atoms, whereas larger copper nanostructures did not show emission. In a different
study, a clear relationship between the luminescence properties and the clusters size
was demonstrated in heat-treated [19] and photoactivated [18, 26] silver clusters
confined in zeolitic matrices. Their optical properties were shown to be strongly
influenced by the host crystal structure, the silver content, and the nature of the
remaining counter-balancing ions. Remarkably, these silver cluster zeolite composites displayed high photo- and chemical-stability and high external quantum efficiencies (the external quantum efficiency of a luminescent material is defined as the
ratio of the number of photons emitted to the number of photons absorbed by a
material, this measure takes into account losses associated with the absorption of
photons by non-emissive species, like, for instance, in this case, silver ions and
impurities associated with the starting material) [27], making them very attractive,
for instance, as phosphor materials for several applications, such as in fluorescent
lamps and as upconverter materials in solar cells.
1.1 Synthesis Strategies Developed for the Stabilization
of Metal Clusters
The selective synthesis of metal clusters is heavily hindered by the inevitable
sintering tendency of metal clusters leading to the formation of larger nanoparticles
with the subsequent loss of their peculiar optical properties. Throughout the years,
many strategies have been explored to synthesize well-defined metal clusters. For
instance, mass-selected oligoatomic silver clusters in gas phase have been widely
investigated [1, 2, 28, 29]. Combining experimental and theoretical analysis, the
optical properties and molecular transitions of such small isolated clusters have been
studied. However, as these synthesis conditions are not ideal for most common
applications, alternative stabilization protocols are required. The high coordination
affinity of silver atoms with lone-pair electrons such as in nitrogen has been
successfully exploited by using cytosine-rich DNA strands as a scaffold for small
silver clusters [15]. Similarly, polyphosphates, peptides, or polymers have been
utilized as stabilizing agents for the fabrication of silver clusters. Recently, the use
of rigid confinement scaffolds, such as glasses and zeolites, have been also demonstrated. These strategies will be further discussed in the following sections using
representative examples focusing on silver clusters.
1.2 Luminescent DNA Encapsulated Silver Clusters
Deoxyribonucleic acid (DNA) strands, especially the cytosine base, have shown
strong affinity for Ag ions. In the pioneering work of Petty and Dickson [15], the
fabrication of the first DNA-templated luminescent Ag nanoclusters was demonstrated (Fig. 1). Small silver subnanoclusters (<10 atoms) were produced by
78
E. Coutino-Gonzalez et al.
study, a clear relationship between the luminescence properties and the clusters size
was demonstrated in heat-treated [19] and photoactivated [18, 26] silver clusters
confined in zeolitic matrices. Their optical properties were shown to be strongly
influenced by the host crystal structure, the silver content, and the nature of the
remaining counter-balancing ions. Remarkably, these silver cluster zeolite composites displayed high photo- and chemical-stability and high external quantum efficiencies (the external quantum efficiency of a luminescent material is defined as the
ratio of the number of photons emitted to the number of photons absorbed by a
material, this measure takes into account losses associated with the absorption of
photons by non-emissive species, like, for instance, in this case, silver ions and
impurities associated with the starting material) [27], making them very attractive,
for instance, as phosphor materials for several applications, such as in fluorescent
lamps and as upconverter materials in solar cells.
1.1 Synthesis Strategies Developed for the Stabilization
of Metal Clusters
The selective synthesis of metal clusters is heavily hindered by the inevitable
sintering tendency of metal clusters leading to the formation of larger nanoparticles
with the subsequent loss of their peculiar optical properties. Throughout the years,
many strategies have been explored to synthesize well-defined metal clusters. For
instance, mass-selected oligoatomic silver clusters in gas phase have been widely
investigated [1, 2, 28, 29]. Combining experimental and theoretical analysis, the
optical properties and molecular transitions of such small isolated clusters have been
studied. However, as these synthesis conditions are not ideal for most common
applications, alternative stabilization protocols are required. The high coordination
affinity of silver atoms with lone-pair electrons such as in nitrogen has been
successfully exploited by using cytosine-rich DNA strands as a scaffold for small
silver clusters [15]. Similarly, polyphosphates, peptides, or polymers have been
utilized as stabilizing agents for the fabrication of silver clusters. Recently, the use
of rigid confinement scaffolds, such as glasses and zeolites, have been also demonstrated. These strategies will be further discussed in the following sections using
representative examples focusing on silver clusters.
1.2 Luminescent DNA Encapsulated Silver Clusters
Deoxyribonucleic acid (DNA) strands, especially the cytosine base, have shown
strong affinity for Ag ions. In the pioneering work of Petty and Dickson [15], the
fabrication of the first DNA-templated luminescent Ag nanoclusters was demonstrated (Fig. 1). Small silver subnanoclusters (<10 atoms) were produced by
78
E. Coutino-Gonzalez et al.