Chemistry Reference
In-Depth Information
4.1.2 Au
144
(SR)
60
and Au
144-x
Ag
x
(SCH
2
CH
2
Ph)
18
Alloy Nanomolecules
The two-phase Brust-Schiffrin method [
5
] published in 1994 offers a way to
synthesize stable thiol-protected gold nanoparticles. Using comprehensive analyt-
ical tools such as high-resolution transmission electron microscopy, NMR, small-
angle X-ray scattering, and thermo gravimetric analysis, Murray and coworkers
were able to determine the approximate cluster size, number of gold atoms, and
core diameter of certain gold nanoparticles. Different sizes of nanoparticles were
observed ranging from 1.5 to 5.2 nm, containing 100-4,800 Au atoms [
42
]. They
observed nanoparticles with 140 gold atoms as one of the major products in the
experiment conducted at 0
C, twofold excess thiol and rapid NaBH
4
(reduction
agent) addition. Since then, the synthesis of Au
140
(later corrected as Au
144
) was
established with optimized experimental conditions and interesting optical, mono-
layer, and electrochemical properties were revealed [
10
-
12
]. Later Whetten and
coworkers identify Au
140
as gold nanoclusters with 29 kDa core mass based on laser
desorption ionization mass spectrometry and other supporting evidence
[
43
]. Tsukuda and coworkers using electrospray mass spectrometry reported that
this 29 kDa cluster contains 144 gold atoms and 59 ligands, namely Au
144
(SR)
59
[
44
]. However DFT calculations suggested that the composition is Au
144
(SR)
60
[
45
]. Later in 2009, Murray [
8
] and Jin [
46
] achieved high-resolution ESI MS data
for the 29 kDa nanoclusters and assigned the composition to be Au
144
(SR)
60
,as
shown in Fig.
4
. Today, the synthesis of Au
144
(SR)
60
is well established and
research is in progress toward applications in drug delivery and catalysis.
In this section, we introduce the Ag alloy of the 144-Au atom species, namely
the Au
144-
x
Ag
x
(SR)
60
. The synthesis of the Au
144-
x
Ag
x
(SR)
60
[
47
] also follows the
direct synthesis protocol. However, it involves three steps. The first step involves
the direct synthesis of Au
144-
x
Ag
x
(SR)
60
crude product that contains polydisperse
Au-Ag clusters. The second step is the etching of the crude product with excess
thiol to remove meta-stable alloy nanomolecules to obtain Au
144-
x
Ag
x
(SR)
60
in significant quantities. In this case, we used thermochemical treatment as
further purification method. Final step is the isolation of highly monodisperse
Au
144-
x
Ag
x
(SR)
60
nanomolecules using solvent fractionation.
Figure
5
shows the negative mode ESI mass spectra of the nanomolecules
focused on the 3- (triply charged) ions. The bottom curve (black) shows the
ESI-MS of monometallic Au
144
(SR)
60
. Introduction of silver precursor in different
molar ratio in the initial synthesis indicates the formation of Au
144-
x
Ag
x
(SR)
60
alloys with different
x
values(silver atoms). However, interestingly the total
number of metal atoms remains constant at 144. The envelope of peaks observed
in the mass spectra is due to the different number of Ag atoms incorporated.
The mass difference between the consecutive peaks in the mass spectra agrees
with the mass difference between gold and silver (Au
¼
196.97 Da, Ag
¼
107.87
Da,
m
89.1 Da). Experiments at higher silver ratios (e.g., Au:Ag, 1:1) do not
produce stable Au
144-
x
Ag
x
(SR)
60
alloys and decompose upon etching. Synthesized
products using 1:0.25 and 1:0.66 incoming Au:Ag mole ratio show additional
¼
