Co,N,S co-doped hollow carbon with efficient oxidase-like activity for the detection of Hg2+ and Fe3+ ions

Most nanozymes have limited catalytic ability, and constructing high performance nanozymes is important and challenging. Metal-carbon hybrid materials obtained by integrating transition metals into carbon can not only generate rich active sites, but also enhance their electrical conductivity. Heteroatom doping is shown to be an efficient strategy to improve the catalytic activity of carbon-based nanozymes. Here, by taking advantage of the structural merits of MOFs-derived porous carbon materials combined with sulfur doping strategy, the Co,N,S co-doped hollow carbon material, i.e. Co-N/S-HCN, was fabricated by pyrolyzing a thiomalic acid (TA) modified zeolitic imidazolate framework ([email protected]) in a nitrogen atmosphere. It exhibits hollow dodecahedron shape with rich defects, high BET specific surface area of 240.75 m2/g and pore volume of 0.27 cm3/g. The electrochemical characterization reveals the doping of S reduces the charge transfer resistance of the Co-N-HCN, accelerates electron transfer rate, and finally boosts its catalytic activity. The Co-N/S-HCN shows superior oxidase-like activity and can convert colorless 3,3′,5,5′-tetramethylbenzidine (TMB), o-phenylenediamine and 2,2′-azo-bis (3-ethylbenzothiazolin-6-sulfonic acid) diammonium salt into visible color substrates without hydrogen peroxide. The oxidase-like activity of Co-N/S-HCN was 2.1 times that of Co-N-HCN. The quenching trials revealed the generation of O2•−, •OH and 1O2 during catalytic TMB oxidation by Co-N/S-HCN. The Co-N/S-HCN oxidase-like nanozyme exhibited good affinity to TMB with a Km value of 0.25 mM and Vmax of 4.19 × 10−7 M s−1. In terms of Soft-Hard Acid-Base principle, Hg2+ combines easily with the sulfur. Thus, the introduction of Hg2+ ions can restrain TMB oxidation catalyzed by Co-N/S-HCN, thereby inhibiting the color reaction. In addition, the addition of 8-hydroxyquinoline can restrain TMB oxidation catalyzed by Co-N/S-HCN, resulting in a decrease in absorbance at 652 nm. However, the color reaction recovers in the presence of Fe3+, resulting in an increase in absorbance at 652 nm. In terms of above fact, a colorimetric sensing platform for determining Hg2+ and Fe3+ ions was established and successfully used for detecting Hg2+ in water samples and Fe3+ in liquors. This work offers a different way to prepare nanozymes with efficient oxidase-like activity and is of great significance for detecting heavy metal ions in environmental and food samples.