Novel lignin-derived carbon dots nanozyme cascade-amplified for ratiometric fluorescence detection of xanthine oxidase and intracellular imaging

The development of lignin-derived carbon dots with dual functionality of fluorescence and enzyme-like activity was realized by hydrochloric acid mediated carbon dots modification, and then a ratiometric fluorescence sensing strategy to detect xanthine oxidase activity was constructed. Sodium lignosulfonate (LS) was used as the carbon source and modified lignin carbon dots (CDs) with both fluorescence and peroxidase-like activities were prepared. DFT theoretical calculations and experiments demonstrated that carbonyl, carboxyl, and chloride on the surface of carbon dots were potential active sites to catalyze production of ·OH from H2O2. Xanthine oxidase generated H2O2 in the process of catalyzing the production of uric acid from the substrate xanthine. The ·OH produced via LS-CDs catalyzed H2O2 oxidized residual o-phenylenediamine (OPD) groups on surface of OPD-CDs to enhance yellow fluorescence, accompanied by blue fluorescence quenching of LS-CDs. The realization of a highly sensitive, highly selective, and visualized ratiometric fluorescence sensing strategy was achieved. The linear range of the established fluorescence sensing system for xanthine oxidase was 0.5-120 U/L with detection limit of 0.13 U/L. The cascade reaction signal transduction improves the designability of sensing method. Finally, the system successfully realized the sensing of xanthine oxidase in serum and cells.