High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 231

100 nm

2000 nm

FIGURE 10.1 TEM image shows CNTs (dark areas) within a cell nucleus.

10.2.2 i Maging t ecHniques
This group of techniques is extremely advantageous in that they have potential to yield highly
resolved images, which allows direct visualization of nanoparticles and thus will yield information
on size and shape. It is a general consensus that properties related to the three-dimensional structure
of nanoparticles have a profound inluence on their toxicity and the small size has the potential to
enter cells and ultimately cause cell death (Porter et al., 2007), as shown in Figure 10.1.
The ability to provide images on the nanoscale, with sensitivity that reaches the individual
nanoparticle level, is a powerful tool used in the study of cellular interactions of nanomaterials.
Imaging techniques have been widely used for investigating structure and morphology of
nanoparticles. The amount of detail present in the image will ultimately be dependent on the
instrument's spatial resolution; a high-resolution microscope, for example, will result in images
that can reveal very small defects or anomalies. Although these tools are extremely useful, they all
share the same disadvantages, in that they do not have a wide ield of view, are relatively expensive
techniques (to purchase and maintain), require the need of specially trained analysts, and have no
potential for automation.

10.2.2.1 Electron-Based Microscopies
These tools are able to produce highly magniied, resolved images of objects with a much greater
depth of ield in comparison to conventional optical microscopes (Egerton, 2005). Scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) are the two most
common techniques for nanoparticle characterization. SEM creates an image by scanning a tightly
focused electron beam over the sample and detecting the secondary electrons from the sample onto
the screen; each point on the screen will then correspond to a pixel (picture element). TEM, on
the other hand, forms an image using a system of lenses. Unlike SEM, the electron beam passes
entirely through the sample and is subsequently collected to appear on a screen, generating a
“transmission” electron image (Reimer, 1993). There are advantages and disadvantages associated
with both techniques. TEM, for example, has a far greater spatial resolution than SEM, but suffers
from lengthy, time-consuming sample preparation. One dificulty is getting the specimen sample
thin enough for analysis using TEM. Another disadvantage with TEM is the need for a very intense
electron beam (with energy in the range ∼200 to 300 keV) (Kiang et al., 1996) compared to SEM
(∼20 keV); this may pose some challenges to the structural and thermal stability of nanoparticles
during analysis when under the inluence of high-energy electron irradiation.