S. P. Tear

Detection and characterization of engineered nanoparticles in food and the environment.

Nanotechnology is developing rapidly and, in the future, it is expected that increasingly more products will contain some sort of nanomaterial. However, to date, little is known about the occurrence, fate and toxicity of nanoparticles. The limitations in our knowledge are partly due to the lack of methodology for the detection and characterisation of engineered nanoparticles in complex matrices, i.e. water, soil or food. This review provides an overview of the characteristics of nanoparticles that could affect their behaviour and toxicity, as well as techniques available for their determination. Important properties include size, shape, surface properties, aggregation state, solubility, structure and chemical composition. Methods have been developed for natural or engineered nanomaterials in simple matrices, which could be optimized to provide the necessary information, including microscopy, chromatography, spectroscopy, centrifugation, as well as filtration and related techniques. A combination of these is often required. A number of challenges will arise when analysing environmental and food materials, including extraction challenges, the presence of analytical artifacts caused by sample preparation, problems of distinction between natural and engineered nanoparticles and lack of reference materials. Future work should focus on addressing these challenges.

Application of hydrodynamic chromatography-ICP-MS to investigate the fate of silver nanoparticles in activated sludge

Detection and characterisation are two of the major challenges in understanding the fate, behaviour and occurrence of engineered nanoparticles (ENPs) in the natural environment. In a previous paper we described the development of hydrodynamic chromatography coupled to plasma mass spectrometry (HDC-ICP-MS) for detecting and characterising ENPs in aqueous matrices. This paper describes the applicability of the approach, to study the behaviour of silver nanoparticles in a much more complex and relevant environmental system i.e. sewage sludge supernatant. Batch sorption studies were performed at a range of nanosilver concentrations. Following completion, the sludge supernatant was characterised by ICP-MS, HDC-ICP-MS and transmission electron microscopy (TEM). It was found that, after a contact time of 6 h, most of the silver had partitioned to the sewage sludge (>90%). However, of the silver remaining in the supernatant, some of this was in the nanoparticle form, implying that closer consideration should be given to the longer-term impact of the release of silver ENPs into aquatic ecosystems. These preliminary data clearly show the utility of HDC-ICP-MS for studying the occurrence and behaviour of ENPs in complex natural environments.

A robust size-characterisation methodology for studying nanoparticle behaviour in ‘real’ environmental samples, using hydrodynamic chromatography coupled to ICP-MS

A hyphenated methodology has been developed and validated, which utilizes the extensive size separation range of hydrodynamic chromatography (here: 5–300 nm) combined with the multi-element selectivity of ICP-MS. This has been applied to the analysis of metal-based nanoparticles in environmental samples. The quality of the particle sizing data obtained from this exercise was enabled through the production of a range of gold nanoparticles (sterically stabilized to prevent aggregation in environmental matrices), which were validated for use as external size calibration standards as well as internal retention time markers, using TEM. The methodology was then successfully applied to a study where nanosilver was spiked into sewage sludge, preliminary data from which showed that a fraction of nanosilver survived as single nanoparticles in the sludge supernatant. The method was also tested on solutions containing other commonly used nanoparticles (TiO2, SiO2, Al2O3 and Fe2O3). Overall, the data showed that, by using ICP-MS with collision cell technology, the methodology would be helpful in investigating the fate of a significant range of nanoparticle types. Other characteristics of HDC-ICP-MS are: rapid analysis time ( 300 nm), and limited sample pre-treatment required.

Imaging of engineered nanoparticles and their aggregates under fully liquid conditions in environmental matrices.

The increasing industrial production of engineered nanoparticles (ENPs) raises concern over their safety to humans and the environment. There is a lack of knowledge regarding the environmental fate and impact of ENPs and in situ methods are needed to investigate e.g. nanoparticle aggregation and adsorption in the media of concern such as water, sediment and soil. In this study, the application of wet scanning electron microscopy (WetSEM) technology in combination with energy dispersive x-ray spectroscopy (EDS) to visualise and elementally identify metal and metal oxide nanoparticles (Au, TiO(2), ZnO and Fe(2)O(3)) under fully liquid conditions in distilled and lake water as well as in a soil suspension has been investigated. WetSEM capsules comprise an electron transparent membrane enabling the imaging and EDS analysis of liquid samples. Results are compared with conventional SEM images and show that WetSEM/EDS is a promising complementary tool for the in situ investigation of ENPs and their aggregates in natural matrices. In combination with other analytical tools (e.g. HDC- or FFF-ICP-MS, DLS), WetSEM could help to provide a better understanding of the fate and behaviour of ENPs in the environment.

Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Geometry and confinement effects at the nanoscale can result in substantial modifications to a materials properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefiting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic level, we reveal that strain gradients induced in the confined oxide shell by the nanoparticle geometry enhance the transport of diffusing species, ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insight into ionic conductivity in strained nanostructures.

A LEED analysis of the CoO(100) surface

Abstract A preliminary LEED theory/experiment comparison has been performed on the CoO(100) surface. The results clearly indicate that the surface has a structure close to a simple termination of the bulk lattice, to within ±3% of a layer spacing.

On the structure of Si(113)

Abstract The structure of the (113) face of Si has received a large amount of interest recently because of its high stability, despite having a high index. There remains some controversy about the surface reconstruction, various groups having reported observations of both 3 × 1 and 3 × 2 structures. We have obtained atomically resolved STM images of this surface showing areas of 3 × 1 reconstructed surface, the structure of which matches a modified version of a previously proposed model. In addition we have also obtained CITS images showing the complex underlying reconstruction and spectroscopy measurements which are comparable with a recent ARUPS study of the same surface.

STM studies of submonolayer coverages of Ag on Ge(111)

Abstract Submonolayer coverages of Ag on Ge(111) have been studied under ultra-high vacuum using STM. Atomically resolved images of the reconstruction have been obtained. The images obtained from the 4 × 4 structure are strongly dependent upon the bias voltage polarity, indicating a significant influence from electronic effects. A new model is proposed which has the reconstruction occurring within the top bi-layer of an unreconstructed Ge(111) surface. STM images of the reconstruction show a structure very similar to that obtained for Ag/Si(111), indicating that the surface is a honeycomb-chained-trimer structure. Images have also been obtained of 3 × 1 and 5 × 1 regions and possible models for these structures are proposed.

A computer controlled low energy electron diffractometer for surface crystallography

Abstract A new diffractometer has been developed for LEED crystallography. It uses three grid electron optics in front of a large phosphor coated hemispherical glass screen which forms part of the vacuum wall. The LEED pattern on this screen is viewed and intensity data acquired by a computer controlled TV camera. The same computer controls the whole experiment. A predictor-corrector algorithm is used to track the diffracted beams as the primary energy is changed. System evaluation has used a Cu(111) surface and R factor analysis has shown the systematic experimental errors to lead to uncertainty in the fractional surface layer spacing of the order of ±1%. The instrument is thus about twice as accurate and a hundred times faster than conventional LEED systems.

LEED surface crystallography, R-factors and the structure of the (110) surfaces of III–V semiconductors

Investigation of InSb(110) is presented which is used as the basis for a study of the way in which R-factors are used in LEED methods. These are general comments on the nature of R-factors and the methodology of LEED appropriate to many of the complex surfaces such as the zincblende (110) surface now being studied.