Mn-Zn ferrite nanoparticles coated with mesoporous silica as core material for heat-triggered release of therapeutic agents

Hydrothermal synthesis was employed to prepare three samples of Mn-Zn ferrite nanoparticles with the mean size of crystallites of 12–14 nm, whose compositions were accurately determined by X-ray fluorescence spectroscopy to Mn0.82Zn0.21Fe1.97O, Mn0.70Zn0.31Fe1.99O4, and Mn0.62Zn0.41Fe1.97O4. The X-ray diffraction and SQUID magnetometry were used to determine the spinel structure and ferrimagnetic properties of the samples. AC field heating experiments were performed on particles dispersed in glycerol to evaluate the specific absorption rate. Based on the superparamagnetic behaviour and magnetic hyperthermia data, the Mn0.62Zn0.41Fe1.97O4 phase was selected for further studies, including a more detailed characterization of the crystal and magnetic structure by neutron diffraction and Mossbauer spectroscopy, encapsulation into mesoporous silica, determination of the specific absorption rate of coated particles, and analysis of the porosity of the shell. Shortly, the Mn0.62Zn0.41Fe1.97O4 nanoparticles were found to possess high magnetization of M(10kOe) = 48.4 emu/g at room temperature and superparamagnetic behaviour at body temperature on the timescale of magnetic measurements. The coated product was formed by clusters of ferrite crystallites with overall mean size of 52 nm encapsulated into 22 nm thick mesoporous shell and showed heating efficiency of 57 W/g(Mn + Fe) in AC field of 970 kHz and amplitude of 8 mT. The mesoporous shell provided specific surface area of 670 m2/g, and its pores of an average diameter 3 nm occupied specific volume of 0.45 cm3/g. In a a preliminary study, the silica pores were loaded with rhodamine B as a model of a hydrophilic drug and subsequently enclosed by lauric acid that served as a temperature sensitive gatekeeper. The release of the model compound to aqueous medium was determined at room temperature and 60 °C by spectrophotometric analysis. Results have shown that amount of rhodamine B in pores achieved 15 mg per gram of silica-coated ferrite cores and its release was controlled by temperature.