Electron count and ligand composition influence the optical and chiroptical signatures of far-red and NIR-emissive DNA-stabilized silver nanoclusters

Abstract

Near-infrared (NIR) emissive DNA-stabilized silver nanoclusters (Ag_N-DNAs) are promising fluorophores in the biological tissue transparency windows. Hundreds of NIR-emissive Ag_N-DNAs have recently been discovered, but their structure–property relationships remain poorly understood. Here, we investigate 19 different far-red and NIR emissive Ag_N-DNA species stabilized by 10-base DNA templates, including well-studied emitters whose compositions and chiroptical properties have never been reported before. The molecular formula of each purified species is determined by high-resolution mass spectrometry and correlated to its optical absorbance, emission, and circular dichroism (CD) spectra. We find that there are four distinct compositions for Ag_N-DNAs emissive at the far red/NIR spectral border. These emitters are either 8-electron clusters stabilized by two DNA oligomer copies or 6-electron clusters with one of three different ligand compositions: two oligomer copies, three oligomer copies, or two oligomer copies with additional chlorido ligands. Distinct optical and chiroptical signatures of 6-electron Ag_N-DNAs correlate with each ligand composition. Ag_N-DNAs with three oligomer ligands exhibit shorter Stokes shifts than Ag_N-DNAs with two oligomers, and Ag_N-DNAs with chlorido ligands have increased Stokes shifts and significantly suppressed visible CD transitions. Nanocluster electron count also significantly influences electronic structure and optical properties, with 6-electron and 8-electron Ag_N-DNAs exhibiting distinct absorbance and CD spectral features. This study shows that the optical and chiroptical properties of NIR-emissive Ag_N-DNAs are highly sensitive to nanocluster composition and illustrates the diversity of structure–property relationships for NIR-emissive Ag_N-DNAs, which could be harnessed to precisely tune these emitters for bioimaging applications.