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

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 (AgN-DNAs) as water-soluble two-photon absorbers. We investigate 2PA of four different atomically precise AgN-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 (TPEL) technique for a wide wavelength range from 810 to 1400 nm. The AgN-DNAs exhibited reversed strength of corresponding transitions in the two-photon regime, as compared to one-photon, which adds another puzzle to the complex photophysics of these emitters. Maximal 2PA cross-section value (813 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 AgN-DNAs without chlorido ligands, presenting high 2PA 176 GM at 1050 nm. Our findings support the potential of AgN-DNAs as NIR-to-NIR two-photon probes that are both excited and emit in the NIR. Their high σ2 and fluorescence quantum yield results in superior two-photon brightness on the order of ~10^2 GM, significantly higher than water-soluble organic fluorophores.