Needless to say, most of the regular paths for entering into cells are not permitting
the entry of nanoparticles. One exception here may be endocytosis.
Yet, some nanoparticles find their way through cell walls and membranes, and
end up inside cells. Here they represent quite a different type of toxicity than
dissolved ions of the same elements, partly because they constitute discrete particles
rather than diffusive ions that may spread among cells. A particle is likely to stay
inside the cell it has entered and end up in lysosomes (in the case of eukaryotic cells),
but may, e.g., dissolve and represent a steady source of dissolved ions that may be
harmful. The concentration of such ions is likely to be substantially higher in a cell
containing a nanoparticle than in a cell being exposed (along with neighboring cells)
to a similar level of dissolved elements that may move more or less freely within the
exposed tissue.
Nanoparticle toxicity mechanisms include effects of dissolved ions from metallic
nanoparticles (common for Ag nanoparticles that release Ag
+
), induction of reactive
oxygen species (ROS and other types of oxidative stress; such effects have been
shown for iron-based nanoparticles (Lewinski et al. 2008)) and damages directly
related to the surface and shape of nanoparticles (e.g., nanotubes and their asbestoslike induction of damages on cells). For Fe-based nanoparticles, only mechanisms
related to ROS-formation and oxidative stress have been described as direct effects.
These have been reviewed recently (Lei et al. 2018), and will not be detailed further
here. Indirect effects on O 2 availability is another mechanism, but as we argue
below, this is not a nanoparticle-specific toxic effect.
28.2.1 Ageing and Other Time-Dependent Modifications
of Toxicity
A particular aspect of nanoparticle toxicity testing that is relevant for nano-sized
zero-valent iron (nZVI) and other nanoparticles to be used for remediation is
reduction in toxicity over time. While all nanoparticles are subject to changes in
surface properties as a result of interactions with environmental matrices,
nanoparticles for remediation are frequently designed to lose their reactivity by
interacting with environmental pollutants. Including temporal changes in toxicity
during testing is therefore a particularly relevant aspect that should be assessed for
such materials. Reduced toxicity as a result of ageing in soil has indeed been
demonstrated for nZVI aged for 30 days in soil, using growth (body weight) of
earthworms as an endpoint (El-Temsah and Joner 2012a) and partly for rice after
2–4 weeks ageing (Wang et al. 2016). In the natural environment, living organisms
will mostly be exposed to aged nZVI, and not to pristine particles. This is important
to keep in mind when designing toxicity studies. Not only may such tests show that
adverse effects of the nanoparticles are short-lived, but it may also be helpful in
designing nanoparticles for remediation as toxicity and reactivity against pollutants
are likely to be strongly linked (Hjorth et al. 2017).
28 Ecotoxicity of Nanomaterials Used for Remediation
575
the entry of nanoparticles. One exception here may be endocytosis.
Yet, some nanoparticles find their way through cell walls and membranes, and
end up inside cells. Here they represent quite a different type of toxicity than
dissolved ions of the same elements, partly because they constitute discrete particles
rather than diffusive ions that may spread among cells. A particle is likely to stay
inside the cell it has entered and end up in lysosomes (in the case of eukaryotic cells),
but may, e.g., dissolve and represent a steady source of dissolved ions that may be
harmful. The concentration of such ions is likely to be substantially higher in a cell
containing a nanoparticle than in a cell being exposed (along with neighboring cells)
to a similar level of dissolved elements that may move more or less freely within the
exposed tissue.
Nanoparticle toxicity mechanisms include effects of dissolved ions from metallic
nanoparticles (common for Ag nanoparticles that release Ag
+
), induction of reactive
oxygen species (ROS and other types of oxidative stress; such effects have been
shown for iron-based nanoparticles (Lewinski et al. 2008)) and damages directly
related to the surface and shape of nanoparticles (e.g., nanotubes and their asbestoslike induction of damages on cells). For Fe-based nanoparticles, only mechanisms
related to ROS-formation and oxidative stress have been described as direct effects.
These have been reviewed recently (Lei et al. 2018), and will not be detailed further
here. Indirect effects on O 2 availability is another mechanism, but as we argue
below, this is not a nanoparticle-specific toxic effect.
28.2.1 Ageing and Other Time-Dependent Modifications
of Toxicity
A particular aspect of nanoparticle toxicity testing that is relevant for nano-sized
zero-valent iron (nZVI) and other nanoparticles to be used for remediation is
reduction in toxicity over time. While all nanoparticles are subject to changes in
surface properties as a result of interactions with environmental matrices,
nanoparticles for remediation are frequently designed to lose their reactivity by
interacting with environmental pollutants. Including temporal changes in toxicity
during testing is therefore a particularly relevant aspect that should be assessed for
such materials. Reduced toxicity as a result of ageing in soil has indeed been
demonstrated for nZVI aged for 30 days in soil, using growth (body weight) of
earthworms as an endpoint (El-Temsah and Joner 2012a) and partly for rice after
2–4 weeks ageing (Wang et al. 2016). In the natural environment, living organisms
will mostly be exposed to aged nZVI, and not to pristine particles. This is important
to keep in mind when designing toxicity studies. Not only may such tests show that
adverse effects of the nanoparticles are short-lived, but it may also be helpful in
designing nanoparticles for remediation as toxicity and reactivity against pollutants
are likely to be strongly linked (Hjorth et al. 2017).
28 Ecotoxicity of Nanomaterials Used for Remediation
575