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temperature, undergoes a cycloreversion reaction to produce singlet
oxygen. The singlet oxygen decays to the ground-state triplet oxygen,
exciting the encapsulated squaraine, which in turn fluoresces in the near-
IR region. The exact mechanism of energy transfer from singlet oxygen
to squaraine is not known. The system is very convenient, as rotaxane
endoperoxide forms quantitatively upon irradiation of squaraine rotaxane
solution in the presence of air; the resulting peroxides can be indefinitely
stored at
20 C and near-IR fluorescence appears upon warming the
sample to room temperature.
Scherman and coworkers developed organic nanoparticles with long-
lasting (persistent) luminescence. 255 Nanoparticles based on magnesium sil-
icate doped with luminescent cations (Eu 2 þ ,Dy 3 þ , and Mn 2 þ ) emit upon
irradiation red or near-IR luminescence for several hours after irradiation,
which can be easily detected from the animal body several hours after
injection.
7. CONCLUSION
Despite vigorous research efforts, the optimal fluorophore that fulfills
all requirements for in vivo applications has not been created yet. Given the
diverse applications of fluorescence in in vivo imaging, it is rather unlikely
that a single universal molecular platform can be ever found. Searching
for the new fluorescent materials, modification and improvement of the
existing fluorophores, and conjugation of different fluorophores to combine
their properties—these are the three areas that warrant further progress in the
field of fluorophores. The latter approach seems to be particularly powerful
for creating new systems with unique properties, as illustrated by some of the
recent advances highlighted in this review, such as energy-transfer dyads
with tunable Stokes shift, self-illuminating near-IR BRET systems, or
near-IR luminescent squaraine rotaxanes. On the other hand, advances in
related fields, namely, materials science, nanoscience, and biotechnology,
will likely provide the new materials with currently unattainable properties
and tools for improvements and expansion of the properties of currently
existing fluorophores.
ACKNOWLEDGMENT
The author wishes to thank the University of Maryland, Baltimore County, for supporting
this work (start-up funds and SRAIS award).
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