Size-controlled synthesis of superparamagnetic magnetite nanoclusters for heat generation in an alternating magnetic field

Abstract

We report the size-controlled synthesis of monodisperse magnetite (Fe3O4) nanoclusters by a facile and economic solvothermal route by using polyacrylic acid (PAA) as the coordinative ligand. In comparison with previous reports, nanocluster sizes were tuned over a broad range of 640–250 nm while retaining the superparamagnetic behaviour in the present study. The nanocluster sizes are varied by changing the amounts of PAA as well as water added in the reaction medium during the solvothermal treatment. As-synthesized Fe3O4 nanoclusters are characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The radio frequency alternating magnetic field induced heating studies indicate that the as-synthesized Fe3O4 nanoclusters are promising candidates for magnetic fluid hyperthermia applications. In the case of magnetic nanoclusters, consisting of individual superparamagnetic nanocrystals, the field induced heating occurs at two different length scale, viz. Neel-Brown relaxation of the individual superparamagnetic nanocrystals (length scale <10 nm) and Brownian relaxation of the entire nanoclusters (length scale ~250 nm or higher). Experimental findings indicate that the heating efficiency of the magnetic nanoclusters vary significantly with variations in the relaxation time at both length scales. The heating efficiency of the magnetic nanoclusters is also found to increase linearly with the square of the external field amplitudes, in accordance with the linear response theory.