How large is small?

The effect of nanoparticles on the human organs or on the environment has scarcely been investigated up to now. Their behaviour is largely dependent on their particle size: A particle 18 nanometers large can have completely different properties from a particle 35 or 160 nanometers large. The difference in size thus plays an important role in the assessment of the risk of these "midget particles" for humanity and the environment. At the same time, such size-dependent properties offer the possibility of diverse technological applications. Whether we are speaking of health and environmental risk or technological use - to know the size of the nanoparticles precisely is important in any case.

Therefore, PTB has developed a novel measuring method for nanoparticles. It unites the advantages of various types of electron microscopes: Scientists upgraded a scanning electron microscope (SEM) with a transmission detector. This upgrade is far more cost-saving than a transmission electron microscope (TEM). With the aid of the transmission detector, the particle boundaries can in many cases be represented more accurately than with a conventional SEM.

A problem with the highly accurate measurement of nanoparticles is the precise determination of the particle boundary which is "blurred" in electron microscopic images. With which grey scale value does the particle begin and which image pixel still belongs to the background? In order to be able to answer this question a simulation is carried out: A program developed at PTB calculates the detector signal for a particle of a determined size, for example 150 nm, and thereby takes into account the interactions of the electrons with the particle and the characteristics of the detector. Then a comparison is made. If the calculated signal agrees with the measured signal, then it is possible to make conclusions about the real size of the investigated particle from the simulation. If not, then the calculation is continued with another particle size, for example, 151 nm, until there is an agreement between the two signals.

The PTB scientists investigated representatives from the material classes of metals, ceramics and plastics and it was possible to show that the detector signal changed along with the material properties. Thus, the electrons interact, for example with the very dense gold differently than with latex, which is less dense. The customary approach, to use the same criterium for all particles for the data evaluation, regardless of which material it is and how large they are, thus has its weaknesses.

In order to take into account both the size as well as the material of the particles, PTB has developed an automatic evaluation. On the basis of the simulation results, it calculates for each individual particle an individual detector signal for the particle rim. This enables a precise size determination to be made, which is adjusted to the respective particle. In spite of this time-consuming procedure it is possible to evaluate several hundred micrographs in a few minutes. The PTB scientists have furthermore developed a method to be able to automatically take many nanoparticle pictures successively. Thus, they are now able to characterize a sample within one day by measuring and evaluating up to some thousand particles.

The novel PTB measuring method could contribute to the production of certified reference materials within the European Union. Reference materials serve to compare all measurements made Europe-wide with a defined standard. Only in this way is it possible to standardize measuring results of various laboratories.

Contact
Tobias Klein, PTB Working Group 4.22, Quantitative Microscopy,
Tel.: +49 531 592-4229,
e-mail: tobias.klein(at)ptb.de

Original publication:
E. Buhr, N. Senftleben, T. Klein, D. Bergmann, D. Gnieser, C.G. Frase, H. Bosse: Characterization of nanoparticles by scanning electron microscopy in transmission mode, Measurement Science and Technology, Vol. 20, 084025 (9p), 2009 A current publication is in progress