Chemistry Reference
In-Depth Information
surfactant exchange has been improved by the use of dihydrolipoic acid
(DHLA), a bidentate thiol with a carboxylic acid group that is more stable
than the monodentate ligand. Particles of CdSe/ZnS capped with the
bidentate ligand were found to retain quantum yields of up to 20% in
water.
150
-
152
The one drawback is the agglomeration of DHLA-capped nano-
particles in acidic media (due to the carboxylic acid group requiring depro-
tonation by a base to impart solubility in water). This has been overcome by
the use of engineered ligands, based on a DHLA anchoring group linked to
a hydrophilic PEG unit (see Figure 6.3A),
153,154
which can also be followed by
a terminal functional group for further bioconjugation.
149,155,156
QDs passiv-
ated with such ligands have been successfully used in cellular imaging
applications and displayed stability over a range of pH values and ionic
concentrations. As expected with thiol ligand substitution reactions, a drop
in quantum yield from
ca.
70% to
ca.
30% was routinely observed. The
colloidal stability of QDs could be impressively enhanced even further, over
a wider range of pHs (although quenching was observed a
d
n
1
y
4
n
g
|
6
er just a few hours
at pH values <2) for several months by the use of bis(DHLA)PEGylated ligands
which provided four thiol anchoring points per PEG chain for coordination
to the particle surface.
157
QDs capped with the bis(DHLA) ligands could be
conjugated to peptides and used in cell imaging studies.
Earlier studies which showed the use of zwitterions (such as cysteine) as
e
ective capping agents yielding particles with small hydrodynamic
diameters
138
inspired the use of DHLA-based sulfobetaine zwitterions as
.
Figure 6.3
A range of dithiol-based ligands: for biological applications (A), as
a photoreactive switch (B) and as a ferrocene-based photoswitch (C).
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