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Fig. 16 Positive mode ESI-MS mass spectra showing 3+ region of Au
130
(SR)
50
with several
ligands phenylethanethiol (PC2), hexanethiol (C6), dodecanethiol (C12), Au
130-
x
Ag
x
(SR)
50
and
Au
130-
x
Pd
x
(SR)
50
. The purpose of this figure is to show that (a) the core-size conversion works
with various ligands and alloy systems, (b) the 130-atom core has special stability associated with
its structure
5.3.1 Core-Size Conversion in Au-Ag Alloy Nanoclusters
Core-size conversion process is also repeated with the alloy systems. Alloy gold
nanomolecules like Au
144-
x
Ag
x
(SR)
60
and Au
38-
x
Ag
x
(SR)
24
have been reported
previously [
47
,
69
]. Using a mixture of gold (HAuCl
4
) and silver (AgNO
3
) salts,
the crude product for alloy nanoclusters was synthesized. From this crude
product, clusters larger than 40 kDa were isolated using SEC and etched in the
presence of excess thiol at higher temperatures. During these etching reactions,
Au
130-
x
Ag
x
(SR)
50
was observed in the reaction mixtures. Figure
17
shows the
positive mode ESI-MS of Au
130-
x
Ag
x
(SR)
50
formed by two different Au:Ag pre-
cursor ratios. When 1:0.1 Au:Ag ratio was used in the synthesis of crude product,
Au
130
(SCH
2
CH
2
Ph)
50
was the major peak in ESI-MS of purified product, with
about a maximum of three silver atom incorporations. When the precursor Au:Ag
ratio was increased to 1:0.15, the number of silver atoms incorporated has increased
with Au
125
Ag
5
(SCH
2
CH
2
Ph)
50
being the most intense peak in the ESI-MS spectra
of the isolated product. But the maximum number of silver atom incorporations was
increased to 20. With Au:Ag ratio of 1:0.1, the ESI-MS of the isolated product
shows some peaks to the left of Au
130
(SR)
50
. These were assigned to
nanomolecules comprising of 131 metal atoms. When the Au:Ag precursor ratio
was further increased, there was no Au
130-
x
Ag
x
(SR)
50
observed in the reaction
