Size Optimization of Iron Oxide@Noble Metal Core-Shell Nanohybrids for Photothermal Applications

Abstract

The optical properties of several iron oxide@noble metal core-shell nanohybrids (NHs) have been systematically studied by means of electrodynamics simulations applying Mie theory for coated spheres. Focus has been made in analyzing the dependence of the absorption cross section Cabs on the composition as well as on the variables that determine the NHs size, that is, the core radius, R, and the shell thickness, S. The absorption spectra are characterized by an intense peak attributed to a plasmon resonance mode for which the spectral position can be finely tuned in the Vis-NIR range according to the NH size and composition. The absorption cross-section peak intensity, Cabs,max, a key quantity regarding photothermal applications, also presents a strong dependence on the NH size and composition. In general, it is found that α-Fe2O3@Au and α-Fe2O3@Ag lead to larger Cabs,max values than Fe3O4@Au and Fe3O4@Ag core-shell NHs, which is attributed to the lower imaginary refractive index of α-Fe2O3 in comparison to that of Fe3O4. The theoretical Cabs,max values were then used to calculate the temperature change ΔT experienced by the NH when its plasmon resonance mode is excited on resonance. This information has been summarized in diagrams that relate, for each NH composition, the core radius R, the shell thickness S, its resonance wavelength, and the temperature change ΔT experienced at their resonance wavelength. This set of diagrams summarizes relevant information that allows us to predict, for instance, the size and composition that a NH should have to produce the largest ΔT upon illumination at a certain wavelength. The results presented in this work should be helpful to guide and optimize the design of magnetic-plasmonic core-shell NHs with potential photothermal applications.