MXene quantum dots for geochemical ion sensing: mechanism-driven design of integrated platforms.

MXene quantum dots (MQDs) have recently emerged as a versatile class of low-dimensional nanomaterials with unique photophysical, redox, and surface chemical properties, positioning them as promising platforms for geochemical ion sensing. This review provides a comprehensive and mechanism-oriented overview of MQDs for the detection of earth-relevant metal ions and oxyanions, with emphasis on the progression from nano-architectonic design to versatile sensing platforms. We systematically discuss how structural engineering, surface functionalization, heteroatom doping, and defect modulation govern quantum confinement, exciton dynamics, and fluorescence behavior of MQDs. Fundamental ion-quantum dot interaction mechanisms, including electrostatic attraction, coordination adsorption, redox-mediated electron transfer, and energy transfer processes such as inner filter effect and Förster resonance energy transfer, are analyzed. Recent advances in performance-driven applications for the sensing of toxic and environmentally significant species (e.g., Cd2+, Cr(vi), Mn(vii), As3+, Hg2+, and dichromate) are highlighted, with particular focus on dual-function detection-remediation strategies and versatile platforms. Finally, current challenges and future perspectives toward innovative platforms for environmental monitoring are outlined, underscoring the potential of MQDs for next-generation environmental and earth science applications.