Radiative Decay Engineering: Biophysical and Biomedical Applications

Abstract

Fluorescence spectroscopy is a widely used research tool in biochemistry and molecular biology. Fluorescence has also become the dominant method enabling the revolution in medical diagnostics, DNA sequencing, and genomics. To date all the fluorescence observables, including spectral shifts, anisotropies, quantum yields, and lifetimes, have all been utilized in basic and applied uses of fluorescence. In this forward-looking article we describe a new opportunity in fluorescence, radiative decay engineering (RDE). By RDE we mean modifying the emission of fluorophores or chromophores by increasing or decreasing their radiative decay rates. In most fluorescence experiments the radiative rates are not changed because these rates depend on the extinction coefficient of the fluorophore. This intrinsic rate is not changed by quenching and is only weakly dependent on environmental effects. Spectral changes are usually caused by changes in the nonradiative rates resulting from quenching or resonance energy transfer. These processes affect the emission by providing additional routes for decay of the excited states without emission. In contrast to the relatively constant radiative rates in free solution, it is known that the radiative rates can be modified by placing the fluorophores at suitable distances from metallic surfaces and particles. This Review summarizes results from the physics literature which demonstrate the effects of metallic surfaces, colloids, or islands on increasing or decreasing emissive rates, increasing the quantum yields of low quantum yield chromophores, decreasing the lifetimes, and directing the typically isotropic emission in specific directions. These effects are not due to reflection of the emitted photons, but rather as the result of the fluorophore dipole interacting with free electrons in the metal. These interactions change the intensity and temporal and spatial distribution of the radiation. We describe the unusual effects expected from increases in the radiative rates with reference to intrinsic and extrinsic biochemical fluorophores. For instance, the decreased lifetime can result in an effective increase in photostability. Proximity to nearby metallic surfaces can also increase the local field and modify the rate of excitation. We predict that the appropriate localization of fluorophores near particles can result in usefully high emission from "nonfluorescent" molecules and million-fold increases in the number of photons observable from each fluorophore. We also describe how RDE can be applied to medical testing and biotechnology. As one example we predict that nearby metal surfaces can be used to increase the low intrinsic quantum yields of nucleic acids and make unlabeled DNA detectable using its intrinsic metal-enhanced fluorescence.

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“…In addition, significant increases in probe photostability were observed for probes coated on the silver fractal like structures. These results further serve to compliment our recent work on the effects of nobel metal particles with fluorophores, a relatively new phenomenon in fluorescence we have termed both "metal-enhanced fluorescence" [1] and "radiative decay engineering" [2,3]. These results are explained by the metallic surfaces modifying the radiative decay rate (Γ) of the fluorescent labels.…”

“…We have recently reported the favorable effects obtained for fluorophores in close proximity to metallic particles and islands [1][2][3][4][5][6][7][8][9]. These effects include increased quantum yields, decreased lifetimes, increased photostability, and increased energy transfer.…”

“…These effects include increased quantum yields, decreased lifetimes, increased photostability, and increased energy transfer. These effects are due to the interactions of excited-state fluorophores with the surface plasmon resonance (the oscillations of free electrons) on a metals surface [1,2,6,7]. These interactions result in increased rates of excitation, quenching, increased intensities, or quantum yield (Fig.…”

“…For the most favorable effect of enhanced fluorescence, we are mostly concerned with the important prospect of increasing the radiative decay rate (Γ) for fluorophores, which typically occurs when a fluorophore is positioned some ≈50-100 Å from an appropriately sized and shaped metallic particle. We have recently reviewed these criteria [1,2] and refer to the use of fluorophore-metal interactions as radiative decay engineering (RDE) [2,3] or metal-enhanced fluorescence (MEF) [1]. Interestingly, from Fig.…”

“…1 and Eqs. (1)(2)(3)(4), when the radiative decay rate (the spontaneous rate at which a fluorophore emits photons) is increased, the quantum yield increases while the lifetime decreases Eqs. (3,4).…”