Two-photon brightness of NIR-emitting, atomically precise DNA-stabilized silver nanoclusters

Abstract

Near-infrared (NIR) emitters with high two-photon absorption (2PA) cross-sections are of interest to enable in vivo imaging in the tissue transparency windows. This study explores the potential of DNA-stabilized silver nanoclusters (Ag_N-DNAs) as water-soluble two-photon absorbers. We investigate 2PA of four different atomically precise Ag_N-DNA species with far-red to NIR emission and varying nanocluster and ligand compositions. 2PA cross-sections, σ₂, were determined by two-photon excited luminescence (2PEL) technique for a wide wavelength range from 810 to 1400 nm. The Ag_N-DNAs exhibited reversed strength of corresponding transitions in the two-photon regime, as compared to one-photon, which further demonstrates the complex photophysics of these emitters. Maximal 2PA cross-section value (∼582 GM) was observed for (DNA)₃[Ag₂₁]¹⁵⁺, which is stabilized by 3 DNA oligomers. (DNA)₂[Ag₁₆Cl₂]⁸⁺ presented distinct 2PA behavior from the Ag_N-DNAs without chlorido ligands, with a high 2PA of 176 GM at 1050 nm. Our findings support the potential of Ag_N-DNAs as NIR-to-NIR two-photon probes that are both excited and emit in the NIR. Their high σ₂ and fluorescence quantum yield values result in superior two-photon brightness on the order of ∼10² GM, significantly higher than water-soluble organic fluorophores.