Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 300

organic nanoparticles. In the presence of PVP, on the other hand, well-
dispersed DASPE nanoparticles ranged in about 30-100 nm in diameter
were successfully synthesized (Fig. 7b ). The nanoparticles were emissive in
dispersion as expected and seen in Fig. 7c , whose photo was taken under UV-
light irradiation. Absorption of the DASPE nanoparticles exhibited a large
bathochromic shift in comparison with that of the dye monomer in water, and
thus the local polarity of the counteranion matrix (TPB or TFPB) had a strong
influence on the red shift of absorption (Fig. 7d ). Moreover, formation of the
DASPE nanoparticles resulted in enhanced fluorescence as compared to that
of the monomeric dye molecule in water -

20-fold enhancement in the
fluorescence quantum yield (Fig. 7e ). The enhancement was dependent on
the counteranion used (using TFPB is more fluorescent than that of TPB), and
can be attributed to both the high frictional resistance of the internal single-
bond twisting of the ethylpyridinium unit and the matrix polarity effect
around DASPE, which can suppress the deactivation processes from the
excited fluorescence states of the chromophore [ 35 ].

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4 Concluding Remarks and Outlook

In this chapter, a review of organic dye nanoparticles consisting of small functional
chromophores was presented. I first described their versatile preparation strategies;
“reprecipitation method” and “ion-association method.” Then I presented unique
spectroscopic properties of some representatives of the dye nanoparticles prepared
to date. One of the most interesting features is the size-dependent optical properties,
which are completely different from the so-called quantum size effect (or quantum
confinement effect) widely observed in inorganic semiconductor nanomaterials or
quantum dots. In addition, I placed emphasis on their fluorescence properties
(namely, fluorescent organic nanoparticles). The highly emissive characteristics
of the dye nanoparticles are generally based on the AIEE that is brought about by a
control of molecular geometry, arrangement, and morphology as well as relevant
molecular interaction and/or matrix polarity.
Currently, research on organic dye nanoparticles is still in its infancy, and much
remains to be done for their future development. For example, developing methods
for fabrication of organic dye nanoparticles with desired sizes, morphologies, and
structures is still a key task. Indeed, the tiny organic dye nanoparticles whose size is
comparable to the inorganic ones (less than about 10 nm in diameter) are still hard
to synthesize. Next, the design and synthesis of molecules with unique spectro-
scopic properties and their controllable construction of nanostructures are a great
challenge. In particular, chromophores having flexible conformations/configurations
or CT characteristics may be interesting. With regard to fabrication of fluorescent
organic dye nanoparticles and their utilization, the following properties should be
also taken into account [ 11 ]; (1) they possess high fluorescence quantum yield,
(2) they are dispersible (soluble) in relevant buffers, or in some cases, cell culture