Temperature dependent dual-mode switchable fluorescent chemosensor for Hg²⁺ detection

Fluorescent probes are important tools for detecting Hg²⁺. Based on the binding mechanisms between the probe and Hg²⁺, they can be mainly classified into two types: the coordination mode and the reaction mode. Each pattern has its unique advantages as well as inherent limitations. In this work, a dynamic smart fluorescent probe (TPENS) was elaborately designed to achieve temperature-dependent switching between coordination-based and reaction-based modes. At room temperature (20 °C), the probe binds with Hg(II) to form a TPENS-Hg²⁺ complex, resulting in a reversible coordination-based sensor with a turn-off signal output. When the temperature is increased to 60 °C, the probe undergoes a bond cleavage reaction with Hg(II), converting TPENS to TPENO via Hg²⁺-triggered deprotection reaction, thereby constructing a ratiometric reaction-based sensor. Thus, in the aggregated state with water fractions as f_w = 98%, the probe responds specifically to Hg²⁺ ions only, exhibited excellent selectivity and sensitivity toward Hg²⁺ in both coordination and reaction modes, with the LOD as low as 1.5 × 10^-7 M. The mechanisms of this chemosensor under the two distinct modes have been thoroughly validated by ¹H NMR spectroscopy, FTIR spectroscopy and HRMS. Additionally, the probe can detect Hg²⁺ in environmental water samples. This work provided a novel strategy for the development of dynamic smart probes, i.e. by employing rational molecular design to endow the probe with the ability to switch between different sensing modes.