Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 243

investigating environmental colloids. In this technique, the image is formed only
from very high angle, incoherently scattered electrons. The signal shows a strong
dependence on the atomic weight of the specimen and the resulting images are
therefore also referred to as z-contrast images. In addition, the specimen thickness
and the acceleration voltage also infl uence the signal intensity. The thicker the speci-
men and the lower the acceleration voltage, the more the electrons are scattered at
high angles and thus the higher the signal intensity. Applied to the analysis of (engi-
neered) NPs this means that particles of comparable size will appear brighter the
heavier they are, which makes the quest for metal particles much easier. Utunomiya
and Ewing (2003) outline the potential of the technique for the analysis of NPs in
the environment in general. Using this technique, nanoscale particles in rock samples
(Utsunomiya and Ewing, 2003) and within ambient fi ne particles (Utsunomiya et al. ,
2002, 2004) were discovered. In an experimental study Morones et al. (2005) local-
ized individual silver NPs inside E. coli using the HAADF technique.

Sample preparation. In conventional SEM technology, samples have to be electri-
cally conductive to avoid charging of the sample due to the electron bombardment.
This requires that non-conductive samples are coated with a conducting layer, such
as carbon or platinum. Newer SEMs can be operated at low acceleration voltages
(a few kV or less), which also allows investigation of non-conductive samples
without coating. In addition, SEMs that can be operated at low vacuum conditions
(including the ESEM technology) do not require conductive samples anymore.
Thus, depending on the capabilities of the respective SEM, the samples can be
directly investigated without any treatment or need to be coated with a conductive
layer.
However, for TEM investigations, a proper sample preparation procedure is of
the highest importance and often determines the quality of the analysis obtained
afterwards. Therefore, the sample preparation procedure critically depends on the
goal of the respective investigation. Generally, the preparation methods developed
in materials science and biology can be used with slight modifi cations. A special
technique for the investigation of organic materials has been developed and opti-
mized (Leppard, 1995; Santschi et al. , 1998 ; Wilkinson et al. , 1999 ; Perret et al. , 1991 ;
Mavrocordatos and Perret, 1995; Lienemann et al. , 1998 ). To minimize artefacts
related to the drying process in the ultrahigh vacuum in the TEM these authors
strongly recommend the use of ' nanoplast ' , a non - plastifi ed melamine resin. Other
issues such as optimized centrifugation deposition and optimum coverage of the
sample grids are well reviewed by Mavrocordatos et al. (2007) .

6.2.6.2 Atomic Force Microscopy and Other Scanning Probe Microscopy

Atomic force microscopy (AFM) is a scanning probe microscope (SPM) method
that uses an oscillating nanometre structured fi ne tip in the end of a cantilever that
is being scanned over the surface of the sample substrate. Both the oscillating
movement of the cantilever (z axis) and the scanning over the surface (x axis and
y axis) are controlled by piezo-electric actuators. The oscillating movements of the
cantilever are measured by correlating the defl ection of a laser refl ection that falls
onto a quadrant photo diode (Figure 6.16). The laser-balance is very sensitive to