Chemistry Reference
In-Depth Information
sensor by detecting the linear luminescence quenching with a detection
limitof1.5
10
9
M.
193
The overall system is very sensitive and speci
c
but is not water-soluble.
The exchange of surface ligands has also been used in attempts to make
linked nanocomposites. Ligands such as 2,2
0
-bipyrimidine and related
molecules were used to link QDs of CdSe or CdS with PbS with the resulting
cross-linked materials examined by electron microscope, infrared, absorp-
tion and emission spectroscopies.
194,195
Similar bipyridine-type ligands have
also been used to develop a new type of photochromic system, by linking
a bipyridinium dication to a nanoparticle surface (although this time
through a thiol group), which can transfer electrons from the particle to the
accepter ligand by irradiation with visible light.
196
The large extinction
coe
d
n
1
y
4
n
g
|
6
cient harvesting of photons and
hence the injection into the bipyridinium ligand resulted in a colour change.
The large absorption cross-section of a QD has also been exploited with an
amine-based dye, where a single dye molecule attached to a CdSe QD surface
reportedly quenched the excitonic emission of the entire nanoparticle,
transferring the energy to the dye molecule, enhancing the emission
characteristics of the dye.
197
cient of the nanoparticles allowed e
6.6.4 Carboxylic Acid-Based Surfactant Exchange
Although not as common as thiol, amine or phosphine-based transfer
ligands, the stability of carboxylic acid groups bound to a nanoparticle
surface make then ideal phase-transfer reagents and this appears to be
a popular method of modifying nanoparticulate metal and metal oxide
surfaces. In this case, as mentioned above, it seems that oleic acid is almost
unique in that, depending upon reaction conditions, it can be used to render
particles soluble in either polar or non-polar solvents,
113
arguably making
surfactant exchange unnecessary in some cases where the solubility can be
altered chemically rather than by actual ligand replacement. The exchange of
surface-bound oleic acid on Fe
3
O
4
for other carboxylic acids with reactive
hydroxyl groups has been reported by Lattuada and Hatton, and highlights
the simplistic nature of phase transfer using carboxylate species.
198
Phase
transfer, however, is not simply a case of changing solvents
.
—
the phase-
transfer ligand o
en requires a pendant functional group, especially if the
ligand is designed for a biological application. One might therefore design
a ligand with a carboxylic acid group to coordinate to a nanoparticle surface,
leaving a further group exposed. Kasture
et al.
have prepared glycolipid-
terminated cobalt nanoparticles, using a capping agent obtained by
a biosynthetic reaction between oleic acid and glucose. It was reported that
the ligand coordinated to the nanoparticle surface through both the double
bond and the carboxylate group.
199
The strength of carboxylic acid binding has also been exploited to manu-
facture electronically coupled assemblies. Carboxy-type species have been
used to phase-transfer Fe
3
O
4
into water leaving the particles either positively
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