Biomimetic Surface Engineering of Polydopamine-Modified Carbon Quantum Dots Enables Light-Switchable Peroxidase/Catalase Activity

The use of light as an external trigger enables dynamic control over enzyme-mimicking nanomaterials is an attractive method for smart and switchable nanozyme systems. Herein, we report a metal-free, bioinspired nanozyme platform based on polydopamine-functionalized carbon quantum dots (PDA@CQDs) synthesized from recycled polyethylene terephthalate (PET) via a two-step carbonization–hydrothermal strategy followed by controlled surface polymerization. Comprehensive structural and spectroscopic characterizations, including FT-IR, XPS, XRD, DLS, and ¹³ C NMR analyses, clearly confirmed the formation of a nitrogen- and oxygen-rich polydopamine shell on the CQDs. Kinetic studies revealed that PDA@CQDs exhibit outstanding peroxidase-like activity in the dark with a V _max of 11.6 × 10 ^− 7 M·s ^− 1 and an exceptionally low K _m of 0.14 mM, outperforming horseradish peroxidase and many reported nanozymes. Remarkably, upon light irradiation, the catalytic behavior showed a complete and reversible switch to dominant catalase-like activity (V _max = 14.5 × 10 ^− 7 M·s ^− 1 , K _m = 0.83 mM), efficiently decomposing H ₂ O ₂ into H ₂ O and O ₂ while suppressing peroxidase activity. This photo-triggered duality is governed by surface-engineered quinone/semiquinone redox states within PDA and light-driven electron transfer from surface of CQD. The presented work establishes surface-engineered quantum dots as programmable nanozymes, offering a sustainable and tunable strategy for next-generation catalytic, biomedical, and environmental technologies.