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Other nitrogen-based ligands have also been used as surfactants, such as
pentadecanenitrile, which has been used in place of OAm in the preparation
of FePt nanoparticles using standard precursors. Interestingly, the reaction
provided a bimodal size distribution, and it was hypothesised that the nitrile
functionality bonded to surface platinum sites.
78
d
n
1
y
4
n
g
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6
6.4 Thiols
Long-chain thiols are common surfactants and appear to be e
cient capping
agents for most semiconducting and metal nanoparticles. The origin of the
popular use of thiols as passivating agents can be traced back to the work of
Brust on the preparation of gold nanoparticles;
79,80
he chose the surfactant
with reference to the well-known interaction of gold and sulfur.
81
Micic
published the
rst useful reports of thiol-stabilised CdTe particles
82
and
described an in-depth analysis. Studies have been carried out investigating
the potential use of related long-chain selenium and tellurium analogues
such as dodecyl diselenide and dodecyl ditelluride as ligands for passivating
gold nanoparticles (selenols and tellurols are relatively air sensitive and
hence not considered good candidates for passivating ligands).
83
The use of
selenium- and tellurium-based ligands is of interest because of the extended
orbitals relative to the sulfur, possibly resulting in unusual electronic
coupling. However, alkaneselenide-passivated gold particles were found to
be slightly less stable than the almost inde
nitely stable thiolate-passivated
materials, and the alkanetelluride-capped particles were clearly unstable,
with particles precipitating from solution in a matter of days. Medintz has
highlighted that monodentate thiol-based capping agents used in biological
applications of QDs coordinate through dative thiol bonds and are only
stable for days. Bidentate thiols are, however, much more stable, on the order
of years rather than days, and are discussed later.
84
Thiols have been found to
have a binding energy of 0.36 eV (34.7 kJ mol
1
) whereas thiolates have
a binding energy of 13.2 eV (1283 kJ mol
1
).
23
Thiols are therefore generally the surfactant of choice for simple routes to
inert metal nanoparticles that do not utilise organometallic precursors,
although the use of thiols in organometallic-based routes is also becoming
more popular. The actual modes of bonding with the gold surface in particles
prepared by the relatively mild Brust route has been investigated by density
functional theory and this suggests a gold
.
sulfur alloy is formed at the
surface.
85,86
NMR investigations into thiol-capped gold nanoparticles suggest
that the thiol attaches
via
the sulfur atom, and can either keep the sulfur-
bound hydrogen or lose it (forming a thiolate species) depending upon
reaction conditions.
87
Computational modelling of thiols on QD surfaces
con
-
rms that thiolated species bond to surface metal (
i.e.
zinc) and also to
surface sulfur atoms
via
a much weaker bond.
88
Thiols are, however,
extremely reactive and have been known to react with the metal particles in
high-temperature routes, yielding metal sul
de particles.
6
This reactivity of
the thiol has been utilised e
ciently in aqueously prepared CdTe particles,
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