Metal-enhanced fluorescence of rhodamine B isothiocyanate from micrometer-sized silver powders.

We report here the use of micrometer-sized silver (microAg) powders to enhance the fluorescence emission of rhodamine B isothiocyanate (RhBITC). Our findings clearly show that the RhBITC displays up to a 36-fold increase in fluorescence emission intensity when RhBITC-labeled poly(allylamine hydrochloride) (RhBITC-PAH) is assembled onto a polyelectrolyte layer (about 30 nm thick) above microAg, when compared to that measured for RhBITC-PAH assembled directly onto a glass slide. A similar experiment conducted using a vacuum-evaporated Ag film displayed only a 5-fold increase. The enhanced fluorescence observed for microAg powders is obviously due to their ability to concentrate the electric fields. This suggests that metal-enhanced fluorescence (MEF) can potentially be employed to increase the sensitivity and detection limit of various bioassays that employ RhBITC as a fluorescence label. Since microAg powders are also an efficient substrate for surface-enhanced Raman scattering (SERS), molecular sensors operating via both SERS and MEF may then be able to be fabricated using microAg powders.