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Artificial nanozymes have been receiving considerable interest for their outstanding performance and wide application. However, their low activity results in a high concentration of substrates, costs, and environmental pollution. To enhance nanozymic activity, a composite, FexNi1−xS2/hollow carbon spheres (FexNi1−xS2/SC), was facilely synthesized by a solvothermal method. The response surface methodology (RSM) was used to optimize the Ni content in FexNi1−xS2/SC and the experimental conditions, where Fe0.75Ni0.25S2/SC exhibited the highest activity. The Km (Michaelis–Menten's constant) values of Fe0.75Ni0.25S2/SC are 0.025 and 0.021 mM with H2O2 and oxidized 3,3′,5,5′-tetramethylbenzidine (TMB) as the substrates, respectively, which are 148 times and 20.5 times lower than those with HRP, 1.88 and 7.19 times lower than those of FeS2/SC, and 1.88 and 10.52 times lower than those of Fe0.8Ni0.2S2, meaning a strong affinity of Fe0.75Ni0.25S2/SC for the substrate. The catalytic efficiency (Kcat/Km) of Fe0.75Ni0.25S2/SC was 5.4 (H2O2) and 27.4 times (TMB), and 9.7 (H2O2) and 66.2 times (TMB) higher than those of FeS2/SC and Fe0.8Ni0.2S2, respectively. The effects of the synergistic interaction between Fe and Ni, the S–C bond formation, and the hollow carbon spheres on the activity were studied. A nanozymic mechanism was proposed. Fe0.75Ni0.25S2/SC could be used to detect cysteine (Cys) at room temperature in 1 min with a detection limit (LOD) of 0.049 μM. [ABSTRACT FROM AUTHOR]

Pyrite FeS2 has been applied as a peroxidase due to its easy preparation and low cost. However, the low peroxidase-like (POD) activity limited its wide application. A hollow sphere-like composite (FeS2/SC-5.3%) composed of pyrite FeS2 and sulfur-doped hollow sphere-shaped carbon was synthesized by a facile solvothermal method, where the S-doped carbon was in situ formed during FeS2 formation. The synergistic action such as the defects at the carbon surface and the formation of S–C bonding improved the nanozyme activity. The S–C bonding was a bridge between the carbon and the Fe atom in FeS2, which enhanced the electron transfer between the Fe atom and the carbon and accelerated the conversion from Fe3+ to Fe2+. The optimum experimental conditions were obtained by the response surface methodology (RSM). The POD-like activity of FeS2/SC-5.3% was significantly improved compared to that of FeS2. The Michaelis–Menten constant (Km) of FeS2/SC-5.3% is 80 times lower than that of horseradish peroxidase (HRP, natural enzyme). FeS2/SC-5.3% can be used to detect cysteine (Cys) with a limit of detection (LOD) as small as 0.061 μM at room temperature in only 1 min. [ABSTRACT FROM AUTHOR]