Environmental Engineering Reference
In-Depth Information
reactions associated with different NMs for different purposes, such as photocatalytic
degradation of contaminants with NPs (see Chapter 3 of this topic; Nagaveni et al.,
2004), disinfection of water or wastewater (Rincon and Pulgarin, 2003), cytotoxicity of
NMs (Rancan et al., 2002; Shvedova et al., 2003; Sayes et al., 2004; long et al., 2006;
Poland et al., 2008). In most of these studies, light irradiated NMs exhibit strong
activity because of the associated release of some free radicals. For example, Ceo, on
photosensitization, can induce significant lipid peroxidation and other forms of oxidative
damage in biological membranes; this phenomenon can be greatly moderated by
endogenous antioxidants and scavengers of reactive oxygen species (Kamat et al., 1998).
TiO2 photocatalysis promoted peroxidation of the polyunsaturated phospholipids'
component of the lipid membrane initially and induced major disorder in the
E. coli
cell
membrane. Subsequently, essential functions that rely on intact cell membrane
architecture, such as respiratory activity, were lost, and cell death was inevitable
(Maness et al., 1999). For these reasons, NMs are being considered as a new disinfectant
that can be used for disinfection of water or wastewater (Kiihn et al., 2003). Long et al.
(2006) reported that the biological response of brain microglia (BN2) to noncytotoxic
(2.5-120 ppm) concentrations of TiO2 (P25) was rapid (< 5 min) and sustained (120
min) release of reactive oxygen species (CV). The time course of the released CV"
indicated that P25 not only stimulated the immediate "oxidative burst" response in
microglia but also interfered with mitochondrial energy production.
Numerous
in vitro
studies prove that some nano techno structured catalysts (e.g.,
TiCh) cause OS-mediated toxicity in diverse cell types (Chapter 3; references in Long et
al., 2006). DNA nicking from aqueous dispersions of the TiO2 NPs was previously
attributed to the generation of singlet oxygen ('02) and hydroxyl free radicals by
photogenerated charge carriers. A similar mechanism for DNA damage would be
expected for CdSe/ZnS semiconductor QDs (comprised of cadmium selenide capped
with a shell of zinc sulfide, complete with biotin surface functionality). The ZnS shell
only confines the hole to the core of the QD, while the electron extends over the entire
structure. Thus, while protecting the emitting core from oxidation, the ZnS shell should
not inhibit electron induced free radical generation in water (Green and Howman, 2005).
However, surface modification of QDs can change their physicochemical properties,
which changes the cytotoxicity of QDs. Therefore, the properties of QDs are not only
related to those of QD-core materials but also to molecules covering the surface of QDs,
such as crystallinity, surface area, surface hydroxyl groups, and properties related to
optical absorption at higher wavelength (Hoshino et al., 2004; Nagaveni et al., 2004).
The mechanisms involved in these photocatalytic degradations or reactions may
be different when different NMs are involved in different systems. In general,
photoactivation (generation of electrons and holes) is required. Taking nano-TiC>2 as an
example, when nano-TiC>2 is irradiated with UV, photoactivation occurs in femtoseconds
(Draper and Fox, 1990; Sivalingam et al., 2003; Nagaveni, et al., 2004):
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