Functionalization of Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ nanoparticles with polyethylene glycol (Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ @PEG) (at $x$ = 0, 0.1, 0.2, 0.4 and 0.6) for biomedical applications

Extensive research on magnetic nanoparticles (MNPs) has shown their enormous potential for use in various fields, including biomedicine. However, the created MNPs must have long-term colloidal stability, which is not an easy task, since chemical, physical, biological factors and conditions must be taken into account when synthesizing and functionalizing MNPs for a specific application. By regulating the nature of the core (particle), shell (coating) and ligands (coating material), it is possible to create nanocomposites (MNCs) with MNPs-term colloidal stability for a wide range of applications, including for the diagnosis and therapy of various diseases with the required biocompatibility and functionality. In this regard, the work is devoted to the synthesis of MNP Co$_{1-x}$Zn$_x$Fe$_2$O$_4$, the functionalization (coating) of particles with polyethylene glycol (PEG) and studies of the effect of coating on the properties of the obtained MNCs Co$_{1-x}$Zn$_x$Fe$_2$O$_4$@PEG. Fourier transform infrared spectroscopy (IR-PF), X-ray diffraction (XRD) and Mossbauer spectroscopy (MSp) were used to study the properties of MNPs and MNCs. The formation of a PEG layer on particles has been confirmed by infrared spectroscopy. The analysis of the Mossbauer spectra and distribution functions of effective magnetic fields suggests that during the functionalization of MNPs Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ particles with approximately the same properties combine and form clusters coated with a surfactant. Heating of MNC Co$_{1-x}$Zn$_x$Fe$_2$O$_4$@PEG (0 $\le x\le$ 0.4) the temperature required for hyperthermic therapy (44–46$^\circ$C) is reached in 60 seconds when an external alternating magnetic field with a frequency of 2.0 MHz and a strength of 4.5 kA/m is applied. Synthesized and PEG-coated MNCs Co$_{1-x}$Zn$_x$Fe$_2$O$_4$@PEG can be used for magnetic resonance imaging,