Biomedical Engineering Reference
In-Depth Information
a
b
9000
0.3
3
7200
1
2
λ
reg
= 720 nm
5400
0.2
2
2
1
3600
3
0.1
1
1800
0
0
0
5
10
15
20
450
500
550
600
650
700
Molar ratio, x
Wavelength, nm
c
1,1
0,14
1,0
0,12
0,9
0,8
0,10
2
0,7
0,6
0,08
0,5
0,06
0,4
0,3
0,04
1
0,2
0,02
0,1
0,0
0,00
02468 0 2 4 6 8 0
Molar ratio, x
Fig. 4.15
Experimental comparison of TOPO-capped CdSe/ZnS QD PL quenching and (m-Pyr)
4
-
H
2
P fluorescence enhancement in toluene at 295 K. (
a
) Integrated fluorescence intensity of
(m-Pyr)
4
-H
2
P(
1
) and enhanced integrated fluorescence intensity of (m-Pyr)
4
-H
2
P in the presence
of CdSe/ZnS QDs (
d
CdSe
=
3.00 nm,
n
ZnS
=
2 monolayers) (
2
) at the same molar concentrations of
porphyrin and excitation conditions (
465 nm) for both cases. (
b
) Comparison of the absorp-
tion spectrum (
1
) and porphyrin fluorescence excitation spectrum (
2
,
λ
=
exc
720 nm, corrected for
excitation spectral response) for a solution of CdSe/ZnS QD with (m-Pyr)
4
-H
2
P molecules at molar
ratio
x
λ
=
reg
4.0; (
3
) is fluorescence excitation spectrum for alone porphyrin at the same porphyrin
concentration and experimental conditions as for case (
2
). Spectra (
1
), (
2
), and (
3
) are normalized
with respect to the intensity of Q(0,0) porphyrin band (
=
647 nm). (
c
) QD relative integrated
PL intensities
I
/
I
0
(
1
,
left scale
) and efficiency of Foerster resonance energy transfer, based on
the porphyrin fluorescence enhancement (
2
,
right scale
) for CdSe/ZnS QDs upon titration by
(m-Pyr)
4
-H
2
P molecules as function of the molar ratio
x
λ
=
4.3.1
Foerster Resonant Energy Transfer
Usually, for FRET “QD
Dye” in nanoassemblies, the direct verification of
the energy transfer process as a real reason of QD PL quenching should be
done via the direct comparison of the experimental quenching efficiency of the
donor (QD) PL and the efficiency of the acceptor (dye) fluorescence enhancement
(sensitization).
Figure
4.15
collects the corresponding results of this comparison carried out
for TOPO-capped CdSe/ZnS QDs and (m-Pyr)
4
-H
2
P porphyrin molecules upon
variation of the molar ratio
x
.
It should be mentioned that the fluorescence quantum efficiency (
→
) for attached
(m-Pyr)
4
-H
2
P molecules remains almost the same as that for free porphyrin [
62
]. At
ϕ
