2.4 Surface Charge
The surface charge contains significant information on the colloidal stability, transport, and importantly the NPs interactions with biological system in the environment. In the previous in vitro studies, the surface charge of CNPs has been reported
to dictate CNPs cellular uptake and to control their cytotoxicity (Alpaslan et al.
2015; Asati et al. 2010; Nel et al. 2009). The toxicity of cationic CNPs in general
exhibited more toxic than their neutral or anionic counterparts. The overall charge of
the cell membrane is negative, and hence CNPs (positively charged or neutral in
nature) can be easily uptaken and get simply bound to the cell membrane through
electrostatic interaction, leading to the damage of cell membrane integrity and
resulting in the lysis of cells (Asati et al. 2010; Collin et al. 2014; He et al. 2012;
Spielman-Sun et al. 2017).
In addition to the cellular uptake, the subsequent subcellular localization of
surface-charged CNPs takes a key part in the cytotoxicity profile of CNPs. Asati
et al. found that CNPs (+) displayed significant toxicity as they entered the lysosomes of the cells, while minimal toxicity is observed when the CNPs (0) and CNPs
(À) localized in the cytoplasm or did not enter the cells (Asati et al. 2010). Similarly,
Spielman-Sun et al. concluded that the tissue localization and transformation of
CNPs within Triticum aestivum were significantly affected by their different surface
charge (Spielman-Sun et al. 2017). Further, risks associated with CNPs exposure
will be determined partially by the environmental processes. Collin et al. (Collin
et al. 2014) and He et al. (2012) proposed respectively to add humic acid (HA) and
phosphate (P) in the exposure media to attenuate the toxicity. Both HA and P could
adjust the positive surface charge density by conferring a net negative surface charge
and thus decrease CNPs accumulation and toxicity.
Overall, it is clear from the few studies that the roles of surface charge on CNPs
interactions with cell surface and subsequent toxicity must be considered. However,
to gain a deeper understanding, the key components mediating surface charge of
CNPs in environmental systems and their consequent localization in biological
substructure require more precise control of variables, which will contribute to
eliminate possible associated toxic effects.
2.5 Coating
The surface of any material is the prominent route of interaction with the cellular
environment. Therefore, studies have been conducted to develop coatings around
NPs with the aims of providing steric, electrostatic, or electrosteric repulsive forces
among particles to resist agglomeration and dissolution (Collin et al. 2017; Ganguly
et al. 2018; Levard et al. 2012). Various types of compounds including NOM,
carboxylic acid, polysaccharides, and polymers have been applied to functionalize
CNPs surface. Fulvic acid (FA) and HA are ubiquitous component of water and
172
G. You et al.
The surface charge contains significant information on the colloidal stability, transport, and importantly the NPs interactions with biological system in the environment. In the previous in vitro studies, the surface charge of CNPs has been reported
to dictate CNPs cellular uptake and to control their cytotoxicity (Alpaslan et al.
2015; Asati et al. 2010; Nel et al. 2009). The toxicity of cationic CNPs in general
exhibited more toxic than their neutral or anionic counterparts. The overall charge of
the cell membrane is negative, and hence CNPs (positively charged or neutral in
nature) can be easily uptaken and get simply bound to the cell membrane through
electrostatic interaction, leading to the damage of cell membrane integrity and
resulting in the lysis of cells (Asati et al. 2010; Collin et al. 2014; He et al. 2012;
Spielman-Sun et al. 2017).
In addition to the cellular uptake, the subsequent subcellular localization of
surface-charged CNPs takes a key part in the cytotoxicity profile of CNPs. Asati
et al. found that CNPs (+) displayed significant toxicity as they entered the lysosomes of the cells, while minimal toxicity is observed when the CNPs (0) and CNPs
(À) localized in the cytoplasm or did not enter the cells (Asati et al. 2010). Similarly,
Spielman-Sun et al. concluded that the tissue localization and transformation of
CNPs within Triticum aestivum were significantly affected by their different surface
charge (Spielman-Sun et al. 2017). Further, risks associated with CNPs exposure
will be determined partially by the environmental processes. Collin et al. (Collin
et al. 2014) and He et al. (2012) proposed respectively to add humic acid (HA) and
phosphate (P) in the exposure media to attenuate the toxicity. Both HA and P could
adjust the positive surface charge density by conferring a net negative surface charge
and thus decrease CNPs accumulation and toxicity.
Overall, it is clear from the few studies that the roles of surface charge on CNPs
interactions with cell surface and subsequent toxicity must be considered. However,
to gain a deeper understanding, the key components mediating surface charge of
CNPs in environmental systems and their consequent localization in biological
substructure require more precise control of variables, which will contribute to
eliminate possible associated toxic effects.
2.5 Coating
The surface of any material is the prominent route of interaction with the cellular
environment. Therefore, studies have been conducted to develop coatings around
NPs with the aims of providing steric, electrostatic, or electrosteric repulsive forces
among particles to resist agglomeration and dissolution (Collin et al. 2017; Ganguly
et al. 2018; Levard et al. 2012). Various types of compounds including NOM,
carboxylic acid, polysaccharides, and polymers have been applied to functionalize
CNPs surface. Fulvic acid (FA) and HA are ubiquitous component of water and
172
G. You et al.