Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 290

fraction of the injected solution, temperature [ 18 ], ultrasonication [ 19 ], time for
aging [ 20 ], and stabilizer concentrations [ 21 ]. The reprecipitation method is also
called the Ouzo effect [ 22 ]. The Ouzo effect is a phenomenon seen when water is
added to ouzo (anise-flavored Greek liqueurs), a cloudy oil-in-water microemul-
sion form.

2.2 Ion-Association Method

Despite the simplicity in the operation of the reprecipitation method, it is fraught
with some difficulties; for instance, some kinds of “organic” solvents are used for
controlling precipitation processes. The search for ways of controlling size, shape,
and the physical/chemical properties of organic dye nanoparticles in pure aqueous
media without using specific organic cosolvents is still a challenge, and certainly
provides an important aspect in the development of environmental nanoscience.
The present author has developed a novel method called “ion-association
method.” This is also a simple and versatile method for the preparation of ion-
based organic dye nanoparticles in pure aqueous solution by the ion association
approach [ 23 ]. It utilizes the control of hydrophilicity/hydrophobicity of the ionic
material itself via ion-pair formation; for example, addition of a cationic target dye
solution into aqueous solution containing a certain kind of hydrophobic anions
forms an electrically neutral ion-pair because of the strong electrostatic attraction,
followed by aggregation of ion-pair species originated from van der Waals attrac-
tive interactions between them to produce nuclei and the subsequent nanoparticles
(Fig. 3 ). In this case, hydrophobic but water-soluble anions, such as tetraphenyl-
borate (TPB) or its derivatives (tetrakis(4-fluorophenyl)borate (TFPB), tetrakis[3,5-

Fig. 3 Concept of the “ion-association method” for fabricating ion-based organic dye nanoparti-
cles in pure aqueous media. The approach is based on ion-pair formation between the ionic dye
(for example, cationic dye) and the hydrophobic counterion that is soluble in water [for example,
tetraphenylborate (TPB) or its derivative anion], which gives rise to a hydrophobic phase in water.
For preparation, organic cosolvent is unnecessary. The size of the dye nanoparticles can be
controlled by adjusting the interionic interaction between the dye cation and the associative
hydrophobic counteranion