Magnetophoretic mobility of iron oxide nanoparticles stabilized by small carboxylate ligands

Iron oxide nanoparticles were synthesized by the co-precipitation method and subjected to an adsorption and functionalization process at controlled pH with five different small carboxylate ligands, namely: citrate, tartrate, gluconate, succinate and beta-alanine. The efficiency of the functionalization was evaluated by infrared spectroscopy and thermogravimetric measurements, finding that the presence of hydroxyl groups in the organic molecule is essential to achieve good levels of adhesion. In the three cases with higher levels of adhesion, the nanoparticles remain permanently stabilized and the system behaves like a ferrofluid at neutral pH. Colloidal properties such as the mean hydrodynamic diameter and the surface electrokinetic potential were studied over a wide pH range, being able to establish a clear correlation between the speciation equilibria of the adsorbed species, the surface charge and the formation of agglomerates between the particles. Finally, a magnetophoretic experiment was designed to evaluate and compare the mobility of the nanoparticles under a magnetic field gradient. In systems with individually stabilized nanoparticles, no magnetophoretic mobility was observed due to the low limit velocity reached by the particles. This result is consistent with a theoretical limit speed calculated considering the different parameters of the experiment and the ferro-fluids. In systems with agglomerated nanoparticles it was found that the magnetophoretic mobility and the limit speed increase with the hydrodynamic diameter. In turn, this property varies strongly with the pH of the system according to the protonation / deprotonation equilibria of the surface of the nanoparticles and the formation or rupture of the agglomerates.