Cerium‐Engineered Nanozyme With Oxidase‐Like Activity for Upconversion Luminescence‐based Sensing of Ascorbic Acid

Currently, coupling catalytic activity with programmable photonic responses in upconverting nanostructures remains a key challenge for advanced optical sensing. Here, we present a Cerium‐engineered upconverting nanozyme (CS‐UCNP@PAA‐Ce) that integrates oxidase‐mimicking catalysis with near‐infrared (NIR)‐excited luminescence modulation. The nanozyme is constructed from red‐emitting NaYF 4 :Yb 3 + ,Er 3 + @CaF 2 core‐shell upconverting nanoparticles (CS‐UCNP), where the epitaxial CaF 2 shell suppresses surface quenching to amplify red emission, while poly(acrylic acid) (PAA) enables aqueous stability and effective anchoring of Ce 4 + catalytic sites. Distinct from conventional nanozyme systems, the Ce 4 + ‐functionalized interface drives H 2 O 2 ‐free oxidation of 3,3′,5,5′‐tetramethylbenzidine (TMB), generating oxidized TMB (ox‐TMB) whose absorption spectrally overlaps with the nanozyme emission that induces efficient emission attenuation via an inner filter effect. Leveraging this mechanism, a NIR‐operated “OFF→ON” sensing platform is established for ascorbic acid, achieving limits of detection of 1.10 µM in water and 0.88 µM in PBS buffer (pH 5.5), together with high selectivity and strong anti‐interference against diverse co‐existing species. Benefiting from NIR excitation, the nanozyme system minimizes autofluorescence and photodamage while enabling reliable sensing in complex matrices. The practical utility is validated in tap water, bovine serum albumin medium, and fruit samples, demonstrating its potential for optical sensing in complex food, environmental, and biological matrices.