Electrochemical Behaviour of the AA2024 Aluminium Alloy Modified with Self-Assembled Monolayers/Polyaniline Double Films

The electrochemical characteristics of the AA2024 aluminium alloy modified with octadecyltrimethoxysilane (ODTMS) + polyaniline (PANi) and propiltrimethoxysilane (PTMS) + (PANi) were studied in the present work. The results show that the different protective coatings shift the values of corrosion and pit potentials to more positive values making the system nobler and indicate that the double film ODTMS + PANi present the best protection against corrosion characteristics, that is probably due to the two contributions: anodic protection associated with the barrier effect.

Superhydrophobic modification of polyimide films based on gold-coated porous silver nanostructures and self-assembled monolayers

We present a facile and effective method for controlling the surface hydrophobicity of polyimide films from sticky to superhydrophobic properties by tailoring their topographies. Nanostructured silver layers were produced on polyimide films by treatment with aqueous KOH and AgNO3, followed by thermal treatment at 200 degrees C or higher temperatures. Further modification of the gold-coated silver layers with n-dodecanethiol led to hydrophobic surfaces. Different morphologies of the silver layers at the micro- and nano-meter scales, which result in the variety of hydrophobicity, can be tailored by controlling the thermal treatment temperature. Surfaces prepared at 320 degrees C showed a sticky property that water drops did not slide off even when the sample was held upside down. Superhydrophobic surfaces were obtained when the temperature was above 340 degrees C. A remarkable superhydrophobicity, as evidenced by a very large water contact angle of 162 degrees and a very small sliding angle of 7 degrees, was achieved by heating the modified polyimide films at 360 degrees C. This is also the first example for superhydrophobic modification of polyimide films.

Acetylene plasma polymerized surfaces for covalent immobilization of dense bioactive protein monolayers

Smooth polymerized surfaces, suitable for biochemical and biomedical applications, were deposited using a modified plasma enhanced chemical vapour deposition method with acetylene as a reaction precursor. Horseradish peroxidase (HRP) activity assays showed that the protein immobilized on the plasma polymerized surfaces maintained its biological function for a much longer period of time compared to that on uncoated surfaces. The kinetics of HRP attachment to the plasma polymerized surfaces were analyzed using quartz crystal microbalance with dissipation analysis. Spectroscopic ellipsometry and attenuated total reflection Fourier transform infrared spectroscopy were used to determine the thickness and the quantity of the attached protein. The results showed that the plasma polymerized surfaces provided a high density of attachment sites to covalently immobilize a dense monolayer of proteins.

Stepwise synthesis of Gly−Gly−His on gold surfaces modified with mixed self-assembled monolayers

Peptide-modified electrode surfaces have been shown to have excellent recognition properties for metal ions. An efficient method of screening a potential peptide for its selectivity for a given metal would involve the synthesis of the peptide directly on the electrode surface. This paper outlines a procedure in which the tripeptide Gly−Gly−His was synthesized one amino acid at a time on a gold surface modified with a self-assembled monolayer of the mixed alkanethiolates 3-mercaptopropionic acid (MPA) and 3-mercaptopropane (MP). Electrochemistry and high-resolution mass spectrometry were used to elucidate the structure of the adsorbed species and follow the synthesis. The amino acids can be attached only to MPA, but the presence of a diluting unreactive molecule of MP reduces steric crowding about the reaction center. The maximum coverage of synthesized tripeptide occurs at a ratio of MPA/MP of 1:1.

Characterisation of gold electrodes modified with self-assembled monolayers of l-cysteine for the adsorptive stripping analysis of copper

Electrochemical sensors for copper ions in environmental samples were prepared by modifying gold electrodes with l-cysteine by self-assembly. The adsorption of l-cysteine on gold electrodes was studied by electrochemical reductive desorption in 0.5 M KOH, and the interaction of l-cysteine with copper ions was investigated by cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy. At low concentrations the ratio of l-cysteine to bound Cu(II) is 2:1. At higher concentrations (0.1 M) copper reacts with adsorbed cysteine forming copper sulfide on the electrode surface. On a modified l-cysteine gold electrode, Osteryoung square wave voltammetric determination of Cu(II) with a detection limit below 5 ppb has been demonstrated.

Self-assembled monolayers into the 21st century : recent advances and applications

The modification of an interface on a molecular level with more than one molecular ‘building block' is essentially an example of the ‘bottom–up' fabrication principle of nanotechnology. The fabrication of such integrated molecular systems in electrochemistry has seen rapid progress in recent years via the development of sensing interfaces fabricated using self-assembled monolayers (SAMs). This review outlines recent advances and applications of self-assembled monolayers for modifying electrodes with an emphasis on the development of integrated molecular systems. First, some basic issues regarding fabricating integrated molecular systems, such as the role of the surface topography of the electrode and patterning surfaces, are discussed. Subsequently an overview of recent developments in pH, inorganic and bio sensing involving the use of SAMs is given. Finally emerging trends in using molecular building blocks in the fabrication of integrated molecular systems, such as nanotubes, dendrimers and nanoparticles, are reviewed.

Parameters important in fabricating enzyme electrodes using self-assembled monolayers of alkanethiols

The fabrication of enzyme electrodes using self-assembled monolayers (SAMs) has attracted considerable interest because of the spatial control over the enzyme immobilization. A model system of glucose oxidase covalently bound to a gold electrode modified with a SAM of 3-mercaptopropionic acid was investigated with regard to the effect of fabrication variables such as the surface topography of the underlying gold electrode, the conditions during covalent attachment of the enzyme and the buffer used. The resultant monolayer enzyme electrodes have excellent sensitivity and dynamic range which can easily be adjusted by controlling the amount of enzyme immobilized. The major drawback of such electrodes is the response which is limited by the kinetics of the enzyme rather than mass transport of substrates. Approaches to bringing such enzyme electrodes into the mass transport limiting regime by exploiting direct electron transfer between the enzyme and the electrode are outlined.

Atom probe microscopy of self-assembled monolayers : preliminary results

We have achieved three-dimensional imaging of decanethiol self-assembled monolayers (SAMs) on metal surfaces by atom probe tomography (APT). The present Letter provides preliminary results on Ni [001] and Au [111], shows the analytical potential of APT analysis of SAMs, and details developments in specimen preparation and in data-treatment methodologies. Importantly, the investigation of the mass spectra from analysis of the SAMs revealed no combination of sulfur and hydrogen at the interface between the metal substrates and the organic materials, potentially providing insight about the bonding of the thiols on the substrate.

Self-assembled monolayers on mercury probed with a modified surface force apparatus

Self-assembled monolayers (SAMs) of three thiol compounds formed on mercury are investigated by a combination of cyclic voltammetry, electrocapillary curves, and a novel method of measuring electrical doublelayer properties. The last method involves a modified surface force apparatus in which a flat mica surface is pressed down toward a fixed mercury drop held beneath it, while both are immersed in aqueous electrolyte solution. Optical interference measurements are made of the mica-mercury separation as a function of electrical potential applied to the mercury, which yields information on the double-layer interaction between the two surfaces. Mercury is decorated by SAMs of 11-mercapto-1-undecanoic acid, which is shown to bring negative charge to the mercury/aqueous interface due to dissociation of the carboxylic acid groups; 11-mercapto-1- undecanol, which although it is uncharged changes the dipole potential of the interface; and 1-undecanethiol, which likewise changes the dipole potential, but by a different amount. The difference between the changes in dipole potential (90 mV) can be related to the different terminal groups of these two SAMs, -CH3 compared to -OH, that are in contact with the aqueous phase.

Self-assembly of functional, amphipathic amyloid monolayers by the fungal hydrophobin EAS

The hydrophobin EAS from the fungus Neurospora crassa forms functional amyloid fibrils called rodlets that facilitate spore formation and dispersal. Self-assembly of EAS into fibrillar rodlets occurs spontaneously at hydrophobic:hydrophilic interfaces and the rodlets further associate laterally to form amphipathic monolayers. We have used site-directed mutagenesis and peptide experiments to identify the region of EAS that drives intermolecular association and formation of the cross-β rodlet structure. Transplanting this region into a nonamyloidogenic hydrophobin enables it to form rodlets. We have also determined the structure and dynamics of an EAS variant with reduced rodlet-forming ability. Taken together, these data allow us to pinpoint the conformational changes that take place when hydrophobins self-assemble at an interface and to propose a model for the amphipathic EAS rodlet structure.

Reduced graphene oxide directed self-assembly of phospholipid monolayers in liquid and gel phases

The response of cell membranes to the local physical environment significantly determines many biological processes and the practical applications of biomaterials. A better understanding of the dynamic assembly and environmental response of lipid membranes can help understand these processes and design novel nanomaterials for biomedical applications. The present work demonstrates the directed assembly of lipid monolayers, in both liquid and gel phases, on the surface of a monolayered reduced graphene oxide (rGO). The results from atomic force microscopy indicate that the hydrophobic aromatic plane and the defect holes due to reduction of GO sheets, along with the phase state and planar surface pressure of lipids, corporately determine the morphology and lateral structure of the assembled lipid monolayers. The DOPC molecules, in liquid phase, probably spread over the rGO surface with their tails associating closely with the hydrophobic aromatic plane, and accumulate to form circles of high area surrounding the defect holes on rGO sheets. However, the DPPC molecules, in gel phase, prefer to form a layer of continuous membrane covering the whole rGO sheet including defect holes. The strong association between rGO sheets and lipid tails further influences the melting behavior of lipids. This work reveals a dramatic effect of the local structure and surface property of rGO sheets on the substrate-directed assembly and subsequent phase behavior of the supported lipid membranes.

Interaction of small amounts of bovine serum albumin with phospholipid monolayers investigated by surface pressure and atomic force microscopy

The influence of small amounts of bovine serum albumin (BSA) (nM concentration) on the lateral organization of phospholipid monolayers at the air-water interface and transferred onto solid substrates as one-layer Langmuir-Blodgett (LB) films was investigated. The kinetics of adsorption of BSA onto the phospholipid monolayers was monitored with surface pressure isotherms in a Langmuir trough, for the zwitterionic dipalmitoylphosphatidyl ethanolamine (N,N-dimethyl-PE) and the anionic dimyristoylphosphatidic acid (DMPA). A monolayer of N,N-dimethyl-PE or DMPA incorporating BSA was transferred onto a solid substrate using the Langmuir-Blodgett technique. Atomic force microscopy (AFM) images of one-layer LB films displayed protein-phospholipid domains, whose morphology was characterized using dynamic scaling theories to calculate roughness exponents. For DMPA-BSA films the surface is characteristic of self-affine fractals, which may be described with the Kardar-Parisi-Zhang (KPZ) equation. on the other hand, for N,N-dimethyl-PE-BSA films, the results indicate a relatively flat surface within the globule. The height profile and the number and size of globules varied with the type of phospholipid. The overall results, from kinetics of adsorption on Langmuir monolayers and surface morphology in LB films, could be interpreted in terms of the higher affinity of BSA to the anionic DMPA than to the zwitterionic N,N-dimethyl-PE. Furthermore, the effects from such small amounts of BSA in the monolayer point to a cooperative response of DMPA and N,N-dimethyl-PE monolayers to the protein. (c) 2005 Elsevier B.V. All rights reserved.

Study of the diclofenac/phospholipid interactions with liposomes and monolayers

The interaction of diclofenae sodium (SD) with soya phosphatidylcholine (SPC) has been studied with floating Langmuir monolayers and liposomes. SD was either introduced into the subphase of SPC monolayers or co-spread with SPC on an aqueous subphase. In both cases, SD caused the surface pressure isotherm to become more expanded, thus demonstrating the affinity between SD and SPC. The incorporation of SD caused SPC liposomes to have a decreased diameter according to light scattering experiments. When SPC liposomes were injected into an aqueous subphase, their destruction yielding surface-active monomers could be monitored by changes in surface pressure. SD-loaded liposomes displayed a much faster kinetics when the surface density of surface-active monomers was plotted against time, with rate constants increasing significantly with the SD concentration. The kinetic profile can be quantitatively analyzed by plotting In[1 - (Gamma/Gamma(infinity))] versus t(1/2) (C) 2004 Elsevier B.V. All rights reserved.

Electrochemical and spectroscopic characterization of screen-printed gold-based electrodes modified with self-assembled monolayers and Tc85 protein

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Isolation of bovine coronavirus (BCoV) in monolayers of HmLu-1 cells

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)