Achieving smartphone-based colorimetric assay for Hg2+ with a bimetallic site strategy based on Hg2+-triggered oxidase-like catalytic activity of NSC/Co6Ni3S8 nanocomposite

Abstract

Modulation of the nanozyme's catalytic activity is crucial for its real applications in detecting target analytes. Herein, we fabricated the nanocomposite (NSC/Co₆Ni₃S₈) of N, S co-doped carbon and Co₆Ni₃S₈ by a facile sol-gel approach. Compared to NSC/Ni₉S₈, NSC/Co₆Ni₃S₈ with bimetallic active sites displayed better enzyme-mimetic activity. This nanocomposite could catalyze O₂ to form ·O₂^- and oxidize colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into blue oxTMB. The other two free radicals (h⁺ and ·OH) played minor roles during the catalytic reaction. Hg²⁺ could integrate with S^2- to form HgS and the surface charges of O₂ were transferred to Hg²⁺ to promote O₂ adsorption. DFT theoretical calculations highlight that the main reasons for the enhancing effect of Hg²⁺ on color development results from electron transfer and increased adsorption energy of O₂ molecules onto the surface of NSC/Co₆Ni₃S₈. By employing the oxidase-like activity of NSC/Co₆Ni₃S₈ and Hg²⁺-triggered promoting effect, a colorimetric sensing platform was established for Hg²⁺ assay with a linear range of 10-200 μg/L and detection limit of 3 μg/L. Through integration with a smartphone-based APP "Thing Identify" software, a visual colorimetric assay for Hg²⁺ was constructed with a detection limit of 5 μg/L. Compared to the data detected by the mercury vapor meter, the relative recoveries of 92.4-108.1% evidenced the higher accuracy of this smartphone-based visual detection. Overall, the NSC/Co₆Ni₃S₈-based colorimetric assay is convenient, rapid, and visual, and can be applied for routine monitoring of Hg²⁺ in real-world waters under outdoor conditions.

Keywords: Hg(2+) detection; NSC/Co(6)Ni(3)S(8) nanocomposite; Oxidase-mimetic activity; Smartphone-based visual colorimetric assay; Superoxide anion.