soils, which favor to stabilize dispersed NPs (Collin et al. 2014; Schwabe et al.
2013). Citric acid (CA) and acrylic acid (AA) are prevalent carboxylic acid serving
as capping or reducing agents (Barton et al. 2014; Ould-Moussa et al. 2014). Polysaccharides are common coatings including gum arabic (GA), dextran, and alginate
(Alg) (Schwabe et al. 2013; Spielman-Sun et al. 2017; Zhao et al. 2012a, b). Polymer
such as polyvinylpyrrolidone (PVP) has also been introduced to functionalize CNPs
surface (Taylor et al. 2016).
The nature of the coating plays an important part in various levels of toxicity, as
listed in Table 1. For instance, surface modification of CNPs with HA, CA, and AA
constitute a protection against toxic cellular effects by decreasing the potential
surface reactivity (Barton et al. 2015; Heckert et al. 2008) and reducing the ROS
generation (Marie et al. 2014), which both mitigated the toxicity (Barton et al. 2014;
Marie et al. 2014; Trujillo-Reyes et al. 2013). In respect to the surface
functionalization with FA and GA, though the plant growth and translocation factor
of CNPs exhibited no difference, the content of CNPs associated with root (bare >
FA > GA) was influenced (Schwabe et al. 2013). On the contrary, other studies
indicated that CA coating promoted the internalization of CNPs in cells and the
concentrations of dissolved Ce ions in water column, leading to greater toxicity than
the uncoated ones (Garaud et al. 2016; Ould-Moussa et al. 2014; Tella et al. 2015).
The conflicting results might be deemed from the different exposure system as well
as the inspected endpoints in the individual studies. Similar conclusions with higher
toxicity were drawn by Zhao et al. (2012a, b), Booth et al. (2015), and Dowding et al.
(2013) when CNPs were stabilized by Alg, poly-AA, and hexamethylenetetramine
(HMT), respectively. The different effects of various coatings have been generally
concluded as the hydrophobic coating imposed high levels of toxicity and vice versa
for hydrophilic coatings (Yin et al. 2005).
Several CNPs were synthesized by coating with biocompatible polymers such as
dextran (Alili et al. 2011; Barkam et al. 2015), oleic acid (Lee et al. 2013),
2-ethylhexanoic acid (Dowding et al. 2013), sodium bis(2-ethylhexyl)sulfosuccinate
(Chaudhury et al. 2013), and polyethylene glycol (PEG) (Cimini et al. 2012; Vincent
et al. 2009). As a result, the surface stability of CNPs can be greatly improved, which
is a vital aspect to be considered for biological applications and cytotoxicity. The
surface functionalization of CNPs for biocompatibility can protect CNPs from
interacting with ions and create more effective means in the presence of biological
environment. Alili et al. (2011) found that concentrations of dextran-coated CNPs
being nontoxic for normal (stromal) cells show a cytotoxic effect on squamous
tumor cells. Similarly, Cimini et al. (2012) reported that PEG-coated and anti-Aβ
antibody-conjugated antioxidant nanoparticles (Aβ-CNPs-PEG) specifically target
the Aβ aggregates, and concomitant rescue neuronal survival better than Aβ-CNPs,
by modulating the brain-derived neurotrophic factor signaling pathway.
Consequently, such hermetic coatings may define the surface of CNPs and in part
or significantly affect their behavior in the environment. But it is important to
consider that the various molecular weight and chemical structure of coating compounds will change how well they stabilize the CNPs against agglomeration,
interaction, and ultimately toxicity in the environment.
Surface Properties and Environmental Transformations Controlling the. . .
173
2013). Citric acid (CA) and acrylic acid (AA) are prevalent carboxylic acid serving
as capping or reducing agents (Barton et al. 2014; Ould-Moussa et al. 2014). Polysaccharides are common coatings including gum arabic (GA), dextran, and alginate
(Alg) (Schwabe et al. 2013; Spielman-Sun et al. 2017; Zhao et al. 2012a, b). Polymer
such as polyvinylpyrrolidone (PVP) has also been introduced to functionalize CNPs
surface (Taylor et al. 2016).
The nature of the coating plays an important part in various levels of toxicity, as
listed in Table 1. For instance, surface modification of CNPs with HA, CA, and AA
constitute a protection against toxic cellular effects by decreasing the potential
surface reactivity (Barton et al. 2015; Heckert et al. 2008) and reducing the ROS
generation (Marie et al. 2014), which both mitigated the toxicity (Barton et al. 2014;
Marie et al. 2014; Trujillo-Reyes et al. 2013). In respect to the surface
functionalization with FA and GA, though the plant growth and translocation factor
of CNPs exhibited no difference, the content of CNPs associated with root (bare >
FA > GA) was influenced (Schwabe et al. 2013). On the contrary, other studies
indicated that CA coating promoted the internalization of CNPs in cells and the
concentrations of dissolved Ce ions in water column, leading to greater toxicity than
the uncoated ones (Garaud et al. 2016; Ould-Moussa et al. 2014; Tella et al. 2015).
The conflicting results might be deemed from the different exposure system as well
as the inspected endpoints in the individual studies. Similar conclusions with higher
toxicity were drawn by Zhao et al. (2012a, b), Booth et al. (2015), and Dowding et al.
(2013) when CNPs were stabilized by Alg, poly-AA, and hexamethylenetetramine
(HMT), respectively. The different effects of various coatings have been generally
concluded as the hydrophobic coating imposed high levels of toxicity and vice versa
for hydrophilic coatings (Yin et al. 2005).
Several CNPs were synthesized by coating with biocompatible polymers such as
dextran (Alili et al. 2011; Barkam et al. 2015), oleic acid (Lee et al. 2013),
2-ethylhexanoic acid (Dowding et al. 2013), sodium bis(2-ethylhexyl)sulfosuccinate
(Chaudhury et al. 2013), and polyethylene glycol (PEG) (Cimini et al. 2012; Vincent
et al. 2009). As a result, the surface stability of CNPs can be greatly improved, which
is a vital aspect to be considered for biological applications and cytotoxicity. The
surface functionalization of CNPs for biocompatibility can protect CNPs from
interacting with ions and create more effective means in the presence of biological
environment. Alili et al. (2011) found that concentrations of dextran-coated CNPs
being nontoxic for normal (stromal) cells show a cytotoxic effect on squamous
tumor cells. Similarly, Cimini et al. (2012) reported that PEG-coated and anti-Aβ
antibody-conjugated antioxidant nanoparticles (Aβ-CNPs-PEG) specifically target
the Aβ aggregates, and concomitant rescue neuronal survival better than Aβ-CNPs,
by modulating the brain-derived neurotrophic factor signaling pathway.
Consequently, such hermetic coatings may define the surface of CNPs and in part
or significantly affect their behavior in the environment. But it is important to
consider that the various molecular weight and chemical structure of coating compounds will change how well they stabilize the CNPs against agglomeration,
interaction, and ultimately toxicity in the environment.
Surface Properties and Environmental Transformations Controlling the. . .
173