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.

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.

No abstract

Luminescent metal nanoclusters (NCs) stabilized by natural proteins are of special interest in bioimaging applications. However, the detailed structure of the protein-templated NCs and the nature of their emissive states remain poorly understood. A fair amount of nonluminescent metal ions and clusters complexed to the proteins hinders probing of the structure of the emitting clusters using mass spectroscopy, infrared, or other conventional spectroscopy methods. In this respect, only luminescent excitation spectra distinguish the emitting NCs. In this experimental and theoretical joint study, we modeled the fluorescent excitation and excitation anisotropy spectra of protein-based silver (Ag) NCs. We varied the synthesis conditions and studied the spectral properties of Ag clusters on bovine serum albumin (BSA) and lysozyme, which had already been used as templates, as well as on HMG box (HMGB1) and histone H1 (H1) proteins. We also calculated the electronic spectra of quantum mechanics-optimized Ag−thiolate, Ag−semiquinone, and Ag−formaldehyde complexes with two confined electrons using second-order algebraic diagrammatic construction [ADC(2)] and resolution-of-identity approximate coupled-cluster singles-and-doubles (RI-CC2) methods and compared them with the experimental spectra. We propose a model for the fluorescent Ag−protein complexes in which two reduced Ag atoms are sufficient to form the fluorescent core of the complex. The proposed structural model of the luminescent centers in the Ag−protein complexes differs from the common view that the fluorescent metal NCs in proteins contain about 10 or more metal atoms. The fluorescent Ag clusters formed on the four investigated natural protein matrices exhibited two different spectral and structural patterns. Deprotonated free cysteine residues stabilized the fluorescent Ag 3 +1 core formed in the BSA matrix. The second type of fluorescent center was realized in the H1, HMGB1, and lysozyme protein matrixes. In this case, tyrosine residues probably stabilize the fluorescent Ag 2 centers.

The application of the AgNCs as a luminescent probe allows quantitative determination of the [Ig]/[HSA] ratio in the mixtures.