“Kill two birds with one stone” role of Ag-BC@Cu nanozyme: Orthogonal colorimetric discrimination and determination of Cr(III) and Cr(VI) in water through independently regulated dual enzyme-mimicking pathways

A new Ag/Cu metal-organic nanozyme with MOF-like features was developed. • Dual ligands and Ag/Cu co-incorporation enable dual enzyme-mimicking activities. • “Kill two birds with one stone”: one nanozyme discriminates Cr(III) and Cr(VI). • Orthogonal channels allow direct Cr(III)/Cr(VI) quantification without subtraction. • Smartphone-assisted analysis enables chromium speciation in river water. Chromium predominantly exists as Cr(III) and Cr(VI) in aquatic environments, which differ markedly in physicochemical properties and toxicity. Rapid and direct discrimination of Cr(III) and Cr(VI) remains challenging because conventional chromium speciation methods often require redox pretreatment, multiple recognition elements, or indirect calculation. Herein, a new Ag/Cu-containing metal-organic nanozyme with MOF-like structural characteristics (Ag-BC@Cu) was developed. Ag-BC@Cu exhibited laccase-like and peroxidase-like activities and was used to construct an orthogonal colorimetric sensor for discrimination of Cr(III) and Cr(VI). Cr(III) selectively inhibited the laccase-like pathway, whereas Cr(VI) enhanced the peroxidase-like pathway, producing independent colorimetric responses at 510 and 652 nm, respectively. Generally, both chromium species were directly discriminated and quantified using a single nanozyme without redox pretreatment, multiple probes, or subtraction-based calculations. The detection limits of the orthogonal colorimetric sensor were 0.05 mg·L -1 for Cr(III) and 0.0047 mg·L -1 for Cr(VI). Smartphone-assisted analysis further enabled portable visual detection of chromium speciation. The colorimetric sensor exhibited good selectivity and was successfully applied to river-water samples, with results consistent with LC-ICP-MS. This work provides a simple “kill two birds with one stone” strategy for chromium speciation, with potential for portable water-quality monitoring. A dual enzyme-mimicking Ag-BC@Cu nanozyme enabled the orthogonal colorimetric detection of Cr(III) and Cr(VI). Cr(III) selectively inhibited the laccase-like activity, whereas Cr(VI) enhanced the peroxidase-like activity, enabling successful application to river-water samples. Download: Download high-res image (297KB) Download: Download full-size image