2.6 Surface Oxidation State and Oxygen Vacancy (V o
ÁÁ
)
The surface oxidation state of CNPs can vary depending on synthesis methods and
conditions. Accordingly, the synthesis process conducted at high temperatures yields
CNPs with lower Ce
3+ /Ce
4+ ratio and larger particle size (Chen and Stephen Inbaraj
2018). Different chemicals such as sodium hydroxide, ammonium hydroxide,
and hexamethylenetetramine used during synthesis could produce CNPs with
lower Ce
3+ /Ce
4+ ratio (Dowding et al. 2013). Besides, the reaction of CNPs with
elements such as zirconium (Zr) and platinum (Pt) was shown to lead to an increase
in surface Ce
3+ /Ce
4+ ratio (Zhang et al. 2006). Several trivalent metal ions such as
lanthanum, samarium, gadolinium, yttrium, and neodymium have been used as
dopants to increase oxygen vacancy concentration (Babu et al. 2009; Grulke et al.
2014; Patil et al. 2006; Shehata et al. 2014). With transforming between Ce(IV) and
Ce(III) and leaving V o
ÁÁ in the lattice (shown in Fig. 1), CNPs easily form
nonstoichiometric compositions of CeO 2–x , which makes CNPs acting as quenchers
or producers of ROS (Lu et al. 2016; Pulidoreyes et al. 2015). A correlation between
the size and lattice parameter of CNPs was established by Deshpande et al. (2005)
who claimed that the smaller the CNPs particle size, the higher the surface Ce(III)/Ce
(IV) ratio. In this regard, the ratio of Ce(III)/Ce(IV) at CNPs surface is the key to
understand their potential toxicity (Collin et al. 2017), as exhibited in Table 1.
As reported, the CNPs’ antioxidant and prooxidant properties are closely related
to the % surface Ce(III) values (Deshpande et al. 2005; Lu et al. 2016; Wu et al.
2018a, b). Lu et al. (2016) reported that at the same concentration of 10 μM, CNPs
(15–20 nm, 27.55% of Ce(III)) displayed an excellent antioxidant ability and thus an
obvious protection effect, whereas CNPs (5–10 nm, 30.74% of Ce(III)) behaved in
the opposite manner. Similarly, Pulidoreyes et al. (Pulidoreyes et al. 2015) found
that the % surface Ce(III) is the main driver of CNPs toxicity in the case of where
CNPs did not internalize in the alga. Through oxidative reactions, CNPs could
abiotically generate H 2 O 2 (Xia et al. 2008; Zhao et al. 2012a, 2012b), while Ce
(III) was reported to be able to redox-cycle with H 2 O 2 to form ROS such as hydroxyl
(
Á OH) (Seal 2008). A step forward understanding demonstrates that
Á OH and CNPs
can create a high amount of Ce(III), which could scavenge additional
Á
OH, ultimately
strengthening the antioxidant activity of CNPs (Deshpande et al. 2005; Lee et al.
2013). However, when the quantity of Ce(III) reaches a certain level, CNPs convert
their antioxidant activity to oxidant activity (Lu et al. 2016), which can partially
explain the contradictory results in medicinal applications and toxicological
research.
. Detailed information about the ROS generation is reviewed in Sect. 5.
As to the V o
ÁÁ , they are thought to make CNPs more effective at generating Ce(III)
than the equivalent bulk material and act as “active center” of various redox
reactions exhibited by CNPs (Lee et al. 2013). On the other side, V o
ÁÁ can significantly alter biological interactions and allocate oxygen moieties from biological
molecules, inhibiting a set of biological antioxidant effects and inducing toxicity
response (Gupta et al. 2016). Celardo et al. (2011) demonstrated that V o
ÁÁ did not
174
G. You et al.
ÁÁ
)
The surface oxidation state of CNPs can vary depending on synthesis methods and
conditions. Accordingly, the synthesis process conducted at high temperatures yields
CNPs with lower Ce
3+ /Ce
4+ ratio and larger particle size (Chen and Stephen Inbaraj
2018). Different chemicals such as sodium hydroxide, ammonium hydroxide,
and hexamethylenetetramine used during synthesis could produce CNPs with
lower Ce
3+ /Ce
4+ ratio (Dowding et al. 2013). Besides, the reaction of CNPs with
elements such as zirconium (Zr) and platinum (Pt) was shown to lead to an increase
in surface Ce
3+ /Ce
4+ ratio (Zhang et al. 2006). Several trivalent metal ions such as
lanthanum, samarium, gadolinium, yttrium, and neodymium have been used as
dopants to increase oxygen vacancy concentration (Babu et al. 2009; Grulke et al.
2014; Patil et al. 2006; Shehata et al. 2014). With transforming between Ce(IV) and
Ce(III) and leaving V o
ÁÁ in the lattice (shown in Fig. 1), CNPs easily form
nonstoichiometric compositions of CeO 2–x , which makes CNPs acting as quenchers
or producers of ROS (Lu et al. 2016; Pulidoreyes et al. 2015). A correlation between
the size and lattice parameter of CNPs was established by Deshpande et al. (2005)
who claimed that the smaller the CNPs particle size, the higher the surface Ce(III)/Ce
(IV) ratio. In this regard, the ratio of Ce(III)/Ce(IV) at CNPs surface is the key to
understand their potential toxicity (Collin et al. 2017), as exhibited in Table 1.
As reported, the CNPs’ antioxidant and prooxidant properties are closely related
to the % surface Ce(III) values (Deshpande et al. 2005; Lu et al. 2016; Wu et al.
2018a, b). Lu et al. (2016) reported that at the same concentration of 10 μM, CNPs
(15–20 nm, 27.55% of Ce(III)) displayed an excellent antioxidant ability and thus an
obvious protection effect, whereas CNPs (5–10 nm, 30.74% of Ce(III)) behaved in
the opposite manner. Similarly, Pulidoreyes et al. (Pulidoreyes et al. 2015) found
that the % surface Ce(III) is the main driver of CNPs toxicity in the case of where
CNPs did not internalize in the alga. Through oxidative reactions, CNPs could
abiotically generate H 2 O 2 (Xia et al. 2008; Zhao et al. 2012a, 2012b), while Ce
(III) was reported to be able to redox-cycle with H 2 O 2 to form ROS such as hydroxyl
(
Á OH) (Seal 2008). A step forward understanding demonstrates that
Á OH and CNPs
can create a high amount of Ce(III), which could scavenge additional
Á
OH, ultimately
strengthening the antioxidant activity of CNPs (Deshpande et al. 2005; Lee et al.
2013). However, when the quantity of Ce(III) reaches a certain level, CNPs convert
their antioxidant activity to oxidant activity (Lu et al. 2016), which can partially
explain the contradictory results in medicinal applications and toxicological
research.
. Detailed information about the ROS generation is reviewed in Sect. 5.
As to the V o
ÁÁ , they are thought to make CNPs more effective at generating Ce(III)
than the equivalent bulk material and act as “active center” of various redox
reactions exhibited by CNPs (Lee et al. 2013). On the other side, V o
ÁÁ can significantly alter biological interactions and allocate oxygen moieties from biological
molecules, inhibiting a set of biological antioxidant effects and inducing toxicity
response (Gupta et al. 2016). Celardo et al. (2011) demonstrated that V o
ÁÁ did not
174
G. You et al.