5,10,15,20-tetrakis (4-carboxylphenyl) porphyrin functionalized NiCo2S4 yolk-shell nanospheres: Excellent peroxidase-like activity, catalytic mechanism and fast cascade colorimetric biosensor for cholesterol

5,10,15,20-tetrakis (4-carboxylphenyl) porphyrin functionalized NiCo2S4 yolk-shell nanospheres (Por-NiCo2S4) were fabricated by a facile and efficient solvothermal strategy and demonstrated to possess excellent peroxidaselike activity. Compared with pure NiCo2S4 nanospheres, Por-NiCo2S4 exhibits the superior peroxidase activity, which was evaluated by the fast oxidation of 3,3',5,5'-tetramethylbenzidine (a chromogenic substrate, TMB) by H2O2 to form the blue product (oxTMB) only in 10 s. The systematic studies including radical scavenger, fluorescence probe, and electrochemical experiments confirm that superoxide free radicals (center dot O-2(-)) play a key role in catalytic reactions, in addition to the synergistic effect of holes (h(+)). Based on the developed pemxidase activity of Por-NiCo2S4 , and combined with cholesterol oxidase (Chox), a fast cascade reaction sensing platform has been constructed to quantitatively determination cholesterol in a wide linear range of 0.1-9.0 mM with the detection limit of 19.36 mu M. The Por-NiCo2S4 based colorimetric biosensing platform has been used for determining the levels of total cholesterol in serum samples, indicating Por-NiCo2S4 is expected to be used as the promising materials for fast visual colorimetry in clinical medical detection.