RESEARCH PAPER PEGylation of SPIONs by polycondensation reactions: a new strategy to improve colloidal stability in biological media Wesley Renato Viali • Eloiza da Silva Nunes • Caio Carvalho dos Santos • Sebastia ˜o William da Silva • Fermin Herrera Arago ´n • Jose ´ Antonio Huamanı ´ Coaquira • Paulo Ce ´sar Morais • Miguel Jafelicci Jr. Received: 14 March 2013 / Accepted: 27 June 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract In this study, we report on a new route of PEGylation of superparamagnetic iron oxide nano- particles (SPIONs) by polycondensation reaction with carboxylate groups. Structural and magnetic charac- terizations were performed by X-ray diffractometry (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and vibrating sample magnetometry (VSM). The XRD confirmed the spinel structure with a crystallite average diameter in the range of 3.5–4.1 nm in good agreement with the average diameter obtained by TEM (4.60–4.97 nm). The TGA data indicate the presence of PEG attached onto the SPIONs’ surface. The SPIONs were super- paramagnetic at room temperature with saturation magnetization (M S ) from 36.7 to 54.1 emu/g. The colloidal stability of citrate- and PEG-coated SPIONs was evaluated by means of dynamic light scattering measurements as a function of pH, ionic strength, and nature of dispersion media (phosphate buffer and cell culture media). Our findings demonstrated that the PEG polymer chain length plays a key role in the coagulation behavior of the Mag-PEG suspensions. The excellent colloidal stability under the extreme conditions we evaluated, such as high ionic strength, pH near the isoelectric point, and cell culture media, revealed that suspensions comprising PEG-coated SPION, with PEG of molecular weight 600 and above, present steric stabilization attributed to the polymer chains attached onto the surface of SPIONs. Keywords SPIONs  PEG  Iron oxide  Surface modification  Magnetic fluid  Colloidal stability Introduction Nanoparticles are submicron particles (diameter rang- ing from 1 to 100 nm) made of inorganic or organic materials, which exhibit, for instance, new structural (Gubin et al. 2003), magnetic (Kodama 1999), elec- tronic (Sampaio et al. 2001), and catalytic properties (Kamat 2002) when compared with their bulk coun- terparts. In nanosized particles, the novel properties Electronic supplementary material The online version of this article (doi:10.1007/s11051-013-1824-x) contains supplementary material, which is available to authorized users. W. R. Viali  E. da Silva Nunes  C. C. dos Santos  M. Jafelicci Jr. (&) Laborato ´rio de Materiais Magne ´ticos e Coloides, Departamento de Fı ´sico-quı ´mica, Instituto de Quı ´mica, Universidade Estadual Paulista, Araraquara, SP 14801-970, Brazil e-mail: jafeli@iq.unesp.br S. W. da Silva  F. H. Arago ´n  J. A. H. Coaquira  P. C. Morais Instituto de Fı ´sica, Nu ´cleo de Fı ´sica Aplicada, Universidade de Brası ´lia, Brası ´lia, DF 70910-900, Brazil P. C. Morais Department of Control Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 123 J Nanopart Res (2013) 15:1824 DOI 10.1007/s11051-013-1824-x