Atmospheric pitting corrosion of 304L stainless steel: the role of highly concentrated chloride solutions.

The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl(2) droplets in relation to changes in relative humidity (RH) and chloride deposition density (CDD). It was found that highly reproducible morphologies occur that are distinct at different RH. Pitting at higher concentrations, i.e. lower RH, resulted in satellite pits forming around the perimeter of wide shallow dish regions. At higher RH, these satellite pits did not form and instead spiral attack into the shallow region was observed. Increasing CDD at saturation resulted in a very broad-mouthed pitting attack within the shallow dish region. Large data sets were used to find trends in pit size and morphology in what is essentially a heterogeneous alloy. Electrochemical experiments on 304 stainless steel wires in highly saturated solutions showed that the passive current density increased significantly above 3 M MgCl(2) and the breakdown pitting potential dropped as the concentration increased. It is proposed that the shallow dish regions grow via enhanced dissolution of the passive film, whereas satellite pits and a spiral attack take place with active dissolution of bare metal surfaces.

Diamond Light Source: status and perspectives.

Diamond Light Source, a third-generation synchrotron radiation (SR) facility in the UK, celebrated its 10th anniversary in 2012. A private limited company was set up in April 2002 to plan, construct and operate the new user-oriented SR facility, called in brief Diamond. It succeeded the Synchrotron Radiation Source in Daresbury, a second-generation synchrotron that opened in 1980 as the world's first dedicated X-ray-providing facility, closing finally in 2008, by which time Diamond's accelerators and first beamlines were operating and user experiments were under way. This theme issue of Philosophical Transactions of the Royal Society A gives some examples of the rich diversity of research done in the initial five years, with some glimpses of activity up to 2014. Speakers at the 10 year anniversary symposium were drawn from a small number of major thematic areas and each theme was elaborated by a few speakers whose contributions were placed into a broader context by a leading member of the UK academic community in the role of rapporteur. This introduction gives a summary of the design choices and strategic planning of Diamond as a coherent user facility, a snapshot of its present status and some consideration of future perspectives.

Energy-resolved electron-yield XAS studies of nanoporous CoAlPO-18 and CoAlPO-34 catalysts.

Energy-resolved electron-yield X-ray absorption spectroscopy is a promising technique for probing the near-surface structure of nanomaterials because of its ability to discriminate between the near-surface and bulk of materials. So far, the technique has only been used in model systems. Here, the local structural characterization of nanoporous cobalt-substituted aluminophosphates is reported and it is shown that the technique can be employed for the study of open-framework catalytically active systems. Evidence that the cobalt ions on the surface of the crystals react differently to those in the bulk is found.

Interfacial phenomena during salt layer formation under high rate dissolution conditions.

Interfacial phenomena occurring during high metal dissolution rates, in an environment with diffusion-limited transport of dissolution products, have been investigated using time-resolved X-ray diffraction (XRD), small-angle X-ray scattering (SAXS) and fast radiography. Time resolved SAXS data reveal that highly anisotropic interfacial X-ray scattering always precedes salt nucleation. The correlation between the interfacial scattering the presence of salt crystals indicates that the interface is between the metal electrode and the concentrated NiCl2 electrolyte and can therefore be interpreted as reflectivity or Porod scattering. Using fast radiography, we show that continued crystal nucleation and growth results in formation of a crystal-containing salt layer, which initially extends far from the interface (>20 µm), until the NiCl2 concentration decreases below saturation. Dissolution of this thick salt layer occurs mainly at the furthest boundary from the interface until, the salt layer thickness decreases to a steady state value, resulting in a steady state limiting current. These results show that the presence of a crystalline salt layer at a dissolving interface causes microscopic roughening which has implications for understanding both the role of salt films in pitting corrosion and electrochemical processing.

Vancomycin dimer formation between analogues of bacterial peptidoglycan surfaces probed by force spectroscopy.

Functionalised thiols presenting peptides found in the peptidoglycan of vancomycin-sensitive and -resistant bacteria were synthesised and used to form self-assembled monolayers (SAMs) on gold surfaces. This model bacterial cell-wall surface mimic was used to study binding interactions with vancomycin. Force spectroscopy, using the atomic force microscope (AFM), was used to investigate the specific rupture of interfacial vancomycin dimer complexes formed between pairs of vancomycin molecules bound to peptide-coated AFM probe and substrate surfaces. Clear adhesive contacts were observed between the vancomycin-sensitive peptide surfaces when vancomycin was present in solution, and the adhesion force demonstrated a clear dependence on antibiotic concentration.

Nanomechanical detection of antibiotic-mucopeptide binding in a model for superbug drug resistance.

The alarming growth of the antibiotic-resistant superbugs methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) is driving the development of new technologies to investigate antibiotics and their modes of action. We report the label-free detection of vancomycin binding to bacterial cell wall precursor analogues (mucopeptides) on cantilever arrays, with 10 nM sensitivity and at clinically relevant concentrations in blood serum. Differential measurements have quantified binding constants for vancomycin-sensitive and vancomycin-resistant mucopeptide analogues. Moreover, by systematically modifying the mucopeptide density we gain new insights into the origin of surface stress. We propose that stress is a product of a local chemical binding factor and a geometrical factor describing the mechanical connectivity of regions activated by local binding in terms of a percolation process. Our findings place BioMEMS devices in a new class of percolative systems. The percolation concept will underpin the design of devices and coatings to significantly lower the drug detection limit and may also have an impact on our understanding of antibiotic drug action in bacteria.

Investigating the specific interactions between carbonic anhydrase and a sulfonamide inhibitor by single-molecule force spectroscopy.

In this communication, we report on the interaction landscape of an active site-specific enzyme-inhibitor complex by single-molecule force spectroscopy. Electrostatic immobilization was employed to orient a carbonic anhydrase enzyme on a positively charged surface so its active site is pointing upward. This approach to immobilization effectively increases the number of specific interactions measured between the zinc ion of the active site on carbonic anhydrase and a sulfonamide inhibitor tethered to an atomic force microscope (AFM) probe. Further, it reduces the time required for data collection and thereby minimizes the possible mechanical damage to the probe and contamination of the enzyme surface. The rupture force measured at various loading rates is interpreted in terms of a single energy barrier for the carbonic anhydrase enzyme-sulfonamide inhibitor complex from which the kinetic and thermodynamic parameters were estimated on the basis of microscopic models and were compared to the Bell-Evans model. The dissociation rate for the enzyme-inhibitor complex was found to be significantly faster (~35 times) than the natural spontaneous dissociation rate.

Macroscopic 2D networks self-assembled from nanometer-sized protein/DNA complexes.

We demonstrate the self-assembly of DNA and DNA binding proteins into two-dimensional networks that are then addressable by sending a second protein to a specific recognition site on the DNA network. These networks cover centimeters in area but can be addressed with nanometer precision. This hierarchical self-assembly of specific DNA protein complexes will be the basis for complex positioning of single molecules in two and three dimensions.

Electrostatic orientation of enzymes on surfaces for ligand screening probed by force spectroscopy.

In this letter, we show that electrostatic immobilization provides a simple but effective approach for the immobilization and orientation of carbonic anhydrase onto charged surfaces. The enzyme is oriented differently on oppositely charged surfaces, with the majority of active sites facing upward on a positively charged surface and downward on a negatively charged surface. An array of negatively charged microscale surface patterns within a positively charged background was prepared by microcontact printing and used as the substrate to immobilize the enzymes. This enabled the probing of the enzyme orientations on the two differently charged surface regions by force spectroscopy with the same atomic force microscopy (AFM) probe modified with a thiolated sulfonamide inhibitor. The unbinding forces between the inhibitor tip and the enzyme immobilized on the two differently charged surfaces were measured. Two control experiments, blocking of the enzyme active site with a competitive inhibitor and removal of the zinc ion from the enzyme catalytic center, were employed to distinguish between specific and nonspecific interactions and to further verify the differences in enzyme orientation. Autocorrelation analysis of the force histograms was carried out to evaluate the specific single enzyme-inhibitor interaction force.

Calibration of AFM cantilever spring constants.

In this paper we present two simple, reliable and readily applicable methods for calibrating cantilevers and measuring the thickness of thin gold films. The spring constant calibration requires knowledge of the Young's modulus, density of the cantilever and resonant frequency. The thickness of thin gold layers was determined by measuring changes in the resonant frequency and Q-factor of beam shaped AFM cantilevers before and after coating. The techniques for measuring the spring constant and thin film thickness provide accuracy on the order of 10-15%.

Chiral discrimination of basic and hydrophobic molecules by chemical force spectroscopy.

Chemical force spectroscopy (CFS) is becoming a powerful technique with which to measure intermolecular forces. The prime advantage over other techniques is the ability to map spatial distribution of ligands on the surface. We have used CFS to explore the binding force of a variety of different chiral molecules in order to understand more about the nature of what is a fundamental aspect of chemical biology and of central importance in pharmacology.

Systematic manipulation of surface chemical reaction on the nanoscale: a novel approach for constructing three-dimensional nanostructures.

Nanoscale patches, created by nanografting a maleimide-terminated thiol into a self-assembled monolayer, were elaborated by sequential chemical reactions. Each stage in the nanofabrication was followed by atomic force microscopy (AFM), providing a controlled approach to the fabrication of novel three-dimensional (3D) surface nanostructures.

Application of gas microstrip detectors for X-ray absorption spectroscopy in common process gases.

We report upon the design of a new gas microstrip detector (GMSD) for use in X-ray absorption spectroscopy applied to the study of catalysis and material science. We show that GMSDs can operate not only with the gas mixtures normally used in proportional counters but also with the majority of gas mixtures used in common catalytic reactions. The detector functions well in the presence of water vapor. EXAFS investigations of a test system of NiO on Ni metal are discussed in which it is demonstrated that depth profiling using electron yield X-ray absorption spectroscopy is possible in a wide variety of gaseous environments. Electron detection of XAS using GMSDs is applicable to metals, semiconductors, and insulators presented in almost all forms of sample including films, pellets, powders, crystals, and liquids.

Role of Surface Perfection in Chemical Force Microscopy.

In this paper we investigate the effect that the quality of the gold substrate has upon the distribution of adhesion force measurements in chemical force microscopy. Gold samples have been prepared by two protocols which give either predominantly single-crystal Au(111) or polycrystalline gold films on mica. Gold-coated tips and surfaces were functionalized with self-assembled monolayers terminating in carboxylic groups, and more that 500 force-distance plots were taken. Analysis of these plots revealed a 50% narrower distribution of adhesion forces between monolayers prepared on single-crystal surfaces and monolayers prepared on polycrystalline gold, when measurements were repeated at a single point. However when measurements were taken over a 1-µm2 area, the distribution of adhesion forces was similar for both samples. An explanation for this may lie in the size of the domains on the gold surface relative to the contact area of the tip.