Chemistry Reference
In-Depth Information
In biological applications, this is usually viewed as detrimental as surfactant
exchange is known to reduce emission quantum yield, for example, and the
nanoparticle
'
s magnetic or
luminescent attributes should ideally be
maintained.
One method of phase-transferring particles to water is to encapsulate the
entire particle, including the original ligand, in a micelle structure.
250
This
has the disadvantage of signi
d
n
1
y
4
n
g
|
6
cantly increasing the hydrodynamic diam-
eter,
251
but the increased protection provided by the extra layer of surfactant
and the preservation of the interface integrity by avoiding damage to the
surface by surfactant exchange results in the stabilisation of the nano-
particles
'
properties, notably the optical characteristics. Micelle formation is
also relatively inexpensive, quick and simple, unlike many other methods of
preparing water-soluble luminescent materials.
In the seminal work in QD encapsulation reported by Dubertret
et al.
CdSe/
ZnS QDs capped with TOPO were overcoated with phospholipids, speci
cally
n
-poly(ethylene glycol) phosphatidylethanolamine (PEG-PE) and phosphati-
dylcholine (PC), an extremely simple and e
ective method.
252,253
The PEG-PE
was found to interdigitate with ligands on the nanoparticle surface, leaving
the PEG groups pendant in solution, and PC was found to e
ectively control
the e
ective spacing of the PEG-PE molecules. The resulting particles were
stable in aqueous solution for months, with each micelle usually containing
just one nanoparticle with emission quantum yields of 24%. The initial
nanoparticles were 3
4 nm in diameter, ideally suited for encapsulation
within PEG-PE, but were found to be 10
-
-
15 nm in diameter as determined by
.
microscopy a
er encapsulation. Replacement of PEG-PE with an amino-
functionalised PEG-PE allowed further linking to DNA, opening the potential
for targeted delivery. Investigations into labelling
Xenopus
embryos were
undertaken, with essentially no toxicity observed for what was considered
normal injection concentrations (2
10
9
QDs per cell). At concentrations in
10
9
QDs per cell, abnormalities were observed. Other biological
molecules, such as bovine serum albumin (BSA) have been attached to the
lipid, demonstrating the versatility of the capping layer.
254
This technique has been extended to the use of 1,2-dioctanoyl-
sn
-
glycero
-3-
phosphocholine to phase-transfer CdSe/CdS QDs, which were used to label
rat hippocampal neurons.
255
A nice example of the use of QDs encapsulated
in phospholipids is in their use as a model for the uptake of vasoactive
intestinal peptide (VIP)-gra
excess of 5
ed phospholipids as drug delivery platforms for
breast cancer cells.
256
Although the use of phospholipids appears to be an excellent way of sta-
bilising particles, the structure was only maintained by hydrophobic inter-
actions. Micelles with a more stable shell were achieved by using specially
designed phospholipids that included a norbornene group, which was then
oligomerised to form a phospholipid hexamer.
257
The hexamers were then
used in the same way as simple monomeric phospholipids to stabilise
hydrophobic nanoparticles, although encapsulation required signi
cantly
more hexameric material to ensure phase transfer. Stability tests showed that
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