2.2 Morphology
Recent developments in synthesizing NPs can enable preparation of CNPs with
kinds of morphologies including spheres, rods, cubes, wires, octahedrons, and
polygon (Chen and Stephen Inbaraj 2018; Dowding et al. 2013). More specifically,
by adopting hydrothermal synthesis methods, [Ce(NO 3 ) 3 Á4H 2 O] as a precursor
yielded coarse particles, whereas cerium(IV) salts [Ce(SO 4 ) 2 Á4H 2 O and Ce
(NH 4 ) 4 (SO 4 ) 2 Á2H 2 O] produced fine powders (Hirano and Kato 1996). Commonly,
nanoparticle size and morphology are responsible for their transformation and
translocation in biological system, which is highly related to their toxicity and the
biological response (Gatoo et al. 2014; Tong et al. 2013; Zhang et al. 2017). It has
been widely accepted that CNPs with higher aspect ratios (including tube, belt, rod,
and wire vs polyhedron) lead to more severe cytotoxicity to cells (Forest et al. 2016;
Li et al. 2014; Lu et al. 2016). Forest et al. (2016) reported that rodlike CNPs could
increase the toxicity in macrophages from RAW264.7 cell line, while the cubic and
octahedral CNPs exhibited no such effects. Moreover, in cellular environment,
CNPs were reported to transform from sphere into urchin-shaped structures and
set in motion a series of events triggering cell lysis (Li et al. 2014). Nevertheless,
another study suggested that compared with nanorods, nanowire bundles and
nanocubes with sharp edges and corners in their crystal structure may induce more
mechanical damage to cell membrane (Ji et al. 2012). They could lead to misbalance
of the ionic concentration as well as redox state inside and outside the cell. Thereby,
it is speculated that the morphological biotransformation of CNPs is of key significance and more difficult to depict in their toxicological effects.
The important role of morphology in the intrinsic chemical reactivity of CNPs is
an additional factor that must be considered. Zhang et al. compared the uptake and
transformation of octahedral (O-CNPs), cubic (C-CNPs), rod (R-CNPs), and irregularly shaped CNPs (I-CNPs) in cucumber plants. They found that the reactivity of
CNPs decreased in the order of R-CNPs > I-CNPs > C-CNPs > O-CNPs, which
brought the efficient accumulation, translocation, and transformation of R-CNPs in
the plants (Zhang et al. 2017). Dowding et al. also proposed that CNPs with
morphologies of spherical (S-CNPs, 5–8 nm) and polygonal (P-CNPs, 8–10 nm)
did not take part in the decreased cell viability in 48 h, whereas S-CNPs are more
active than P-CNPs toward surface oxygen release (Dowding et al. 2013). Taken
together, the shape of CNPs determines their surface reactivity, thus impacting their
environmental behavior and also the organisms-CNPs interaction, which is believed
to underlie their toxicity effects in the chronic exposure environment (Thill et al.
2006; van Hoecke et al. 2009).
As a general rule, the smaller the size and the larger the aspect ratio of CNPs
would result in the greater chance for the cells to uptake. Additionally, CNPs with
specific morphology can mechanically damage cells because of their sharp edges and
redox reaction activity. Thus, the effect of size and morphology of CNPs on toxicity
does not seem to provide uniform results, since the agglomeration characteristics, the
properties of CNPs, the cell type, and also culture environment may lead to prominent variation in the outcome of size- and morphology-dependent studies. The
observed inconsistency requires more systematic and in-depth investigation.
170
G. You et al.
Recent developments in synthesizing NPs can enable preparation of CNPs with
kinds of morphologies including spheres, rods, cubes, wires, octahedrons, and
polygon (Chen and Stephen Inbaraj 2018; Dowding et al. 2013). More specifically,
by adopting hydrothermal synthesis methods, [Ce(NO 3 ) 3 Á4H 2 O] as a precursor
yielded coarse particles, whereas cerium(IV) salts [Ce(SO 4 ) 2 Á4H 2 O and Ce
(NH 4 ) 4 (SO 4 ) 2 Á2H 2 O] produced fine powders (Hirano and Kato 1996). Commonly,
nanoparticle size and morphology are responsible for their transformation and
translocation in biological system, which is highly related to their toxicity and the
biological response (Gatoo et al. 2014; Tong et al. 2013; Zhang et al. 2017). It has
been widely accepted that CNPs with higher aspect ratios (including tube, belt, rod,
and wire vs polyhedron) lead to more severe cytotoxicity to cells (Forest et al. 2016;
Li et al. 2014; Lu et al. 2016). Forest et al. (2016) reported that rodlike CNPs could
increase the toxicity in macrophages from RAW264.7 cell line, while the cubic and
octahedral CNPs exhibited no such effects. Moreover, in cellular environment,
CNPs were reported to transform from sphere into urchin-shaped structures and
set in motion a series of events triggering cell lysis (Li et al. 2014). Nevertheless,
another study suggested that compared with nanorods, nanowire bundles and
nanocubes with sharp edges and corners in their crystal structure may induce more
mechanical damage to cell membrane (Ji et al. 2012). They could lead to misbalance
of the ionic concentration as well as redox state inside and outside the cell. Thereby,
it is speculated that the morphological biotransformation of CNPs is of key significance and more difficult to depict in their toxicological effects.
The important role of morphology in the intrinsic chemical reactivity of CNPs is
an additional factor that must be considered. Zhang et al. compared the uptake and
transformation of octahedral (O-CNPs), cubic (C-CNPs), rod (R-CNPs), and irregularly shaped CNPs (I-CNPs) in cucumber plants. They found that the reactivity of
CNPs decreased in the order of R-CNPs > I-CNPs > C-CNPs > O-CNPs, which
brought the efficient accumulation, translocation, and transformation of R-CNPs in
the plants (Zhang et al. 2017). Dowding et al. also proposed that CNPs with
morphologies of spherical (S-CNPs, 5–8 nm) and polygonal (P-CNPs, 8–10 nm)
did not take part in the decreased cell viability in 48 h, whereas S-CNPs are more
active than P-CNPs toward surface oxygen release (Dowding et al. 2013). Taken
together, the shape of CNPs determines their surface reactivity, thus impacting their
environmental behavior and also the organisms-CNPs interaction, which is believed
to underlie their toxicity effects in the chronic exposure environment (Thill et al.
2006; van Hoecke et al. 2009).
As a general rule, the smaller the size and the larger the aspect ratio of CNPs
would result in the greater chance for the cells to uptake. Additionally, CNPs with
specific morphology can mechanically damage cells because of their sharp edges and
redox reaction activity. Thus, the effect of size and morphology of CNPs on toxicity
does not seem to provide uniform results, since the agglomeration characteristics, the
properties of CNPs, the cell type, and also culture environment may lead to prominent variation in the outcome of size- and morphology-dependent studies. The
observed inconsistency requires more systematic and in-depth investigation.
170
G. You et al.