Citric acid coated iron oxide nanoparticles — Structural and magnetic properties

Magnetic nanoparticles have attracted broad attention due to their potential biomedical applications, such as contrast agents for magnetic resonance imaging (MRI), heating mediators for cancer therapy (hyperthermia), and magnetic labels for biosensing. For nanoparticles to be utilized for biomedical applications these nanoparticles must acquire superparamagnetic behavior with relatively high value of saturation magnetization [1-3]. However, the main challenge in the use of these nanoparticles is the functionalization of particle surface [4-5]. Magnetite (Fe₃O₄) nanoparticles, with diameter of ~20nm, are synthesized using sol-gel method. Uncoated Fe₃O₄ nanoparticles show superparamagnetic behavior as shown in Fig. 1. For making these nanoparticles useful in MRI contrast agent, nanoparticles were coated with citric acid. Citric acid (C₆H₈O₇) is a short chained molecule containing three carboxylate groups [6-8]. Citric acid gets adsorbed on the surface of superparamagnetic iron oxide nanoparticles (SPION) coordinating through one or two carboxylic group therefore leaving at least one carboxylic acid group unattached. This not only provides additional functionality but also prevents the agglomeration and oxidation of nanoparticles [1, 6]. Concentration of the coating solution is changed from 0.1M to 0.5M. The mixture was ultrasonically agitated and the residual mass was then centrifuged and dried to obtain citric acid coated iron oxide nanoparticles [Fig. 2(a-e)]. XRD, FTIR, SEM and VSM measurements show that the changes in concentration of citric acid strongly affect the magnetic and structural properties. As the concentration of citric acid is increased the saturation magnetization of nanoparticles slightly decreases as a consequence of encapsulation of nanoparticles by a non-magnetic coating. However, the superpara-magnetic behavior is preserved [Fig. 2(f)] in the coating process thus makin- them a potential candidate as MRI contrast agent.