In situ Hg 2+ improved the peroxidase-like activity and triggered "ON" the oxidase-like activity of yolk-shell Co 3 S 4 microspheres for the detection of Hg 2 .

Exploring highly active nanozymes is an important task to realize the real-time detection of some heavy metal ions in water. In this work, yolk-shell Co 3 S 4 microspheres have been verified to possess excellent peroxidase-like activity, which can be further improved by adding Hg 2+ . Very interestingly, Hg 2+ can trigger "ON" the oxidase-like activity of Co 3 S 4 microspheres. The dual peroxidase-/oxidase-like activity of the yolk-shell Co 3 S 4 microspheres is evaluated by using the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB). Furthermore, comprehensive studies verify that the enhanced peroxidase-like activity, together with the "ON" oxidase-like activity of the yolk-shell Co 3 S 4 microspheres, is attributed to the in situ generation of HgS on the surface of Co 3 S 4 microspheres and then the release of more active sites. Importantly, the in situ generated HgS on the surface of Co 3 S 4 microspheres can form a heterojunction, which also accelerates the catalytic process. During the catalytic reaction, some active species (O 2 - and h + ) can be detected by ESR. Thus, a colorimetric sensing platform based on Hg 2+ -triggered signal amplification has been successfully constructed, which can be validated by the detection of Hg 2+ residue in environmental water.