Biomedical Engineering Reference
In-Depth Information
and that even information on the geometry and stability of the assembly can be
obtained. Obviously, non-FRET quenching does not depend on the electronic details
of the attached dye molecules but on the properties of the solvent or embedding
environment. These observations prompt us to suggest that dye molecules influence
and modify the structure and dynamics of the (TOPO or amine) surfactant (ligand)
shell. Since ligands saturate the dangling bonds at the surface [
48
,
56
-
58
]theyalso
modify the intra-band states or near band edge states of a QD [
48
,
56
-
58
]. This
implies that we have to investigate the competition of dye attachment and ligand
coordination. We will approach this task in the following section.
4.4
Porphyrin and Ligand Exchange Dynamics
in Nanoassemblies
A comprehensive description of dye attachment dynamics in relation to PL quench-
ing processes is of crucial importance for the elucidation of mechanisms of
photoinduced processes in “QD-H
2
P” nanoassemblies (as well as for other dyes)
as a step to an investigation of the chemical topography of a QD surface. In fact,
dye molecules that quench the PL of the QD in a controlled way are an indirect
measure for the ligand dynamics at the QD surface as well as they may influence
surface-related processes.
While the formation of assemblies is manifested via QD PL quenching three
issues remain open: (1) the identification of the average number of dye molecules
attached to the QD surface, (2) the formation dynamics of such nanoassemblies,
and (3) the closely related dynamics of ligands. In each case, the dynamics of
nanoassembly formation is closely related to the ligand dynamics, since the dye
molecule either has to find freely accessible sites on the QD surface or partly
replaces the existing ligand shell. Papers on ligand dynamics [
32
-
34
,
106
,
125
-
128
] mostly deal with uncapped CdSe QDs, since the effects on the PL quenching
following ligand dynamics are much more pronounced for uncapped than for capped
systems, e.g. CdSe/ZnS QDs. In the present section, we present results on capped
CdSe/ZnS QDs which generally exhibit much less variation of PL intensities.
Moreover, the application of dyes offers the possibility to investigate surface
attachments on an extremely low concentration level not accessible by conventional
experiments on ligand dynamics. Additionally, as will be shown later, the use of
dyes allows for an investigation of the microscopic aspects of the quenching process
itself.
Here, we focus on nanoassemblies based on TOPO-capped CdSe/ZnS QDs (as
well as CdSe QDs in some cases) and (
m
-Pyr)
4
-H
2
P molecules showing (like
(
p
-Pyr)
4
-H
2
P) among a series of
meso
-pyridyl substituted free-base porphyrins the
most effective PL quenching of QDs at the same titration conditions (Sect.
4.2.1
,
Fig.
4.6
a). Typically, in titration experiments that study the PL intensity of CdSe or
CdSe/ZnS QDs as a function of the added amount of H
2
P molecules, the spectra
