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 Co3 S4 microspheres have been verified to possess excellent peroxidase-like activity, which can be further improved by adding Hg2+ . Very interestingly, Hg2+ can trigger "ON" the oxidase-like activity of Co3 S4 microspheres. The dual peroxidase-/oxidase-like activity of the yolk-shell Co3 S4 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 Co3 S4 microspheres, is attributed to the in situ generation of HgS on the surface of Co3 S4 microspheres and then the release of more active sites. Importantly, the in situ generated HgS on the surface of Co3 S4 microspheres can form a heterojunction, which also accelerates the catalytic process. During the catalytic reaction, some active species (O2 - and h+ ) can be detected by ESR. Thus, a colorimetric sensing platform based on Hg2+ -triggered signal amplification has been successfully constructed, which can be validated by the detection of Hg2+ residue in environmental water.