A Highly Charged Ag6 4+ Core in a DNA-Encapsulated Silver Nanocluster
Small metal clusters fill the gap between the atomic scale and the metallic state with its distinctive bulk phenomena. Besides being of high fundamental interest, this intermediate character of metal clusters also gives rise to unique and potentially useful electronic, magnetic, and optical properties.[ 2] To bring these properties to real-world applications, however, the clusters must be stabilized and prevented from spontaneous aggregation and other decomposition reactions. One successful strategy relies on the immobilization of metal clusters on surfaces. Another approach stabilizes the clusters by the attachment of protective ligands. Quite often the same ligands can also serve as templates for the formation of the metal clusters. This favorable situation is realized for a manifold of silver nanoclusters, which are synthesized by the reduction of silver salts in the presence of templates, such as poly(amidoamine) dendrimers, poly- (acrylic acid) derivatives, poly(methacrylic acid), peptides, or DNA. The thus formed nanoclusters contain only a few Ag atoms and hence exhibit molecule-like properties,[ 10] in contrast to the conventional larger nanoparticles, which more closely resemble the metallic state. In particular, the strong fluorescence of silver nanoclusters has attracted significant attention because of its potential practical applications.[5–10] Among the various species investigated so far, silver nanoclusters encapsulated by the single-stranded oligonucleotide dC12 arguably constitute the most promising system thanks to their very high photoemission rates and their excellent photostability.