Sensitive colorimetric assay of hydrogen peroxide and glucose in humoral samples based on the enhanced peroxidase-mimetic activity of NH2-MIL-88-derived FeS2@CN nanocomposites compared to its precursors.

By employing NH2-MIL-88 as a template, we synthesized the intermediate Fe@CN under high-temperature calcination and further fabricated the FeS2@CN nanocomposites in the presence of sulfur powder. Under varying temperatures (300-600 °C) and Fe@CN-to-S ratios (1:3-6), FeS2@CN500-5 nanocomposites had the highest peroxidase-mimetic activity. Under optimized conditions (incubation temperature 40 °C; solution pH 4.0 and nanocomposite concentration 10 µg/mL; 652-nm absorption), the Michaelis-Menten constant (Km) of FeS2@CN was much lower than that of horseradish peroxidase (HRP), therefore demonstrating that it had a higher affinity for both chromogenic substrates than conventional HRP. The limits of detection for H2O2 and glucose were 0.15 and 0.30 µmol/L, respectively, and the recoveries for glucose were 91.8-103% with RSDs <5.2%. The novelty of this study lies in (1) the FeS2@CN was confirmed to possess stronger enzyme-mimetic activity than its precursors (NH2-MIL-88 and Fe@CN); (2) the enhanced activity resulted from the unsaturated sites of N and S doping and the plentiful defects on the porous carbon surface; and (3) free radical trapping experiments evidenced that •OH played a major role in the catalytic reaction, while h+ and •O2- simultaneously participated in the catalytic process. These convincing performance metrics lead us to postulate that the FeS2@CN-based colorimetric biosensor provides a promising approach for several real-world applications, such as point-of-care diagnosis and workplace health evaluations.