<p>Sensitive and rapid determination of ascorbic acid (AA) is essential for clinical diagnostics, oxidative stress assessment, and food quality monitoring. Herein, a novel synergistic SrO/MnO₂ nanozyme with oxygen vacancies and mixed valences of manganese for fast and colorimetric detection of AA is reported without using any chemical agents and without using chromogenic substrate or oxidant in the assay procedure. In contrast with the current nanozyme-based assays, which depend on the use of chromogenic substrates and oxidants for AA sensing, this proposed sensing strategy is based on the natural redox reaction of AA with the nanozyme SrO/MnO₂, allowing direct detection of AA without using 3,3ʹ,5,5ʹ-tetramethylbenzidine (TMB) or hydrogen peroxide (H₂O₂). Under optimum conditions, the developed nanozyme offered an extensive linear dynamic range from 1 to 100&#xa0;µM, a low limit of detection of 0.77&#xa0;µM, a low limit of quantitation of 2.5&#xa0;µM, and the whole test took just 2&#xa0;min. Besides, the developed sensor proved to be highly selective toward AA, exhibiting good discrimination between AA and other potentially interfering biomolecules and inorganic ions. Moreover, the SrO/MnO₂ nanozyme possessed high catalytic stability under various pHs and temperatures and could be used multiple times, pointing to its high stability and cost-effectiveness as opposed to enzymatic sensors. Practical applicability of the designed sensing system was confirmed through successful detection of AA in human serum, commercial vitamin C tablets, and beverages with no negative matrix effects. These results clearly indicate that the heterostructure SrO/MnO₂ nanozyme can form a fast, sensitive, and eco-friendly TMB- and H₂O₂-free colorimetric sensor, having great potential in clinical, food safety, and point-of-care testing of antioxidants.</p> Graphical abstract <p></p> <p>Graphical representation of ascorbic acid detection without the use of enzymes or any other material.</p>