Effect of antibiotics on mechanical properties of Bordetella pertussis examined by atomic force microscopy.

In recent years, the coevolution of microorganisms with current antibiotics has increased the mechanisms of bacterial resistance, generating a major health problem worldwide. Bordetella pertussis is a bacterium that causes whooping cough and is capable of adopting different states of virulence, i.e. virulent or avirulent states. In this study, we explored the nanomechanical properties of both virulent and avirulent B. pertussis as exposed to various antibiotics. The nanomechanical studies highlighted that only virulent B. pertussis cells undergo a decrease in their cell elastic modulus and height upon antimicrobial exposure, whereas their avirulent counterparts remain unaffected. This study also permitted to highlight different mechanical properties of individual cells as compared to those growing in close contact with other individuals. In addition, we analyzed the presence on the bacterial cell wall of Filamentous hemagglutinin adhesin (FHA), the major attachment factor produced by virulent Bordetella spp., under different virulence conditions by Force Spectroscopy.

Morphological stabilization and KPZ scaling by electrochemically induced co-deposition of nanostructured NiW alloy films.

We have assessed the stabilizing role that induced co-deposition has in the growth of nanostructured NiW alloy films by electrodeposition on polished steel substrates, under pulsed galvanostatic conditions. We have compared the kinetic roughening properties of NiW films with those of Ni films deposited under the same conditions, as assessed by Atomic Force Microscopy. The surface morphologies of both systems are super-rough at short times, but differ at long times: while a cauliflower-like structure dominates for Ni, the surfaces of NiW films display a nodular morphology consistent with more stable, conformal growth, whose height fluctuations are in the Kardar-Parisi-Zhang universality class of rough two-dimensional interfaces. These differences are explained by the mechanisms controlling surface growth in each case: mass transport through the electrolyte (Ni) and attachment of the incoming species to the growing interface (NiW). Thus, the long-time conformal growth regime is characteristic of electrochemical induced co-deposition under current conditions in which surface kinetics is hindered due to a complex reaction mechanism. These results agree with a theoretical model of surface growth in diffusion-limited systems, in which the key parameter is the relative importance of mass transport with respect to the kinetics of the attachment reaction.

Localization of adhesins on the surface of a pathogenic bacterial envelope through atomic force microscopy.

Bacterial adhesion is the first and a significant step in establishing infection. This adhesion normally occurs in the presence of flow of fluids. Therefore, bacterial adhesins must be able to provide high strength interactions with their target surface in order to maintain the adhered bacteria under hydromechanical stressing conditions. In the case of B. pertussis, a Gram-negative bacterium responsible for pertussis, a highly contagious human respiratory tract infection, an important protein participating in the adhesion process is a 220 kDa adhesin named filamentous haemagglutinin (FHA), an outer membrane and also secreted protein that contains recognition domains to adhere to ciliated respiratory epithelial cells and macrophages. In this work, we obtained information on the cell-surface localization and distribution of the B. pertussis adhesin FHA using an antibody-functionalized AFM tip. Through the analysis of specific molecular recognition events we built a map of the spatial distribution of the adhesin which revealed a non-homogeneous pattern. Moreover, our experiments showed a force induced reorganization of the adhesin on the surface of the cells, which could explain a reinforced adhesive response under external forces. This single-molecule information contributes to the understanding of basic molecular mechanisms used by bacterial pathogens to cause infectious disease and to gain insights into the structural features by which adhesins can act as force sensors under mechanical shear conditions.

Optimization of the surface properties of nanostructured Ni-W alloys on steel by a mixed silane layer.

Ni-W nanostructured coatings electrodeposited on steel by galvanostatic pulses were functionalized by tetraethoxysilane (TEOS) and octadecyltrichlorosilane (OTS) in a two-step procedure. A silica-rich layer is formed by the reaction of TEOS with the metal coating surface oxides, which allows a further reaction with OTS forming a hydrocarbon-silica outer network. This mixed silane layer provides hydrophobicity and improves the corrosion behavior of the Ni-W surface coatings without modifying their excellent mechanical properties.

Adhesin contribution to nanomechanical properties of the virulent Bordetella pertussis envelope.

Adherence to a biological surface allows bacteria to colonize and persist within the host and represents an essential first step in the pathogenesis of most bacterial diseases. Consequently, the physicochemical properties of the outer membrane in bacteria play a key role for attachment to surfaces and therefore for biofilm formation. Bordetella pertussis is a Gram-negative bacterium that colonizes the respiratory tract of humans, producing whooping cough or pertussis, a highly infectious disease. B. pertussis uses various adhesins exposed on its surface to promote cell-surface and cell-cell interactions. The most dominant adhesin function is displayed by filamentous hemagglutinin (FHA). B. pertussis Tohama I wild-type (Vir+) strain and two defective mutants, an avirulent (Vir-) and a FHA-deficient (FHA-) B. pertussis strains were studied by AFM under physiological conditions to evaluate how the presence or absence of adhesins affects the mechanical properties of the B. pertussis cell surface. Quantitative information on the nanomechanical properties of the bacterial envelope was obtained by AFM force-volume analysis. These studies suggested that the presence of virulence factors is correlated with an increase in the average membrane rigidity, which is largely influenced by the presence of FHA. Moreover, for this system we built a nanoscale stiffness map that reveals an inhomogeneous spatial distribution of Young modulus as well as the presence of rigid nanodomains on the cell surface.

Complex surface chemistry of 4-mercaptopyridine self-assembled monolayers on Au(111).

The adsorption of 4-mercaptopyridine on Au(111) from aqueous or ethanolic solutions is studied by different surface characterization techniques and density functional theory calculations (DFT) including van der Waals interactions. X-ray photoelectron spectroscopy and electrochemical data indicate that self-assembly from 4-mercaptopyridine-containing aqueous 0.1 M NaOH solutions for short immersion times (few minutes) results in a 4-mercaptopyridine (PyS) self-assembled monolayer (SAM) with surface coverage 0.2. Scanning tunneling microscopy images show an island-covered Au surface. The increase in the immersion time from minutes to hours results in a complete SAM degradation yielding adsorbed sulfur and a heavily pitted Au surface. Adsorbed sulfur is also the main product when the self-assembly process is made in ethanolic solutions irrespective of the immersion time. We demonstrate for the first time that a surface reaction is involved in PyS SAM decomposition in ethanol, a surface process not favored in water. DFT calculations suggest that the surface reaction takes place via disulfide formation driven by the higher stability of the S-Au(111) system. Other reactions that contribute to sulfidization are also detected and discussed.

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system.

Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several "hot topics" that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule-gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Restricted surface mobility of thiolate-covered metal surfaces: a simple strategy to produce high-area functionalized surfaces.

We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. These surfaces are highly irregular with a fractal dimension close to 2.5. Auger electron spectroscopy and voltammetric data indicate that thiol self-assembly with a maximum surface coverage approximately 1/3 takes place, the same result as that found for smooth gold surfaces. Therefore, neither curvature effects, which would promote higher coverage, nor excluded volume effects, which would result in lower coverage, are present in these irregular surfaces. The high surface area of the bare electrodes exhibits a rapid surface decay in different liquid media that is hindered by alkanethiolate chemisorption. The presence of thiolate SAMs reduces markedly the mass transport surface diffusion of gold adatoms, hindering surface area decay and freezing the system in a metastable state for days. This effect cannot be explained by considering only hydrocarbon-hydrocarbon chain interactions, because it is also observed for ordered arrays of adsorbed S atoms. Therefore, interactions between ordered chemisorbed species at high coverage seem to be responsible for the observed behavior. The thiol-covered high-area metallic substrates can be used to efficiently anchor a large number of molecules, biomolecules, or nanostructures, improving the performance of SAM-based optical and electrochemical devices.

Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics.

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.

Complex surface dynamics during anodic dissolution of Ni.

The evolution of the surface roughness during the anodic dissolution of polycrystalline Ni was investigated by means of ex situ AFM in acid phosphate solutions. To characterize the time and spatial scaling behavior of surface roughness, the interface width and the power spectral density of the surface at different dissolution stages were analyzed in terms of dynamic scaling theories. The time dependence of global surface roughness, W(L,t), shows an unstable behavior characterized by a continuous increase without saturation following the relation W approximately t(beta), where beta > 0.5. The unstable behavior results from the development of wide grooves that originates a surface consisting of mounds. Two scaling regimes at scales shorter and larger than the mound dimensions (l(c)) were observed. For l < l(c), we found alpha approximately 1 consistent with mounds exhibiting smooth (faceted) walls, whereas an anomalous scaling behavior with a proper local roughness exponents (alpha(loc) < 1) dominates at l > l(c). The introduction of nitrite in the solution, a common additive used in phosphating baths, leads to some changes in the scaling behavior as a consequence of different generated chemical surface conditions during dissolution. The different dissolution rates of the exposed crystal orientations and surface diffusion of adatoms were identified as the physical processes that govern the interface dynamic for this system.

Role of surface heterogeneity and molecular interactions in the charge-transfer process through self-assembled thiolate monolayers on Au(111).

A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n = 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negative-charged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate- terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.

Influence of the adsorption of N species on the anodic dissolution of Ni.

The dissolution and passivation of Ni in nitrite-containing acid solutions are investigated by Auger spectroscopy, atomic force microscopy, and conventional electrochemical techniques. The dissolution/ passivation of the Ni surface is consistent with a competition between adsorbed OH- and nitrogen-containing species with a potential-dependent surface coverage. Nitrogen-containing species hinder the passivation of the Ni surface, shifting the formation of the complex nickel hydroxide/oxide film to more positive potential values. The dynamics of the dissolving interface, followed by atomic force microscopy, reflect first the competition of adsorbed species, leading to the development of protrusions and cavities, and finally the formation of the passive film that promotes surface smoothening by a preferential dissolution of the protrusion tips under ohmic control.

Following adsorption kinetics at electrolyte/metal interfaces through crystal truncation scattering: sulfur on Au(111).

Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffraction allowed study of the structure and kinetics of S/Au(111) electrodes in aqueous electrolytes under potential control. Integrated intensities of a particular crystal truncation rod at anti-Bragg conditions were used to trace the sulfur adsorption and desorption as a function of electrode potential in real time. The S desorption is a first order process and the adsorption follows a Langmuir isotherm. A weakly bound S layer is found on the surface before charge transfer, and then specific adsorption occurs.