Effect of Silver Nanoparticles on Plant Growth
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Nanoparticles are becoming increasingly used as materials in over 2000 consumer products due to their unique chemical, physical and electrical properties. A nanometer is one billionth of a meter and nanoparticles can be 1-100 nm in size. Silver nanoparticles are used for their antibacterial properties in many every day products such as food storage containers, air filtration systems and bandages. Nanomaterials are structures, devices, and systems at the nanometre scale. They are fast becoming an important material that can range from better and faster electronics to more efficient fuel usage, drug discovery and stronger, more resistant materials (Whatmore, 2006).
The demand for engineered nanomaterials is a rapidly growing industry which was expected to reach a market size of approximately 2.6 trillion dollars by 2015 (Lee et al., 2010), however there is little knowledge on whether nanomaterials have an adverse effect on the environment or to human health and what the extent of these effects could be. Engineered nanoparticles have a wide range of chemical, physical and electrical properties such as conducting heat with low resistance and being stronger and lighter than other bulk materials (Tolaymat et al., 2017). The broad number of products that contain nanomaterials for consumers may lead to the release of an increased quantity of engineered nanoparticles in to the environment, which display different physiochemical properties than larger materials. (Geisler-Lee et al., 2012). While the benefits of nanomaterials are broadcasted, their potential effects to the environment and to human health from their widespread use in consumer products are just becoming recognized. (Hoet et al., 2004).
There are a number of ways that nanoparticles can be released in to the environment as shown in Figure 1. There are a number of different entry points for engineered nanomaterials into the environment, including wastewater treatment plant (WWTP) effluent, and WWTP sludge, however, it is difficult to estimate the relevant concentrations of nanoparticles that will be released in to the environment (Maurer-Jones et al., 2013).
Once nanoparticles enter the environment there can be movement throughout the environment. One way this could happen is through food webs. If nanoparticles are consumed by organisms on a low trophic level thy may begin to accumulate in organisms at higher trophic levels. One of the challenges for working out the dangers associated with nanomaterial release in to the environment is the concern related to how clear our knowledge of how the properties of nanomaterials change once they interact with the environment. Also, nanoparticle properties can be affected by conditions, such as soil chemistry, pH, and organic matter. (Darlington et al., 2009)
One of these effects to the environment could be the release of nanomaterials, through different pathways, in to bodies of water including lakes, rivers, and streams which could also cause run off in to soils and in to the air. Recent research (Das et al., 2012) showed that AgNPs rapidly but temporarily inhibited natural bacterioplankton production. Nanoparticles can affect biological behaviour at the cellular, subcellular and protein levels of a plant.
The effect of nanomaterials on plant species is a topic that is being widely researched however there is still no conclusive answer on whether nanomaterials, specifically silver nanoparticles, have a negative impact on plant species, however metallic engineered nanoparticles may have stimulatory and inhibitory effects on plants. Arabidopsis thaliana is widely used in scientific research and was used in this study to further investigate the effects of silver nanoparticles on germination of seeds and also chlorophyll fluorescence after treatment with differing concentrations of nanomaterials.
The silver nanoparticles used in this experiment were capped with PVP; this is because capped nanoparticles are less likely to aggregate in the solution over time and are more stable than uncapped nanoparticles (Tejamaya et al., 2012). Due to this a control of PVP had to be used to show that the capping had no effect on the plant species itself.
Two mutations of A. thaliana seed were used in this experiment to test the effects of silver nanoparticles. The two sizes of silver nanoparticles were dissolved in distilled water which also meant that distilled water had to be used as a control to show that, on its own, it had no effect on the plant germination. Silver nitrate was also used at differing concentrations as a third control to show any differences between nanoparticles and as silver nitrates can be reduced, with PVP as a stabilizer, to synthesize silver nanoparticles (Samadi et al., 2010). As silver nanoparticles are smaller in size than silver nitrate particles, there will be a higher abundance of nanoparticles within the solution at a given concentration than silver nitrates.
The effect of silver nanoparticles on plant species is important due to the many ways that nanoparticles can be dispersed in the environment. Relatively few studies have investigated the toxicological and environmental effects of engineered nanoparticles (Smita et al., 2012). However, the concentrations used in this experiment would generally be higher than the concentrations of these nanoparticles in the environment, although accurate concentrations in the environment are still not fully known. This is because their concentration in the environment will depend on factors such as the amount of the material released over time. The nanoparticles may become physically or chemically altered by environmental conditions such as temperature and salinity of water and also these factors may alter the form of the nanoparticles, exposure, and transport through the environment. There is still concern over the potential impacts of engineered nanoparticles in the environment on aquatic and terrestrial organisms. Although some data indicates that current risks of engineered nanoparticles in the environment may be low, what we know of the potential impacts of engineered nanoparticles in the environment is still limited. There is still a demand for continued work to further understand the exposure levels for engineered nanoparticles in environmental systems and try and further our knowledge on the significance of these levels in terms of the environment which is what has been addressed in this project (Boxall et al., 2007).
A similar study was carried out by (Obaid, 2016) which evaluated the impact of capped silver nanoparticles on terrestrial and aquatic plants, one of the terrestrial plants being A. thaliana . In this study chlorophyll fluorescence and gaseous exchange of the plants were measured to analyse the effects of the capped silver nanoparticles. The study showed that the capped silver nanoparticles displayed varying toxicity to the plants at higher concentrations, with particular interest to how they effected the germination of A. thaliana, with inhibition of germination at a concentration of 100mg/l of capped silver nanoparticles. The outcome of this study found that there are many factors that have significance on the toxicity of silver nanoparticles which includes exposure method, released ions, plant species, light intensity and growth mediums. However the concentrations used in the study by (Obaid, 2016), much like the concentrations used in this project, are exaggerated and concentrations as high as these will not be present in the environment as yet although it is important to test high concentrations due to large quantities of nanoparticles being used in every day products therefore such concentrations may be present in the environment in the very near future.
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