Colorimetric Assay Using Mesoporous Fe-Doped Graphitic Carbon Nitride as a Peroxidase Mimetic for the Determination...

Iron can enter the electron-rich cavities of graphitic carbon nitride (g-C₃N₄). On account of this phenomenon, Fe-doped g-C₃N₄ (Fe-g-C₃N₄) was prepared as a peroxidase mimetic by using one-step pyrolysis of urea and FeCl₃·6H₂O. Compared to g-C₃N₄, Fe-g-C₃N₄ has a large specific surface area due to the presence of mesopores and cracks, a smaller band gap, and a high loading of Fe in its structure. Thus, Fe-g-C₃N₄ exhibits greater peroxidase activity with a more obvious color change when using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate in the presence of hydrogen peroxide (H₂O₂). The color of a mixture of TMB and Fe-g-C₃N₄ gradually deepens with increasing concentrations of H₂O₂. Accordingly, a rapid, sensitive, and low-cost colorimetric assay for the detection of H₂O₂ was developed. After optimization, this method boasts a wide linear dynamic range for H₂O₂ detection from 0.005 to 400 μM (r² = 0.9971) with a detection limit of 0.005 μM. Because H₂O₂ is a main product of glucose oxidation by glucose oxidase (GOx), a colorimetric assay for glucose detection was also realized, with a linear dynamic range of 1-1000 μM (r² = 0.9996) and a detection limit of 0.5 μM. These assays were applied to the quantitative detection of H₂O₂ in milk and glucose in human serum, respectively.