Hybrid Mg _x /Mn _{1– x} Fe ₂ O ₄ /Oxidized MWCNT Nanofluids for Controllable Magnetic and Cyclic Viscoelasticity in Hyperthermia Application

Conventional ferrofluid requires repeated dosage to maintain the therapeutic window of 42–45 °C in magnetic fluid hyperthermia (MFH). To reduce the therapeutic dosage, superparamagnetic PEGylated Mg x Mn 1– x Fe 2 O 4 nanoparticles (12–20 nm) were uniformly anchored onto oxidized MWCNTs via a one-pot sonochemical-assisted hydrothermal method. The Mg x Mn 1– x Fe 2 O 4 nanostructure exhibits mixed cubic–spherical morphology, while surface oxidation of MWCNTs introduces −OH, −NH 2, and −COOH functional groups, enhancing the hydrophilicity of Mg x Mn 1– x Fe 2 O 4 nanoparticles. These functional groups impart cyclic colloidal stability via electrostatic repulsion, with a ζ-potential of −44 mV for repetitive utilization in MFH. The magnetorheological measurements confirm the cyclic stability by retaining the initial viscosity of 9 Pa.s at 0.8 T after rupturing the MWCNT@Mg x Mn 1– x Fe 2 O 4 magnetic nanofluid (MNFs) with a shear rate of 0–500 s –1 for 5 consecutive cycles. The MNFs show shear-thinning behavior expressed by the power law (η = c γ n + η ∞ ), where the flow behavior index n ranges from −0.69 to −0.94, exhibits viscoelastic behavior. The MWCNT@Mg 0.2 Mn 0.8 Fe 2 O 4 (CNTMg0.2) hybrid magnetic nanofluids (MNFs) displayed narrow hydrodynamic size distribution of 92 nm and saturation magnetization of 47 emu/g, outperforming Mg x Mn 1– x Fe 2 O 4 MNFs. To optimize magnetic induction heating within Hergt’s biological safety limit, MNFs at concentrations of 5 and 10 mg/mL were tested under alternating magnetic fields of 12 mT at constant frequencies of 264.1, 372.8, and 586.7 kHz. The CNTMg0.2 nanofluids achieved the highest specific absorption rate (SAR) of 168.4 W/g and intrinsic loss power (ILP) of 3.179 nHm 2 /kg. The induction heating parameters are comparable or higher than the clinically approved magnetic fluid (ILP range: 0.15–3.1 nHm 2 /kg) with improved cyclic stability at varying shear rate 0–500 s –1 and magnetic field strength of 0 −0.8 T. Additionally, CNTMg0.2 revealed excellent biocompatibility, having cell survival rates of 70.9%, 79.9%, and 74.5% for HeLa, MDA-MB-231, and DU145 cells, respectively, even at the highest concentration of 1000 μg/mL.