Surface-Enhanced Fluorescence within a Metal Nanoparticle Array: The Role of Solvent and Plasmon Couplings

Plasmon resonances of metal nanoparticles arranged in arrays are known to be coupled, yielding optical properties that are not the simple sum of those of the individual nanoparticles. On top of this, when the nanoparticle array is immersed in a solvent or in a dielectric matrix, the environment is modifying the position and the intensity of the single-particle plasmon resonances and also their coupling. In this article, by means of a hybrid quantum-mechanical/continuum model, we study the consequences of solvent effects and plasmon couplings on the fluorescence intensity of a dye (naphthalene monoimide) positioned within a four-metal nanoparticle square array. In particular, we analyze the effects of the metal nature (Ag vs Au), interparticle distance, dye-particle distance, dye orientation, and addition of a solvent (dimethylformamide) on the surface-enhanced fluorescence intensity of the dye and on the various quantities that determine such enhancement (radiative and nonradiative decay times, absorption). The conditions that make the effects of the four particles additive are explored, and the results of the hybrid model are tested against those of a fully quantum mechanical model for a dye plus two metal clusters (Ag-20) system.