Magnetically triggered heating in poly acrylic acid stabilized magnetic nanoemulsions: influence of surfactant conformation on global heating efficiency

Oil-in-water magnetic nanoemulsions (size ~ 200 nm) containing superparamagnetic nanoparticles (MNPs) in the oil phase are potential candidates for multi-modal hyperthermia therapy due to the possibility of loading with chemotherapeutic and photo-active agents. However, the effect of stabilizing agent and its conformation, especially in the acidic environment near cancerous cells, on the heating efficiency needs to be probed in detail. With this objective, experiments were performed on poly acrylic acid stabilized magnetic nanoemulsions containing Fe₃O₄ MNPs (size ~ 9.6 nm) in the oil phase. Under exposure to an oscillating magnetic field, the MNPs underwent Neel-Brown relaxation and the dissipated thermal energy travelled across the oil-water interface causing a global increase in fluid temperature. On increasing solution pH from 3 to 9, it was observed that heating efficiency decreased by 50%, which was attributed to the pH-dependent conformational changes of the adsorbed PAA molecules that influenced the interfacial heat transfer. At higher pH, the globular conformation of the PAA molecules reduced the overlap of vibrational density of states between oil and PAA molecules that ultimately resulted in a lower interfacial heat transfer leading to a reduced heating efficiency.