Magnetite nanoparticles were functionalized with carboxymethyldextran (CMDx) covalently attached to the particles to prevent polymer desorption in biomedical applications. Carbodiimide chemistry was used to react carboxylic groups (–COOH) present in the CMDx molecule with amine end groups (NH2) previously grafted onto the nanoparticle surface by condensation of aminopropylsilane (APS) molecules. This method produces highly stable suspensions of magnetite nanoparticles with electrostatic and steric repulsion and no particle precipitation in the presence of electrolytes.