Carbon nanotubes (CNTs) are engineered nanomaterials (a nanometer is one-billionth of a meter) shaped as cylinders made up of carbon atoms. CNTs are stronger than steel but are only about one-sixth the weight. The unique thermal, magnetic and electrical properties of CNTs make them useful in a multitude of products we use daily, including electrical (batteries), renewable energy (solar cell technologies), construction (rebar for concrete reinforcement), transportation (planes, boats, car parts), biomedical (imaging technology, pharmaceutical carriers, implants), cosmetic (anti-aging products, sunscreen), and recreational applications (golf clubs, bicycles, skis, toys).
CNTs can be made of a single sheet of graphite material (single-walled carbon nanotubes; SWCNTs), or can be made o f multiple l ayers to form multi-walled carbon nanotubes (MWCNTs). MWCNTs are exceptionally strong and can vary in length, shape and metal composition, making the assessment of environmental risk particularly difficult. Yet MWCNTs are the most commonly produced carbonaceous nanomaterials, with an estimated global production of 9,400 tons in 2015 alone (De Volder et al., 2013).
An increase in the number of new applications for CNTs has led to a rapid increase in their production (De Volder et al., 2013). With increasing production and use of CNTs, and their subsequent release during the life cycle of CNT-based products, these engineered nanomaterials are likely to accumulate in surface waters, wastewater treatment plant effluents, biosolids, sediments and soils. Subsequent and unintended uptake of CNTs by agricultural crops could increase the risk of human exposure through the food chain (Das et al., 2018; Miralles et al., 2012). It is estimated that more than 50 percent of released CNTs will enter soils, resulting in an annual release of 20−40 tons to soils by 2030 (Gottschalk et al., 2013). Still, relatively little is known about the effects of CNT on soils and water (Ge et al., 2016). While toxicology studies are underway to identify specific CNT risks to human health, the focus is primarily on occupational exposure and biomedical applications (Donaldson et al., 2006). Therefore, accurate detection of CNTs in agricultural plants is necessary to understand potential pathways for human exposure through the food chain and to assess impacts on the environment.
A recent study shows that more than 1.25 milligram per kilogram (mg/kg) MWCNTs can accumulate in leaves of rice (Oryza sativa), maize (Zea mays) and soybean (Glycine max) plants when these plants are grown in hydroponic systems with 2.25 milligram per liter (mg/L) of MWCNTs (Lin et al., 2009; Zhao et al., 2017). The chemical properties of CNTs, among many other factors, can influence their uptake and translocation in plants. For instance, dispersion and accumulation patterns in maize tissue and cells were different for pristine MWCNT (p-MWCNT), without any reactive groups on tube surface and MWCNTs containing carboxylic functional groups (c-MWCNT) on the surface (Zhai et al., 2015). Interactions between CNTs and soil components also greatly influence their uptake in plants.
Nanotechnology has received recent attention for helping to control agricultural production input costs while encouraging sustainable agriculture through improved targeted fertilizer delivery and release (Raliya et al., 2017). In addition, its use in accelerating and regulating plant growth may improve the production of biofuel and food crops, thereby improving both energy and food security at a global scale (Mukesh and Jha, 2017). However, responses of plants to CNTs depend on species, growth medium and environmental conditions, as well as CNT concentration and type (Mukherjee et al., 2016). Despite nanotechnology being used more frequently in food production, its potential effects on the environment and human health are not clear. Additional scientific research is needed to better understand these risks (Raliya et al., 2017).
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