Biomedical Engineering Reference
In-Depth Information
Finally, the dynamically controlled attachment process may be an additional reason
for PL quenching in QD-Dye nanoassemblies [
30
,
31
,
33
,
39
,
44
,
106
]. It turned
out that in case of FRET in “QD-Dye” nanoassemblies, the quantitative verification
of an energy transfer process as the dominant reason for QD PL quenching is
only provided by the comparison of FRET efficiencies obtained, on the one hand,
via the donor (QD) PL quenching and, on the other hand, via the sensitization
of the acceptor (dye) fluorescence. This is often missing in many publications
resulting in incorrect assignments of processes and erroneous data evaluation. In
this respect, experimental evidence for such QD PL quenching processes without
a corresponding fluorescence enhancement of dye molecules attached to QD
surfaces was also reported for complexes of QDs and tetramethylrhodamine-labeled
streptavidin [
46
], but mechanisms have not been quantitatively discussed.
With these ideas in mind, we will present in this contribution a comparative
and quantitative analysis of recent quenching and sensitization results for “QD-
Dye” nanoassemblies based on steady-state and PL picosecond time-resolved
measurements showing the existence of a strong competitive non-FRET quenching
process together with the evaluation of the related mechanism. This contribution
is organized as follows. Available data based on a combination of ensemble
and single molecule/particle spectroscopy for “QD-Dye” nanoassemblies will be
discussed. In Sect.
4.2
the basic formation principles for self-assembled “QD-Dye”
nanoassemblies (based on TOPO- and amine-capped CdSe or CdSe/ZnS QDs and
functionalized dyes such as pyridyl substituted porphyrins and perylene diimids) as
well as their spectral-kinetic properties will be highlighted. Section
4.3
is devoted to
the analysis of exciton relaxation pathways and QD photoluminescence quenching
in “QD-Dye” nanoassemblies including FRET and non-FRET processes. Temporal
dynamics of ligand exchange as well as the influence of the solvent and temperature
effects will be discussed in Sects.
4.4
and
4.5
. It will be demonstrated that the QD
surface is inhomogeneous with respect to the involved attachment and detachment
processes, i.e. the formation of “QD-Dye” nanoassemblies is in competition with
exchange dynamics of TOPO or amine ligands and functionalized organic dye
molecules. It will be shown also that very few or even only one attached dye
molecule change the distribution and/or presence of dye-related surface trap states
considerably. The “decoration” of QDs by dye molecules makes a phase transition
of the QD capping ligand shell (at low temperatures) highly visible or even amplifies
this phase transition.
We would like to emphazise that this contribution should be viewed as a
review of a likewise comparative characterization of the non-covalent self-assembly
possibilities and the influence of interface properties on the excitation dynamics
in “QD-Dye” nanoassemblies. It may be pointed out that properties of “QD-
Dye” nanoassemblies are not only interesting in themselves but also provide a
valuable tool to investigate surface-related phenomena in QDs on an extremely low
level of surface modification, thus providing the data for a further development
of defined multi-component structures for exploitation such as artificial light-
harvesting complexes, electro- and photochemical devices or nanosensors.
