Radiative Decay Engineering: Metal-Enhanced Fluorescence

Abstract

In the preceding chapters we described the wide-ranging applications of fluorescence. All these applications relied upon the spontaneous emission of fluorophores in free space. By free space we mean optically transparent nonconducting media. In these final chapters we describe a new topic called radiative decay engineering (RDE). The term RDE is used because the environment around the fluo-rophore is modified or engineered to change the radiative decay rate of the fluorophore. In Chapter 1 we showed that the radiative decay rate (Γ) is determined by the extinction coefficient of the fluorophore. Extinction coefficients do not change substantially in different environments. Similarly, the radiative rates remain nearly the same under most conditions. The changes in quantum yield or lifetime displayed by fluorophores in different environments are due to changes in the non-radiative decay rates. In this chapter we describe the effects of conducting metallic silver particles on fluorescence. A fluorophore in the excited state has the properties of an oscillating dipole. The excited fluorophore can induce oscillations of the electrons in the metal. The electric field created by the metal can interact with the excited fluorophore and alter its emission. This interaction is almost certainly bidirectional so that light-induced oscillations in the metal can affect the fluorophore. The interactions of fluorophores with metallic surfaces can have a number of useful effects, including increased quantum yields, increased photostability, increased distances for resonance energy transfer, and decreased lifetimes. These changes can result in increased sensitivity, increased photostability, and decreased interference from unwanted background emission. These effects are called metal-enhanced fluorescence (MEF).