Annealed Silver-Island Films for Applications in Metal-Enhanced Fluorescence: Interpretation in Terms of Radiating Plasmons

Abstract

The effects of thermally annealed silver island films have been studied with regard to their potential applicability in applications of metal-enhanced fluorescence, an emerging tool in nano-biotechnology. Silver island films were thermally annealed between 75 and 250 • C for several hours. As a function of both time and annealing temperature, the surface plasmon band at ≈420 nm both diminished and was blue shifted. These changes in plasmon resonance have been characterized using both absorption measurements, as well as topographically using Atomic Force Microscopy. Subsequently, the net changes in plasmon absorption are interpreted as the silver island films becoming spherical and growing in height, as well as an increased spacing between the particles. Interestingly, when the annealed surfaces are coated with a fluorescein-labeled protein, significant enhancements in fluorescence are osbserved, scaling with annealing temperature and time. These observations strongly support our recent hypothesis that the extent of metal-enhanced fluorescence is due to the ability of surface plasmons to radiate coupled fluorophore fluorescence. Given that the extinction spectrum of the silvered films is comprised of both an absorption and scattering component, and that these components are proportional to the diameter cubed and to the sixth power, respectively, then larger structures are expected to have a greater scattering contribution to their extinction spectrum and, therefore, more efficiently radiate coupled fluorophore emission. Subsequently, we have been able to correlate our increases in fluorescence emission with an increased particle size, providing strong experiment evidence for our recently reported metal-enhanced fluorescence, facilitated by radiating plasmons hypothesis.KEY WORDS: Metal-enhanced fluorescence; radiative decay rate; increased excitation rate; radiative decay engineering; surface-enhanced fluorescence; silver island films; fluorescence spectroscopy.

Plasmonic engineering of singlet oxygen generation

“…According to our current interpretation of MEF (shown in Fig. 1A), nonradiative energy transfer occurs from excited distal fluorophores to the surface plasmon electrons on noncontinuous films (10). The surface plasmons in turn radiate the photophysical characteristics of the coupling fluorophores.…”

A Strategy for the Maximum Fluorescence Enhancement Based on Tetrahedral Amorphous Carbon-Coated Metal Substrates

“…1 Plasmon excitation results in significantly enhanced local electric fields at the nanoparticle surfaces, which gives rise to fundamentally interesting phenomena and technologically important applications [2][3][4][5][6][7][8][9] such as MEF. [10][11][12][13][14][15][16][17] MEF is the modification of the fluorescence intensities and lifetimes for fluorophores placed near a metal surface, [18][19][20] which has attracted special attention from the viewpoints of academic as well as industrial interests. There are two competitive mechanisms at work which are both dependent on the separation distance between the fluorophores and metal surfaces.…”

First observation of surface plasmon-coupled chemiluminescence (SPCC)

“…We reported that under appropriate conditions, close proximity of fluorophores to silver nanoparticles can lead to increased quantum yields and radiative decay rates, increased photostability and decreased lifetimes [10]. We explained the effects of metals on fluorescence using a simple concept based on radiating plasmons called the radiating plasmon model (RPM) [11][12]. In addition, we have also reported the first observation of metal-enhanced chemiluminescence (MEC) where Silver Island films (a non-continuous surface) in close proximity to chemiluminescing species, significantly enhanced the luminescence intensity [13][14].…”