Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 123

photostability. Conjugated polymers [ 75 ] being used as FRET donors also offer
advanced properties, such as high brightness and superquenching ability.
In the designed nanocomposites, the organic dyes are expected to maintain their
role as FRET acceptors. This is due to their small size and the well developed
chemistry of their covalent attachment to any molecular unit [ 76 ]. In this role, they
keep their strong advantage over other emitters, offering a variety of possibilities of
providing fluorescence reporting in intensity, lifetime, anisotropy and, especially,
spectroscopic changes.

6.2 Plasmonic Enhancement

The spectral properties of fluorophores can be dramatically altered by the near-field
(subwavelength distance) interactions with the free electrons present in metals. The
presence of electron clouds in a metallic structure can alter the optical properties of
a molecule by increasing the excitation field and by modifying the radiative and
nonradiative decay mechanisms [ 77 , 78 ]. It can be seen that on this interaction the
emission lifetime, , increases, contrary

to the commonly expected decrease [ 78 ]. This increase, by 1-2 orders of magni-
tude, is significant, so it can convert weak fluorescence emitters into good ones.
This fact can be of great practical significance.
The generality of this phenomenon observed on metallic nanoparticles (espe-
cially those composed of silver, gold, and copper) is also important. The interac-
tion with metal nanoparticles produces emission enhancement of not only organic
dyes but also Quantum Dots [ 79 , 80 ], conjugated polymers [ 81 ], and metal-
chelator luminophores [ 82 ]. It is characteristic of not only fluorescence but
also phosphorescence and chemiluminescence. In organic dyes, photostability is
increased by decreasing the lifetime. It dramatically influences the FRET effi-
ciency in a donor-acceptor system made of organic dyes adsorbed on silver or
gold nano-island films [ 83 ].
Much effort has been made to establish the optimum distance between the dye
and the surface of the metal particle. If fluorescence from a molecule or nanoparti-
cle is directly adsorbed onto the surface of a metallic particle, it is strongly
quenched due to electron transfer, and if the distance is too large, the enhancement
effect has again to vanish; there should, therefore, be an optimal distance for the
observation of the enhancement effect. Based on experiments with organic dyes
[ 84 , 85 ] and Quantum Dots [ 80 ], this distance should be 5-10 nm.
The optimal enhancement effect is observed when the localized surface plasmon
resonance is tuned to the emission wavelength of a locally situated fluorophore
[ 86 ]. This is consistent with the model suggesting a greatly increased efficiency for
energy transfer from fluorophores to surface plasmons [ 78 ]. Since resonance energy
transfer is involved, the important factors affecting the intensity of fluorescence
emission must also be the orientation of the dye dipole moments relative to the

t R , decreases, whereas the quantum yield, F