## 3 Copper and Copper Nanoparticles  
Copper is one of the most abundant elements found on Earth. It has played an important part in history, given its many properties, like good electrical and thermal conductivity, high corrosion resistance and increased malleability. It has been used in ornaments, weapons and coins since the early 14 th century [13]. Copper is an essential nutrient required for the normal functioning of the human body. The Food and Nutrition Board at the National Academies of Sciences, engineering and medicine have published the following recommended dietary allowance for copper, as represented in Table 2.  
The human body can obtain copper from various sources, like shellfish, seeds and nuts, cereals, whole grain products and chocolate. Liquids like tap water can also contain copper; however, the copper concentration depends on the source of the water. Although the human body can obtain the daily copper allowance from a multitude of sources, studies have shown that the average adult loses approximately 1.3 mg/day of copper. Copper can be found in more than 30 types of protein, and it plays an important part in living organisms' metabolism. Numerous enzymes containing copper contribute to different body functions, such as oxygen transportation and iron homeostasis [13]. In addition, copper is also found in skin, bones and different body organs [15]. If it is ingested in higher quantities that exceed the human tolerance, copper becomes toxic and can cause hemolysis, jaundice, abdominal pain, nausea and, in extreme cases, death [16; 17]. A common source of copper intoxication is tap water due to the piping used in water distribution either containing copper or being made of copper alloys. On the other hand, copper deficiency is quite rare in humans. Different studies on both animals and humans show the results of copper deficiency include connective tissue disorders, osteoporosis and other bone problems, as well as increased risk of infection [18]. Another study highlighted that copper deficiency can lead to anemia and improper fetal development [15]. In addition to playing an important role for the human body, copper is also essential to the development of plants. Copper helps with protein regulation, photosynthetic electron transport, mitochondrial respiration and cell wall metabolism. When plants have a copper deficiency they present curled leaves, petioles bend downwards and permanent loss of turgor in young leaves. Contrarily, a high concentration of copper leads to toxicity, growth inhibition, photosynthesis interferences and oxidative stress [19]. This action will be further described in a later section (Mechanism of toxicity). Copper nanoparticles (CuNPs) have obtained public interest due to their mechanical, electrical, magnetic and thermal properties and they have been used in water treatment, heat transfer systems and antimicrobial coating for surgical tools [20; 21]. An advantage to using copper is that it is cheap and widely available, thus obtaining CuNPs is cost effective. One of the downsides of CuNPs is that when exposed to aqueous environments, they are susceptible to oxidation. Copper transforms into CuO and Cu\({}_{2}\)O, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Age** & **Male** & **Female** \\ \hline
0–6 months & 200 \(\mu\)g & 200 \(\mu\)g \\
7–12 months & 200 \(\mu\)g & 200 \(\mu\)g \\
1–3 years & 340 \(\mu\)g & 340 \(\mu\)g \\
4–8 years & 440 \(\mu\)g & 440 \(\mu\)g \\
9–13 years & 700 \(\mu\)g & 700 \(\mu\)g \\
14–18 years & 890 \(\mu\)g & 890 \(\mu\)g \\
19+ years & 900 \(\mu\)g & 900 \(\mu\)g \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Recommended daily copper allowance [14]. and converts to Cu\({}^{2+}\) during preparation, imposing challenges in further synthesizing CuNPs in an ambient environment [22]. [PERSON] et al. analyzed the oxidation of copper nanoparticles in relation to temperature.