Tuning magnetic heating efficiency of colloidal dispersions of iron oxide nano-clusters by varying the surfactant concentration during solvothermal synthesis

Abstract

Magnetic fluid hyperthermia (MFH) is being actively sought as a supplementary cancer therapy, where enhancing the MFH efficiency is essential for reducing dosage and field exposure. Owing to the superior magnetic properties, magnetic nano-clusters (MNCs) are being developed as MFH agents, where the role of synthesis routes, colloidal stability and magneto-structural properties on MFH efficiency requires further attention. In this article, we report the tuning of MFH efficiency of water and diethylene glycol-based magnetic nanofluids containing iron-oxide MNCs of sizes ∼ 125 to 539 nm, obtained by varying the surfactant concentration. Magnetite MNCs are prepared via solvothermal route, where the primary nano-crystallite (MNP) size is varied from 7.4 ± 0.7 to 25.0 ± 0.8 nm by reducing the sodium citrate (surfactant) concentration from 29 to 0.29 mg/mL. With decreasing surfactant concentration, the amount of sodium citrate absorbed on the surface of the freshly formed nano-seeds reduces, which lowers the electrostatic stabilization in polar media and favours the growth of larger MNPs that results in comparatively larger MNCs. Transmission electron microscopy and atomic force microscopy based studies indicate the monodisperse spherical morphology of the MNCs, consisting of several primary nano-crystallites. Room temperature isothermal magnetization measurements indicate high saturation magnetization and superparamagnetic nature of the MNCs, which are beneficial for MFH applications. Magneto-calorimetry experiments are performed for the accurate estimation of the MFH efficiency of the colloidal dispersions of the MNCs. Experimental findings indicate that the heating efficiency increases from 15.24 ± 1.22 to 193.8 ± 5.2 W/gFe (i.e., an enhancement of ∼ 12.7 times), at a field exposure condition of 33.1 kA/m and 126 kHz, when the primary nano-crystallite size is increased by ∼ 3.4 times by reducing the surfactant concentration. It is observed that the heating efficiency decreases linearly with the logarithm of surfactant concentration in all the cases, which is attributed to the variations in primary nano-crystallite sizes and the resultant magnetic losses. Magneto-calorimetric experiments, using MNCs immobilized in high viscosity agar matrix, indicate insignificant contributions from the whole-scale Brownian relaxation, and the heating efficiency is found to be solely dependent on the magnetic losses of the primary nano-crystallites. With increasing primary nano-crystallite size, augmentation of heating efficiency is due to the increase in magnetization and dynamic coercivity, which is confirmed from theoretical calculations using the high frequency hysteresis loops and sweeping rate modified Stoner Wohlfarth model. The obtained results indicate the possibility of tuning hyperthermia efficiency of MNCs by varying the surfactant concentration, which is beneficial for designing colloidally stable dispersions of MNCs with improved MFH efficiency.