pH-Dependent Oxidase- and Hydrolase-Mimicking Activity of Nanoceria and Its Composite with Nitrogen-Doped Carbon Quantum Dots for Sensitive Detection of Dopamine, Glutathione, and Methyl Paraoxon

Nitrogen-doped carbon quantum dot-cerium oxide (NCQD-CeO₂) nanocomposites were developed as an enzyme-free, multi-analyte sensing platform based on fluorescence resonance energy transfer (FRET). The sensing mechanism arises from the redox-active Ce³⁺/Ce⁴⁺ couple of cerium oxide, which catalyzes analyte-specific reactions and generates in situ FRET acceptors that modulate NCQD fluorescence. CeO₂ nanoparticles catalyze the oxidation of dopamine to aminochrome (quinone), whose absorption overlaps with NCQD emission, resulting in fluorescence quenching in both acidic and basic conditions. Dopamine self-polymerization under basic conditions is suppressed by glutathione through its antioxidant activity, while nanoceria enhances electron-transfer kinetics. Glutathione further regulates the Ce³⁺/Ce⁴⁺ redox equilibrium, enabling indirect detection via modulation of FRET efficiency. In the case of methyl paraoxon, CeO₂ exhibits phosphatase-like activity and catalyzes its conversion to p-nitrophenol, which acts as a FRET acceptor and quenches NCQD fluorescence. The analyte-dependent formation of distinct FRET acceptors allows selective signal transduction, establishing the NCQD-CeO₂ nanocomposite as a versatile platform for multi-analyte sensing.