Platinum nanoparticles (PtNPs) are classical peroxidase-like nanozyme; self-agglomeration of nanoparticles leads to the undesirable reduction in stability and catalytic activity. Herein, a hybrid peroxidase-like nanocatalyst consisting of PtNPs in situ growing on g–C₃N₄ nanosheets with enhanced peroxidase-mimic catalytic activity (PtNP@g–C₃N₄ nanosheets) was prepared for H₂O₂ and oxidase-based colorimetric assay. g–C₃N₄ nanosheets can be used as carriers to solve the problem of poor stability of PtNPs. We observed that the catalytic ability could be maintained for more than 90 days. PtNP@g–C₃N₄ nanosheets could quickly catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), and the absorbance of blue color oxidized TMB (oxTMB) showed a robust linear relationship with the concentration of H₂O₂ (the detection limit (LOD): 3.33 μM). By utilizing H₂O₂ as a mediator, this strategy can be applied to oxidase-based biomolecules (glucose, organophosphorus, and so on, that generate or consume hydrogen peroxide) sensing. As a proof of concept, a sensitive assay of cholesterol that combined PtNP@g–C₃N₄ nanosheets with cholesterol oxidase (ChOx) cascade catalytic reaction was constructed with an LOD of 9.35 μM in a widespread range from 10 to 800 μM (R² = 0.9981). In addition, we also verified its ability to detect cholesterol in fetal bovine serum. These results showed application prospect of PtNP@g–C₃N₄ nanosheets-based colorimetry in sensing and clinical medical detection.

Nucleocapsid protein (N protein) is an appropriate target for early determination of viral antigen-based severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We have found that β-cyclodextrin polymer (β-CDP) has shown a significant fluorescence enhancement effect for fluorophore pyrene via host–guest interaction. Herein, we developed a sensitive and selective N protein-sensing method that combined the host–guest interaction fluorescence enhancement strategy with high recognition of aptamer. The DNA aptamer of N protein modified with pyrene at its 3′ terminal was designed as the sensing probe. The added exonuclease I (Exo I) could digest the probe, and the obtained free pyrene as a guest could easily enter into the hydrophobic cavity of host β-CDP, thus inducing outstanding luminescent enhancement. While in the presence of N protein, the probe could combine with it to form a complex owing to the high affinity between the aptamer and the target, which prevented the digestion of Exo I. The steric hindrance of the complex prevented pyrene from entering the cavity of β-CDP, resulting in a tiny fluorescence change. N protein has been selectively analyzed with a low detection limit (11.27 nM) through the detection of the fluorescence intensity. Moreover, the sensing of spiked N protein from human serum and throat swabs samples of three volunteers has been achieved. These results indicated that our proposed method has broad application prospects for early diagnosis of coronavirus disease 2019.