Surface-state–engineered MXene quantum dots for fluorescent heavy metal ion sensing: Photophysical foundations and mechanistic insights into electronic and redox interactions

MXene quantum dots (MQDs) have emerged as versatile fluorescent nanomaterials for heavy metal ion sensing owing to their quantum-confined electronic structure, abundant surface terminations, tunable photoluminescence, and high surface reactivity. This review presents a mechanistic overview of the photophysical principles and surface-state engineering strategies governing fluorescence modulation in MQD-based sensing systems. The origins of MQD photoluminescence are first discussed, with emphasis on quantum confinement, defect-mediated emission, surface functional groups, interfacial charge redistribution, and trap-state dynamics that regulate optical behavior. Building on these foundations, the review analyzes ion-specific sensing mechanisms for environmentally relevant heavy metal ions including Pb 2+ , Cd 2+ , Hg 2+ , As 3+ , and chromium species. Key fluorescence response pathways—including charge-transfer-induced quenching, trap-state modulation, static surface complexation, and redox-coupled electronic interactions—are critically examined in relation to MQD surface chemistry and electronic structure. Representative studies further demonstrate how heteroatom doping, heterojunction engineering, and matrix integration improve selectivity, sensitivity, and response kinetics. By correlating MQD photophysics with ion-dependent electronic interactions, this review establishes a unified framework for the rational design of next-generation fluorescent MQD sensors for environmental monitoring applications.