A series of viologen complexes containing thiophene and Br− dual fluorescent chromophores for continuous visual sensing of pH and Hg2+

Abstract

The mercury ion (Hg²⁺) is a typical high-toxicity substance that can cause severe damage to the environment and human bodies. For the detection of Hg²⁺, there are still significant challenges in the detection range and limit of detection (LOD). In this study, three viologen-based fluorescent probes are developed, CdCl₄(Btybipy) (1), ZnBr₄(Btybipy) (2), CdBr₄(Btybipy) (3) (Btybipy = bis-1-thiophen-3-ylmethyl-[4,4′]-bipyridinyl) through conventional solvent methods for detecting pH and Hg²⁺. 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 Hg²⁺ (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 Hg²⁺. 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 Hg²⁺ can provide a potential strategy for the exploitation of promising sensing platforms for monitoring Hg²⁺.