Evidence of hydrogen trapping at second phase particles in zirconium alloys.

Zirconium alloys are used in safety-critical roles in the nuclear industry and their degradation due to ingress of hydrogen in service is a concern. In this work experimental evidence, supported by density functional theory modelling, shows that the α-Zr matrix surrounding second phase particles acts as a trapping site for hydrogen, which has not been previously reported in zirconium. This is unaccounted for in current models of hydrogen behaviour in Zr alloys and as such could impact development of these models. Zircaloy-2 and Zircaloy-4 samples were corroded at 350 °C in simulated pressurised water reactor coolant before being isotopically spiked with 2H2O in a second autoclave step. The distribution of 2H, Fe and Cr was characterised using nanoscale secondary ion mass spectrometry (NanoSIMS) and high-resolution energy dispersive X-ray spectroscopy. 2H- was found to be concentrated around second phase particles in the α-Zr lattice with peak hydrogen isotope ratios of 2H/1H = 0.018-0.082. DFT modelling confirms that the hydrogen thermodynamically favours sitting in the surrounding zirconium matrix rather than within the second phase particles. Knowledge of this trapping mechanism will inform the development of current understanding of zirconium alloy degradation through-life.

3D morphology of the human hepatic ferritin mineral core: new evidence for a subunit structure revealed by single particle analysis of HAADF-STEM images.

Ferritin, the major iron storage protein, has dual functions; it sequesters redox activity of intracellular iron and facilitates iron turn-over. Here we present high angle annular dark field (HAADF) images from individual hepatic ferritin cores within tissue sections, these images were obtained using spherical aberration corrected scanning transmission electron microscopy (STEM) under controlled electron fluence. HAADF images of the cores suggest a cubic morphology and a polycrystalline (ferrihydrite) subunit structure that is not evident in equivalent bright field images. By calibrating contrast levels in the HAADF images using quantitative electron energy loss spectroscopy, we have estimated the absolute iron content in any one core, and produced a three dimensional reconstruction of the average core morphology. The core is composed of up to eight subunits, consistent with the eight channels in the protein shell that deliver iron to the central cavity. We find no evidence of a crystallographic orientation relationship between core subunits. Our results confirm that the ferritin protein shell acts as a template for core morphology and within the core, small (approximately 2 nm), surface-disordered ferrihydrite subunits connect to leave a low density centre and a high surface area that would allow rapid turn-over of iron in biological systems.

Direct electron transfer to a metalloenzyme redox center coordinated to a monolayer-protected cluster.

A strategy for establishing electrical contact to the metal center of a redox metalloenzyme, galactose oxidase (GOase), by coordination of a linker attached to a monolayer-protected gold cluster is presented. The cluster-enzyme hybrid system was first prepared in solution and characterized by high-angle annular dark-field scanning transmission electron microscopy. Electrochemical communication between a gold electrode and GOase was achieved by first modifying the electrode surface with a biphenyl dithiol self-assembled monolayer followed by reaction with gold clusters capped with thioctic acid. GOase was then immobilized by replacement of the H(2)O molecule at the Cu(II) exogenous site by coordination of a carboxylate-terminated gold cluster. This chemical attachment ensured electrical contact between the redox center and the electrode, leading to direct mediatorless electron transfer to the protein. Hybrid systems can find applications in biosensors and biofuel cells and for studying electrochemically the catalytic mechanism of reactions for which free radicals and electron-transfer reactions are involved. The present results can be extended to other metalloenzymes.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking.

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

Three-dimensional shapes and structures of lamellar-twinned fcc nanoparticles using ADF STEM.

Small particles with face-centred cubic structures can have non-single-crystallographic shapes. Here, an approach based on annular dark-field scanning transmission electron microscopy (STEM) is used to obtain information about the crystal sub-units that make up supported and unsupported twinned Pt, Pt alloy and Au nanoparticles. The three-dimensional shapes of two types of lamellar-twinned particles (LTPs) of Pt are obtained using high-angle annular dark-field STEM. Possible growth mechanisms of the LTPs and origins for the contrast features in the recorded images are discussed.

A transmission electron microscopy study of Fe-Co alloy nanoparticles in silica aerogel matrix using HREM, EDX, and EELS.

Magnetic nanocomposite materials consisting of 5.5 wt% Fe-Co alloy nanoparticles in a silica aerogel matrix, with compositions Fe(x)Co(1-x) of x = 0.50 and 0.67, have been synthesized by the sol-gel method. The high-resolution transmission electron microscopy images show nanoparticles consisting of single crystal grains of body-centered cubic Fe-Co alloy, with typical crystal grain diameters of approximately 4 and 7 nm for Fe(0.5)Co(0.5) and Fe(0.67)Co(0.33) samples, respectively. The energy dispersive X-ray (EDX) spectra summed over areas of the samples gave compositions Fe(x)C(o1-x) with x = 0.48 +/- 0.06 and 0.68 +/- 0.05. The EDX spectra obtained with the 1.5 nm probe positioned at the centers of approximately 20 nanoparticles gave slightly lower concentrations of Fe, with means of x = 0.43 +/- 0.01 and x = 0.64 +/- 0.02, respectively. The Fe(0.5)Co(0.50) sample was studied using electron energy loss spectroscopy (EELS), and EELS spectra summed over whole nanoparticles gave x = 0.47 +/- 0.06. The EELS spectra from analysis profiles of nanoparticles show a distribution of Fe and Co that is homogeneous, i.e., x = 0.5, within a precision of at best +/-0.05 in x and +/-0.4 nm in position. The present microscopy results have not shown the presence of a thin layer of iron oxide, but this might be at the limit of detectability of the methods.

Imaging methods for determining uptake and toxicity of carbon nanotubes in vitro and in vivo.

Demand for carbon nanotubes (CNTs) is increasing rapidly in electrical, mechanical, and health and medical applications due to their thermal, electrical conductive and other properties. The continued commercial up-scaling of CNT production and application needs to be accompanied by an understanding of the occupational health, public safety and environmental implications of these materials. An increasing volume of literature on the toxicity of CNTs is being published; however, the results of these studies are frequently inconclusive. Due to the enormous number of permutations of nanoparticle shape, dimensions, composition and surface chemistry, only a fundamental understanding of the processes by which CNTs interact with cells will allow a realistic, practical assessment of the risks of the wide range of possible products. Alternatively, by understanding how the physicochemical properties of CNTs relate to their interaction with cells, it will be possible to design 'medical grade' CNTs, which can be used as diagnostic agents or as vectors to deliver therapeutic agents to cell and tissue targets. This article discusses the challenges associated with characterizing the toxicity of CNTs and the need for complimentary nanometrology techniques to relate their physicochemical properties to their toxicity.

Cs corrected STEM EELS: Analysing beam sensitive carbon nanomaterials in cellular structures.

Identification of individual single wall nanotubes (SWNTs) within a cellular structure can provide vital information towards understanding the potential mechanisms of uptake, their localisation and whether their structure is transformed within a cell. To be able to image an individual SWNT in such an environment a resolution is required that is not usually appropriate for biological sections. Standard transmission electron microscopy (TEM) techniques such as bright field imaging of these cellular structures result in very weak contrast. Traditionally, researchers have stained the cells with heavy metal stains to enhance the cellular structure, however this can lead to confusion when analysing the samples at high resolution. Subsequently, alternative methods have been investigated to allow high resolution imaging and spectroscopy to identify SWNTs within the cell; here we will concentrate on the sample preparation and experimental methods used to achieve such resolution.

In situ preparation of network forming gold nanoparticles in agarose hydrogels.

Gold nanoparticles are obtained by reduction of a Au(iii) precursor within an agarose hydrogel where they form percolating networks upon partial dehydration and shrinkage of the gel.

Uptake of noncytotoxic acid-treated single-walled carbon nanotubes into the cytoplasm of human macrophage cells.

Water-soluble single-walled nanotubes (SWNTs) are being tested as contrast agents for medical imaging and for the delivery of therapeutically active molecules to target cells. However, before they become used commercially, it will be essential to establish their subcellular distribution and whether they are cytotoxic. Here we characterize uptake of unlabeled, acid-treated, water-soluble SWNTs by human monocyte derived macrophage cells using a combination of Raman spectroscopy and analytical electron microscopy and compare our findings to previous work on unpurified SWNTs. Raman spectroscopy demonstrated that acid-treated SWNTs had a greater number of functional groups on the carbon walls than nontreated SWNT. The acid-treated SWNTs were less aggregated within cells than unpurified SWNTs. Bundles, and also individual acid-treated SWNTs, were found frequently inside lysosomes and also the cytoplasm, where they caused no significant changes in cell viability or structure even after 4 days of exposure.

Free-standing graphene at atomic resolution.

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the pi --> pi* transition and the pi + sigma plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.

High-resolution detection of Au catalyst atoms in Si nanowires.

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

Macroscopic graphene membranes and their extraordinary stiffness.

The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal, and other characteristics of this one-atom-thick material. Here, we describe a new and highly reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by only one side do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.

Biochemical and structural insights into bacterial organelle form and biogenesis.

Many heterotrophic bacteria have the ability to make polyhedral structures containing metabolic enzymes that are bounded by a unilamellar protein shell (metabolosomes or enterosomes). These bacterial organelles contain enzymes associated with a specific metabolic process (e.g. 1,2-propanediol or ethanolamine utilization). We show that the 21 gene regulon specifying the pdu organelle and propanediol utilization enzymes from Citrobacter freundii is fully functional when cloned in Escherichia coli, both producing metabolosomes and allowing propanediol utilization. Genetic manipulation of the level of specific shell proteins resulted in the formation of aberrantly shaped metabolosomes, providing evidence for their involvement as delimiting entities in the organelle. This is the first demonstration of complete recombinant metabolosome activity transferred in a single step and supports phylogenetic evidence that the pdu genes are readily horizontally transmissible. One of the predicted shell proteins (PduT) was found to have a novel Fe-S center formed between four protein subunits. The recombinant model will facilitate future experiments establishing the structure and assembly of these multiprotein assemblages and their fate when the specific metabolic function is no longer required.

Direct imaging of single-walled carbon nanotubes in cells.

The development of single-walled carbon nanotubes for various biomedical applications is an area of great promise. However, the contradictory data on the toxic effects of single-walled carbon nanotubes highlight the need for alternative ways to study their uptake and cytotoxic effects in cells. Single-walled carbon nanotubes have been shown to be acutely toxic in a number of types of cells, but the direct observation of cellular uptake of single-walled carbon nanotubes has not been demonstrated previously due to difficulties in discriminating carbon-based nanotubes from carbon-rich cell structures. Here we use transmission electron microscopy and confocal microscopy to image the translocation of single-walled carbon nanotubes into cells in both stained and unstained human cells. The nanotubes were seen to enter the cytoplasm and localize within the cell nucleus, causing cell mortality in a dose-dependent manner.

Visualizing the uptake of C60 to the cytoplasm and nucleus of human monocyte-derived macrophage cells using energy-filtered transmission electron microscopy and electron tomography.

Concerns have been raised over the release of C60 nanoparticles into the environment and the potential risk to human health. To address these concerns it is essential to understand the pathways by which nanoparticles enter the cell, where they migrate to, and to establish whether the particles are transformed or modified within the cell. Imaging the subcellular distribution of carbon-based nanoparticles is particularly challenging. It is difficult to achieve high spatial resolution with sufficient image contrast to enable the nanoparticles to be identified within the cell. We have exposed human monocyte-derived macrophages (HMMs) to C60 and used energy filtered transmission electron microscopy (EFTEM) to image the distribution of C60 aggregates within intracellular compartments. We demonstrate that images recorded using low-loss electrons provide a significant improvement in contrast between the cellular material and the C60 allowing a clear differentiation between C60 and unstained cellular compartments and also between ordered and disordered forms of aggregated C60. We confirm that C60 is taken up by HMMs in vitro and is sequestered at several sites within the cell. These sites include the cytoplasm, lysosomes, and most significantly the cell nuclei.

Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum.

Nanocrystals are under active investigation because of their interesting size-dependent properties and potential applications. Silicon nanocrystals have been studied for possible uses in optoelectronics, and may be relevant to the understanding of natural processes such as lightning strikes. Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au and CdSe clusters. However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase. In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size-selection.

Four-dimensional spectral tomography of carbonaceous nanocomposites.

There is considerable interest in the adhesion of polymers to carbon nanotubes for nanocomposite applications.(1-4) One example is multiwalled carbon nanotubes (MWCNTs) dispersed in nylon 6,6.(5) We will show that high-contrast tomographic reconstructions can be created from plasmon-loss electrons that show the three-dimensional structural complexity of the MWCNT-nylon composite at the nanoscale. Further, by recording a series of energy-loss images at successive tilts, it is possible to interrogate subvolumes to extract energy-loss spectra from the reconstructed "volume spectra".