Synthesis and characterization of stable dicarboxylic pegylated magnetite nanoparticles Sara Gil a,b , Emilio Castro a,b , João F. Mano a,b,n a 3B's Research Group—Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, 4806-909 Taipas, Guimarães, Portugal b ICVS/3B's—PT Government Associate Laboratory, Braga/Guimarães, Portugal article info Article history: Received 20 December 2012 Accepted 11 March 2013 Available online 20 March 2013 Keywords: Magnetic nanoparticles MRI Nanomedicine Superparamagnetism Biomaterials abstract The coating of implantable nano- or micro-objects with polyethylene glycol (PEG) enhances its biocompatibility and biodistribution. Herein, we describe a new protocol that enhances and maintains MNPs stability in biological media, simulating multiple conditions to which they would be subjected in the human body. Magnetite nanoparticles (MNPs) prepared via a facile way at room temperature by co- precipitation reaction, were coated with dicarboxylic polyethylene glycol (DCPEG) via covalent bonds. The surface of the nanoparticles was first coated with 3-aminopropyl trimethoxysilane by a silanization reaction and then linked with DCPEG of different molecular weight (Mw¼5000, 10,000 and 20,000 g mol -1 ). The uncoated magnetite nanoparticles, with an average size of 20 nm, exhibited superparamagnetism, high saturation magnetization and a negative surface charge (with a zeta potential value of -40 mV). Increase of Mw enhances the colloidal stability of MNPs and makes them more suitable to tolerate high salt concentrations (1M NaCl) and wide pH (from 5.5 to 12) and temperature ranges (24 1C to 46 1C). The results indicate that magnetite nanoparticles coated with DCPEG with Mw¼20,000 have improved properties over their counterparts, making them our best choice for biomedical studies. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The stability of magnetic nanoparticles (MNPs) in aqueous media results from the equilibrium established between attractive and repulsive forces. Attractive forces come from van der Waals and magnetic dipolar attractions, while repulsive forces mainly originate from electrostatic and steric repulsions [1]. In aqueous solutions, the Fe atoms coordinate with water, which dissociates readily to leave the iron oxide surface hydroxyl functionalized. These hydroxyl groups are amphoteric and may react with acids and bases [2]. Depending upon the pH of the solution, the surface of the magnetite will be positively or negatively charged. For superparamagnetic iron oxide nanoparticles (SPIONs) to be stable in aqueous media at physiological pH (7.35–7.45 for human blood), it is necessary to bring in additional charges to the nanoparticle surface. This is very important for electrostatically stabilizing the colloids due to the fact that the isoelectric point (IEP) of “naked” SPIONs is observed at pH 6.8 [3]. Around this point of zero charge (PZC), the surface charge density is too small and the particles are no longer stable in water and flocculate. Matching both electrostatic and steric stabilization, allows to obtain stable SPIONs [4–6]. Nevertheless, satisfying colloidal stability around neutral pH is not enough for magnetic nanoparticles to be useful as magnetic- field-directed drug targeting and as contrast agents for magnetic resonance imaging administrated intravenously; they should also have good enough colloidal stability at physiological ionic strength around 0.17 M. Phosphate buffered saline (PBS) is often used to mimic the pH and ionic strength of physiological conditions as the osmolarity and ion concentrations of the PBS buffer match those of the human body [1]. However, in biological media, electrostatically stable MNPs are prone to aggregation due to neutralization of the surface charge [7]. Therefore, polymers are often used as stabiliz- ing agents as they provide steric repulsion to the MNPs in addition to electrostatic repulsion, thus being able to reduce the influence of ionic strength on the colloidal stability. Under physiological conditions, the effective minimization of SPIONs aggregation, caused by protein adsorption, must also be taken into account. Thus, anti-biofouling polymers are preferred to modify the MNPs for producing reticuloendothelial system evad- ing nanoparticles with long blood half-lifes [8]. Among anti- biofouling polymers, polyethylene glycol (PEG) is one of the good Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.058 n Corresponding author at: 3B's Research Group—Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, 4806-909 c, Guimarães, Portugal E-mail address: jmano@dep.uminho.pt (J.F. Mano). Materials Letters 100 (2013) 266–270