Chemistry Reference
In-Depth Information
a single
functional group on the polymer chain such as a thiol that could be used
to attach a further functionality, which therefore did not require the usual
EDC coupling chemistry.
268
This handle could be used to attach dye
molecules and other biological moieties, notably any material that
contains a maleimide group as the double bond is known to form a stable
C
PAA could also be amended to include a
'
chemical handle
'—
d
n
1
y
4
n
g
|
6
Sbond.
When one considers the structure of an amphiphilic polymer, essentially
carboxylic acid groups along a polymer backbone with hydrophobic tails,
comparisons to the long-chain carboxylic acids are obvious
-
it could be
suggested that the polymers are simply a polymeric version of fatty acid
capping agents. Smith and Nie have developed a new synthetic pathway
based on previous work done with amphiphilic polymers as the
in situ
capping agent rather than a reagent added a
—
er nanoparticle synthesis.
269
In
the case of QD synthesis, metal salts were mixed with the polymer (termed
'
, a term given to amphiphilic polymers used to stabilise integral
membrane proteins that predates the use to phase-transfer QDs
270
), a non-
coordinating solvent (low molecular weight PEG) and a chalcogen source
such as TOPTe/PEG, and injected into the reaction solution at elevated
temperatures. The amphipol released the metal ions during the synthesis
yielding the nanoparticle, to which the amphipol bound through the
carboxylic acid groups. The particles could then be directly added to non-
polar solvents as the pendant alkyl chains allowed dissolution. The particles
could also be directly added to polar solvents, as the amphipols released
from the metal ions during the reaction formed a bilayer structure, exposing
the polar carboxylic acid groups to solution and again allowing dissolution.
The particles were, however, only soluble in one medium at a time and were
not truly amphiphilic
amphipols
'
.
once isolated from a polar solvent, they could not be
redispersed in a non-polar solvent, and
vice versa
(without further process-
ing). The quality of the nanoparticles prepared was comparable to the stan-
dard organometallic route and could be extended to metals and metal oxides.
An interesting observation from what was termed the
—
'
amphibious bath
'
reaction
was the precipitation of particle from the reaction mixture when
methoxy-terminated PEG was used and the reaction mixture cooled to below
50
C. This potentially allowed easy isolation of the particles. Similar reac-
tions with poly(maleic anhydride) have been explored where cysteamine
groups have been gra
ed onto the polymer backbone, yielding a polymer
with a pendant thiol group available for coordination to the particle.
271
Interestingly, chemical modi
cation of 40% of the maleic anhydride groups
was found to be optimum (similar to the modi
ed PAA described above),
giving
ca.
20 thiol groups per polymer chain. QDs with standard ligands
(such as TOPO and long-chain amines) could have them exchanged for the
thiolated polymer with mild heat treatment, and further treatment of the
polymer/QD with a PEG ligand containing amine and hydroxyl groups
resulted in e
cient phase-transfer to water, forming a tightly bound polymer
layer. The resulting particles were notably smaller than those encapsulated in
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