Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 288

nanoparticles, extensive work is being carried out to extend research on nanostruc-
tured materials of many organic compounds.
This chapter focuses on zero-dimensional nanomaterials of organic dye mole-
cules, that is, organic dye nanoparticles. Note that nanoparticles of organic poly-
mers or micelles are excluded because of their vast studies and fundamental
differences. In Sect. 2 , preparation strategies and protocols that have been devel-
oped for obtaining organic dye nanoparticles are reviewed in detail. The organic
dye nanoparticles are composed mostly of neutral (that is, noncharged) molecules,
but many ionic dyes are also available. Taking the diversity of ionic dye molecules
into consideration, therefore, the present author has developed a simple and versa-
tile method, called “ion-association method,” which can be applied for constructing
ion-based organic dye nanoparticles in aqueous solution. By introducing some
examples of organic dye nanoparticles, their unique optical properties that involve
the size effect are described in Sect. 3 . More attention is paid to the novel and
interesting fluorescent behavior. In conclusion, my personal view of future prospect
in this area is provided.
2 How to Synthesize Organic Dye Nanoparticles: Preparation
Strategies
It is of fundamental and technological interest to understand how physicochemical
properties of organic dye nanoparticles develop as a function of size. The first step
to reach this goal is to synthesize well-defined organic dye nanoparticles in a
controlled manner.
Typically, the organic dye nanoparticle system can be obtained in two ways
(1) top-down approach by mechanical milling of the raw organic materials by wet
or dry milling processes and (2) bottom-up approach by precipitation or condensa-
tion of the organic products in certain solvents [ 16 ]. In method (2), a mixed solvent
system with changes in its composition is frequently used. A purely aqueous system
has been developed by the present author for ion-based organic dye nanoparticles,
which can be classified in category (2). In both cases, various stabilizers, such as
surfactants or polymers, are added if necessary and the function of boundary layer
of nanoparticles by adsorption is taken over. Milling processes (category (1)) are in
principle unsuitable for the production of nanodispersed systems with narrow size
distribution because with decrease in the particle size, it becomes increasingly more
difficult to use applied mechanical energy in the form of shearing and cavitation
forces for particle milling without simultaneously including particle agglomeration
[ 16 ]. Note that a recent development in laser ablation technology may overcome
such a mechanical energy problem in the milling process, although it needs an
expensive short-pulse laser system [ 17 ]. For example, laser ablation of organic
microcrystalline powders, such as red dye quinacridone, dispersed in a poor solvent
is applied for the synthesis of the dye nanoparticles with tuning laser wavelength,