Production of MBs and quality control. The MBs were washed in several steps (part I, A). MBs in the upper phase were separated by centrifugation, isolating the plain MBs with a positively charged surface. These MBs were then functionalized by different ligands using the layer-by-layer technique (part I, B). As quality control, DTPA-SPION MBs were visualized by the confocal laser microscope (Leica, resolution 550 μm × 550 μm; part II), and flow cytometry (Partec CyFlow ML flow cytometer) was used to determine the size distribution of DTPA-MBs (part III) using forward scattering versus side scattering dot plot.