  Global geometry optimization and time-dependent density functional theory
calculations have been used to study the structural evolution and optical
properties of (AgAu)n (n=2-6) nanoalloys both as individual clusters and as
clusters stabilized with the fragments of DNA of different size. We show that
alloying can be used to control and tune the level of interaction between the
metal atoms of the cluster and the organic fragments of the DNA ligands. For
instance, gold and silver atoms are shown to exhibit synergistic effects in the
process of charge transfer from the nucleobase to the cluster, with the silver
atoms directly connected to the nitrogen atoms of cytosine increasing their
positive partial charge, while their more electronegative neighbouring gold
atoms host the excess negative charge. This allows the geometrical structures
and optical absorption spectra of small bimetallic clusters to retain many of
their main features upon aggregation with relatively large DNA fragments, such
as a cytosine-based 9-nucleotide hairpin loop, which suggests a potential
synthetic route to such hybrid metal-organic compounds, and opens up the
possibility of bringing the unique tunable properties of bimetallic nanoalloys
to biological applications.
