Chemistry Reference
In-Depth Information
and exposure reaction has been reported by Smith
et al.
, who highlighted the
problems associated with such systems.
119
With the development of the original synthetic method came the
rst
ligand-exchange reactions. Murray elegantly described the use of pyridine as
a labile intermediate when replacing TOPO with other Lewis base species
such as TBP and tributyl phosphine oxide (TBPO), pyrazine and tricetyl-
phosphate.
120
The use of pyrazine, a molecule with two ring nitrogens,
induced bridging of the nanoparticles, resulted in
d
n
1
y
4
n
g
|
6
occulation. Heating
pyridine-capped particles under vacuum reportedly removed all capping
agents, yielding particles with no passivation at all. Pyridine was also used as
an intermediate for the deposition of an inorganic shell. In early work
describing the synthesis of CdSe/CdS core/shell particles, the core particles
had the TOPO ligands replaced with pyridine prior to the epitaxial growth of
the CdS shell,
121
although this procedure appears to be extraneous and the
deposition of the shell can be achieved with the original ligands. This,
however, highlights that the coordination of ligands to a nanoparticle surface
is clearly a dynamic process, in which ligands absorb and desorb, allowing
further interactions such as further shell growth. Later work demonstrated
that up to 80% of surface ligands could be removed from oleic acid-capped
CdSe by repeated exposure to pyridine, although solar cells made from the
resulting QDs decreased in e
ciency due to the resulting increased number
of trap states and aggregation.
122
Murray also alluded to the use of Lewis acid
species, such as tributylborane and trioctylaluminium, as capping agents,
although further details were not reported.
120
A more in-depth report inves-
tigating surface exchange reactions with 4-picoline, 4-(tri
.
uromethyl)thio-
phenol and tris(2-ethylhexyl)phosphate revealed that despite rigorous
processing, 10
-
15% of the original capping agent remained on the particle
surface a
er surfactant exchange.
123
To be an ideal phase-transfer agent, the
resulting transferred materials should exhibit colloidal stability in the
chosen solvent (water for biological applications, over a wide range of pH and
over an extended time period), maintain its inherent optical or magnetic
properties, maintain its original size, provide opportunities for further
functionalisation, and in the case of biological applications, exhibit
non-speci
c binding and avoid unwanted side reactions.
Not all exchange reactions are mass action driven; surface-ligand substi-
tution can be achieved by reaction-based exchange.
21
For example, addition
of bis(trimethylsilyl)selenide/sul
de to alkylphosphonate-passivated CdSe/
ZnSe resulted in the elimination of
O
,
O
-bis(trimethylsilyl)octadecylphos-
phonic acid and the precipitation of the nanoparticles as the new surface cap
failed to stabilise them. Adding related reagents such as
S
-trimethylsilyl-
2,5,8,11-tetraoxatridecane-13-thiol resulted in the cap exchange where the
phosphonate was replaced with the thiolate, giving QDs passivated with
a long polar chain making them soluble in polar solvents, although the
emission was quenched. Similarly, addition of chlorotrimethylsilane and
tridecyltrimethylammonium chloride resulted in chlorine-terminated QDs,
although the emission was quenched only slightly. The use of silylated
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