Simple, scalable production of ricinoleic acid-functionalized superparamagnetic nanoparticles, size- and shape-tunable, hydrophobic and hydrophilic

Materials properties at reduced dimensions are heavily influenced by both size and shape and, consequently, the synthetic approach. The synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) has been extensively explored in the literature; however, preparative routes that allow for precise particle size and shape tunability with the scope for surface functionalization and industrial scalability remain challenging. Secondly, taking into account the importance of green chemistry in nanomaterial synthesis, a synthetic protocol that limits the use of harmful chemicals is highly solicited. In this work, we outline a simple one-pot decomposition route that avoids the complexities involved in the preparation, separation, and purification of precursor complexes required of traditional preparative routes and, consequently, is easily scalable. Ricinoleic acid was employed as a greener alternative to oleic acid as both the metal complexing and the capping agent. The modified thermal decomposition route effectively bypasses the demanding precursor synthesis steps of the traditional thermal decomposition route while retaining its advantages. Minor modifications in the preparation scheme allow for the control of particle size and shape. Additionally, a simple and general NTA-mediated ligand-exchange protocol was outlined that can directly transfer the hydrophobic nanoparticles from the non-aqueous reaction mixture to an aqueous phase without the need for product separation from the crude reaction mixture. The general approach reported was extended to prepare monodispersed binary and ternary ferrite nanoparticles. Hydroxylation of the surface-attached ricinolate ligands was explored as a ligand modification strategy to render the nanoparticles hydrophilic and, thus, water-dispersible. X-ray diffraction indicated the presence of magnetite and maghemite phases. TEM images showed monodispersed nanoparticles with a narrow size distribution. EDS mapping showed the uniformity in cation distribution over the nanoparticles. XPS spectra were recorded for elemental analysis. The nanoparticles were superparamagnetic with a saturation magnetization of 41 emu/g.