Iron oxide nanoparticles (Fe3O4, γ-Fe2O3 and FeO) as photothermal heat mediators in the first, second and third biological windows

Nanotherapies are gaining increased interest for the treatment diverse diseases, particularly cancer, since they target the affected area directly, presenting higher efficacy and reduced side effects than traditional therapies. A promising nanotherapy approach is hyperthermia, where the nanoparticle can induce a local temperature increase by an external stimulus in the sick tissue to selectively kill the malignant cells. Among the diverse hyperthermia methods, photothermia is based on the absorption of light by the nanoparticles and further conversion into heat. Within the very wide range of nanostructured photothermal agents, iron oxides offer remarkable features since they are already approved by the FDA/EMA for various biomedical applications, they are biodegradable, easily manipulated using magnetic fields and can be imaged by diverse techniques. Here, we summarize the advantages of using the second biological window, both from the perspective of the skin and the optical properties of iron oxides. Further, we review the photothermal performance of iron oxide nanoparticles in the first, second and third biological windows. Overall, the results show that, for different types of iron oxide nanoparticles (Fe₃O₄, $\gamma$-Fe₂O₃, wüstite-FeO), both the heating capacity (i.e., induced temperature increase) and the photothermal conversion efficiency, $\eta$, vary in a complex way with the light wavelength, depending critically on the measurement conditions and physiochemical properties of the materials. Despite the spread in the reported photothermal properties of iron oxides, Fe₃O₄ particles tend to perform better than their $\gamma$-Fe₂O${}_3$ counterparts, particularly in the second biological window. Interestingly, FeO, which has not been exploited so far from a photothermal perspective, shows very appealing absorption properties. Our preliminary studies using FeO/Fe₃O₄ core/shell nanoparticles evidence that they have excellent photothermal properties, outperforming Fe₃O₄ in both first and second biological windows. Finally, some applications beyond cancer treatment of iron oxide nanoparticles, exploiting the enhanced properties in the second spectral window, are discussed.