Abstract
Magnetoplasmonics based on composite nanostructures is widely used in many biomedical applications. Nanostructures, consisting of a magnetic core and a gold shell, exhibit plasmonic properties, that allow the concentration of electromagnetic energy in ultra-small volumes when used, for example, in imaging and therapy. Magnetoplasmonic nanostructures have become an indispensable tool in nanomedicine. The gold shell protects the core from oxidation and corrosion, providing a biocompatible platform for tumor imaging and cancer treatment. By adjusting the size of the core and the shell thickness, the maximum energy concentration can be shifted from the ultraviolet to the near infrared, where the depth of light penetration is maximum due to low scattering and absorption by tissues. A decrease in the thickness of the gold shell to several nanometers leads to the appearance of the quantum effect of spatial dispersion in the metal. The presence of the quantum effect can cause both a significant decrease in the level of energy concentration by plasmon particles and a shift of the maxima to the short-wavelength region, thereby reducing the expected therapeutic effect. In this study, to describe the influence of the quantum effect of spatial dispersion, we used the discrete sources method, which incorporates the generalized non-local optical response theory. This approach made it possible to account for the influence of the nonlocal effect on the optical properties of composite nanoparticles, including the impact of the asymmetry of the core-shell structure on the energy characteristics. It was found that taking spatial dispersion into account leads to a decrease in the maximum value of the concentration of electromagnetic energy up to 25%, while the blue shift can reach 15 nm.
Highlights
Localized surface plasmon resonance (LSPR) is a collective oscillation of conduction electrons at the interface between plasmon metal and dielectric upon incident light excitation
We will be interested in the analysis of the absorption cross-section, which is responsible for the concentration of electromagnetic energy inside the particle: σabs (θ0, λ) = −Re
We start our research with spherical core-shell particles and estimate the spatial dispersion effect on the position and amplitude of LSPR
Summary
Localized surface plasmon resonance (LSPR) is a collective oscillation of conduction electrons at the interface between plasmon metal and dielectric upon incident light excitation. The fact is that when the characteristic size of the metal shell becomes comparable to the Fermi wavelength of electrons in this metal (~5 nm for gold and silver), the so-called spatial dispersion of the metal arises In this case, the conventional local relations between the electric field and the displacement included in the system of Maxwell’s equations are proved to be insufficient for a rigorous description of the electromagnetic properties, since the quantum effect of spatial dispersion emerges [24,25]. There are more popular approaches that allow one to consider the emerging quantum effects, while remaining within the framework of Maxwell’s electromagnetic theory One of such approaches, accounting for the arising spatial dispersion of the plasmonic material, is the Drude hydrodynamic model [27,28] and its modifications, which are applicable to core-shell particles [29].
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