Hydrogen peroxide (HO) is an abundant molecule associated with biological functions and reacts with natural enzymes, such as catalase. Even though direct HO measurement can be used to diagnose pathological conditions, such as infection and inflammation, HO quantification further enables the detection of disease biomarkers in enzyme-linked assays (e.g., ELISA) in which enzymatic reactions may generate or consume HO. Such a quantification is often measured optically with organic dyes in biological media that suffer, however, from poor stability. Currently, the optical HO 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 CeO nanoparticles that become luminescent upon their Eu doping. We vary systematically their diameter from 4 to 16 nm and study their catalase-mimetic antioxidant activity, manifested as catalytic HO decomposition in aqueous solutions, revealing a strong nanoparticle surface area dependency. The interaction with HO influences distinctly the particle luminescence rendering them highly sensitive HO biosensors down to 0.15 μM (5.2 ppb) in solutions for biological assays. Our results link two, so far, unrelated research domains, the CeO 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.

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