5.2.2 Atomically Precise Clusters of Gold and Silver—
Synthetic Routes
The production of silver and gold nanoclusters can be performed following several
routes. The metal ions from dissolved metal salts can be reduced, either by a
chemical reductant (e.g., sodium borohydride), (Fig. 5.3), or by light (photoreduction with near-ultraviolet light), or by c-rays (by radiolysis of water). The
chemical reduction and the photoreduction are the most commonly used methods
[32]. The specific properties of metal nanoclusters, such as the composition, stability, and fluorescence quantum yield, depend largely on the scaffold used during
reduction.
Thiols are frequently used on noble metal substrates because of the strong affinity
of sulfur for these metals. And thiolated ligands (-SR) have appeared to be extremely
good candidates to produce ultrasmall nanocluster sizes, in particular for gold [33].
Following the pioneering work of Brust et al. [34] based on the reduction of the
metal precursors and the formation of metal core, thiol-containing small molecules
were extensively used to stabilize gold and silver nanoclusters in the aqueous
solution [4, 35]. The use of thiol-containing small molecules as stabilizers permits to
better control the production of gold NCs (AuNCs) than phosphine-capped ones,
contributing to the stronger Au-S covalent bonding. Generally, the method of synthesizing thiolate-capped AuNCs proceeds as follows. Gold salts [AuCl 4 ]
− are
dissolved in water and then transferred to an organic solvent by phase transfer agent;
the thiols are added to the mixture inducing reduction of Au
3+ ions into Au
+ ions and
form Au
+
–SR complexes or polymers; then, the Au
+ polymers are reduced by
adding the reducing agent leading to thiolate-protected gold nanoclusters.
Glutathione (GSH), a ubiquitous low-molecular-weight thiol, played a significant role in producing gold NCs which showed good water solubility, bioactive
surface, and high stability. Whetten and coworkers have proposed an unprecedented
thiol-protective AuNC route by using the GSH (N-c-glutamyl-cysteinyl-glycine) as
the stabilizer. The as-synthesized AuNCs were fractionated by using polyacrylamide gel electrophoresis (PAGE) and characterized by mass spectrometry (MS)
[37]. Tsukuda and colleagues have also reported the characterization of fractionated
AuNCs protected by GSH monolayers. The as-prepared AuNCs were isolated into
single-sized Au n (SR) m clusters by the PAGE method and characterized using
electrospray mass spectrometry [38, 39]. Then, Kumar et al. [36] bridged the gap to
use glutathione as a ligand to produce protected AgNCs that were isolated into
single-sized Ag n (SR) m clusters by the PAGE method and characterized using
electrospray mass spectrometry.
While the routes for producing metal NCs lead to a mixture of Au n (SR) m
(Ag n (SR) m ) [40] cluster size, achieving atomic precision and molecular purity is
challenging because the nanocluster growth is extremely complicated and remains
poorly understood. Nevertheless, a systematic methodology called “size focusing”
for achieving atomically precise clusters of gold and silver with molecular purity
has been proposed. This methodology consists of two primary steps [41]:
144
R. Antoine
Synthetic Routes
The production of silver and gold nanoclusters can be performed following several
routes. The metal ions from dissolved metal salts can be reduced, either by a
chemical reductant (e.g., sodium borohydride), (Fig. 5.3), or by light (photoreduction with near-ultraviolet light), or by c-rays (by radiolysis of water). The
chemical reduction and the photoreduction are the most commonly used methods
[32]. The specific properties of metal nanoclusters, such as the composition, stability, and fluorescence quantum yield, depend largely on the scaffold used during
reduction.
Thiols are frequently used on noble metal substrates because of the strong affinity
of sulfur for these metals. And thiolated ligands (-SR) have appeared to be extremely
good candidates to produce ultrasmall nanocluster sizes, in particular for gold [33].
Following the pioneering work of Brust et al. [34] based on the reduction of the
metal precursors and the formation of metal core, thiol-containing small molecules
were extensively used to stabilize gold and silver nanoclusters in the aqueous
solution [4, 35]. The use of thiol-containing small molecules as stabilizers permits to
better control the production of gold NCs (AuNCs) than phosphine-capped ones,
contributing to the stronger Au-S covalent bonding. Generally, the method of synthesizing thiolate-capped AuNCs proceeds as follows. Gold salts [AuCl 4 ]
− are
dissolved in water and then transferred to an organic solvent by phase transfer agent;
the thiols are added to the mixture inducing reduction of Au
3+ ions into Au
+ ions and
form Au
+
–SR complexes or polymers; then, the Au
+ polymers are reduced by
adding the reducing agent leading to thiolate-protected gold nanoclusters.
Glutathione (GSH), a ubiquitous low-molecular-weight thiol, played a significant role in producing gold NCs which showed good water solubility, bioactive
surface, and high stability. Whetten and coworkers have proposed an unprecedented
thiol-protective AuNC route by using the GSH (N-c-glutamyl-cysteinyl-glycine) as
the stabilizer. The as-synthesized AuNCs were fractionated by using polyacrylamide gel electrophoresis (PAGE) and characterized by mass spectrometry (MS)
[37]. Tsukuda and colleagues have also reported the characterization of fractionated
AuNCs protected by GSH monolayers. The as-prepared AuNCs were isolated into
single-sized Au n (SR) m clusters by the PAGE method and characterized using
electrospray mass spectrometry [38, 39]. Then, Kumar et al. [36] bridged the gap to
use glutathione as a ligand to produce protected AgNCs that were isolated into
single-sized Ag n (SR) m clusters by the PAGE method and characterized using
electrospray mass spectrometry.
While the routes for producing metal NCs lead to a mixture of Au n (SR) m
(Ag n (SR) m ) [40] cluster size, achieving atomic precision and molecular purity is
challenging because the nanocluster growth is extremely complicated and remains
poorly understood. Nevertheless, a systematic methodology called “size focusing”
for achieving atomically precise clusters of gold and silver with molecular purity
has been proposed. This methodology consists of two primary steps [41]:
144
R. Antoine