DNA Cage Holds Silver Atoms Still For Scrutiny

Using two DNA sequences of ten nucleotides each to hold together exactly 16 silver atoms, researchers in Denmark and Japan seek to better understand the optical properties of silver nanoclusters.

AsianScientist (Oct. 9, 2019) – In a study published in the journal Angewandte Chemie International Edition, an international team of scientists has studied the structure of silver nanoclusters to better understand their fluorescent nature.

Nanoclusters are little ‘heaps’ of a few atoms that often have interesting optical properties, depending on their size. In particular, nanoclusters made of silver (Ag) fluoresce very brightly, but stabilizing them in solution to study their properties was tricky.

Now, scientists in Japan and Denmark have crystallized a nanocluster of exactly 16 Ag atoms using two DNA sequences of ten nucleotides each. The magenta crystals emit light in the near infrared when irradiated with green light, with nearly identical spectra in crystal form or in solution.

Structural analysis revealed that the Ag16 nanoclusters have a diameter of about 7 Å and a height of about 15 Å (1 Å is one ten millionth of a millimeter). Each nanocluster is tightly wrapped and almost completely shielded by two DNA strands in a horseshoe conformation.

The two DNA strands are primarily bound by interactions with the Ag atoms and to some extent by a few hydrogen bonds. Surprisingly, none of the Watson-Crick base pairing typically observed for DNA is found in this case. Additionally, novel silver-silver interactions were observed within the cluster.

Packing of the DNA-silver nanoclusters into the crystal is promoted by various interactions, including those between phosphate groups and calcium ions, and π-stacking between neighboring thymine nucleobases. The latter plays an important role in the crystallization process. Additionally, loosely associated silver cations are present within the crystal; some form a bridge between DNA bases, while others interact only with silver atoms within the core of the clusters.

These new insights could help to explain the relationship between the structural and emission properties of nanoclusters, said the researchers. They intend to develop a method for synthesizing other monodisperse, biocompatible, water-soluble silver clusters with advantageous photophysical features for applications such as biomedical imaging.

The article can be found at: Cerretani et al. (2019) Crystal Structure of a NIR‐Emitting DNA‐Stabilized Ag16 Nanocluster.


Source: Sophia University, Japan. Photo: Sophia University.
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