Facile synthesis of folic acid-functionalized iron oxide nanoparticles with ultrahigh relaxivity for targeted tumor MR imaging

We present the polyethyleneimine (PEI)-assisted synthesis of folic acid (FA)-functionalized iron oxide (Fe₃O₄) nanoparticles (NPs) with ultrahigh relaxivity for in vivo targeted tumor magnetic resonance (MR) imaging. In this work, water-dispersible and stable Fe₃O₄ NPs were synthesized in the presence of PEI via a facile mild reduction approach. The surface PEI coating afforded the formed Fe₃O₄ NPs with the ability to be functionalized with polyethylene glycol (PEG)-linked FA and fluorescein isothiocyanate (FI). A further acetylation step to neutralize the remaining PEI surface amines gave rise to the formation of multifunctional FA-functionalized Fe₃O₄ NPs, which were subsequently characterized via different methods. We show that the developed FA-functionalized Fe₃O₄ NPs have a good water-dispersibility, good colloidal stability, ultrahigh r₂ relaxivity (475.92 mM⁻¹ s⁻¹), and good hemocompatibility and cytocompatibility in the studied concentration range. The targeting specificity of the FA-modified Fe₃O₄ NPs to FA receptor (FAR)-overexpressing HeLa cells (a human cervical carcinoma cell line) was subsequently validated by flow cytometry and confocal microscopy. Significantly, the developed FA-modified Fe₃O₄ NPs can be used as a nanoprobe for targeted MR imaging of HeLa cells in vitro and the xenografted tumor model in vivo via an active FA-mediated targeting strategy. The developed multifunctional FA-modified Fe₃O₄ NPs with an ultrahigh r₂ relaxivity may be used as an efficient nanoprobe for the targeted MR imaging of various kinds of FAR-overexpressing tumors.