Critical Role of Shell in Enhanced Fluorescence of Metal-Dielectric Core-Shell Nanoparticles

Large-scale simulations are performed by means of the transfermatrix method to reveal optimal conditions for metal-dielectric core-shell particles to induce the largest fluorescence on their surfaces. With commonly used plasmonic cores (Au and Ag) and dielectric shells (SiO2, Al2O3, ZnO), optimal core and shell radii are determined to reach maximum fluorescence enhancement for each wavelength within 550-850 nm (Au core) and 390-500 nm (Ag core) bands, in both air and aqueous hosts. The peak value of the maximum achievable fluorescence enhancement factors of core-shell nanoparticles, taken over an entire wavelength interval, increases with the shell refractive index and can reach values up to 9 and 70 for Au and Ag cores, within 600-700 and 400-450 nm wavelength ranges, respectively, which is much larger than that for corresponding homogeneous metal nanoparticles. Replacing air by an aqueous host has a dramatic effect of nearly halving the sizes of optimal coreshell configurations at the peak value of the maximum achievable fluorescence. In the case of Au cores, the fluorescence enhancements for wavelengths within the first near-infrared biological window (NIR-I) between 700 and 900 nm can be improved 2fold compared to a homogeneous Au particle when the shell refractive index is n(s) greater than or similar to 2. As a rule of thumb, the wavelength region of optimal fluorescence (maximal nonradiative decay) turns out to be red-shifted (blue-shifted) by as much as 50 nm relative to the localized surface plasmon resonance wavelength of the corresponding optimized core-shell particle. Our results provide important design rules and general guidelines for enabling versatile platforms for imaging, light source, and biological applications.