Linear and nonlinear optical properties of GSH-stabilized Ag2S QDs and the effects of amino acids on their fluorescence

We demonstrate that a simple post-synthetic amino acid addition can effectively enhance and tune the emission properties of Ag 2 S/GSH quantum dots (QDs) solely through surface chemistry modification. The addition of thiol-containing amino acids (AAs) at low millimolar concentrations leads to pronounced fluorescence enhancement accompanied by narrowing of the emission band, indicating efficient surface passivation and suppression of non-radiative recombination pathways, the fluorescent quantum yield (QY) increased from 2.17% to 4.82%. In contrast, higher amino acid concentrations gradually reduce the emission intensity due to surface oversaturation and formation or modification of additional trap states within the bandgap, which affects both emission wavelength and decay dynamics. These results demonstrate that the type, concentration, and chirality of thiol-containing AAs added to the Ag 2 S QDs solution constitute an effective strategy for controlling surface defect passivation and tuning the emission properties of Ag 2 S QDs. To complement the linear optical characterization, the third-order nonlinear optical properties were investigated using the femtosecond Z-scan technique across a broad spectral range (650-1600 nm). The results demonstrated high two-photon absorption cross-section (σ 2 ) value σ 2 = 1779 ± 591 GM (Goeppert-Mayer units) at λ EXC. = 700 nm, highlighting the excellent nonlinear optical performance of the colloidal Ag 2 S/GSH QDs.