Chemistry Reference
In-Depth Information
Fig. 15 MALDI mass spectra of the samples collected at several time intervals from etching of
larger (
40 kDa) clusters. Note that (
1
)Au
130
(SR)
50
is absent initially. Au
130
(SR)
50
is formed by
the core-size conversion of larger (
>
~200 atom) nanoclusters, (
2
)Au
144
(SR)
60
is not present in
starting material or in the final product. So Au
144
(SR)
60
is not a product of core-size conversion in
this study. The absence of Au
144
(SR)
60
in the final product, enables the preparative scale SEC
separation and isolation of pure Au
130
(SR)
50
. Asterisk indicates meta-stable clusters. Peak labeled
# in 12, 36, and 48 h samples corresponds to 76.3 kDa nanomolecule [
60
], which is stable
throughout the etching process
>
formation of Au
130
(SR)
50
was observed with different ligands like dodecanethiol
and hexanethiol.
The molecular weight of Au
130
(SCH
2
CH
2
Ph)
50
is 32,467 Da. Electrospray
ionization mass spectrometry (ESI-MS) generally yields multiply charged peaks
in the mass spectra. These multiply charged peaks appear at a lower mass than the
molecular ions and are better resolved, thereby allowing the assignment of compo-
sition of nanomolecules. In Fig.
16
, positive mode ESI-MS spectrum of the 3+
region of Au
130
(SCH
2
CH
2
Ph)
50
nanomolecules is shown, at 10,822 m/z (32,467/3).
Figure
16
shows all the different Au
130-
x
(Metal)
x
(SR)
50
nanomolecules prepared by
core-size conversion reactions, where R
¼
CH
2
CH
2
Ph, -C
6
H
13
,-C
12
H
25
and
Metal
Pd and Au. Using the mass difference between the peaks corresponding to
Au
130
(SR)
50
with different ligands, the number of ligands in the nanomolecule was
confirmed to be 50. In both hexanethiol and dodecanethiol ligands, the number of
ligands was 50, which adds confidence to the assigned formula. This also proves
that the same core-size conversion is valid in all three ligand systems. Composi-
tional isomers, in short “
Composomers
,” of this 130-metal atom nanomolecule
were prepared by the same core-size conversion reactions. Composomers have
the same number of total metal atoms and ligands, while the number of individual
type of metal atoms varies [
71
]. Au
110
Ag
20
(SR)
50
and Au
115
Ag
15
(SR)
50
are exam-
ples for 130-metal atom composomers. Here, the total number of metal atoms is
130. But the numbers of gold and silver atoms vary. Alloys of several
nanomolecules are evident in the literature [
25
,
33
,
34
,
47
,
69
,
72
].
¼
