Johannes Mielke

How reliably can a material be classified as a nanomaterial? Available particle-sizing techniques at work

Currently established and projected regulatory frameworks require the classification of materials (whether nano or non-nano) as specified by respective definitions, most of which are based on the size of the constituent particles. This brings up the question if currently available techniques for particle size determination are capable of reliably classifying materials that potentially fall under these definitions. In this study, a wide variety of characterisation techniques, including counting, fractionating, and spectroscopic techniques, has been applied to the same set of materials under harmonised conditions. The selected materials comprised well-defined quality control materials (spherical, monodisperse) as well as industrial materials of complex shapes and considerable polydispersity. As a result, each technique could be evaluated with respect to the determination of the number-weighted median size. Recommendations on the most appropriate and efficient use of techniques for different types of material are given.Graphical Abstract

Reliable nanomaterial classification of powders using the volume-specific surface area method

AbstractThe volume-specific surface area (VSSA) of a particulate material is one of two apparently very different metrics recommended by the European Commission for a definition of “nanomaterial” for regulatory purposes: specifically, the VSSA metric may classify nanomaterials and non-nanomaterials differently than the median size in number metrics, depending on the chemical composition, size, polydispersity, shape, porosity, and aggregation of the particles in the powder. Here we evaluate the extent of agreement between classification by electron microscopy (EM) and classification by VSSA on a large set of diverse particulate substances that represent all the anticipated challenges except mixtures of different substances. EM and VSSA are determined in multiple labs to assess also the level of reproducibility. Based on the results obtained on highly characterized benchmark materials from the NanoDefine EU FP7 project, we derive a tiered screening strategy for the purpose of implementing the definition of nanomaterials. We finally apply the screening strategy to further industrial materials, which were classified correctly and left only borderline cases for EM. On platelet-shaped nanomaterials, VSSA is essential to prevent false-negative classification by EM. On porous materials, approaches involving extended adsorption isotherms prevent false positive classification by VSSA. We find no false negatives by VSSA, neither in Tier 1 nor in Tier 2, despite real-world industrial polydispersity and diverse composition, shape, and coatings. The VSSA screening strategy is recommended for inclusion in a technical guidance for the implementation of the definition. Graphical abstractWe evaluate the extent of agreement between classification by electron microscopy (EM) and classification by Volume-Specific Surface Area (VSSA) on a large set of diverse particulate substances. These represent the challenges anticipated for identification of nanomaterials by the European Commission recommendation for a definition of nanomaterials for regulatory purposes.

Assessment of different electron microscopy techniques for particle size quantification of potential nanomaterials

While nano-scaled intermediate and consumer products are omnipresent in many industries, one challenge consists in the development of methods that reliably identify, characterize and quantify nanomaterials both as a substance and in various matrices. For product registration purposes, the European Commission proposed a definition for nanomaterial which requires a quantitative size determination of the primary particles in a sample down to sizes of 1 nm. According to a material is defined as nano if 50% of the primary particles are observed to comprise a smallest dimension <100 nm. The NanoDefine project was set up to develop and validate a robust, readily implementable and cost-effective measurement approach to obtain a quantitative particle size distribution and to distinguish between nano and non-nano materials according to the definition Among the available particle sizing techniques, electron microscopy was found to be one option meeting most of the requirements of the regulation. However, the use of electron microscopy for particle sizing is often limited by cost per sample, availability in industry, particle agglomeration/aggregation, extremely broad size distributions, 2D materials and operator bias in case of manual evaluation.

Evaluation of Electrospray as a Sample Preparation Tool for Electron Microscopic Investigations: Toward Quantitative Evaluation of Nanoparticles

The potential of electrospray deposition, for the controlled preparation of particles for imaging in electron microscopes, is evaluated on various materials: from mono-modal suspensions of spherical particles to multimodal suspensions and to real-world industrial materials. It is shown that agglomeration is reduced substantially on the sample carrier, compared with conventional sample preparation techniques. For the first time, it is possible to assess the number concentration of a tri-modal polystyrene suspension by electron microscopy, due to the high deposition efficiency of the electrospray. We discovered that some suspension stabilizing surfactants form artifact particles during electrospraying. These can be avoided by optimizing the sprayed suspension.

Improved Spatial Resolution of EDX/SEM for the Elemental Analysis of Nanoparticles

The interest in nanoparticles remains at a high level in fundamental research since many years and increasingly, nanoparticles are incorporated into consumer products to enhance their performance. Consequently, the accurate and rapid characterization of nanoparticles is more and more demanded. Electron microscopy (SEM, TSEM and TEM) is one of the few techniques which are able to image individual nanoparticles. It was demonstrated recently that the transmission electron microscopy at a SEM can successfully be applied as a standard method to characterize accurately the size (distribution) and shape of nanoparticles down to less than 10 nm [1, 2].

Electrospray as a sample preparation tool for electron microscopic investigations: Toward quantitative evaluation of nanoparticles

The potential of electrospray deposition, for the controlled preparation of particles for imaging in electron microscopes, is evaluated on various materials: from mono-modal suspensions of spherical particles to multimodal suspensions and to real-world industrial materials. It is shown that agglomeration is reduced substantially on the sample carrier, compared with conventional sample preparation techniques. For the first time, it is possible to assess the number concentration of a tri-modal polystyrene suspension by electron microscopy, due to the high deposition efficiency of the electrospray. We discovered that some suspension stabilizing surfactants form artifact particles during electrospraying. These can be avoided by optimizing the sprayed suspension.

Electrospray Deposition of Nanoparticles on TEM Grids

Although there are many experimental techniques for measuring particle sizes and size distributions, electron microscopy (EM) is still considered as the gold standard in this field, especially, when it comes to particles in the nano range (1 nm – 100 nm). This is the case, because EM cannot only resolve the size of individual particles accurately, but also the particles shape and morphology and can – to a limited extend – also address constituent particles in aggregates and agglomerates.