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

Modulation of the nanozyme's catalytic activity is crucial for its real applications in detecting target analytes. Herein, we fabricated the nanocomposite (NSC/Co 6 Ni 3 S 8 ) of N, S co-doped carbon and Co 6 Ni 3 S 8 by a facile sol-gel approach. Compared to NSC/Ni 9 S 8 , NSC/Co 6 Ni 3 S 8 with bimetallic active sites displayed better enzyme-mimetic activity. This nanocomposite could catalyze O 2 to form ·O 2 − 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 2+ could integrate with S 2− to form HgS and the surface charges of O 2 were transferred to Hg 2+ to promote O 2 adsorption. DFT theoretical calculations highlight that the main reasons for the enhancing effect of Hg 2+ on color development results from electron transfer and increased adsorption energy of O 2 molecules onto the surface of NSC/Co 6 Ni 3 S 8 . By employing the oxidase-like activity of NSC/Co 6 Ni 3 S 8 and Hg 2+ -triggered promoting effect, a colorimetric sensing platform was established for Hg 2+ 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 2+ 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/Co 6 Ni 3 S 8 -based colorimetric assay is convenient, rapid, and visual, and can be applied for routine monitoring of Hg 2+ in real-world waters under outdoor conditions.