Reversible color changing response of pyrenylated charge-transfer probes towards Hg²⁺: linker-driven modulation of sensitivity and selectivity

Two easily synthesizable prenylated charge transfer probes with a terminal pyridine unit were designed. The compounds differ based on the nature of the spacer: rigid olefinic (1) and flexible hydrazo (2). Both compounds show exclusive interaction with Hg²⁺ ions, leading to changes in visible color. Experimental analyses showed that compound 1 outperformed compound 2 in terms of sensitivity (<i>A</i>/<i>A</i> ₀ ∼ 13-fold <i>vs.</i> ∼2-fold) and detection limit (1.2 ppm <i>vs.</i> 5.8 ppm), attributed to its rigid linker and enhanced binding affinity. Moreover, ratiometric responses (<i>r</i> ² > 0.99) and excellent selectivity for Hg²⁺ were demonstrated. This study highlights the development of robust, reusable, and cost-effective tools for mercury detection, thus addressing critical environmental and health concerns associated with mercury contamination. Environmental robustness of compound 1 was validated across solvents, counter-ions, and water mixtures, with decreasing efficacy in high-water-content systems owing to solvation effects and probe aggregation. Mechanistic studies confirmed pyridine nitrogen as the primary coordination site with reversible binding, enabling probe reuse. Practical applications were explored using paper strips modified with the probes, which facilitated rapid, cost-effective, and quantitative detection of Hg²⁺ <i>via</i> image analysis.