Biomedical Engineering Reference
In-Depth Information
Tabl e 4. 4
Comparison of
time-dependent luminescence
intensities for DAP and
DTTP (structures are
presented in Fig.
4.3
)for
variable concentration ratios
x
.
I
end
corresponds to the
relative luminescence
intensity for long times, τ
q
is
the time to reach
I
end
Dye molecule
x
I
end
τ
q
[min]
DAP
1.6
0.75
±
0.02
43
±
8
3.2
0.71
±
0.01
30
±
2
5.4
0.67
±
0.02
20
±
5
7.5
0.59
±
0.02
17
±
3
10.5
0.62
±
0.02
11
±
2
DTPP
1
0.74
±
0.04
103
±
31
3
0.66
±
0.04
81
±
22
5
0.53
±
0.02
62
±
9
7
0.63
±
0.01
34
±
5
10
0.56
±
0.01
28
±
2
up to 66% [
140
]. To investigate this strongly solvent-dependent FRET efficiency in
more detail, we have carried out a series of experiments for these nanoassemblies in
a toluene/TEHOS mixture at different relative ratios
R
(Figs.
4.30
and
4.31
).
From the comparison of results (Fig.
4.31
) obtained after titration with DTPP
at
x
5-7 and presented in Fig.
4.30
two observations are immediately obvious:
(1) the time scales of QD PL quenching and DTPP fluorescence increase are much
faster for the mixture of the solvents (
R
=
50:50) as for TEHOS samples discussed
before and (2) the DTPP fluorescence first increases strongly during about 9 min
and thereafter decreases again on a much longer time scale. The letter process is
accompanied by a red shift of the DTPP fluorescence band maximum by about 5 nm
(see Fig.
4.30
). This red shift equals the shift of the fluorescence maximum when
measuring DTPP in TEHOS or in toluene. This observation might be explained by
a time-dependent separation of the two solvent components in such a way that at
the beginning “QD-Dye” nanoassemblies are predominantly solvated in a TEHOS
and later on in a more toluene rich environment. This is supported by the finding
that FRET efficiency decreases as is expected for a nanoassembly in toluene. For
a solvent mixture at
R
=
20:80 DTPP fluorescence properties are more closely to
those in TEHOS, while they are more close to a dye fluorescence in toluene for
R
=
80:20. Correspondingly, we find a strong “overshoot” of luminescence intensity
for
R
=
80:20.
In titration experiments the excitation wavelength was
=
20:80 which is completely lost for
R
=
412 nm, which
corresponds to an absorption minimum for DTPP and a 100 times stronger QD
absorption. For this reason directly excited DTPP emission is very low and almost
only established via FRET. Figure
4.31
shows a comparison of emission intensities
of “QD-DTPP” nanoassemblies at different times after the titration step as a
function of the solvent mixture ratio
R
.
It can be clearly seen from Fig.
4.30
that the DTPP fluorescence is very low both
for
R
λ
ex
=
0:100 even after long waiting times. On the contrary,
strong DTPP emission at
R
=
100:0 and for
R
=
50:50 is immediately present after titration and
further increases in time. Comparing the related time scales of increasing FRET
in Fig.
4.31
a-c reveals that they are similar as already shown in Table
4.3
for
QD PL quenching and DTPP fluorescence increase. For
R
=
=
100:0 (only toluene)
