Heteroatom-doped MXene quantum dots for selective transition metal ion sensing: from atomic-level design to...

Heteroatom-doped MXene quantum dots (MQDs) have emerged as promising fluorescent nanoplatforms for the selective detection of transition metal ions such as Fe³⁺, Cu²⁺, Zn²⁺, and Mn²⁺. Their tunable electronic structure, high quantum yield, and versatile surface chemistry enable precise modulation of optical properties and binding interactions with metal ions. This review provides a comprehensive overview of recent advances in the design and application of heteroatom-doped MQDs for transition metal ion sensing. Particular emphasis is placed on atomic-level engineering strategies, including dopant-host electronic coupling, defect-dopant synergy, single-atom doping, selective functionalization at edge versus basal-plane sites, and multi-element doping (e.g., S, P, B, and halogens). These structural modifications enable tailored control over charge distribution, redox activity, and coordination environments, thereby improving sensitivity and ion selectivity. Beyond conventional fluorescence quenching mechanisms, emerging sensing strategies are also discussed, including ratiometric detection, stimuli-responsive probes, multimodal sensing systems integrating optical, electrochemical, and visual signals, and logic-gated or data-assisted sensing approaches designed to improve analytical reliability in complex matrices. Representative sensing behaviors are highlighted, such as redox-mediated quenching for Fe³⁺ and Cu²⁺, fluorescence enhancement for Zn²⁺, and dual-emission ratiometric recognition for Mn²⁺. Finally, current challenges-including synthesis scalability, selectivity in competitive environments, matrix interference, and translation toward deployable sensing devices-are critically evaluated, and future directions for portable sensing platforms and intelligent analytical systems are discussed.