Chemistry Reference
In-Depth Information
phase-transfer agents, and the resulting phase-transferred QDs were stable
for months (unlike cysteine-capped materials) but increased the hydrody-
namic diameter to a larger degree than cysteine.
158
Quantum yields of up to
70% using multishell QDs were obtained, and signi
cantly, the resulting
materials were insensitive to solution pH and the materials could be used
routinely in cell imaging applications with non-speci
c adhesion, a problem
d
n
1
y
4
n
g
|
6
normally addressed by the use of PEG-based ligands.
The use of dithiol-based ligands is not restricted to biological applications.
The addition of an azobenzene chromophore to the PEG group (Figure 6.3B)
on a dithiol-based ligand, which switched conformation from
trans
to
cis
upon activation with UV light and could be reversed by thermal reisomer-
isation has led to a simple photoswitch. The polymeric analogue of the ligand
(Figure 6.4) was prepared, and when attached to the QDs, signi
cantly
reduced the photoluminescence quantum yield when transformed to the
cis
isomer, which was again reversed upon restoration of the con
guration.
159
Similar ligands, which shi
ering pH values,
have been coordinated to QD surfaces by dithiol groups. The emission from
the QDs can be either reversibly increased by electron transfer or quenched
across a pH range of 3
absorption characteristics at di
11.
160,161
The concept of a photoswitch using a QD as
the signal has been extended further, using dithiol ligands containing
a ferrocene unit (Figure 6.3C). Addition of the ligand to CdSe/ZnS QDs
quenched emission, which could be restored by addition of
-
uorine ions that
facilitated photoinduced electron transfer.
162
Dithiol-based linking mole-
cules connected to a silica substrate through a terminal siloxy group have
also been used to tether QDs to an optical
.
bre.
163
Other molecular species with two sulfur groups have also been explored as
phase-transfer agents; dithiocarbamates, prepared
in situ
by addition of the
required amine and carbon disul
de to a QD solution, have been shown to be
e
ective surfactant exchange reagents.
164
QDs with multiple shells, such as
CdSe/CdS/CdZnS/ZnS which had the initial hydrophobic shell exchanged for
a glycine-based dithiocarbamate, exhibited a bathochromic shi
in emission
and absorption spectra, along with a substantial drop in emission quantum
yield of
ca.
40%. A very similar reaction was reported using simple TOPO-
capped CdSe QDs, where the initial quantum yield of
ca.
3% was, unusually,
found to increase to 15% upon phase transfer using a glycine-based dithio-
carbamate; this was attributed to the improved surface passivation.
165
The
addition of phenyldithiocarbamate to a pre-existing solution of TOPO-
capped CdSe QDs has been found to shi
the optical bandgap by up to 220
meV, which was attributed to relaxation of the exciton hole into the ligand
shell.
166
Addition of
N
-2,4,6-trimethylphenyl-
N
-methyldithiocarbamate at
room temperature to a solution of oleic acid-capped PbS QDs resulted in
ligand exchange, blue-shi
le due to
surface etching. The resulting particles were found to be more resistant to
oxidation, allowing the fabrication of e
ing and broadening the absorption pro
cient solar cells using a simple
solution process.
167
The phase transfer of Fe
3
O
4
nanoparticles to water (for
MRI applications) using dimercaptosuccinic acid, a ligand with two thiol
Search WWH ::
Custom Search