Nitrogen-Doped Carbon Quantum Dots for FRET-Assisted and Smartphone-Enabled Fluorescent Detection of Melatonin in...

Melatonin (N-acetyl-5-methoxytryptamine, MLT) is an endogenous neurohormone that regulates circadian rhythms, exhibits potent antioxidant activity, and has been implicated in cancer suppression and neuroprotection. In this study, we report the design and development of a highly sensitive optical nanosensor based on nitrogen-doped carbon quantum dots (N-CQDs) for the quantitative detection of MLT in human blood. The N-CQDs were synthesized via a facile pyrolysis route, yielding uniform nanodots that exhibit strong blue photoluminescence. Comprehensive structural, morphological, and optical characterizations were conducted using X-Ray Diffraction (XRD), transmission electron microscopy (TEM), and fluorescence spectroscopy, confirming the successful incorporation of nitrogen functionalities with a photoluminescence quantum yield of 36.6%. The synthesized N-CQDs exhibit a highly selective and sensitive detection towards MLT, and the sensing mechanism is plausibly governed by Förster resonance energy transfer (FRET)-mediated quenching of N-CQD emission upon interaction with MLT. This developed probe of N-CQDs achieved an impressive detection limit of 4.68 nM, demonstrating its potential for biomedical diagnostic applications. Furthermore, the system was successfully integrated into a smartphone-assisted sensing platform for portable and real-time MLT monitoring. Quantitative analysis of spiked human blood samples exhibited excellent recovery values ranging from 96.1% to 104.2%, validating the reliability and applicability of the proposed N-CQD-based sensor for clinical and point-of-care analysis.