Spectral Distortions in Metal-Enhanced Fluorescence: Experimental Evidence for Ultra-Fast and Slow Transitions

Metal-enhanced fluorescence (MEF) has become an increasingly important technology in recent years, with thorough research addressing the fundamentals of MEF. In many studies, spectral distortion is observed in the enhanced spectra as compared to free-space fluorescence emission profiles. Despite this observation, very little experimentation has hitherto been undertaken to investigate the mechanistic underpinnings of spectral distortion in MEF. Herein, we investigate MEF spectral distortion using Rose Bengal and fluorescein on silver nanoparticle substrates, subsequently isolating the coupled fluorescence spectrum for a deeper understanding of the spectral modifications. Clear experimental evidence for bathochromic distortion is reported. Remarkably, we also report hypsochromic distortion in one of the first experimental observations of plasmonic coupling to high-energy excited states. Additionally, the coupled fluorescence spectra from other published literature have also been both extracted and examined, and the subsequent spectral distortions are reported here. The previously asserted theory of radiative decay rate modification for spectral distortion is discussed in the context of both plasmonic properties as well as fluorophore photophysical characteristics including lifetime and quantum yield. The dual enhancement mechanism of MEF is also explored in the context of spectral distortion. The results and discussion reported herein subsequently provide one of the first comprehensive examinations of spectral distortion in MEF to date.