Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 272

enced by factors including the type of organism (e.g. trophic level, uni- or
multicellular, aquatic or terrestrial) and the interactions of a particular organism
with its environment. For terrestrial organisms, exposure routes will be much the
same as those studied in human toxicology, namely via inhalation, ingestion or
dermal absorption (Oberdorster et al. , 2005). Exposure to atmospheric contami-
nants is covered in the area of environmental toxicology and usually relies on
human exposure data. In aquatic animals, however, uptake across the gill and other
external surface epithelia is also possible, and interactions with plants may include
adsorption onto the root surface, incorporation into the cell wall or diffusion into
the intercellular space (Nowack and Bucheli, 2007). The possible mechanisms by
which nanoparticles may interact with biological systems are illustrated schemati-
cally in Figure 7.7 .
At the cellular level, most internalisation of nanoparticles by eukaryotic organ-
isms will occur via endocytosis (Moore, 2006). Endocytosis is a process by which
particulate material may enter a cell without passing through the cell membrane.
The membrane folds around material outside the cell, resulting in the formation of
a sac-like vesicle into which the material is incorporated. Bacteria are not able to
endocytose and there are three possible mechanisms through which nanoparticles

UV

O 2

Electron-donor/acceptor
active groups

O 2 -

Example:

O 2

HO OH

C

e -

C R

Material
composition, e.g.,
discontinuous crystal
planes and defects,
generating active
electonic conligurations

O

h +
UV activation of
electron hole pairs
leading to bond
splitting and radical
formation
Media interactions
by particle dissolution,
coating, passivation
and hydrophobicity/
hydrophilicity

e -

O 2 -

HO

C R

O

e -

Redox cycling and
catalytic chemistry
via coating metals
(e.g., Fe)
and organics
(e.g., quinones)

Dissolution

Q -

Q

Coating may protect the
surface, change cellular
uptake or could lead to
release 01 toxic chemicals

Fe ++

O 2

O 2 -

Passivation

Q = quinone
Q - = semiquinone

H 2 O 2

H 2 O

H 2 O

Hydrophobicity → interactions
with cell membranes, determining uptake
Hydrophilicity → water suspendability

Fenton chemistry

OH

Figure 7.7 Possible mechanisms by which nanoparticles may interact with biological
systems. (From A. Nel, T. Xia, L. Madler and N. Li, Toxic potential of materials at the nano-
level, Science , 311 , 2006, 622-7. Reprinted with permission from AAAS.)