Phytotoxicity and upper localization of Ag@CoFe2O4 nanoparticles in wheat plants

Environmental concern related to Ag+ release from conventional AgNPs is expected to be prevented once contained into a magnetic core like magnetite or CoFe2O4. Accordingly, we obtained CoFe2O4 NPs by microwave-assisted synthesis, which AgNO3 addition rendered Ag@CoFe2O4 NPs. NPs were characterized, and before exploring potential applications, we carried out 7-day wheat toxicity assays. Seed germination and seedling growth were used as toxicity endpoints and photosynthetic pigments and antioxidant enzymes as oxidative stress biomarkers. Total Fe, Co, and Ag determination was initial indicative of Ag@CoFe2O4 NPs uptake by plants. Then NPs localization in seedling tissues was sought by scanning electron microscopy (SEM) and darkfield hyperspectral imaging (DF-HSI). Not any silver ion (Ag+) was detected into the ferrite structure, but results only confirmed the presence of metallic silver (Ag0) adsorbed on the CoFe2O4 NPs surface. Agglomerates of Ag@CoFe2O4 NPs (~10 nm) were fivefold smaller than CoFe2O4 NPs, and ferrimagnetic properties of the CoFe2O4 NPs were conserved after the formation of the Ag@CoFe2O4 composite NPs. Seed germination was not affected by NPs, but root and shoot lengths of seedlings diminished 50% at 54.89 mg/kg and 168.18 mg/kg NPs, respectively. Nonetheless, hormesis was observed in roots of plants exposed to lower Ag@CoFe2O4 NPs treatments. Photosynthetic pigments and the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX) indicated oxidative damage by reactive oxygen species (ROS) generation. SEM suggested NPs presence in shoots and roots, whereas DF-HSI confirmed some Ag@CoFe2O4 NPs contained in shoots of wheat plants.