Biomedical Engineering Reference
In-Depth Information
distribution. For ensembles containing nonspherical, polydispersed, or par-
tially agglomerated particles, more descriptive parameters and their accord-
ing distributions have to be determined for a comprehensive morphological
characterization. For example, for nonspherical particles such as nanorods and
nanoplates, more than one distribution is required for a complete ensemble
description, for example, distributions for diameter and length, or thickness
and two lateral extensions. Sometimes particle symmetries allow for a simpli-
fied description. For hierarchically constructed objects such as multiwalled
nanotubes, for instance, data on the number of walls and their according chi-
rality provides complex structure information in a compact, numeric form.
Achieving a complete description of a nanoparticle ensemble is only pos-
sible with data from further parameter distributions, including porosity, pore
size, defect density, chirality, crystallinity, and surface-crystallographic orien-
tations. However, the dominance of surface atoms of unknown binding state
may prohibit efficient information reduction, for instance by crystallographic
classification of nanoscale crystallites. Already the structure description of a
nanoparticle ensemble requires a large number of parameter distributions
and thus significantly complicates the nanotoxicological identification of
structure-effect relations. Further, the characterization of the chemical com-
position, as described below, will add additional material descriptors.
To reduce the necessary information for structure description, and since
many nonmicroscopic characterization techniques are unable to mea-
sure shape parameters, nanoparticles are frequently
assumed
to be spheri-
cal and then described by
equivalent
diameters. Such equivalent diameters
correspond to spherical particles showing the same behavior as the (pos-
sibly) nonspherical particles under investigation. For instance, gas mobil-
ity analysis determines aerodynamic diameters, centrifugation determines
sedimentation diameters, dynamic light scattering (DLS), and nanoparticle
tracking analysis (NTA) determines hydrodynamic diameters, whereas
BET (Brunauer-Emmett-Teller) analysis allows for the analysis of porosity
and specific surface area that can be used to derive an averaged diameter
(per weight) for nonporous spherical model particles. However, even for
truly spherical particles, these
apparent
diameters do not necessarily cor-
respond to the geometric particle diameter and must be treated with care.
The aerodynamic diameter, for instance, depends on the orientation of the
particles during flight, whereas the hydrodynamic diameter is affected by a
pH-dependent ion sheath, which may surround surface-charged particles.
The reduced information content provided by equivalent particle diameters
must be kept in mind if particle size data are used for particle ensemble
characterization. Any research on structure-toxicity relations must therefore
comprise particle size assessment and particle shape analysis.* Small-angle
*
For elongated particles, for example, the extension of the particle in one dimension may
exceed a critical threshold and could render it into a toxic fiber-like particle that may cause
frustrated phagocytosis [8].
