The aim of this work is to synthesize manganese zinc ferrite nanoparticles () by the effective and environmental friendly hydrothermal method and investigate the effect of magnetic ferrofluids (MFFs) containing these on MRI contrast. Moreover, the effect of different parameters such as Zn substitution and citric acid presence as a chelator on spinel phase formation, magnetization and size were investigated. To improve colloidal stability of synthesized for biomedical applications, acid treatment were done followed by coating with different biocompatible polymers such as polyethylene glycol (PEG), dextran and chitosan. The prepared were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic light scattering (DLS) and vibrating sample magnetometer (VSM). The results showed prepared magnetic crystallized mainly in the spinel phase. Nevertheless, without citrate ions presence, hematite phase was appeared in product. Magnetic measurments showed that all samples have nearly zero remanence and coercivity at room temperature. Furthermore, synthesized magnetic with core size of 14 and 7 nm, had high saturation magnetizations (M s ) of 63.6 and 52.6 emu/g respectively. Relaxometric results revealed that proton relaxation rates strongly depend on the magnetic size, coating hydrophilicity and the aggregation state of the particles. Thus, while PEG coating reduces particle aggregation by increasing inter-particle spacing leading to reduction of both r 1 and r 2 relaxations, dextran and chitosan lead to an increase mainly in r 2 values due to the aggreation of particles in bigger clusters where they are in closed contact. Values of r 2 /r 1 , which are also indicative of contrast agent effectiveness, for all prepared MFFs are higher than the reported values for commercial samples. PEG-coated preincubated with Hela cells, were visualized mainly outside the cells, and their biocompatibility at 0.2 mg Fe/ml was demonstrated by alamar blue assay. In conclusion, PEG-coated which have suitable hydrodynamic size, high relaxivity, low cytotoxicity and cell internalization in Hela cell, could be the best potential as MRI contrast agent.
