Chloride Ligands on DNA-Stabilized Silver Nanoclusters

DNA-stabilized silver nanoclusters (Ag_N-DNAs) are magic-sized nanoclusters of 10-30 silver atoms stabilized by one to three short DNA oligomers. The stabilizing DNA ligand’s sequence sculpts the size and shape of the tiny nanocluster, thereby selecting its shape-tuned fluorescence excitation and emission peaks. The combinatorially large space of DNA sequence enables DNA ligands to stabilize a wide range of nanocluster structures with sequence-tuned optical properties. These emitters are promising for sensing and biomedical imaging applications, but until recently, the inorganic-organic interface of Ag_N-DNAs was not well-understood. Here, we report our recent work on characterizing the surface chemistry of a new class of Ag_N-DNAs with hybrid ligands and superior optical properties.<br/>We investigate a set of five atomically precise Ag_N-DNA species with near infrared emission and previously reported X-ray crystal structures. Using high-resolution mass spectrometry, we determine the molecular formula of these nanoclusters to be (DNA)₂[Ag₁₆Cl₂]⁸⁺. This is the first time that chloride ligands have been ever reported for Ag_N-DNAs. [1] Chloride ligands can be exchanged for bromide ligands, leading to a redshift in the optical spectra of the emitters. DFT calculations confirm the stability of chloride ligands on the Ag₁₆ nanocluster core, yield qualitative agreement between computed and measured UV-vis absorption spectra, and provide interpretation of the ³⁵Cl-nuclear magnetic resonance spectrum of the (DNA)₂[Ag₁₆Cl₂]⁸⁺. A reanalysis of the X-ray crystal structure confirms that two previously assigned low-occupancy silvers are, in fact, the two chloride ligands. Finally, using the unusual stability of (DNA)₂[Ag₁₆Cl₂]⁸⁺ in biologically relevant saline solutions as an indicator of chloride ligands on Ag_N-DNAs, we use high-throughput screening to identify an additional new Ag_N-DNA with mixed DNA/chloride ligand chemistry. Inclusion of chlorides on Ag_N-DNAs presents a promising new route to expand the diversity of Ag_N-DNA structure-property relationships and to imbue these emitters with significantly enhanced stabilities for <i>in vivo</i> biomedical imaging applications. This work also demonstrates the power of combining high-resolution mass spectrometry, optical spectroscopy, nuclear magnetic resonance, and DFT to characterize the ligand interface of metal nanoclusters and to understand how this interface influences optical properties of these novel emitters.<br/>[1] A. Gonzàlez-Rosell, et al., <i>J. Am. Chem. Soc</i>. 2023, 145, 19, 10721–10729