Nanoparticles for in-vitro and in-vivo biosensing and imaging

38 Principles
the time spent in S 1 : for a single experimental decay the 63% relax at t < τ and 37% at
t > τ.
Figure 2.2: A simplified Jablonski diagram.
An opportunity to control the radiative rates arises from the interactions of fluorophores
with nearby metallic surfaces or particles. Nearby metal surfaces can respond
to the fluorophore oscillating dipole and modify the rate of emission and the spatial distribution
of the radiated energy. The electric field felt by a fluorophore is affected by the
interactions of the incident light with the nearby metal surface and also by interaction
of the fluorophore oscillating dipole with the metal surface (excitation enhancement).
Additionally, the fluorophores oscillating dipole induces a field in the metal (emission
enhancement). These interactions can increase or decrease, depending on distance, the
field incident on the fluorophore, its radiative decay rate and the spatial distribution of
the radiated energy.
2.2 Radiative decay engineering
We refer to the interactions of fluorophores with novel metal particles as radiative decay
engineering (RDE) because the radiative decay rates of the fluorophores are modified by
placing them in close proximity to the metal [6] [7] [8].
Prior to describing the unusual effects of metal surfaces on fluorescence it is valuable to
describe what we mean by Radiative Decay Engineering (RDE) and in particular spectral
changes expected for increased radiative decay rates. Spectroscopists are accustomed to
perform experiments using 5 to 10 A ◦ -sized fluorophores in macroscopic solutions, typically
transparent to the emitted radiation. There may be modest changes in refractive
index, such as for a fluorophore in a membrane, but such changes have only a minor
effect on the fluorescence spectral properties. In such nearly homogeneous solution, the
fluorophores emit into free space and are observed in the far field. The spectral properties
are well described by Maxwells equations for an oscillating dipole radiating into free
space. However, the interactions of electromagnetic radiation with physical objects can