Small Silver Clusters Ligated by DNA Bases: Computational Study of Electrochemical and Structural Properties

Silver clusters grown in single-stranded DNA are promising new fluorophores for applications in bio-labels and bio-imaging, due to their highly emissive properties from visible to the near-infrared range and good biocompatibility. Photophysical properties of small silver clusters are largely size, charge, and structure dependent. We have performed Density Functional Theory (DFT) calculations to study 4 to 20 atoms Ag cluster. Calculations show that the geometry of clusters and their interactions with bases depend on the oxidation state. For uniform capping, the cytosines exhibit stronger binding to the cluster than guanines. Redox potentials predict higher stability for singlet configurations of clusters with the even number of Ag atoms, while doublet configurations are more energetically favorable for clusters with the odd number of atoms. The difference in the redox potential between clusters in their singlet and doublet spin configurations decreases with increasing the cluster size. Calculated absorption spectra show that the doublet optical transitions are significantly red-shifted, compared to singlets. However, the optical intensity of the first doublet peak decreases with the cluster size increase, and become completely optically forbidden when increases up to 17-20 atoms.