β-Cyclodextrin capped ZnS nanoparticles for CER-assisted colorimetric and spectrophotometric detection of Pb⁺, Cu⁺, and Hg⁺ in an aqueous solution.

Herein, simple, low-cost, and room-temperature synthesis of beta-cyclodextrin (β-CD) stabilized zinc sulfide nanoparticle (ZnS NP) through the chemical precipitation method has been reported for cation exchange reaction (CER) based colorimetric sensing of Pb, Cu, and Hg. Formation of β-CD stabilized ZnS NPs (ZnS@β-CD) was verified by physicochemical characterization techniques such as XRD, XPS, FE-SEM, and TEM. ZnS@β-CD NPs showed color change selectively for the metal ions Pb⁺, Cu⁺, and Hg⁺ among the various metal ions including Sn⁺, Cr³⁺, Mn⁺, Fe³⁺, Co⁺, Ni⁺, and Cd⁺. The solubility product of reactants and the transformed products are the reason for selective CER of ZnS@β-CD NPs towards Pb⁺, Cu⁺, and Hg⁺ ions. ZnS@β-CD NPs dispersion revealed rapid color change from white to orange, black, and bright yellow on the addition of higher concentrations of Pb⁺, Cu⁺, and Hg⁺ respectively. This color change is due to the formation of complete CER-transformed nanostructures such as PbS, CuS, and HgS in higher concentrations (10⁻- 10⁻³ M) of corresponding metal ions. The partial CER altered products Zn,PbS, ZnCuS and ZnHgS were detected due to the appearance of pale color in the lower metal ions concentrations of 10⁻⁴ - 10⁻⁶ M. This CER assisted transformation was also monitored through spectrophotometric methods. Moreover, infrared spectroscopic analysis was used to testify the structure of CER transformed product. The synthesized ZnS@β-CD NPs act as an efficient CER-based sensor for distinguishing and determining Pb⁺, Cu⁺, and Hg⁺ at different level concentrations in the aqueous solution.