Hydrogen peroxide detection has been widely applied in the fields of biology, medicine, and chemistry. Colorimetric detection of hydrogen peroxide has proven to be a fast and convenient method. In this work, 5,10,15,20-tetrakis(4-chlorophenyl) porphyrin modified Co[Formula: see text]S[Formula: see text] nanocomposites (H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text] were prepared via a facile one-step hydrothermal method. H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text] nanocomposites were demonstrated to possess an enhanced mimetic peroxidase activity toward the substrate, 3,3[Formula: see text],5,5[Formula: see text]-tetramethylbenzidine (TMB), which can be oxidized to oxTMB (oxidized TMB) in a buffer solution of hydrogen peroxide with a color change from colorless to blue. The catalytic activity of H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text] was further analyzed by steady-state kinetics, and H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text] had high affinity towards both TMB and H[Formula: see text]O[Formula: see text]. Furthermore, fluorescence and ESR data revealed that the catalytic mechanism of the peroxidase activity of H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text] is due to hydroxyl radicals generated from decomposition of H[Formula: see text]O[Formula: see text]. Based on the catalytic activity of H[Formula: see text]TClPP-Co[Formula: see text]S[Formula: see text], a sensitive colorimetric sensor of H[Formula: see text]O[Formula: see text] with a detection limit of 6.803 [Formula: see text]M as well as a range of 7–100 [Formula: see text]M was designed.
