Chemistry Reference
In-Depth Information
the QD emission, which was stable for an hour, a
er which time it dropped
to 75% of the initial value. The limit of detection was found to be
ca.
9.0
m
M.
280
6.9 Calixarenes and Related Macrocyclic Compounds
Amphiphilic polymers and phospholipids are not the only species to induce
phase transfer by interdigitation; unfunctionalised cyclodextrins also
induced water solubility in oleic acid-capped nanoparticles of metals and
metal oxides by forming inclusion complexes between the hydrophobic
surface species and the hydrophobic cavity of the molecule.
281
Calix[4]arene
carboxylic acids have also been used to phase-transfer QDs into water
extremely e
d
n
1
y
4
n
g
|
6
ectively,
282
while retaining the high quantum yields of
ca.
30%.
In this case, the carboxylic acids were deprotonated to induce water solu-
bility, so the mode of bonding was not clear, although hydrophobic inter-
actions were assigned as the coordination mechanism. The fact that the
carboxylic acids required deprotonation suggested intercalation of the
molecule interior with the surface ligand, although addition of large
concentrations of Na
+
or K
+
quenched the emission by 10%, suggesting
sensing behaviour. Interestingly, the emission of the QDs could be signi
-
cantly shi
ed towards the red end of the visible spectrum simply by using
a larger calixarene,
283
and the exchange of surface sulfur atoms for hydroxyl
groups was suggested as a possible explanation based on similar work with
cyclodextrins.
284
This does seem to point to the direct coordination of the
carboxylic acid group with the QD surface, so clearly the mode of bonding
remains unresolved.
More typical for calixarenes and related macrocyclic compounds is the
use of functional groups, as demonstrated by the use of monothiolated
b
.
-cyclodextrin on CdSe/CdS particles to induce water solubility through
ligand exchange,
285
which has the advantage of freeing the interior of the
bowl-shaped molecule for potential sensing applications. Crown
compounds (15-crown-5) have been attached to CdSe/ZnS particles by
dithiol linkers, and detected potassium ions at levels as low as 10
6
M.
286
Similarly, thioethoxy-functionalised calix[4]arene has been attached to the
surface of CdSe/ZnS QDs, and been used to sense mercury ions
via
uo-
rescence quenching with a limit of detection of 15 nM.
287
More compli-
cated structures have also been reported, such as tris(phenylureido)calix[6]
arene with three pendant long-chain thiols to facilitate coordination to
QDs. This structure was found to decrease the emission quantum yield by
30% when attached to CdSe/ZnS QDs, and induced a bathochromic shi
of 5 nm in the emission.
288
The paper also reported the quenching e
ect
of bipyridinium cations on the QDs emission. Related to this is the use of
pyridyl-substituted free-base porphyrins which have been coordinated to
CdSe/ZnS QDs through amine groups, quenching the emission by a FRET
mechanism.
289
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