Case Study Challenges in Human Health Hazard and Risk Assessment of Nanoscale Silver - Safety of Nanomaterials along Their Lifecycle - page 428

19.7 CHALLENGES CAUSED BY LIMITATIONS IN
NANOPARTICLE MEASUREMENT TECHNIQUES
Methods to characterize nanomaterials and their limitations are discussed in detail
in Section I, Chapters 1-4 of this topic. Airborne particles can be measured with a
variety of instruments (Kuhlbusch et al. 2011). The instruments generally provide
qualitatively comparable measurements, each with its specific limitations (Asbach
et al. 2012; Kaminski et al. 2013). Most methods are useful to characterize the
virgin materials because background and contaminations cannot be differentiated
and nanosilver particles in a mixed exposure atmosphere cannot be distinguished
from other nanomaterials (Kuhlbusch et al. 2011). Currently, the aerosol mass spec-
trometer is the only instrument sizing and chemically analyzing nanoscale particles
online. However, this instrument does not detect metals such as nanosilver or metal
oxides. Similarly, light scattering methods to characterize nanoparticles in aque-
ous dispersions exhibit clear strengths and limitations, including that only simple
dispersions can be measured—subpopulations of different material or size cannot
be, or at least reliably, detected (Landsiedel et al. 2012; Roebben et al. 2011). Noble
metal particles such as silver nanoparticles exhibit plasmon resonance and can
be detected using characteristic UV-Vis spectra, in which the shift in the adsorp-
tion maximum acts as an indicator of particle size—a property that still remains
to be exploited for routing measurement of nanosilver (Hagendorfer et al. 2012).
An important consideration for lifecycle evaluations is also the limited range of
detected size for the different methods. In the case of partial homo- or heteroag-
glomeration of nanosilver, this might lead to incorrect measurement of total particle
numbers. Because of the size to mass relationship this can lead to severe over- or
underestimations in the dose metrics. Hence, a combination of methods is recom-
mended to characterize the material (Linsinger et al. 2012). For the detection of
the material in the environment, or bodies or organs, measurement of total silver
is usually possible. However, detection of the physical state, that is, the presence of
nanoparticles is extremely difficult. The method of choice for the identification of
nanomaterials in situ , for example, in organs of animals from a toxicokinetic study
or products presumably containing nanosilver, is electron microscopy coupled with
single particle chemical analysis such as EDX. However, taking into account the
aforementioned plasticity of silver within organisms and the environment, detected
electron-dense particles do not necessarily represent the exogenously introduced
nanoparticles. Silver ions form nanoscale Ag 2 S and Ag 2 Se deposits, and might be
derived from other sources and/or the introduced nanoparticles (Liu et al. 2012).
Moreover, electron microscopy requires extensive sample preparation, which is
not always compatible with the specimen, although some methods in development
allow for minimally processed samples (Dudkiewicz et al. 2011). Often, the distri-
bution or dilution is so high that the likelihood of finding a nanoparticle for instance
in an organ using electron microscopy after ultrathin slicing is extremely small
and defies quantification. Conversely, due to these limitations current technology
may not allow for a definitive proof of absence of nanomaterials, even if automated
particle identification software is used (Kuhlbusch et al. 2011).