Ligand-stabilized luminescent metal clusters, in particular, DNA-based Ag clusters, are now employed in a host of applications such as sensing and bioimaging. Despite their utility, the nature of their excited states as well as detailed structures of the luminescent metal-ligand complexes remain poorly understood. We apply a new joint experimental and theoretical approach based on QM/MM-MD simulations of the fluorescence excitation spectra for three Ag clusters synthesized on a 12-mer DNA. Contrary to a previously proposed "rod-like" model, our results show that (1) three to four Ag atoms suffice to form a partially oxidized nanocluster emitting in visible range; (2) charge transfer from Ag cluster to DNA contributes to the excited states of the complexes; and (3) excitation spectra of the clusters are strongly affected by the bonding of Ag atoms to DNA bases. The presented approach can also provide a practical way to determine the structure and properties of other luminescent metal clusters.

The rapidly developing field of bionanotechnology requires detailed knowledge of the mechanisms of interaction between inorganic matter and biomolecules. Under conditions different from those in an aqueous solution, however, the chemistry of these systems is elusive and may differ dramatically from their interactions in vitro and in vivo. Here, we report for the first time a photoemission study of a metal silver-DNA interface, formed in vacuo, in comparison with DNA-Ag and fluorescent DNA-Ag complexes formed in solution. The high-resolution photoelectron spectra reveal that in vacuo silver atoms interact mainly with oxygen atoms of the phosphodiester bond and deoxyribose in DNA, in contrast to the behavior of silver ions, which interact preferentially with the nitrogen atoms of the bases. This offers new insight into the mechanism of DNA metallization, which is of importance in creating metal-bio interfaces for nanotechnology applications.

The ordered structure of UV chromophores in DNA resembles photosynthetic light-harvesting complexes in which quantum coherence effects play a major role in highly efficient directional energy transfer. The possible role of coherent excitons in energy transport in DNA remains debated. Meanwhile, energy transport properties are greatly important for understanding the mechanisms of photochemical reactions in cellular DNA and for DNA-based artificial nanostructures. Here, we studied energy transfer in DNA complexes formed with silver nanoclusters and with intercalating dye (acridine orange). Steady-state fluorescence measurements with two DNA templates (15-mer DNA duplex and calf thymus DNA) showed that excitation energy can be transferred to the clusters from 21 and 28 nucleobases, respectively. This differed from the DNA–acridine orange complex for which energy transfer took place from four neighboring bases only. Fluorescence up-conversion measurements showed that the energy transfer took place within 100 fs. The efficient energy transport in the Ag–DNA complexes suggests an excitonic mechanism for the transfer, such that the excitation is delocalized over at least four and seven stacked bases, respectively, in one strand of the duplexes stabilizing the clusters. This result demonstrates that the exciton delocalization length in some DNA structures may not be limited to just two bases.

In this experimental and theoretical joint study, we used single amino acids as model systems for studying protein−cluster interactions. We probed 12 natural amino acids with different functional groups as potential templates of fluorescent silver (Ag) nanoclusters obtained by sodium borohydride reduction of Ag ions. We also calculated the Gibbs free energies of the complexes formed between Ag + ions, Ag atoms, and two-atom Ag clusters with the amino acids' various functional groups. Only cysteine and tyrosine could form fluorescent complexes with Ag clusters. This agrees with the calculated Gibbs free energies for the Ag cluster−amino acid complexes. We also show that the tyrosine-based fluorescent Ag cluster could be obtained using a green synthetic method in which tyrosine, at alkali pH, acts as a reducing agent. The optimized structure of a complex of Ag 3 + cluster with three semiquinone tyrosine rings is proposed. These results can be used in designing and synthesizing new peptidetemplated biolabels.

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