Chemistry Reference
In-Depth Information
Fig. 6 Some cluster
aggregation reactions of
small molecular gold cluster
cations
Cluster aggregation reactions
[Au
8
(PPh
3
)
8
]
2+
+
Au(NO
3
)(PPh)
3
[Au
9
(PPh
3
)
8
]
3+
2X
-
(X= Cl or SCN)
[Au
11
Cl
2
(PPh
3
)
8
]
+
[Au
9
(PPh
3
)
8
]
3+
+
[Au
13
(PMePh
2
)
10
Cl
2
]
3+
[Au
11
(PMePh
2
)
10
]
3+
+
[AuCl(PMePh
2
)]
on having a suitable initial mixture of colloid particles and the “ageing” and
“etching” processes have the effect of weeding out the less stable particles and
thereby increasing the concentration of the more stable component. They have
suggested that this procedure is adaptable to other Au
n
(SR)
m
particles. Pradeep and
his co-workers [
90
] have characterised some Au
15
nanoclusters anchored to cyclo-
dextrin via core “etching” reactions of larger clusters Au(SG) by treating them with
a tripeptide based on
N
-
γ
-glutamyl-cysteinyl-glycine (GSH) and cyclodextrin in
water at 70
C for 48 h. The GSH acts as the etching agent and cyclodextrin as the
host cavity. Dass has used “etching” processes based on thiophenol in THF to
convert mixtures containing primarily Au
68
and Au
102
clusters and required
refluxing at 80
C with vigorous stirring. In this way [Au
36
(SPh)
23
] was purified
and characterised using MALDI-TOF mass spectrometric techniques [
81
,
91
]. Jin's
group [
92
,
93
] has been particularly effective in using size-exclusion chromato-
graphy (SEC) for separations which have resulted in monodispersed [Au
40
(SR)
24
]
from [Au
38
(SR)
24
] clusters and Zhu has separated [Au
18
(SePh)
14
] from related
clusters with 15, 19, 20 clusters using HPLC with an SEC column [
94
].
Tsukuda and Whetten have used gel electrophoresis (PAGE) to effectively
develop the chemistry of gold colloids capped by GSH [
95
-
101
]. Recent develop-
ments in the field include the development of procedures which can be scaled up
[
102
], and the development of catalytically active nanoparticles supported on
polymers [
103
,
104
], one-pot synthesis [
79
] and simplification by Dass [
47
,
105
]
who demonstrated that almost no fragmentation of Au nanoclusters was observed in
the MALDI-TOF mass spectra of Au
25
(SCH
2
CH
2
Ph)
18
clusters using
trans
-2-
[3-(4-
tert
-butylphenyl)-2-methyl-2-propenyldidene] malononitrile (abbreviated as
DCTB) as matrix with low laser pulse intensity (just above the threshold intensity).
Since then, more gold nanoclusters with various compositions have also been
characterised by the efficient MALDI-TOF MS technique.
Figure
7
indicates that pure Au
25
nanoclusters [
79
] may be prepared by a
two-step procedure from H[AuCl
4
] which is first reduced to Au(I) by thiols and
subsequently reduced by NaBH
4
to metallic clusters. Low temperatures and slow
stirring conditions are critical for the formation of the right type of Au(I):SR
intermediate species, which lead to the formation of Au
25
in high yield in the
second reduction step. The high purity of Au
25
nanoclusters achieved using this
