Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 301

media, (3) they are sufficiently stable and thus provide reproducible quality,
(4) they have functional groups for site-specific labeling, and (5) they have suitable
size for suitable interactions between the nanoparticle and the environment. Note
that a different group of organic nanomaterials, such as vesicles and dendrimers,
has been applied to in vivo fluorescence imaging; for instance, polyamidoamine
dendrimer-based fluorogenic substrates are designed to image tumor-associated
matrix metalloproteinase-7 (one of an extracellular matrix-degrading metallopro-
teinase) in vivo [ 40 ]. This enables us to anticipate the future practical use of organic
dye nanoparticles as a new probe. For further broad utilization, novel performances
such as optical waveguides, thin film, and pattern fabrications [ 41 ] on solid
substrates are also important.
The uniqueness of organic dye nanoparticles can also make them the building
blocks for many potential applications, such as light harvesting, drug delivery,
chemical and biochemical sensors, and so forth. In particular, light harvesting,
which is the trapping of energy via peripheral chromophores and funneling to a
central point where it is converted back into visible light, is highly anticipated
because of large amounts of absorbing chromophores on the surface periphery in
the nanoparticle, providing a high probability for the capture of light. When the
ideally designed syntheses of dye nanoparticles are successful, the relatively short
distance from the periphery to the core would allow for highly efficient energy
transfer as dendrimers do [ 42 ]. The mechanism for light harvesting in organic dye
nanoparticles will begin with the periphery chromophore molecules capturing the
energy of photons from light. Resonance energy transfer or migration between
chromophores then occurs mostly by the F¨rster excitation transfer mechanism,
where the energy is transferred through-space via dipole-dipole interactions.
Through energy transfer in the nanoparticles, we can use the energy (for example,
fluorescence enhancement) that is channeled to the nanoparticle core.
In relation to the resonance energy transfer phenomenon, doping technology
may help to improve emission efficiency and tune emission colors, which have been
widely used in electroluminescent (EL) devices [ 43 ]. In a doped system, the slight
variation of the content of energy acceptor will result in significant emission color
change. In addition, the energy transfer significantly increases the fluorescence
quantum yield of the energy acceptor [ 44 ]. A pioneer work on doped organic dye
nanoparticles with tunable emission has been reported for the DCM-doped
1,3,5-triphenyl-2-pyrazoline (TPP) nanoparticles prepared by a simple reprecipita-
tion of the binary mixture, with DCM as the energy acceptor and TPP the donor
[ 45 ]. DCM, 4-dicyanomethylene-2-methyl-6-(4-dimethylaminostyryl)-4 H -pyran,
is a well-known fluorescent dye with red emission widely used in EL devices.
The emission colors of the doped dye nanoparticles evolved from blue to red with
increasing DCM concentration. Other than tuning emission by size, the work
provides an alternative method to control the emission of organic dye nanoparticles
by employing the doping technique.
Finally, it should be noted that, as the concentration of the absorbing chromo-
phores is increased to the point where efficient energy transfer takes place between
them, the energy can be dissipated by an alternative route known as “concentration