In this study, SPIONs (primary diameter of about 10.7 nm providing large specific surface area of 115.1 m 2 g -1 ) were decorated with three different phosphonic acid derivates. PAC3Imi+, PAC16 and PAC10Ph bind to the nanoparticle surface covalently forming SAMs in the process as shown in Figure and Figure (see supplementary information) via attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis. PAC3Imi+ renders the SPION surface more positively charged in comparison to uncoated particles as indicated by an increase in ζ-potential at pH 7, while the other two molecules reduce it (see Figure , supplementary information). However, these positive charges repel each other leading to a lower grafting density on the surface of 1.21 ± 0.08 nm compared to the hydrophobic-rendering PAC16 (2.25 ± 0.06 nm -2 ) and PAC10Ph