Iron oxide nanoparticles prepared by co-precipitation and thermal decomposition in PLGA: a magnetic comparison

The potentiality of magnetic nanoparticles for a large range of biomedical applications is largely claimed and demonstrated. In particular, it is well known that their ability as hyperthermia agents may be exploited for cancer therapy or/and thermally activated drug release. To improve the internalization efficiency and in vivo stability, numerous research efforts aim at the creation of composite materials, consisting of iron oxide nanoparticles (NPs) incorporated into a biocompatible matrix [1,2]. The magnetic behavior of the final system and the thermal efficiency are strongly determined by the amount of loaded NPs, by their size as well as by the nature and strength of magnetic interactions among them, which ultimately reflect their arrangement inside the matrix. Some of these parameters are strictly connected to the method employed for the NPs preparation and establishing a clear relationship will allow a tuning of the performances of the composite samples. In this context, we have prepared two types of iron oxide NPs by two of the most used techniques: co-precipitation (CP NPs) and thermal decomposition (TD NPs). The NPs were characterized by TEM microscopy in relation to their shape and size (large size distributions are observed in both cases, with mean values of 10-15 nm). Then, the two types of NPs have been embedded into PLGA (PLGA=Poly(lactic-co-glycolic acid)) following an identical procedure, based on the oil-in-water emulsion solvent extraction method [1]. Both the as-prepared NPs and the composite samples have been studied in the 5-300 K temperature range, with a SQUID magnetometer, by measuring hysteresis loops, the thermal dependence of the magnetization in zero-field-cooling and field-cooling mode and the field-dependent isothermal and demagnetized remanence. Moreover, the samples have been exposed to a time-varying magnetic field in order to evaluate their potential in generating magnetic hyperthermia. The two types of NPs have a similar saturation magnetization (~ 70 emu/g), exhibit superparamagnetic relaxation and have a comparable hyperthermia response. However, they possess a different crystalline degree, higher in TD NPs, and exhibit a different tendency to form aggregates, more marked in CP NPs. This last property seems to play a crucial role in determining the characteristics of the composite samples. In fact, CP NPs tend to keep a similar state of aggregation also after being dispersed in PLGA. Hence, magnetic inter-particle interactions of similar strength rule their magnetic behavior and the overall magnetic properties do not change substantially. TD NPs are more efficiently dispersed in PLGA, which reduces magnetic interactions effects, but a smaller amount of them is loaded in the polymeric matrix (~ 4 wt.% against ~ 7 wt.% of the sample made with CP NPs). In particular, our results indicate that the smaller TD NPs are no loaded in the composite sample, suggesting that they get lost during the synthesis process. [1] M. R. Ruggiero et al., Nanotechnology, 27 (2016) 285104. [2] L. Del Bianco et al. Mater. Res. Express. 2 (2015) 065002.