Are the green synthesized nanoparticles safe for environment? A case study of aquatic plant Azolla filiculoides as an indicator exposed to magnetite nanoparticles fabricated using microwave hydrothermal treatment and plant extract

It has been claimed that the green synthesized NPs possess no toxicity in comparison to the NPs fabricated via conventional protocols like reduction by sodium borohydride. Therefore, it is necessary to test the toxic effects of NPs on environment. In the current study, we report the binding of Fe₃O₄ NPs to galate ions containing biomaterial namely "galate bio-capping agent". The bio-capping agent is simply mixed with the Fe³⁺ cations at pH 8 to produce negatively charged bio-capped Fe₃O₄ NPs. Finally, the toxic effects of the Fe₃O₄ NPs were investigated on some growth and developmental indices of the aquatic plant species Azolla filiculoides. The relative frond number and relative growth rate were calculated after treatment of plants with different concentrations of bio-capped Fe₃O₄ NPs. In addition, the content of phenolics as well as antioxidant enzymes' activity including superoxide dismutase and peroxidase were assessed. The Fe₃O₄ NPs led to growth reduction and significant changes in total phenol and flavonoid content as well as in antioxidant enzymes' activity. All these findings confirm reactive oxygen species formation due to the nanoparticle toxicity. In consequence, the enzymatic and non-enzymatic antioxidant defense systems of plant were stimulated against oxidative stress.