Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 193

polycaprolactone and terminal PEG has been found to be an extremely
e ective phase-transfer agent for QDs, giving very stable solutions due to the

intermediate hydrophobic polycaprolactone moiety shielding the binding
group from solution. 221
Other simple polymers used include poly(dimethylaminoethyl methacry-
late) (PDMAEMA), which was used to replace TOPO on CdSe/ZnS QDs,
making them soluble in toluene and methanol. 222 Notably, the radius of the
particle/polymer conjugate almost doubled in diameter from 3 to 5.9 nm,
and the emission dropped 30% in intensity. Pyrene-functionalised
PDMAEMA has also been used to determine the number of polymer strands
attached to a QD using size exclusion chromatography, by monitoring the
amount of free polymer removed from solution upon addition of TOPO-
capped QDs. 223 QDs of CdSe which emitted at 550 nm were found to have on
average 12.5

d n 1 y 4 n g | 6

1.9 polymer molecules attached, whereas the smaller particles
of CdSe which emitted at 520 nm had on average 4.6 0.6 polymer molecules

attached.
Poly(allylamine), poly(sodium styrene sulfonate) and PAA have all been
used to transfer hydrophobic nanoparticles to water using a high-
temperature solution method. 224 A diethylene glycol solution of the polymer
was heated under an inert atmosphere, followed by injection of a toluene
solution of the nanoparticles, forming a cloudy solution. This was followed
by heating at ca. 240 C until the solution cleared, indicating a successful
phase transfer. The particles were isolated by addition of a dilute solution of
acid causing the nanoparticles to precipitate, although they could be redis-
persed in water by deprotonating the carboxylic acid groups. Another simple
polymer, poly( N -isopropylacrylamide) (PNiPAm), a thermoresponsive mate-
rial, has also been used to phase-transfer luminescent QDs, by simply adding
a chloroform solution and shaking, followed by addition of the resulting
solid to water and storage in a refrigerator. 225 The quantum yields dropped
only slightly although the overall size did increase. The resulting material
could also be cross-linked. Not all phase-transfer protocols with polymers
result in simple ligand exchange; some protocols result in thick polymer
shells or polymer particles doped with smaller nanoparticles. 226 - 232
There are, however, only a few polymers that are commercially available
and present the desired range of linking functionalities, solubility and
potential for immediate use. Most polymers have to be prepared speci

.

cally
for use with nanoparticles. Again, the chemistry does not have to compli-
cated; the simple gra

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ing of mercaptoethylamine onto a PAA backbone
yielded a polymer with pendant thiol groups available to coordinate to a QD
surface, with the remaining carboxylic acid groups available for further
conjugation. 233 When used to phase-transfer QDs with multiple shells, the
resulting water-soluble material exhibited emission quantum yields that
were actually higher than the initial hydrophobic sample, although the
hydrodynamic diameter increased notably, suggesting the polymer had
a thickness of about 3 nm. In another example, 234 as shown in Figure 6.7,
a polymeric ligand of ca. 290 repeat units (esters) was functionalised with

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