Synthesis and characterization of polymer-coated SPIOs with different surface charge due to PEGylation with mono- or bifunctional PEGs. Monodisperse oleic acid stabilized iron oxide cores (11 nm iron oxide core, see electron micrograph) were used as starting material. Whereas our polymer coated model SPIOs (A) is negatively charged due to free carboxyl groups (25 nm, hydrodynamic diameter), reaction with methoxy-PEG amine resulted in a more neutral particle (B), reaction with PEG-bisamine in an even cationic particle (C) as seen in electrophoresis (left Quantum dots, right SPIOs with the same polymer-coating and the same pegylation). Modification of the EDC concentration resulted in gradually PEGylated products, which can be detected by increasing size (arrows) in size-exclusion-FPLC and DLS. The FPLC was calibrated with human plasma by DLS-analysis of proteins in collected fractions (closed circles).