A simple, scalable protocol for the synthesis of ricinoleic acid-functionalised superparamagnetic nanoparticles with tunable size, shape, and hydrophobic or hydrophilic properties.

Abstract

Vegetable oils such as oleic acid have been widely used in the synthesis of nanomaterials as they are environmentally benign, cheap, and biodegradable. Ricinoleic acid (RA), which differs from oleic acid by the presence of an additional hydroxyl group, has surprisingly remained unexplored in the preparation of metal oxide nanoparticles, although it offers the advantage over oleic acid of easy functionalization due to the presence of the hydroxyl group. Here is a simple one-pot procedure for the synthesis of a variety of superparamagnetic nanoparticles, iron oxides and ferrites, using RA both as a precursor complexing agent and as a capping agent outlined. This procedure overcomes the challenges associated with the traditional thermal decomposition method, which demands separate precursor preparation and purification steps, thus promoting a simple yet scalable economic production of various magnetic nanoparticles. Minor changes in the reaction conditions allowed for the production of nanoparticles with different sizes, ranging from 5 to 17 nm, as well as different shapes, spherical and cuboid. Iron oxide nanospheres with an average particle size of 10 nm were superparamagnetic at room temperature with a saturation magnetization of 41 emu g^-1. The as-prepared RA-coated nanoparticles are hydrophobic and dispersible in non-polar solvents but may easily be rendered hydrophilic and water dispersible; epoxidation, followed by alkaline ring-opening, produced hydroxylated nanoparticles with a positive zeta potential of 31 eV, whereas exchange of the capping RA with nitrilotriacetic acid (NTA) gave nanoparticles with a negative zeta potential of -25 eV. The present study highlights the uniqueness of using RA in the preparation of magnetic nanoparticles; apart from the ease and economics of scaling, it offers the possibility of the nanoparticles being either hydrophobic or hydrophilic.