Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 293

between PDDP molecules in the nanoparticles, whereas the CT peak evolution
was probably due to exciton confinement.
2. In DAP nanoparticles (40-160 nm), whose average diameter could be controlled
by varying the aging time, the absorption at the lower-energy side experienced a
bathochromic shift with an increase in the particle size as a result of a change in
the intermolecular interactions, while the higher-energy bands of anthracene
moiety split due to the electronic coupling between the pyrazoline ring of one
molecule and the anthracene unit of the neighboring one. The nanoparticle
emission in the blue region from the pyrazoline chromophore shifted to shorter
wavelengths (from about 480 to 430 nm) with an increase in particle size,
accompanied by a gradual dominance of the emission at about 540 nm from
the exciplex (

heterodimeric molecule formed from two species at an electronic
excited state) formed by the pyrazoline ring of one molecule and the anthracene
moiety of the neighboring molecule.

¼

The reprecipitation strategy lies in the conversion of the products dissolved in a
suitable organic solvent into nanodispersed systems in a different medium by a
precipitation/condensation procedure. On the other hand, the ion-association strat-
egy can produce ion-based dye nanoparticles in pure aqueous media by utilizing a
water-insoluble ion-pair formation reaction. The following example shows the size-
dependent absorption properties for the cation-based pseudoisocyanine (PIC; see
the chemical structure in Fig. 4 ) dye nanoparticles.

3. PIC nanoparticles with average diameters ranging from 64 to 125 nm were
prepared in pure aqueous media using the ion-association method [ 23 ]. Figure 4
shows typical transmission electron microscopy (TEM) images and size distri-
butions of the PIC nanoparticles. Rapid addition of aqueous PIC bromide
solution into the ultrasonicated aqueous solution of sodium tetraphenylborate
(NaTPB) produced the dye nanoparticle dispersion; that is, ion association
between PIC + and TPB and subsequent nucleation and growth of the ion-pair
species (PIC + TPB ) led to water-insoluble nanoparticle formation. The particle

size could be controlled by changing the molar ratio (or, charge ratio) of the
loaded TPB to PIC + (

[TPB ]/[PIC + ]), and were correlated with the surface
charge density of the nanoparticles. On the basis of Gibbs' adsorption equation,
G ¼

¼

, R ,or T are the surface excess of adsorbate,
solute concentration, surface tension, gas constant, or temperature, respectively,
the increase in surface adsorption of ions (

C /( RT )(d g /d C ), where G , C , g

accompanied by an
increase in C ) brings about the reduction of surface tension of nanoparticles (that
is, d

¼ the increase in G

0), resulting in decrease in particle size. The spectroscopic features
exhibit that (a)absorption of nanoparticles was quite similar to that of the dye
monomer, indicating that the PIC chromophores did not aggregate themselves in
the nanoparticle (Fig. 4 ) and (b) the absorption peak originated from the 0-0
band of PIC was red-shifted compared to that of monomer in water. Such a large
red shift essentially comes from the “solvent (or matrix) effect” that is related
to the matrix polarizability. This behavior is a characteristic feature of ion-based
organic nanoparticles. (c) Most interestingly, as the average nanoparticle size

g /d C

<