*2.2. Determination of the Iron Core Size, Polydispersity Index and Hydrodynamic Size*

USPIONs iron core size was checked using transmission electron microscope (TEM) Jeol-1200FX (JEOL Ltd., Tokyo, Japan). As indicated in Figure 1, the average size of USPIONs was 29.8 ± 0.2 nm (mean ± standard error of the mean—SEM), which was in the expected range.

**Figure 1.** Transmission electron microscope (TEM) figure of polyethylene glycol (PEG)-coated ultra-small superparamagnetic iron oxide nanoparticle (USPION) dispersion. Size of nanoparticles iron core was 29.8 ± 0.2 nm (mean ± standard error of the mean—SEM), and no aggregation of nanoparticles was found.

Dynamic light scattering (DLS) measurements were performed using Zetasizer Nano-ZS (Malvern Instruments, Malvern, UK) equipped with a helium/neon laser (λ = 633 nm) and thermoelectric temperature controller at a scattering angle of 173◦ and 25 ◦C. All of the data analyses were made in automatic mode. The measured size was presented as the average value of 20 runs, with triplicate measurements within each run.

For the hydrodynamic size analysis, diluted USPIONs (0.01 mg/1 mL) were used. As indicated in Figure 2, the particle size calculated from the number average was 51.3 ± 16.3 d.nm with a dispersity of 0.147. During the measurement, no formation of NPs aggregates was observed.

**Figure 2.** Distribution of USPIONs according to their size.
