6.1 Oberflächen- und Dünnschichtanalyse
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Accurate characterisation of the morphology (size) and chemistry of graphene-related 2D materials (GR2M) is key in understanding their extraordinary functionalities. Hence, not only the tailoring of these functionalities aiming at applications of increased-performance becomes possible, but also the correlation of the physico-chemical properties with the understanding of the potential toxicity eventually enables a safe and sustainable development of the GR2M for new applications. Whilst AFM and Raman Spectroscopy are recommended to measure the thickness of GO flakes, Scanning Electron Microscopy (SEM) is the most suited method to assess their lateral size, which varies between tens of µm down to below 100 nm. In this paper, procedures for the accurate determination of lateral size of graphene oxide (GO) flakes by SEM are presented. The prerequisite for accurate flake size analysis is the proper sample preparation, i.e. deposition of ideally isolated flakes on a substrate, with the flakes being unfolded, non-overlapped, parallel with the substrate, and having a high coverage density. Examples of optimum image caption conditions and image analysis procedures will be presented. The size descriptors and their measurement are described in the context of the corresponding analysis approach: i) length and width of the flakes with a quick and rough, but robust procedure, and ii) exact contouring of the flakes as part of a highly accurate, but more time-consuming measurement approach. The possibility of application of automated image analysis is discussed as the alternative to the manual flakes analysis.
BAM is currently building up a platform of novel nanoRMs relying on iron oxide nanoparticles of different shape, size and surface chemistry. Iron oxide was chosen as a core material because of its relevance to the material and life sciences.
As a first candidate of this series, we present cubic iron oxide nanoparticles with a nominal edge length of 8 nm. These particles were synthesized by thermal decomposition of iron oleate in high boiling organic solvents adapting well-known literature procedures. After dilution to a concentration suitable for electron microscopy (TEM and SEM) as well as for small-angle X-ray scattering (SAXS) measurements, the candidate nanoRM was bottled and assessed for homogeneity and stability by both methods following the guidelines of ISO 17034 and ISO Guide 35.
The particle sizes obtained by both STEM-in-SEM and TEM are in excellent agreement with a minimum Feret of 8.3 nm ± 0.7 nm. The aspect ratio (AR) of the iron oxide cubes were extracted from the images as the ratio of minimum Feret to Feret resulting in an AR of 1.18 for TEM to 1.25 for SEM. Alternatively, a rectangular bounding box was fitted originating from the minimum Feret and the longest distance through the particle in perpendicular direction. This led to AR values of 1.05 for TEM and 1.12 for SEM, respectively. The results confirm the almost ideal cubic shape.
ACEnano is an EU-funded project which aims at developing, optimising and validating methods for the detection and characterisation of nanomaterials (NMs) in increasingly complex matrices to improve confidence in the results and support their use in regulation. Within this project, several interlaboratory comparisons (ILCs) for the determination of particle size and concentration have been organised to benchmark existing analytical methods. In this paper the results of a number of these ILCs for the characterisation of NMs are presented and discussed. The results of the analyses of pristine well-defined particles such as 60 nm Au NMs in a simple aqueous suspension showed that laboratories are well capable of determining the sizes of these particles. The analysis of particles in complex matrices or formulations such as consumer products resulted in larger variations in particle sizes within technologies and clear differences in capability between techniques. Sunscreen lotion sample analysis by laboratories using spICP-MS and TEM/SEM identified and confirmed the TiO2 particles as being nanoscale and compliant with the EU definition of an NM for regulatory purposes. In a toothpaste sample orthogonal results by PTA, spICP-MS and TEM/SEM agreed and stated the TiO2 particles as not fitting the EU definition of an NM. In general, from the results of these ILCs we conclude that laboratories are well capable of determining particle sizes of NM, even in fairly complex formulations.
A brief perspective of BAM on nanocarriers is presented including examples with special emphasis on the characterization of such materials and underlying challenges. In this respect, also ongoing activities at BAM on different types of core/shell nanomaterials and related systems are briefly summarized.
The main outcomes of the EMPIR project nPSize are presented and the suitability of the new capabilities (e.g. reference materials, measurement procedures, VAMAS inter-laboratory comparisons) to be standardized and implemented in accredited analytical laboratories is discussed. Complementation and/or filling gaps of published and ongoing standardisation projects on size, shape and number concentration measurements under ISO/TC 229/JWG 2 are offered.
In the present work a series of design rules are developed in order to tune the morphology of TiO2 nanoparticles through hydrothermal process. Through a careful experimental design, the influence of relevant process parameters on the synthesis outcome are studied, reaching to the develop predictive models by using Machine Learning methods. The models, after the validation and training, are able to predict with high accuracy the synthesis outcome in terms of nanoparticle size, polydispersity and aspect ratio. Furthermore, they are implemented by reverse engineering approach to do the inverse process, i.e. obtain the optimal synthesis parameters given a specific product characteristic. For the first time, it is presented a synthesis method that allows continuous and precise control of NPs morphology with the possibility to tune the aspect ratio over a large range from 1.4 (perfect truncated bipyramids) to 6 (elongated nanoparticles) and the length from 20 to 140 nm.
Accurate and reproducible measurement of the structure and properties of high-value nanoparticles is extremely important for their commercialization. A significant proportion of engineered nanoparticle systems consist of some form of nominally core-shell structure, whether by design or unintentionally. Often, these do not form an ideal core-shell structure, with typical deviations including polydispersity of the core or shell, uneven or incomplete shells, noncentral cores, and others. Such systems may be created with or without intent, and in either case an understanding of the conditions for formation of such particles is desirable. Precise determination of the structure, composition, size, and shell thickness of such particles can prove challenging without the use of a suitable range of characterization techniques. Here, the authors present two such polymer core-shell nanoparticle systems, consisting of polytetrafluoroethylene cores coated with a range of thicknesses of either polymethylmethacrylate or polystyrene. By consideration of surface energy, it is shown that these particles are expected to possess distinctly differing coating structures, with the polystyrene coating being incomplete. A comprehensive characterization of these systems is demonstrated, using a selection of complementary techniques including scanning electron microscopy, scanning transmission electron microscopy, thermogravimetric analysis, dynamic light scattering, differential centrifugal sedimentation, and X-ray photoelectron spectroscopy. By combining the results provided by these techniques, it is possible to achieve superior characterization and understanding of the particle structure than could be obtained by considering results separately.
Coming as response to the needs expressed by The European Commission mandating CEN, CENELEC and ETSI to develop European standards for methods that can characterize reliably manufactured nanomaterials, a new European metrology research project ‘nPSize - Improved traceability chain of nanoparticle size measurements’ has received funding for the next three years. The project will develop methods, reference materials and modelling to improve the traceability chain, comparability and compatibility for nanoparticle size measurements to support standardization.
nPSize has selected only those nanoparticle sizing techniques which are able to provide traceable results: electron microscopy (SEM, TSEM and TEM), AFM and SAXS. Metrologists from national metrological or designated institutes (PTB, LNE, LGC, VSL, SMD and BAM) will work together with scientists with know-how in development of new nano reference nanoparticles (CEA, University of Turin, LGC, BAM) and with experts in advanced data processing, e.g. by machine learning (POLLEN). With the support of DIN, the project outcomes will be channelized to standardization bodies such as ISO/TC 229 ‘Nanotechnologies’/JWG 2 ‘Nanoparticle Measurement and Characterization’ (SEM, TSEM and TEM), CEN/TC 352 ‘Nanotechnologies’ (SEM, TSEM and TEM), ISO/TC 201/SC 9 (AFM), ISO/TC 24/SC 4 (SAXS).
Three technical work packages will ensure input for impact to standardization community, nanoparticle manufacturers, instrument manufacturers, and (accredited) service laboratories:
- WP1 Performance and traceability of characterization methods
- WP2 Reference materials - Preparation and Characterization
- WP3 Modelling and development of measurement procedures
Well-defined non-spherical nanoparticles shapes such as cubes, platelets, bipyramids, rods/acicular will be developed, with mono- and polydisperse size distribution, as well as with accurate particle number concentration (by SAXS and isotopically enrichment for ICP-MS).
Physical modelling of the signal for TSEM, SEM, 3D-AFM and SAXS will be used to feed machine learning modeling from a-priori measurement data. Further, data fusion will be developed for hybrid sizing techniques: SEM with TSEM/TEM, SEM/TSEM with AFM, SEM/TSEM with SAXS with the final aim of improving the true shape and size of non-spherical nanoparticles by a better estimation of the measurement uncertainties.
In the second half-time of the project dedicated workshops (focused on method improvement and reference materials development) will be organized to disseminate the gained knowledge to end-users. Further, a data library with relevant tagged measurement data is planned to be organized and made publicly available. Inter-laboratory comparisons based on the newly developed multi-modal nano reference materials will be organized preferably within VAMAS/TWA 34 ‘Nanoparticle populations’.