Wavelength Dependence of the Fluorescence Quenching Efficiency of Nearby Dyes by Gold Nanoclusters and Nanoparticles: The Roles of Spectral Overlap and Particle Size

The efficiency of the glutathione monolayer-protected gold nanocluster (NC) Au-25 (1.2 nm metal core diameter (d)) in quenching the emission of dyes intercalated into DNA is compared with that of 2 and 4 nm gold nanoparticles (NPs). In all cases, the DNA/dye moieties and the gold particles are not covalently conditions, steady-state measurements reveal that the quenching efficiency of Au-25 is a factor of 10 lower than that of plasmonic 4 nm gold NPs but comparable to that of 2 nm particles, which do not show a distinct plasmon band. Nonetheless, significant emission quenching is observed even at very low (nanomolar) concentrations of Au-25. The quenching efficiency of the 4 nm NPs is significantly higher for dyes emitting near the wavelength of the plasmon peak, whereas that of the 2 nm gold NPs is well-described by the nanosurface energy transfer (NSET) model proposed by the Strouse group (J. Am. Chem. Soc. 2005, 127, 3115). Interestingly, for Au-25, the maximum quenching efficiency occurs for dyes emitting in the same wavelength range as that of the 2 and 4 nm NPs (490-560 nm), where it shows no discrete absorption features, rather than for wavelengths coincident with its HOMO-LUMO, intraband, or interband transitions. The fluorescence quenching properties of Au-25 NCs are therefore found to be distinct from those of larger NCs and NPs but do not appear to conform to theoretical predictions advanced thus far.