Monodisperse magnetite nanoparticles modified with poly(ethylene glycol) (PEG) were synthesized using a silane functionalized PEG obtained by reacting 3-aminopropyl triethoxysilane with carboxylic acid-methoxy PEG (mPEG-COOH) Using amide reactions. Transmission electron microscopy (TEM) dynamic, light scattering (DLS), and zeta potential measurements show the particles are monodisperse (sigma(gv) similar to 0.2) and stable in water for pH of 3-9 and ionic strengths, up to 0.3 M NaCl. Thermogravimetric analysis Coupled with TEM and DLS indicates formation of a dense graft layer oil the particle surface. An analysis of the interparticle interaction energy indicates that the particles are stabilized by strong steric repulsions between PEG chains on their surface. (C) 2008 Elsevier Inc. All rights reserved.