Near-infrared fluorescent Ag2S quantum dots stabilized by penicillamine enantiomers: influence of ligand chirality on linear and nonlinear optical properties and toxicity

Abstract

We present hydrothermal synthesis and comprehensive physicochemical characterization of water-soluble Ag₂S quantum dots (QDs) emitting in the second near-infrared biological window (NIR-II), stabilized by chiral ligands: L- and D-penicillamine (Pen). By systematically optimizing reaction temperature, time, and the Ag : S ratio, we significantly improved the photophysical properties of the QDs. The obtained Ag₂S/Pen QDs exhibited NIR-II emission (λ_EM. ∼ 1040 nm) with a fair quantum yield (QY = 1.1%), high colloidal stability, low cytotoxicity, and fluorescence lifetime reaching 84 ns in aqueous media. The use of enantiomerically pure and racemic variants of Pen allowed us to investigate the influence of stereoisomer configuration on morphology and consequently, linear and nonlinear optical (NLO) properties of the QDs. Spectrally-resolved NLO study using a femtosecond laser Z-scan technique showed the presence of two-photon absorption (2PA) with a peak cross section reaching σ₂ ∼ 511 GM (Goeppert–Mayer units) for Ag₂S/D-Pen excited at λ_EXC. = 824 nm and 260 GM (λ_EXC. = 950 nm) for Ag₂S/L-Pen, respectively. Biocompatibility studies in THP-1 macrophages and HLMEC endothelial cells revealed favorable tolerance profiles for Ag₂S/Pen QDs, particularly in immune cells. Notably, macrophages maintained high viability and even showed enhanced metabolic activity, while endothelial cells exhibited good tolerance at lower concentrations, supporting the potential of these QDs for biomedical applications involving immune and vascular systems. These findings demonstrate that the structure of surface ligands plays a key role in controlling the structural and optical properties of Ag₂S QDs, providing insights for the design of NIR fluorescent nanomaterials for biomedical and photonic applications.