## 9 Biosafety of Nanomaterials in Sustainable Agriculture  
Assessment of safety of nanomaterials is always a difficult issue as the materials depend on many factors: substances or salts used, process method, biological substrates, size and structure, and reactions in medium of applications [101]. During toxicity tests, the prepared stable nanoparticles may become unstable due to aggregation, which modifies the required administered doses. Such behavior would rip the nanoparticles of their unique behaviors and they become different materials [152]. The commonly studied nanomaterials include fullerene (C\({}_{70}\)); fullerol (C\({}_{60}\)(OH)\({}_{20}\)); carbon nanotubes; and ZnO, TiO\({}_{2}\), Fe\({}_{3}\)O\({}_{4}\), CeO\({}_{2}\), Au, Ag, Cu, and Fe nanoparticles. They influence in one or more ways soil fertility and plant nutrients fortification [114]. Nanomaterials may not be completely safe depending on the method of synthesis. Chemically-, physically- and biologically-synthesized nanomaterials have being reported as toxic. Chemically-synthesized nanoparticles pose greater toxicity due to the slow release of the chemical agents used in their synthesis. The biologically-engineered nanomaterials are more biocompatible and relatively safer to the environment and organisms. Since the synthesis of nanomaterials involve different methods and reaction conditions, the materials are formed in various sizes and morphologies, which gives them different properties. As a result, there are many contrasting results obtained from these nanomaterials. Different factors affect their behavior and toxicity: particle sizes, the materials of fabrication, etc. Nano-scaled materials can be safe but, when in contact with other media, some inconsistent reactions may be triggered, making them unsafe. To make this new technology safe and sustainable, there is a recent trend from using chemical methods for nanomaterial synthesis to biological methods. This is necessary because the materials of synthesis greatly affect the nanoparticles, which need to be observed individually. Individual bionanomaterials are composed of biomolecules that may be harmful, and they behave differently  
Figure 3: Improving the quality of soil and crops using appropriate nanoparticles Adapted from [148; 149; 150; 151]. when used in material synthesis. It is therefore important to continue to study the behaviors of these nanomaterials and their decomposition products in organisms to ascertain their level of toxicity. Fullerene (C\({}_{60}\)) NPs have been reported to be non-toxic to microorganisms in the soil, but are toxic in aquatic environments, which clearly indicates that the environment affects the reactions and behaviors of nanomaterials [96; 153]. At the moment, chemicalized nanoparticles may contribute some toxicity to the environment or where applied due to the medium of preparation, but the toxicity of the bionanomaterials are still under intensive study. Thus far, the level of toxicity of bionanomaterials is still uncertain due to the different materials used in the preparation and the various reactions with the medium of application. However, it is promising to use these nanoparticles for future development of the agraran sector. Researchers at the School of Agricultural, Food and Biosystems Engineering (ETSIAAB) from Universidad Politecnica de Madrid (UPM) have recently reported that zinc oxide nanoparticles serve as the source of zinc micronutrient and can be well utilized as fertilizer feedstock without excessive toxicity [154; 155]. On the other hand, [PERSON] et al. reported more lethal effects of using zinc oxide nanoparticles in acidic soil than in calcareous soils [156]. In their experiment, using tomato and bean plants, there was higher photosynthetic pigmentation of the plants with increasing zinc concentration in the calcareous soil. In a nutshell, the toxicity of zinc oxide nanoparticles greatly depends on the soil pH. Plants interaction with zinc ions and zinc oxide nanoparticles are different.