464
D. P. Singh et al.
5.3.2 Förster Energy Transfer (FRET)
FRET is a mechanism where a donor chromophore, initially in its electronic excited
state, may transfer its excitation energy to an acceptor chromophore through nonradiative long range dipole-dipole coupling. The distance at which resonance energy
transfer (RET) is 50% efficient is called the Förster distance (R o ). There are some
other factors that influence in the FRET process. Dipole-dipole interaction and relative orientation of donor and acceptor, the proximity of guest nanomaterial like
QDs to each other in the host LCM matrix, acceptor concentration relative to donor
concentration, re-absorption of FRET signals by host material may be some intrinsic
factors where as the distance between probes, overlapping integral and wrong choice
of probes are the experimental sources of error [4]. Usually, dynamic quenching is the
signature for the occurrence of FRET which is a commonly observed phenomenon
in the LCM when any semiconducting QDs are dispersed in the host matrix. The
fluorescence lifetime of the LCM changes with the addition of QDs; however, being
an absolute parameter, lifetime doesn’t vary with the concentration of QDs. Lifetime
is also not influenced by the fluctuations in excitation source intensity and hence
it is a precise parameter to confirm the FRET between donor and acceptor. In case
of bi-exponential decay time, one can obtain a shorter and a longer life time. The
shorter decay component reflects the emission from radiative relaxation of excited
electron to ground state and the possible origin of longer decay component remains
relatively uncertain. It is reported that longer decay component in the fluorescence
can be attributed to radiative decay from electron-hole recombination on the surface
involving surface localized states. Quenching of fluorescence properties of donor and
subsequent enhancement in fluorescence properties of acceptor confirm the occurrence of FRET between donor and acceptor. One can also estimate the occurrence of
FRET if the lifetimes of donor-acceptor molecules change significantly. Hirst et al.
[58] have reported that FLS provides a unique method for characterizing the distribution of fluorescent quantum dots (QDs) in the LCM, as energy-transfer processes,
such as FRET, can be detected. In this case, if two particles are within the close proximity of <12 nm, FRET produces an average red-shift in the CdSe/ZnS core/shell
QDs peak emission as a function of the particle separation. In other study, Singh
et al. [4] have investigated the FRET mechanism in the ferroelectric LCM/ CdSe
QDs composites and they found that the FRET was absent in the composites due the
factors like relative orientation and long-range dipolar interaction between the LCM
molecules and the QDs along with the proximity of QDs (>10 nm) in composites.
Therefore, to observe the FRET mechanism effectively, the aforementioned factors
should be taken into consideration during the measurement.
D. P. Singh et al.
5.3.2 Förster Energy Transfer (FRET)
FRET is a mechanism where a donor chromophore, initially in its electronic excited
state, may transfer its excitation energy to an acceptor chromophore through nonradiative long range dipole-dipole coupling. The distance at which resonance energy
transfer (RET) is 50% efficient is called the Förster distance (R o ). There are some
other factors that influence in the FRET process. Dipole-dipole interaction and relative orientation of donor and acceptor, the proximity of guest nanomaterial like
QDs to each other in the host LCM matrix, acceptor concentration relative to donor
concentration, re-absorption of FRET signals by host material may be some intrinsic
factors where as the distance between probes, overlapping integral and wrong choice
of probes are the experimental sources of error [4]. Usually, dynamic quenching is the
signature for the occurrence of FRET which is a commonly observed phenomenon
in the LCM when any semiconducting QDs are dispersed in the host matrix. The
fluorescence lifetime of the LCM changes with the addition of QDs; however, being
an absolute parameter, lifetime doesn’t vary with the concentration of QDs. Lifetime
is also not influenced by the fluctuations in excitation source intensity and hence
it is a precise parameter to confirm the FRET between donor and acceptor. In case
of bi-exponential decay time, one can obtain a shorter and a longer life time. The
shorter decay component reflects the emission from radiative relaxation of excited
electron to ground state and the possible origin of longer decay component remains
relatively uncertain. It is reported that longer decay component in the fluorescence
can be attributed to radiative decay from electron-hole recombination on the surface
involving surface localized states. Quenching of fluorescence properties of donor and
subsequent enhancement in fluorescence properties of acceptor confirm the occurrence of FRET between donor and acceptor. One can also estimate the occurrence of
FRET if the lifetimes of donor-acceptor molecules change significantly. Hirst et al.
[58] have reported that FLS provides a unique method for characterizing the distribution of fluorescent quantum dots (QDs) in the LCM, as energy-transfer processes,
such as FRET, can be detected. In this case, if two particles are within the close proximity of <12 nm, FRET produces an average red-shift in the CdSe/ZnS core/shell
QDs peak emission as a function of the particle separation. In other study, Singh
et al. [4] have investigated the FRET mechanism in the ferroelectric LCM/ CdSe
QDs composites and they found that the FRET was absent in the composites due the
factors like relative orientation and long-range dipolar interaction between the LCM
molecules and the QDs along with the proximity of QDs (>10 nm) in composites.
Therefore, to observe the FRET mechanism effectively, the aforementioned factors
should be taken into consideration during the measurement.