Biomedical Engineering Reference
In-Depth Information
issue of CNTs is the removal of catalytic metals from the production pro-
cess. Otherwise, toxicity assessment may reflect toxic effects of by-products
or residues of CNT synthesis rather than of the CNT material.* On the other
hand, any postsynthesis treatment may alter the material characteristics of a
sample, especially the CNT sample's length distribution and surface reactivity.
Thus, purification must be balanced against modification.
Another challenge is the dispersion of CNT samples for
in vivo
or
in vitro
toxicological assessment. Research on fundamental interactions of CNTs and
cell or organs may require fully dispersed (high purity) CNTs. Overall toxic-
ity assessment may require more realistic exposition scenarios using unpuri-
fied, as-produced samples with dispersion degrees adapted to the emission
source.
Next, an appropriate choice of characterization methods must be made in
order to cover the following metrics necessary to determine relevant physi-
cochemical properties of CNTs: size (aspect ratio, distribution), shape, sur-
face area (area/mass ratio), composition (element analysis), surface chemistry
(functionalization), crystallinity, agglomeration (dispersion), porosity, het-
erogeneity, stability (thermal), impurities, defects, solubility, chirality, and
conductivity. The selection of techniques should consider the fact that previ-
ously acquired data on CNT material properties must be comparable at least
with reference to the chosen technique. Minimal requirements for character-
ization and the commonly used methods should be extracted from the litera-
ture, e.g. [30,31]; however, other classes of nanoparticles may require different
sets of techniques. In particular, Raman spectroscopy, SEM, and TEM can be
considered as base techniques for CNT characterization. Additionally, TGA,
UV-Vis/NIR, and XPS are necessary assets for a complete characterization.
Electron diffraction and EDX are usually included in advanced TEM devices
and may thus be available for elemental analysis without additional instru-
mentation. Local probe techniques that characterize CNTs on an individual
basis, such as AFM, STM, and STS, appear not suitable for routine character-
ization of CNTs and thus are not considered in this selection.
Last, not least, comparative assessment of CNT standard materials, whenever
available, will set a nanotoxicological study into a broader context, allow com-
parability of results, and promote the understanding of observed effects [32].
2.6 Conclusion and Perspectives
The characterization of nanoparticles for toxicological testing is an essen-
tial but complex task. Owing to the manifold characteristics of nanoparticles
*
Frustrated phagocytosis in macrophages resulting from the physical CNT characteristics [8]
must be distinguished from chemical effects [9].
