## 3 Results and Discussion  
Iron oxide nanoparticles are very efficient hyperthermia probes, but their efficiency drops when aggregated [1; 4]. To avoid detrimental dipolar interaction in nanostructured systems, we mixed iron oxide nanoparticles with non-magnetic cerium oxide ones. Both particle types were synthesized and coated with respect to the existing procedures described in materials and methods. PAA-coated nanoparticles exhibit particular good colloidal stability in various complex media such as brine solutions, ionic liquids, or cell culture media [17; 21; 22]. In particular, we demonstrated in our previous work that it was possible to avoid the aggregation of PAA-coated nanoparticles and positive polyelectrolytes by the addition of salt and allow the aggregation by dilution or dialysis [17; 19]. It was achieved for iron oxide or cerium oxide nanoparticles coated with PAA, separately. The present work is dedicated to the co-assembly of iron oxide and cerium oxide nanoparticles into aggregates that could be a model for iron oxide nanoparticles diluted into nanostructured matrices. The objective was to highlight the role of dipolar interaction when nanoparticles are immobilized in aggregates. We chose to use electrostatic complexation because the interactions were conducted between both types of nanoparticles and the positives polyelectrolytes. The process engaged in the complexation did not depend on the nature of the inorganic core of the particles, but only on the interactions between PAA chains and the positive polyelectrolyte ones. As a confirmation, Figure 2 shows the hydrodynamic diameter of the mixture between iron oxide, cerium oxide, and a 1:1 mixture of both particles with PTEA\({}_{11\text{K}}\)-b-PAM\({}_{30\text{K}}\) versus the NH\({}_{4}\)Cl concentration. When dispersions of nanoparticles at 0.5 M NH\({}_{4}\)Cl of PAA-Fe\({}_{2}\)O\({}_{3}\), PAA-CeO\({}_{2}\), and a mixture of PAA-Fe\({}_{2}\)O\({}_{3}\) and PAA-CeO\({}_{2}\) in the ratio 1:1 with PTEA\({}_{11\text{K}}\)-b-PAM\({}_{30\text{K}}\) were diluted with water, the aggregation state occurred at the same critical NH\({}_{4}\)Cl concentration. From 0.5 M down to 0.38 M NH\({}_{4}\)Cl, nanoparticles were well-dispersed and the hydrodynamic diameter corresponded only to the ones of the particles. For a NH\({}_{4}\)Cl concentration lower than 0.38 M, the diameters of the objects in solution increased, demonstrating the aggregation through electrostatic interactions. The aggregation occurred at the same salt concentration for both particles and their mixtures, which allowed for the formation of assemblies containing the two particle types. This was confirmed by electron energy loss spectroscopy (EELS) analysis on TEM microscope. We showed a uniform  
Figure 2: Hydrodynamic diameter versus ionic strength for mixture of PTEA\({}_{11\text{K}}\)-b-PAM\({}_{30\text{K}}\) with Fe\({}_{2}\)O\({}_{3}\) (circles), CeO\({}_{2}\) (triangles), and a mixture of Fe\({}_{2}\)O\({}_{3}\) and CeO\({}_{2}\) (squares) with the ratio 1:1.  
repartition of both iron oxide and cerium oxide nanoparticles in the aggregates (Figure 3). Furthermore, the ratio between cerium and iron particles in the aggregate corresponded to 67.5% and 32.5%, respectively. This is in good agreement with the introduced ratios in the mixture in brine (16.7% of PAA-Fe\({}_{2}\)O\({}_{3}\) and 83.3% of PAA-CeO\({}_{2}\)). The second ratio is due to the stoichiometry of maghemite compared to the one of cerium oxide nanoparticles. The SLP was measured for well dispersed iron oxide and cerium oxide nanoparticles. Complementarily, aggregated samples containing increasing concentrations of iron oxide (Fe\({}_{2}\)O\({}_{3}\)/CeO\({}_{2}\) ratios from 0.17 to 1.5) were stimulated under an alternative magnetic field. Figure 4 represents the variation of temperature of the samples submitted to an alternative magnetic field. Magnetic field was applied after 60 s of recording of the temperature to ensure that the temperature was stable.