*2.1. Nanoparticles*

Commercially available water dispersion of polyethylene glycol (PEG)-coated USPIONs were purchased from Sigma-Aldrich (Bratislava, Slovakia, cat. No. 747408). USPIONs' concentration was 1 mg Fe/mL and they were dispersed in water. The size of USPIONs confirmed by the transmission electron microscope was 28–32 nm, the zeta potential was −12 mV, polydispersity index was 0.1, and the hydrodynamic size was about 45 nm (all parameters declared by the manufacturer). Before i.v. administration to rats, USPIONs were autoclaved at 121 ◦C for 30 min and mixed with sterile saline to reach a final dose of 1 mg of Fe/kg of body weight.
