Achieving smartphone-based colorimetric assay for Hg with a bimetallic site strategy based on Hg-triggered oxidase-like catalytic activity of NSC/CoNiS nanocomposite.

Modulation of the nanozyme's catalytic activity is crucial for its real applications in detecting target analytes. Herein, we fabricated the nanocomposite (NSC/CoNiS) of N, S co-doped carbon and CoNiS by a facile sol-gel approach. Compared to NSC/NiS, NSC/CoNiS with bimetallic active sites displayed better enzyme-mimetic activity. This nanocomposite could catalyze O to form ·O 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. Hg could integrate with S to form HgS and the surface charges of O were transferred to Hg to promote O adsorption. DFT theoretical calculations highlight that the main reasons for the enhancing effect of Hg on color development results from electron transfer and increased adsorption energy of O molecules onto the surface of NSC/CoNiS. By employing the oxidase-like activity of NSC/CoNiS and Hg-triggered promoting effect, a colorimetric sensing platform was established for Hg 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 Hg 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/CoNiS-based colorimetric assay is convenient, rapid, and visual, and can be applied for routine monitoring of Hg in real-world waters under outdoor conditions.