S2--Mediated Regulation of Polysulfides Drives the Formation of Stable Sulfur Quantum Dots for Bilirubin Sensing

Stable fluorescent sulfur quantum dots (SQDs) were generated directly through a one-step process via S2--mediated regulation (SMR) of residual polysulfides present in analytical-grade sodium thiosulfate, eliminating the need for the conventional acid-induced formation of elemental sulfur from sodium thiosulfate. During this process, S2- not only hydrolyzes to produce OH-, which promotes the cleavage of sulfur chains in the residual polysulfides, but also rapidly captures the generated elemental sulfur and converts it into abundant polysulfides. These polysulfide intermediates subsequently undergo nucleation and structural reorganization, ultimately yielding highly stable SQDs enriched with surface negative charges. Steady-state and transient fluorescence spectroscopy reveals that the SQDs produced via SMR (denoted as S2--SQDs) exhibit a quantum yield of 11.13% and a fluorescence lifetime of 9.24 ns, together with significantly enhanced long-term photostability compared with SQDs prepared under alkaline regulation alone. Furthermore, the S2--SQDs enable rapid and selective fluorescence sensing of bilirubin, with a detection limit as low as 0.23 μM. This work provides new mechanistic insights into the fabrication of stable sulfur-based quantum emitters. More importantly, it highlights the often-overlooked role of residual sulfur species in commercial reagents and underscores their substantial contribution to the formation and luminescence behavior of SQDs.