Simulating Plasmon Enhancement of Optical Properties in Hybrid Metal-Organic Nanoparticles

Hybrid organic-inorganic Nanoparticles (HNPs) are very interesting and widely studied materials, for their versatile applications in biotechnology and medicine, with high potential in biomedical imaging, gene and drug delivery, and photothermal cancer therapy, making them one of the most promising materials for early and accurate cancer diagnosis and effective cancer therapy. However, computing their physico-chemical properties in details proves to be a challenge. While the nature of the organic component of the HNPs necessitates a full quantum chemical treatment, the size of the inorganic component renders this treatment computationally too expensive to be assessed with an homogeneous technique. For this reason hybrid models have been developed combining a QM level treatment and a classical electromagnetism approach, respectively, for molecules and the inorganic nano-structures upon which they are adsorbed. In particular, the inorganic component, usually a metal, is considered as a classical continuous body, characterized by its own frequency dependent dielectric function, for which the Maxwell equations are solved using the Boundary Element Method (BEM), while excitation energies due to the energy transfer from the molecule to the metal is evaluated exploiting Time Dependent Density Functional Theory (TDDFT). After proving that the polarization charges distribution, used in BEM, well describe the optical properties of bare inorganic Nanoparticles, reproducing experimental spectra of bare Gold Nanoparticles using BEM tools, we described the interactions of the organic molecular frame with the Nanoparticles. In order to reproduce the desired spectroscopic properties for the hybrid system we evaluated on one side the effects introduced by the presence of a Nanoparticle over the energies and the excited state dynamics of the isolated organic dyes (radiative and non radiative deactivation constants). On the other side we computed how the organic layer of dye and surfactant around the gold Nanoparticle affects the plasmonic excitation of the metallic subsystem adding this layer to the electrodynamic computations.