Distance and plasmon wavelength dependent fluorescence of molecules bound to silica-coated gold nanorods.

Plasmonic nanoparticles can strongly interact with adjacent fluorophores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. This dipolar coupling is dependent upon nanoparticle composition, distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore's absorbance/emission and the surface plasmon band of the nanoparticles. In this work, we examine the distance and plasmon wavelength dependent fluorescence of an infrared dye ("IRDye") bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11-26 nm thick. IRDye is covalently attached to the nanoparticle surface via a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed with 17 nm shells when the nanorod plasmon maximum is resonant with IRDye absorption. Time-resolved photoluminescence reveals multiexponential decay and a sharp reduction in fluorescence lifetime with decreasing silica shell thickness and when the plasmon maximum is closer to IRDye absorption/emission. Control experiments are carried out to confirm that the observed changes in fluorescence are due to plasmonic interactions, is simply surface attachment. There is no change in fluorescence intensity or lifetime when IRDye is bound to mesoporous silica nanoparticles. In addition, IRDye loading is limited to maintain a distance between dye molecules on the surface to more than 9 nm, well above the Förster radius. This assures minimal dye-dye interactions on the surface of the nanoparticles.