Chemistry Reference
In-Depth Information
QDs transferred using the hexamer were stable in bu
er solution at pH 6 and
9, whereas the same materials transferred using the monomer displayed
a reduction in emission. QDs transferred by both materials showed a signif-
icant reduction of emission in 5 M NaCl solution, although the species
transferred by oligomer showed less of a drop-o
. Similar work has been
reported by Kim
et al.
258
where a PEG-10,12-pentacosadiynoic acid conjugate
was used to phase transfer QDs in a reverse micelle. Exposure to UV light
resulting in cross-linking across numerous pentacosadiynoic acid groups,
resulting in stabilised micelles,
ca.
150 nm in diameter.
The use of phospholipids was not limited to semiconductor QDs.
Magnetic nanoparticles such as MnFe
2
O
4
were encapsulated in either
1,2-distearoyl-
sn
-
glycero
-3-phosphoethanolamine-
N
-[biotinyl(poly(ethylene
glycol))2000], 1,2-distearoyl-
sn
-
glycero
-3-phosphoethanolamine-
N
-[methox-
y(poly(ethylene glycol))2000] or 1,2-distearoyl-
sn
-
glycero
-3-phosphoethanol-
amine-
N
-[maleimide(poly(ethylene glycol))2000] by simple dissolution in
chloroform along with the nanoparticles.
259
Once the chloroform had evap-
orated, the solid was redispersed in phosphate bu
d
n
1
y
4
n
g
|
6
er solution and heated to
a
ect a clear stable solution. Strands of DNA then could be attached to the
maleimide-functionalised phospholipid. The particles,
ca.
12 nm in diam-
eter, were substantially larger than the QDs described above and might
initially be considered too large for PEG-PE type micelles, although particles
up to 15 nm in diameter were found to be e
ectively passivated. To
demonstrate that the DNA-linked and biotinylated particles were still bio-
logically active, they were attached to substrates through the functional
groups, while the nanoparticles retained their magnetic properties. Fe
3
O
4
particles, capped with oleic acid have also been phase-transferred using
PEG-PE-based phospholipids, speci
.
cally 1,2-distearoyl-
sn
-
glycero
-3-phos-
phoethanolamine-
N
-[methoxy(poly(ethylene glycol))2000], and used in MRI
experiments.
260
In this case, the non-toxic particles were 8
10 nm in diameter
without the PEG-PE capping, and determined to be
ca.
22 nm by light
scattering a
-
er phase transfer. Upon incubation in 1 M NaCl solution, the
PEG-PE capped particles showed no increase in size, whereas iron oxide
particles capped with other polymeric ligands almost doubled in diameter,
indicating the stability of particles capped with the PEG-PE system.
One of the advantages of the ligand system is that numerous phospho-
lipids and mixtures thereof can be easily utilised, allowing the preparation of
multifunctional materials. The phospholipid system has also been extended
to incorporate a gadolinium-containing lipid, Gd-DTPA-bis(stearylamide).
261
Inclusion of this ligand with 1,2-distearoyl-
sn
-
glycero
-3-phosphoethanol-
amine-
N
-[methoxy-(poly(ethylene glycol))] (PEG-DSPE) in the phase transfer
of CdSe/ZnS QDs gave a luminescent water-soluble material with para-
magnetic character and also having the bene
ts associated with the encap-
sulation of the particles in PEG-terminated micelles. The resulting particles
had a high relaxivity of almost 2000 mM
1
s
1
. The ligand could be further
conjugated to Arg-Gly-Asp (RGD) peptides and used in imaging speci
c
endothelial cells using both MRI and optical techniques. Another method of
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