Facile Synthesis of Iron and Nitrogen Co-Doped Carbon Dot Nanozyme as Highly Efficient Peroxidase Mimics for Visualized Detection of Metabolites

Visual detection based on nanozymes has great potential for the rapid detection of metabo-lites in clinical analysis or home-based health management. In this work, iron and nitrogen co-doped carbon dots (Fe,N-CDs) were conveniently synthesized as a nanozyme for the visual detection of glucose (Glu) or cholesterol (Chol). Using inexpensive and readily available precursors, Fe,N-CDs with peroxidase-like activity were conveniently prepared through a simple hydrothermal method. Co-doping of Fe and N atoms enhanced the catalytic activity of the nanozyme. The nanozyme had a low Michaelis constant (Km) of 0.23 mM when hydrogen peroxide (H2O2 ) was used as the substrate. Free radical trapping experiments revealed that the reactive oxygen species (ROS) generated in the nanozyme-catalyzed process were superoxide anion radicals (.O2-), which can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue oxidation product (ox-TMB) with character-istics absorbance at 652 nm. Based on this mechanism, a colorimetric sensor was constructed to detect H2O2 ranging from 0.1 uM to 200 uM with a detection limit (DL) of 75 nM. In the presence of glucose oxidase (Gox) or Chol oxidase (Chox), Glu or Chol was oxidized, respectively, and generated H2O2 . Based on this, indirect detection of Glu and Chol was realized with linear detection ranges of 5-160 uM and 2-200 uM and DLs of 2.8 uM and 0.8 uM, respectively. A paper-based visual detection platform was fabricated using Fe,N-CDs as nanozyme ink to prepare testing paper by inkjet printing. Using a smartphone to record the RGB values of the testing paper after the reaction, visual detection of Glu and Chol can be achieved with linear detection ranges of 5-160 uM (DL of 3.3 uM) and 2-200 uM (DL of 1.0 uM), respectively.