High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 230

10.2 CHARACTERIZATION

10.2.1 c Hallenges to tHe c Haracterization oF H arn
In general, challenges that exist with the characterization of nanoparticles also exist with HARN,
which will ultimately have a big impact on the success of the analysis and the end results obtained.
It is reasonable to assume that such challenges can be magniied when it comes to HARN because
of the higher aspect ratio property compared to other nanoparticles. The high aspect ratio property
implies a much increased surface area, which will mean increase in surface interaction and activity,
compared to other nanoparticles.
There are boundaries as to what we classify as ibers that are to be considered an inhalation
hazard. The World Health Organization (WHO) deines a iber as a particle longer than 5 μm, less
than 3 μm in width and having an aspect ratio of >3:1 (WHO/EURO Technical Committee for
Monitoring and Evaluating MMMF, 1985; WHO, 1997). This is not a health-based criterion but the
one based on a practical deinition of a respirable iber.
Sample preparation is one of the most critical steps toward successful characterization of
nanoparticles, in which there are many variables to consider when designing a method for
preparation. The irst step to consider in sample preparation is the need to have “reliable” sampling,
such that “sample collected from bulk represents the physical and chemical characteristics of the
entire sample” (NIST 960-1, 2001). Most nanoparticles are received in the powder form or in liquid
form, that is, the form of a stable suspension. Powder sampling is more dificult, as there is a
natural tendency for nanoparticles to aggregate and unlike in solution phase, it is more dificult to
control surface charges on particles; some general guidelines on powder sampling can be found in
Allen (2004a,b). The next steps in the sample preparation will be governed by the requirements of
individual methods, which may require specialized conditions for measurement; each will represent
their own unique challenges. Ideally, samples for analysis should be free from (1) the inherent
aggregation problems associated with nanoparticles and (2) other contaminants not associated with
the nanoparticles being characterized. However, to achieve such goals is not trivial. For example, to
successfully disperse nanoparticles in a liquid media, sonication methods often need to be employed.
However, this has the potential to change the size distribution of the HARN and introduce defects
(e.g., Islam et al., 2001). Furthermore, the stability of dispersions over time (Vaisman et al., 2006)
also inluences the outcome of the analysis as does the “state” of the sample required for analysis,
that is, whether the nanoparticles should be ixed onto a solid substrate, suspended in liquid media,
or aerosolized (solid or liquid aerosols). Last, some surface techniques (such as conventional electron
microscopy, Brunauer, Emmert, and Teller [BET], and x-ray photoelectron spectroscopy [XPS]) are
not applicable to the analysis of samples dispersed in liquids.
Characterization of nanomaterials can be complex, as there are many different material
attributes that need to be considered. The need to characterize nanoparticles for toxicological
evaluation is made even more complicated by the need to analyze as close as possible to the
“as-dosed” form, that is, to what is required under the toxicological investigation. This is not
easy as (1) quantities used in the analysis are normally smaller and (2) the state of particles is
likely to change under the conditions of the analysis. Overall, it is the experimental conditions
used in toxicological studies, which will eventually determine the choice of techniques used
for characterization. For example, an inhalation study may require the need to characterize the
nanoparticles in dry powder aerosol form.
The main challenge is to identify those techniques deemed to be most “suitable” for characteriza-
tion. This overview focuses on those techniques that are well established or commercially available
analytical techniques, and those techniques that yield chemical/physical property information that
have been linked/hypothesized in some way to toxicological activities. The techniques highlighted
fall into two categories: imaging (high resolution, with the capability of probing individual HARN)
and nonimaging techniques (often involves the measurement of a collection/ensemble of HARN).