Preparation and Characterization of Manganese Ferrite Nanoparticles via Co-precipitation Method for Hyperthermia

1. Introduction

      Soft magnetic oxides, MnFe₂O₄, where Mn is a divalent cation, have a spinel structure, and then named spinel ferrites too, and in the bulk form have many applications in telecommunication and electronics [1]. Nanoparticles of these magnetic oxides have different characteristics in comparison with the bulk ones [2]. The use of magnetic particles to induce hyperthermia in biological tissues is an important factor for tumor therapy [3, 4]. Hyperthermia is a therapeutic procedure, which is used to raise the temperature of a region of the body affected by cancer to 42-46 °C. This method involves the introduction of ferromagnetic nanoparticles into tissues, and their subsequent irradiation with an alternating electromagnetic field. Hyperthermia is a promising approach in cancer therapy. The challenge in this method is to restrict local heating of the tumor surrounding [5]. This goal can be partially accomplished by the physical phenomenon of losses when magnetic nanopowders are injected within the cancer tissue and then heated in an alternating electromagnetic field [6]. There are different loss mechanisms in bulk magnetic materials and magnetic nanoparticles. Relaxation loss is the main portion of losses in particles with dimensions below 100 nm [6]. This means that by controlling the particle size of a magnetic nanoparticle, we can adjust the heat generation under an oscillating magnetic field [4]. In this work, Mn ferrite nanopowders were prepared via coprecipitation method for hyperthermia application.

2. Material and methods

      A 500 ml solution, containing 0.51 molar FeSO₄.7H ₂O and a 500 ml solution, containing 0.24 molar MnSO₄ .H₂O were prepared. These solutions were mixed and added to a 6 molar NaOH solution at the temperature ~14 °C (pH of the medium was about 14). After stirring at the same temperature for 10 min., the obtained precipitate was washed off several times by distilled water while a pH=7 was obtained. After each washing, an ultrasonic bath was used to extract the ions through the precipitates. To dry the washed precipitate, a magnetic stirrer with hot plate was used. This procedure was done at 70 °C for 1.5 h and a continuous stream of oxygen gas was run into the precipitate all the time. At this time a black dry powder was obtained.

       The crystal structure of the powders was characterized by an X-ray diffractometer (Bruker, Advanced D8), using CuKαradiation (λ=1.54 A). The particle size was calculated by Scherrer's formula. Particle morphology of the sample was investigated by a transmission electron microscope (TEM), Philips CM12. Magnetization curve of the powder was obtained by a vibrating sample magnetometer (VSM).

      To measure the temperature increase, a mixture of one gram of Mn ferrite nanoparticles and 100 ml of distilled water was prepared. Ten ml of the mixture was then placed at the center of a 20 turns RF coil and an AC current (f=400 kHz) was applied. A sensitive thermometer was used to measure the temperature increase.

3. Results and discussion

       Figure 1 shows XRD pattern of the co-precipitated nanopowders. As can be seen, all main peaks are related to a single-phase spinel structure (10-0319 Jacobsite, Syn.). Mean particle size of the nanopowders is about 5 nm that was obtained by Scherrer's formula and necessary corrections. Figure 2 shows TEM photograph of the single-phase nanopowders and as can be seen there is a uniform distribution of particle size with mean particle sizes between 3 and 10 nm, which is in agreement with the Scherrer's result.

      Figure  3  shows  the  room  temperature hysteresis loop of the coprecipitated Mn ferrite and as can be seen the saturation magnetization of the nanopowders is 54.2 emu/g, which is less than the saturation magnetization of bulk Mn ferrite (60 emu/g) [7]. This difference can be explained by core-shell coupling model elsewhere [8]. In this model, it is supposed that each particle consists of a core with ferrimagnetic order and a constant thickness nonmagnetic shell with spin glass order. It is obvious that by decreasing the particle size, the surface-to-volume ratio of a particle will increase, which leads to a reduction of magnetization of the particles [8]. Temperature increase measurement due to a mixture of the nanoparticles and distilled water (10 g/l) in presence of an AC magnetic field (f=400 kHz) show that a 5 °C temperature increase is achievable after 20 min. as the particle size of the nanoparticles is less than 10 nm, this can be due to relaxation loss [6].