Chloride Ligands on DNA-Stabilized Silver Nanoclusters

Abstract

DNA-stabilized silver nanoclusters (Ag_N-DNAs) are known to have one or two DNA oligomer ligands per nanocluster. Here, we present the first evidence that Ag_N-DNA species can possess additional chloride ligands that lead to increased stability in biologically relevant concentrations of chloride. Mass spectrometry of five chromatographically isolated near-infrared (NIR)-emissive Ag_N-DNA species with previously reported X-ray crystal structures determines their molecular formulas to be (DNA)₂[Ag₁₆Cl₂]⁸⁺. Chloride ligands can be exchanged for bromides, which red-shift the optical spectra of these emitters. Density functional theory (DFT) calculations of the 6-electron nanocluster show that the two newly identified chloride ligands were previously assigned as low-occupancy silvers by X-ray crystallography. DFT also confirms the stability of chloride in the crystallographic structure, yields qualitative agreement between computed and measured UV-vis absorption spectra, and provides interpretation of the ³⁵Cl-nuclear magnetic resonance spectrum of (DNA)₂[Ag₁₆Cl₂]⁸⁺. A reanalysis of the X-ray crystal structure confirms that the two previously assigned low-occupancy silvers are, in fact, chlorides, yielding (DNA)₂[Ag₁₆Cl₂]⁸⁺. Using the unusual stability of (DNA)₂[Ag₁₆Cl₂]⁸⁺ in biologically relevant saline solutions as a possible indicator of other chloride-containing Ag_N-DNAs, we identified an additional Ag_N-DNA with a chloride ligand by high-throughput screening. 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 favorable stability for biophotonics applications.