Ecotoxicological properties of functionalized magnetic graphene oxide and multiwall carbon nanotubes in Daphnia magna

In this investigation, we analyzed the nanotoxicity of two magnetic nanohybrids made of nanomaghemite dispersed in activated multiwall carbon nanotubes (JA1) and in graphene oxide (JA2), and we studied their influences on the water flea Daphnia magna that is part of the zooplankton, base of the food chain and of an utmost importance for the ecological balance. Before the toxicological studies, both nanohybrids were structurally, vibrationally, hyperfine, and magnetically characterized respectively with X-ray diffraction, scanning and transmission electron microscopy, Raman spectroscopy, ⁵⁷Fe Mössbauer spectrometry, and vibrating sample magnetometry. Transmission electron microscopy gave the particle size distribution histogram with mean particle sizes of 13.8 (6) and 10.4 (2) nm for the nanomaghemite phase in the JA1 and JA2 nanohybrids, respectively. Raman analysis showed the presence of the nanomaghemite phase that has a threshold laser power of 0.71 mW for the γ→α structural phase transition. It was demonstrated that an increase of laser power produced defects and misalignments of the multi-walled carbon nanotubes causing nanomaghemite thermal activation in the JA1 nanohybrid, while the graphene oxide phase was detected by Raman spectroscopy and scanning electron microscopy, this last showed that graphene oxide sheets acting as a template for nanomaghemite growth in the JA2 nanohybrid. ⁵⁷Fe Mössbauer spectrometry and magnetization data confirmed the presence of nanomaghemite nanoparticles with high magnetocrystalline anisotropy. Daphnids were cultured and exposed to different concentrations of both nanohybrids to obtain the median lethal concentration and morphological data. The lethal concentration values were 381.8 mg L⁻¹ for the magnetic JA1 and 0.94 mg L⁻¹ for JA2 nanohybrid. The results of the morphological measurements showed a negative influence for high concentrations of the nanohybrids, while low concentrations have shown biocompatibility with aquatic life.