Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 191

or negatively charged. In this case, either (3-carboxypropyl)trimethylammo-
nium chloride or 2-carboxyethyl phosphonate was simply attached to the
nanoparticle surface, giving either a negative or positively charged surface
species respectively, with the cationic species used to label stem cells. 200 CdSe
QDs have been linked to carboxylated polypyridine ruthenium complexes,
which were oxidised upon transfer of a photoexcited hole. 201
In related work, phenyl groups were gra

d n 1 y 4 n g | 6

ed to CdSe QDs via a family of



carboxylic acids with di ering chain lengths. It was found that ligands with

shorter chain lengths quenched emission more than the longer-chain acids,
which was attributed to charge transfer to the phenyl ring. 202 CdSe QDs have
also been functionalised with electroactive carboxy-terminated thiophene-
based molecules, such as 10-((3-methyl-3,4-dihydro-2 H -thieno[3,4- b ][1,4]
dioxepin-3-yl)methoxy)-10-oxodecanoic acid, for inclusion in solar energy
conversion applications. 203 Notably, the particles required pretreatment with
pyridine before ligand substitution.
Related to carboxylate-induced phase transfer is the transfer using
hydroxyl groups. Dopamine-based molecules, with a triazine central unit and
a terminal PEG group, have been linked to Fe 3 O 4 particles via the diol
functionality, making the initially oleic acid-capped particles water-
soluble. 204,205 Similarly, hyaluronic acid/dopamine conjugates have been
attached to the surface of Fe 3 O 4 nanoparticles. In this case, the nanoparticles
were transferred to water prior to the attachment of the conjugate, and the
ligand attached a erwards, again via the diol functionality. The particles

were then used in labelling experiments with the hyaluronan receptor CD44,
a receptor whose overexpression is linked with various tumours. 206 Dopa-
mine has also been used to link PEG to oleic acid-capped Fe 3 O 4 particles,
giving water-soluble materials with a terminal carboxylic acid group. 207 The
resulting particles were stable in water and were found not to be taken up by
macrophage cells in labelling experiments. Other molecules with ketone
groups have been used to exchange ligands on Fe 3 O 4 nanoparticles, such as
deprotonated quinone-based compounds, which have been used to form
organometallic coordination polymers of nanoparticles. 208
Polymer-based carboxylic acids such as poly(acrylic acid), PAA, have also
been found to be e



.

ective phase-transfer reagents, and can be used to
exchange with oleic acid on Fe 2 O 3 at high temperatures. 209 The bene

ts of
a polymeric capping agent are numerous, and are discussed in the next
section.



6.6.5 Polymers and Dendrimer-Based Surfactant Exchange
As stated, monodentate ligands are relatively unstable, and if the bond to the
particle surface breaks, the functionality is lost. A way round this is to attach
the functional groups to a polymer. Polymers are fast becoming the favoured
way of protecting QDs in biological applications, 210 despite the fact they
increase the hydrodynamic diameter. Numerous papers explore the inter-
action of QDs and polymers (normally conjugated) for applications such as