The Role of a Double Molecular Anchor on the Mobility and Self-Assembly of Thiols on Au(111): The Case of Mercaptobenzoic Acid

The dynamics of the self-assembly process of thiol molecules on Au(111) is affected by the interplay between molecule–substrate and molecule–molecule interactions. Therefore, it is interesting to explore the effect of a second anchor to the gold surface, in addition to the S atom, on both the order and the feasibility of phase transitions in self-assembled monolayers. To assess the role of an additional O anchor, we have compared the adsorption of two mercaptobenzoic acid isomers, 2-mercaptobenzoic acid (2-MBA) and 4-mercaptobenzoic acid (4-MBA), on Au(111). Results from scanning tunneling microscopy, X-ray photoelectron spectroscopy, electrochemical techniques, and density functional theory calculations show that the additional O anchor in 2-MBA hinders surface mobility, reducing domain size and impeding the molecular reorganization involved in phase transition to denser phases on the Au(111) substrates. This knowledge can help to predict the range order and molecular density of the thiol SAM depending on the chemical structure of the adsorbate.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Auto-asociación del ácido okadaico como vía de entrada a las células

[ESP] El ácido okadaico (OA), una toxina de naturaleza polieter producida por dinoflagelados, fue el primer inhibidor natural de proteínas fosfatasas que actúan sobre los residuos de serina y treonina, y que juegan un papel fundamental en la regulación de muchos procesos esenciales en las células. Usando técnicas de “scanning tunneling microscopy (STM)” se puede observar claramente la auto-asociación que sufre dicha toxina, y que parece crucial para su trasporte a través de las membranas celulares [1,2]. Una de las toxinas marinas de distribución mundial, y que causa graves trastornos tanto desde el punto de vista sanitario como socioeconómico, es el ácido okadacio 1 (OA). Sustancia cuyo origen se encuentra en los dinoflagelados de los géneros Dinophysis y Prorocentrum y que produce los síntomas del síndrome diarreico cuando pasa a la cadena trófica a través de marisco contaminado. Estas toxinas cobran una extraordinaria importancia ya que son potentes inhibidores selectivos de las proteínas fosfatasas de tipo 1 y 2A y que las convierte en extraordinarias herramientas farmacológicas para estudiar como se regulan la mayoría de los procesos intracelulares. A pesar de que han sido ampliamente estudiadas desde el punto de vista estructural y biosintético, y que como se ha mencionado, se utilizan a diario como material indispensable en la mayoría de los laboratorios de farmacología, se conoce muy poco de cómo es realmente el mecanismo mediante el cual son capaces de atravesar las membranas biológicas y penetrar en el interior de las células vivas [3]. Hace relativamente poco tiempo en nuestro laboratorio describimos la habilidad que tienen este tipo de sustancias para formar estructuras capaces de albergar un catión metálico, en este caso potasio, y formar dímeros relativamente estables [4,5]. Mediante el uso de STM hemos conseguido establecer cuales son las condiciones propicias para la formación del complejo dimérico con potasio y poderlo observar mediante adsorción sobre monocapas de 111Au (Fig. 1) [6,7]. Utilizando esta tecnología de observación y un modelo sencillo de membrana formado por una bicapa lipídica de fosfatidil colina, hemos conseguido proponer una hipótesis razonable de cómo este tipo de toxinas penetran al interior de la células vivas y ejercen su acción donde la forma dimérica que alberga un catión potasio juega un factor fundamental

Spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles.

The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efficiency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in conditions of long-term storage but not in the operation of devices with such contacts. In this regard, the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim of the present work is to understand the reactions occurring at the surfactant/metal interface, which may help to improve the fabrication of novel electrodes

Surface Morphology Evolution of Chemical Vapor-Deposited Tungsten Films on Si(100)

Chemical vapor deposition (CVD) is used to prepare research-grade heterostructures and to produce the majority of industrially important thin films.[1] In particular, CVD tungsten films are used for many technological applications.[2,3] In CVD an external source maintains a fixed concentration of reactant molecules at a distance above the film surface.[4] Then, gas diffusion drives the molecules through the diffusion layer[2] towards the film surface. At the film interface a reaction must occur before new material is incorporated into the solid. Kinetic studies show that two growth regimes are usually present in CVD. At a low deposition temperature (low rate, regime I) the kinetics is controlled by the surface reaction, whereas at a high temperature (high rate, regime II), mass transport of reactants to, or reaction products from, the surface is the rate-controlling step.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrochemical Self-Assembly of Alkanethiolate Molecules on Ni(111) and Polycrystalline Ni Surfaces

In this work, the electrochemical formation of alkanethiolate self-assembled monolayers (SAMs) on Ni(111) and polycrystalline Ni surfaces from alkanethiol-containing aqueous 1 M NaOH solutions was studied by combining Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that alkanethiolates adsorb on Ni concurrent with NiO electroreduction. The resulting surface coverage depends on the applied potential and hydrocarbon chain length. Electrochemical and XPS data reveal that alkanethiolate electroadsorption at room temperature takes place without S-C bond scission, in contrast to previous results from gas-phase adsorption. A complete and dense monolayer, which is stable even at very high cathodic potentials (-1.5 V vs SCE), is formed for dodecanethiol. DFT calculations show that the greater stability against electrodesorption found for alkanethiolate SAMs on Ni, with respect to SAMs on Au, is somewhat related to the larger alkanethiolate adsorption energy but is mainly due to the larger barrier to interfacial electron transfer present in alkanethiolate-covered Ni. A direct consequence of this work is the possibility of using electrochemical self-assembly as a straightforward route to build stable SAMs of long-chained alkanethiolates on Ni surfaces at room temperature.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrocatalytic and Magnetic Properties of Ultrathin Nanostructured Iron–Melanin Films on Au(111)

We have prepared ultrathin, nanostructured melanin films on Au- ACHTUNGTRENUNG(111) by means of electrochemical selfassembly. These films were characterized by using Auger electron spectroscopy, X-ray absorption near-edge structure spectroscopy, scanning tunneling microscopy, magnetic force microscopy, and electrochemical techniques. Two types of nanostructures are present in the film: melanin nanoparticles and Fe3O4 nanoparticles. The melanin nanoparticles contain Fe bonded to oxygen-containing phenolic groups in an octahedral configuration similar to that found in Fe2O3. The inorganic– organic composite exhibits magnetic properties and catalyzes the electroreduction of hydrogen peroxide in alkaline and neutral electrolyte solutions. The electrocatalytic activity depends on the Fe-bound melanin and appears to be similar to that found for Fe–porphyrins.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Self-Association of Okadaic Acid: Structural and Pharmacological Significance

Okadaic acid (OA) has been an invaluable pharmacological tool in the study of cellular signaling. The great affinity of this polyether for its targets together with its high specificity to inhibit certain protein phosphatases enables the differential study of these proteins. Crystallographic structures of protein phosphatases in complex with OA show a 1:1 protein to toxin ratio. Nevertheless, it has been found that OA is able to self-associate under certain conditions although very little is known about the importance of this phenomenon. Here we review the available knowledge on the latter topic and we report on the existence of an unusual self-associated tetrameric form. The structure of these oligomers is proposed based on spectroscopic data and molecular modeling calculations.Peer reviewe

Self-Assembly of Okadaic Acid as a Pathway to the Cell

The polyether toxin okadaic acid (OA) inhibits several protein serine/threonine phosphatases that play central roles in the regulation of many essential cellular processes. The use of scanning tunneling microscopy (STM) shows that dimerization of such toxins is crucial to understand the mechanism of toxin transport across model membranes.Peer reviewe

Melanin films on Au(1 1 1): Adsorption and molecular conductance

We have studied the adsorption and electronic properties of thin melanin films on Au(1 1 1) by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional (DF) calculations. We have found that the minimum melanin unit detected under different adsorption conditions is consistent with the structural model for eumelanin protomolecules based on tetramer macrocycles formed by four monomer units (hydroquinone, indolequinone and its tautomers) with an inner porphyrin ring. DF calculations reveal that the entire π structure of the tetramers is implied in the chemisorption process through its frontier orbitals (HOMO and LUMO), a fact that is reflected in the change of intramolecular bonds. Also van der Waals interactions give an important contribution to the adsorption energy (≈0.02 eV/Å 2). Dried thin melanin films (1 monolayer in thickness) exhibit good electronic conductance due to the presence of localized states near the Fermi level while dried thicker films exhibit a semiconductor-like behavior. Illumination of the thicker films with white light results in significant photo-induced tunneling currents when the melanin-covered Au is made negative with respect to the tip.Fil: Orive, Alejandro Gonzalez. Universidad de La Laguna; EspañaFil: Hernández Creus, Alberto. Universidad de La Laguna; EspañaFil: Carro, Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad de La Laguna; EspañaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin