Correlating Metal-Enhanced Fluorescence and Structural Properties in Ag@SiO2 Core-Shell Nanoparticles

Metal@silica concentric nanoparticles capable of metal-enhanced fluorescence (MEF) represent a powerful means to improve the brightness and stability of encapsulated organic fluorophores and are finding numerous applications in biology, analytical chemistry, and medical diagnostics. The rational design of MEF-enabled labels and sensors often involves comparing fluorescence enhancement factors (EF) between nanostructures having different structural properties (e.g., metal core diameter, silica shell thickness, extent of spectral overlap between plasmon band and fluorophore). Accurate determination of EFs requires the measurement of fluorescence emission intensity in the presence and absence of the plasmonic core while minimizing the impact of physical and chemical artifacts (e.g., signal variations due to scattering, adsorption, sedimentation). In this work, Ag@SiO2@SiO2 + x (where x is fluorescein, eosin, or rhodamine B) nanostructures were synthesized with excellent control of core size, silica spacer shell thickness and fluorophore concentration. Using UV-VIS spectrometry, spectrofluorimetry, time-resolved fluorometry, and transmission electron microscopy, we investigated the influence of these key structural factors on fluorescence emission intensity, and the results were used to develop a generalized methodology for the determination of fluorescence enhancement factors in Ag@SiO2 core-shell nanoparticles. This methodology should be of general importance to designing MEF-enabled nanostructures, sensors, and related analytical techniques.