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

Abstract

Large-scale simulations are performed by means of the transfer-matrix 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 core–shell 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 2-fold compared to a homogeneous Au particle when the shell refractive index is n s ≳ 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.

Layer-by-Layer-Stabilized Plasmonic Gold-Silver Nanoparticles on TiO2: Towards Stable Solar Active Photocatalysts

“…This near field is intensified by the presence of an optimally thin dielectric shell around the nanoparticle due to its polarizability resulting from the bound electrons. The intensification of the near field increases with the dielectric constant (ε), thus favoring the polymers over air (ε >2 vs. ~1 for air at 25 °C in the UV─vis light frequency range) [42]. Importantly, upon increase of the dielectric shell thickness beyond the optimum, the near field amplitude again drops proportionately, since the distance effect becomes dominant.…”

Nanoantenna‐Enhanced Light‐Emitting Diodes: Fundamental and Recent Progress

“…For instance, metal core–dielectric shell structure utilizes the dielectric shell to effectively suppress quenching caused by the strong absorption of metal, while still producing a strong local electric field due to the surface plasmon resonance of metal. [ 111 ] With the development of micro/nanofabrication technology, it will become a trend to integrate hybrid nanoantenna into LED. For traditional LEDs, the hybrid nanoantenna array constructed in different parts of the LED will produce better enhancement than the pure metal or dielectric structures.…”

Raman and Fluorescence Enhancement Approaches in Graphene-Based Platforms for Optical Sensing and Imaging

“…The emission rate is related to the coupling of the emitter with the metal NP that involves many parameters such as the radiative and non-radiative decay rates of the emitter and of the NP, the quenching of the emitter by the metal, and the decay of the emitter by exciting plasmons [ 114 ]. Moreover, the interaction processes are strongly dependent on the emitter location relative to the NPs [ 115 , 116 ]. A reduction of the emission measured lifetime is usually observed due to new non-radiative de-excitation channels provided by the metal so that the fluorescent molecules have to be separated from the NPs by several nanometers to avoid total quenching.…”

“…The metallic surface, the NPs, or both are coated with a dielectric layer to avoid electric contact and the delocalization of the plasmons. This insulating layer is also required to avoid fluorescence quenching [ 116 ]. Complex experimental systems and mathematical treatments are being developed for single-molecule optical studies using plasmonic nanocavities [ 133 , 134 ] that allow nanometric spatial resolution, well below the theoretical diffraction limit for visible light (around 200 nm for 500 nm excitation wavelength).…”