177
nanoparticles size and shape, besides being a simple methodology. On the other
hand, it can be harmful to the environment, because of the use of toxic solvents and
chemical reducing agents like sodium borohydride and diethylenetriamine, among
others. In this method, typically the reduction of metal salts is performed in a solvent and a separate reducing agent (Dang et al. 2011).
Solution-based methods: these are common and effective methods to obtain copper oxide nanoparticles with controllable shape, composition, and reproducibility
(Zhang et al. 2014). The reaction usually has low temperature and large-scale production, and synthesis parameters control the compositions, sizes, and dimensions
of the copper oxide nanostructures. Hydrothermal and chemical precipitation techniques are two of the most used techniques to synthesize copper oxide
nanostructures.
Solid-state thermal conversion of precursors: this method is employed for obtaining copper oxide nanostructure by thermal conversion of precursors, and it is similar
to the solution-based chemical precipitation method. However, the thermal dehydration of cupric precursors is accomplished in solid state using higher temperature
(Zhang et al. 2014).
Electrochemical method: nanostructured transition metal oxides (MOs) or nanoporous MOs, including copper oxide nanostructure, can be prepared using electrochemical method because of its simplicity and the operation process that involves
low temperature. Some advantages of this method include nanostructures with
higher growth orientation and controlled morphology and size (Zhang et al. 2014).
Thermal oxidation method: it can be employed to synthesize 1D copper oxide
nanostructures, which consists of heating Cu substrates in air, during which the
reaction between Cu and oxygen (O 2 ) occurs. The parameters that can change the
morphologies of copper oxide are the oxidation temperature, growth time, and others (Singh and Ali 2010; Li et al. 2012; Filipič and Cvelbar 2012).
Biosynthesis and green synthesis: currently, the use of “green” synthesis has also
been pursued for producing nanomaterials with less environmental impact, where
the nanoparticles can be recovered from natural resources, for example, from microorganisms and plants (Kulkarni and Muddapur 2014), such as neem, alfalfa,
Cinnamomum camphora, Emblica officinalis, lemon grass, tamarind, and Euphorbia
tirucalli (Naika et al. 2015).
7.2.3 Graphene
Graphene is composed by sheets of carbon atoms possessing sp
2
hybridization, presenting two-dimensional hexagonal structure with a high surface area (Novoselov
et al. 2004; Nogueira et al. 2015). They possess remarkable thermal, electronic, and
mechanical properties finding applications in the design of supercapacitors, nanoelectronics, nanosensors, and drug delivery systems (Facure et al. 2020; Novoselov
et al. 2004; Pérez et al. 2009; Wang 2011; Baptista-Pires et al. 2014; Liu et al. 2016;
Andre et al. 2017; Mercante et al. 2017). Owing to the widespread use of
7 Toxicity of Engineered Nanostructures in Aquatic Environments
nanoparticles size and shape, besides being a simple methodology. On the other
hand, it can be harmful to the environment, because of the use of toxic solvents and
chemical reducing agents like sodium borohydride and diethylenetriamine, among
others. In this method, typically the reduction of metal salts is performed in a solvent and a separate reducing agent (Dang et al. 2011).
Solution-based methods: these are common and effective methods to obtain copper oxide nanoparticles with controllable shape, composition, and reproducibility
(Zhang et al. 2014). The reaction usually has low temperature and large-scale production, and synthesis parameters control the compositions, sizes, and dimensions
of the copper oxide nanostructures. Hydrothermal and chemical precipitation techniques are two of the most used techniques to synthesize copper oxide
nanostructures.
Solid-state thermal conversion of precursors: this method is employed for obtaining copper oxide nanostructure by thermal conversion of precursors, and it is similar
to the solution-based chemical precipitation method. However, the thermal dehydration of cupric precursors is accomplished in solid state using higher temperature
(Zhang et al. 2014).
Electrochemical method: nanostructured transition metal oxides (MOs) or nanoporous MOs, including copper oxide nanostructure, can be prepared using electrochemical method because of its simplicity and the operation process that involves
low temperature. Some advantages of this method include nanostructures with
higher growth orientation and controlled morphology and size (Zhang et al. 2014).
Thermal oxidation method: it can be employed to synthesize 1D copper oxide
nanostructures, which consists of heating Cu substrates in air, during which the
reaction between Cu and oxygen (O 2 ) occurs. The parameters that can change the
morphologies of copper oxide are the oxidation temperature, growth time, and others (Singh and Ali 2010; Li et al. 2012; Filipič and Cvelbar 2012).
Biosynthesis and green synthesis: currently, the use of “green” synthesis has also
been pursued for producing nanomaterials with less environmental impact, where
the nanoparticles can be recovered from natural resources, for example, from microorganisms and plants (Kulkarni and Muddapur 2014), such as neem, alfalfa,
Cinnamomum camphora, Emblica officinalis, lemon grass, tamarind, and Euphorbia
tirucalli (Naika et al. 2015).
7.2.3 Graphene
Graphene is composed by sheets of carbon atoms possessing sp
2
hybridization, presenting two-dimensional hexagonal structure with a high surface area (Novoselov
et al. 2004; Nogueira et al. 2015). They possess remarkable thermal, electronic, and
mechanical properties finding applications in the design of supercapacitors, nanoelectronics, nanosensors, and drug delivery systems (Facure et al. 2020; Novoselov
et al. 2004; Pérez et al. 2009; Wang 2011; Baptista-Pires et al. 2014; Liu et al. 2016;
Andre et al. 2017; Mercante et al. 2017). Owing to the widespread use of
7 Toxicity of Engineered Nanostructures in Aquatic Environments