Biomedical Engineering Reference
In-Depth Information
crystallinity, size, shape, diameter, length, layer number, chirality, surface
chemistry, specific surface area, to mention only a few. However, to compile
such a complete atomic particle data set for a disperse many-particle ensem-
ble is practically impossible for most materials and contemporary analytical
techniques.
Analytical techniques for nanoparticles can be classified into individual
(single) particle analysis (IPA) and ensemble-averaging (integral) analysis
(EAA) techniques. They derive material information of nanoparticle ensem-
bles in two principally different ways.
Individual particle analysis techniques rely on small random subsamples
of an ensemble, consisting of a few particles only. These are prepared and
analyzed in detail on a particle-by-particle basis. Individual properties of
the observed particles are then compiled into distributions. However, this
approach may produce unreliable property distributions if laborious analy-
sis impedes characterization of a sufficiently large number of particles; that
is, lacks an adequate amount of statistical significance. In addition, subsam-
ple preparation artifacts may affect the resulting property distributions; for
instance, if large particle agglomerates are filtered off the ensemble or are
omitted during microscopic analysis.
Ensemble-averaging techniques, on the other hand, allow determination of
material property distributions of large ensembles. This may help improve
statistical significance of ensemble averages. Because of their integral, non-
microscopic character, however, they may fail to detect unexpected morpho-
logical features of individual particles or particle subsets of the ensemble. For
instance, they may fail to unambiguously distinguish porous large particles
from similar-sized small particle aggregates. For a complete and meaningful
analysis, which should be the basis of any nanotoxicological study aiming
at reliable and traceable results, a combination of both approaches is recom-
mended here, in order to avoid systematic misinterpretation of ensemble-
averaging results owing to unexpected individual particle features or shapes.
2.2.1 Primary Nanoparticles
The characterization of nanoparticulate matter requires answering basic
questions on its nature. First, whether it is composed of primary particles or
of so-called secondary particles, which are themselves composed of several
primary particles, called agglomerates or aggregates, as it will be discussed
in Section 2.2.2. The second important question is that of primary particle
size, which is closely related to particle shape and is subject of the following
subsections.
2.2.1.1 Particle Structure, Shape, and Size
Only ensembles of nonagglomerated compact spherical particles allow a
complete
morphological description solely on the basis of a particle diameter
