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the main-band excitation we have the “directed transfer” resulting in the shift of
fluorescence spectra to longer wavelengths. The dyes excited in the Red-Edge
conditions possess an excitation energy lower than the mean in this ensemble and
the probability of their serving as donors is low.
Finally, it should be noted that homo-FRET, which is just the exchange of
energies between the same dyes, is undetected by common spectroscopic or lifetime
measurements and needs the “hetero-FRET probing” for its detection. The Red-
Edge effect allows the easy distinguishing of the decrease of anisotropy due to
FRET (static effect) from that occurring due to rotational freedom of fluorophores
(dynamic effect), which does not depend on excitation wavelength.
6 Collective Effects in Nanocomposites
The design of fluorescent nanocomposites is a broad area of research and develop-
ment. Different combinations of materials, including semiconductor nanocrystals,
luminescent complexes of lanthanoids, and conjugated polymers can be chosen for
the development of advanced reporters. The combination of organic dyes with other
luminescent materials allows providing many improvements in sensing technolo-
gies, especially because organic dyes are much more “responsive”, but they are
often behind these emitters in photostability, lifetime, and brightness. Two illustra-
tions of these new possibilities are presented below.
6.1 FRET Modulation of Reporter Lifetime
In a donor-acceptor system connected by FRET, the donor lifetime determines the
lifetime of the acceptor [
70
], which follows from the kinetic scheme of this reaction
[
71
]. This fact has serious practical implications. Since the energy transfer occurs
during the lifetime of the donor, its proper choice (e.g., as luminescent lanthanide
complexes) allows increasing dramatically in the acceptor lifetime compared to the
lifetime observed on direct excitation. The long lifetimes allows extending in the
time window for the observation of decays and application of simple instruments
for their recording. This provides a simple approach to discriminating the directly
excited short-lived emission of the acceptor and the background fluorescence in the
time domain [
72
].
With the choice of Quantum Dots as FRET donors [
73
,
74
], the efficiency of
the fluorescence response can be optimized in different directions. These emitters
exhibit a very high degree of brightness that produces a strong antenna effect, the
ability to modulate the emission wavelength within the same material by varia-
tion of particle sizes (for wavelength converting), and the lifetimes of
10
8
s,
which are typically longer than those of organic dyes. They display excellent
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