Mechanisms of spectral profile modification in surface-enhanced fluorescence

We study the importance of an often-overlooked aspect of surface-enhanced fluorescence (SEF): the spectral profile modification (SPM) that should accompany any plasmon-resonance-mediated enhancement or quenching of the fluorescence close to metal surfaces. We argue theoretically and demonstrate experimentally that under SEF conditions the plasmon resonances affect not only the fluorescence intensity (enhancement or quenching) but also its spectral profile, sometimes to a point where the original fluorescence spectrum may no longer be recognizable as such. Moreover, we argue that the modified excited-state decay rate may, under certain conditions, become comparable to the internal relaxation rate. This effect leads to a new fluorescence mechanism, which we call fast-dynamics SEF (FDSEF), where the spectral profile is affected by the relaxation dynamics in the excited state (in addition to the plasmon resonance). Finally, we propose that the so-called surface-enhanced Raman scattering (SERS) continuum may, in many instances, be interpreted in terms of a SPM of the SEF (in its simplest form or as FDSEF). Experimental evidence using highly uniform nanolithography-prepared gold nanostructures are provided to support these ideas.