Biosynthesis of magnetite and cobalt ferrite nanoparticles using extracts of "hairy" roots: preparation, characterization, estimation for environmental remediation and biological application.

The "green" synthesis of magnetite and cobalt ferrite nanoparticles (FeO-NPs and CoFeO-NPs) using extracts of L "hairy" roots was proposed. In particular, the effect and role of important variables in the 'green' synthesis process, including the metal-salt ratio, various counter ions in the reaction mixture, concentration of total flavonoids and reducing power of the extract, were evaluated. The morphology and size distribution of the magnetic nanoparticles (MNPs) depended on the metal oxidation state and ratio of Fe(iii) : Fe(ii) in the initial reaction mixture. MNPs obtained from divalent metal salts in the reaction mixture were non-uniform in size with high aggregation level. Samples obtained by the FeCl/FeSO mixture with a ratio of Fe(iii) : Fe(ii) = 1 : 2 showed an irregular shape of the nanoparticles and high aggregation level. MNPs obtained by the FeCl/FeSO/CoCl mixture showed a regular shape with slight aggregation, and were in the nanosize range (10-17 nm). Thus, this mixture as a metal-precursor was used for MNP biosynthesis in the subsequent experiments. The XRD data showed that the magnetic specimens contained mainly spinel type phase. The data of EDX and XPS analysis indicated that the product of the "green" synthesis was magnetite with some impurities, owing to the obtained ratio of Fe : O being similar to the theoretical atomic ratio of magnetite (3 : 4). The FeO-NP samples were superparamagnetic with high magnetization (until 68 emu g). The Co-containing MNPs demonstrated low ferromagnetic properties. The MNPs with pure magnetite phase, very good magnetization and uniform size distribution ( 12-14 nm) were prepared by the "hairy" root extract characterized by the highest amount of total flavonoids. According to the FTIR data, the synthesized FeO-NPs had a core-shell like structure, in which the core was composed of FeO, and the shell was formed by bioactive molecules. The presence of several organic compounds (such as flavonoids or carboxylic acids) plays a key role in the suppression of FeO-NP aggregation without addition of a stabilizing agents. Synthesized FeO-NP samples effectively removed Cu(ii) and Cd(ii) with the maximum adsorption capacity, reaching 29.9 mg g and 33.5 mg g, respectively. It is probable that the presence of organic components in extracts plays an important role in the adsorption properties of biosynthesised MNPs. The obtained MNPs were successfully applied to the removal of heavy metal ions in the environmental water samples. FeO-NPs also negatively affected plant growth in the case of using "hairy" roots as a test model, and the greatest inhibitory activity (99.56 wt%) was possessed by MNPs with high magnetic properties.