Is the dopant concentration plays a role for this transfer of energy? What are the possible reason if we do not find the energy transfer (absence of fret)? actually with the addition of 0.1 to 0.3 wt% of CdSe QDs in FLC (Felix 16/100 and SCE4), we didn't observed FRET. So I am confused for the phenomenon behind it. Size of QDs is 3.5 nm.
I am not familiar with ferroelectric liquid crystals so my answer will be pretty generic: possible reasons are distance and orientation.
For the distance: in a solution with free donor/acceptor and under normal irradiatio condition, the distance between an emitting donor and an acceptor will depend on the acceptor concentration alone (Principle of Fluorescence Spectroscopy, Lackowicz).
The other problem that can occur and is often overlooked is the relative orientation of donors and acceptors. It's the kappa^2 factor that is assumed equal to 2/3 to reflect a random configuration. It is often overlooked because it is difficult to control, I would guess that in a liquid crystal relative orientation can become a problem.
Make sure your donor concentration is not higher than your acceptor concentration.
I think you may have to check also overlap area between donor emission and acceptor absorption.
May be you can check the medium in which you are doing the experiment. There could be a compounds in the medium that could quench the energy from the donor rather than by acceptor.
Because as mentioned by others, for the FRET to happen
1.you only need Donor and Acceptor at a right distance (2-6nm) and in the right concentration.
2.More importantly check how much spectral over lap you have between Donor and Acceptor.
3.Quantum efficiency of Donor and acceptor
One additional reason could be quenching of acceptor fluorescence by QDs via, e.g. charge transfer process. So, even if FRET is efficient between donor and acceptor, quenching process could be so strong, that the acceptor fluorescence is completely desappear. You should also check, if the donor fluorescence quantum yield changes.
There is a couple of papers where QDs are quenched by different mechanisms than FRET, e.g. electron transfer (e.g. Abey Issac) or non-FRET quenching of CdSe QDs. I studied such effects on CdSe/ZnS and Perylenebisimide molecules. My former boss Christian von Borczyskowski worked a lot on such systems. You can check his literature for such phenomena. QDs photolumenscence (quantum yield) and its surface passivation are directly related, both can be affected by solvent polarity. There is plenty of possibilities to observe no FRET in CdSe type QDs...;-)
Thanks dear Danny. Can you send me your some papers regarding this problem on [email protected]. Thanks in advance.
FRET is based on interaction of optical transition dipoles of a donor and an acceptor molecule. This works in general for singlet transitions of an organic molecule, but not necessarily for a quantum dot.
The typical reasons of the lack of FRET are :
i) the distance between the probes (too big)
ii) overlap integral close to 0
iii) wrong choice of probes (distance vs. Foerster radius)
iv) other factors like e.g. acceptor fluorescence quenching caused by buffer composition
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