Biomedical Engineering Reference
In-Depth Information
x-ray scattering (SAXS) can provide first ensemble-averaged indications of
deviations from spherical shape and initiate subsequent shape inspection by
microscopic techniques. Other methods for particle size or mass determina-
tion include static light scattering, differential mobility analysis, and mass
spectrometric techniques such as time-of-flight mass spectrometry (ToF-MS,
MALDI-ToF-MS, ESI-ToF-MS) and asymmetric flow-field flow fractionation
(AF4). The shape and size of individual nanoparticles can be analyzed by
scanning probe microscopic techniques (SPM) such as scanning tunneling
microscopy (STM) and atomic force microscopy (AFM), as well as electron
beam-based microscopic techniques such as scanning electron microscopy
(SEM) or TEM. The latter also allows investigation of individual crystallite
structures, whereas x-ray diffraction (XRD) can determine crystallographic
data of particle ensembles.
2.2.1.2 Particle Composition and Impurities
The chemical composition of a nanomaterial is of utmost importance, in par-
ticular for any toxicological assessment. Individual nanoparticles may be
chemically analyzed by SEM or scanning transmission electron microscopy
(STEM) paired with energy dispersive x-ray analysis (SEM/EDX, STEM/
EDX). Common and important methods for the characterization of the ele-
mentary composition of nanoparticle ensembles are inductively coupled
plasma mass spectrometry (ICP-MS) and ICP optical emission spectrometry
(ICP-OES), atomic absorption spectrometry (AAS),* x-ray fluorescence analy-
sis (XFA), x-ray photoelectron spectroscopy (XPS), and other less frequently
used elemental analysis techniques.
For nanoparticles, the release rate of soluble material not only depends on
the type, composition, and pH value of the immersing medium, but will also
show significant particle size dependence. In addition, particle solubility is
strongly related to its surface chemistry, as discussed below. For a proper
toxicity assessment, the surface properties of persistent nanoparticles and
the release rate of soluble toxic nanoparticles need to be studied in detail.
Special care is further required for the characterization of soluble impurities,
which may have toxic effects that must not be falsely attributed to the par-
ent nanoparticle. An example is the observed toxicity of catalyst metals in
chemically inert carbonaceous materials [9].
2.2.1.3 Surface Composition and Chemistry
The surface of a nanoparticle governs all particle-medium interactions.
Considering their high specific surface area, nanoparticles require devoting
*
Using methods such as ICP-MS, ICP-OES and AAS, the samples with the analyte will be
atomized before measurement, limiting the obtained data exclusively to the elementary com-
position of the sample.
