a layered silicate [52]. Both aromatic compounds are not very soluble in water and
were solubilized by encapsulating their molecules in the cavities. At the same time,
the incorporation of the dyes in the cavitand protected them against molecular
aggregation and prevented fluorescence quenching. Highly efficient FRET was
achieved, even with relatively high loadings of the encapsulated dye molecules
[52]. In similar work, the pair of tetracationic Zn-porphyrin and neutral aromatic
molecule of 2-acetyl-anthracene encapsulated inside the cavity of a cationic organic
cavitand was used [67]. Under the condition of an intermolecular distance of only
2.4 nm, an almost 100% efficiency of FRET was observed. In some cases, the
modification using ionic surfactants makes the materials suitable for the adsorption
of neutral molecules. Hostasol red is a hydrophobic and strongly luminescent dye in
aprotic solvents, but its luminescence is significantly quenched in polar solvents. Its
quenching was also observed when the dye was adsorbed onto a zeolite L surface
[69]. An increase in luminescence quantum yield could be achieved if the potassium
cations in the zeolite were exchanged for imidazolium cations. The dye molecules
embedded within the channels protected by perylene dye co-adsorbed onto the
external surface, resulting in highly luminescent materials in which FRET from
the perylene dye to Hostasol red occurred. Effective FRET was also applied at an
ED/EA ratio of 100:1 [69]. Another example is hydrogels, which were based on Lap,
cationic cyclodextrins, pseudorotaxenes, and organic dyes exhibiting high quantum
yields and efficient FRET [70]. Adjusting the various ratios of the interacting dyes
led to different colors of emitted light, including white light composed of a proper
spectrum of polychromatic light. The tunable luminescent properties of the hydrogel
will open up new possibilities for the preparation of new luminescent materials.
4.2.5 Covalently Bound Dye Molecules
Relatively stable hybrid materials can be prepared in particular by covalently
attaching chromophoric groups to the surface of nanoparticles. This method requires
the use of reactive dyes or precursors that can react with specific groups on the
surface of the particles. An example can be the coupling of reactive silanes with
chromophore groups by reacting with the OH groups on the surface of silicate
nanoparticles [51, 71]. Occasionally, such reactive precursors may be used as part
of reaction mixtures for the synthesis of hybrid nanoparticles [72, 73]. Surface
heterogeneity at the nanometer scale is a very important factor in the modification
of nanoparticles with dyes. An example is the selective modification of silicate
particle edges [51, 71]. Some such systems exhibited very efficient FRET [51, 72,
73]. The pair of coumarin and cyanine dye derivatives were used in an energy
transfer study [72]. The coumarin moieties were incorporated into the phyllosilicate
structure during the synthesis of the silicate. A reactive silane derivative with a
coumarin group was used. The second dye was intercalated into the coumarin/
silicate complex (Fig. 9). In this work, the phenomenon of energy transfer was
investigated in detail, and it was possible to distinguish the transfer by both radiation
and non-radiation mechanisms [72]. The photophysical interaction could be
226
J. Bujdák
were solubilized by encapsulating their molecules in the cavities. At the same time,
the incorporation of the dyes in the cavitand protected them against molecular
aggregation and prevented fluorescence quenching. Highly efficient FRET was
achieved, even with relatively high loadings of the encapsulated dye molecules
[52]. In similar work, the pair of tetracationic Zn-porphyrin and neutral aromatic
molecule of 2-acetyl-anthracene encapsulated inside the cavity of a cationic organic
cavitand was used [67]. Under the condition of an intermolecular distance of only
2.4 nm, an almost 100% efficiency of FRET was observed. In some cases, the
modification using ionic surfactants makes the materials suitable for the adsorption
of neutral molecules. Hostasol red is a hydrophobic and strongly luminescent dye in
aprotic solvents, but its luminescence is significantly quenched in polar solvents. Its
quenching was also observed when the dye was adsorbed onto a zeolite L surface
[69]. An increase in luminescence quantum yield could be achieved if the potassium
cations in the zeolite were exchanged for imidazolium cations. The dye molecules
embedded within the channels protected by perylene dye co-adsorbed onto the
external surface, resulting in highly luminescent materials in which FRET from
the perylene dye to Hostasol red occurred. Effective FRET was also applied at an
ED/EA ratio of 100:1 [69]. Another example is hydrogels, which were based on Lap,
cationic cyclodextrins, pseudorotaxenes, and organic dyes exhibiting high quantum
yields and efficient FRET [70]. Adjusting the various ratios of the interacting dyes
led to different colors of emitted light, including white light composed of a proper
spectrum of polychromatic light. The tunable luminescent properties of the hydrogel
will open up new possibilities for the preparation of new luminescent materials.
4.2.5 Covalently Bound Dye Molecules
Relatively stable hybrid materials can be prepared in particular by covalently
attaching chromophoric groups to the surface of nanoparticles. This method requires
the use of reactive dyes or precursors that can react with specific groups on the
surface of the particles. An example can be the coupling of reactive silanes with
chromophore groups by reacting with the OH groups on the surface of silicate
nanoparticles [51, 71]. Occasionally, such reactive precursors may be used as part
of reaction mixtures for the synthesis of hybrid nanoparticles [72, 73]. Surface
heterogeneity at the nanometer scale is a very important factor in the modification
of nanoparticles with dyes. An example is the selective modification of silicate
particle edges [51, 71]. Some such systems exhibited very efficient FRET [51, 72,
73]. The pair of coumarin and cyanine dye derivatives were used in an energy
transfer study [72]. The coumarin moieties were incorporated into the phyllosilicate
structure during the synthesis of the silicate. A reactive silane derivative with a
coumarin group was used. The second dye was intercalated into the coumarin/
silicate complex (Fig. 9). In this work, the phenomenon of energy transfer was
investigated in detail, and it was possible to distinguish the transfer by both radiation
and non-radiation mechanisms [72]. The photophysical interaction could be
226
J. Bujdák