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previously been reported and FRET experiments explored.
236
Multifunctional
designer polymers have also been used with magnetic iron oxide nano-
particles for biological applications.
237
In this example, a polymer with
various subunits has been utilised, with a dopamine group for attachment to
the iron oxide particle, a dye molecule to allow cellular imaging and a free
amine group for further conjugation to other species including double
strands of RNA/polymer conjugates, which allowed the particles to mark the
cells wall of a Caki-1 cell line. The preparation of such polymeric species
allowed nanoparticles to retain their inherent optical or magnetic properties
while providing additional groups that induced water solubility and allowed
conjugation to di
d
n
1
y
4
n
g
|
6
ering molecules of interest to biologists, while inhibiting
non-speci
c binding and maintaining a small hydrodynamic diameter, all
key factors in biological labelling.
One notable feature of the general use of polymers is the resulting increase
in hydrodynamic diameter (in some cases up to 40 nm
119
)
—
an undesirable
side e
ect of phase transfer, as the renal clearance limit of nanoparticles is
suggested to be 5.5 nm.
139
An elegant way round this has been described by
Smith and Nie, who replaced
ca.
35% of the carboxylic acid groups on PAA
with amine and thiol functionalities.
238
QDs of CdTe were then initially
phase-transferred to water using thioglycerol, which was then replaced with
the polymer. This combination of thiol and amine reportedly resulted in the
polymer wrapping round the QD, giving a shell thickness of 1.5
-
2nm,
signi
cantly smaller than if the parent PAA polymer alone was used. In this
case, the photoluminescent stability of the CdTe was maintained (probably
due to the use of thiol groups) with quantum yields of up to 50% if the
optimum capping ratio was used.
A related family of structures, dendrons, have also been used as capping
agents for semiconductor QDs.
239
Hydroxy-terminated dendrons (genera-
tions 1
.
-
3) with a thiol focal point were prepared from the parent disul
de
dendrimer and were linked to TOPO-capped CdSe particles by re
uxing an
alkaline solution of methanol, nanoparticles and dendron in the dark.
96
A two-phase procedure was also found to be e
ective, where the hydrophobic
nanoparticles were dissolved in ethyl ether, the dendrons dissolved in water
at pH 10.3 and both solvents mixed, e
ecting a phase transfer
via
surfactant
exchange. The resulting structures, where the TOPO is replaced by the den-
drons, were water-soluble and extremely stable to chemical processing and
resistant to photo-oxidation. The use of higher-generation dendrons resulted
in increased protection, attributable to the steric e
orded by the
increasingly branched structures. The dendron terminal groups could then
be functionalised further by an amide coupling reaction, resulting in struc-
tures that were soluble in either water or organics, depending on the func-
tionality. This is signi
ects a
cant, as the dendrons present a wide range of
coupling chemistries comparable to organic reagents used in biochemistry.
Another type of third-generation dendron investigated was notable for the
eight alkene terminal functionalities (Figure 6.8), which were cross-linked by
ring closing metathesis (RCM) reactions which resulted in a dendron
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