Fluorescence News Enhanced and Localized Multiphoton Excited Fluorescence Near Metallic Silver Islands: Metallic Islands Can Increase Probe Photostability

Recently we reported that close-proximity metallic mum enhancement in the incident electric field has been calculated to be a factor of 140 near appropriately sized silver islands or colloids can alter the radiative decay metallic ellipsoids [8]. Because the incident intensity is rate, ⌫ m , and/or excitation rate, E m , of fluorophores [1–5]. the square of the incident field strength for a 1-photon We have shown that the quantum yield of low quantum process, enhancements in excitation rates by a factor of yield fluorophores can be increased, with a maximum up to 2*10 4 are possible. It is this phenomenon that one predicted increase of 1/Q 0 , (Q 0-quantum yield in absence can typically attribute to increases in observed apparent of metal), whereas significant increases in emission inten-quantum yields near metallic particles to greater than sity from high quantum yield species, in the absence of unity. However, a much more dramatic enhancement is any non-radiative rate modifications, can only be ob-possible for multiphoton excitation. For a 2-photon served by substantial increases in E m. Complimentary to absorption process the rate of excitation is proportional our previous results and interpretations, we can now to the square of the incident intensity. This suggests that report that enhanced and localized multiphoton excitation 2-photon excitation could be enhanced by a factor of 3.8* of rhodamine B (RhB) fluorescence occurs near metallic 10 8. Such an enhancement in the excitation rate is thought silver islands. to provide selective excitation of fluorophores near to The increase in fluorescence emission intensity for metal islands or colloids, even if the solution contains a RhB molecules adjacent to metallic silver islands (Fig. considerable concentration of other fluorophores that 1) is accompanied by a reduction in lifetime, compared could undergo 2-photon excitation at the same wave-to that observed using 1-photon excitation. Given the length, but are more distant from the metals surface (Fig. high quantum yield of RhB (Q 0 ϭ 0.48), these results 3). This interpretation is borne out by the fact that given can be explained by the metallic particles significantly the overwhelming excess of high quantum yield RhB in increasing the E m of the RhB molecules. Moreover, given this sample geometry (Ϸ96% of solution is too distant the sample geometry (Fig. 2) and the absence of any for any fluorophore–metal effect), the fluorescence life-notable increase in emission intensity using 1-photon time is still …