Principles of Fluorescence Spectroscopy

856 RADIATIVE DECAY ENGINEERING: METAL-ENHANCED FLUORESCENCE
what types of particles are most effective for MEF and the
properties of these particles that result in metal-enhanced
fluorescence. We have developed a conceptually simple
model that we hope has predictive value for MEF. 57 Mie
theory can be used to calculate the optical properties of
metal colloids when the colloids are smaller than the incident
wavelength. These calculations show that the extinction
of colloids is due to both light absorption and light
scattering. The relative contribution of absorption and scattering
depends on the size and shape of the colloids. In general,
larger particles and non-spherical particles show larger
relative contributions of scatter to the total extinction. At
present we believe the scattering contribution of the total
extinction is the origin of MEF. The scattering component
represents far-field radiation from the induced oscillating
dipole. In a sense this effect is similar to emission. This
similarity can be seen in Figure 25.10, where the scattered
light appears to be visually similar to fluorescence. It seems
logical that the excited fluorophore and nearby metal particle
cooperate in producing far-field radiation at the emission
wavelength. We refer to this concept as the radiating
plasmon (RP) model. 57 If correct, the RP model provides a
rational approach for the design of metal particles for MEF.
The particle or structure should be selected for a high crosssection
for scattering and for a scattering component that is
dominant over the absorption.
25.9. PERSPECTIVE ON RET
At present relatively few laboratories are performing studies
on MEF. 58–65 The early results are confirming the results
from our laboratory. If this trend continues MEF will
become widely used in sensing, biotechnology, and forensics.
The studies described in this chapter were performed
using SIFs, which have a heterogeneous distribution of particle
sizes. In the future we can expect MEF to use betterdefined
particles. Silver and gold colloids can be made with
a variety of shapes, 66–79 some of which may be more useful
for MEF. And, finally, it seems likely that MEF will be performed
using regular particulate surfaces of a type prepared
using nanosphere lithography, 80–82 dip-pen lithography, 83
microcontact printing, 84 and other emerging methods for
nanolithography.
Our vision for the future of RDE is shown in Figure
25.32. At present almost all fluorescence experiments are
performed using the free-space emission. This emission is
mostly isotropic and the radiative decay rates are mostly
constant (top). The use of RDE will allow the design of
Figure 25.32. Comparison of free-space fluorescence emission with
emission modified and directed by metallic structures.
metallic structures that interact with the excited fluorophore.
These interactions can result in modified spectral
properties and directional emission (bottom). In some cases
the directionality will be the result of the excited fluorophores
creating plasmons in the metal, which in turn
result in far-field radiation. The ability to control the emission
process represents a paradigm shift for the field of fluorescence
spectroscopy.
REFERENCES
1. Strickler SJ, Berg RA. 1962. Relationship between absorption intensity
and fluorescence lifetimes of molecules. J Chem Phys 37:814–
822.
2. Toptygin D, Savtchenko RS, Meadow ND, Roseman S, Brand L.
2002. Effect of the solvent refractive index on the excited state lifetime
of a single tryptophan residue in a protein. J Phys Chem B
106(14):3724–3734.
3. Ford GW, Weber WH. 1984. Electromagnetic interactions of molecules
with metal surfaces. Phys Rep 113:195–287.
4. Chance RR, Prock A, Silbey R. 1978. Molecular fluorescence and
energy transfer near interfaces. Adv Chem Phys 37:1–65.
5. Axelrod D, Hellen EH, Fulbright RM. 1992. Total internal reflection
fluorescence. In Topics in Fluorescence Spectroscopy, Vol. 3:
Biochemical applications, pp. 289–343. Ed JR Lakowicz. Plenum
Press, New York.