Enzyme-Mimetic Antioxidant Luminescent Nanoparticles for Highly Sensitive Hydrogen Peroxide Biosensing

Hydrogen peroxide (H₂O₂) is an abundant molecule associated with biological functions and reacts with natural enzymes, such as catalase. Even though direct H₂O₂ measurement can be used to diagnose pathological conditions, such as infection and inflammation, H₂O₂ quantification further enables the detection of disease biomarkers in enzyme-linked assays (e.g., ELISA) in which enzymatic reactions may generate or consume H₂O₂. Such a quantification is often measured optically with organic dyes in biological media that suffer, however, from poor stability. Currently, the optical H₂O₂ biosensing without organic-dyes in biological media and at low, submicromolar, concentrations has yet to be achieved. Herein, we rationally design biomimetic artificial enzymes based on antioxidant CeO₂ nanoparticles that become luminescent upon their Eu³⁺ doping. We vary systematically their diameter from 4 to 16 nm and study their catalase-mimetic antioxidant activity, manifested as catalytic H₂O₂ decomposition in aqueous solutions, revealing a strong nanoparticle surface area dependency. The interaction with H₂O₂ influences distinctly the particle luminescence rendering them highly sensitive H₂O₂ biosensors down to 0.15 μM (5.2 ppb) in solutions for biological assays. Our results link two, so far, unrelated research domains, the CeO₂ nanoparticle antioxidant activity and luminescence by rare-earth doping. When these enzyme-mimetic nanoparticles are coupled with alcohol oxidase, biosensing can be extended to ethanol exemplifying how their detection potential can be broadened to additional biologically relevant metabolites. The enzyme-mimetic nanomaterial developed here could serve as a starting point of sophisticated in vitro assays toward the highly sensitive detection of disease biomarkers.