Biodetection Based on Fluorescence Quenching and Surface-Enhanced Fluorescence Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 226

10.2.2.2 Localized enhancements
The EM ield of a luorophore can be signiicantly enhanced near
metal nanomaterials when their sizes are much smaller than the
illumination and/or detected wavelengths. When metal objects
are much smaller than the wavelength, the EM ields are localized,
giving rise to inhomogeneities in the EM ield distribution. In fact,
any discontinuity between a metal and a dielectric gives rise to
inhomogeneities in the EM ield distribution, a factor which is useful
for the speciic excitation of a given luorophore. 50 Any localization of
EM intensity near metal nanomaterials that couples to propagating
modes increases the luorescence of a luorophore by several orders
of magnitude. These high EM effects have been realized in various
non-linear phenomena such as second harmonic generation 51-54
and Raman scattering. 55
Researchers have studied the spatial localization of light in
a number of metal systems, including single metal NPs, metal
aggregates, and metal ilms. 56-58 Each system has its speciic
enhancement properties linked closely to its geometry. Single metal
ellipsoids, for instance, are sources for generating an exceptionally
large luorescence enhancement (up to 50-fold). The luorescence
of a given luorophore can be increased by more than 2 orders of
magnitude by varying the gap (usually less than 50 nm) between
two metal NPs, a procedure which is also wavelength dependent.
Inhomogeneous metal ilms, given their random nature, are expected
to enhance the luorescence of various luorophores simultaneously,
allowing multiple luorescence imaging.
In addition to the composition and shape, the size of the NPs
plays an important role in determining SEF. There are many different
interactions that have dramatically different effects on SEF, which
are dependent on the luorophore emission frequency ( ω ), on the
distance ( d ) between the luorophore and the metal, and on the size
of the metal NPs. 59
(i) When distance d is on the order of 1 nm, for the low frequency
regime, ω ≤ ω SP , where ω SP is the surface plasmon frequency.
The dominating phenomenon is Landau damping, 60 an
eficient damping of the luorescence.
(ii) When d ≥ 1 nm, for ω ≈ ω SP , coherent excitation of the surface
plasmon occurs on metal NPs that act as a nanoantenna,
enhancing the strength of the dipole oscillator and inducing
enhanced luorescence. It is worth noting that these