single crystal X-ray diffraction analysis. A metal core containing Ag 6
4+ species
encapsulated within a cubic arrangement of the thiolated template was then
proposed.
1.4 Polymer-Encapsulated Silver Clusters
One of the most versatile strategies to fabricate luminescent water-soluble silver
clusters is based on the utilization of synthetic polymers as encapsulating and
stabilizing agents. The first evidence of the fabrication of polymer-encapsulated
silver clusters was presented by Henglein [43], in this study silver ions were reduced
in the presence of sodium poly(phosphate), as consequence colloidal particles
containing silver clusters were generated. In subsequent reports, this concept was
further developed, and synthetic polymers of different nature were used
[44, 45]. However, the first study dealing with the fabrication of luminescent longlived polymer-encapsulated silver clusters was carried by Dickson and Zheng [46],
by using second and fourth-generation OH-terminated dendrimers as scaffolds and
photon irradiation as activation procedure. They were able to synthesize luminescent
silver nanoclusters (Ag 2 –Ag 8 ) with emission maxima ranging between 500 and
650 nm. The use of these materials as biological labels was then suggested. In a
different report carried by Diez and co-workers [47], the tunability of the emission of
silver nanoclusters encapsulated in poly(methyl methacrylic acid, PMAA) by varying the polarity of the solvent was demonstrated. This work set the basis for the use
of these materials for molecular sensing. In the majority of the previous examples,
UV-Vis irradiation was mostly used as activation tool to generate well-defined silver
clusters with narrowed size distribution. The use of chemical reducing agents was
employed to a lesser extent since less specificity in the formation of the silver cluster
was observed (mixture of silver clusters and nanoparticles). An example of an
alternative and selective method to synthesize fluorescent polymer-protected Ag
clusters was presented by Xu and Suslik [48]. By using a sonochemical approach
and varying the synthetic conditions, it was possible to control the optical properties
of silver clusters encapsulated in PMAA polymers. Due to the recent developments
in the synthesis of polymer-encapsulated luminescent silver clusters, multiple applications have been proposed for these materials such as bioimaging, chemical and
biosensing, single molecule studies, and catalysis.
1.5 Silver Clusters Confined in Hard Template-Based
Inorganic Systems
Inspired by the work performed by scientists centuries ago on stained glass, several
research groups started to explore the fabrication of well-defined silver clusters in
Highly Luminescent Metal Clusters Confined in Zeolites
81
4+ species
encapsulated within a cubic arrangement of the thiolated template was then
proposed.
1.4 Polymer-Encapsulated Silver Clusters
One of the most versatile strategies to fabricate luminescent water-soluble silver
clusters is based on the utilization of synthetic polymers as encapsulating and
stabilizing agents. The first evidence of the fabrication of polymer-encapsulated
silver clusters was presented by Henglein [43], in this study silver ions were reduced
in the presence of sodium poly(phosphate), as consequence colloidal particles
containing silver clusters were generated. In subsequent reports, this concept was
further developed, and synthetic polymers of different nature were used
[44, 45]. However, the first study dealing with the fabrication of luminescent longlived polymer-encapsulated silver clusters was carried by Dickson and Zheng [46],
by using second and fourth-generation OH-terminated dendrimers as scaffolds and
photon irradiation as activation procedure. They were able to synthesize luminescent
silver nanoclusters (Ag 2 –Ag 8 ) with emission maxima ranging between 500 and
650 nm. The use of these materials as biological labels was then suggested. In a
different report carried by Diez and co-workers [47], the tunability of the emission of
silver nanoclusters encapsulated in poly(methyl methacrylic acid, PMAA) by varying the polarity of the solvent was demonstrated. This work set the basis for the use
of these materials for molecular sensing. In the majority of the previous examples,
UV-Vis irradiation was mostly used as activation tool to generate well-defined silver
clusters with narrowed size distribution. The use of chemical reducing agents was
employed to a lesser extent since less specificity in the formation of the silver cluster
was observed (mixture of silver clusters and nanoparticles). An example of an
alternative and selective method to synthesize fluorescent polymer-protected Ag
clusters was presented by Xu and Suslik [48]. By using a sonochemical approach
and varying the synthetic conditions, it was possible to control the optical properties
of silver clusters encapsulated in PMAA polymers. Due to the recent developments
in the synthesis of polymer-encapsulated luminescent silver clusters, multiple applications have been proposed for these materials such as bioimaging, chemical and
biosensing, single molecule studies, and catalysis.
1.5 Silver Clusters Confined in Hard Template-Based
Inorganic Systems
Inspired by the work performed by scientists centuries ago on stained glass, several
research groups started to explore the fabrication of well-defined silver clusters in
Highly Luminescent Metal Clusters Confined in Zeolites
81