Few-Atom Silver Clusters as Fluorescent Reporters - Advanced Fluorescence Reporters in Chemistry and Biology - page 309

clusters exhibit electrogenerated chemiluminescence (also called electrochemilu-
minescence) [ 20 ]. In summary, silver clusters have an appealing set of features that
complements the conventional fluorophores.
In this chapter, we discuss first how the silver clusters relate to silver atoms and
silver nanoparticles. Then we overview the formation of fluorescent silver clusters
in aqueous solution, using silver salts as precursors and various scaffolds as
stabilizers. Finally we discuss applications of silver clusters in fluorescent labeling
of biological tissues, and their use as fluorescent probes for sensing of molecules.
2 Metal Clusters: The Missing Link Between Single Atoms
and Plasmonic Nanoparticles
The behavior of metals goes through several noticeable transitions when their size
is varied (Fig. 1 ). Metals of macroscopic dimensions, such as silver jewelry and
copper wires, have a mirror-like luster and are good electrical conductors. This
Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous
band of energy levels, the limited number of atoms in metal clusters results in discrete energy
levels, allowing interaction with light by electronic transitions between energy levels. Metal
clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the
energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not
yet firmly established