Spiropyran-based optical approaches for mercury ion sensing: Improving sensitivity and selectivity via cooperative ligation interactions using cysteine

Spiropyrans are an attractive starting point in design of optical approaches for metal ions sensing. However, the high background in aqueous solution and non-specific chelation of the spiropyran with heavy metal ions has hindered their application as reliable sensors for environmental and biological species. Here, we report on a new spiropyran-based approach for sensitive and selective sensing of Hg²⁺ in aqueous solution, based on cooperative ligation interactions among the spiropyran probe, an intermediate, cysteine, and the metal ion. To test the feasibility of this design, three spiropyran scaffolds, L1-L3, with different ligation functions at the 8′-position were examined as model systems. The results demonstrate that by using cysteine, a potential ligand of Hg²⁺, the spiropyran could detect 1.0 × 10^-7 M Hg²⁺ in aqueous solution. Due to the specific metal-amino acid interaction, the approach exhibits selective response toward Hg²⁺ over other metal ions and anions, although possible interference from Cu²⁺ has to be considered at the high level of the metal ion. This approach has been used for the determination of Hg²⁺ in water samples containing potential interferents with satisfactory recovery.