Sergio Lozano-Perez

Filled and glycosylated carbon nanotubes for in vivo radioemitter localization and imaging

Functionalization of nanomaterials for precise biomedical function is an emerging trend in nanotechnology. Carbon nanotubes are attractive as multifunctional carrier systems because payload can be encapsulated in internal space whilst outer surfaces can be chemically modified. Yet, despite potential as drug delivery systems and radiotracers, such filled-and-functionalized carbon nanotubes have not been previously investigated in vivo. Here we report covalent functionalization of radionuclide-filled single-walled carbon nanotubes and their use as radioprobes. Metal halides, including Na(125)I, were sealed inside single-walled carbon nanotubes to create high-density radioemitting crystals and then surfaces of these filled-sealed nanotubes were covalently modified with biantennary carbohydrates, improving dispersibility and biocompatibility. Intravenous administration of Na(125)I-filled glyco-single-walled carbon nanotubes in mice was tracked in vivo using single-photon emission computed tomography. Specific tissue accumulation (here lung) coupled with high in vivo stability prevented leakage of radionuclide to high-affinity organs (thyroid/stomach) or excretion, and resulted in ultrasensitive imaging and delivery of unprecedented radiodose density. Nanoencapsulation of iodide within single-walled carbon nanotubes enabled its biodistribution to be completely redirected from tissue with innate affinity (thyroid) to lung. Surface functionalization of (125)I-filled single-walled carbon nanotubes offers versatility towards modulation of biodistribution of these radioemitting crystals in a manner determined by the capsule that delivers them. We envisage that organ-specific therapeutics and diagnostics can be developed on the basis of the nanocapsule model described here.

Nanometre-scale evidence for interfacial dissolution–reprecipitation control of silicate glass corrosion

Silicate glasses are durable solids, and yet they are chemically unstable in contact with aqueous fluids-this has important implications for numerous industrial applications related to the corrosion resistance of glasses, or the biogeochemical weathering of volcanic glasses in seawater. The aqueous dissolution of synthetic and natural glasses results in the formation of a hydrated, cation-depleted near-surface alteration zone and, depending on alteration conditions, secondary crystalline phases on the surface. The long-standing accepted model of glass corrosion is based on diffusion-coupled hydration and selective cation release, producing a surface-altered zone. However, using a combination of advanced atomic-resolution analytical techniques, our data for the first time reveal that the structural and chemical interface between the pristine glass and altered zone is always extremely sharp, with gradients in the nanometre to sub-nanometre range. These findings support a new corrosion mechanism, interfacial dissolution-reprecipitation. Moreover, they also highlight the importance of using analytical methods with very high spatial and mass resolution for deciphering the nanometre-scale processes controlling corrosion. Our findings provide evidence that interfacial dissolution-reprecipitation may be a universal reaction mechanism that controls both silicate glass corrosion and mineral weathering.

Atom probe tomography today

This review aims to describe and illustrate the advances in the application of atom probe tomography that have been made possible by recent developments, particularly in specimen preparation techniques (using dual-beam focused-ion beam instruments) but also of the more routine use of laser pulsing. The combination of these two developments now permits atomic-scale investigation of site-specific regions within engineering alloys (e.g. at grain boundaries and in the vicinity of cracks) and also the atomic-level characterization of interfaces in multilayers, oxide films, and semiconductor materials and devices.

A guide on FIB preparation of samples containing stress corrosion crack tips for TEM and atom-probe analysis.

The preparation of samples containing stress corrosion crack tips for 3D atom-probe tomography and transmission electron microscopy is of ultimate importance for understanding the mechanisms controlling crack propagation. In this paper, it will be shown that a focused ion beam machine equipped with an in situ micromanipulator is an ideal tool to systematically prepare such demanding samples. The methodology is described and discussed in detail, and several results are presented to demonstrate the potential of the technique.

Achieving sub-nanometre particle mapping with energy-filtered TEM

A combination of state-of-the-art instrumentation and optimized data processing has enabled for the first time the chemical mapping of sub-nanometre particles using energy-filtered transmission electron microscopy (EFTEM). Multivariate statistical analysis (MSA) generated reconstructed datasets where the signal from particles smaller than 1 nm in diameter was successfully isolated from the original noisy background. The technique has been applied to the characterization of oxide dispersion strengthened (ODS) reduced activation FeCr alloys, due to their relevance as structural materials for future fusion reactors. Results revealed that most nanometer-sized particles had a core-shell structure, with an Yttrium-Chromium-Oxygen-rich core and a nano-scaled Chromium-Oxygen-rich shell. This segregation to the nanoparticles caused a decrease of the Chromium dissolved in the matrix, compromising the corrosion resistance of the alloy.

Nuclear reactor materials at the atomic scale

With the renewed interest in nuclear energy, developing new materials able to respond to the stringent requirements of the next-generation fission and future fusion reactors has become a priority. An efficient search for such materials requires detailed knowledge of material behaviour under irradiation, high temperatures and corrosive environments. Minimizing the rates of materials degradation will be possible only if the mechanisms by which it occurs are understood. Atomic-scale experimental probing as well as modelling can provide some answers and help predict in-service behaviour. This article illustrates how this approach has already improved our understanding of precipitation under irradiation, corrosion behaviour, and stress corrosion cracking. It is also now beginning to provide guidance for the development of new alloys.

Examinations of oxidation and sulfidation of grain boundaries in alloy 600 exposed to simulated pressurized water reactor primary water

High-resolution characterizations of intergranular attack in alloy 600 (Ni-17Cr-9Fe) exposed to 325°C simulated pressurized water reactor primary water have been conducted using a combination of scanning electron microscopy, NanoSIMS, analytical transmission electron microscopy, and atom probe tomography. The intergranular attack exhibited a two-stage microstructure that consisted of continuous corrosion/oxidation to a depth of ~200 nm from the surface followed by discrete Cr-rich sulfides to a further depth of ~500 nm. The continuous oxidation region contained primarily nanocrystalline MO-structure oxide particles and ended at Ni-rich, Cr-depleted grain boundaries with spaced CrS precipitates. Three-dimensional characterization of the sulfidized region using site-specific atom probe tomography revealed extraordinary grain boundary composition changes, including total depletion of Cr across a several nm wide dealloyed zone as a result of grain boundary migration.

Overview: Recent progress in three-dimensional atom probe instruments and applications

Over the last few years there have been significant developments in the field of three-dimensional atom probe (3DAP) analysis. This article reviews some of the technical compromises that have led to different instrument designs and the recent improvements in performance. An instrument has now been developed, based around a novel reflectron configuration combining both energy compensation and focusing elements, that yields a large field of view and very high mass resolution. The use of laser pulsing in the 3DAP, together with developments in specimen preparation methods using a focused ion-beam instrument, have led to a significant widening in the range of materials science problems that can be addressed with the 3DAP. Recent studies of semiconductor materials and devices are described.

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.

Quantitative EELS analysis of zirconium alloy metal/oxide interfaces.

Zirconium alloys have been long used for fuel cladding and other structural components in water-cooled nuclear reactors, but waterside corrosion is a primary limitation on both high fuel burn-up and extended fuel cycle operation. Understanding the processes that occur at the metal/oxide interface is crucial for a full mechanistic description of the oxidation process. In this paper we show that reliable quantification of the oxygen content at the metal/oxide interface can be obtained by Electron Energy Loss Spectrometry (EELS) if enough care is taken over both the preparation of Transmission Electron Microscopy (TEM) samples and the methodology for quantification of the EELS data. We have reviewed the accuracy of theoretically calculated inelastic partial scattering cross-sections and effective inelastic mean-free-paths for oxygen and zirconium in oxidized Zr-alloy samples. After careful recalibration against a ZrO₂ powder standard, systematic differences in the local oxygen profile across the interface in different zirconium alloys were found. The presence of a sub-stoichiometric oxide layer (a suboxide) was detected under conditions of slow oxide growth but not where growth was more rapid. This difference could arise from the different corrosion resistances of the alloys or, more likely, as a result of the transition in oxidation behaviour, which refers to a sharp increase in the oxidation rate when the oxide is a few microns thick.