Chemistry Reference
In-Depth Information
various unique and efficient synthetic methods have been developed to prepare gold
NCs with different core sizes. In general, two entirely independent methods, gas
phase and solution phase syntheses, have been established to obtain monodispersed
gold nanoclusters. In comparison with the unstable metal clusters obtained from gas
phase, the nanoclusters synthesized from solution phase have relatively higher
stability due to the surface passivation by protecting ligands. On the other hand,
through chemical reactions in the solution phase, the core size and the composition
of metal NCs can be easily manipulated by tuning the reaction conditions, such as
the reaction temperature, relative strengths of reducing agents, the ratio of metal
precursors to protecting ligands, and the choice of protecting ligands with different
functional groups. Based on these considerations, modern syntheses of metal
clusters have been mostly achieved in solution phase through chemical processes.
Among the various solution phase-based syntheses, the Brust-Schiffrin or
modified methods have been widely applied to the preparation of gold nanoclusters
due to their simplicity and versatility. In the syntheses, different types of organic
compounds have been used as capping ligands to protect gold nanoclusters, such as
thiolate, phosphine, selenolate, carbonyl, alkyne, DNA, protein cages, and so
on. Generally, there are two most widely employed variants: the original
Brust-Schiffrin method characterized with two-phase system in water and an
organic solvent (toluene) and a modified one-phase system. In the two-phase
method, gold salts (usually HAuCl
4
) dissolved in water were first transferred to
the toluene phase by phase-transferring reagents, such as tetraoctylammonium
bromide (TOABr). Subsequently, organic protecting ligands were added to form
Au(I)SR intermediates, followed by addition of NaBH
4
to produce gold
nanoclusters. Briefly, the two-phase Brust-Schiffrin synthesis includes two pro-
cesses: phase transfer of metal precursors and the reduction of metal ions. The
two-step reaction mechanism can be summarized as follows [
17
]:
AuC1
4
C
8
H
17
Þ
4
C
8
H
17
Þ
4
AuC1
4
ð
aq
Þþ
N
ð
ð
toluene
Þ!
N
ð
ð
toluene
Þ
(1)
m
AuCl
4
3
m
e
ð
toluene
Þþ
n
C
12
H
25
SH
ð
toluene
Þþ
4
m
Cl
ð
!
aq
Þþð
Au
m
Þð
C
12
H
25
SH
Þ
n
ð
toluene
Þ
(2)
where BH
4
is the reducing agent and
m
and
n
are the mole numbers of the metal
precursor and the protecting ligand, respectively. In the one-phase Brust-Schiffrin
method [
18
], polar solvents, such as THF and methanol, are usually used for
preparing gold nanoclusters. In recent years, various modified Brust-Schiffrin
methods have been developed for the synthesis of gold nanoclusters with different
core sizes. For instance, by tuning the stirring rate for the formation of Au(I)SR
intermediates and changing the reaction temperature, Jin and coworkers[
19
] have
successfully synthesized Au
25
nanoclusters in high yield. In another report, Wu
et al. [
20
] introduced a facile, single-phase THF procedure that produced
monodispersed Au
25
nanoclusters via a “size focusing” process.
Except for the atom-precise synthesis of gold nanoclusters, their unique optical
and chemical properties make them potentially useful
in a wide variety of
