Chemistry Reference
In-Depth Information
After the Murray's group report on the synthesis of Au
25
(SR)
18
nanomolecules
several other synthetic protocols were reported to increase the final yield of the
nanomolecules. Wu et al. reported a one-phase THF method [
29
]. In this method,
gold salt and thiol are added to THF, without any water, as both the gold salt and
ligand are nicely soluble in THF. Then the reducing agent NaBH
4
is added to the
reaction mixture and the reaction is monitored by UV-vis spectroscopy [
29
].
We have used MALDI-TOF mass spectrometry to monitor the size evolution of
Au
25
(SR)
18
in one-phase THF method. In this report, the aliquots from the reaction
were collected at different time intervals and analyzed by MALDI-TOF mass
spectrometry. While Jin's work showed that Au
25
(SR)
18
is formed in the reaction
with time, this work nicely shows several gold nanoclusters that are formed in the
synthesis and how the proportion of various nanomolecules evolves with time to
yield Au
25
(SR)
18
. Figure
1
(left) shows the MALDI-TOF mass spectra of the
samples collected at different time intervals.
Murray's group recently published another protocol to synthesize anionic
Au
25
(SR)
18
in high yield using phenylethanethiol ligand [
30
]. In this modified
protocol, tetraoctylammoniumbromide (TOABr) is added to the THF along with
the gold salt. Addition of TOABr aids the formation of anionic Au
25
(SR)
18
in high
yields (~50% yield) and stability (see Figure
1
right). Murray has recently summa-
rized all the published work on Au
25
(SR)
18
in the form of a review article [
1
].
Besides the synthesis and characterizations, atomic structures of Au
25
(SR)
18
nanomolecule were reported independently by both Murray and Jin research groups
[
13
,
31
]. In the Au
25
(SR)
18
family, several alloys have been reported by
different research groups including Au
25-
x
Ag
x
(SR)
18
[
24
], Au
24
Pd(SR)
18
[
32
],
Au
25-
x
Cu
x
(SR)
18
[
33
], and Au
24
Pt(SR)
18
[
34
]. Interestingly, Ag atom doping results
in maximum heteroatom incorporation, i.e. up to 12 silver atoms.
The fundamental question arising from Au
25-
x
Ag
x
(SR)
18
alloy nanomolecules is:
Where are the silver atoms incorporated in the atomic structure? This question can
be addressed in two different ways. 1
. Computational/theoretical studies
. Lin and
Mingos provided general rules for understanding the geometric preferences of alloy
clusters in the 1980s and more recently DFT calculations have been used to
interpret these preferences in thiolate clusters [
35
]. For example, Walter
et al. [
36
] and Guidez et al. [
37
] have shown that the surface of the icosahedral
core is energetically preferred for silver. Others [
38
] have also predicted the same.
Jiang and coworkers theroretically predicted the doping of the icosahedral gold core
of MAu
24
(SR)
18
, with several heteroatoms [
35
]. The synthesis and structural
characterization of icosahedral [Au
9
M
IB
4
Cl
4
(PMePh
2
)
8
][C
2
B
9
H
12
] alloy and its
substitutional preferences were discussed [
39
,
40
]. 2.
Experimental determination
using single crystal X-ray analysis
. Here, X-ray crystallography analysis was used
to determine the location of silver doping.
Au
25-
x
Ag
x
(SCH
2
CH
2
Ph)
18
alloy nanomolecules were synthesized using the
direct synthesis methodology reported by Negishi [
24
]. Several batches of Au:Ag
atomic combinations were synthesized using different Au:Ag incoming molar
ratios. Crystallization of alloy nanomolecules were set up using vapor diffusion
method. Crystals were obtained in dried form in the batch of Au:Ag 1:0.25
