Chemistry Reference
In-Depth Information
established before. The concept of etching was first used in self-assembled mono-
layers [
58
]. Murray's group and Tsukuda's group employed etching reactions to
synthesize highly stable gold nanomolecules [
44
,
59
]. More recently, many
research groups have used etching as a synthetic route for making several stable
nanomolecules [
47
,
60
,
61
].
When a mixture of gold nanoclusters is etched in the presence of excess thiol,
gold nanoclusters can follow two possible routes. The first route is the conversion of
unstable gold nanoclusters to Au-SR polymers [
7
]. The Au-SR aggregates formed
in these reactions are not soluble in any solvent systems and are found as insoluble
material in the reaction flasks. The formation of Au-SR aggregates was proved in
the seminal work by Whetten and coworkers [
7
]. The second route is the conversion
of gold nanoclusters to stable smaller sizes. This process of conversion of larger
gold nanoclusters to smaller sizes is called as
core-size conversion.
During the core-
size conversion reactions, the larger gold nanoclusters in the crude product for
etching are not stable during etching and breakdown to smaller sizes. The smaller
sizes formed are highly stable and accumulate in large quantities in these core-size
conversion reactions.
5.1 Core-Size Conversion Synthesis of Au
38
(SR)
24
and Au
40
(SR)
24
Along with the two-phase Brust-Schiffrin method, one-phase THF method is also
widely used for the synthesis of gold nanoclusters. Murray and coworkers have
reported a one-phase THF/TOABr method for the synthesis of Au
25
(SR)
18
gold
nanomolecules in 2004 [
55
]. In 2008, similar one-phase method in THF for the
synthesis of Au
25
in high yield was reported by Jin and coworkers [
29
]. Recently,
Dass's group studied the synthesis of Au
25
by one-phase THF method by MALDI-
TOF mass spectrometry to understand the nanoparticle size evolution during
synthesis [
2
]. In the one-phase THF method, there are several species observed
between Au
25
and Au
144
, namely Au
67
,Au
103-105
, and a series of clusters smaller
than Au
67
.Au
102
was isolated and crystallized with
p
-mercaptobenzoic acid
[
50
]. However, Au
102
(SR)
44
is not observed with phenylethanethiol and
hexanethiol. Instead, Au
103
(SR)
45
,Au
104
(SR)
45
,Au
104
(SR)
46
, and Au
105
(SR)
46
were observed with phenylethanethiol and hexanethiol ligands. These clusters
together are referred to as Au
103-105
(SR)
45-46
for simplicity throughout
this
chapter [
62
].
When the crude products obtained from one-phase THF reaction were etched in
excess ligand (phenylethanethiol or hexanethiol) at higher temperatures, Au
38
and
Au
40
were observed in MALDI-MS data of the aliquots collected from the etching
reactions. However, the following questions could not be answered from these
etching reactions. Are these clusters formed during the synthesis and survive the
etching process or larger clusters convert to Au
38
and Au
40
? If these nanomolecules
