Nitrogen-doped MXene quantum dots: Mechanistic insights and engineered turn-off fluorescence for advanced multi-analyte sensing

Nitrogen-doped MXene quantum dots (N-MQDs) have emerged as a promising class of fluorescent nanomaterials owing to their tunable electronic structure, high surface activity, and excellent photostability. This work provides a comprehensive mechanistic analysis of turn-off fluorescence behavior in N-MQDs and highlights their potential in advanced sensing applications. Key quenching pathways, including photoinduced electron transfer (PET), Förster resonance energy transfer (FRET), trap-state-mediated nonradiative decay, aggregation-induced quenching, and environmentally modulated processes, are systematically discussed. Nitrogen doping plays a central role by introducing mid-gap states and active surface sites, enabling precise control over quenching efficiency, selectivity, and response kinetics. Furthermore, functional modulation strategies such as chemical functionalization, bioconjugation, heteroatom co-doping, and hybrid material integration are reviewed as effective approaches to enhance sensitivity and multi-analyte discrimination. The applicability of N-MQDs is demonstrated across a broad range of targets, including neurotransmitters, heavy metal ions, antibiotics, oxidative stress biomarkers, and environmental pollutants, often achieving nanomolar detection limits. By correlating surface chemistry, electronic structure, and fluorescence dynamics, this study provides design guidelines for next-generation turn-off fluorescent sensors with high selectivity, robustness, and real-world applicability in biomedical and environmental monitoring.