Chemistry Reference
In-Depth Information
In an example of a more complicated structure based on calixarenes,
290
CdSe/ZnS QDs were phase-transferred with glutathione, and linked with
(3-aminophenyl)boronic acid and then
-cyclodextrin. Rhodamine B was
then inserted into the cyclodextrin cavity. The dye absorbed at the emission
wavelength of the QD, so excitation of the QD resulted in emission of the
dye
via
a FRET process. In the presence of a speci
b
c analyte, the dye was
displaced and emission from the QD dominated upon excitation. The QD/
cyclodextrin without the dye could also be used as a sensor for various
organic groups, as long as the analyte acted as an electron transfer
quencher.
Related to this is the reversible phase transfer of QDs based on the for-
mation of complexes between ferrocene and
d
n
1
y
4
n
g
|
6
b
-cyclodextrin.
291
In this reac-
tion, TOPO-capped CdSe/ZnS QDs were dissolved in an organic solution
containing either 6-ferrocenyl-1-hexanethiol or 11-ferrocenyl-1-undecanethiol
and gently heated under an inert atmosphere. Reaction of a chloroform
solution of the ferrocene-modi
er solution of
cyclodextrin resulted in phase transfer as the ferrocene unit threaded into
the hydrophilic cyclodextrin, and QD emission was quenched by the cyclo-
dextrin. This was reversed by adding a solution of sodium-2-naphthalene
sulfonate and adamantine carboxylate, known to displace ferrocene from
the hydrophobic interior of the cyclodextrin. Similarly, adamantaneacetic
acid-capped nanoparticles of the up-converting phosphor NaYF
4
:Yb,Er
were phase-transferred from chloroform into water using simple
ed QDs with a phosphate bu
b
-
cyclodextrin, exhibiting no notable loss of emission and with the
nal
.
water-soluble material showing colloidal stability over the pH range 3
-
10.
292
Interestingly, the phase transfer was not successful when
a
-cyclodextrin
was used.
6.10 Biological Molecules as Capping Agents
In some cases, when a new ligand is added to a nanoparticle surface, it is with
the aim of attaching a further biological molecule. However, the use of
a capping agent as a bridge to a biological molecule
135
is not always neces-
sary. The successful use of thiols has resulted in direct ligand exchange for
thiolated biological reagents such as peptides,
293
lipids,
294
glycoconjugates,
295
glycopeptide antibiotics,
296,297
neurotransmitters,
298
-
301
chemotherapeutic
drugs,
302
hormones
299
and DNA,
303
and has now become a common and
simple method of bioactivating semiconductor QDs, based on previous
studies of gold nanoparticle/DNA interactions.
304
-
306
How the biological
molecule of interest orientates itself once linked to a particle is obviously
a key parameter, and reports exist on how for, example, DNA bases interact
with a nanoparticle surface.
307
The coordination of a single strand of DNA
onto a particle surface opens obvious possibilities
attachment of, for
example, a dye molecule to a complementary strand of DNA allowed
investigations in FRET.
308,309
—
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