the electric double layer around colloidal particles, which concentrate both ions of
positive and negative charge. The association of ions of opposite charges or the
formation of particle-inorganic cation-anionic dye bridges cannot also be neglected.
This phenomenon has not been studied in depth, although FRET would be an ideal
method. The effect of pH indicates that the charge of dye molecules plays an
important role in their adsorption and the stability of the system, which was
confirmed by FRET [47]. It was proven directly by FRET that the adsorption of
zwitterionic RB is much weaker than that of cationic R6G [31]. This interpretation
was based on the repulsive electrostatic forces between the negatively charged group
of the RB and the negatively charged surface of the particles [31]. Interestingly, this
only occurred at high dye concentrations with respect to the adsorption capacity of
the particles. At low surface concentrations, both the RB and the cationic R6G were
adsorbed quantitatively and did not compete with each other. The modification of the
surface of nanoparticles is a strategy for creating adsorption sites for both types of
ions. For example, a phyllosilicate with amino-alkyl groups on the surface proved to
be an optimal type of material to form complexes with dye molecules with anionic
groups [48].
4.2.2 Quenching by Molecular Aggregates
Planar heteroaromatic systems have a large tendency for molecular aggregation and
strong metachromatic properties. The result of their molecular aggregation is not
only a loss of dye photoactivity but also strong photo-quenching properties. The
formation of molecular aggregates is a relatively complex phenomenon that takes
place in hybrid colloidal systems. The high charge density leads to a high density of
the adsorbed counterions of dye, which are favorable conditions for the formation of
H-aggregates with a sandwich-type structure. To date, various types of strategies
have been developed to avoid the formation of molecular aggregates in hybrid
systems with layered nanoparticles. For ionic dyes, the choice of a layered host of
a relatively low surface charge density often helps to reduce the dye molecular
aggregation. The main literature on this topic was reviewed recently [5], and only the
most important facts are mentioned here. The use of surfactants as the third component is a very effective and commonly used strategy to reduce the concentration of
dye molecules, thereby preventing the formation of molecular aggregates. An
effective method is also to select dyes whose molecular structure is non-planar and
contains bulky groups. Competitive FRET between photoactive species and the
aggregates in the systems with layered silicates has been reported [49]. Similar
processes are illustrated in the scheme explaining competitive fluorescence
quenching and FRET occurring in systems of rhodamine and oxazine dyes
(Fig. 3). The molecular aggregation rarely leads to improved photophysical properties or an increased photoactivity. Rare cases involve the formation of J-aggregates
and the phenomenon of aggregation-induced emission, which occur in special cases
of dyes with very specific structures [5].
218
J. Bujdák
positive and negative charge. The association of ions of opposite charges or the
formation of particle-inorganic cation-anionic dye bridges cannot also be neglected.
This phenomenon has not been studied in depth, although FRET would be an ideal
method. The effect of pH indicates that the charge of dye molecules plays an
important role in their adsorption and the stability of the system, which was
confirmed by FRET [47]. It was proven directly by FRET that the adsorption of
zwitterionic RB is much weaker than that of cationic R6G [31]. This interpretation
was based on the repulsive electrostatic forces between the negatively charged group
of the RB and the negatively charged surface of the particles [31]. Interestingly, this
only occurred at high dye concentrations with respect to the adsorption capacity of
the particles. At low surface concentrations, both the RB and the cationic R6G were
adsorbed quantitatively and did not compete with each other. The modification of the
surface of nanoparticles is a strategy for creating adsorption sites for both types of
ions. For example, a phyllosilicate with amino-alkyl groups on the surface proved to
be an optimal type of material to form complexes with dye molecules with anionic
groups [48].
4.2.2 Quenching by Molecular Aggregates
Planar heteroaromatic systems have a large tendency for molecular aggregation and
strong metachromatic properties. The result of their molecular aggregation is not
only a loss of dye photoactivity but also strong photo-quenching properties. The
formation of molecular aggregates is a relatively complex phenomenon that takes
place in hybrid colloidal systems. The high charge density leads to a high density of
the adsorbed counterions of dye, which are favorable conditions for the formation of
H-aggregates with a sandwich-type structure. To date, various types of strategies
have been developed to avoid the formation of molecular aggregates in hybrid
systems with layered nanoparticles. For ionic dyes, the choice of a layered host of
a relatively low surface charge density often helps to reduce the dye molecular
aggregation. The main literature on this topic was reviewed recently [5], and only the
most important facts are mentioned here. The use of surfactants as the third component is a very effective and commonly used strategy to reduce the concentration of
dye molecules, thereby preventing the formation of molecular aggregates. An
effective method is also to select dyes whose molecular structure is non-planar and
contains bulky groups. Competitive FRET between photoactive species and the
aggregates in the systems with layered silicates has been reported [49]. Similar
processes are illustrated in the scheme explaining competitive fluorescence
quenching and FRET occurring in systems of rhodamine and oxazine dyes
(Fig. 3). The molecular aggregation rarely leads to improved photophysical properties or an increased photoactivity. Rare cases involve the formation of J-aggregates
and the phenomenon of aggregation-induced emission, which occur in special cases
of dyes with very specific structures [5].
218
J. Bujdák