Carbon-Nitrogen Conjugates Composited Cu1.8S with Enhanced Peroxidase-like Activity in Colorimetric Detection of Hydrogen Peroxide and Glutathione

In this work, cystine–glucose Maillard conjugates were composited with Cu1.8S microspheres (Cu1.8S–cgmc) to achieve higher sensitivity for the colorimetric analysis. Enhanced peroxidase-like activity was obtained in Cu1.8S–cgmc with a carbon elemental ratio of 1.83% compared to bare Cu1.8S. The catalytic activity of Cu1.8S–cgmc followed Michaelis–Menten kinetic behavior. The Michaelis–Menten constants of Cu1.8S–cgmc on the substrate was over 2-fold lower than that of the bare Cu1.8S, indicating the higher affinity of Cu1.8S–cgmc. The adsorption equilibrium constant of Cu1.8S–cgmc on the substrate was 9.89-fold higher than that of bare Cu1.8S based on thermodynamic investigations. The conjugated structure and carboxyl, hydroxyl and amino groups on Cu1.8S–cgmc improved its hydrophilicity and adsorption on the substrate. The affinity-improved Cu1.8S–cgmc was applied as a peroxidase mimic in colorimetric detection with promoted sensitivity. Compared to bare Cu1.8S, Cu1.8S–cgmc had a 25.5-fold and 19.8-fold lower LOD for H2O2 and glutathione, respectively. The Cu1.8S–cgmc-based colorimetric method showed excellent sensitivity and accuracy in practical colorimetric detection.