A dual-mode biosensing platform for the detection of melatonin based on zinc-nitrogen co-doped carbon dots with nanozyme activity

A straightforward yet sensitive dual-signal strategy was developed for the detection of melatonin (MT), based on the fluorescence properties and peroxidase-like activity of zinc‑nitrogen co-doped carbon dots (Zn,N-CDs). Through hydrogen bonding, electron transfer, as well as coordination interactions with MT, Zn,N-CDs undergo fluorescence quenching driven by the synergistic effects of static and dynamic mechanisms. Meanwhile, the generated singlet oxygen oxidizes 3,3′,5,5′-tetramethylbenzidine (TMB) into the blue-colored oxidized product (oxTMB), thus producing a ‘triggered’ dual-signal response. This platform enables simultaneous fluorescence and colorimetric analyses, offering wide linear ranges (fluorescence: 2–600 μM; colorimetry: 5–500 μM) and remarkably low detection limits (0.020 μM and 0.999 μM, respectively). Furthermore, the method exhibits excellent selectivity and has been successfully applied to pharmaceutical sample analysis, highlighting its promise for visual quality monitoring of relevant products.