Biomedical Engineering Reference
In-Depth Information
Fig. 4.18
Bottom
: Scheme of DTPP orientation on the QD surface (
a
), spectral (
b
), and intensity
(
c
) time traces of “QD-DTPP” nanoassemblies.
To p
: Scheme of TEHOS and of the sample
composition
for single QD-DTPP nanoassemblies under these conditions [
140
]. Samples were
prepared by dip-coating the silicon substrates (with 100 nm thick thermal oxide,
Crystec, Berlin) into solutions of TEHOS in hexane with highly diluted CdSe/ZnS
QDs and DTPP molecules resulting in 10 nm thick films of TEHOS with nanomolar
concentrated CdSeZnS QDs and DTPP at
=
488 nm.
Similar to PP and CdSe/ZnS in toluene [
64
,
74
], mixing CdSe/ZnS and DTPP
in a thin TEHOS film results both in assemblies and in single constituents.
Thus, luminescent spots observed by confocal microscopy show three types of
emission spectra related to either nanoassemblies, QDs or DTPP. About 10% of
the luminescent spots show spectra of both constituents. Figure
4.18
shows the
intensity fluctuations of luminescence for CdSe/ZnS and DTPP as a function of
an observation time of several minutes. Typically, strong intensity intermittency
(blinking [
124
]) for QD PL in combination with spectral fluctuations (spectral
diffusion) is quite obvious [
141
]. In addition, an enhanced blinking of DTPP is
observed in contrast to the almost negligible blinking known for isolated single
molecules [
110
].
A closer inspection reveals that the respective QD and DTPP emission intensities
are strongly (positively) correlated, as can be directly seen in Fig.
4.18
c, when
plotting the corresponding intensities on top of each other: the strong blinking of QD
is imposed onto DTPP, which now blinks very often. DTPP fluorescence is nearly
completely “off” as soon as the QD photoluminescence is turned “off” according to
a blinking process. When DTPP is photobleached or detached from the QD (at 77 s)
the QD PL increases on average by about a factor of 3. There are only three times at
which intensity correlation fails, namely at 56, 66, and 72 s, respectively, for which
DTPP fluorescence decreases while the QD photoluminescence increases. This can
λ
exc
