A precise control of the particle size of dextran-coated magnetite nanoparticles (Dex-M NPs) was successfully performed by combination of co-precipitation and hydrothermal synthesis methods. The Dex-M NPs, in the size range 3.1-18.9 nm, were used to fabricate biocompatible magnetic fluids for application in magnetic hyperthermia therapy (MHT). The effects of the carrier fluid viscosity, particle size, and applied magnetic field strength (H_appl) on the specific loss power (SLP) of the Dex-M NPs were investigated at a fixed magnetic field frequency (f). The experimental results show that SLP of the larger Dex-M NPs significantly decreases for a highly viscous carrier fluid. Moreover, regardless of the carrier fluid viscosity, the particle size strongly affects the heating efficiency of the Dex-M NPs. SLP ranges from zero for the smallest Dex-M NPs (with particle size d=3∙1 nm) to 55.21 W⁄g for the largest ones (d=18∙9 nm) at H_appl=28 kA⁄m and f=120 kHz. The most important finding in our research is that, at a fixed frequency, the optimal size of the Dex-M NPs (the size that maximizes SLP) shows a rising trend by enhancing H_appl. In fact, the highest values of SLP at H_appl=11 kA⁄m, 13-17∙5 kA⁄m, and 19-28 kA⁄m are obtained for the Dex-M NPs with d=11∙5 nm, 15 nm, and 18∙9 nm, respectively. The shift of optimal size to the higher values by increasing H_appl could shed light on the correlated effects of the particle size and H_appl on the heating efficiency of magnetic nanoparticles (MNPs) and pave a new way toward the better tuning of them for an effective and biologically safe treatment.
