The Influence of Coating and Agglomeration on Specific Absorption Rate of Iron Oxide Nanoparticles
Authors: Yuan Yuan , Borca-Tasciuc Diana-Andra 
Published: May 2012

Magnetic nanofluids can be remotely heated by alternating magnetic field and have significant potential for cancer hyperthermia therapy. The heat generated by magnetic nanoparticles is typically quantified by the specific absorption rate (SAR), which represents the thermal power per unit mass of magnetic material generated in the presence of an alternating magnetic field. During hyperthermia treatment, heat dosage of tumor tissue correlates with slowing tumor growth. The therapeutic ratios of cancer can be increased with the use of biofunctionalized magnetic nanoparticles that have higher SAR for modest amplitudes of magnetic field[1]. Hence, understanding the factors that control the heat generation of magnetic nanoparticle suspensions is important to design fluids with optimized biocompatibility and functionality. In all biomedical applications, the nanoparticles must be coated on the surface to prevent their agglomeration [2], enhance biocompatibility and allow targeting to a specific area. Existing studies have shown that the SAR of nanoparticles may change in the presence of functional coating[3–5]. However, while these studies show that the coating may affect the heat generation rate, there is a limited understanding on the mechanisms that cause that changes of SAR. Hence, it is important to carry out a systematic investigation of nanoparticles similar in size but with different organic coating relevant to biomedical applications to obtain a more complete picture of the mechanisms contributing to changes in SAR. In this work, we present a review of our efforts in this area. Specifically, in our studies we are investigating the correlation between the magnetic and physical properties of commercially available nanoparticles systems and their heat generation rate. The susceptibility and SAR of suspensions of coated and uncoated iron oxide nanoparticles of similar particle size are measured. The coatings selected are highly relevant to biomedical applications and include amine and carboxyl functionalization as well as bioaffine ligands such as protein and biotin. The particle and cluster size was determined from transmission electron microscope (TEM), X-Ray diffraction (XRD) and Dynamic light scattering (DLS). TEM and DLS studies suggested that clusters exist in samples. A summary of all morphological properties together with pH of each suspension is shown in Table.1. The AC magnetic susceptibility of the suspensions was measured as a function of frequency with an in-house made apparatus. Finally SAR was determined by heating the suspension in a commercial induction system and measuring the temperature rise as function of time with a fiber optic sensor. Following these measurements, the SAR values were predicted in two ways: 1) based on measured AC susceptibility and 2) based on particle physical and magnetic properties, starting from Debye model for susceptibility. The normalized predicted and experimental SAR values for all samples are also shown in Table 1. From Table 1, it was found that pH may influence aggregation as described in Ref [6], which indicated that at pH about 2 nanoparticles are highly charged preventing their aggregation while in pH in 6–10 suspensions aggregations are more significant. Normalized SAR of nanoparticle system with aggregations seems to be not related to concentration, different from the well dispersed system[7]. The carboxyl coated sample has smallest diameter and show the lowest SAR, as reported in Ref[8]. The results of suspensions of uncoated iron oxide nanoparticles as well as particles coated with amine groups show that normalized experimental SAR (NSARE) agrees relatively well with calculated SAR using experimental susceptibility (NSARC_χ″E); poor agreement was found when experimental susceptibility was substituted with calculated one (NSART_χ″C) using Debye model, which is developed for non-interacting magnetic particles. These results suggest that the coating do not have a direct effect on SAR. On the other hand, agglomeration, which was present in both samples, may lead to dipolar interaction between nanoparticles and enhancement in magnetic properties and SAR. For carboxyl coated sample which has negligible clustering, showed no temperature increase and zero imaginary part of susceptibility. Therefore, good agreement between Debye-model based predictions of SAR and experimental results were obtained in this sample. However, unexpected results were obtained for bioaffine ligands coated sample, where the experimental SAR values are higher than the SAR values determined based on experimental susceptibility. Protein coated sample, which has the larger clusters among the two samples, has a heat generation rate is 6 times higher than the prediction. Meanwhile, the biotin coated sample which has relatively smaller clusters show only a small increase in heat generation rate. A possible explanation for these results is the loss of superparamagnetic character and an opening in hysteresis loop at test frequency for suspensions with large clusters, which may increase the dissipated power above that produced by the relaxation heat losses [9]. Above results show that coating had little effect on SAR. On the other hand, aggregations and clusters may significantly affect SAR, possibly due to dipolar interaction between nanoparticles in suspensions with relatively small clusters or loss of superparmagnetic characters when very large clusters are present.