Synthesis of Water-Soluble Ag 2 S 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 Zn ²⁺ and Cd ²⁺ 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 Ag ₂ S 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 Zn ²⁺ or Cd ²⁺ 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 Zn ²⁺ and 546 nM for Cd ²⁺ at pH = 7.4. Furthermore, the as-synthesized Ag ₂ S 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 Zn ²⁺ in biology and Cd ²⁺ in environment via the NIR-II fluorescent Ag ₂ S QDs.