A series of viologen complexes containing thiophene and Br - dual fluorescent chromophores for continuous visual sensing of pH and Hg 2 .

The mercury ion (Hg 2+ ) is a typical high-toxicity substance that can cause severe damage to the environment and human bodies. For the detection of Hg 2+ , there are still significant challenges in the detection range and limit of detection (LOD). In this study, three viologen-based fluorescent probes are developed, CdCl 4 (Btybipy) (1), ZnBr 4 (Btybipy) (2), CdBr 4 (Btybipy) (3) (Btybipy = bis-1-thiophen-3-ylmethyl-[4,4']-bipyridinyl) through conventional solvent methods for detecting pH and Hg 2+ . Reversible discoloration and fluorescence response behaviour in the pH range of 4-12.8 is demonstrated by viologen-based fluorescent probes, which exhibit "ON-OFF-ON" signal changes. Compared with complex 1, it is surprising to find that complexes 2-3 display both fluorescence enhancement and fluorescence quenching simultaneously with the addition of different concentrations of Hg 2+ (0-20 and 25-400 μM). There is broad linearity in the range of 0-20 and 50-300 μM with LODs of 2.14 and 3.13 nM, respectively. This occurrence of dual-signal modes is attributed to the participation of Br - and the thiophene S atom as dual chromophores in the coordination reaction of Hg 2+ . Dual-signal mode output, high sensitivity, wide detection range, and low LODs are exhibited by these fluorescent probes. The unique coordination reaction between Br - and the thiophene S atom with Hg 2+ can provide a potential strategy for the exploitation of promising sensing platforms for monitoring Hg 2+ .