Progress and analysis of carbon nanotube applications in crop research

Abstract: The promotion and biotoxic effects of carbon nanotubes (CNTs) on crop cultivation have drawn much attention over the last decade. The studied species have covered common crops, such as rice, corn, wheat, tomato, and grapes. There was great inconsistency in the previous experiments, including the various species, growth stages, and cultivation methods of crops as well as the various types, dispersion, application, and concentrations of CNTs, resulting in the diversification of test results in the literature. To establish a relatively unified experiment framework for carbon nanotubes, this study first reviewed the progress of the application of carbon nanotubes in following four crop research directions: crop cultivation, stress resistance, physiological index monitoring, and agricultural genetic engineering, and summarized the carbon nanotube parameters used in the literature and their application effects. It was found that the ultrasonic method or carboxylation covalent modification was commonly used to disperse the CNTs for crop cultivation research, which would result that the effective concentration of carbon nanotubes in crop research being unknown. Meanwhile, this kind of non-selectively modified CNTs was used as the type of nano-sized material. There was no significant difference in the effects between NPs and CNTs on the crops. It is still lacking in the unique effects of CNTs on crop growth. Then, the non-covalently modified CNTs were proposed to establish the unique role of CNTs in crop cultivation, for example, they can aggregate at the target sites after CNTs were modified by macromolecules that can be decomposed by special plant enzymes. Biomolecules (e.g., proteins, and nucleic acids), and polymers (e.g., chitosan, PEG, PEI, and PAMAM) can non-covalently be modified on the surface of CNTs, and further used to adsorb the molecules with negative charges, which can be used as carrier or sorbent. In addition, the non-covalent modification caused no defects on the CNT surface, indicating the fluorescence properties of SWCNTs were retained. So, CNTs were often used for DNA/RNA delivery and the development of biosensors. The working principles and application scenarios of selectively modified CNTs were also summarized as: 1) The CNTs were undoubtedly an important research direction for the applications as the carriers of gene transfer. However, the DNAs that were carried by CNTs were difficult to achieve stable gene transformation, in terms of transgenic applications. By contrast, the siRNA-CNTs complexes shared unique advantages, in terms of administration cycle, plant cell uptake, and protection of RNA from degradation during non-transformative gene delivery. One of the future development directions was in the field of agricultural non-transgenic insect-resistant. 2) The surface of SWCNTs was non-covalently modified with a variety of organic substances. The target molecules were then bound to the organic substances. A variety of sensors were obtained using their fluorescence or electrochemical properties. Some of them were more suitable as implantable sensors for the real-time online monitoring of multiple chemicals in plants, compared with the in vitro sensors. The reason was that the CNTs were used to penetrate the cell walls, indicating excellent near-infrared fluorescence properties. There was no biotoxic effect on the crops at appropriate concentrations (and even beneficial to crop growth). Furthermore, the toxicity problem was caused by the accumulation of exogenous toxic substances in the plant, due to the adsorption or ion exchange with the modified CNTs. It was more worthy of investigation than the toxicity problem of CNT itself. Finally, suggestions for the CNTs experiment program in crop research are given, including the selection of carbon nanotubes, the preparation of medium and characterization methods. To sum up, unlike the previous crop cultivation using pristine and covalently modified CNTs, in the future, using non-covalently modified CNTs or non-covalent stacking of CNTs for crop research is an important development direction.