Chemistry Reference
In-Depth Information
Fig. 9 Positive MALDI mass spectra of the samples collected from etching of pure Au
67
in the
presence of excess thiol. After 50 h, Au
40
(SR)
24
is the only product in the reaction mixture. Please
note that no Au
40
(SR)
24
is present in the initial sample. This indicates that Au
40
(SR)
24
is
exclusively formed via a core-size conversion process from Au
67
. For a discussion on the
intermediates observed in the core conversion reactions, refer to later sections. The scheme depicts
the conversion of Au
67
to Au
40
, with a possible loss of (Au-SR)
m
by-products
reaction, the 10 h sample shows several meta-stable species in the 13-17 kDa mass
region. However, most of the Au
67
has disappeared from the reaction mixture and
peaks corresponding to Au
40
(SR)
24
were observed in the MALDI mass spectrum. In
the samples examined at 23 and 39 h, the amount of meta-stable clusters has
decreased and no peaks corresponding to Au
67
were observed. With the progress
of the core size conversion reaction, Au
40
(SR)
24
is the only species observed in the
reaction mixture. Au
38
(SR)
24
could not be observed in the final product. These
results prove that Au
40
(SR)
24
is formed by core-size conversion of Au
67
(SR)
35
.
To study the etching and core-size conversion reactions, the Au
103-105
fraction
obtained from the crude product after SEC was reacted with excess thiol separately.
Upon etching, this fraction has core-converted to Au
40
(SR)
24
. Figure
10
shows the
progress of this etching reaction monitored by MALDI mass spectrometry. In the
MALDI mass spectrum of 3 h sample, there were some meta-stable clusters
observed around ~14 kDa mass range. With time, all the Au
103-105
clusters in
the reaction mixture disappeared and only the meta-stable species were observed
in the reaction mixture. In the 8 h sample, Au
40
(SR)
24
was observed along with
some other meta-stable clusters. After 11 h, Au
40
(SR)
24
was the only product in the
reaction mixture, suggesting that Au
103-105
has core-converted to Au
40
(SR)
24
.
Some insoluble residue was also observed in these etching reactions. These are
presumably Au-SR aggregates, as discussed in an earlier report by Whetten and
coworkers [
7
]. This suggests that 100% of the metal atoms and ligands present in
the Au
103-105
sample do not convert to Au
40
. This is also the case with other core-size
conversion reactions. Some of the starting material forms Au-SR by-products. So far,
