Crown@ZIF-67/MXene Sensor for Ultrasensitive and Selective On-Site Tl⁺ Monitoring

Thallium (Tl) is a highly toxic priority pollutant, yet reliable monitoring remains challenging because many Tl sensors lack sufficient selectivity, portability, and durability in complex waters. A tightly coupled Crown@ZIF-67/MXene composite was assembled and integrated onto a screen-printed carbon electrode (SPCE) to create a portable electrochemical sensor for Tl⁺. Crown@ZIF-67/MXene was synthesized via an in situ crystallization route, in which 18-crown-6 was introduced during ZIF-67 growth so that ZIF-67 nucleated and grew around the crown ether while simultaneously anchoring onto MXene to form a tightly coupled composite. Under optimized square-wave anodic stripping voltammetry (SWASV), Crown@ZIF-67/MXene/SPCE provides a wide linear range (0.1-300 μg L^-1) and a low limit of detection (0.036 μg L^-1), while maintaining accurate quantification in the presence of coexisting ions (measurement error <2.8%). The performance is attributed to crown-ether-based Tl⁺ recognition, ZIF-67-enabled high-surface-area enrichment, and MXene-facilitated charge transport. Validation in tap water, Yangtze River water, Minzhu Lake water, and zinc-oxide-plant wastewater demonstrates close agreement with ICP-MS, supporting the applicability of this method in environmentally relevant water matrices. After continuous exposure to real wastewater for 5 days, the response deviates by <10% relative to a fresh electrode, indicating antifouling behavior and operational stability. Overall, this work establishes a practical electrochemical platform for the selective and trace-level ⁺ monitoring of Tl in complex environmental waters.