Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 124

normal metal surface and the overlap of the absorption and emission bands of the
dye with the plasmon band of the metal.
An immense variety of potentially useful nanocomposites can be constructed
exhibiting enhanced emission.
7 Realization of Collective Effects in Sensing
and Imaging Technologies
In conventional fluorescence sensor techniques, single dye molecules are used as
reporters. The introduction of supramolecular structures containing multiple dyes
can increase the output signal and thus achieve lower limits of detection. A
convenient technique to create these structures is to produce nanoparticles of
inorganic [ 8 ] and organic [ 7 ] polymer origin that are “soaked” with dyes. There
may be different additional benefits of such incorporation, such as the isolation of
the dyes from molecular contact with the test medium and protection against
perturbation of their properties. The possibility that the degradation of dyes
in aggressive media and in living cells may be avoided is also important. The
dyes usually exhibit much higher fluorescence intensity, quantum yield, and life-
time in solid environments than in liquid media. This is due to the stronger
restrictions imposed by a solid matrix on the rotation of dye segments, intermolec-
ular collisions, and dielectric relaxations - all the factors that quench fluorescence.
Porosity of nanoparticles is another property that can be modulated and optimized.
The larger surface of nanoparticles may allow the attachment of several functional
units - instead of one - that can allow facile bioconjugation, immobilization, and
incorporation into various assays. Additional benefits can be gained with the
exploration of the collective effects described above. They can include:
l Shifting of wavelength by engineering the local environment of dye molecules,
their complexation, and directed homo-FRET.
l Selection of the most convenient excitation wavelength and shifting of wave-
length in a broader range using cascade hetero-FRET.
l Engineering the rigidity of the dye environment and switching off the homo-
FRET by variation of the excitation wavelength due to Red-Edge effect.
l Superenhancement due to light harvesting in FRET and plasmonic effects and
superquenching due to homo-FRET with the participation of a quencher.
7.1 Collective Effects in Sensing
To determine many analytes simultaneously in solutions, the nanoparticles or larger
microspheres carrying particular sensors should be recognizable in their mixtures
by distinct spectroscopic properties [ 87 ]. This allows the mixing of the uniquely