Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 120

In the studies on blended conjugated polymer nanoparticles [ 60 ], it was shown
that about 90% of conversion of blue light into green, yellow, and red emissions can
be achieved by the addition of less than 1% of polymer emitting at these wave-
lengths. It was stated that because of such a composition, the fluorescence bright-
ness of the blended polymer nanoparticles can be much higher than that of
inorganic quantum dots and dye-loaded silica particles of similar dimensions.
Direct application of such wavelength converters is found in multiplex assays
(simultaneous analysis of many targets). In homogeneous-type assays, the possibil-
ity of distinguishing several types of sensor-target interactions can be realized if we
are able to label every type of sensor molecule with a certain “ barcode ” that could
be recognizable in chromatography or flow cytometry. Dyes (and nanoparticles)
emitting at different wavelengths may serve as these barcodes. The requirement for
their excitation with the same light source can be easily satisfied with the same dye
as the FRET donor but with a different cascade of acceptors [ 61 ].
It is easy to intervene into a cascade system by removing and then adding the
intermediates in the FRET process. When the donor and the acceptor are in
proximity but their spectral overlap is insufficiently small for the transfer, the
introduction of a third partner that can serve as an acceptor to the primary donor
and as a donor to the terminal acceptor results in an efficient transfer. Such “ FRET-
gating ” can be used in generating the fluorescence response, as it was shown in the
DNA assay, in which fluorescein-labeled testing DNA was used as a “gate” in the
cascade transfer between conjugated polymer and ethidium bromide intercalated
into a double-helical structure [ 62 ]. New versions of this technology have been
reported [ 63 ].
5.3 Light-Harvesting (Antenna) Effects
The FRET can be directed in such a way that a large number of strong light-
absorbing donors, when excited, transfer their energy to a much smaller number of
acceptors. Because they are excited via efficient energy transfer from many donors,
the fluorescence of acceptors can be increased dramatically. This principle of light-
harvesting is used in the natural systems of photosynthesis that collect an enormous
amount of solar energy by exciting the so-called antenna pigments and redirecting it
to reaction centers. The donors serve as “antennas” to provide the most efficient
collection and transfer of energy to an acceptor. By providing amplification of
acceptor emission, such “ antenna effects ” can be used for optimizing the fluores-
cence properties of many molecular and supramolecular systems, from dimers of
organic dyes to complex nano-composites [ 64 , 65 ].
The antenna effect is illustrated in Fig. 6 . The necessary conditions for its
efficient implementation are the high molar absorbance of antenna dyes, efficient
energy transfer to acceptor dye, and high quantum yield of emission of the latter.
At the same time, while evaluating the highly increased apparent brightness of the