Synthesis of Water-Soluble Ag2S Quantum Dots with Fluorescence in the Second Near-Infrared Window for Turn-On Detection of Zn(II) and Cd(II)

The second near-infrared window emission (1.0-1.7 μm, NIR-II) has received extensive attention with the advantages of negligible tissue scattering, reduced autofluorescence, and less background noise. Here a novel analysis platform based on quantum dots (QDs) for highly selective detection of Zn²⁺ and Cd²⁺ with an enhanced NIR-II fluorescence is reported for the first time. We have developed a facile two-step route to synthesize the water-soluble Ag₂S QDs, constituting of a green hydrothermal process and followed surface ligands exchange. Surface passivation was proposed to be the mechanism for the enhanced fluorescence. The added Zn²⁺ or Cd²⁺ could react with the surface thioglycollic acid to form Zn-thiol or Cd-thiol complex passivation shell, which restored surface defects and suppressed nonradiative recombination pathway. The detection platform exhibited a linear relationship between the ion concentrations and enhanced fluorescence and had a detection limit as low as 760 nM for Zn²⁺ and 546 nM for Cd²⁺ at pH = 7.4. Furthermore, the as-synthesized Ag₂S QDs showed good robustness in real sample matrix and were demonstrated to be able to detect exogenous Zn(II) in cells. These properties suggest potential applications of detection of Zn²⁺ in biology and Cd²⁺ in environment via the NIR-II fluorescent Ag₂S QDs.