Corresponding authors: E. Illés (illese@chem.hu) and E. Tombácz (tombacz@chem.u-szeged.hu) Elsevier ECIS2013 Best Poster Prize awarded contribution Present address: A. Jedlovszky-Hajdú, Laboratory of Nanochemistry, Department of Biophysics and Radiation Biology, Semmelweis University, H-1089 Budapest, Nagyvárad tér 4, Hungary. Graphical abstract Highlights• The mechanisms of oleate adsorption on as-synthesized and purified MNPs are different.• PEG adsorption increases the weight of steric component in electrosteric stabilization by oleate.• MNPs become clustered upon OA adsorption; cluster size is larger for as-synthesized MNPs.• PEG-surfacted MNPs do not show cytotoxicity and hemolytic potential on human blood.• MRI T2 enhancement of as-synthesized MNPs supports higher degree of clustering. AbstractThe surface of oleate double layer coated (surfacted) magnetite nanoparticles (OA@MNPs) was coated by PEG (poly(ethylene glycol) of Mw=1000, 4000 or 20000 Da, respectively, to get core-shell structured nanomagnets. The oleate bilayers were prepared in two different ways; (i) oleic acid was added directly into the magnetite co-precipitation mixture containing the MNPs to obtain OA@s-MNP samples -s-MNP standing for "as-synthesized MNP" and (ii) sodium oleate (oleate anion, OA) was added to the purified MNPs to obtain OA@p-MNP samples -p-MNP standing for "purified MNP". The effect of the surfactant addition method on the pH-and ionic strength-dependent stability (dynamic laser light scattering and laser-Doppler electrophoresis experiments), the biomedical applicability (MRI measurements) and the biocompatibility (blood sedimentation and blood smear tests) of the coreshell MNPs was studied. Different mechanisms of oleate adsorption were found in ATR FT-IR experiments (inner sphere surface complexation via ligand exchange for the s-MNPs and additional H-bonding for the p-MNPs), suggesting different behaviour. The colloidal stability and salt tolerance of the two kinds of OA@MNPs were similar, but the hydrodynamic diameter of the OA@s-MNP was considerably larger than that of OA@p-MNP. In accordance with this, the r2 relaxation was also higher for the s-MNP samples (~400 and ~ 200 mM -1 s -1 , respectively). The physico-chemical tests indicate that the OA-coated MNPs form clusters and the degree of clustering of OA@s-MNPs is significantly 2 greater than that of OA@p-MPNs. PEGylation does not appear to affect colloidal stability and salt tolerance meaningfully. The adsorption of PEG was proved experimentally. We have found that the PEG top layer decreases the electrostatic contribution, nevertheless increases the steric contribution of the original electrosteric stabilization caused by the OA double layer. However, an increase in the molecular weight above 1000 Da and the amount of added PEG above 5 mmol/g gradually reduces the salt tolerance of the samples. The results indicate strong potential for biomedical application and biocompatibility of the PEGylated MNPs.