Effects Of Functionalized Multi-walled Carbon Nanotubes On Seed Germination And Physiological And Biochemical Characteristics Of Rice Seedlings

With the rapid development of nanotechnology, carbon nanotubes (CNTs) as a new type of unique nanomaterials, have been widely used in many fields. They will get into the ecological environment inevitably and will further result in ecological risk. Recently, the biological safety problems about environmental risk and ecological impact of nanomaterials have attracted great attention of many researchers from all over the world. The biological effect of CNTs is closely related to the physiochemical properties of CNTs, however, there is seldom any study compared the biological effect of CNTs with different physiochemical properties. And if there is any, they are on cells, algae or animals. Thus, in this study, three kinds of CNTs (RAW-MWCNT, COOH-MWCNT and PEG-MWCNT) with different physiochemical properties, such as hydrophilicity, water solubility, surface functional groups and biocompatibility, were tested and their effects on rice (a typical agricultural crop) were compared. The effects of different CNTs on seed germination, growth, antioxidase activities and cell damage of rice seedlings were explored by hydroponic experiments. The surface morphologies, particle sizes and functional groups of three CNTs were examined as well. The regularities and mechanisms of the CNTs' effects on the plant were discussed, and the factors related to nanomaterials'environmental risk and ecological impact were revealed. The results are as follows:1. As seen from the characterization of three CNTs, the morphology of CNTs obviously changes before and after the treatment of modification. Modified CNTs show smaller particle size, better dispersibility and greater solubility in water. RAW-MWCNT are tangled and the tubes were longer, greater in bending degree and tube integrity; while modified CNTs are fluffy and show smaller particle size in water; the tubes are cut down to be shorter and the tube surface seems smoother. The results of FT-IR show that negatively charged groups such as carboxyl group (-COOH) are brought to the surface of COOH-MWCNT and the Z eta potential of COOH-MWCNT decreases at the same time. The PEG modification introduces-COCl,-CO- and -OH groups to PEG-MWCNT, however the Zeta potential of PEG-MWCNT is higher than that of RAW-MWCNT. The effective particle size increases in the following order: PEG-MWCNT< COOH-MWCNT< RAW-MWCNT; while the dispersibility and water solubility are the opposite.2. It can be found from the results of seed germination that functionalized CNTs have stronger inhibition effect on rice germination than non-functionalized CNTs. The inhibition rate follows the order of PEG-MWCNT> COOH-MWCNT> RAW-MWCNT. Hydrophilic groups such as -COOH and-OH, was introduced to the surface of functionalized CNTs, thus they get smaller particle size and better water solubility. These poperties are advantages for functionalized CNTs to block the holes of seed coat during the water uptaking process of seed and will lead to inhibition of seed germination as a result.3. The growth and physiological and biochemical index of rice seedlings show that functionalized CNTs increase the root shoot ratio and that comparing with non-functionalized MWCNT, functionalized CNTs have stronger inhibition effect on plant protein content, greater influence on antioxidant enzyme activities such as Superoxide dismutase(SOD), Peroxidase(POD) and Catalase(CAT), and more serious damage to root cells. Protein content and antioxidant enzyme activities of the root are sensitive to PEG-MWCNT and the root cell damage decreases in the order of PEG-MWCNT> COOH-MWCNT> RAW-MWCNT. However, the shoot is more sensitive to COOH-MWCNT and strongest oxidative damage is found for the shoot under the exposure of COOH-MWCNT. Due to the smaller particle size and better water solubility, functionalized CNTs are more likely to be adsorbed on the surface of the root, aggregate there and even enter into the root system. During the transportation of water and nutrient, functionalized CNTs may be transported to part above ground and cause stronger oxidative damage comparing to RAW-MWCNT. Consequently, the protein content decreases; the lipid peroxidative product (MDA) accumulates and the cell damage increases. The growth of the shoot is inhibited more seriously than the growth of root, as a result the shoot root ratio increases under the exposure of functionalized CNTs.In conclusion, functionalized CNTs show greater inhibition effect and biological risk on rice growth than non-functionalized CNTs. The toxic effect increases in the order of RAW-MWCNT< COOH-MWCNT< PEG-MWCNT. Considering the characterization results of the physiochemical properties of the CNTs, it can infer that the effect of CNTs on plant growth is closely related to the CNTs' surface structures and physicochemical properties, such as particle size, dispersibility, water solubility, functional groups and so on. CNTs with smaller size, better dispersibility and stronger water solubility will affect the growth of plant greater. Thus, when we explore the environmental risk and ecological impact of CNTs, the surface structures and physicochemical properties of the CNTs should be considered.