Monodisperse magnetite nanoparticles coated with oleic acid were synthesized by the thermodecomposition method. These nanoparticles were modified via ligand exchange with amine-silane molecules, which allowed the expression of amine (NH2) end groups onto the nanoparticle surface. Carbodiimide chemistry was used to react these amine groups with carboxylic groups (-COOH) present in carboxy-methyl dextran (CMDx) molecules. We used CMDx with degrees of substitution ranging from 0.35 to 1 to study the colloidal stability of the functionalized nanoparticles by zeta potential measurements. Fluorescent nanoparticles were also obtained by incorporation of fluorescein-amine groups during the synthesis of magnetite-amino-silane-CMDx nanoparticles. These fluorescent nanoparticles were used to visualize the internalization in CaCo-2 and MCF-7 cancer cell monolayers through confocal laser scanning microscopy (CLSM). The cytotoxic effects of these nanoparticles were study to determine their potential application for cancer treatment.