Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 233

6.2.6 Microscopic Methods

Microscopic methods enable the investigation of physical and chemical properties
at the level of individual particles as well as aggregates thereof. Due to the limited
resolution of light microscopes, which is in the order of 0.1

m (lambda/2 for uv
light), individual NPs cannot be detected using conventional light microscopes. In
the following discussion the focus is therefore on electron and atomic force micro-
scopes. The extreme wealth of information that can be obtained on the individual
particle level, however, is very time consuming. Thus, the number of particles that
can be investigated is rather limited and also depends on the kind of information
desired. Pure morphological information used to derive size distributions can be
obtained by combination with image analysis tools. In this way, several microscopic
images can be processed semi-automatically, resulting in several hundreds to thou-
sands of particles that are evaluated (Mavrocordatos et al. , 2000 ). If, however,
chemical or crystallographic information is required, the number of particles that
can be analysed within a reasonable time is reduced to a few tens. Microscopic
investigations, therefore, are carried out on a very small subset of the particle
ensemble; thus, they should be used in combination with other methods that can
be performed on bulk samples and also deliver information on bulk properties of
the sample. However, engineered NPs in the aqueous environment might occur
only in very low amounts compared to the naturally occurring colloids, which ques-
tions the signifi cance of data derived from bulk methods. On the other hand, it
might be very challenging, if possible at all, to detect enough NPs within an over-
whelming number of natural colloids of comparable size, to derived statistically
meaningful results (Vigneau et al. , 2000 ).

µ

6.2.6.1 Electron Microscopy

Using electrons instead of light increases the resolution power of microscopes by
several orders of magnitudes and enables the investigation of materials at the
atomic level. There are two major electron microscope families: The scanning
(SEM) and the transmission electron microscope (TEM) (Figure 6.10). It is possible
to equip the TEM with a scanning unit, which then leads to a STEM (scanning
transmission electron microscope), and more recently also TEM detectors are avail-
able for the SEM. The interaction of the electron beam with the specimen produces
a variety of signals that can be used to obtain information such as chemical com-
position, morphology and structure of the specimen. Although the interaction of
the electrons with matter is independent of the microscope, different sample prop-
erties (such as thickness) and different operational conditions of the microscopes
(acceleration voltage) constrain the possible signals that can be detected. In the
following, both SEM and TEM techniques are briefl y reviewed, with a special focus
on the particle characterization (both engineered and natural).
The immense number of articles dealing with the analysis of nanosized materials
documents the potential of the electron microscopic methods but at the same time
renders an exhaustive literature review very challenging. Therefore, the focus here
is on a selection of important articles, whereby the selection of the articles surely
refl ects in part the author's view. Due to the rapid dilution of engineered NPs in