Temperature- and pH-Sensitive Nitrogen and Sulfur Codoped Carbon Quantum Dots for Sequential Detection of Fe³⁺ and H₂S

In this study, nitrogen and sulfur codoped carbon quantum dot (N,S-CQD) fluorescent nanoprobes were synthesized through a one-step hydrothermal method using l-cysteine and α-methacrylic acid as raw materials, and an “on–off–on” fluorescence mechanism was designed using N,S-CQDs for sequential determination of Fe3+ and H2S in human serum. The quenching process (on–off) and mechanism of N,S-CQDs by Fe3+ were studied via fluorescence spectroscopy, zeta potential, cyclic voltammetry, and orbital energy level simulation. Meanwhile, an “off–on” strategy for the N,S-CQD/Fe3+ fluorescence sensing platform was constructed for the determination of the H2S-based “on–off” mechanism. This study not only achieved highly selective detection of Fe3+ and H2S over a wide concentration range of 0–250, 250–500, and 2.5–900 μM, respectively, but also improved our understanding on the specific identifications between N,S-CQDs, Fe3+, and H2S. What is more, it was found that N,S-CQDs also possessed pH and temperature sensing performance. Finally, N,S-CQD fluorescent nanoprobes were successfully applied for the determination of Fe3+ and H2S in spiked serum samples, which also proved their potential application in medical diagnosis and biosensors.