Optical absorption of bimetallic nanoclusters complexed with nucleobases

In this study, the structural and optical properties of complexes formed by DNA and mono and bimetallic nanoclusters (NCs) of Au and Ag were carried out using time dependent density functional theory. We focused on NCs of composition Ag_nAu_m(n+m=3). The electronic transitions associated with light absorption across these NCs are meticulously examined, highlighting numerous transitions from bonding to antibonding orbitals. This analysis contributes insights into the electronic structure relevant to understanding photochemical processes. The binding energy assessments reveal that silver-rich NCs are more stable when placed on top of hydrogen bonds, whereas gold-rich NCs show a preferential affinity for cytosine. The absorption spectra of the DNA:NC complexes suggest that compositions comprising solely of one metal exhibit absorption peaks across a wide range of the visible spectrum, attributable to symmetry-breaking changes induced by the presence of the DNA base pairs. In the case of mixed clusters, transitions in the visible band are observed at the violet end of the spectrum, with the exception of the Ag₂Au cluster that presents a modest-intensity green-band peak when associated with the guanine base. Furthermore, a notable splitting of absorption peaks is recorded both in the low and middle ultraviolet ranges due to the symmetry reduction upon formation of the complexes, leading to complexes with a rich structure of absorption peaks in the far ultraviolet range. The analysis of the electronic orbitals involved in the absorption transitions of the DNA:NC complexes indicates that the majority of the absorption processes in the DNA:NC complexes involve charge transfer from the NC to the DNA. Finally, the spectra presented here provide useful information for determining the stoichiometry of Au and Ag NCs in experiments, their affinities with DNA bases, as well as the properties of DNA:NC biomarkers at the molecular level.