored protein structural motifs. These two strategies are combined in the present report.We present a synthetic scheme for a new 4-alpha-helix bundle carboprotein built on a galactopyranoside derivative with a thiol anchor aglycon suitable for surface immobilization on gold. The galactopyranoside with thiol anchor and the thiol anchor alone were prepared for comparison. Voltammetry of the three molecules on Au(111) showed reductive desorption peaks caused by monolayer adsorption via thiolate-Au bonding. In situ STM of the thiol anchor disclosed an ordered adlayer with clear domains and molecular features. This holds promise, broadly for single-molecule voltammetry and the SPM and scanning tunnelling microscopy (STM) of natural and synthetic proteins.     

Platinum nanofilm formation by EC-ALE via redox replacement of UPD copper: studies using in-situ scanning tunneling microscopy

The growth of Pt nanofilms on well-defined Au(111) electrode surfaces, using electrochemical atomic layer epitaxy (EC-ALE), is described here. EC-ALE is a deposition method based on surface-limited reactions. This report describes the first use of surface-limited redox replacement reactions (SLR(3)) in an EC-ALE cycle to form atomically ordered metal nanofilms. The SLR(3) consisted of the underpotential deposition (UPD) of a copper atomic layer, subsequently replaced by Pt at open circuit, in a Pt cation solution. This SLR(3) was then used a cycle, repeated to grow thicker Pt films. Deposits were studied using a combination of electrochemistry (EC), in-situ scanning tunneling microscopy (STM) using an electrochemical flow cell, and ultrahigh vacuum (UHV) surface studies combined with electrochemistry (UHV-EC). A single redox replacement of upd Cu from a PtCl(4)(2-) solution yielded an incomplete monolayer, though no preferential deposition was observed at step edges. Use of an iodine adlayer, as a surfactant, facilitated the growth of uniformed films. In-situ STM images revealed ordered Au(111)-(square root 3 x square root 3)R30 degrees-iodine structure, with areas partially distorted by Pt nanoislands. After the second application, an ordered Moiré pattern was observed with a spacing consistent with the lattice mismatch between a Pt monolayer and the Au(111) substrate. After application of three or more cycles, a new adlattice, a (3 x 3)-iodine structure, was observed, previously observed for I atoms adsorbed on Pt(111). In addition, five atom adsorbed Pt-I complexes randomly decorated the surface and showed some mobility. These pinwheels, planar PtI(4) complexes, and the ordered (3 x 3)-iodine layer all appeared stable during rinsing with blank solution, free of I(-) and the Pt complex (PtCl(4)(2-)).     

In situ scanning tunneling microscopy of 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol adsorbed on pt(111) electrodes

Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine four dithiol molecules, including 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO(4). The open-circuit potential (OCP) of Pt(111) electrodes decreased substantially from 0.95 to 0.3 V (versus reversible hydrogen electrode) upon the adsorption of dithol molecules, which indicates that these adsorbates injected electrons into the Pt electrode. For all dithiol molecules, ordered adlattices of p(2 x 2) and (square root 3 x square root 3)R30 degrees were formed when the dosing concentration was lower than 150 microM and the potential of Pt(111) was more negative than 0.5 V. Raising the potential of Pt(111) from 0.1 to 0.4 V or more positive values could transform p(2 x 2) to (square root 3 x square root 3)R30 degrees before it turned disarray. The insensitivity of the structure of dithiol adlayers with their chemical structures was explained by upright molecular orientation with the formation of one Pt-S bond per dithiol molecule. This molecular orientation was independent of the coverage of dithiol molecules, as nucleation seeds produced at the beginning of adsorption were also constructed with p(2 x 2). The triangular-shaped STM molecular resolution suggested 3-fold binding of sulfur headgroup on Pt(111). All dithiols were adsorbed so strongly on Pt(111) electrodes that switching the potential negatively to the onset of hydrogen evolution in 0.1 M HClO(4) or water reduction in 1 M KOH could not displace dithiol admolecules.     

Schiff碱N-aete-N在Au(111)上自组装单分子膜的电化学及STM研究

利用电化学技术及扫描隧道显微镜(STM),于0.1mol/LHClO₄溶液中研究了Schiff碱N-aete-N在单晶Au(111)面上所形成的自组装单分子膜(SAMs)的电化学性质及结构.N-aete-N在Au(111)电极表面的吸附抑制了金的阳极氧化,同时使固/液界面双层电容明显降低.观察到N-aete-NSAMs的高分辨STM图像.N-aete-N分子在Au(111)表面上以(6×7)结构单胞呈二维有序排列,其表面浓度为5.5×10^-11mol/cm².     

烷基链长对紫精分子在Cu(100)电极表面吸附结构的影响

用循环伏安法(CV)和原位扫描隧道显微镜(STM)研究了烷基取代的紫精分子在Cu(100)电极上的氧化还原行为及其吸附结构对电极电势的依赖性. 对乙基紫精(DHV)和庚基紫精(DEV)在含有KCl电解质溶液中进行循环伏安曲线的测定, 两者呈现出不同的氧化还原行为. 从STM所得图像观察, 二价庚基紫精在Cl-c(2×2)-Cu(100)电极上呈现出二维有序的点阵组装结构,而二价乙基紫精却未出现任何的吸附结构. 降低电极电势至单电子转移反应发生时, 形成的自由基庚基紫精在电极表面呈现出稳定的条带状组装结构, 而自由基乙基紫精出现的条带组装结构比较密集且不能稳定存在. 继续降低电极电势, 庚基紫精的吸附结构会随之出现明显的变化,而乙基紫精不会有吸附结构改变的响应.