Biomedical Engineering Reference
In-Depth Information
Fig. 4.17
, of s streched exponential function and PL
intensity I PL for 90 single isolated (isol) QDs and single 30 “QD-PP” nanoassemblies (ass).
Adapted from [ 74 ]
Probability densities ( t bin =
20 s) of
τ
,
β
compared to isolated QDs clearly shifted to shorter decay times
. Similar effects
are observed for the probability density of intensities I PL for isolated QDs and “QD-
PP” nanoassemblies. The related quenching efficiency of one dye molecule per QD
is obviously broadly distributed between weak and near complete PL quenching.
Based on this statistical analysis, calculation of the mean value of
τ
¯
τ
D by
A
D
averaging over all nanoassemblies leads to the value ¯
.
1 ns for the decay
time for QD PL in single “QD-PP” nanoassemblies while it is ¯
τ
13
D
D
.
3ns for
isolated QDs. From this we obtain an average effective quenching rate k DA to be
τ
21
1
τ DA =
1
τ
1
τ DD
k DA =
D
(4.8)
A
k DA =
¯
D
10 7 s 1 .
which results in 1
/
τ
=
2
.
9
×
10 7 s 1 obtained in bulk experiments for the
corresponding “QD-PP” nanoassemblies in titration experiments at x
This result is very close to k q =
3
.
5
×
1 in toluene
[ 74 , 94 ]. From this we conclude that the nature of quenching processes (including
low efficiencies of FRET) is very similar in ensemble and single nanoassembly
experiments. Both experimental approaches being applied for “QD-H 2 P” and “QD-
PDI” nanoassemblies evidently show the existence of strong non-FRET quenching
pathways competing effectively with FRET.
It should be noted that for given CdSe/ZnS or CdSe QDs, ligand dynamics and
non-FRET processes depend critically on the kind of solvent [ 62 - 65 , 74 , 75 , 90 ,
94 , 101 , 127 ]. Most of the FRET investigations on “QD-Dye” nanoassemblies
discussed above have been performed in toluene. Adding polar acetone increases
considerably the QD PL quenching [ 62 ], which indicates the involvement of charged
QD states. On the other hand, quenching has been effectively suppressed in non-
polar n-octane [ 64 ]. To show the influence of solvent properties qualitatively we
discuss here in short the properties of “QD-PDI” nanoassemblies based on TOPO-
capped CdSe/ZnS QDs and DTPP molecules (see structure in Fig. 4.3 ) in a thin
film of the solvent TEHOS (tetrakis-2-ethylhexoxy-silane has long aliphatic chains,
see the structure on the top of Fig. 4.18 ) since efficient FRET has been observed
=
Search WWH ::




Custom Search