Abstract
We explore the spectral dependence of fluorescence enhancement and the associated lifetime modification of fluorescent molecules coupled to single metal nanoparticles. Fluorescence lifetime imaging microscopy and single-particle dark-field spectroscopy are combined to correlate the dependence of fluorescence lifetime reduction on the spectral overlap between the fluorescence emission and the localised surface plasmon (LSP) spectra of individual gold nanoparticles. A maximum lifetime reduction is observed when the fluorescence and LSP resonances coincide, with good agreement provided by numerical simulations. The explicit comparison between experiment and simulation, that we obtain, offers an insight into the spectral engineering of LSP mediated fluorescence and may lead to optimized application in sensing and biomedicine.
Highlights
The pioneering work of Purcell[1] identified that the intensity and lifetime of spontaneous emission are determined by the intrinsic characteristics of the emitter, and by the material environment in which the emitter is located, or in other words the local electromagnetic density of states
We have used fluorescence lifetime imaging microscopy (FLIM) to provide high-resolution spatial and temporal lifetime imaging of fluorescent molecules in the vicinity of gold nanoparticles (GNPs), and correlated these measurements with the localized surface plasmon resonance (LSPR) of the GNPs characterized by dark field spectroscopy
Wide-field dark field microscopy was used to identify the locations of single GNPs, while fluorescence microscopy identifies that Nile Red emission is enhanced in the vicinity of each GNP, with a one-to-one correspondence (Fig. 1(b,c))
Summary
The pioneering work of Purcell[1] identified that the intensity and lifetime of spontaneous emission are determined by the intrinsic characteristics of the emitter, and by the material environment in which the emitter is located, or in other words the local electromagnetic density of states. When emitters are placed near a metallic nanostructure, the emission intensity can be drastically enhanced, accompanied by a reduction of emission lifetime[2,3,4,5,6] This phenomenon, referred to as metal-enhanced fluorescence (MEF), is attributed to the excitation of plasmonic modes within the nanostructure. To enable more efficient localized surface plasmon resonance (LSPR) related application, a variety of nanostructures have been studied, such as nanorods[15], nanoshells[16], silver island films[17] and silver nanoprisms[18,19] to derive a more complete picture of the role of the relative spectral overlap of excitation, LSPR and emission wavelengths on fluorescence intensity enhancement and lifetime reduction, as well as emitter non-radiative line broadening[20]. Range of LSPR resonance was achieved through the variation of nanoparticle shape, the more controlled restriction of nanoparticle geometry in the present work to spherical nanoparticles provides the opportunity to perform a direct comparison between experimental measurements and numerical simulation, to provide an in-depth understanding of the effect of spectral overlap on fluorescence modification
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