Bimetallic Fe3O4@Co3O4/CN as a Nanozyme with Dual Enzyme-Mimic Activities for the Colorimetric Determination of Mercury(II)

Colorimetric biosensor-based nanozymes have received considerable attention in various fields thanks to the advantages of the simple preparation, good stability, and regulable catalytic activity of nanozymes. In this study, a bimetallic nanozyme Fe3O4@Co3O4/CN was prepared via the high-temperature calcination of Fe3O4-PVP@ZIF-67. The material retained its skeletal structure before calcination, which prevented the aggregation of nanoparticles and exposed more active sites of the nanozyme, substantially enhancing the intrinsic dual enzyme-mimetic activities, including peroxidase- and oxidase-like activities. In particular, Fe3O4@Co3O4/CN with oxidase-like activity catalyzed the colorless tetramethylbenzidine (TMB) to become blue oxTMB with oxygen. Reducing glutathione (GSH) could inhibit the above oxidation reaction. In contrast, with respect to the existence of mercury(II), GSH bound to mercury(II) due to the strong affinity between mercury(II) and -SH, thus eliminating the inhibition and restoring the oxTMB signal. A simple and effective colorimetric sensor was fabricated to detect mercury(II) based on the above principles. The proposed measurement had a linear range of 0.1–15 μM and a limit of detection (LOD) of 0.017 μM. It was shown that the established colorimetric sensing system could be successfully applied to detect mercury(II) in water samples, and the Fe3O4@Co3O4/CN nanozyme proved to be a promising candidate for biosensing application.