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Scanning Electron Microscopy (SEM) represents the mostly widespread method available in analytical laboratories dedicated to the characterization of physical properties such as morphology of various solid materials from ‘micro-’ down to the nanometre scale. The use of secondary electrons excited by a sharply, nm-focussed primary electron beam enables at any modern SEM to image objects with high in-depth and lateral sensitivity, i.e. with high spatial resolution. Hence, e. g. nanoparticles (NPs) are able to be easily characterized with respect to their individual size and shape, but also to the morphology of their surface.
By preparing the nano-objects on thin membranes as electron transparent samples it is possible to perform electron microscopy in the transmission mode (TEM and TSEM). The corresponding transmission (i.e. mass-thickness) contrast reveals in-depth information, but is also well suited for dimensional measurements in the 2D projection image. Both the surface sensitive mode and the transmission one are meanwhile available at any modern SEM. If an X-ray spectrometer is attached to an electron microscope, it is possible to analyse the characteristic X-rays induced by electron bombardment. Most electron microscopes have attached an energy dispersive X-ray spectrometer (EDX) so that EDX elemental maps can be carried out. Recent technological developments of high throughput EDS detectors and their advantages regarding high-resolution X-ray analysis down to the nm range are reviewed.
High-resolution micrographs (SEM, TSEM, TEM) and corresponding X-ray elemental maps on various representative nanoparticles, but also layered samples prepared in cross-section, will be presented and discussed. The importance of selecting the best suited analysis conditions will be highlighted. Also other, often challenging, topics such as sample preparation and image data processing will be critically addressed by practical examples. Further analytical techniques able to be employed at an electron microscope, like cathodoluminescence (CL) or micro-X-ray fluorescence (µXRF) will be briefly surveyed.
Titanium dioxide (TiO₂) nanoparticles (NPs) are one of the main sources of the nanoparticulate matter to which humans are directly exposed and several studies have demonstrated their potential toxic effects. The in vivo detailed spatial distribution of TiO₂ NPs is investigated herein for the first time, using a 2D chemical imaging analysis based on confocal Raman spectroscopy. The invertebrate nematode C. elegans was employed as a prototypical model of living organisms. Rod, bipyramidal and quasispherical engineered TiO₂ NPs with different primary particle sizes and agglomeration states were prepared, characterized and then administered to nematodes. Exploiting the typical fingerprint of TiO₂ in the Raman spectrum, we monitored the biodistribution of NPs inside the worm using a non-invasive, label-free method. The high spatial resolution chemical imaging and the specificity of the Raman technique in the localization of TiO₂ NPs helped in the design of behavioral C. elegans studies aimed at elucidating the relationship among the size, shape, and agglomeration state of NPs and their ability to induce specific toxic effects. Rod-shaped NPs were the most toxic, greatly impairing pharyngeal function, reproduction and larval growth; this indicates that the rod shape, more than the bipyramidal and spherical shapes, enables NPs to interact with biological systems. These findings indicate that this Raman-nematode combined approach represents a step forward in the field of detection of NPs in living organisms, and being rapid and inexpensive enough, it can be applied as the first screening for the ability of NPs to biodistribute and exert toxicological properties in vivo.
The authors have tested the prototype of an electrospray deposition system developed by the company RAMEM under its trademark IONER. To test the prototype and assess its performance, several materials have been sprayed onto TEM grids and the resulting particle distributions were compared to more traditional sample preparation strategies like the “drop on grid” method. Operation parameters such as the sample flow-rate, capillary – substrate distance, electric field strength and sampling period length have been optimized. It was found that the particles deposited by electrospray generally show a much more homogeneous spatial distribution on the substrate and a substantial increase of the number of single particles, which are much better suited to an automatic image evaluation procedure than the agglomerated particles observed otherwise. The applicability of the technique to a broad range of materials is demonstrated by various examples, but also the influence of the substrate, the choice of the particular TEM grid, on the obtained spatial particle distribution is assessed.
The content of the paper is the assessment of the performance of (conventional) measurement techniques (MTs)with respect to the classification of disperse materials according to the EC recommendation for a definition of nanomaterial. This performance essentially refers to the accurate assessment of the number weighted median of (the constituent) particles. All data and conclusions are based on the analytical study conducted as real-world performance testing.
It comprised different types of MTs (imaging, counting, fractionating, spectroscopic and integral) as well as different types of materials. Beside reference materials with well-defined size distribution the study also included several commercial powders (variation of particle composition, morphology, coating, size range and polydispersity). In order to ensure comparability of measurement results, the participants were guided to use uniform protocols in sample preparation, conducting measurements, data analysis and in reporting results. Corresponding documents have been made public, in order to support the reviewing process of the paper, respectively to ensure the reproducibility of data by other users under the same conditions.
The scientific paper relies on a comprehensive set of revised measurement data reported in uniform templates, completely describes the experimental procedures and discusses the MTs’ performance for selected materials in detail. Even more, the study is summarised and evaluated, which leads to recommendations for the use of MTs within a tiered approach of NM characterisation. In addition, the paper critically examines the factors that may affect the outcome of such a comparison among different MTs.
The TiO2 material selected for the present inter-laboratory comparison (ILC) was the bipyramidal anatase UT001 batch synthesized hydrothermally by University of Turin in the frame of the EU/FP7 Project SETNanoMetro and carefully prepared on TEM grids by BAM to be distributed to the ILC participants. A clear measurement protocol including data analysis and reporting (including an xls reporting template) has been also prepared by BAM. 14 of 18 participants have measured and reported results which have been already checked by BAM as being conform to the agreed protocol. After receiving the last results (of 4 participants) the ILC data evaluation will be carried out by BAM in line with agreed ISO/TC229 procedures as for the other case sudies.
A proposal of an inter-laboratory study (ILC) on determination of size and shape distribution of TiO2 nanoparticles (NPs) by transmission electron microscopy is presented. The anatase NPs synthesized in a controllable fashion within the EU/FP7 project SETNanoMetro can be considered as shape-defined (bipyramidal) and are offered to complete the list of case studies already in progress within ISO/TC229/JWG2. The main points of the measurement procedure are presented as well as a proposed procedure to evaluate the size and shape according to the standard operation procedure already developed within SETNanoMetro is discussed. Potential ILC participants and a plan with next step to be carried out are proposed.
Evaluation of electron microscopy techniques for the purpose of classification of nanomaterials
(2016)
One current and much-debated topic in the characterization of nanomaterials (NM) is the implementation of the recently introduced recommendation on a definition of a nanomaterial by the European Commission.
All currently available sizing techniques able to address nanoparticles were systematically evaluated. It was demonstrated that particle sizing techniques like: analytical centrifugation, particle tracking analysis, single-particle inductively coupled plasma mass-spectrometry, differential electrical mobility analysis, dynamic light scattering, small angle X-ray scattering, ultrasonic attenuation spectrometry, but also gas Adsorption analysis based on the BET-method can be applied for a screening classification. However, the quality of the results depends on the individual material to be classified. For well-dispersed, nearly spherical (nano)particles most of the sizing techniques can be applied in a quick and reliable way. In contrast, the classification of most real-world materials is a challenging task, mainly due to non-spherical particle shape, large polydispersity or strong agglomeration/ aggregation of the particles. In the present study it was shown that these issues can be resolved in most cases by electron microscopy as a confirmatory classification technique.
Electron microscopy techniques such as TEM, STEM, SEM or TSEM transmission in SEM) are capable of assessing the size of individual nanoparticles accurately (see Figures 1 and 2). Nevertheless the challenging aspect is sample preparation from powder or liquid form on the substrate, so that a homogeneous distribution of well-separated (deagglomerated) particles is attained. The systematic study in this work shows examples where the extraction of the critical, smallest particle dimension - as the decisive particle parameter for the classification as a NM - is possible by analysing the sample after its simple, dry preparation. The consequences of additional typical issues like loss of information due to Screening of smaller particles by larger ones or the (in)ability to access the constituent particles in aggregates are discussed.
Advances in scanning electron microscopy (SEM) enable the high-resolution imaging of single nanoparticles (NPs) with sizes well below 10 nm. The SEM analysis in transmission mode (T-SEM) of NPs on thin film supports has many benefits when compared to the analysis of NPs on bulk substrates. The enhanced material (mass – thickness) contrast of the T-SEM imaging mode is well suited for in-depth and, particularly valuable, to very accurate, traceable, lateral dimensional measurements of NPs. Compared to samples prepared on bulk substrates, T-SEM with energy dispersive X-ray spectroscopy (EDS) achieves a drastically improved spatial resolution of the emitted X-rays. The poor signal-to-noise ratio of the X-ray spectra emitted by a single nanoparticle (NP) can be improved by the use of high-sensitivity (high collection solid angle) silicon drift (SDD), energy-dispersive X-ray spectrometers (EDS). The EDS spectral imaging of a single NP with a spatial resolution below 10 nm has become possible. This is demonstrated by means of various examples of nanostructures. Advanced data processing of T-SEM/EDS results sets the stage for the automated classification of NPs by feature analysis. This method combines the detection of morphological structures of interest by image processing of T-SEM micrographs with the chemical classification by EDS.