Examples in Sensor Applications
Systems with luminescent polymers are often designed to serve as sensors for
various analytical purposes. For example, a hybrid material based on LbL assemblies
composed from perylene, poly(N-vinylcarbazole), and LDH nanoparticles exhibited
a reversible switching of the emission of two colors: violet and blue [83]. The
switching was achieved by modulating FRET from poly(N-vinylcarbazole) to
perylene by changing the concentration of vapors of organic volatile solvents. The
phenomenon was explained by the migration of solvent molecules inside the films
leading to swelling and increased distances between the ED and EA. This led to the
inhibition of the FRET, which was reversible upon the repeated removal or increase
in the vapor pressure of the solvents [83]. For this purpose, proper polymers must be
chosen to sensitively respond to the presence of solvent vapors. Films of similar
properties were designed on the same type of polymer and the complex tris
[2-(4,6-difluorophenyl)pyridinato-C
2 ,N]Ir
III [84]. Cyan luminescence from the Ir
III
complex due to triplet Ir
III -to-ligand and ligand-to-Ir
III charge transfer and also under
conditions of the selective excitation of the polymer proved the occurrence of FRET.
The process was described as two-dimensional and achieved high efficiency. The
presence of the vapors of various volatile organic solvents could interrupt the energy
transfer process [85]. Some of the developed polymeric materials can be also applied
in cancer therapy (Fig. 10). Microcapsules based on MoS 2 and a pH-responsive
polymer (a copolymer of 2-(diethylamino)-ethyl methacrylate and butyl methacrylate) with fluorescent rhodamine end groups are a typical example of a sensor which
can easily penetrate inside the cells. The MoS 2 inside the capsules played the role of
an efficient FRET quencher. The changes in pH in the environment of the microcapsules induced the conformation changes in the polymer chains, which sensitively
affected the efficiency of FRET [86].
4.2.7 Molecular Orientation and Anisotropy
The spatial orientation of EA and ED molecules is a parameter that significantly
affects FRET efficiency. It is a challenge to synthesize systems with a high degree of
organization of fluorophores with controlled intermolecular distances and orientation and a high photoactivity. The difficulties occur due to the limits of the size of
assemblies that can be applicable for devices. Large ordered systems might be
produced quite easily using a supramolecular organization and self-assembly in
two or three dimensions. However, the functionality of such systems often suffers
from luminescence quenching due to the formation of molecular aggregates.
The orientation of dye molecules on the surface of nanoparticles is quite important for FRET efficiency. Determining it for colloidal hybrid systems is quite
problematic, but the orientation of dye molecules in oriented films can be determined
relatively easily using linearly polarized spectroscopy, e.g., [35, 87–89]. The optical
anisotropy of the oriented hybrid films is due to the two-dimensional nature of the
particles and their alignment on the surface and at the same time a preferential
230
J. Bujdák
Systems with luminescent polymers are often designed to serve as sensors for
various analytical purposes. For example, a hybrid material based on LbL assemblies
composed from perylene, poly(N-vinylcarbazole), and LDH nanoparticles exhibited
a reversible switching of the emission of two colors: violet and blue [83]. The
switching was achieved by modulating FRET from poly(N-vinylcarbazole) to
perylene by changing the concentration of vapors of organic volatile solvents. The
phenomenon was explained by the migration of solvent molecules inside the films
leading to swelling and increased distances between the ED and EA. This led to the
inhibition of the FRET, which was reversible upon the repeated removal or increase
in the vapor pressure of the solvents [83]. For this purpose, proper polymers must be
chosen to sensitively respond to the presence of solvent vapors. Films of similar
properties were designed on the same type of polymer and the complex tris
[2-(4,6-difluorophenyl)pyridinato-C
2 ,N]Ir
III [84]. Cyan luminescence from the Ir
III
complex due to triplet Ir
III -to-ligand and ligand-to-Ir
III charge transfer and also under
conditions of the selective excitation of the polymer proved the occurrence of FRET.
The process was described as two-dimensional and achieved high efficiency. The
presence of the vapors of various volatile organic solvents could interrupt the energy
transfer process [85]. Some of the developed polymeric materials can be also applied
in cancer therapy (Fig. 10). Microcapsules based on MoS 2 and a pH-responsive
polymer (a copolymer of 2-(diethylamino)-ethyl methacrylate and butyl methacrylate) with fluorescent rhodamine end groups are a typical example of a sensor which
can easily penetrate inside the cells. The MoS 2 inside the capsules played the role of
an efficient FRET quencher. The changes in pH in the environment of the microcapsules induced the conformation changes in the polymer chains, which sensitively
affected the efficiency of FRET [86].
4.2.7 Molecular Orientation and Anisotropy
The spatial orientation of EA and ED molecules is a parameter that significantly
affects FRET efficiency. It is a challenge to synthesize systems with a high degree of
organization of fluorophores with controlled intermolecular distances and orientation and a high photoactivity. The difficulties occur due to the limits of the size of
assemblies that can be applicable for devices. Large ordered systems might be
produced quite easily using a supramolecular organization and self-assembly in
two or three dimensions. However, the functionality of such systems often suffers
from luminescence quenching due to the formation of molecular aggregates.
The orientation of dye molecules on the surface of nanoparticles is quite important for FRET efficiency. Determining it for colloidal hybrid systems is quite
problematic, but the orientation of dye molecules in oriented films can be determined
relatively easily using linearly polarized spectroscopy, e.g., [35, 87–89]. The optical
anisotropy of the oriented hybrid films is due to the two-dimensional nature of the
particles and their alignment on the surface and at the same time a preferential
230
J. Bujdák