Surface chemistry matters: how ligand–core interactions control magnetic and thermal properties of iron oxide nanoparticles?

• Ligand structure and purity control magnetic properties of iron oxide NPs. • Transfer yield correlates with anchor affinity and terminal polarity. • Impurity adsorption unexpectedly increases transverse relaxivity r 2 . • Specific Absorption Rate increases with ligand-core affinity and H-bonding to water. Iron oxide nanoparticles (IONPs) are one of the most developed magnetic nanomaterials due to their potential applications as magnetic resonance imaging (MRI) contrast agents and in magnetic hyperthermia. Most studies have focused on developing synthetic methods to control particle size and morphology to enhance their magnetic performance. In this work, the influence of surface chemistry on the magnetic properties of IONPs by employing a series of purified PEGylated ligands with systematic variations in their anchoring and terminal groups is reported. The ligands were synthesized and used to functionalize IONPs via a ligand exchange approach. These functionalized IONPs exhibited marked differences in their physicochemical and magnetic properties depending on the nature of the ligand. Specifically, the efficiency as MRI T 2 contrast agents and the heating capacity under alternating magnetic fields were found to correlate with the anchor group-Fe binding constant, the absolute value of the ξ-potential, and the presence of surface impurities. These findings reveal that subtle chemical changes in the surface coating exert a crucial and quantifiable influence on the overall magnetic performance. This work highlights surface engineering as a rational strategy for developing IONPs with enhanced magnetic properties.