Optical probes for thallium ion detection: Recent advances in design strategies and performance regulation

Thallium, a highly toxic heavy metal, exists mainly as Tl⁺ and Tl³⁺, posing severe threats to environmental safety and human health. The distinct chemical properties of Tl⁺ and Tl³⁺ have long hampered the development of selective detection methods. In the past decade (2015-2025), fluorescent and colorimetric probes have emerged as promising tools for rapid, on-site thallium analysis, attracting growing interest in environmental monitoring and toxicology. In this review, we highlight recent advances in optical probes for Tl⁺ and Tl³⁺, focusing on design strategies including aggregation regulation, conformational recognition, ionophore chelation, and reaction-based mechanisms. Particular attention is paid to their core sensing mechanisms, analytical performance, inherent limitations, and practical applications. Finally, critical challenges and promising directions for future research are also discussed, including novel porous material exploitation, reaction-based mechanism innovation, aqueous probe fabrication, and detection-remediation integration. This review aims to provide a focused overview and guidance for the rational design of next-generation thallium sensors with improved sensitivity, selectivity, and portability.