Bimetal loaded graphitic carbon nitride with synergistic enhanced peroxidase-like activity for colorimetric detection of p-phenylenediamine†

In recent years, great progress has been made on the study of nanozymes with enzyme-like properties. Here, bimetallic Fe and Ni nanoclusters were anchored on the nanosheets of nitrogen-rich layered graphitic carbon nitride by one-step pyrolysis at high temperature (Fe/Ni–CN). The loading content of Fe and Ni on Fe/Ni–CN is as high as 8.0%, and Fe/Ni–CN has a high specific surface area of 121.86 m² g⁻¹. The Fe/Ni–CN can effectively oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂, and exhibits efficient peroxidase-like activity, leading to a 17.2-fold increase compared to pure graphitic carbon nitride (CN). Similar to the natural horseradish peroxidase (HRP), the Fe/Ni–CN nanozyme follows catalytic kinetics. The Michaelis–Menten constant (K_m) value of the Fe/Ni–CN nanozyme for TMB is about 8.3-fold lower than that for HRP, which means that the Fe/Ni–CN nanozyme has better affinity for TMB. In addition, the catalytic mechanism was investigated by combination of free radical quenching experiments and density-functional theory (DFT) calculations. The results show that the high peroxidase-like activity is due to the easy adsorption of H₂O₂ after bimetal loading, which is conducive to the production of hydroxyl radicals. Based on the extraordinary peroxidase-like activity, the colorimetric detection of p-phenylenediamine (PPD) was constructed with a wide linear range of 0.2–30 μM and a low detection limit of 0.02 μM. The sensor system has been successfully applied to the detection of residual PPD in real dyed hair samples. The results show that the colorimetric method is sensitive, highly selective and accurate. This study provides a new idea for the efficient enhancement of nanozyme activity and effective detection of PPD by a bimetallic synergistic strategy.